S8 Mastication - Learning Objectives
1. Describe the significant bony landmarks and openings that relate to mastication, the temporomandibular joint (TMJ), and passages for key neurovasculature.
2. What type of joint is the temporomandibular joint (TMJ)? What actions occur at this joint?
The temporomandibular joint (TMJ) is a compound synovial hinge joint formed by the mandibular fossa of the temporal bone receiving the condyle of the mandible. These structures are separated by a fibrocartilaginous articular disc, which compounds the articulation into superior and inferior joints (and cavities).
The superior joint allows for protrusion and retraction of the mandible, while the inferior joint allows for elevation and depression of the mandible with minor pivoting actions. The muscles of mastication produce the movements at the TMJ. The TMJ is primarily afferently innervated by the auriculotemporal n. and vascularized by branches from both the maxillary a. and the superficial temporal a.
3. What major ligaments and other connective tissues limit the range of motion of the temporomandibular joint (TMJ)?
Stylomandibular ligament is often considered a reinforcing ligament of the TMJ; however, its alignment is such that it does not seem to afford any mechanical constraints to the the joint. This ligament extends from the styloid process of the temporal bone to the posterior border of the ramus and angle of the mandible.
The lateral (temporomandibular) ligament is important in restricting posterior and lateral movement of the condylar process of the mandible. It attaches the articular tubercle of the temporal bone to the neck of the condylar process of the mandible. It completely covers the condylar process of the mandible and is intimately related to the articular capsule.
The sphenomandibular ligament extends from the lingula of the mandibular foramen to the spine of the sphenoid. This ligament is tense when the TMJ is in an intermediate position (slightly opened).
The retro-articular tissue (retrodiscal pad/zone) is located between the posterior portion of the articular disc and the articular capsule, containing neurovasculature. This area is often a site of edema in temporomandibular disorder (TMD). Typically, pain associated with TMD typically sources from the retro-articular tissue.
4. Describe the attachments, innervation, and actions of the muscles of mastication.
5. What are temporomandibular disorders (TMD)? What occurs with dislocation of the jaw (subluxation of the temporomandibular joint)?
Temporomandibular disorder (TMD) is a general term for a pathology related to the temporomandibular joint, including muscles of mastication, ligaments, bone, articular disc, and neurovasculature. Possible causes of TMD are disease, arthritis, occlusal changes, and muscle spasms, among others. Crepitus (clicking) is a common complaint associated with TMD.
Dislocations of the jaw (subluxation of the TMJ) typically occurs in an anterior direction - the mandibular condyle moves anterior to or onto the articular tubercle of the temporal bone. Dislocations can occur in traumatic blows to the side of the jaw or due to weakening of protective structures of the TMJ, such as ligaments, articular capsule, etc. In these cases, simple actions, such as yawning or opening the mouth wide while eating, can cause dislocation.
6. What are the main branches of V3 (mandibular n.) when considering mastication, and what do they innervate? Which branches are affected by an inferior alveolar nerve block?
The mandibular n. (V3) is a division of the trigeminal n. (CN V) and exits the cranial cavity through the foramen ovale of the sphenoid bone into the infratemporal fossa. It consists of a main trunk, an anterior division, and a larger posterior division. V3 serves several important functions, chiefly: efferent to muscles of mastication and afferent to mandible, mandibular teeth & gingiva, and skin superficial to a portion of the temporal region. V3 is one of the deepest structures in the infratemporal fossa. Branches of V3 will often accompany similarly named branches of the maxillary a., and may be located concurrently.
Trunk of mandibular n. (V3):
- Nerve to medial pterygoid - efferent innervation to medial pterygoid m.
Anterior division of mandibular n. (V3):
- Masseteric n. - efferent innervation to masseter m.
- Deep temporal n. - efferent innervation to temporalis m,; variable number of branches
- Nerve to lateral pterygoid - efferent innervation to lateral pterygoid m.; typically a branch of the buccal n.
- Buccal n. (long buccal n.) - afferent innervation to skin overlying buccinator and buccal mucous membrane; can be located passing between the two heads of the lateral pterygoid m.
Posterior division of mandibular n. (V3):
- Auriculotemporal n. - afferent innervation to TMJ, to a portion of the skin of external acoustic meatus, to the skin of the anterior auricle of the ear, and to the skin of the superior V3 dermatome (anterior and anterosuperior to the ear)
- Postganglionic parasympathetic fibers from the otic ganglion travel with the auriculotemporal n. to the parotid gland
- Lingual n. - afferent innervation to presulcal part (anterior 2/3rds, excepting vallate papillae) of the tongue and the mucosa of the floor of the oral cavity
- Joined by the chorda tympani in the infratemporal fossa
- Inferior alveolar n. - afferent innervation via branches to all mandibular teeth and gingiva
- Gives off nerve to mylohyoid; enters the mandibular foramen
- Forms inferior dental plexus and 2 terminal branches: incisive br. and mental n.
Anesthetic may be injected around the vicinity of the mandibular foramen in a procedure known as an inferior alveolar nerve block. When correctly performed, all mandibular teeth on the ipsilateral side of the mandible are anesthetized, in addition to skin of lower lip and chin, gingivae, and mucosa associated with the lower lip and mandibular teeth (afferent innervation from branches of inferior alveolar n.). Issues may arise if injection of anesthetic occurs in structures adjacent to the mandibular foramen, such as the medial pterygoid m. or parotid gland.
7. What key autonomic ganglia and pathways are associated with the mandibular n. (V3)?
The otic ganglion is closely associated with the trunk of the mandibular n. (V3). The otic ganglion is a parasympathetic ganglion associated in location with the deep portion of the mandibular n. (V3) trunk, just inferior to the foramen ovale. The otic ganglion receives preganglionic parasympathetic fibers from the lesser petrosal n. (glossopharyngeal n. → tympanic n. → lesser petrosal n.). Postganglionic fibers are transmitted to the parotid gland via the auriculotemporal n. (of V3).
The chorda tympani is a branch of the facial n. (CN VII) that enters the infratemporal fossa via the petrotympanic fissure. Its fibers join the lingual n. (a branch of V3) en route to the submandibular ganglion. Postganglionic fibers from the submandibular ganglion innervate the submandibular and sublingual glands. Chorda tympani also conducts special sensation (taste) from the presulcal part (anterior 2/3rds) of the tongue.
8. What is the pterygoid venous plexus?
The pterygoid plexus of veins is located around the muscles of mastication and surrounding the 2nd part of the maxillary a. (pterygoid part). These veins are typically large in caliber and obvious during dissection.
This plexus has numerous tributaries, including alveolar brs., buccal, deep temporal, greater palatine, masseteric, middle meningeal, and sphenopalatine.
The pterygoid plexus of veins communicates with three adjacent regions through common venous routes for spread of infectious materials, including:
- Cavernous sinus
- Via small veins that pass through the foramen lacerum and ovale, as well as emissary foramina
- Face through the facial v.
- Via the deep facial v.
- Via the inferior ophthalmic v.
The maxillary v. is formed by the coalescence of veins from the pterygoid plexus of veins, accompanies the maxillary a., first part (mandibular part), and joins with the superficial temporal v. to form the retromandibular v.
9. What are the major branches of the maxillary a. that supply the muscles of mastication, the meninges, and TMJ?
Arteries that supply muscles of mastication
- Deep temporal aa. - supply to temporalis m.; anterior and posterior branches
- Masseteric a. - supply to masseter m.; may be additional supply to TMJ
- Pterygoid aa. - supply to lateral & medial pterygoid mm.
Arteries that supply meninges
- Middle meningeal a. - supply to bones of cranial vault and meninges
- Typically the first branch of the maxillary a.
- Passes between auriculotemporal n. fibers
- Enters skull via foramen spinosum
- Accessory meningeal a. (when present) - small supply to meninges; more notable in supply to pterygoid mm. and tensor veli palatini m.
- Enters skull via foramen ovale
Arteries that supply TMJ
- Maxillary a.
- Many branches of the maxillary a. will supply TMJ medially
- Superficial temporal a.
- Branches of superficial temporal a. will supply TMJ laterally
The maxillary a. and superficial temporal a. are the terminal branches of the external carotid a. (ECA).