13 In both cases, catabolic degradation was above normal levels,

13 In both cases, catabolic degradation was above normal levels, suggesting that loads within a physiological range are necessary for maintenance of cartilage integrity and growth. The increased expression of VEGF is in

agreement with the results of Tanaka et al.,14 who observed abundant presence of VEGF in the mandibular condyle after mechanically induced TMJ osteoarthrosis. In that study, the percentage of VEGF immunopositive chondrocytes significantly increased with the period of applied mechanical stress. During mechanical overloading, reduced oxygen tension activates the hypoxia-induced transcription factor-1, which is linked to the expression of VEGF.15 The results of our study showed no difference for the level of type II collagen after bilateral teeth extraction. As previously mentioned, it was expected a decreased expression of type II collagen following buy GSK2118436 up-regulation of IL-1β and VEGF. In rabbits, unilateral removal of teeth10 and surgically created disc displacement9 resulted in increased and decreased expression of type II collagen in the condylar cartilage, respectively. Besides differences between animal models, these contrasting results suggest that the type of loading

is an important factor in type II collagen expression. Basically, three types of loading can be distinguished: PDK4 compression, tension, and shear. Tensile forces correspond more to fibroblastic activity, leading to the production of BMS-734016 type I collagen, while compressive forces tend to be correlated with chondrocytes and the increased production of type II collagen.16 During joint loading the cartilage layers are sheared

to adapt their shape to the incongruent articular surfaces. Excessive shear, however, can cause a fatigue, which irreversibly may lead to damage of cartilage. Furthermore, excessive shear stress is associated with a breakdown of joint lubrication through a reduction of hyaluronic acid molecular weight.4 We speculate that bilateral symmetrical loss of posterior teeth may keep mandibular stability, since both TMJs will be similarly loaded. However, this would be accompanied by increased shear stress. Is has been shown that loss of posterior occlusal support leads to a noticeable cranial condyle movement during clenching.17 This may lead to a more intimate contact between the articular surfaces, causing excessive shear stress. In contrast to bilateral tooth loss, the increased expression of IL-1β after unilateral extraction was accompanied by an increase in type II collagen on both sides of the jaw. This different response was probably due to differences in the nature and magnitude of the forces applied to the TMJs in these distinct biomechanical situations.

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