Histologic Changes Associated With the Herbst Appliance in Adult Rhesus Monkeys (Macaca mulatta)

Although most Class II orthopedic treatments have been recommended for use in juveniles and adolescents, orthopedic intervention with the Herbst appliance has recently been suggested for use in young adults. Previous experimental studies have suggested that the potential for adaptation in the temporomandibular joint (TMJ) region of the young adult, while diminished compared with that of adolescents, still may be sufficient to allow for the resolution of Class II problems. The purpose of the current study was to evaluate histologically the response of the mandibular condyle, glenoid fossa, and posterior border of the ramus of adult 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 14 female adult animals. The animals were killed at 3-, 6-, 12-, and 24-week intervals after appliance placement. Seven female adult Rhesus monkeys served as controls. The TMJ regions were removed and analyzed histologically qualitatively and quantitatively by way of the Bioquant IV measuring system (Bioquant Image Analysis Corp, Nashville, TN). Significant variation was observed in the TMJ regions of the control animals, with some animals having identifiable prechondroblastic and chondroblastic layers of condylar cartilage and others with more fibrous coverings of the mandibular condyle. In the experimental animals, adaptive changes in the condylar cartilage were evident as early as 3 weeks, with the dimensions of the condylar cartilage increasing gradually throughout the experimental period. Only minor changes were noted in the articular tissue. All adult control animals had a bony cap, and the bony cap persisted in the experimental animals. Significant bone deposition occurred along the anterior surface of the postglenoid spine only in the 6and 12-week experimental groups. No significant areas of bone deposition and resorption were noted along the posterior border of the ramus. The result of any study in animals, even in nonhuman primates, obviously cannot be applied directly to human functional orthopedic treatment. Because the tissue types and the morphology of the TMJ generally are similar between monkey and man, however, the findings of the present study may provide some insight concerning the changes occurring within the mandibular condyle and glenoid fossa when a Herbst appliance is used clinically in young adult patients. (Semin Orthod 2003;9:26-40.) Copyright 2003, Elsevier Science (USA). All rights reserved.