Osteology

Ethmoid Bone

Ethmoid Bone Terminology

‘Ethmoid’ is a Greek word that means ‘sieve-like’. Ethmoid is so named because it possesses a perforated (sieve-like) plate called a cribriform plate.

Ethmoid Bone Location

• The single ethmoid bone is situated in the anterior part of the base of the cranium between the orbits.
• It forms part of the medial wall of the orbits and part of the bony septum, roofs, and lateral walls of the nasal cavities.

Ethmoid Bone Features And Attachments

Ethmoid bone consists of a cribriform plate, a perpendicular plate, and two lateral masses called labyrinths.

Ethmoid Bone Cribriform Plate

1. It is the median part of the superior surface of the ethmoid.
2. It contributes to the median portion of the anterior cranial fossa (anterior part of the interior of the base of the skull).
3. It occupies the ethmoidal notch of the frontal bone.
4. It possesses a median triangular upward projection called crista galli (named because it resembled the crown of a cock, the zoological name of which is Gallus domesticus).
5. The posterior sloping border of the crista galli gives attachment to the falx cerebri.
6. The anterior border of the crista galli has two alae which articulate with frontal bone to complete foramen caecum. An emissary vein passes through this foramen.
7. On each side of the crista galli, the cribriform plate shows several perforations through which pass about 15-20 filaments of the olfactory nerve. This part is also related to the olfactory bulb superiorly.
8. Just lateral to the anterior part of the crista galli there is a slit-like passage for a process of dura mater.
9. Just lateral to the anterior end of the slit there is a foramen for the passage of the anterior ethmoidal nerve.

Ethmoid Bone Perpendicular Plate

1. It is a quadrangular flat plate projecting downwards from the midline of the cribriform plate.
2. Its anterior border articulates with the nasal process of the frontal bone and the crest of the nasal bones.
3. Its posterior border articulates with a sphenoidal crest above and vomer below.
4. Its superior border is attached to the cribriform plate.
5. Its inferior border receives the attachment of septal cartilage.
6. Its surfaces are mainly smooth except in the upper parts where there are grooves for filaments of olfactory nerves.

Ethmoid Bone Labyrinths

A large number of air-filled spaces (ethmoidal air cells) constitute the labyrinth. These air cells are divisible into anterior, middle, and posterior ethmoidal sinuses by bony plates.

Many of these air cells open on the surface and are completed only when articulating with the adjacent bones. Each labyrinth may be considered to have six surfaces.

1. Upper Surface

• It has several open-air cells which are completed only after articulation with the edges of the ethmoidal notch.
• It has two grooves which are converted into anterior and posterior ethmoidal canals by articulation with frontal bone.

2. Lower Surface

It articulates with the upper part of the nasal surface of the maxilla to complete the ethmoidal air cells from below.

3. Anterior Surface

It possesses half-cut air sinuses which are completed by the frontal process of maxilla and lacrimal bone.

4. Posterior Surface

It articulates with the sphenoidal concha and orbital process of palatine bone to complete the posterior ethmoidal sinus.

5. Lateral Surface

• It is a thin and smooth plate called an orbital plate.
• It covers the middle and posterior ethmoidal sinuses.
• It forms a large part of the medial wall of the orbit.
• It is quadrangular in shape and articulates as follows:
  1. Superiorly with the orbital plate of the frontal bone.
  2. Inferiorly with the maxilla and orbital process of the palatine bone.
  3. Anteriorly with lacrimal bone.
  4. Posteriorly with sphenoid bone.

6. Medial Surface

1. It forms part of the lateral wall of the corresponding half of the nasal cavity.
2. Its upper part is marked by numerous vertical grooves which lodge filaments of the olfactory nerve.
3. Its posterior part is marked by an anteroposterior fissure called the superior meatus.
4. The posterior ethmoidal sinus opens into the superior meatus.
5. The superior meatus is bounded above by a curved plate called the superior nasal concha.
6. Below and in front of the superior meatus is another curved plate of bone called the middle nasal concha.
7. The lateral surface of the middle concha is concave and forms the medial wall of the middle meatus.
8. The lateral wall of the middle meatus is marked by a swelling produced by the middle. ethmoidal air cells. This swelling is called bulla ethmoidalis.
9. The middle ethmoidal sinus opens on the surface of the bulla or immediately above it.
10. A thin bar of bone called an uncinate process projects downwards and backward from the anterior part of the labyrinth.
11. The curved gap between the uncinate process and bulla is called hiatus semilunaris.
12. The upper end of the hiatus semilunaris is continuous with a curved canal called the ethmoidal infundibulum.
13. Anterior ethmoidal sinus opens into the infundibulum.
14. In 50% of cases, the infundibulum continues superiorly as the frontonasal duct to reach the frontal sinus.

Ethmoid Bone Ossification

1. At the age of 3rd month of intrauterine life, the walls of the nasal cavity are marked by a cartilaginous framework called a cartilaginous nasal capsule.
2. The cartilaginous nasal capsule consists of two lateral regions and a median nasal part.
3. A single center appears for each labyrinth in the lateral region of the nasal capsule at about 5th month of intrauterine life.
4. Perpendicular plate and crista galli ossify from a single center which appears in the median septal part of the nasal capsule at the age of 1st year after birth.
5. The labyrinths fuse with perpendicular plates in the region of the cribriform plate at about 2 years of age.
6. The ethmoid air cells begin to develop during intrauterine life and are present in the form of narrow pouches at birth.

Ethmoid Bone Applied Anatomy

1. A severe impact on the nasal bridge may involve frontal processes of maxillae and two orbital plates of ethmoid bones.
2. In case of head injury, a discharge of CSF from the nose (CSF rhinorrhoea) is indicative of a fracture of the cribriform plate of ethmoid in the anterior cranial fossa.
3. If the basilar fracture involves the cribriform plate, it may result in anosmia (loss of smell sensation) due to damage to olfactory nerve filaments.
4. Fracture of cribriform plates may cause meningitis if it opens into the nasal cavity.
5. The bony septum of the nose (perpendicular plate of ethmoid and vomer) is paper thin and does not resist much to forces responsible for the fracture.
6. The ethmoid is spared in Le Fort 1 fractures while involved in Le Fort 2 and 3 fractures.
7. Since the ethmoid bone is clothed in mucosa over large areas of its surfaces, its fractures open into the nasal cavity or ethmoid air cells with the potential risk of infection.
8. Nasal ethmoidectomy is performed in several operations involving the ethmoidal labyrinth.
9. In this, an artificial opening is made in the ethmoidal labyrinth to drain the sinuses, for example. after removal of frontoethmoidal mucoceles or optic nerve decompression in the optic canal.

Zygomatic Bones

Zygomatic Bones Terminology

The term zygomatic is derived from the Greek word ‘zyg’ which means ‘yoke’. Hence the zygomatic refers to a bone that is shaped like a yoke uniting the frontal, maxilla, and temporal bones.

1. The zygomatic bone is also called ‘malar bone’ because it forms prominence of the cheek which is called ‘mala’ in Latin.
2. The term ‘zygoma’ is used by clinicians which includes both ‘zygomatic bone’ and ‘zygomatic arch’. Anatomists use the term ‘zygoma’ for ‘zygomatic arch’.
3. The term ‘zygomatic complex’ implies to zygomatic bone and other bones adjacent to it, i.e. maxilla and the zygomatic process of frontal bone.

Zygomatic Bones Location

Zygomatic bones are present in the upper and lateral parts of the face.

Zygomatic Bones Features And Attachments

Each zygomatic bone has three surfaces. (lateral, temporal, and orbital), five borders (anterosuperior, anteroinferior, postero- superior, posteroinferior and posteromedial) and two processes (frontal and temporal).

Zygomatic Bones Surfaces

1. Lateral Surface

1. It is convex.
2. Zygomaticofacial foramen is present near the orbital (anterosuperior) border. It transmits zygomaticofacial nerve and vessels.
3. The area below the zygomaticofacial foramen gives origin to two muscles:
   • Zygomaticus major (posteriorly)
   • Zygomaticus minor (anteriorly).

2. Temporal Surface

1. Its anterior part is rough for articulation with the maxilla.
2. Its posterior larger part is smooth and forms the anterior boundary of the temporal fossa.
3. Close to the posteroinferior border, this surface provides attachment to the masseter muscle.
4. The zygomaticotemporal foramen present on this surface transmits the zygomaticotemporal nerve and vessels.

3. Orbital Surface

1. It partly contributes to the lateral wall and floor of the orbit.
2. It possesses zygomatic orbital foramina which transmit:

• • Zygomaticotemporal and zygomaticofacial nerves.
  • Zygomatic branches of the lacrimal artery.

Zygomatic Bones Borders

1. Anterosuperior Border

• This is also called the orbital border.
• It provides attachment to the orbital septum.

2. Anteroinferior Border

• This is also called the maxillary border.
• Levator labii superioris arises partly from this border near the orbital border.

3. Posterosuperior Border

• It is also called the temporal border.
• Temporal fascia is attached to this border.

4. Posteroinferior Border

Masseter muscle originates from this border.

5. Posteromedial Border

It articulates with the greater wing of the sphenoid above and the maxilla below.

Zygomatic Bones Processes

1. Frontal Process

• It articulates with the zygomatic process of the frontal bone (to form
  frontozygomatic suture) superiorly and greater wing of sphenoid bone posteriorly.
• Whitnall’s tubercle is present on its orbital aspect about 1 cm below the frontozygomatic suture. The following structures are attached to this tubercle:
  • Lateral check ligament.
  • Lateral palpebral ligament.
  • Suspensory ligament of the eyeball.
  • Aponeurosis of levator palpebrae superioris.

2. Temporal Process

• It is directed backward.
• It articulates with the zygomatic process of the temporal bone to complete the zygomatic arch.
• Its inferior margin and medial surface provide attachment to the masseter muscle.

Zygomatic Bones Ossification

Zygomatic bone ossifies in the membrane.

1. Usually single centre appears at the age of the 8th week of intrauterine life.
2. Sometimes a horizontal suture divides the bone into an upper larger and a lower smaller segments.

Zygomatic Bones Applied Anatomy

1. Malar flush is the redness of the skin over zygomatic prominence. This is observed in tuberculosis, mitral stenosis (narrowing of the left atrioventricular orifice), and rheumatic fever.
2. The zygomatic bone is of great clinical importance due to its functional significance
   • It protects the globe of the eye.
   • It gives origin to the masseter muscle.
   • It transmits part of the masticatory forces to the cranium
   • It absorbs the force of an impact before it reaches the brain.
3. When a rapidly moving object hits the zygomatic bone, a comminuted fracture results in the displacement of bone fragments.
4. Tripod fracture means fracture of the zygomatic complex. The zygomatic bone is like a legged stool.
5. The seat of the stool is the main portion of the zygomatic bone while the legs are the frontal process, inferior orbital margin, and zygomatic buttress.
6. The frontozygomatic suture, zygomatic prominence, zygomatic buttress, and 1st molar tooth, all lie in the same vertical line.
7. In the majority of cases, the zygomatic complex fracture is associated with rotation along this axis.
8. Zygomatic bone forms one of the principal means by which occlusal stress is transmitted from the maxilla and spreads over the base of the skull.
9. Fracture of the zygoma (zygomatic arch and zygomatic bone) is second among the most common fractures of the middle 3rd of the face.
10. The junctions of the frontal and temporal processes of the zygomatic bone form important landmarks in the treatment of maxillofacial injuries.
11. The periosteum and attachment of strong temporal fascia limit the displacement of zygomatic bone following injury.
12. In a depressed fracture of the zygomaticomaxillary complex, there is the displacement of the zygomatic bone without its fracture.
13. Le Fort 3 fracture of the face is a horizontal fracture just below the base of the skull. The fracture line passes laterally through frontozygomatic sutures.
14. As there is a concurrent fracture of zygomatic arches, the maxillae, and zygomatic bones are separated from the rest of the skull.

Occipital Bone

Occipital Bone Terminology

The word ‘occipital’ is derived from the Greek words ‘ob’ (meaning back) and ‘cipit’ (meaning head). Hence referred to the back of the head.

Occipital Bone Location

The occipital bone occupies the posterior part of a skull and plays a major role in the formation of the posterior cranial fossa.

Occipital Bone Features And Attachments

The largest foramen of the skull called foramen magnum, is located in the occipital bone. The components of occipital bone are better described in relation to this foramen.

It consists of 1 basilar part (above and in front of the foramen magnum), 2 lateral parts (lateral to the foramen magnum), and 1 squamous part (above and behind the foramen magnum).

Foramen Magnum

1. It is located on the floor of the posterior cranial fossa.

2. It provides communication between the posterior cranial fossa and vertebral canal.

3. Its margins provide attachments to the following structures:

• Anterior margin: Anterior atlanto-occipital membrane.
• Posterior margin: Posterior atlanto-occipital membrane.
• Lateral margins: Alar ligament.

4. Structures passing through the foramen magnum are as follows:

• Anterior Part
  1. Apical ligament of dens.
  2. Superior longitudinal band of cruciform ligament.
  3. Membrana tectoria.
• Posterior Part
  1. Medulla oblongata.
  2. Meninges.
  3. Spinal roots of accessory nerves.
  4. Meningeal branches of upper cervical nerves (C_1-3)
  5. Vertebral arteries.
  6. Sympathetic plexuses around vertebral arteries.
  7. Anterior and posterior spinal arteries.

Basilar Part

It extends upwards and forwards to meet the body of the sphenoid. Before 25 years of age growth cartilage intervenes between the sphenoid and basilar part of the occipital bone but after this period the two bones fuse. The basilar part has two surfaces (superior and inferior) and two lateral margins.

• Superior Surface
  1. It is smooth.
  2. It forms a clivus with the body of the sphenoid.
  3. It is related to medulla oblongata.
  4. Its lower part receives attachments of the following structures from above downwards:
     • Membrana tectoria.
     • Superior longitudinal band of cruciform ligament.
     • Apical ligament of dens.
  5. Its lateral margins are grooved by inferior petrosal sinuses.
• Inferior Surface
  1. Its middle is marked by a tubercle called the pharyngeal tubercle. This tubercle is approximately 1 cm anterior to the foramen magnum.
  2. Attached to this tubercle is the upper part of the pharyngeal raphe (pharyngeal ligament).
  3. The anterolateral to pharyngeal tubercle is the area for longus capitis.
  4. Posterolateral to pharyngeal tubercle (just in front of condyle) is the attachment of rectus capitis anterior.
• Lateral Margins
  • These are rough and articulate with petrous parts of the temporal bones.

Lateral Parts (Right And Left)

Each can be divided into a broader medial portion (adjacent to the foramen magnum) and a narrower lateral portion (jugular process).

1. Medial Portion

• Inferior surface
  1. It is marked by an articular occipital condyle.
  2. The articular surface of the occipital condyle is oval and convex to articulate with the concave superior articular process of the atlas.
  3. The occipital condyle is located lateral to the anterior half of the foramen magnum.
  4. Behind the condyle is the condylar fossa which may have a condylar canal for the emissary vein from the sigmoid sinus.
  5. Lateral to the anterior part of the condyle is the outer opening of the hypoglossal canal.
• Medial aspect
  1. It has got a tubercle and a foramen.
  2. The tubercle is situated on the medial aspect of the condyle and provides attachment to the alar ligament.
  3. The foramen located just above the tubercle forms the inner opening of the hypoglossal canal.
  4. The following structures pass through the hypoglossal canal:
     • Hypoglossal nerve.
     • Meningeal branch of ascending pharyngeal artery.
• Superior surface
  1. It is marked by an oval eminence called a jugular tubercle.
  2. The jugular tubercle overlies the hypoglossal canal.
  3. The posterior part of the jugular tubercle often presents a shallow groove for 9, 10, and 11 cranial nerves.

2. Jugular Process

1. Its lateral end presents a rough area that joins the jugular surface of the temporal bone by a growth cartilage. The cartilage ossifies at the age of 25 years.
2. Its anterior margin is notched and completes the jugular foramen with a similar notch on the posterior border of the petrous part of the temporal bone.
3. Its superior surface is marked by a deep groove that lodges the terminal part of the sigmoid sinus.
4. Its inferior surface provides attachment to rectus capitis lateralis.

Squamous Part

It has two surfaces (external and internal), three angles (one superior and two lateral), and four borders (two lambdoid and two mastoid).

1. Surfaces

• External Surface
  1. The middle of the external surface is marked by a projection called external occipital protuberance.
  2. Two lines extend laterally on each side from the external occipital protuberance.
  3. A superior faint line is called the highest nuchal line. This provides attachment to the occipital belly of occipitofrontalis
  4. An inferior well-defined line is called a superior nuchal line. This receives attachments of the trapezius and sternomastoid in its medial and lateral parts respectively.
  5. A midline ridge extends from the external occipital protuberance to the foramen magnum. This is called the external occipital crest. It gives attachment to ligament nuchae.
  6. Running laterally on each side from the middle of the external occipital crest is another ridge called the inferior nuchal line.
  7. There are two muscles attached between the superior and inferior nuchal lines on both sides, i.e. semispinalis capitis (medially) and obliquus capitis superior (laterally).
  8. Similarly, there are two muscles attached to each side of the midline below the inferior nuchal line, i.e. rectus capitis posterior major (laterally) and rectus capitis posterior minor (medially).
     • Note: Remember that the rectus capitis muscles are named after their relations with the occipital condyle. The one lying anterior to the condyle is rectus capitis anterior.
     • The muscle lying lateral to the condyle is called rectus capitis lateralis. Rectus capitis posterior will naturally be located behind the condyle.
     • Since the latter muscle is two in number, these are further qualified by adding ‘major’ and ‘minor’ to their names.
     • It is interesting to note that rectus capitis anterior, lateral, and posterior are attached to three components of the occipital bone, i.e. basilar part, lateral part, and squamous part respectively.
• Internal Surface
  1. Its middle is marked by an elevation called internal occipital protuberance which corresponds with the external occipital protuberance on the external surface.
  2. Four grooves diverge from the internal occipital protuberance, one upwards, one downwards, and two laterally whose margins provide attachments to falx cerebri, falx cerebelli, and tentorium cerebelli respectively.
  3. The groove running upwards is produced by the superior sagittal sinus.
  4. The groove running downwards is occupied by the occipital sinus.
  5. The groove extending laterally is produced by the corresponding transverse sinus.
  6. The internal surface is marked by a depression in the midline near the posterior margin of the foramen magnum. This is vermian fossa related to inferior vermis of cerebellum.
  7. Midline elevation extending from internal occipital protuberance to the posterior margin of foramen magnum is called internal occipital crest. Falx cerebelli is attached to it.
  8. On each side of the internal occipital crest, a hollow is related to the cerebellar hemisphere.
  9. The internal surface above the grooves for part of the corresponding temporal bone to form transverse sinuses is related to the occipital lobes of the cerebrum.

2. Angles

• The superior angle reaches lambda which during intrauterine life is membranous (posterior fontanelle).
• The lateral angle on each side meets with the mastoid part of the corresponding temporal bone to form an asterion.

3. Borders

• Lambdoid Border
  • It extends on each side from the superior angle to the lateral angle and articulates with the posterior margin of the corresponding parietal bone to form the lambdoid suture.
• Mastoid Border
  • It extends on each side from a lateral angle to the jugular process and articulates with a mastoid occipitomastoid suture.

Occipital Bone Ossification

1. Origin
   1. Part of the occipital bone above the highest nuchal line develops in the membrane.
   2. The rest of the occipital bone ossifies in cartilage.
2. Appearance Of Centres
   1. Usually, seven centers appear in the 8th week of intrauterine life as follows:
   2. Four for the squamous part (one for each half of the membranous and cartilaginous parts).
      • Two for lateral parts.
      • One for the basilar part.

• Fusion
  1. Membranous and cartilaginous portions fuse with each other when the baby starts holding the neck, i.e. 3rd month.
  2. Squamous part fuses with lateral parts when primary dentition is completed, i.e. at 2 years.
  3. Lateral parts fuse with the basilar part when the permanent dentition begins, i.e. at 6 years.
  4. The basilar part fuses with the sphenoid and the lateral part fuses with the temporal bone at the age of 25 years.
     • Note: Remember, the occipital condyle is contributed partly from the lateral part and partly from the basilar part of the occipital bone.

Occipital Bone Applied Anatomy

The squamous part of the occipital bone contributing to the vault is prone to both fissured and depressed fractures but the portions lying in the base of the skull always show a linear fracture.

1. Margins of the foramen magnum form natural thick bony buttresses at the base of the skull, therefore, the fracture lines often converge towards the foramen magnum.
2. A crack in the inner table of the squamous part of the occipital bone may damage the large diploic vein and produce a small epidural hematoma.
3. Almost invariably fractures of the squamous part of the occipital bone in children are associated with rupture of the dura mater.
4. Basal fracture of the skull involving the hypoglossal canal will damage the hypoglossal nerve.
5. A gap in the squamous part of the occipital bone forming the cranial vault is usually filled with tantalum or titanium due to a lack of regeneration in this part whose periosteum is devoid of the cambium layer.

Auditory Ossicles Terminology

There are three ossicles malleus, incus and stapes.

These names are Latin in origin, the meanings of which are as follows:

• Malleus: Hammer
• Incus: An anvil
• Stapes: A stirrup

Note: Remember that MIS is situated between the tympanic membrane and the oval window where M-Malleus, 1-Incus and S-Stapes.

Auditory Ossicles Features And Attachments 

Features And Attachments Of Malleus

It consists of a head, neck and handle.

• Malleus Head
  1. It is the large upper end of the bone.
  2. It is located within the epitympanic recess.
  3. Its posterior surface articulates with the body of the incus.
• Malleus Neck
  1. It is the constricted part below the head.
  2. Its medial surface is crossed by chorda tympani nerve.
• Malleus Handle
  1. It is the lower elongated part of the malleus.
  2. It is embedded in the tympanic membrane and moves with it.
  3. Its upper end (root) shows the following features:
     • A slight projection on the medial aspect provides attachment to the tendon of the tensor tympani.
     • Anterior process projects forward. Anterior ligament of the malleus is attached to it. This ligament extends into the petrotympanic fissure.
     • The lateral process projects laterally from where extend anterior and posterior malleolar folds to the ends of the tympanic sulcus.

Auditory Ossicles

Features And Attachments Of Incus

It has a large body and two processes (long and short).

• Incus Body
  1. It is cubical in shape.
  2. Its anterior surface is concave and articulates with the head of the malleus.
• Incus Processes
  1. Long Process
     • It projects downwards parallel to the handle of the malleus.
     • Its lower end (lenticular process) bears an articular surface on the medial aspect for articulation with the head of stapes.
  2. Short Process
     • It is directed backwards.
     • It is attached by a ligament to the fossa incudis just below the aditus.

Features And Attachments Of Stapes

It has a head, a neck, two limbs (anterior and posterior) and a foot plate (base).

• Stapes Head
  1. It is rounded
  2. It articulates with the long process of incus.
• Stapes Neck
  1. It is a constricted part adjacent to the head.
  2. The Tendon of the stapedius is attached to its posterior surface.
• Stapes Limbs (Crura)
  1. Anterior and posterior limbs diverge from the neck.
  2. These two limbs are attached to the footplate.
• Stapes Foot Plate (Base)
  1. It is oval in shape.
  2. It fits into the fenestra vestibuli.

Auditory Ossicles

Auditory Ossicles Ossification

1. Malleus and incus develop from the dorsal end of Meckel’s cartilage.
2. Stapes develop from the dorsal end of the hyoid arch cartilage.
3. Malleus ossifies by two centres:
   • One endochondral centre near the neck.
   • One centre for the anterior process appears in dense connective tissue.
     1. Appearance
        • 4th month of intrauterine life.
     2. Fusion
        • 6th month of intrauterine life.
4. Incus ossifies by the single endochondral centre in the upper part of the long process. This centre appears in the 4th month of intrauterine life.
5. Stapes ossify by a single endochondral centre which appears in the base at 4th month of intrauterine life.
6. At birth, the auditory ossicles are of almost adult size.

Auditory Ossicles Functions

The malleus functions as a lever as it is attached to the tympanic membrane.

1. The base of stapes is considerably smaller than the tympanic membrane.
2. Due to this fact, the vibratory force of the stapes is about 10 times that of the tympanic membrane.
3. Thus the auditory ossicles increase the force but decrease the amplitude of vibrations transmitted from the tympanic membrane.

Auditory Ossicles Applied Anatomy

1. Treacher-Collins syndrome is a condition in which there are abnormalities of ossicles and craniofacial skeleton.
2. This may be one of the causes of congenital conductive deafness.
3. Damage to ossicles in cases of head injury with fracture of temporal bone leads to very severe and permanent conductive deafness.
4. Late conductive deafness due to aseptic necrosis of the long process of incus can occur some years after head injury.
5. Ankylosis of stapes is a common occurrence in cases of otosclerosis.

Vertebrae

Vertebrae General Considerations

1. The vertebral column is made up of a number of irregular bones called vertebrae.
2. The vertebral column forms the central axis of the body.
3. There are 33 vertebrae.
4. Vertebrae are named according to the regions they belong to.
5. Following is the classification of vertebrae:
   • Cervical vertebrae – 7
   • Thoracic vertebrae – 12
   • Lumbar vertebrae – 5
   • Sacral vertebrae – 5 (these fuse to form a single sacrum)
   • Coccygeal vertebrae – 4 (these fuse to form a single coccyx)
6. Vertebrae are mobile or fixed.
7. Mobile vertebrae are called true vertebrae while fixed vertebrae are called false vertebrae.
8. Movable vertebrae are cervical, thoracic, and lumbar.
9. Sacral and coccygeal vertebrae are immobile.

Curvatures Of Vertebral Column

1. Primary Curvatures

1. During intrauterine life, the whole vertebral column is concave ventrally and convex dorsally. This is primary curvature.
2. In adults, primary curvatures are retained only in thoracic and sacral regions.
3. These are mainly due to the shape of vertebrae.

2. Secondary Curvatures

1. Secondary curvatures are convex forwards.
2. These develop after birth.
3. These develop due to posture.
4. These are mainly due to the shape of intervertebral discs.
5. Secondary curvatures are observed in cervical and lumbar regions.
6. Cervical curvature appears around 6 – 9 months when the child starts holding his head by himself.
7. Lumbar curvature appears at about 12 – 18 months when the child starts walking.

Movements Of Vertebral Column

The vertebral column shows the following movements:

1. Flexion: Forward bending.
2. Extension: Backward bending.
3. Lateral flexion: Side bending.
4. Rotation: Twisting of the trunk.
5. Circumduction: Combination of all the above movements.

Features Of A Typical Vertebra

A typical vertebra is made up of 2 parts, the body and the vertebral arch.

1. Body

1. It is the ventral part of a vertebra.
2. It is cylindrical in shape.
3. It has four surfaces, anterior, posterior, superior, and inferior.
4. The anterior surface is convex from side to side and concave from above downwards.
5. The posterior surface is slightly concave from side to side but flat from above downwards. It has a number of foramina for the exit of basivertebral veins. It forms the anterior boundary of the vertebral foramen.
6. The upper and lower surfaces are rough for the intervertebral discs.

2. Vertebral Arch

It consists of a pair of pedicles, a pair of laminae, and seven processes (one spinous, four articular, and two transverse).

1. Pedicles
   • These are pairs of short thick processes that project backward from the body.
   • Between the adjacent pedicles are intervertebral foramina.
2. Laminae
   • These are bony plates extending backward and medially from the posterior end of the pedicles.
   • Posteriorly they fuse to form a spine in the midline.
   • The body, pedicles, and laminae together enclose the foramen of the vertebra called the vertebral foramen.
3. Transverse Process
   • It projects laterally on each side from the junction of the pedicle and lamina.
4. Articular Processes
   • There are two on each side and four in total.
   • These are superior and inferior articular processes projecting upwards and downwards respectively from the junction of the pedicle and lamina.
5. Spinous Process (Spine)
   • It projects backward in the midline from the meeting point of the laminae.

Distinguishing Features

1. The cervical vertebra is characterized by the presence of a foramen in each transverse process. This foramen is named as foramen transversarium.
2. The thoracic vertebra is recognized by the presence of costal facets on the sides of the body.
3. The Lumbar vertebra is larger in size and lacks both foramen transversarium as well as costal facets.
4. There is no isolated sacral vertebra. Five sacral vertebrae fuse to form a single piece of triangular and curved sacrum.
5. Similarly, there is no isolated coccygeal vertebra. Four coccygeal vertebrae fuse to form a single piece of coccyx. The coccyx is relatively much smaller in size than the sacrum.

Regional Vertebrae Cervical Vertebrae

These are classified as typical and atypical. 3rd to 6th cervical vertebrae are typical. 1st, 2nd and 7th cervical vertebrae are atypical.

1. Typical Cervical Vertebra

It has a body and a vertebral arch. These enclose a vertebral foramen.

1. Body
   1. It is the smallest among all vertebrae.
   2. It is narrower anteroposteriorly.
   3. It has 4 surfaces; superior, inferior, anterior, and posterior.
   4. Its superior surface is concave from side to side with an upward projecting lip on either side. This surface is mainly related to the intervertebral disc.
   5. The inferior surface is convex from side to side. The anterior border of the inferior surface projects downwards to hide the intervertebral disc. The inferior surface, like the superior surface, is also related to the intervertebral disc.
   6. The anterior surface provides attachments to an anterior longitudinal ligament in the middle and longus colli muscle on either side of it.
   7. The posterior surface has a number of foramina for basivertebral veins. Its superior and inferior margins provide attachments to the posterior longitudinal ligament.
2. Vertebral Foramen
   1. It is triangular in shape.
   2. It is bigger than the body.
3. Vertebral Arch
4. It is comprised of pedicles, laminae, the superior and inferior articular processes, the transverse processes, and the spine.
   1. Pedicles
      1. These are directed backward and laterally. It is this direction which is responsible for the triangular shape of the vertebral foramen.
      2. Above and below the pedicles are superior and inferior vertebral notches respectively. These notches are equal in size.
   2. Laminae
      1. These are long and narrow.
      2. The superior border is thinner than the inferior border.
      3. Ligamentum flavum is attached to its superior border and the lower part of its anterior surface.
   3. Articular Processes
      1. Superior and inferior articular processes are located on each side, above and below the junction of the pedicle and lamina.
      2. Superior articular process faces upwards and backward.
      3. The inferior articular process is directed downwards and forwards.
      4. The superior articular process of a vertebra articulates with the inferior articular process of the vertebra above.
      5. Articular processes lie in one line forming an articular pillar.
   4. Transverse process
      1. It has a foramen called foramen transversarium, which forms a characteristic feature of the cervical vertebra.
      2. The vertebral artery, vertebral vein, and sympathetic nerves pass through the foramen transversarium (vertebral artery passes through upper 6 foramina only).
      3. The foramen transversarium is bounded anteriorly and posteriorly by anterior and posterior roots respectively.
      4. The lateral ends of the anterior and posterior roots are connected by the costotransverse bar or intertubercular lamella.
      5. Junctions of anterior and posterior roots with costotransverse bars are marked by anterior and posterior tubercles respectively.
      6. The enlarged anterior tubercle of the sixth cervical vertebra is called the carotid tubercle. This is related to the common carotid artery.
      7. Anterior tubercles give origins to scalenus anterior, longus capitis and longus colli muscles.
      8. Posterior tubercles provide attachments to levator scapulae, scalenus medius, scalenus posterior, and some deep muscles of the back.
      9. The anterior root, anterior tubercle, costotransverse bar, posterior tubercle, and adjoining (lateral part of) posterior root represent the costal element while the medial part of the posterior root represents the transverse element of the developing vertebra.
   5. Spine
      1. It is small and bifid.
      2. Ligamentum nuchae is attached to the spinous notch.
      3. Interspinous ligaments are attached to their superior and inferior borders.
      4. Sides provide attachments to deep muscles of the back.

2. Atypical Vervical Vertebrae

First Cervical Vertebra Terminology

It is also named an atlas because it supports the skull. According to Greek mythology, Atlas is the God who supported the earth on his shoulders.

Distinguishing Features

1. It is ring-shaped with narrow anterior and posterior arches.
2. It has nobody.
3. It has no spine.
4. It has a large lateral mass on either side.
5. The two transverse processes are widest apart relative to other cervical vertebrae.

Normal Anatomical Position

1. Two arches lie in the same horizontal plane.
2. The anterior arch is smaller than the posterior arch.
3. Superior articular facets on lateral masses are elongated.

Features And Attachments

Atlas has an anterior arch, a posterior arch, and two lateral masses.

1. Anterior Arch
   1. It is smaller than the posterior arch.
   2. It connects the two lateral masses.
   3. The anterior tubercle is present on its anterior aspect in the midline. The midline part of the upper end of the anterior longitudinal ligament is attached to it.
   4. Its anterior surface on each side of the anterior tubercle provides attachment to longus colli muscle.
   5. An oval facet is present on its posterior surface in the midline for articulation with dens of the 2nd cervical vertebra to form the atlantoaxial joint.
   6. The anterior atlantooccipital membrane is attached to the upper border of the anterior arch.
   7. The lateral part of the upper end of the anterior longitudinal ligament is attached to the lower border of the anterior arch.
2. Posterior Arch
   1. It is longer than the anterior arch.
   2. The midline posterior tubercle on its posterior surface represents the spine.
   3. The Ligamentum nuchae is attached to the posterior tubercle.
   4. On each side of the posterior tubercle is attached the rectus capitis posterior minor.
   5. The vertebral artery (3rd part) and first cervical nerve lie in the shallow groove on the superior surface of the posterior arch just behind the lateral mass.
   6. The posterior atlanto-occipital membrane is attached to the superior border behind the grooves.
   7. Ligamentum flavum is attached to its lower border on each side of the midline.
3. Lateral Masses
   1. Each lateral mass has two articular facets (superior and inferior), two surfaces (anterior and medial), and a transverse process.
   2. The superior articular facet is concave and elongated. It articulates with the occipital condyle from the atlanto-occipital joint.
   3. Note: Remember, we say ‘No’ at the atlantoaxial joint, i.e. move the head from side to side while we say ‘Yes’ at the atlantooccipital joint, i.e. perform nodding movement of the head.
   4. The inferior articular facet is flat and circular. It articulates with an axis.
   5. The medial surface has a tubercle for the transverse ligament of the atlas.
   6. The anterior surface gives origin to the rectus capitis anterior.
   7. The transverse process is long and strong. It has foramen transversarium which transmits vertebral artery, vertebral vein, and sympathetic nerve.
   8. Rectus capitis lateralis, levator scapulae, and superior oblique muscles are attached to its superior aspect around the foramen transversarium.
   9. The inferior oblique muscle is attached to its inferior surface. The anterior aspect of the transverse process is related to the ventral ramus of 1st cervical nerve and accessory nerve.

Second Cervical Vertebra

Terminology

It is also called an axis because the atlas carrying the skull rotates on it.

Peculiarities

1. It is the strongest of the cervical vertebrae.
2. It is easily identified by the presence of an odontoid process (dens) which is a strong tooth-like projection from the superior surface of the body.

Features And Attachments

1. Body And Odontoid Process
   1. The apex of the odontoid process gives attachment to the apical ligament.
   2. On each side of the apex, the sloping gives attachment to the alar ligament.
   3. The anterior surface of the odontoid process possesses an oval facet for articulation with the anterior arch of the atlas.
   4. The posterior surface of the odontoid process is grooved to lodge the transverse ligament of the atlas.
   5. The inferior surface of the body is related to the intervertebral disc.
   6. The anterior surface of the body gives attachments to the anterior longitudinal ligament in the midline and longus colli muscle on each side.
   7. The posterior surface of the body provides attachments to the following three structures from above downwards:
      1. The lower vertical limb of the cruciform ligament.
      2. Membrana tectoria.
      3. Posterior longitudinal ligament.
   8. The superior articular facet (for articulation with the inferior facet of the atlas) is situated lateral to the odontoid process, partly over the body and partly on the pedicle.
2. Vertebral Arch
   1. The pedicle passes backward from the upper part of the body.
   2. The superior articular facet is large, flat, and circular. It is directed upwards and laterally.
   3. The inferior articular facet is situated posterior to the transverse process and is directed downwards and forwards.
   4. The spine is short, thick, and strong. Its tip is bifid and receives attachment of ligamentum nuchae.
   5. Ligamentum flavum is attached to the superior border and lower part of the anterior surface of the lamina on each side.
   6. The side of the spine provides attachment to rectus capitis posterior major.
   7. The external surface of the lamina is meant for the attachment of the inferior oblique in its upper part and the deep muscles of the back in its lower part.
   8. Transverse processes are very small. They represent the true posterior tubercles
      only.
   9. The tip of the transverse process receives attachments of the following 3 muscles from anterior to posterior:
      1. Scalenus medius
      2. Levator scapulae
      3. Deep muscles of the back

Seventh Cervical Vertebra

Terminology

It is also called vertebra prominent because it has a very long spine which may be palpated under the skin of the lower part of the back of the neck.

Peculiarities

1. The spine is long, horizontal, and nonbifid.
2. The transverse process is large with a prominent posterior tubercle.
3. Foramen transversarium is smaller and sometimes may be absent.

Important Attachments And Relations

1. The spine provides attachments to ligamentum nuchae, trapezius, rhomboids minor, and deep muscles of the back.
2. The posterior tubercle of the transverse process receives attachments of the suprapleural membrane and scalenus minimus.
3. Foramen transversarium transmits an accessory vertebral vein.
   • Note: The vertebral artery occupies the foramina transversal of the upper 6 cervical vertebrae only

Thoracic Vertebrae

There are 12 thoracic vertebrae.

Peculiarities

1. Articular facets are present by the side of the body and in front of transverse processes.
2. The body is heart-shaped.
3. Vertebral foramen is circular.
4. The spinous process is long, pointed, and directed. downwards.
5. The pedicle is attached to the upper part of the body making the inferior vertebral notch deeper.

Lumbar Vertebrae

There are 5 lumbar vertebrae.

Peculiarities

1. A lumbar vertebra has a massive body.
2. Vertebral foramen is triangular.
3. Spine is quadrangular.
4. The superior articular facet is concave.
5. The inferior articular facet is convex.
6. The posterior part of the root of the transverse process has a rough elevation called the accessory process.

Sacrum

General Form

The sacrum is a wedge-shaped triangular bone. The base of the wedge is superior and forms the base of the sacrum. The edge of the wedge forms the inferior apex.

It has 4 surfaces, pelvic (anterior), dorsal (posterior), and 2 lateral. The canal of the sacrum is called the sacral canal.

Anatomical Position

1. The sacrum is a midline bone placed between hip bones (on each side), the 5th lumbar vertebra (superiorly), and the coccyx (inferiorly).
2. The superior surface of the body of 1st sacral vertebra slopes forward at an angle of 30°.
3. The anterior surface of the sacrum faces downwards and forwards.
4. The upper end of the sacral canal is directed upwards.

Coccyx

Normal Anatomical Position

Coccyx is directed downwards and forwards.

Features

A coccyx is formed by the fusion of four coccygeal vertebrae. It is triangular in shape. with the base upwards and apex downwards. It has two surfaces (pelvic and dorsal) and two lateral borders (right and left).

Hyoid

‘Hyoid’ is a Greek word that means ‘U’ shaped.

Hyoid Level

Hyoid lies at the level of the 3rd cervical vertebra.

Hyoid Location

1. The Hyoid is situated in the anterior midline of the neck above the thyroid cartilage.
2. Its body (the bend of ‘U’) is the first resistant structure felt in the midline of the neck, inferior to the chin.
3. The tip of the greater cornu (the limb of ‘U’) of the hyoid can be palpated in the relaxed neck near the anterior border of the sternocleidomastoid muscle midway between the laryngeal prominence and mastoid process.

Hyoid Features And Attachments

Hyoid bone consists of a central body, a pair of greater cornua, and a pair of lesser cornua.

Hyoid Body

It has two surfaces (anterior and posterior), two borders (upper and lower), and two lateral ends.

1. Surfaces

1. Anterior Surface
   1. It is convex.
   2. A median ridge divides it into two lateral halves.
   3. Geniohyoid and mylohyoid muscles are inserted on this surface in its upper and lower parts respectively.
   4. Hyoglossus partly originates from anterior surface.
   5. The investing layer of the cervical fascia is attached below the insertion of the mylohyoid.
2. Posterior Surface
   1. It is concave.
   2. It is related to the following structures.
      • Bursa
      • Thyrohyoid membrane
      • Epiglottis

2. Borders

1. Upper Border
   • It provides attachment to 3 structures from anterior to posterior
     1. Genioglossus muscle
     2. Hyoepiglottic ligament
     3. Thyrohyoid membrane.
2. Lower Border
   • Two muscles are mainly attached to this border on each side of the midline from medial to lateral.
     1. Sternohyoid
     2. Omohyoid

3. Ends

1. Each end continues posteriorly as greater cornu.
2. Lesser cornu projects upwards at the junction of the body and greater cornu.

Greater Cornua (Singular Greater Cornu)

Greater cornu has two surfaces (upper and lower), two borders (medial and lateral), and a tubercle (at the posterior end).

1. Surfaces

1. Upper Surface
   1. It has the following attachments from medial to lateral.
   2. Middle constrictor along the whole length.
   3. Hyoglossus along the whole length.
   4. The stylohyoid muscle at the junction of lesser and greater cornua.
   5. Fibrous loop of digastric muscle-lateral to attachment of stylohyoid muscle.
2. Lower Surface
   • Fibroareolar tissue separates this surface from the thyrohyoid membrane.

2. Borders

1. Medial Border
   • It receives attachment of the thyrohyoid membrane.
2. Lateral Border
   • The thyrohyoid muscle is attached to this border anteriorly.

Lesser Cornua (Singular Lesser Cornu)

1. It is a small conical projection attached to the bone at the junction of the body and greater cornu by fibrous tissue.
2. It may form a synovial joint with the greater cornu.
3. It has the following attachments:
   • Stylohyoid ligament at the tip.
   • Middle constrictor-posterolaterally.

Hyoid Ossification

1. The hyoid ossifies from ventral portions of the cartilages of the 2nd and 3rd arches.
2. Lesser cornua and the upper part of the body are developed from the 2nd arches.
3. Greater cornua and the lower part of the body are developed from 3rd arches.
4. The appearance of centres – 6 centers of ossification appears, 2 for the body and 1 for each cornu, as follows:
   • Greater cornu just before birth Body-just after birth
   • Lesser cornu-puberty
5. The cartilage at the tip of each greater cornu persists up to the 3rd decade.

Hyoid Applied Anatomy

1. Some congenital anomalies associated with developing thyroid are commonly observed adjacent to the hyoid, for example. suprahyoid thyroid, infrahyoid thyroid, and thyroglossal cyst.
2. Sometimes a muscular band connects the body of the hyoid with the isthmus or pyramidal lobe of the thyroid gland. This is called levator glandular thyroid.
3. The lingual artery arises from the external carotid artery posteroinferior to the tip of the greater cornu.
4. The latter thus forms an important surgical landmark for locating the lingual artery which is ligated essentially in radical surgery of the tongue.
5. The hyoid bone is of great medical importance. In suspected cases of death, a fracture of the hyoid bone suggests death by throttling or strangulation.

Palatine Bones

Palatine Bones Terminology

‘Palatine’ bone is so named because of its contribution to the ‘hard palate’. The word ‘palate’ is derived from ‘plate’ because it forms a ‘plate-like’ partition between nasal and oral cavities.

Palatine Bones Location

Each palatine bone is located between the maxilla and pterygoid process of the sphenoid in the posterior part of the nasal cavity.

Palatine Bones Features And Attachments

Each palatine bone is ‘L’ shaped in appearance and consists of two plates (horizontal and perpendicular) and three processes (pyramidal, orbital, and sphenoidal).

Palatine Bones Plates

1. Horizontal Plate

It projects medially from the lower end of the perpendicular plate. It has two surfaces (nasal and palatine) and four borders (anterior, posterior, lateral, and medial).

1. Surfaces
   1. Nasal Surface
      1. It faces superiorly.
      2. It is concave from side to side.
      3. It forms the posterior part of the floor of the nasal cavity.
   2. Palatine Surface
      1. It faces inferiorly.
      2. With the corresponding surface of the opposite side, it forms posterior 1/4th of the hard palate.
      3. Near its posterior border, this surface presents a curved ridge called a palatine crest. This crest and the area behind it give attachment to palatine aponeurosis.
2. Borders
   1. Anterior Border
      • It articulates with the posterior border of the palatine process of the maxilla to form a palatomaxillary suture.
   2. Posterior Border
      1. It is concave.
      2. It is free.
      3. This gives attachment to palatine aponeurosis (the aponeurosis is also attached to the palatine crest and the area behind it).
      4. Its medial end projects backward and with that of the opposite side forms the posterior nasal spine. To this spine is attached the musculus uvulae.
   3. Lateral Border
      1. It is attached to the lower border of the perpendicular plate.
      2. Its lower end is marked by a greater palatine groove.
   4. Medial Border
      1. It articulates with that of the opposite bone to form an interpalatine suture.
      2. Articulating medial borders of horizontal plates of two palatine bones project upwards to form a nasal crest.
      3. The nasal crest articulates with the posterior part of the lower border of the vomer and is continuous anteriorly with the nasal crests of maxillae.

2. Perpendicular Plate

It has two surfaces (maxillary and nasal) and four borders (anterior, posterior, superior, and inferior).

1. Surfaces
   1. Maxillary Surface
      1. It faces laterally.
      2. Its major part is rough to articulate with the nasal surface of the maxilla.
      3. Its upper and posterior part is smooth and forms the medial wall of the pterygopalatine fossa.
      4. Its anterior part is also smooth and forms the posterior part of the medial wall of the maxillary sinus.
      5. Its posterior part shows a vertical groove (greater palatine groove) which is converted into a greater palatine canal by the maxilla in the articulated skull. Greater palatine vessels and nerves pass through the greater palatine canal.
   2. Nasal Surface
      1. It faces medially.
      2. It has two horizontal crests. The lower crest is called the conchal crest because it articulates with the inferior concha. The upper one is named as the ethmoidal crest because of its articulation with the middle concha of the ethmoid.
      3. The area below the conchal crest forms the inferior meatus of the nose.
      4. The area between the two crests contributes to the middle meatus of the nose.
      5. The area above the ethmoidal crest takes part in the formation of the superior meatus of the nose.
2. Borders
   1. Anterior Border
      1. Its lower part articulates with the maxillary process of the inferior concha and assists in the formation of the medial wall of the maxillary sinus.
      2. Its upper part forms the posterior boundary of the maxillary hiatus.
   2. Posterior Border
      • It articulates with the anterior border of a medial pterygoid plate of the sphenoid.
   3. Superior Border
      1. It supports the orbital process in front and the sphenoidal process behind.
      2. Between the orbital and sphenoidal processes is the sphenopalatine notch which is converted into a sphenopalatine foramen by the inferior surface of the body of the sphenoid.
      3. Sphenopalatine foramen is the communication between the pterygopalatine fossa and the posterior part of the superior meatus of the nose.
      4. Sphenopalatine foramen transmits sphenopalatine vessels and posterior superior nasal nerves.
   4. Inferior Border
      1. It is continuous with the lateral border of the horizontal plate.
      2. In front of the pyramidal process, it is marked by the lower end of the greater palatine groove.

Palatine Bones Processes

1. Pyramidal Process

• It projects downwards, backward, and laterally from the junction of two plates of palatine bone.
• It fits into the pterygoid fissure of the pterygoid process of sphenoid.
• Its posterior surface completes the lower part of the pterygoid fossa.
• Its lateral surface is rough anteriorly and smooth posteriorly. The rough part articulates with maxillary tuberosity. The smooth part forms the lower part of the infratemporal fossa.
• Its inferior surface presents lesser palatine foramina for lesser palatine nerves and vessels.

2. Orbital Process

It projects upwards and laterally from the anterior part of the upper border of the perpendicular plate. A constricted neck connects it with the perpendicular plate.

It has three articular surfaces (anterior, posterior, and medial) and two non-articular surfaces (superior and lateral).

1. Articular Surfaces
   1. Anterior Surface
      • It articulates with maxilla.
   2. Posterior Surface
      • It articulates with a sphenoidal body.
   3. Medial Surface
      • It articulates with ethmoidal bulla.
2. Non-Articular Surfaces
   1. Superior Surface
      • It forms the posterior part of the floor of the orbit.
   2. Lateral Surface
      • It forms part of the medial wall of the pterygopalatine fossa. The border between lateral and posterior surfaces is prolonged downwards as the anterior boundary of the sphenopalatine notch.

3. Sphenoidal Process

It is directed upwards and medially from the posterior part of the upper border of the perpendicular plate. It has three surfaces (superior, inferomedial, and lateral) and three borders (posterior, anterior, and medial).

1. Surfaces
   1. Superior Surface
      1. It articulates with the undersurface of the sphenoidal concha and root of the medial pterygoid plate.
      2. It is grooved to complete the palatovaginal canal.
   2. Inferomedial Surface
      • It contributes to the roof and lateral wall of the nasal cavity.
   3. Lateral Surface
      1. Its posterior part articulates with the medial pterygoid plate.
      2. Its anterior part contributes to the medial wall of the pterygopalatine fossa.
2. Borders
   1. Posterior Border
      • It articulates with the vaginal process of the medial pterygoid plate.
   2. Anterior Border
      • It forms the posterior boundary of the sphenopalatine notch.
   3. Medial Border
      • It articulates with a lot of vomer.

Palatine Bones Ossification

1. Palatine bone ossifies in the membrane.
2. A single center appears in the perpendicular plate during the 8th week of intrauterine life.
3. The ossification spreads into the processes and horizontal plate.
4. At birth, the height of the perpendicular plate is equal to the transverse width of the horizontal plate.
5. The length of the perpendicular plate becomes double the transverse width of the horizontal plate at puberty.

Palatine Bones Applied Anatomy

1. The palatine bone may be involved in the fracture of the mid-facial skeleton.
2. Palatine bone receives adequate blood supply from periosteal arteries and, therefore, all the fragments of fractured bone retain a periosteal blood supply.
3. Palatine bone is clothed in mucosa over large areas of its surfaces. Its fracture may open into the nasal or oral cavities or maxillary sinus with the potential risk of infection.
4. Le Fort fractures of the midfacial skeleton always involve the perpendicular plates of palatine bones.
5. Guerin’s fracture (Le Fort 1 fracture) involves the lower 1/3rd of the perpendicular plates of palatine bones. In cases of Le Fort 2 and 3 fractures, the upper parts of the perpendicular plates are affected.
6. A horizontal plate of palatine bone may be fractured in an uncommon central split of the palate. It is actually paramedian in nature because median sutures (intermaxillary and interpalatine) are relatively strong.

Mandible Terminology

The word ‘mandible’ is derived from the Greek word ‘mandere’ which means to masticate or chew. The Latin word ‘mandibula’ means lower jaw.

Mandible Peculiarities

1. It is a ‘U’ shaped bone.
2. It is also called the ‘lower facial skeleton’.
3. The mandible is the largest and strongest bone of the face.
4. It forms the skeleton of the lower jaw.

Mandible Features And Attachments

The mandible has a body and two rami.

Mandible Body

It is shaped like a horseshoe and has 2 surfaces (external and internal) and 2 borders (upper and lower).

1. Surfaces

• External Surface
• It has the following features:
  1. Symphysis menti
     • It is a faint ridge on the upper part of the midline indicating the fusion of two halves of the mandible.
  2. Mental protuberance
     • It is a triangular area in the lower part of the midline. The upper angle of the triangle marks the lower end of symphysismenti.
  3. Mental tubercles
     • The lower angles of the triangular mental protuberance are marked by tubercles called mental tubercles.
     • Note: Remember that mental protuberance is characteristic of the human jaw.
  4. Mental foramen
     • It is located below the 2nd premolar or junction between two premolar teeth. Mental nerves and vessels pass through it.
  5. Incisive fossa
     • It is a shallow fossa below the incisor teeth. Mentalis and orbicularis oris originate from this fossa.
  6. Oblique line
     • It is a continuation of the anterior border of the ramus on the external surface of the body. It is a faint ridge. It runs downwards and forwards to reach the mental tubercle. The following muscles are attached to it from anterior to posterior:
       1. Depressor labii inferioris.
       2. Depressor anguli oris.
       3. Buccinator (below the molar teeth).
       4. Note: Junction of body and ramus is marked by the courses of facial artery and facial vein.

• Internal Surface

It has the following features:

• 1. Mylohyoid line
     • It is an oblique ridge. It extends downwards and forwards from behind the 3rd molar tooth (1 cm below the alveolar border) to the midline near the lower border between digastric fossae. Mylohyoid muscle is attached to it.
  2. Submandibular fossa
     • It is present below the posterior part of the mylohyoid line. It lodges the following structures:
       1. Submandibular salivary gland.
       2. Facial artery.
       3. Submandibular lymph nodes.
  3. Sublingual fossa
     • It is an area above the anterior part of mylohyoid line. It lodges the sublingual salivary gland.
  4. Genial tubercles
     1. These are irregular elevations on either side of the midline just above the anterior ends of mylohyoid lines. Upper genial tubercle. provides attachment to the genioglossus muscle while the lower genial tubercle gives origin to the geniohyoid muscle.
     2. Note: Genial tubercles are for genial muscles, since the tongue is higher as compared to the hyoid bone, the upper tubercle is for genioglossus and the lower is for geniohyoid.
  5. Attachment of superior constrictor of pharynx
     • The superior constrictor originates from the area above the posterior end of mylohyoid line.
  6. Attachment of pterygomandibular raphe
     • This raphe is attached to the inner surface of the body in continuation with the origin of the superior constrictor just behind the 3rd molar tooth.
  7. Relation of lingual nerve
     • The lingual nerve is related to the mandible between the origin of the superior constrictor and the posterior end of the mylohyoid line.

2. Borders

• Upper Border
  1. It is also called the alveolar part of the mandible.
  2. It is hollowed out by sixteen sockets for the roots of permanent teeth.
  3. The sockets vary in size and depth.
  4. The sockets may be single or subdivided by septa according to the teeth which they contain.
• Lower Border
  1. It is also called the base of the mandible.
  2. The digastric fossa is a depression at its anterior (mesial) end on each side of the
     midline. It receives attachment of the anterior belly of the digastric.
  3. The Investing layer of deep cervical fascia is attached to the whole length of the base.
  4. Platysma is inserted into the lower border near the outer surface.

Mandible Ramus

The Ramus of the mandible has 2 surfaces (lateral and medial), 4 borders (upper, lower, anterior and posterior) and two processes (coronoid and condylar).

1. Surfaces

• Lateral Surface
  1. A small posterosuperior area is related to the parotid gland.
  2. The remaining major area provides attachment to the masseter.
     • Note: Remember, the house of the Prime Minister is located in the lateral area. P stands for Parotid and M stands for Masseter.
• Medial Surface
  1. Mandibular Foramen And Canal: The mandibular foramen is located a little above the centre of the medial surface. It leads into the mandibular canal which curves downwards and forwards into the body, to open on the external surface at the mental foramen. Inferior alveolar nerve and vessels enter the mandibular canal through the mandibular foramen.
  2. Lingula: It is a tongue-shaped projection near the anterior margin of the mandibular foramen. The sphenomandibular ligament is attached to the lingula.
  3. Mylohyoid Groove: It begins at the lower end of the mandibular foramen behind the lingula and continues downwards and forwards to reach the inner surface of the body. Mylohyoid nerve and vessels occupy the mylohyoid groove.
  4. The medial surface of the ramus between the mylohyoid groove and the angle of the mandible is marked by ridges. This area is meant for the attachment of the medial pterygoid.
  5. The area in front of the mylohyoid groove is related part (head) and a lower part (neck) to the lingual nerve.

2. Borders

• Upper Border
  1. It is thin.
  2. It forms a mandibular notch or incisure.
  3. Masseteric nerves and vessels cross the mandibular notch.
• Lower Border
  1. It is a backward continuation of the base of the mandible.
  2. It meets with the posterior border of the ramus to form the angle of the mandible.
• Anterior Border
  1. It is continuous above with the coronoid process and below with the alveolar border of the body.
  2. Temporalis muscle is inserted on this border and adjoining medial surface.
• Posterior Border
  1. It is continuous above with the condylar process.
  2. It meets with the lower border to form the angle of the mandible.
  3. It is related to the parotid gland.

3. Processes

• Coronoid Process
  1. It is a triangular upward projection from the anterosuperior part of the ramus.
  2. Its anterior border is continuous with the anterior border of the ramus and its posterior border bounds the mandibular notch.
  3. The Temporalis muscle gets inserted on the medial surface, apex and margins of the coronoid process.
• Condylar process
  1. It is an upward projection from the posterosuperior part of the ramus. It consists of an upper part (head) and a lower part (neck).
  2. Head
     • It is side to side side-expanded part of the condylar process.
     • It articulates with the temporal bone to form a temporomandibular joint.
  3. Neck
     • It is a constricted part below the head.
     • It provides attachment to the capsule in its upper part.
     • The lateral ligament of the temporomandibular joint is attached to its lateral part.
     • The pterygoid fovea is a depression in its anterior aspect. The lateral pterygoid muscle is inserted on the pterygoid fovea.
     • Medially the neck is related to auriculo- temporal nerve above and the maxillary artery below.

Mandible Ossification

1. Mandible is intramembranous as well as endochondral in origin.
2. The membrane involved is the mesenchymal sheath on the lateral aspect of both Meckel’s cartilages. A centre appears on each side of this sheath during the 7th week of intrauterine life.
3. Cartilages contributing to the mandible are as follows:
   1. Anterior ends of Meckel’s cartilage
      • These are invaded by bone from parent centres at the 10th week of intrauterine life.
   2. Coronoid cartilages
      • These appear at the 10th week of intrauterine life and disappear before birth.
   3. Condylar cartilages
      • These appear at the 10th week of intrauterine life and persist till the 3rd decade.
   4. Cartilaginous nodules
      • One or two of these nodules appear on each side of the symphysismenti at about the 10th week of intrauterine life. These ossify to form mental ossicles at about the 7th month of intrauterine life and fuse with the body at the age of one year.
4. Parts of the mandible which are derived from cartilage are:
   • The incisive part below the incisor teeth.
   • Coronoid and condylar processes.
   • Part of ramus above the mandibular foramen.
   • Note: Remember that the names of all the parts of the mandible which ossify from cartilage start with C, i.e. coronoid process, condylar process, cranial part of ramus and chin part of body related to cutting or incisor teeth.
5. At birth, the mandible consists of two halves connected at symphysis menti. Bony union starts from below upwards during 1st year of age and is completed at the end of 3rd year.

Age Changes In Mandible

Some of the differentiating features in different age groups are as follows:

1. Children

• The body of the mandible is more like a shell having sockets for both deciduous and permanent teeth.
• The angle of the mandible measures about 140°.
• The coronoid process is above the level of the condylar process.
• The mandibular canal and mental foramen are close to the lower border of the body.

2. Adult

• The alveolar and subalveolar parts of the body are of equal depths.
• The angle of the mandible measures about 110°.
• The condylar process projects above the level of the coronoid process.
• The mandibular canal runs parallel to the mylohyoid line.
• The mental foramen is situated midway between the upper and lower borders of the body.

3. Old age

• Loss of teeth is a usual feature.
• The alveolar part is absorbed.
• The angle of the mandible measures about 140°
• The neck of the mandible is bent backwards making the level of the coronoid process higher than the condylar process.
• The mandibular canal and mental foramen are close to the upper border of the body.

 

Mandible Applied Anatomy

1. The mandible occupies a prominent and exposed position in the facial skeleton and, therefore, forms a common site of violent injuries.

2. The slender neck of the mandible is liable to fracture as a result of violence received at the mental prominence.

 

3. The alveolar process is weaker than the rest of the mandible and, therefore, an independent alveolar fracture may occur.

4. The tendency of the mandible to fracture with advancing age is due to resorption of the alveolar portion of bone when the teeth are lost.

5. The elongated root of the canine tooth reduces the bony substance and makes the mandible weaker at this site. The canine region is, therefore, the commonest site of fracture.

6. Thick periosteum over the mandible prevents gross displacement of fractured bones after fracture.

7. Impacted 3rd molar, mental foramen and missing teeth also contribute to the weakness in the mandible.

8. Strong muscles attached to the mandible play a very important role in the displacement of fractured segments of the mandible. Such muscles are divided into 3 groups.

1. The depressor group
   • It includes geniohyoid and digastric muscles. They cause posterior and inferior displacement of a fractured anterior mandibular segment.
2. The elevator group
   • The masseter, temporalis and medial pterygoid muscles belong to this group. Contraction of these muscles causes upward displacement of the fractured segment if the fracture occurs in the region of the angle.
3. The protrusion group
   • It includes lateral pterygoid muscle which causes forward displacement of the head in cases of fracture of the mandibular neck.

9. The following is the general classification of mandibular fractures:

1. Simple
   • Single fracture without exposure to exterior.
2. Compound
   • The fractured site is exposed to the exterior.
3. Comminuted
   • It is multiple fractures of the mandible at the same site. It may be both simple and compound.
4. Complicated
   • It is a fracture associated with injury of teeth, nerves or vessels.
5. Impacted
   • In this fracture, one fragment has been driven into the substance of another fragment.
6. Greenstick
   • In this fractured site bends without displacement.
7. Pathological
   • Fracture is due to underlying diseases like osteomyelitis or tumours.

10. Clinical classification of mandibular fractures.

1. Class 1
   • Teeth are present on both sides of the fractured line.
2. Class 2
   • Teeth are present on one side of the fractured line.
3. Class 3
   • Fragments are edentulous (without teeth)

11. Depending upon the number of sites, mandibular fractures may be of the following types:

• Single unilateral
• Double unilateral
• Bilateral
• Multiple

12. The mandible can be divided into the following regions to simplify the sites of lesions, for example. fractures:

• Condylar
• Coronoid
• Ramus
• Angle
• Alveolar.
• Body.
• Symphysis.

13. Tumours of the hard tissue of the lower jaw can originate in teeth (odontogenic tumours) or mandible (osteogenic tumours)

• Odontogenic Tumours
• These can be of the following two types:
  • Odontoma
  • Arising from teeth proper. Odontomas may be of the following three types according to their structures:
    1. Calcified odontoma
       • It has dentine.
    2. Simple enamel pearl.
       • It has enamel.
    3. Cementoma
       • It has cementum.
  • Ameloblastoma
    • Arising from the embryonal (ameloblast) cells of developing teeth.
• Osteogenic Tumours
• The following are the common osteogenic tumours:
  • Osteoma
  • Fibro-osteoma
  • Myxoma
  • Chondroma
  • Sarcoma
  • Ewing’s tumour
  • Multiple myeloma.
  • Central giant cell tumour

Maxillae Terminology

Maxilla is a Latin word meaning ‘cheek’ or ‘jaw’. The word is commonly used in reference to the upper jaw.

Maxillae Location

1. There are two maxillae which form a major part of the upper facial skeleton.
2. The whole of the upper jaw is formed by two maxillae.
3. The junction of two maxillae is marked by intermaxillary suture visible in the hard
   palate and face in the midline.

Maxillae Features And Attachments

Each maxilla consists of a body and four processes (zygomatic, frontal, alveolar and palatine).

Maxillae Body

It has four surfaces (anterior, infratemporal, orbital and nasal).

1. Anterior Surface

• It is directed forward and laterally.
• There is a vertical elevation at the site of the socket for the canine root. This is called canine eminence.
• Medial to canine eminence is a depression called incisive fossa which gives origin to depressor septi.
• The anterior surface below the incisive fossa gives attachments to the incisivus superior and orbicularis oris.
• Just above the incisive fossa, there is the attachment of the nasalis muscle.
• Lateral to canine eminence is another fossa called canine fossa. Levator anguli oris originates from the canine fossa.
• Above the canine fossa is a foramen called infraorbital foramen. It transmits the infraorbital nerve and vessels.
• Above the infra-orbital foramen is a sharp infra-orbital margin which gives origin to levator labii superioris.
• Its upper part is limited medially by a deep notch called a nasal notch.

2. Infratemporal Surface

• It faces backwards and laterally.
• It forms anterior wall of infratemporal fossa.
• It shows 2-3 openings of alveolar canals which transmit posterior superior alveolar nerves and vessels.
• Its anteroposterior part is marked by maxillary tuberosity which articulates with the pyramidal process of palatine bone.

3. Orbital Surface

• It forms the floor of the orbit.
• Running forward is an infraorbital groove in the middle of its posterior part. The groove continues with the infraorbital canal which opens on the anterior surface as the infraorbital foramen. It is meant for infraorbital nerves and vessels.
• Anteromedially it gives origin to the inferior oblique muscle.
• It has three borders (medial, posterior and anterior).

Medial Border

It is marked anteriorly by a lacrimal notch. Behind this notch, this border provides attachments to lacrimal bone, orbital plate of ethmoid and orbital process of palatine bone from before backwards.

Posterior Border

It forms the anterior border of the inferior orbital fissure.

Anterior Border

It contributes to the medial part of the infra-orbital margin.

4. Nasal Surface

• It forms the lateral wall of the nasal cavity.
• A large opening (maxillary hiatus) is the most prominent feature of this surface.
• Maxillary hiatus leads into the maxillary sinus, a large air space within the body of the maxilla.
• Maxillary hiatus is greatly reduced in size in the articulated skull by ethmoid (uncinate process) and lacrimal bone above, inferior concha below and perpendicular plate of palatine bone behind.
• Below the hiatus, this surface forms the inferior meatus of the nasal cavity.
• The posterior part of the nasal surface has an oblique groove which is converted into a greater palatine canal by a perpendicular plate of palatine bone. Greater palatine nerves and vessels pass through this canal.
• In front of the hiatus is the nasolacrimal groove. This is converted into the nasolacrimal canal by the lacrimal bone and inferior concha. This canal is meant for the nasolacrimal duct.
• An oblique ridge called the conchal crest is present in front of the nasolacrimal groove. It articulates with inferior concha.

 

Maxillae Processes

1. Zygomatic Process

It has three surfaces, anterior, posterior and superior. The latter is rough for articulation with the zygomatic bone.

2. Frontal Process

It possesses an upper end, two surfaces (lateral and medial) and two borders (anterior and posterior).

• Upper End
  • It articulates with the nasal notch of the frontal bone.
• Surfaces
  • Lateral Surface
    1. It has a vertical ridge (anterior lacrimal crest) in the middle for the attachment of the medial palpebral ligament.
    2. The area in front of the crest gives attachments to orbicularis oculi and levator labii superioris alaeque nasi.
    3. The area behind the lacrimal crest contributes to the anterior half of the lacrimal groove.
  • Medial Surface
    1. It has a horizontal ridge (ethmoidal crest) in its middle. It articulates with the middle nasal concha.
    2. A roughened area above the crest articulates with ethmoid to complete anterior ethmoidal air cells.
    3. The area below the ethmoidal crest forms the atrium of the middle meatus.
  • Borders
    1. Anterior border
       • It articulates with the nasal bone.
    2. Posterior border
       • It articulates with lacrimal bone.

3. Alveolar Process

• It is an arched lower border of the body.
• It has sockets for the upper teeth.
• Buccinator originates from the posterior part of the outer surface over the sockets for permanent molar roots.

4. Palatine Process

It is a horizontal bracket-like projection from the lower part of the medial surface of the body. It forms the anterior 3/4th of the hard palate. It has two surfaces (superior and inferior) and three borders (medial, posterior and lateral).

• Surfaces
  • Superior Surface
    1. It is concave and smooth.
    2. It forms the floor of the nasal cavity.
  • Inferior Surface
    1. It has depressions for palatine glands.
    2. It has several nutrient foramina for nutrient vessels.
    3. A greater palatine groove for greater palatine nerve and vessels is present in its posterolateral part.
    4. When two maxillae meet, the incisive fossa is noticed behind the incisor teeth.
    5. The incisive canal is the communication between the incisive fossa and the nasal cavity. It transmits the greater palatine artery and nasopalatine nerve.
• Borders
  • Medial Border
    1. It meets with a similar border of the opposite maxilla to form an intermaxillary suture.
    2. This border is raised into a ridge called the nasal crest. Nasal crests of two sides enclose a groove to receive the vomer.
    3. Its anterior end is prolonged and meets with a similar prolongation of the opposite side to form the anterior nasal spine.
  • Posterior Border
    • It articulates with the anterior border of the horizontal plate of palatine bone to form a palatomaxillary suture.
  • Lateral border
    • It fuses with the body.

Maxillae Ossification

1. The maxilla is intramembranous in origin.
2. It develops in the mesenchyme just superficial to the nasal capsule.
3. Three centres of ossification appear:
   • One centre appears for the main mass just above the canine fossa at about 6th week of intrauterine life.
   • Two centres appear for os incisivum (premaxillary part).
     • Note: Remember that the premaxilla is that part of the maxilla which holds incisor teeth and is a separate bone in most mammalian upper jaws.
4. The maxillary sinus appears on the nasal aspect as a groove at about 4th month of intrauterine life

Age Changes In Maxilla

1. At birth

1. Vertical diameter is lesser than both the transverse and anteroposterior diameters.
2. The body is mainly occupied by sockets for the teeth.
3. The maxillary sinus is seen as a shallow groove on the nasal aspect.

2. Adult

1. Vertical diameter is greater than the transverse and anteroposterior diameters.
2. The maxillary sinus has greatly developed within the body.

3. Old Age

1. Due to the falling of teeth and resorption of the alveolar margin, the vertical diameter is again greatly reduced.
2. The alveolar margin is reduced in thickness at the expense of the labial wall.

Maxillae Applied Anatomy

1. Maxillary sinus (antrum of Highmore)

• It is the air space in the body of the maxilla. It is pyramidal in shape with a base towards the nasal cavity and an apex towards the zygomatic process.
• Its height and anteroposterior measurements are 1.5 inches each while width is 1 inch only. It is very important clinically due to the following facts:
  1. It is the largest paranasal sinus and is commonly involved during the inflammation process (maxillary sinusitis).
  2. It drains into the middle meatus which is higher than its floor. The latter is about 1.25 cm below the floor of the nasal cavity.
  3. To facilitate the drainage of pus in the maxillary sinus an opening is made in the inferior meatus by operative procedures like antral puncture or antrostomy.
  4. Maxillary tumours can produce a bulging in adjacent related surroundings, i.e. superiorly in the floor of the orbit, inferiorly in the roof of the oral cavity.
  5. Anteriorly in the face, posteriorly in the infratemporal fossa and medially in the lateral wall of the nasal cavity.

2. Maxillary fractures

• Unilateral fracture of the maxilla usually involves its alveolar process.
• Bilateral maxillary fractures are classified into the following three types:
  1. Le Fort 1 (Guerin’s fracture)
     1. It is a horizontal fracture along the floor of the nose and below the zygomatic bone.
  2. Le Fort 2
     1. In this fracture line passes through orbits and then runs medial to and below the zygomatic bones towards the alveolar margins.
  3. Le Fort 3
     • In this, the fracture line runs through nasal bones and orbits above the zygomatic bone. This fracture is also called craniofacial disjunction as the face separates from the cranium.

Frontal Bone Terminology

The term ‘frontal’ is derived from the Latin word ‘frons’ which means ‘brow’ or ‘forehead’.

Frontal Bone Location

The frontal bone forms the forehead, the greater part of the roof of each orbit and most of the floor of the anterior cranial fossa.

Frontal Bone Features And Attachments

The frontal bone has a main part (frontal squama) and orbital parts.

Frontal Bone Squama (Main Part)

It has an external surface, right and left temporal surfaces, an internal surface, a nasal part and a margin (parietal or posterior).

1. Surfaces

• External Surface
  1. Supra-Orbital Margins.
     • These are lower limits of the external surface on each side.
     • They form the upper borders of the orbital opening.
  2. Supra-Orbital Notch Or Foramen
     • The junction of the lateral two-thirds of the supra-orbital margin (sharp) with the medial one-third (rounded) is marked by a supra-orbital notch (sometimes foramen).
     • This is meant for the passage of the supraorbital nerve, supraorbital artery and a communicating vein between angular and superior ophthalmic veins.
  3. Superciliary Arch
     • This is an arched prominence just above the supra-orbital margin.
  4. Glabella
     • It is the median prominence between superciliary arches.
  5. Frontal Eminence
     • On each side, about 3 cm above the supra-orbital margin, there is an elevated area called frontal eminence or tuberosity.
     • It is usually more marked in females.
  6. Metopic Suture
     • The frontal bone is bilateral in origin and the junction of the two halves is called frontal or metopic suture. Its remains can be seen even in adults in the region of glabella.
  7. Zygomatic Process
     • Supra-orbital margin extends laterally on each side into a zygomatic process.
     • The zygomatic process articulates with the frontal process of the zygomatic bone.
  8. A line curves upwards and backwards from the zygomatic process. The line soon divides into two lines called superior and inferior temporal lines.
• Temporal Surfaces
  1. An area on each side below and behind the temporal lines is called the temporal surface.
  2. It contributes to the anterior part of the temporal fossa on the lateral aspect of the skull (norma lateralis).
  3. The superior temporal line gives attachment to the temporal fascia.
  4. Inferior tempoRal Line And Temporal Surface Of the Frontal Bone Give Origin To the Temporalis Muscle.
• Internal Surface
  1. This surface shows depressions and elevations for cerebral gyri and sulci respectively.
  2. Sagittal Sulcus
     • It is a midline sulcus in the upper part of the internal surface.
     • Its margins provide attachments to falx cerebri.
     • The sulcus itself lodges the superior sagittal sinus.
  3. Frontal crest
     • Margins of the sagittal sulcus meet in the midline in the lower part and continue as a frontal crest.
     • This also gives attachment to falx cerebri.
  4. A notch below the frontal crest is converted into foramen caecum by articulation with ethmoid bone. An emissary vein passing through it connects the vein of the nose with the superior sagittal sinus.
  5. Several depressions (granular foveolae) on each side of the sagittal sulcus are produced by arachnoid granulations.

2. Nasal Part

1. It is a downward projection of the frontal bone between two supra-orbital margins.
2. Its lower serrated part is known as the nasal notch.
3. Each half of the nasal notch articulates with the following three bones from anterior to posterior:
   • Nasal bone.
   • Frontal process of the maxilla.
   • Lacrimal bone.
4. The nasal spine is a midline downward continuation of the nasal part.
5. On each side of the nasal spine, there is a grooved area which forms the roof of the nasal cavity.
6. The nasal spine itself articulates with the crest of the nasal bone anteriorly and the perpendicular plate of the ethmoid posteriorly.

3. Posterior Margin

1. This is also called the parietal margin because its major part articulates with parietal bones.
2. The lower part of this margin is triangular and rough for articulation with the greater wing of the sphenoid.

Frontal Bone Orbital Parts

Orbital parts consist of two triangular laminae (orbital plates) separated by a gap called an ethmoidal notch.

1. Orbital Plate

It possesses two surfaces, orbital and internal.

• Orbital Surface
  1. It faces downwards.
  2. It forms the roof of the orbit.
  3. Its anterolateral part has a fossa for the lacrimal gland.
  4. Its anteromedial part (trochlear fovea) provides attachment to the fibrocartilaginous pulley for the tendon of the superior oblique muscle.
• Internal surface
  1. It faces upwards.
  2. It contributes to the anterior cranial fossa.
  3. It has impressions for the gyri of the frontal lobe of the cerebral hemisphere.
  4. It has grooves for meningeal vessels.

2. Ethmoidal notch

1. It is a ‘U’ shaped gap occupied by a cribriform plate of ethmoid.
2. Under surfaces of its lateral margins possess several incomplete air cells which complete the ethmoidal air cells when ethmoid bone is in position.
3. Two grooves on the undersurface of each margin are converted into anterior and posterior ethmoidal canals by similar grooves on the superior surface of the ethmoidal labyrinth. These canals are meant for passages of anterior and posterior ethmoidal nerves and vessels.
4. The undersurface of the anterior margin of the notch possesses openings for frontal sinuses (one on each side of the nasal spine).

Frontal Bone Frontal sinus

variable size. It is situated between the outer and Each frontal sinus is an irregular cavity of the inner tables of the frontal bone. They are separated from each other by a bony septum which is usually deviated to one side.

Frontal Bone Ossification

1. The frontal bone ossifies in the membrane.
2. Two primary centres appear one for each half of the frontal bone, in the region of the frontal tuberosity.
3. Primary centres appear during the 8th week of intrauterine life.
4. Ossification extends upwards to form the frontal squama, backwards to form the orbital part and downwards to form the nasal part.
5. At birth frontal bone is made up of two halves separated by frontal or metopic suture.
6. The union between two parts begins in 2nd year and completes at 8th year.

Frontal Bone Applied Anatomy

1. Frontal squama is soft and pliable in neonates which can withstand a considerable amount of compression and moulding, a fact clinically important during childbirth.
2. Frontal squama is prone to depressed or fissured fractures. In neonates and infants, the depressed fracture is often like a dimple in the bone. In adults, a depressed fracture is always associated with an irregular line of fracture.
3. A severe impact at the root of the nose leads to fractures of the frontal sinus walls. If the fracture involves the inner table forming the posterior wall of the frontal sinus, then the air may enter into the cranial cavity (aerocele) causing meningitis and brain. abscess.
4. Fracture of the orbital plate of the frontal bone causes haemorrhages in the orbit. The haemorrhage acquires a triangular shape under the conjunctiva whose apex is towards the corneoscleral junction and base towards the orbital margin.
5. A crack in the inner table of the frontal squama may damage a large diploic vein and produce a small epidural haematoma.
6. Almost invariably the fractures of frontal squama in children are associated with rupture of the dura mater.
7. A gap in the frontal squama will not be regenerating and has to be filled with tantalum or titanium, a procedure called cranial prosthesis.