The temporomandibular joint is a synovial joint between the head (condyle) of the mandible and the mandibular fossa on the undersurface of the squamous part of the temporal bone. The mandible is a single bone with a horizontal horseshoe-shaped body, which is continuous at its posterior ends with a pair of vertical rami, each ramus being surmounted by a head or condyle. The cranium, with which the mandible articulates, is also mechanically a single component, with a mandibular fossa on each side. This complex is in effect one functioning joint, as movement cannot take place at one temporomandibular joint without a concomitant movement occurring at the joint on the opposite side. The temporomandibular joints are thus the bilateral components of a craniomandibular articulation.
The joint is separated into upper and lower cavities by a fibrocartilaginous disc within it. Both bone surfaces are covered with a layer of fibrocartilage identical with that of the disc. Though termed fibrocartilage, the articular cartilage and disc consist mainly of collagen fibres with few cartilage cells. There is no hyaline cartilage in this joint, so it is an atypical synovial joint.
The capsule is attached high up on the neck of the mandible anteriorly, near the articular margin of the head, but lower down the neck posteriorly. Above, it is attached anteriorly just in front of the articular eminence of the temporal bone (Fig. 6.35), posteriorly to the squamotympanic fissure, and medially and laterally to the margins of the mandibular fossa. It is lax above the disc, but taut below. The synovial membrane lines the inside of the capsule and the intracapsular posterior aspect of the neck of the mandible.
The articular disc is attached around its periphery to the inside of the capsule and to the medial and lateral poles of the head of the mandible. Its upper surface is anteroposteriorly concavoconvex in the sagittal plane to fit the articular eminence and fossa; the inferior surface is concave in adaptation to the condyle (Fig. 6.64). Anteriorly the disc is continuous through its capsular attachment with the tendon of lateral pterygoid. Posteriorly the disc divides into two laminae. The upper fibroelastic lamina is attached to the margin of the mandibular fossa; the lower non-elastic fibrous lamina is attached to the neck of the mandible. Between the two laminae is a pad of loosely textured tissue containing many blood vessels and sensory nerve endings. The disc has two transverse thickened bands, the posterior being thickest; between these bands it is thinnest and relatively avascular.
|
|
|
Figure 6.64 Sagittal section of the temporomandibular joint. |
The lateral temporomandibular ligament is a stout band of fibrous tissue passing obliquely down and back from the articular tubercle of the zygomatic arch (see p. 505) to the lateral surface and posterior border of the neck of the mandible (Fig. 6.65). On its deep aspect a narrow band runs transversely from the articular tubercle to the lateral pole of the mandibular head.
|
|
|
Figure 6.65 Capsule and ligaments of the left temporomandibular joint: lateral aspect. |
The sphenomandibular ligament, running between the spine of the sphenoid and the lingula of the mandible (Fig. 6.66), is an accessory ligament of the joint.
|
|
|
Figure 6.66 Capsule and ligaments of the left temporomandibular joint: medial aspect. |
The nerve supply of the joint is from the auriculotemporal nerve and the nerve to masseter.
Stability
The joint is much more stable with the teeth in occlusion than when the jaw is open.
In occlusion the teeth themselves stabilize the mandible on the maxilla and no strain is thrown on the joint when an upward blow is received on the mandible. In the occluded position apart from the stabilizing effect of the teeth, forward movement of the condyle is discouraged by the prominence of the articular eminence and by contraction of the posterior fibres of temporalis, while backward movement is prevented by the fibres of the lateral ligament and by contraction of the lateral pterygoid.
In the open position the joint is less stable as the condyle lies forward on the slope of the articular eminence. Forward dislocation is the most common form of displacement. Forward dislocation is normally opposed by the articular eminence, by the tension of the lateral ligament and by contraction of the masseter, temporalis and medial pterygoid muscles. But when the condyle is dislocated forwards, reduction is prevented by spasm of these same muscles, which hold the dislocated jaw open with the condyle in front of the eminence. The spasm must be overcome (with or without an anaesthetic) by the operator's thumbs pressing downwards on the molar teeth or alveoli, before the condyle can be guided back into the fossa. Anterior dislocation readily occurs in the edentulous. In addition to the loss of stability resulting from the lack of proper occlusion in the elderly, upward tilting of the edentulous mandibular body lowers the mandibular head and neck and elongates the lateral ligament.
Movements
There are three sets of mandibular movements at the temporomandibular joint. These are depression and elevation (opening and closing the jaws), side-to-side (grinding) movements, protraction and retraction (protrusion and retrusion). The group of muscles commonly classified as the muscles of mastication—temporalis, masseter and medial and lateral pterygoids—play major roles in these movements; others taking part can be called accessory muscles of mastication (Fig. 6.67).
|
|
|
Figure 6.67 Muscles producing movements of the temporomandibular joint. The direction of their actions is indicated by the arrows. |
When the mouth is opened, the mandibular head rotates around a horizontal axis in a hinge-like movement that occurs in the lower compartment of the temporo-mandibular joint, between the head and the inferior aspect of the disc, while a gliding movement occurs in the upper compartment between the disc and the mandibular fossa of the temporal bone. In this sequence of events, the mandible is depressed by the digastric, mylohyoid and geniohyoid muscles, while the infrahyoid muscles act to stabilize the hyoid bone. Forward movement of the mandibular head on to the articular eminence of the temporal bone is effected by the lateral pterygoid muscle, principally its inferior head, while the superior head draws the disc forwards.
Elevation of the mandible (closing the jaw) is produced by the masseters, medial pterygoids and temporalis muscles.
Side-to-side movements are the result of medial and lateral pterygoid activity on one side, alternating with similar activity on the other side. Simultaneous contraction of lateral and medial pterygoid muscles of one side rotates the mandible in the horizontal plane around a vertical axis passing a little behind the mandibular head on the opposite side, which moves slightly laterally, while the head on the side of the contracting muscles is drawn forwards on to the articular eminence.
During protraction (as when jutting the chin forwards), all four pterygoid muscles contract, such that the head and disc are drawn forwards, without depression or elevation of the mandibular body. The normal position is restored by passive recoil of stretched joint structures, aided by contraction of the posterior horizontal fibres of temporalis and the posterior deep fibres of masseter.
