4.1 Cranial Bones, Temporomandibular Joint (TMJ) & Muscles of Mastication

Cranial Bones, Temporomandibular Joint (TMJ) & Muscles of Mastication

Quick Reference: The human cranium houses and protects the central nervous system, while the facial skeleton provides structural anchorage for the dentition, airway, and masticatory apparatus. The temporomandibular joint (TMJ) functions as a complex ginglymoarthrodial articulation permitting both rotational (hinge) and translational (gliding) movements. The four primary muscles of mastication—innervated by Cranial Nerve V3—coordinate jaw elevation, depression, protrusion, retrusion, and lateral excursion.

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Osteology of the Cranium and Facial Skeleton

The human skull consists of 22 bones (excluding the six middle ear auditory ossicles and the single hyoid bone). Anatomically, these bones are categorized into two major divisions: the neurocranium (8 cranial bones) that encloses and protects the brain, and the viscerocranium (14 facial bones) that forms the anterior framework of the face, nasal cavity, orbits, and dental arches.

The Cranial Bones (Neurocranium)

The eight cranial bones are joined by immovable, fibrous articulations termed sutures:

1. Frontal Bone (1): Forms the forehead, the superior rims of the orbital cavities (supraorbital margins), the anterior floor of the cranial vault, and contains the bilateral frontal paranasal sinuses.
2. Parietal Bones (2): Paired bones that articulate at the midline sagittal suture to form the bulging roof and superior lateral walls of the cranium. They articulate with the frontal bone anteriorly at the coronal suture and with the occipital bone posteriorly at the lambdoid suture.
3. Occipital Bone (1): Forms the posteroinferior base and back of the cranium. It features the foramen magnum, the large opening through which the spinal cord transitions into the brainstem, and the bilateral occipital condyles, which articulate with the first cervical vertebra (atlas, C1) to allow head nodding.
4. Temporal Bones (2): Paired, complex bones situated at the lateral skull base below the parietals, articulating at the squamosal sutures. Each temporal bone comprises distinct portions and anatomical landmarks essential to dentistry:
   • Squamous Portion: Flat, fan-like plate forming the temple; gives rise to the anteriorly projecting zygomatic process, which articulates with the temporal process of the zygomatic bone to form the zygomatic arch.
   • Glenoid (Mandibular) Fossa: An oval depression on the inferior surface that receives the condyle of the mandible to form the TMJ.
   • Articular Eminence: A smooth, rounded bony ridge located immediately anterior to the glenoid fossa. The mandibular condyle and articular disc glide forward and downward along this slope during jaw opening.
   • External Acoustic Meatus: The ear canal opening situated immediately posterior to the TMJ.
   • Mastoid Process: A rounded bony projection posterior to the ear canal serving as the insertion for the sternocleidomastoid muscle.
   • Styloid Process: A slender, needle-like spine projecting anteroinferiorly, providing attachment for styloid ligaments and muscles.
5. Sphenoid Bone (1): A complex, butterfly-shaped bone situated at the base of the skull anterior to the temporal and occipital bones, functioning as the structural keystone binding the neurocranium to the facial skeleton. Critical features include:
   • Greater and Lesser Wings: Flaring lateral processes contributing to the skull base and posterior orbital walls.
   • Sella Turcica: A saddle-shaped central depression in the superior surface of the sphenoid body housing the pituitary gland.
   • Pterygoid Processes: Paired inferior projections, each dividing into a medial pterygoid plate and a lateral pterygoid plate. The pterygoid plates and the intervening pterygoid fossa provide the muscular origins for the medial and lateral pterygoid muscles.
   • Cranial Foramina: Houses vital neurovascular openings, including the foramen rotundum (transmitting the maxillary nerve, V2), the foramen ovale (transmitting the mandibular nerve, V3), and the foramen spinosum (transmitting the middle meningeal artery).
6. Ethmoid Bone (1): A light, spongy bone positioned in the anterior cranial floor between the orbits. It features the cribriform plate with olfactory foramina for CN I, the superiorly projecting crista galli, the vertical perpendicular plate that forms the superior portion of the bony nasal septum, and the superior and middle nasal conchae.

The Facial Bones (Viscerocranium)

The facial skeleton consists of 14 bones, 12 of which are paired and two of which are single (the vomer and the mandible):

1. Maxillae (2): Paired bones that fuse at the intermaxillary suture to form the upper jaw, the floor of the orbits, the lateral boundaries of the nasal cavity, and the anterior two-thirds of the hard palate (via the palatine processes). Each maxilla contains the large pyramidal maxillary sinus (antrum of Highmore). Key dental landmarks include:
   • Alveolar Process: The thickened, porous ridge containing the sockets (alveoli) for the maxillary teeth.
   • Incisive Foramen: Located palatally at the anterior midline immediately behind the central incisors, transmitting the nasopalatine nerve and vessels.
   • Infraorbital Foramen: Located on the anterior facial surface below the inferior orbital rim, transmitting the infraorbital nerve and vessels.
   • Canine Eminence: A prominent facial bulge of bone overlying the prominent root of the maxillary canine.
   • Maxillary Tuberosity: A rounded bony prominence located distal to the last maxillary molar, serving as an important injection landmark and prosthetic boundary.
2. Mandible (1): The largest, strongest, and only movable bone of the skull. It supports the lower dentition and articulates bilaterally with the temporal bones at the TMJ. It consists of a curved horizontal body and two vertical rami:
   • Mental Protuberance: The triangular prominence forming the chin.
   • Mental Foramen: A bilateral opening on the external surface of the body, typically located below the apices of the mandibular premolars, through which the mental nerve and blood vessels exit.
   • Mandibular Angle (Gonion): The posteroinferior junction where the horizontal body meets the vertical ramus.
   • Mandibular Notch (Sigmoid Notch): The deep concave depression between the coronoid and condyloid processes atop the ramus.
   • Coronoid Process: The thin, triangular anterior projection of the ramus providing attachment for the temporalis muscle.
   • Condyloid Process (Condyle): The thick, rounded posterior projection consisting of a neck and an elliptical articular head that seats into the glenoid fossa of the temporal bone.
   • Mandibular Foramen: Located on the internal (medial) surface of the ramus, protected anteriorly by a sharp spine of bone called the lingula. The inferior alveolar nerve and vessels enter here to course through the mandibular canal.
   • Mylohyoid Line (Ridge): An oblique bony ridge on the internal body providing attachment for the mylohyoid muscle (floor of the mouth).
3. Zygomatic Bones (2): Known as the cheekbones; they form the prominence of the cheeks, lateral orbital rims, and articulate with the maxilla, frontal, and temporal bones.
4. Nasal Bones (2): Small oblong bones joined at the midline to form the bridge of the nose.
5. Lacrimal Bones (2): Delicate, fingernail-sized bones located in the medial anterior orbital wall containing the lacrimal groove.
6. Palatine Bones (2): L-shaped bones situated posterior to the maxillae. Their horizontal plates unite to form the posterior one-third of the hard palate, featuring the greater palatine foramen (posteriorly near the second or third molars) and lesser palatine foramina.
7. Inferior Nasal Conchae (2): Independent scroll-like bones projecting from the lateral nasal walls into the nasal cavity to increase mucosal surface area for air humidification.
8. Vomer (1): A thin, plow-shaped bone that forms the posteroinferior portion of the nasal septum, articulating with the ethmoid perpendicular plate and the palatine bones.

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The Temporomandibular Joint (TMJ)

The temporomandibular joint (TMJ) is a bilateral synovial articulation between the condyloid process of the mandible and the mandibular (glenoid) fossa and articular eminence of the temporal bone. It is classified as a ginglymoarthrodial joint because it executes both rotational (hinge / ginglymoid) and translational (gliding / arthrodial) movements during mastication, swallowing, and speech.

+-------------------------------------------------------------------+
|               TEMPOROMANDIBULAR JOINT (TMJ) ANATOMY               |
+-------------------------------------------------------------------+
| Temporal Bone:                                                    |
|   - Glenoid (Mandibular) Fossa (concave posterior socket)         |
|   - Articular Eminence (convex transverse anterior slope)          |
|                                                                   |
| [ Upper Synovial Compartment ] -> Gliding / Translational Action  |
| ----------------------------------------------------------------- |
| Articular Disc (Meniscus):                                        |
|   - Posterior Band (thick)                                        |
|   - Intermediate Zone (thin, avascular, aneural, pressure-bearing)|
|   - Anterior Band (moderate thickness)                            |
| ----------------------------------------------------------------- |
| [ Lower Synovial Compartment ] -> Hinge / Rotational Action       |
|                                                                   |
| Mandibular Condyle:                                               |
|   - Head of Condyloid Process articulating within lower cavity    |
|   - Attached posteriorly to vascular retrodiscal pad (bilaminar)  |
+-------------------------------------------------------------------+

Anatomical Components of the TMJ

1. Articular Fossa and Articular Eminence: Unlike most synovial joints lined with hyaline cartilage, the articulating surfaces of the temporal bone and mandibular condyle are covered by dense fibrocartilage, which has superior reparative capacity and resistance to mechanical shear forces.
2. Articular Disc (Meniscus): A dense fibrous connective tissue disc positioned between the condyle and temporal bone. It is non-innervated and avascular in its thin intermediate zone, enabling it to withstand substantial masticatory compressive forces without pain. The disc is thicker at its anterior and posterior borders (biconcave shape). Posteriorly, it attaches to the retrodiscal tissue (bilaminar zone), which is rich in blood vessels and sensory nerve endings. Compressive loading of the retrodiscal pad due to anterior disc displacement is a primary source of TMJ arthralgia.
3. Synovial Cavities: The articular disc divides the joint space into two isolated fluid-filled compartments:
   • Lower (Inferior) Synovial Cavity: Bounded by the mandibular condyle and the inferior surface of the disc. Responsible for hinge (rotational) movement around a transverse horizontal axis during the initial 20–25 mm of jaw opening.
   • Upper (Superior) Synovial Cavity: Bounded by the articular eminence/glenoid fossa and the superior surface of the disc. Responsible for gliding (translational) movement forward and downward along the articular eminence as the mouth opens widely (> 25 mm).
4. Synovial Fluid: Secreted by the synovial membrane lining the non-articular inner surfaces of the joint capsule. It lubricates the joint surfaces to eliminate friction and provides metabolic nutrition to the avascular central disc and fibrocartilage.
5. Joint Capsule and Ligaments: A fibrous capsule encloses the joint. Stabilizing ligaments include the temporomandibular (lateral) ligament (prevents posterior and inferior condylar displacement), the sphenomandibular ligament (medial to ramus, extending from sphenoid spine to lingula), and the stylomandibular ligament (from styloid process to mandibular angle).

Mandibular Movements and TMJ Biomechanics

• Depression (Opening): Combines initial rotational hinge movement in the lower compartment followed by anterior-inferior translational gliding of the condyle-disc assembly in the upper compartment. Normal maximum interincisal opening averages 40 to 50 mm (roughly three finger-widths).
• Elevation (Closing): Translational retrusion of the condyle back into the glenoid fossa, followed by terminal hinge closure.
• Protrusion: Forward movement of the mandible. Both condyles and discs translate simultaneously forward and downward along the articular eminences.
• Retrusion: Backward movement of the mandible into the retruded contact position.
• Lateral Excursion: Side-to-side movement. In lateral movement to the right, the right condyle (working side) rotates around a vertical axis in its fossa, while the left condyle (balancing/non-working side) translates forward, downward, and medially along its articular eminence.

Temporomandibular Disorders (TMD)

Temporomandibular disorders comprise a cluster of musculoskeletal and neuromuscular conditions involving the TMJ, masticatory muscles, and associated structures.

• Etiology: Multifactorial, including nocturnal bruxism, clenching, direct trauma, emotional stress, systemic rheumatic conditions, and occlusal disharmonies.
• Signs and Symptoms: Dull, aching preauricular facial pain, masticatory muscle tenderness, limited range of motion or jaw locking, headache, neck pain, and joint sounds.
• Trismus: Tonic spasm of the masticatory muscles resulting in restricted opening (interincisal opening < 35 mm). It can arise from TMD, dental infection, or trauma to the medial pterygoid muscle following an inferior alveolar nerve block.
• Joint Sounds:
  • Clicking / Popping: Audible or palpable sound resulting from anterior disc displacement with reduction. As the condyle translates forward, it pops over the posterior band of the displaced disc to re-establish normal anatomical positioning; upon closing, the disc slips anteriorly again (reciprocal click).
  • Crepitus: A dry, grating, or crunching sound (resembling walking on gravel). Crepitus indicates bone-on-bone contact resulting from advanced osteoarthritic degenerative joint disease, erosion of articular fibrocartilage, or disc perforation.
• Subluxation (Open Lock / Dislocation): Occurs when the patient opens excessively wide (e.g., during a long dental procedure or yawning) and the mandibular condyle glides anteriorly past the crest of the articular eminence and locks in front of it. The surrounding muscles spasm, locking the jaw open.
  • Emergency Reduction Protocol: The dental professional wraps gloved thumbs in sterile gauze, positions them bilaterally over the occlusal surfaces of the mandibular molars with fingers supporting the inferior border of the mandible extraorally, and applies firm downward and backward pressure to allow the condyle to clear the articular eminence and reseat into the glenoid fossa.

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Muscles of Mastication

The four primary muscles of mastication develop embryologically from the first branchial (pharyngeal) arch. Consequently, they are all innervated by the motor root of the mandibular division of the trigeminal nerve (Cranial Nerve V3).

Detailed Anatomy of Masticatory Muscles

1. Masseter Muscle: A powerful, thick quadrilateral muscle superficial to the ramus. It consists of a superficial head originating from the anterior two-thirds of the lower border of the zygomatic arch and a deep head originating from the posterior third and medial surface of the zygomatic arch. Both insert onto the external angle and ramus of the mandible. Contraction powerfully closes the jaws. It is easily palpated during heavy clenching.
2. Temporalis Muscle: A broad, fan-shaped muscle filling the temporal fossa. Its fibers converge beneath the zygomatic arch into a stout tendon that inserts onto the coronoid process and the anterior edge of the mandibular ramus. Its anterior vertical fibers elevate the mandible, while its posterior horizontal fibers retract the mandible back into the glenoid fossa.
3. Medial (Internal) Pterygoid Muscle: Situated on the inner aspect of the mandibular ramus, mirroring the masseter on the outside. Together, the masseter and medial pterygoid form a powerful pterygomandibular sling encircling the mandibular angle, stabilizing the jaw and providing massive crushing force.
4. Lateral (External) Pterygoid Muscle: A short, thick, conical muscle situated deep in the infratemporal fossa. Unlike the other three muscles that close the mouth, the lateral pterygoid is the only muscle of mastication that actively depresses (opens) the mandible (the suprahyoid muscles, especially the digastric, also pull the mandible open). When both inferior heads contract, they pull the condyles and articular discs forward down the articular eminences, initiating wide jaw opening and protrusion. Unilateral contraction pulls that condyle forward and medially, shifting the chin to the opposite side.

       [ Temporalis ]           [ Masseter & Medial Pterygoid ]
     (Elevates & Retrudes)          (Pterygomandibular Sling:
                                      Powerful Elevation)
              \                               /
               \                             /
                +---------------------------+
                |    MANDIBULAR DYNAMICS    |
                +---------------------------+
                             /
                            /
                 [ Lateral Pterygoid ]
              (Only Opener of the Four;
               Protrusion & Excursion)

Key Muscles of Facial Expression Relevant to Dental Assisting

While the muscles of mastication control jaw movements, the muscles of facial expression are innervated by the Facial Nerve (Cranial Nerve VII) and play vital roles in oral competence, food management, and dental isolation:

• Buccinator Muscle: The principal muscle of the cheek, forming the lateral wall of the oral cavity. It originates from the alveolar processes of the maxilla and mandible opposite the molar teeth and the pterygomandibular raphe (a tendinous band extending from the pterygoid hamulus to the mylohyoid line). Its fibers interlace anteriorly with the orbicularis oris at the modiolus.
  • Dental Function: Compresses the cheek against the molar teeth during mastication, preventing food from lodging in the buccal vestibule and keeping the food bolus centered on the occlusal tables.
  • Clinical Significance: The buccinator is pierced by the parotid duct (Stensen's duct) opposite the maxillary second molar. When placing cotton rolls or dry angles in the maxillary vestibule, the assistant must avoid occluding this duct.
• Orbicularis Oris Muscle: A complex circular sphincter muscle encircling the mouth within the lips. It possesses no direct bony attachment.
  • Dental Function: Closes, compresses, puckers, and protrudes the lips, establishing oral seal for swallowing and speech.
• Mentalis Muscle: Originates from the incisive fossa of the mandible and inserts into the skin of the chin.
  • Dental Function: Elevates and wrinkles the skin of the chin and protrudes the lower lip.
  • Clinical Note: Hyperactivity of the mentalis muscle can displace a mandibular impression tray, dislodge a lower denture, or impede placement of the dental dam frame.