Histologically, slightly elevated levels of functional loading lead to thickening of the cartilage on the joint surfaces (Muir 1977. Radin et al. 1978). A further increase in loading interferes with fluid exchange and disrupts the supply of nutrients (Gradishar and Porterfield 1989, Haskin et al. 1995), thereby causing increased tissue degeneration (Ateshian and Wang 1995). Short term loading (less than 2 minutes) of the articular cartilage lowers the coefficient of friction, whereas a load applied for 45 minutes causes a five-fold increase in friction! Cyclic short-term loads allow a high water content in the cartilage and are accompanied by reduced friction (Nickel and McLachlan 1994a). Neither occlusal attrition nor the thickness of the cortical layer of bone as seen in a radiograph provides any reliable indication of the current thickness of the fibrocartilaginous joint surfaces (Pullinger et al. 1990). Contours of the bone seen on the radiograph do not correspond to the actual contours of the joint surfaces!
A noninvasive determination of the stages of regressive adaptation of the joint surfaces can only be made clinically, not by imaging procedures. For this we use the so-called dynamic compression and dynamic medial and lateral translation (sometimes with compression). The fundamentals and clinical procedures are described on the following pages.
163 Examination techniques and their usefulness in differentiating between injuries/lesions of the joint surfaces
Active movements, dynamic compressions, and medial and lateral dynamic translations are all used to serve as nonmanipulated references for specific testing of the joint surfaces. Through the findings from the dynamic compression test it is possible to conclude whether there is an adapted joint surface, osteoarthrosis, osteoarthritis, or capsulitis of the bilaminar zone in the presence of a nonreducing disk displacement (pp. 70–74). Use of the dynamic translation test permits further determination of whether a regressive adaptation and its associated loading vector lie more medially or laterally. This knowledge is essential later during clinical testing of influences to determine whether or not there is a causal relation with occlusal disturbances.
Frequently temporomandibular joints with obvious radiographic changes in the bone show only insignificant clinical symptoms or none at all (Mejersjö and Hollender 1984). Because of this the purpose of a specific functional joint-surface test is only to determine whether or not the joint surfaces are adapted or not. Diagnostically and therapeutically, it is of minor importance how the structures are represented by imaging procedures.
There is a close correlation between regressive adaptations of the functional joint surfaces and crepitus (Boering 1994, Hansson and Nilner 1975, Bates 1993, Pereira et a!. 1994). Controlled studies indicate that crepitus is a reliable clinical sign of osteoarthritis (Holmlund and Axelsson 1996). Within a selected group of temporomandibular joint patients, 3-24% were found to exhibit rubbing sounds (Bates et al. 1994, Zarb and Carlsson 1994).
Sometimes the degeneratively altered joint surfaces are also painful. Even though in the 20th embryonic week the disk is supplied with numerous nerve endings, no innervating structures remain to be seen after birth (Ramieri et al. 1996). Therefore the disk can be excluded as a source of pain. As long as the temporal and condylar joint surfaces are still covered with cartilage, they too are unable to give rise to pain. It is only when subchondral bone is exposed that the nociceptors transmit corresponding pain sensations (Quinn 1989, Kamminishi and Davis 1989).
Conventional clinical examination methods can diagnose initial osteoarthrotic changes only with a low degree of specificity and sensitivity. Therefore manual functional analysis employs not only active protruding and opening movements, but also dynamic compressions and translations, which load the corresponding joint surfaces more heavily during movements. In this way even compensated joint surface changes can be discovered early by provoking a painful response.
Crepitus is a primary examination parameter. The examiner must determine whether at any given moment during manipulation the rubbing sound occurs more loudly and distinctly than during nonmanipulated active movements (protrusion and opening). In addition it should be determined if there is any pain produced by compression. Whenever there are painful alterations in the joint surfaces, the endfeel of inferior traction and anterior translation must be tested before any conclusions are formed about the effectiveness of a possible unloading of the joint. The results of the tests of the joint surfaces allow the following deductions to be made:
• Adaptation: no crepitus and/or no pain during active movements and dynamic tests
• Compensation: crepitus and/or pain only during the dynamic tests
• Decompensation: crepitus and/or pain during active movements and during dynamic tests.
Manifestations of Joint Surface Changes
Unlike other synovial joints the temporomandibular joint has joint surfaces of fibrous cartilage, a highly differentiated connective tissue with no blood vessels or nerve endings. Compared with hyaline cartilage, fibrous cartilage is less easily deformed because of its higher fiber content (Gay and Miller 1978). The mucopolysaccharide content of the synovial fluid is responsible for lubrication of the joint surfaces (Smith 1982). This is why there are normally no noises or pain during active movements and during dynamically influenced movements.
• An increase of crepitus during dynamic compression in the absence of pain indicates osteoarthrosis, a regressively adapted, noninflammatory stage of joint surface damage.
• If crepitus and pain are both provoked there is osteoarthritis, an inflammatory stage of joint surface damage.
The examiner must make certain that the provoked pain is intensified by application of compression and not by a nonspecific joint loading due to increasing jaw opening.
164 A healthy joint
Left: Anatomical preparation of a right temporomandibular joint with the jaws closed. The rounded condyle is covered by a uniform layer of fibrous cartilage. There is no fibrous cartilage on the superior and posterior portions of the fossa. Only the posterior slope and the crest of the articular eminence exhibit functional joint surfaces.
Right: During jaw opening there are no grating sounds or pain.
165 Nonreducing anterior disk displacement
If the disk is displaced anteriorly when the mouth is closed, the top of the condyle will be covered by part of the overstretched bilaminar zone, which at this time acts as the articulating surface. The compression test will intensify pain in this area.
Right: As the jaws are opened, the condyle pushes the disk ahead of itself, thus increasing pain and limitation of movement.
166 Adaptation of the bilaminar zone after anterior disk displacement
Left: Dynamic compressions are associated with pain and limitation of movement only if the bi laminar zone has not adapted as seen in this MRI (light gray, arrows).
Right: This MRI shows an adapted (fibrosed)