3.1 Craniofacial Osteology, Muscles of Mastication & Salivary Glands

3.1 Craniofacial Osteology, Muscles of Mastication & Salivary Glands

Quick Answer: The skull consists of 8 cranial bones protecting the brain and 14 facial bones supporting oral and facial structures. The mandible is the only moveable craniofacial bone, articulating with the temporal bones at the bilateral temporomandibular joints (TMJs). Mandibular movement is powered by the four paired muscles of mastication—masseter, temporalis, medial pterygoid, and lateral pterygoid—all innervated by the mandibular division of the trigeminal nerve (CN V3). Oral moisture and enzymatic digestion are supplied by the parotid (Stensen's duct), submandibular (Wharton's duct), and sublingual (Bartholin's and Rivinus' ducts) salivary glands. Precise clinical delivery of local anesthesia relies on mapping the maxillary (V2) and mandibular (V3) branches of the trigeminal nerve.

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1. Craniofacial Osteology

The human skull is divided into the neurocranium (cranial vault enclosing the brain) and the viscerocranium (facial skeleton supporting the eyes, nasal complex, and oral cavity).

The Cranial Bones (8 Bones)

1. Frontal Bone (1): Forms the forehead, the superior margins of the orbital cavities, and the anterior cranial fossa. Contains the bilateral frontal sinuses.
2. Parietal Bones (2): Paired quadrilateral bones forming the superior roof and lateral walls of the cranium, joined at the midline by the sagittal suture and separated from the frontal bone by the coronal suture.
3. Occipital Bone (1): Forms the posterior and inferior base of the skull. It features the foramen magnum, through which the spinal cord passes, and the bilateral occipital condyles, which articulate with the first cervical vertebra (atlas).
4. Temporal Bones (2): Paired complex bones forming the lateral inferior walls of the cranium. Key anatomical landmarks include:
   • Squamous portion: Flat anterior-superior plate forming the medial wall of the temporal fossa.
   • Zygomatic process: Projects anteriorly to unite with the temporal process of the zygomatic bone, completing the zygomatic arch.
   • Glenoid (mandibular) fossa and articular eminence: Smooth depressions and anterior ridges forming the temporal component of the temporomandibular joint.
   • External acoustic meatus: Ear canal opening.
   • Mastoid process: Posterior projection serving as the insertion for the sternocleidomastoid muscle.
   • Styloid process: Slender inferior needle-like projection anchoring ligaments of the tongue and hyoid apparatus.
5. Sphenoid Bone (1): Central, butterfly-shaped keystone bone of the skull articulating with all other cranial bones. Highlights include the sella turcica (housing the pituitary gland), the greater and lesser wings, and the bilateral pterygoid processes. Each pterygoid process features a medial plate, a lateral plate, and the pterygoid hamulus, serving as primary origins for the pterygoid muscles and tensor veli palatini.
6. Ethmoid Bone (1): Delicate, porous bone situated between the orbits, forming the roof of the nasal cavity (cribriform plate with olfactory foramina), the superior nasal septum (perpendicular plate), the crista galli, and the superior and middle nasal conchae.

The Facial Bones (14 Bones)

• Maxillae (2): Paired bones fused at the intermaxillary suture forming the upper jaw, orbital floor, and nasal floor. Each maxilla contains the large maxillary sinus (antrum of Highmore). Key processes include the alveolar process (housing maxillary dental roots), the palatine process (forming the anterior three-quarters of the hard palate), the frontal process, and the zygomatic process. Important foramina and landmarks include the incisive foramen (posterior to maxillary central incisors), the infraorbital foramen, and the posterior maxillary tuberosity.
• Mandible (1): The largest, strongest, and only moveable bone of the skull. It consists of a horizontal curved body and two vertical rami meeting at the angle of the mandible.
  • Alveolar process: Superior crest of the body supporting the mandibular teeth.
  • Mental foramen: Bilateral openings on the external surface of the body, usually below the apices of the mandibular premolars (teeth 34/35 and 44/45), transmitting the mental nerve and blood vessels.
  • Mental protuberance: The bony prominence of the chin.
  • Coronoid process: Anterior superior projection of the ramus, providing insertion for the temporalis muscle.
  • Condyloid process (condyle): Posterior superior projection articulating within the glenoid fossa of the temporal bone.
  • Mandibular notch (sigmoid notch): Deep depression separating the coronoid and condyloid processes.
  • Mandibular foramen: Opening on the internal medial surface of the ramus, shielded anteriorly by a sharp spine of bone called the lingula, transmitting the inferior alveolar nerve and vessels.
  • Mylohyoid line: Internal ridge running along the body, providing attachment for the mylohyoid muscle.
• Zygomatic Bones (2): The cheekbones, contributing to the lateral orbital walls and completing the zygomatic arch.
• Palatine Bones (2): L-shaped bones whose horizontal plates form the posterior one-quarter of the hard palate. They feature the greater palatine foramen and lesser palatine foramina.
• Nasal Bones (2): Form the bridge of the nose.
• Lacrimal Bones (2): Smallest, most fragile facial bones, situated at the anterior-medial wall of the orbit.
• Inferior Nasal Conchae (2): Independent scroll-like bones projecting into the lower nasal cavity to increase mucosal surface area.
• Vomer (1): Single plowshare-shaped bone forming the posterior-inferior portion of the nasal septum.

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2. The Temporomandibular Joint (TMJ)

The temporomandibular joint (TMJ) is a specialized bilateral synovial joint classified as a ginglymoarthrodial joint, meaning it provides both hinge (ginglymoid) and gliding (arthrodial) movements.

Anatomical Components

1. Condyloid Process of the Mandible: An elliptical, convex bony knob covered with dense fibrous connective tissue (not hyaline cartilage), adapted to resist shear and compressive forces.
2. Glenoid (Mandibular) Fossa and Articular Eminence of Temporal Bone: The fossa is an oval depression situated anterior to the external acoustic meatus; the articular eminence is a transverse bony ridge directly anterior to the fossa that guides the condyle during jaw opening.
3. Articular Disc (Meniscus): A tough, flexible, biconcave fibrous disc interposed between the condyle and the temporal bone. It divides the joint cavity into two distinct compartments:
   • Inferior synovial cavity: Situated between the condyle and the inferior surface of the disc; dedicated to rotational (hinge) movement.
   • Superior synovial cavity: Situated between the articular eminence and the superior surface of the disc; dedicated to translational (gliding) movement.
   • The central bearing zone of the disc is completely avascular and aneural, relying on synovial fluid for metabolic exchange.
4. Capsular Ligament & Synovial Membrane: The fibrous capsule completely encloses the joint space, retaining synovial fluid secreted by the inner synovial lining.
5. Auxiliary Ligaments:
   • Temporomandibular (lateral) ligament: Strengthens the lateral capsule and limits excessive posterior and inferior condylar displacement.
   • Sphenomandibular ligament: Runs from the spine of the sphenoid to the lingula of the mandible.
   • Stylomandibular ligament: Extends from the styloid process to the mandibular angle, stabilizing the joint during extreme protrusion.

                    [ Articular Eminence (Temporal Bone) ]
                                     │
                       Superior Synovial Cavity
                     (Translatory Gliding Movement)
                                     │
                         [ Articular Disc / Meniscus ]
                                     │
                       Inferior Synovial Cavity
                      (Rotational Hinge Movement)
                                     │
                        [ Condyle of the Mandible ]

Joint Kinematics and Movement Phases

• Initial Opening (Hinge Movement): The first 20 to 25 mm of interincisal jaw opening occurs strictly in the lower joint compartment via rotational motion of the condyle against the inferior surface of the disc.
• Full Opening (Gliding Movement): For maximum mouth opening (>25 mm) and protrusion, the condyle and articular disc move together downward and forward along the articular eminence in the upper joint compartment.
• Retrusion & Lateral Excursion: Retrusion returns the condyle into the fossa; lateral excursion involves unilateral condylar translation while the opposite condyle rotates.

Clinical TMJ Dysfunction (TMD)

• Clicking and Popping: Characteristic of internal derangement with anterior disc displacement with reduction; the click occurs when the condyle pops back onto the displaced disc during opening.
• Closed Lock (Trismus): Anterior disc displacement without reduction; the disc blocks condylar translation, restricting jaw opening to approximately 20 mm.
• Crepitus: A dry, grating, or crackling sound caused by bone-on-bone contact from advanced osteoarthritic degradation of the articular tissues.
• Subluxation: Occurs when the condyle translates too far anteriorly over the crest of the articular eminence and becomes temporarily locked in an open position (open lock).

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3. Muscles of Mastication

Four pairs of muscles perform primary masticatory movements. All four are innervated by the mandibular division of the trigeminal nerve (Cranial Nerve V3) and receive blood supply from branches of the maxillary artery.

Clinical Note: The lateral pterygoid is the only primary muscle of mastication that assists in depressing (opening) the mandible. The masseter, temporalis, and medial pterygoid are all jaw elevators (closers). When a patient moves their jaw into left lateral excursion, the right lateral pterygoid is actively firing.

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4. Major Salivary Glands and Ducts

Saliva is essential for lubrication, bolus formation, speech, buffering acids (pH maintenance), remineralization (calcium and phosphate delivery), and enzymatic breakdown of starches via salivary amylase (ptyalin).

Clinical Pathology of Salivary Glands

• Sialolithiasis (Salivary Duct Calculi): Mineralized stones (calcium phosphate deposits) formed within glandular parenchyma or ducts. The submandibular gland (Wharton's duct) is the site of over 80% of all sialoliths due to the duct's long, upward-curving path against gravity and the higher mucin and calcium content of its secretions. Patients report sudden swelling and throbbing pain immediately before and during meals ("mealtime syndrome").
• Xerostomia (Dry Mouth): Subjective sensation of oral dryness resulting from severe salivary hypofunction. Common etiologies include medications (anticholinergics, antihypertensives, tricyclic antidepressants, antihistamines), head and neck radiation therapy, and autoimmune disorders such as Sjögren's syndrome. Untreated xerostomia drastically elevates the risk of rampant cervical and root caries, oral candidiasis, dysphagia, and painful mucosal ulcerations under removable prostheses.

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5. Trigeminal Nerve (Cranial Nerve V) & Dental Local Anesthesia

The trigeminal nerve is the fifth and largest cranial nerve. It is a mixed nerve containing both sensory fibers (supplying the face, scalp, oral mucosa, and teeth) and motor fibers (innervating the muscles of mastication, mylohyoid, anterior belly of digastric, tensor tympani, and tensor veli palatini).

The Three Trigeminal Divisions

1. Ophthalmic Division (V1): Purely sensory. Exits the cranium via the superior orbital fissure. Supplies sensation to the forehead, upper eyelids, cornea, and frontal sinus.
2. Maxillary Division (V2): Purely sensory. Exits the skull via the foramen rotundum into the pterygopalatine fossa. Branches include:
   • Posterior Superior Alveolar (PSA) Nerve: Supplies the maxillary second and third molars, and the distobuccal and lingual roots of the maxillary first molar (teeth 16/26), along with buccal gingiva and maxillary sinus mucosa. Clinical note: In some patients the mesiobuccal root of the maxillary first molar is supplied by the MSA nerve rather than the PSA, so a PSA block alone may not anesthetize that root.
   • Middle Superior Alveolar (MSA) Nerve: Not present in every patient (when it is absent, the PSA and ASA nerves cover its territory). Supplies maxillary premolars and the mesiobuccal root of the first molar, as well as buccal gingiva.
   • Anterior Superior Alveolar (ASA) Nerve: Supplies maxillary central incisors, lateral incisors, and canines (teeth 13-23), labial gingiva, and anterior nasal mucosa.
   • Infraorbital Nerve: Terminal branch exiting the infraorbital foramen, supplying the lower eyelid, side of the nose, and upper lip.
   • Greater Palatine Nerve: Exits the greater palatine foramen to supply the hard palate mucosa and lingual gingiva from the premolars posterior to the molars.
   • Nasopalatine Nerve: Exits the incisive foramen to supply palatal mucosa and lingual gingiva of the six maxillary anterior teeth (canine to canine).
3. Mandibular Division (V3): Mixed (sensory and motor). Exits the cranium via the foramen ovale into the infratemporal fossa. Sensory branches include:
   • Inferior Alveolar Nerve (IAN): Enters the mandibular foramen on the medial surface of the ramus. It courses through the mandibular canal to provide pulpal sensation to all mandibular teeth in that quadrant.
   • Mental Nerve: Exits the mental foramen; provides sensory innervation to the lower lip, chin, and facial gingiva of the anterior teeth and premolars.
   • Incisive Nerve: Remains within the bone anterior to the mental foramen to innervate mandibular canine and incisor pulps.
   • Lingual Nerve: Runs anterior and medial to the IAN; supplies general sensation (touch, pain, temperature) to the anterior two-thirds of the tongue, the floor of the mouth, and the mandibular lingual gingiva.
   • Buccal (Long Buccal) Nerve: Crosses the anterior border of the ramus; provides sensory innervation to the cheek mucosa and buccal gingiva adjacent to the mandibular molars.

Clinical Comparison: Infiltration vs. Nerve Block

MAXILLA: Thin, Porous Cortical Bone
└── Local Infiltration (Supraperiosteal Injection)
    └── Anesthetic solution diffuses readily through porous cortical bone to apex

MANDIBLE: Dense, Thick Cortical Bone Plate
└── Regional Nerve Block Required (Inferior Alveolar Nerve Block / IANB)
    └── Solution deposited in pterygomandibular space near mandibular foramen (behind lingula)
    └── Anesthetizes: IAN (teeth), Lingual Nerve (anterior 2/3 tongue, lingual gingiva)
    └── Supplemental Long Buccal Block needed for buccal soft tissue of molars

• Maxillary Anesthesia: Because maxillary facial cortical bone is thin and porous, pulpal and soft-tissue anesthesia is routinely achieved via local infiltration (supraperiosteal injections) deposited directly over the apices of targeted teeth.
• Mandibular Anesthesia: The mandible features a dense, thick cortical bone plate that prevents anesthetic solutions from diffusing directly to root apices (except in the anterior incisor region). Consequently, the clinician must perform an Inferior Alveolar Nerve Block (IANB). The needle is inserted into the pterygomandibular space, passing posterior to the lingula to deposit anesthetic around the nerve before it enters the mandibular foramen. Because the lingual nerve is positioned slightly anterior and medial to the IAN, it is typically bathed in anesthetic during withdrawal, numbing the anterior two-thirds of the tongue. However, the buccal (long buccal) nerve is a separate branch of V3 that is not anesthetized by a standard IANB; a supplemental long buccal injection into the retromolar area is required whenever restorative work involves clamping or surgical manipulation of mandibular molar buccal gingiva.