Regions of the Head 9. Oral Cavity & Perioral Regions Temporomandibular Joint (TMJ) Zygomatic process of temporal bone Articular tubercle Mandibular Petrotympanic fossa of TMJ fissure Postglenoid tubercle Styloid process External acoustic meatus Mastoid process (auditory canal) Atlanto- occipital joint Fig. 9.27 Mandibular fossa of the TMJ tubercle. Unlike other articular surfaces, the mandibular fossa is cov- Inferior view of skull base. The head (condyle) of the mandible artic- ered by fi brocartilage, not hyaline cartilage. As a result, it is not as ulates with the mandibular fossa of the temporal bone via an articu- clearly delineated on the skull (compare to the atlanto-occipital joints). lar disk. The mandibular fossa is a depression in the squamous part of The external auditory canal lies just posterior to the mandibular fossa. the temporal bone, bounded by an articular tubercle and a postglenoid Trauma to the mandible may damage the auditory canal. Head (condyle) of mandible Pterygoid Joint fovea capsule Neck of Coronoid mandible Lateral process ligament Neck of mandible Lingula Mandibular Stylomandibular foramen ligament Mylohyoid groove AB Fig. 9.28 Head of the mandible in the TMJ Fig. 9.29 Ligaments of the lateral TMJ A Anterior view. B Posterior view. The head (condyle) of the mandible is Left lateral view. The TMJ is surrounded by a relatively lax capsule that markedly smaller than the mandibular fossa and has a cylindrical shape. permits physiological dislocation during jaw opening. The joint is stabi- Both factors increase the mobility of the mandibular head, allowing lized by three ligaments: lateral, stylomandibular, and sphenomandib- rotational movements about a vertical axis. ular (see also Fig. 9.30). The strongest of these ligaments is the lateral ligament, which stretches over and blends with the joint capsule. 194 7050003C09.indd 194 10/20/09 12:28:10 PM Regions of the Head 9. Oral Cavity & Perioral Regions Pterygoid process, Articular lateral plate tubercle Articular disk Pterygospinous Postglenoid ligament tubercle Mandibular Joint notch capsule Sphenomandib- Head (condyle) ular ligament of mandible Stylomandibular ligament Stylomandibular Pterygoid process, ligament medial plate Fig. 9.30 Ligaments of the medial TMJ Fig. 9.31 Opened TMJ Left medial view. Note the sphenomandibular ligament. The variable Left lateral view. The TMJ capsule begins at the articular tubercle and pterygospinous ligament is also present. extends posteriorly to the petrotympanic fi ssure (see Fig. 9.27). Inter- posed between the mandibular head and fossa is the fi brocartilaginous articular disk, which is attached to the joint capsule on all sides. Posterior division Auriculotemporal nerve Mandibular nerve (CN V3) Anterior division Deep temporal nerve, posterior branch Masseteric nerve Fig. 9.32 Dislocation of the TMJ Fig. 9.33 Sensory innervation of the TMJ capsule The head of the mandible may slide past the articular tubercle when Superior view. The TMJ capsule is supplied by articular branches aris- the mouth is opened, dislocating the TMJ. This may result from heavy ing from three nerves from the mandibular division of the trigeminal yawning or a blow to the opened mandible. When the joint dislocates, nerve (CN V3): the mandible becomes locked in a protruded position and can no longer • Auriculotemporal nerve be closed. This condition is easily diagnosed clinically and is reduced by • Deep temporal nerve, posterior branch pressing on the mandibular row of teeth. • Masseteric nerve 195 7050003C09.indd 195 10/20/09 12:28:15 PM Regions of the Head 9. Oral Cavity & Perioral Regions Temporomandibular Joint (TMJ): Biomechanics Transverse axis through Retrusion head of mandible (axis of rotation) 150° Head of mandible A Median plane B Protrusion Axis of Axis of rotation rotation Resting condyle Swinging condyle Balance side Working side (mediotrusion) (laterotrusion) Working side Balance side C Bennett angle D Fig. 9.34 Movements of the mandible in the TMJ B Translation. In this movement the mandible is advanced (protruded) Superior view. Most of the movements in the TMJ are complex motions and retracted (retruded). The axes for this movement are parallel to that have three main components: the median axes through the center of the mandibular heads. C Grinding movements in the left TMJ. In describing these lateral • Rotation (opening and closing the mouth) movements, a distinction is made between the “resting condyle” • Translation (protrusion and retrusion of the mandible) and the “swinging condyle.” The resting condyle on the left working • Grinding movements during mastication side rotates about an almost vertical axis through the head of the A Rotation. The axis for joint rotation runs transversely through mandible (also a rotational axis), whereas the swinging condyle on both heads of the mandible. The two axes intersect at an angle of the right balance side swings forward and inward in a translational approximately 150 degrees (range of 110 to 180 degrees between movement. The lateral excursion of the mandible is measured in de- individuals). During this movement the TMJ acts as a hinge joint grees and is called the Bennett angle. During this movement the (abduction/depression and adduction/elevation of the mandible). mandible moves in laterotrusion on the working side and in medio- In humans, pure rotation in the TMJ usually occurs only during sleep trusion on the balance side. with the mouth slightly open (aperture angle up to approximately D Grinding movements in the right TMJ. Here, the right TMJ is the 15 degrees). When the mouth is opened past 15 degrees, rotation working side. The right resting condyle rotates about an almost ver- is combined with translation (gliding) of the mandibular head. tical axis, and the left condyle on the balance side swings forward and inward. 196 7050003C09.indd 196 10/20/09 12:28:20 PM Regions of the Head 9. Oral Cavity & Perioral Regions Lateral pterygoid muscle, Articular superior head tubercle Mandibular fossa Articular disk Head of mandible Joint capsule Lateral pterygoid muscle, inferior head A Lateral pterygoid muscle, superior head Articular disk Head of mandible Joint capsule Axis of rotation Lateral pterygoid 15° muscle, inferior head B Lateral pterygoid muscle, superior head Mandibular fossa Articular disk Joint capsule Lateral pterygoid muscle, >15° inferior head C Fig. 9.35 Movements of the TMJ A Mouth closed. When the mouth is in a closed position, the head Left lateral view. Each drawing shows the left TMJ, including the articu- of the mandible rests against the mandibular fossa of the temporal lar disk and capsule and the lateral pterygoid muscle. Each schematic bone with the intervening articular disk. diagram at right shows the corresponding axis of joint movement. The B Mouth opened to 15 degrees. Up to 15 degrees of abduction, the muscle, capsule, and disk form a functionally coordinated musculo- head of the mandible remains in the mandibular fossa. disco-capsular system and work closely together when the mouth C Mouth opened past 15 degrees. At this point the head of the man- is opened and closed. Note: The space between the muscle heads is dible glides forward onto the articular tubercle. The joint axis that greatly exaggerated for clarity. runs transversely through the mandibular head is shifted forward. The articular disk is pulled forward by the superior part of the lateral pterygoid muscle, and the head of the mandible is drawn forward by the inferior part of that muscle. 197 7050003C09.indd 197 10/20/09 12:28:21 PM Regions of the Head 9. Oral Cavity & Perioral Regions Muscles of Mastication: Overview The muscles of mastication are derived from the fi rst branchial arch innervation from the mandibular division of the trigeminal nerve (CN and are located at various depths in the parotid and infratemporal re- V3). The muscles of the oral fl oor (mylohyoid and geniohyoid) are found gions of the face. They attach to the mandible and receive their motor on pp. 178, 203. Table 9.6 Masseter and temporalis muscles Muscle Origin Insertion Innervation* Action Masseter ① Superfi cial Zygomatic bone (maxillary Mandibular angle and Masseteric n. Elevates mandible; also head process) and zygomatic arch ramus (lower posterior (anterior division assists in protraction, (lateral aspect of anterior ⅔) lateral surface) of CN V3) retraction, and side-to-side motion Middle head Zygomatic arch (medial Mandibular ramus aspect of anterior ⅔) (central part) ② Deep head Zygomatic arch (deep Mandibular ramus (upper surface of posterior ⅓) portion) and lateral side of coronoid process Temporalis ③ Superfi cial Temporal fascia Coronoid process of Deep temporal nn. Vertical (anterior) fi bers: head mandible (apex and (anterior division Elevate mandible medial and anterior of CN V3) Horizontal (posterior) fi bers: ④ Deep head Temporal fossa (inferior surfaces) Retract (retrude) mandible temporal line) Unilateral: Lateral movement of mandible (chewing) *The muscles of mastication are innervated by motor branches of the mandibular nerve (CN V3), the 3rd division of the trigeminal nerve (CN V). ⑤ ④ ③ ⑥ ① ⑦ ② ⑧ Fig. 9.36 Masseter Fig. 9.37 Temporalis Fig. 9.38 Pterygoids Table 9.7 Lateral and medial pterygoid muscles Muscle Origin Insertion Innervation Action Lateral ⑤ Superior head Greater wing of sphenoid bone Mandible (pterygoid Mandibular n. Bilateral: Protrudes pterygoid (infratemporal crest) fovea) and (CN V3) via lateral mandible (pulls temporomandibular ptery goid n. (from articular disk