- Email: [email protected]
Anatomic and mechanical properties of the lateral disc attachment of the temporomandibular joint
1168
DISCUSSION
J Oral Maxiiiofac 56:l 168-1 169,
Surg 1998
Discussion Anatomic and Mechanical Properties of the lateral Disc Attachment of the Temporomandibular Joint Cordula Schmolke, MD Assistant
Professor,
Anatomical
Institute,
Bonn,
Germany
Many functionally important aspects of the human temporomandibular joint anatomy have been reported in a controversial manner even in the recent literature. This may be because of the high degree of interindividual variations in human anatomy, the different technical approaches for visualizing morphologic details in complicated threedimensional regions such as the temporomandibular joint, or the fact that most anatomic studies are performed in cadavers of relatively aged persons that may show an augmented proportion of pathologic alterations of the joint. Therefore, it has to be appreciated that a multidisciplinary group of researchers with experience concerning the investigation of the anatomy of the temporomandibular joint region with different techniques should most effectively elucidate basic structural problems regarding quantitative parameters of the articular disc. The study of Ben Amor et al is based on macroscopic dissections of 21 pairs of fresh human temporomandibular joints in which tension tests were performed with a tensioncompression screw machine that enabled measurements to be performed with biologic material under conditions standardized as far as possible. The techniques adopted by the authors of this study are appropriate for approaching the main questions put in their report. All parameters determined either directly from the preparations or during the tension tests were statistically tested with respect to sex and age. The range of ages of the dissected cadavers was relatively wide. This might explain why no significant differences with respect to age were obtained for any of the tested parameters. The question of the lateral disc attachment (LDA) and its mechanical properties is the center of the study. Until now, this has not been investigated, although it appears to be of clinical interest in cases of temporomandibular joint disorders. I would like to add some anatomic details about the relationships between the articular disc and the adjacent jaw muscles which make clear that studies of the strength of the LDA are of interest also for a general understanding of the functional role of the articular disc in the temporomandibular joint. Several studies have been published where the authors saw the disc attached to the temporal is muscle.1.3 Results elaborated in my own laboratory have shown that in 26 of 32 specimens the temporalis fascia was not attached to the bone when it passed the zygomatic arc, but remained movable up to the point where it merged with the lateral ligament.*~5 In these specimens it seems likely that the temporahs fascia transfers part of the force that occurs during temporalis muscle contraction to the lateral joint capsule. Moreover, different investigators have found the lateral side of the disc attached to the perimysial sheath of
the masseter muscle.l-* Collagenous fibers of the bilaminar zone and of the anterior and posterior band of the disc extend into the deep part of the masseter muscle, where these collagenous fibers run perpendicularly to the muscle fibers (Fig 1). It must be suspected that forces exerted by the deep part of the masseter muscle might be tranferred to the LDA. For instance, tension of the masseter muscle during wide mouth opening,6 tension caused by contraction of this muscle when closing the widely opened mouth, are likely to have an effect on the lateral part of the articular disc. The physiologic transfer of stress arising from temporalis or masseter muscle activity on the LDA should be more extensive in cases where this attachment is relatively loose than in cases where the LDA is tough. The most important finding of Ben Amor et al is that there are two morphologically different types of LDA whereby, with a degree of statistical significance, LDA type 2 (LDA2) is weaker and obviously combined with a higher degree of mobility than LDA type 1 (LDAI). The authors show that the mean strength to failure in the specimens of LDA2 is less than in specimens of LDAl, and that the points of attachment in the specimens of LDA2 are situated below those in the cases of LDAl. Therefore, when considering possible effects of the temporalis or masseter muscle on the articular disc, these should be more extensive in the case of LDA2 than in the case of LDAl. On the anteromedial side of the joint, Ben Amor et al showed that the ratio between the insertion of the lateral pterygoid muscle and the transverse anterior band length was greater in women than in men. However, a relationship of statistical significance between the type of LDA and the extent of the anteromedial insertion of the lateral pterygoid muscle was not found. I believe that the quantitative data presented in this study will be of great clinical significance when Interpreting
FIGURE
1. Lateral view of a human temporomandibular joint after removal of the lateral joint capsule to expose the upper joint compartment. The arrowheads indicate collagenous fibers extending from masseter muscle the lateral part of the articular disc (D) into the dee (MM]. The subdivisions on the bar correspond to a cr [stance of 1 mm.
1169
CORDULA SCHMOLKE various medially or laterally localized functional and structural temporomandibular joint disorders. However, it has to be taken into account that the traction axis that Ben Amor et al adjusted during their tension tests in the mechanical apparatus might not be identical with the axes of physiologic forces acting on the lateral part of the disc or on the lateral ligament during the various joint movements. Stress transferred, eg, from the masseter muscle to the lateral part of the disc, would be otiented at right angles to the traction axis chosen by Ben Amor et al in their Figure 1. As far as the structural consistency and strength of the lateral joint capsule and of the lateral ligament is concerned, I agree with the authors that there is wide interindividual variability. However, in the material we have investigated either by macroscopic dissection (32 specimens) or as serial plasticized sections (22 specimens), we found the lateral joint capsule to be several times thicker than all the other capsular elements on the anterior, medial, and posterior sides of the joint in all instances. In all specimens the anterior half of the lateral capsule was reinforced by ligamentous collagenous fibers extending from a part of the zygomatic birch in front of the joint to the condylar neck or to the dorsal margin of the mandibular ramus (Fig 2). I never noticed a complete lack of the lateral ligament even in specimens where the lateral joint capsule, including the lateral ligament, appeared transparent.
Acknowledgment The studies on which these comments by the Deutsche Forschungsgemeinschaft
are based were supported (Schm 1117/l-2).
3. Velasco JRM, Vazquez JFR, Collado JJ: The relationships between the temporomandibular joint disc and related masticatory muscles in humans. J Oral Maxillofac Surg 51:390, 1993 4. Schmolke C: The relationship between the temporomandibular joint capsule, articdar disc and jaw muscles. J Anat 184:335, 1994 5. Ritgen
References 1. Couly G, Hureau J, Vaillam menisque temporo-mandibulaire.
FIGURE 2. Macroscopic view of the lateral caspule of a human temporomandibular joint after removal of the masseter muscle. The width of the lateral ligament reinforcing the anterior half of the lateral joint cas ule is indicated by the arrows. The magnification is identical to that o P Figure 1
JM: Le complexe dynamique du Rev de Stomatol Paris 76:597,
1975
2. Dauber W: Die Nachbarschaftsbeziehungen des Discus articularis des Kiefergelenks und ihre funktionelle Bedeutung. Schweiz Monatsschr Zahmned 97:427, 1987
I: Makroskopische und plastinationshistologische Studie zur funktionellen Anatomie des menschlichen Kiefergelenks unter Beriicksichtigung der Vetinderungen bei zwei verschiedenen Mundiiffnungspositionen. Dissertation, University of Bonn, 1996 6. Schmolke C, Hugger A: The human temporomandibular joint region in different positions of the mandible. Ann Anat 1998 (in press)
DISCUSSION
J Oral Maxiiiofac 56:l 168-1 169,
Surg 1998
Discussion Anatomic and Mechanical Properties of the lateral Disc Attachment of the Temporomandibular Joint Cordula Schmolke, MD Assistant
Professor,
Anatomical
Institute,
Bonn,
Germany
Many functionally important aspects of the human temporomandibular joint anatomy have been reported in a controversial manner even in the recent literature. This may be because of the high degree of interindividual variations in human anatomy, the different technical approaches for visualizing morphologic details in complicated threedimensional regions such as the temporomandibular joint, or the fact that most anatomic studies are performed in cadavers of relatively aged persons that may show an augmented proportion of pathologic alterations of the joint. Therefore, it has to be appreciated that a multidisciplinary group of researchers with experience concerning the investigation of the anatomy of the temporomandibular joint region with different techniques should most effectively elucidate basic structural problems regarding quantitative parameters of the articular disc. The study of Ben Amor et al is based on macroscopic dissections of 21 pairs of fresh human temporomandibular joints in which tension tests were performed with a tensioncompression screw machine that enabled measurements to be performed with biologic material under conditions standardized as far as possible. The techniques adopted by the authors of this study are appropriate for approaching the main questions put in their report. All parameters determined either directly from the preparations or during the tension tests were statistically tested with respect to sex and age. The range of ages of the dissected cadavers was relatively wide. This might explain why no significant differences with respect to age were obtained for any of the tested parameters. The question of the lateral disc attachment (LDA) and its mechanical properties is the center of the study. Until now, this has not been investigated, although it appears to be of clinical interest in cases of temporomandibular joint disorders. I would like to add some anatomic details about the relationships between the articular disc and the adjacent jaw muscles which make clear that studies of the strength of the LDA are of interest also for a general understanding of the functional role of the articular disc in the temporomandibular joint. Several studies have been published where the authors saw the disc attached to the temporal is muscle.1.3 Results elaborated in my own laboratory have shown that in 26 of 32 specimens the temporalis fascia was not attached to the bone when it passed the zygomatic arc, but remained movable up to the point where it merged with the lateral ligament.*~5 In these specimens it seems likely that the temporahs fascia transfers part of the force that occurs during temporalis muscle contraction to the lateral joint capsule. Moreover, different investigators have found the lateral side of the disc attached to the perimysial sheath of
the masseter muscle.l-* Collagenous fibers of the bilaminar zone and of the anterior and posterior band of the disc extend into the deep part of the masseter muscle, where these collagenous fibers run perpendicularly to the muscle fibers (Fig 1). It must be suspected that forces exerted by the deep part of the masseter muscle might be tranferred to the LDA. For instance, tension of the masseter muscle during wide mouth opening,6 tension caused by contraction of this muscle when closing the widely opened mouth, are likely to have an effect on the lateral part of the articular disc. The physiologic transfer of stress arising from temporalis or masseter muscle activity on the LDA should be more extensive in cases where this attachment is relatively loose than in cases where the LDA is tough. The most important finding of Ben Amor et al is that there are two morphologically different types of LDA whereby, with a degree of statistical significance, LDA type 2 (LDA2) is weaker and obviously combined with a higher degree of mobility than LDA type 1 (LDAI). The authors show that the mean strength to failure in the specimens of LDA2 is less than in specimens of LDAl, and that the points of attachment in the specimens of LDA2 are situated below those in the cases of LDAl. Therefore, when considering possible effects of the temporalis or masseter muscle on the articular disc, these should be more extensive in the case of LDA2 than in the case of LDAl. On the anteromedial side of the joint, Ben Amor et al showed that the ratio between the insertion of the lateral pterygoid muscle and the transverse anterior band length was greater in women than in men. However, a relationship of statistical significance between the type of LDA and the extent of the anteromedial insertion of the lateral pterygoid muscle was not found. I believe that the quantitative data presented in this study will be of great clinical significance when Interpreting
FIGURE
1. Lateral view of a human temporomandibular joint after removal of the lateral joint capsule to expose the upper joint compartment. The arrowheads indicate collagenous fibers extending from masseter muscle the lateral part of the articular disc (D) into the dee (MM]. The subdivisions on the bar correspond to a cr [stance of 1 mm.
1169
CORDULA SCHMOLKE various medially or laterally localized functional and structural temporomandibular joint disorders. However, it has to be taken into account that the traction axis that Ben Amor et al adjusted during their tension tests in the mechanical apparatus might not be identical with the axes of physiologic forces acting on the lateral part of the disc or on the lateral ligament during the various joint movements. Stress transferred, eg, from the masseter muscle to the lateral part of the disc, would be otiented at right angles to the traction axis chosen by Ben Amor et al in their Figure 1. As far as the structural consistency and strength of the lateral joint capsule and of the lateral ligament is concerned, I agree with the authors that there is wide interindividual variability. However, in the material we have investigated either by macroscopic dissection (32 specimens) or as serial plasticized sections (22 specimens), we found the lateral joint capsule to be several times thicker than all the other capsular elements on the anterior, medial, and posterior sides of the joint in all instances. In all specimens the anterior half of the lateral capsule was reinforced by ligamentous collagenous fibers extending from a part of the zygomatic birch in front of the joint to the condylar neck or to the dorsal margin of the mandibular ramus (Fig 2). I never noticed a complete lack of the lateral ligament even in specimens where the lateral joint capsule, including the lateral ligament, appeared transparent.
Acknowledgment The studies on which these comments by the Deutsche Forschungsgemeinschaft
are based were supported (Schm 1117/l-2).
3. Velasco JRM, Vazquez JFR, Collado JJ: The relationships between the temporomandibular joint disc and related masticatory muscles in humans. J Oral Maxillofac Surg 51:390, 1993 4. Schmolke C: The relationship between the temporomandibular joint capsule, articdar disc and jaw muscles. J Anat 184:335, 1994 5. Ritgen
References 1. Couly G, Hureau J, Vaillam menisque temporo-mandibulaire.
FIGURE 2. Macroscopic view of the lateral caspule of a human temporomandibular joint after removal of the masseter muscle. The width of the lateral ligament reinforcing the anterior half of the lateral joint cas ule is indicated by the arrows. The magnification is identical to that o P Figure 1
JM: Le complexe dynamique du Rev de Stomatol Paris 76:597,
1975
2. Dauber W: Die Nachbarschaftsbeziehungen des Discus articularis des Kiefergelenks und ihre funktionelle Bedeutung. Schweiz Monatsschr Zahmned 97:427, 1987
I: Makroskopische und plastinationshistologische Studie zur funktionellen Anatomie des menschlichen Kiefergelenks unter Beriicksichtigung der Vetinderungen bei zwei verschiedenen Mundiiffnungspositionen. Dissertation, University of Bonn, 1996 6. Schmolke C, Hugger A: The human temporomandibular joint region in different positions of the mandible. Ann Anat 1998 (in press)