Vascular endothelial growth factor (VEGF) plays an essential role in the angiogenesis of growing cartilage. Although VEGF expression in cartilage vanishes in normal adults, VEGF is known to be expressed in chondrocytes of osteoarthritic (OA) cartilage. As little information is available on the VEGF expression in the cartilage of OA-like lesions of the temporomandibular joint (TMJ), VEGF expression in the condylar cartilage of TMJs of rats affected with OA was examined. To evoke OA, mechanical stress was applied by forced jaw opening for 10 or 20 days. After 20 days, marked OA-like lesions were observed in the condyle. VEGF was expressed in the chondrocytes of the mature and hypertrophic cell layers of the intermediate and posterior region of the condyle. The percentage of VEGF immunopositive chondrocytes significantly increased with the period of applied mechanical stress. Furthermore, tartrate-resistant acid phosphatase (TRAP) staining of the condylar cartilage showed significant increment of osteoclasts in the mineralized layer subjacent to the hypertrophic layer where high VEGF expression could be detected. The results suggest that VEGF plays an important role in the progression of OA.

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.

Shear stress might be an important factor associated with fatigue failure and damage of the temporomandibular joint disc. Little information, however, is available on the dynamic behavior of the disc in shear. Since the disc is an anisotropic and viscoelastic structure, in the present study the dependency of the dynamic shear behavior on the direction and frequency of loading was examined. Ten porcine discs were used for dynamic shear tests. Shear stress was applied in both anteroposterior (A-P test) and mediolateral (M-L test) directions. The dynamic moduli increased as the loading frequency increased. The dynamic elasticity was significantly larger in the A-P test than in the M-L test, although the dynamic viscosity was similar in both tests. The present results suggest that non-linearities, compression/shear coupling, and intrinsic viscoelasticity affect the shear material behavior of the disc, which might have important implications for the transmission of load in the temporomandibular joint.

Shear stress can result in fatigue, damage, and irreversible deformation of the temporomandibular joint disc. Insight into the dynamic shear properties of the disc may give insight into the mechanism inducing tissue failure due to shear. We tested the hypothesis that the dynamic shear properties of the disc depend on the amount of shear and compressive strain. Twenty-four porcine discs were used for dynamic shear tests. The specimens were clamped between the plates of a loading apparatus under compressive strains of 5%, 10%, and 15%. Dynamic shear was applied to the specimen by a sinusoidal strain of, respectively, 0.5%, 1.0%, and 1.5%. Both the dynamic elasticity and viscosity were proportional to compressive strain and inversely proportional to shear strain. These shear characteristics suggest a significant role of compressive and shear strain on the internal friction of the disc.

It is well known that angiogenesis is essential for the replacement of cartilage by bone during skeletal growth and regeneration. To address angiogenesis of endochondral ossification in the condyle, we examined the appearance of vascular endothelial growth factor (VEGF) and its receptor Flt-1 in condylar cartilage of the growing rat. The early expression of VEGF at various sites during condylar cartilage development indicates that VEGF plays a role in the regulation of angiogenesis at each site of bone formation. From the findings of Flt-1 immunoreactivity, the VEGF produced by the chondrocytes of the hypertrophic zone should contribute to the promotion of endothelial cell proliferation and to stimulate migration and activation of osteoclasts in condylar cartilage, resulting in the invasion of these cells into the mineralized zone.

The collagen fibers and proteoglycans in the disc of temporomandibular joint provide resistance to various loadings. Thus far, however, the role of the proteoglycans in determining the viscoelastic properties of the disc has not been investigated. In the present study the hypothesis was tested that the viscoelastic behavior of the disc decreases by the removal of proteoglycans. In 32 bovine discs, dynamic tensile tests with a wide range of frequencies were performed. Before testing, specimens were treated with different concentrations of alpha-amylase to remove proteoglycans. As the frequency increased from 0.1 to 100 Hz, the disc became more viscoelastic. Increasing the concentration of alpha-amylase significantly decreased its viscoelasticity. It was concluded that proteoglycans play an important role in determining the viscoelastic properties of the disc and, therefore, give the disc a greater capacity for distributing and reducing stresses.

Traumatic joint injury is known to produce osteoarthritic degeneration in the temporomandibular joint (TMJ). However, little information, is available on its possible effect on the dynamic viscoelastic behavior of the disc. In the present study, impulsive compression was applied to the disc as a model for traumatic joint injury. This was tested in 32 porcine discs. Prior to a dynamic tensile test, impulsive compression with strain rates of 0.01, 0.1 and 1 s(-1) to a final strain of 30% was applied to these discs. Tensile stress was applied in the antero-posterior direction with a wide range of frequencies (0.1-100 Hz). The dynamic E-moduli increased as the loading frequency increased. The dynamic viscosity and elasticity decreased with an increase of strain rate, although the effect on viscosity was greater than on elasticity. The results suggest that strain rate and subsequent peak stress of impulsive compression affect primarily the dynamic viscosity of the disc, and that impulsive compression at high strain rates reduces the resistance of the TMJ disc to stress, resulting in permanent disc deformation and perforation.

To test the hypothesis that the condylar part of the retrodiscal tissue of the temporomandibular joint exhibits resistance to tensile force, we investigated its viscoelastic properties and stress-relaxation behavior under tension. Ten specimens were tested. Stress-relaxation tests were conducted from four different initial stress levels. The tissue exhibited a non-linear stress-strain relationship, which could be represented by a bilinear relation of two line segments. The stress-relaxation curves showed a marked drop in load during the initial 10 s and after 2 min the stress reached an almost steady non-zero level. This feature can be well represented by Kelvin's model. It is concluded that the condylar part of the retrodiscal tissue (a) exhibits a non-linear strain-dependent viscoelastic behavior (b), has a great capacity for energy dissipation and resistance to tensile forces, and (c) contributes to maintain the position of the disc relative to the condyle during jaw closing.