tjm-causes-1The jaw joint (temporomandibular joint) begins to breakdown when the stresses overcome the tissue ability to function and repair. Usually this involves some type of trauma. This can be micro-trauma or macro-trauma. Micro-trauma by itself does not cause damage, but if repeated enough over time does cause tissue damage. Drumming your fingers on a table does not cause damage, but if done for hours without stopping it can cause an injury. Given enough time, the structures cannot withstand the forces generated and the tissues breakdown and become unable to function adequately. Macro-trauma is usually caused by one incident of such magnitude that it can damage the tissue beyond normal repair capabilities. Smashing your finger with a hammer could be an example of macro-trauma.

In the diagram at the right we observe some of the muscles that help move and support the jaw movements. An injury to the joint would cause these muscles to compensate and this increased function (muscle splinting) can cause knots or a Charlie horse, now termed a trigger point. Trigger points are painful at the source and can also refer pain to other structures and are the primary cause for most patients head and neck pain.

Loading: The human temporomandibular joint is a unique joint. The surfaces that contact are covered with a fibrocartilagenous layer and a fibrocartilagenous disc sits between the ball (condyle) and the socket (glenoid fossa / eminence). The fibrocartilagenous structures are the functional surfaces and should remain in contact during opening and closing movements. Any force that would disrupt the normal loading of the joint can be a pathologic or damaging factor.

tjm-causes-2The best way to understand how this joint works is to observe the form and function of a normal joint. A side view through a normal TMJ is presented in the same manner as the previous illustration with the nose to the right and illustrates the internal joint components. The arrows are positioned over the relative avascular central portion of the disc, the part of the structure best able to withstand normal compressive and functional movements. The fibrocartilagenous fibers in this region are oriented front to back, do they can withstand function as the joint moves forward and backward during opening and closing movements. The fibrocartilagenous fibers in the back of the joint (behind the left arrow) are oriented inside to outside, not front to back, thus this back (posterior) region would not function well if the ball of the joint was working in a front to back motion over these fibers.

The ball of the joint is covered with fibrocartilage from the top to the front aspect and the socket is also covered with fibrocartilage from the top to the front. Forces within the two arrows would enable the fibrocartilage surface of the ball to function against the relatively avascular central region of the disc against the fibrocartilage of the socket. This would be an anatomic stable position. The back of the disc tissue is thicker than the central region of the disc. This region has a higher capacity to “spring back” into shape due to the tissue alignment and the make-up of the tissues and is not meant to be compressed. This region serves as a biomechanical lock to help keep the disc in place as forces are generated by the ball against the front aspect of the disc against the front wall of the socket. The top of the arrows point to the glenoid fossa and eminence (or socket) for the jaw joint. The fibrocartilagenous covering of the fossa starts about at the left arrow and becomes thicker the further we progress to the right. The area between the two arrows establishes a functional envelope of motion for the joint. As long as the force from the condyle is positioned within this zone against the relative avascular central portion of the disc against the eminence of the fossa, the forces are being applied to the region best suited for short term compressive loading. This region remains stable if the forces applied do not exceed the tissue capacity to adapt. The area undergoes degenerative alterations if the loading is greater in magnitude or duration than the tissue is able to manage.

tjm-causes-3The white arrow in the following representation demonstrates the physiological force vector that would support this functional envelope of motion. As long as this force vector is maintained within the functional envelope of motions (adaptive region) of the joint structures, and the load is not excessive, the structural alignment is optimally situated.

This force keeps the fibrocartilagenous condylar surface against the avascular central portion of the disc against the fibrocartilagenous surface of the fossa/eminence. The joint goes into the seated position in the socket when the teeth are brought together by the muscles of mastication. The bite determines the most seated joint position with the condyle against the disc against the fossa/eminence. The seated position in the following video occurs when the teeth are brought completely together.

tjm-causes-4When viewed from above, the top of the condyle (or ball of the joint) is observed at in the center of the slide to the left. This disc is centered over the ball (condyle), and the innervations can be observed. The form of these structures and the functional adaptations of these structures enable them to withstand normal loading and allow tissue rebound and healing. The condyle doesn’t always exist in the “ideal functional position”, but often is found to become slight displaced when the teeth are brought together. The condylar displacement may then exert forces on various regions of the disc, ligaments and fossa assembly that may not be best designed to withstand these forces and pathologic alterations may occur. The area behind the condyle is composed of elastic connective tissue, fat cells, arterioles, and veins and nerve tissue. This tissue is not well adapted to withstanding compressive forces. There is also a junction of two bones in the back of the joint, which is not a structure that would normally withstand compressive forces from an anatomic view.

As the ball of the joint moves up and back (white arrow), it shifts the compressive forces from the central region of the fibrocartilagenous disc where the fibers run front to back to the region where the elastic connective tissue attach to the disc, (the retrodiscal tissue or posterior lamina) where the fibers run side to side. This back (posterior) region in a healthy joint is larger than the central portion of the disc, thus serving as a mechanical lock to help hold the disc in place, but does not appear to be designed to be regularly loaded or compressed by the ball of the joint. As this posterior lamina tissue is compressed and loses its form, the disc can begin to be displaced either to the anterior (forward) [yellow arrow], or to medial (inside). Often this is accompanied by an awareness of a deviation in opening and/or noises such as clicking or popping and occasional pain in this region.

tjm-causes-7Cause = ball movement: The forces directed by the ball of the joint cause changes in the disc structures. First there appears to be a compression of the back of the disc, distorting the biomechanical lock. This would also serve to compress the synovial membrane and this may affect the joint lubrication. Further compression of these delicate tissues as the ball moves further up and back in the socket would serve to move the disc forward and further compress the soft tissues behind the joint that are not able to withstand continuous compressive forces. The nerves in the back of the joint and/or the nerves on the outside would now be positioned in a manner where they may be compressed during chewing. This is a potential cause of pain and inflammation as nerves do not function well when compressed. The white arrow shows one of the nerve fibers positioned between the ball and socket when the disc becomes displaced forward and to the inside. A nerve that is compressed may cause pain at the source, or referred pain as the brain may not be able to interpret the precise cause of the injury.

tjm-causes-8The first of the three diagrams demonstrates what actually occurs in real life.

The condyle is now displaced posterior behind the disc and is now functioning on the posterior elastic ligament tissue. The posterior lamina is situated in front of the condyle. As the patient opens, the condyle may ride up over the posterior lamina, causing an opening click. Often a closing click can be heard as the condyle slips off behind the disc. When the joint is dislocated as viewed here, the pressures are now no longer applied to the fibrocartilagenous structures, but rather to the remaining structures (fat cells, elastic connective tissue, arteries, veins and nerves) in the region of the petrotympanic fissure. This is a non-physiologic relationship that usually becomes more pathologic with time.

tjm-causes-9The primary muscles of mastication apply forces that should maintain the physiologic vector to keep the condyle against the disc against the eminence of the fossa. The arrow over the joint demonstrates the normal physiologic force needed to keep the joint structures within the functional envelope of motion. The arrow over the muscles demonstrates the force vector applied by all the primary elevator muscles of mastication. These forces are in effect as long as the teeth are not occluding (touching). When the teeth occlude, the proprioception (feedback) from the teeth will determine the final movements of the lower jaw and the forces upon the muscles and joints. When the joint is dislocated the muscles can no longer function in a normal manner as the loading is altered, the nerve response from the joint is modified and inflammation may occur.

tjm-causes-11This picture depicts an incidence of malocclusion where the tooth contact will cause the lower jaw to be shifted in the direction of the arrow. When this occurs, normal muscle action is not possible, as the mandible (lower jaw) must be shifted to the left, thus changing the forces that exist within the joint structures. If this is allowed to occur over a long enough period of time, the joint structures may be forced to adapt, causing changes in the shape of the components. The muscles are not able to function normally and may become painful. It is possible for this type of situation to force enough changes for a TMJ problem to become evident.

tjm-causes-10When the muscles in the front of the face are caused to function in an abnormal manner (dislocated joint) the muscles in the back of the neck are also caused to compensate. One was to understand this is to place your elbow on the top of a table and your chin in your hand. The lower jaw is therefore no longer mobile. It is possible to still open by flexing your neck up and down, but this causes the very short muscles in the back of the neck to compensate, which they usually don’t do for long without some degree of discomfort or pain. This can help explain some of the neck, shoulder and upper back pain that patient’s experience when their joint structures are not able to function properly.

Success in treating these type of maladies requires a thorough examination of all of the structures of the upper quadrant, which will necessitate corrective measures to enable these interrelated structures to function in a more normal manner.

Sometimes the joint can become so compromised that is remains dislocated. The means the ball now seats in the back of the joint and the structures that used to be in the back are now moved up over the ball, where they become compressed as they are now functioning as a disc. Scar tissue will now begin to form in these tissues as they attempt to compensate for the abnormal forces placed upon them. Different people will form different types of scar tissue. Sometimes this scar tissue may be fairly resilient, but sometimes it is prone to inflammation and breakdown. Every case is different.

The earlier the dysfunction is recognized and intercepted usually corresponds with an increased ability to manage the dysfunctions.1`