Although tissue engineering of the temporomandibular joint (TMJ) structures is in its infancy, tissue engineering provides the revolutionary possibility for treatment of temporomandibular disorders (TMDs). Recently, several reviews have provided a summary of knowledge of TMJ structure and function at the biochemical, cellular, or mechanical level for tissue engineering of mandibular cartilage, bone and the TMJ disc. As the TMJ enables large relative movements, joint lubrication can be considered of great importance for an understanding of the dynamics of the TMJ. The tribological characteristics of the TMJ are essential for reconstruction and tissue engineering of the joint. The purpose of this review is to provide a summary of advances relevant to the tribological characteristics of the TMJ and to serve as a reference for future research in this field. This review consists of four parts. Part 1 is a brief review of the anatomy and function of the TMJ articular components. In Part 2, the biomechanical and biochemical factors associated with joint lubrication are described: the articular surface topology with microscopic surface roughness and the biomechanical loading during jaw movements. Part 3 includes lubrication theories and possible mechanisms for breakdown of joint lubrication. Finally, in Part 4, the requirement and possibility of tissue engineering for treatment of TMDs with degenerative changes as a future treatment regimen will be discussed in a tribological context.

A switch to a soft diet, associated with reduced forces applied to the mandible during mastication, may result in an alteration of the degree of mineralization in the mandible. This alteration may be regionally different. The aim of this study was to analyze this alteration by examination of the degree of mineralization in the mandible of growing rats fed with a hard or soft diet. Fifteen Wistar male rats were used in this investigation. After weaning, six rats were fed with a hard diet and the remaining nine rats with a soft diet. After 9 weeks, three-dimensional reconstructions of the cortical and trabecular bone of their mandibles were obtained using a microCT system. The degree of mineralization was determined for the trabecular bone in the condyle and for the cortical bone in the anterior and posterior areas of the mandibular body. In both diet groups the degree of mineralization was significantly (p < 0.01) lower in the trabecular than in the cortical bone. In the mandibular body, the anterior area showed a significantly (p < 0.01) higher degree of mineralization than the posterior area in both diet groups. In both areas the soft diet group had a significantly (p < 0.05 or 0.01) higher degree of mineralization than the hard diet group. The trabecular bone in the condyle of the hard diet group showed a significantly (p < 0.01) higher degree of mineralization than in the soft diet group. The present results indicate the importance of proper masticatory muscle function for craniofacial growth and development.

In synovial joints, friction between articular surfaces leads to shear stress within the cartilaginous tissue, which might result in tissue rupture and failure. Joint friction depends on synovial lubrication of the articular surfaces, which can be altered due to compressive loading. Therefore, we hypothesized that the frictional coefficient of the temporomandibular joint (TMJ) is affected by the magnitude and duration of loading. We tested this by measuring the frictional coefficient in 20 intact porcine TMJs using a pendulum-type friction tester. The mean frictional coefficient was 0.0145 (SD 0.0027) after a constant loading of 50 N during 5 sec. The frictional coefficient increased with the length of the preceding loading duration and exceeded 0.0220 (SD 0.0014) after 1 hr. Application of larger loading (80 N) resulted in significantly larger frictional coefficients. In conclusion, the frictional coefficient in the TMJ was proportional to the magnitude and duration of joint loading.

The development of the craniofacial system occurs, among other reasons, as a response to functional needs. In particular, the deficiency of the proper masticatory stimulus affects the growth. The purpose of this study was to relate alterations of muscle activity during postnatal development to adaptational changes in the muscle fibers. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a solid (hard-diet group) or a powder (soft-diet group) diet for 63 days. A radio-telemetric device was implanted to record muscle activity continuously from the superficial masseter, anterior belly of digastric and anterior temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time), the total burst number and their average length exceeding specified levels of the peak activity (5, 20 and 50%). The fiber type composition of the muscles was examined by the myosin heavy chain content of fibers by means of immunohistochemical staining and their cross-sectional area was measured. All muscle fibers were identified as slow type I and fast type IIA, IIX or IIB (respectively, with increasing twitch contraction speed and fatigability). At lower activity levels (exceeding 5% of the peak activity), the duty time of the anterior belly of the digastric muscle was significantly higher in the soft-diet group than in the hard-diet group (P < 0.05). At higher activity levels (exceeding 20 and 50% of the peak activity), the duty time of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.05). There was no difference in the duty time of the anterior temporalis muscle at any muscle activity level. The percentage of type IIA fibers of the superficial masseter muscle in the soft-diet group was significantly lower than that in the hard-diet group (P < 0.01) and the opposite was true with regard to type IIB fibers (P < 0.05). The crosssectional area of type IIX and type IIB fibers of the superficial masseter muscle was significantly smaller in the soft-diet group than in the hard-diet group (P < 0.05). There was no difference in the muscle fiber composition and the cross-sectional area of the anterior belly of the digastric and anterior temporalis muscles. In conclusion, for the jaw muscles of male rats reared on a soft diet, the slow-to-fast transition of muscle fiber was shown in only the superficial masseter muscle. Therefore, the reduction in the amount of powerful muscle contractions could be important for the slow-to-fast transition of the myosin heavy chain isoform in muscle fibers.