Joint Capsule and Articular Ligaments
The articular capsule (capsula articularis) forms an airtight seal around the joint so that slight negative pressure can be created inside the joint cavi-ty.The negative pressure in the joint cavity means that the bony joint components are firmly pressed together. The joint capsule is attached around the mandibular fossa and is loose and wide enough to ensure that the movements of the joint cannot be impeded.
This tendinous coating of connective tissue is made up of a dense outer tissue layer and an inner epithelial layer, known as the synovial layer. The synovial fluid is produced here, which consists of protein, salts, tissue fragments, and water. In the event of a ruptured capsule, the synovial fluid would be an ideal nutrient for bacteria. The thick posterior wall of the capsule arises at the petrotympanic fissure (fissura petrotympani-ca), while the anterior wall is attached to the horizontal surface of the articular tubercle.
The fibers of the joint capsule converge at the neck of the mandible and thus surround the condyle at the rear, roughly 5 mm below the edge of the articular surface; anteriorly the fibers surround the condyle directly at the edge of the joint surface because this is where the lateral pterygoid muscle attaches.The joint capsule of theTMJ is so loose that, if the joint is dislocated, rupture of the capsule will only rarely occur, if at all.
The functions of the capsule are to:
- Hold the parts of the joint together
- Create an airtight seal on the joint cavity
- Produce synovial fluid
To support the loose joint capsule, there are three articular ligaments that have to reinforce the capsule while limiting the movements of the jaw (Figs 6-36 and 6-37). These ligaments are the temporomandibular, sphenomandibular, and stylomandibular ligaments.
The temporomandibular ligament (ligamentum laterale or temporomandibulare) arises from the external surface of the zygomatic process on the temporal bone, close to the articular tubercle, and runs obliquely down and backward to the back of the neck of the mandible; a few vertical fibers are firmly attached to the capsule.
The sphenomandibular ligament (ligamentum sphenomandibulare) has its origin at the spine of the sphenoid bone and the middle edge of the mandibular fossa, from where it projects downward and forward to the inner surface of the neck of the mandible and to the lingula at the mandibular foramen.
The stylomandibular ligament (ligamentum stylomandibulare) comes from the styloid process of the temporal bone (processus styloideus) and runs around to the posterior edge of the ramus of the mandible and as far as the angle of the mandible. It has little importance in terms of joint stability, but in the event of dislocation it prevents the mouth from being closed in order to avoid greater damage.
As a result of overload or strain, the joints can sustain diseased changes of varying severity. These can be divided into sprains, dislocations, arthritis, and osteoarthritis.
A sprain (distortion) denotes overextension of the joint capsule and articular ligaments, which can result in effusion of blood in the joint and can cause the joint capsule to swell up because of increased secretion of fluid.
A dislocation (luxation) refers to the situation where the condyle comes out of the joint socket or fossa and articular ligaments are often torn.The less the condyle is held by the socket, the easier it is for dislocations to happen. In the case of the TMJ, the condyle may slide out of the mandibular fossa when the mouth is opened very wide, without the ligaments being stretched or torn. In the TMJ, if the condyle slides in front of the articular tubercles, for instance, this can cause a painful occlusal lock. The joint can be put back into place (reduced) by firmly applying backward and downward pressure on the mandibular dentition.
Arthritis is an inflammatory change to the joints in which purulent fluid is often secreted into the joint cavity, leading to painful swelling of the joint capsule. It can result in permanent damage to bone and cartilage tissue.
Osteoarthritis is a degenerative joint disease that usually involves wear of the cartilage and fibrillation of the bone, mainly as a sign of aging but also as a result of inappropriate stress on particular joints.