Temporomandibular joint adaptations associated with Herbst appliance treatment in juvenile Rhesus monkeys (Macaca mulatta)

This study evaluated histologically the temporomandibular joint and posterior ramal regions in juvenile Rhesus monkeys whose mandibles had been positioned forward with a Herbst appliance. Acrylic splint Herbst appliances were bonded to the maxillary and mandibular dental arches of 20 male juvenile animals. The animals were killed at 3-,6-,12-, and 24-week intervals after appliance placement. Seven male juvenile Rhesus monkeys served as controls. The TMJ regions, including the posterior border of the rami, then were prepared for qualitative and quantitative histological analysis. Linear measurements were obtained by way of Bioquant microscopic equipment (Bioquant Image Analysis Corp, Nashville, TN). Structural adaptations were noted in the mandibular condyle, infratemporal fossa, and the posterior ramal border of the experimental animals in comparison to controls. Specifically, increased proliferation of the condylar cartilage was noted. These adaptations occurred primarily in the posterior and posterosuperior condylar regions. Bone deposition was noted along the anterior border of the postglenoid spine; bone resorption occurred along its posterior border. Significant bony apposition also was evident along the posterior border of the mandibular ramus during early experimental periods. No gross or microscopic pathological changes were noted in the temporomandibular joint in any of the animals. The results of this study indicate that the response of the temporomandibular joint region in the juvenile animals undergoing forced mandibular protrusion with the Herbst bite-jumping mechanism is similar to previous studies investigating the response to functional protrusion with cast occlusal splints. This study also may provide some insight into the general changes occurring microscopically in the temporomandibular joint regions of growing patients undergoing Herbst appliance therapy.