Chapter 2. Temporomandibular (craniomandibular) joint
Mandibular fossa 10
The temporomandibular joint (TMJ) is the synovial joint whereby the condyle of the mandible attaches to the mandibular fossa at the base of the skull. The joint cavity is subdivided into upper and lower compartments by an intra-articular disc. Movement at this joint is essential for mastication and speech. Movement at one TMJ will have a reaction in the TMJ of the other side.
• know the structure of the TMJ at both the gross and the microscopic levels
• appreciate the differences between the joint of a child and that of an adult
• be able to compare and contrast the TMJ with more typical synovial joints
• appreciate the clinical importance of the joint, which can give rise to pain and restriction of jaw movements.
The temporomandibular joint (TMJ) is the synovial articulation between the condylar process of the mandible and the mandibular (glenoid) fossa of the temporal bone. The joint space is divided into two joint cavities (upper and lower) by an intra-articular disc. The upper joint space allows for gliding movements down a bony prominence (the articular eminence) immediately anterior to the mandibular fossa, while the lower joint space allows for hinge movements. With opening and closing of the jaws, a combination of rotation and translation occurs. During wide opening, about 75% of the movement can be explained by rotation in the lower compartment. Movement of the joint is influenced by the teeth. Movements of the joint can be considered as symmetrical (opening, closing, protrusion, retrusion) or asymmetrical (lateral).
The mandibular fossa is an oval depression in the temporal bone, lying immediately anterior to the external acoustic meatus. Its mediolateral dimension is greater than its anteroposterior one in order to accommodate the mandibular condyle, and it is wider laterally than medially. The bone of the central part of the mandibular fossa is thin. The fossa is bounded anteriorly by the articular eminence, laterally by the zygomatic process, and posteriorly by the tympanic plate.
When viewed from above, the mandibular condyle is roughly ovoid in outline, the anteroposterior dimension (approximately 1 cm) being about half the mediolateral dimension. The medial aspect is wider than the lateral. The long axis of the condyle is not, however, at right angles to the ramus, but angled so that the lateral pole of the condyle lies slightly anterior to the medial pole. The convex anterior and superior surfaces of the head of the condyle are the articular surfaces. The broad articular head of the condyle joins the ramus through a thin bony projection termed the neck of the condyle. A small depression, the pterygoid fovea, marks part of the attachment of the inferior head of the lateral pterygoid muscle.
The capsule of the TMJ is thin and is attached to the margins of the mandibular fossa above and to the neck of the condyle of the mandible below. Posteriorly, the capsule is associated with the thick, vascular but loosely arranged connective tissue of the bilaminar zone of the intra-articular disc (the retrodiscal pad). Internally, the capsule is attached to the intra-articular disc and is lined by synovial membrane. The collagen fibres of the capsule run predominantly in a vertical direction. The capsule is richly innervated.
The synovial membrane lines the inner surface of the fibrous capsule and the margins of the intra-articular disc, but does not cover the articular surfaces of the joint. The synovial membrane secretes the synovial fluid that occupies the joint cavities. Important components of the synovial fluid are the proteoglycans, which aid lubrication of the joint. At rest, the hydrostatic pressure of the synovial fluid has been reported as being subatmospheric, but this is greatly elevated during mastication.
The main ligament strengthening the joint capsule is the temporomandibular (lateral) ligament. It takes origin from the lateral surface of the articular eminence of the temporal bone (at the site of a small bony protrusion, the articular tubercle). The temporomandibular ligament inserts on to the posterior surface of the condyle. This ligament provides the main means of support for the joint, restricting backward and inferior movements of the mandible and resisting dislocation during forward movements. The temporomandibular ligament is reinforced by a horizontal band of fibres running from the articular tubercle to the lateral surface of the condyle. These horizontal fibres restrict posterior movement of the condyle. There is little evidence of any comparable ligament on the medial aspect of the joint capsule, so medial displacement is prevented by the temporomandibular ligament of the opposite side.
The accessory ligaments of the TMJ traditionally described are the stylomandibular ligament, the sphenomandibular ligament and the pterygomandibular raphe. However, only the sphenomandibular ligament is likely to have any significant influence upon mandibular movements.
• The sphenomandibular ligament (a remnant of the perichondrium of Meckel’s cartilage) extends from the spine of the sphenoid bone to the lingula near the mandibular foramen.
• The stylomandibular ligament is a reinforced lamina of the deep cervical fascia as it passes medially to the parotid salivary gland. It extends from the tip of the styloid process and the stylohyoid ligament to the angle of the mandible.
• The pterygomandibular raphe extends from the pterygoid hamulus to the posterior end of the mylohyoid line in the retromolar region of the mandible.
The intra-articular disc (meniscus) is a dense, fibrous structure moulded to the bony joint surfaces above and below. Blood vessels are evident only at the periphery of the intra-articular disc, the bulk of it being avascular. Above, the disc covers the slope of the articular eminence in front while below it covers the condyle. When viewed in sagittal section, the upper surface of the disc is concavo-convex from front to back and the lower surface is concave. The disc is of variable thickness, being thinnest in its central part. In centric occlusal position, the articular surface of the condyle lies against the thinner, intermediate part of the intra-articular disc and faces the posterior slope of the articular eminence.
The margin of the intra-articular disc merges peripherally with the joint capsule. Posteriorly, it is attached to the capsule by a bilaminar zone (retrodiscal tissue/pad). The superior lamina is loose and possesses numerous vascular elements and elastin fibres. The inferior lamina is relatively avascular and less extensible, and is attached to the posterior margin of the condyle.
The nerves providing the rich innervation for the joint are the auriculotemporal, masseteric and deep temporal nerves of the mandibular division of the trigeminal nerve. The largest is the auriculotemporal nerve, supplying the medial, lateral and posterior parts of the joint. The remaining two nerves supply the anterior parts of the joint. Although free nerve endings associated with nociception are found everywhere in the joint capsule, of particular functional importance are more complex endings (i.e. Ruffini-like endings) associated with proprioception and important in the control of mastication. Joint receptors, along with Golgi tendon organs in the tendons and muscle spindles in the muscles, are called proprioceptors because of their role in position sense. When the joint capsule is compressed or stretched during movement of the joint, Ruffini-like (slowly adapting) mechanoreceptor endings will signal not only the position of the joint, but also the direction and velocity of the movement; they will not, however, be able to signal the force developed between the teeth.
The blood supply to the joint is supplied mainly from the superficial temporal and maxillary arteries.
Two unusual histological features of the TMJ are that:
• the articular surfaces are not lined by hyaline cartilage like the majority of synovial joints, but by fibrous tissue (reflecting the intramembranous development of the bones of the joint)
• a secondary cartilage is present in the head of the mandibular condyle until adolescence.
Four distinct layers have been described covering the bony head of the adult condyle:
• The most superficial layer forms the articular surface and is composed of fibrous tissue (mainly collagen, but with some elastin fibres). Fibroblasts/fibrocytes within the surface layer are sparsely distributed.
• Beneath the articular surface layer is a more cellular zone (cell-rich zone).
• Beneath the cell-rich zone is another fibrous layer in which a number of the cells are rounded, and have an appearance reminiscent of cartilage-like cells; this layer is generally referred to as the fibrocartilaginous layer.
• Immediately covering the bone is a thin zone of calcified cartilage, distinguished from the underlying bone of the mandibular condyle by its different staining properties. This calcified cartilage is a remnant of the secondary condylar cartilage.
The articular surface covering the mandibular fossa of the temporal bone is similar to that of the condyle. Although generally thinner, it thickens as it passes over the articular eminence.
The intra-articular disc contains cells embedded in a matrix composed of fibres and ground substance. The majority of fibres consist of type I collagen, although traces of other types of collagen have been recorded. There is also a small quantity of elastin fibres present in the disc.