Osteology of the Skull 1 - IIUM Dentistry Study Notes

Introduction to Osteology and the Skull

• Anatomy: Defined as the study of the structure and function of the skeleton and bony structures.

• Osteology: A specialized branch of anatomy dealing specifically with the bones.

• Skull: The bony skeleton of the head.

• Overview of the Human Skeleton:

  • Mandible

  • Clavicle

  • Sternum

  • Skull

  • Cervical vertebrae

  • Humerus

  • Rib

  • Radius

  • Ilium

  • Ulna

  • Sacrum

  • Ischium

  • Scapula

  • Thoracic vertebrae

  • Lumbar vertebrae

  • Femur

  • Patella

  • Tibia

  • Fibula

Classification of Skull Bones

• The Skull: Includes the mandible.

• The Cranium: Refers to the braincase without the mandible.

• Facial Skeleton:

  • Single Bones: Mandible, Vomer.

  • Paired Bones: Maxilla, Zygomatic, Nasal, Lacrimal, Palatine, Inferior nasal conchae.

• Cranial Vault (Neurocranium):

  • Single Bones: Frontal, Occipital, Sphenoid, Ethmoid.

  • Paired Bones: Temporal, Parietal.

Internal Surface of the Cranial Vault

• Diploe: This term refers to the spongy, cancellous bone found in the calvaria (skullcap).

  • It is situated between two layers of cortical bone: the external table and the internal table.

  • The spongy bone contains bone marrow responsible for producing blood cells.

  • Function: To drain venous blood from the cranial bones to the dural venous sinuses and other intracranial/extracranial veins (emissary veins).

• Vascular Markings and Grooves:

  • The internal surface features grooves for meningeal vessels and the superior sagittal sinus.

  • Venous Sinuses of Dura Mater: A group of blood channels draining venous blood from the cranial cavity to the Internal Jugular Vein (IJV).

• Frontal Crest: A ridge for the attachment of the falx cerebri.

  • Falx Cerebri: A large, crescent-shaped fold of dura mater that separates the cerebral hemispheres. It supports dural sinuses for venous and Cerebrospinal Fluid (CSF) drainage. It attaches anteriorly to the crista galli and blends posteriorly with the tentorium cerebelli.

• Granular Foveolae (Arachnoid Foveolae): Bony depressions or pits located near the sulci, created by adjacent arachnoid granulations.

• Arachnoid Granulations (AGs): Herniations of the arachnoid membrane that protrude through the dura mater into the lateral lacunae and venous sinuses. They are primarily associated with the drainage of CSF into the venous sinuses.

Anterior (Frontal) View of the Skull

• General Proportions: The frontal bone occupies the upper third, while the maxilla and mandible occupy the other two-thirds of the anterior view.

• Frontal Bone (Forehead):

  • Squamous Part: The largest part, forming the forehead area and housing the frontal sinuses.

  • Orbital (Horizontal) Part: Forms the roof of the orbit and the ethmoidal sinuses.

  • Nasal Part: Located anteriorly between the two orbital parts; forms a portion of the nasal cavity roof.

  • Landmarks: Frontal eminence, supraorbital margin, frontal notch (or foramen), supraorbital notch (or foramen), superciliary arch, glabella, temporal line, temporal surface, and two zygomatic processes.

• Zygomatic Bone (Cheek):

  • Surfaces:

  • Lateral (Facial): Smooth and convex. Features the zygomaticofacial foramen (transmitting the zygomaticofacial nerve, artery, and vein). Attachment site for zygomaticus major (anterior half) and zygomaticus minor (posterior half) muscles.

  • Posteromedial (Temporal): Faces the temporal and infratemporal fossae. Articulates with the zygomatic process of the maxilla via the zygomaticomaxillary suture. Features the zygomaticotemporal foramen (transmitting the zygomaticotemporal nerve).

  • Orbital: Smooth and concave, forming the anterolateral floor and lateral wall of the orbit. Features the zygomatico-orbital foramen, which leads to a canal branching into zygomaticofacial and zygomaticotemporal canals.

• Maxilla (Upper Jaw):

  • Features the infraorbital foramen.

  • Includes the canine fossa (origin of levator anguli oris) and incisive fossa (origin of incisive of orbicularis oris).

• Nasal Bone (Bridge of Nose):

  • Two nasal bones articulate at the midline.

  • Borders:

  • Superior: Joins frontal bone at the frontonasal suture.

  • Inferior: Free edge, continues as lateral nasal cartilage.

  • Lateral: Articulates with the frontal process of the maxilla at the nasomaxillary suture.

  • Medial: Articulates with its fellow nasal bone at the internasal suture.

• Mandible (Lower Jaw): Features the mental foramen.

• Surface Landmarks:

  • Superciliary Arch: Rounded curve elevation overlying the frontal sinus; more prominent in males.

  • Glabella: Median elevation connecting the superciliary arches; frontonasal suture lies below it.

  • Nasion: Median point at the root of the nose where internasal meets frontonasal sutures.

  • Rhinion: Lowermost point of the internasal suture.

  • Anterior Nasal Spine: Sharp projection at the lower boundary of the piriform aperture.

Neurovascular Structures of the Face

• Supraorbital Foramen: Transmits the Supraorbital Artery (supplying the scalp; branch of Ophthalmic Artery, itself a branch of the Internal Carotid Artery/ICA) and the Supraorbital Nerve (innervates the front of the scalp; $V_1$ branch of Trigeminal Nerve).

• Infraorbital Foramen: Transmits the Infraorbital Nerve (branch of $V_2$; provides sensation to lower eyelid, upper nose, and upper lip) and the Infraorbital Artery (branch of Maxillary Artery in the Pterygopalatine Fossa).

• Mental Foramen (MF): Transmits the terminal branches of the inferior alveolar nerve, known as the Mental Nerve (sensation to chin, lower lip, and anterior mandibular gums). Also transmits Mental Vessels (Mental Artery and Vein; branches of the Maxillary Artery).

Lateral View of the Skull

• Bones Involved: Frontal, Parietal (wall of cranial vault), Occipital (back of skull), Temporal (Squamous and Mastoid parts), Sphenoid (Greater Wing), Zygomatic, Maxilla, Mandible, Nasal, and Lacrimal (contains the nasolacrimal duct for tears).

• Sutures:

  • Coronal: Between parietal and frontal bones.

  • Squamous: Between parietal and temporal bones.

  • Lambdoid: Between parietal and occipital bones.

  • Zygomatic-related: Fronto-zygomatic (FZ), Temporozygomatic (TZ), and Zygomaticomaxillary (ZM) sutures.

• Zygomatic Arch: Formed by the temporal process of the zygomatic bone (anterior $1/3$) and the zygomatic process of the temporal bone (posterior $2/3$). They are united by the zygomaticotemporal suture.

• Pterion (Clinically Significant):

  • An "H-shaped" junction between the temporal, parietal, frontal, and sphenoid bones.

  • It is the thinnest part of the skull.

  • A fracture here can lacerate the middle meningeal artery (anterior branch of the Maxillary Artery, branch of the External Carotid Artery/ECA).

  • Laceration results in blood forming between the inner skull and outer dura, known as an extradural (epidural) haematoma.

• Asterion: Junction of the occipital, temporal, and parietal bones on the lateral aspect.

• Temporal Fossa: Filled with the temporalis muscle, which attaches to the coronoid process. The temporalis fascia attaches to the upper border of the zygomatic arch.

• Mastoid Process: "Breast-shaped" process of the temporal bone. It contains mastoid air cells that communicate with the middle ear via the mastoid antrum. It serves as an insertion point for:

  • Sternocleidomastoid

  • Splenius capitis

  • Longissimus capitis

  • Posterior belly of digastric

• External Acoustic Meatus (EAM): A sigmoid-shaped tube extending from the concha to the tympanic membrane. The outer $1/3$ is cartilage, and the inner $2/3$ is temporal bone.

Posterior and Superior Views of the Skull

• Posterior View Bones and Landmarks:

  • Occipital Bone: Features the external occipital protuberance (Inion), external occipital crest, and nuchal lines (superior and inferior).

  • Superior Nuchal Line: Attachment for Trapezius, Sternocleidomastoid, and Splenius capitis.

  • Inferior Nuchal Line: Attachment for Rectus capitis posterior major/minor and Obliquus capitis superior.

  • Sutures: Sagittal, Lambdoid, Occipitomastoid, and Parietomastoid.

• Superior View (The Calvaria):

  • Bregma: Meeting point of coronal and sagittal sutures. Site of the anterior fontanelle, which closes at $12-24$ months (no longer palpable by $18$ months).

  • Lambda: Meeting point of sagittal and lambdoid sutures. Site of the posterior fontanelle.

  • Vertex: The highest point on the sagittal suture.

  • Parietal Foramen: Perforates the parietal bone for emissary veins from the superior sagittal sinus.

Internal Surface: Cranial Fossae

• Anterior Cranial Fossa (ACF):

  • Bones: Orbital plate of frontal, cribriform plate of ethmoid, lesser wing of sphenoid.

  • Contents: Frontal lobes of the brain, olfactory bulbs, and olfactory tracts.

  • Foramina:

  • Foramen Cecum: Transmits emissary veins connecting extracranial veins to the superior sagittal sinus.

  • Cribriform Plate: Features small holes for Olfactory Nerves (CN I).

  • Clinical: Fracture may cause CSF rhinorrhea (leak from nose) and "Black Eye" because the frontalis muscle has no bony attachment, allowing blood seepage into eyelids.

• Middle Cranial Fossa (MCF):

  • Bones: Body and greater wings of sphenoid, temporal (petrous and squamous parts), parietal.

  • Median Area (Sella Turcica): Includes the hypophyseal fossa (lodges the pituitary gland) and dorsum sellae. Below it are the sphenoidal air sinuses.

  • Foramina:

  • Optic Canal: Transmits Optic Nerve (CN II) and Ophthalmic Artery.

  • Superior Orbital Fissure (SOF): Transmits CN III, IV, $V_1$ (Lacrimal, Frontal, Nasociliary), and VI, plus ophthalmic veins.

  • Foramen Rotundum: Transmits Maxillary Nerve ($V_2$) to the Pterygopalatine Fossa.

  • Foramen Ovale: Transmits Mandibular Nerve ($V_3$) to the Infratemporal Fossa.

  • Foramen Spinosum: Transmits Middle Meningeal Artery and meningeal branch of $V_3$.

  • Foramen Lacerum: Traversed by the ICA, venous and sympathetic plexus. The greater petrosal and deep petrosal nerves unite here to form the nerve to the pterygoid canal (Vidian nerve).

  • Clinical: Fracture may cause bleeding/CSF discharge through the ear, nose, or mouth. Involvement of the semicircular canal causes vertigo.

• Posterior Cranial Fossa (PCF):

  • Bones: Sphenoid, temporal, occipital, parietal.

  • Contents: Cerebellum, pons, medulla (brainstem).

  • Landmarks: Clivus (sloping surface supporting the pons/medulla), Internal occipital protuberance.

  • Foramina:

  • Foramen Magnum: Transmits the spinal cord, vertebral arteries, and spinal accessory nerve (CN XI).

  • Internal Acoustic Meatus (IAM): Transmits Facial Nerve (CN VII) and Vestibulocochlear Nerve (CN VIII).

  • Jugular Foramen: Transmits CN IX, X, and XI, and the Internal Jugular Vein (IJV).

  • Hypoglossal Canal: Transmits Hypoglossal Nerve (CN XII).

Questions & Discussion

• Identification Exercise (Anterior View):

  • A: Frontal Bone

  • B: Maxilla

  • C: Mandible

  • D: Zygomatic Bone

  • E: Nasal Bone

• Identification Exercise (Lateral View):

  • A: Parietal Bone; B: Occipital Bone; C: Temporal Bone; D: Ext. Auditory Meatus; E: Condyle of Mandible; F: Coronoid Process; G: Mental Foramen; H: Maxilla; I: Greater Wing of Sphenoid; J: Frontal Bone; K: Pterion.

• Identification Exercise (Posterior View):

  • A: Parietal Bone; B: Occipital Bone; C: Lambdoid Suture; D: Sagittal Suture.

• Topic Recap Questions:

  • Identify openings and structures passing through them (Arrows pointed to Supraorbital foramen, Infraorbital foramen, and Mental foramen).

  • Difference between Asterion and Pterion? (Pterion is the H-shaped junction of 4 bones sensitive to injury; Asterion is the junction of 3 bones on the posterior-lateral side).