Reduced Loading-Reloading Model of the Temporomandibular Joint in Mice

Objectives: The National Institute of Dental and Craniofacial Research reported that approximately 10.8 million people in the United States suffer from temporomandibular joint (TMJ) disorders. The exact cause of these disorders is unknown; however, many dentists and physicians believe that increased mechanical loading plays an important role in the etiology of the disease. The purpose of this study is to better understand how reduced mechanical loading and re-loading change the structure and molecular markers of TMJ cartilage. Methods: 21-day old female mice were divided into three groups. The mice were either fed a normal pellet diet (group 1) or a soft diet (group 2 and 3). They also had their incisors trimmed out of occlusion (group 2 and 3). After 14 days, Group 3 mice were switched to a normal diet and no longer had their incisors trimmed. When the mice reached 49-day old they were sacrificed and processed for histology. Hematoxylin and eosin staining (H&E staining) and immunohistochemistry for collagen Type II and collagen Type X were performed on the sections of the TMJ obtained from the mice. Bone mineral density (BMD) of condyle was also measured by X-ray densitometry (PIXIMus, GE Lunar). Results: The histological results showed that reduced loading caused a statistically significant decrease in the thickness of cartilage, as well as collagen type II positive layer. The bone volume under the condylar cartilage was also dramatically decreased. However, re-loading restored above changes to the level comparable to the control group. Conclusion: Reduced loading of the TMJ causes structural and protein expression changes in the mandibular condylar cartilage which can be reversed by re-loading. Using this model on transgenic mice would allow the identification of specific genes involved in the mechanical loading response of the TMJ. This work is supported by NIH grant DE017193-01A1 (SW).