ORIGINAL ARTICLE Three-dimensional changes in the condyle during development of an asymmetrical mandible in a rat: A microcomputed tomography study Haruhisa Nakano, DDS, PhD, a Koutaro Maki, DDS, PhD, b Yoshinobu Shibasaki, DDS, PhD, b and Arthur J. Miller, PhD c Tokyo, Japan, and San Francisco, Calif A rapidly growing postnatal animal model was used to study changes in the calcified tissue of the mandibular condyle during altered muscle function. A maxillary occlusal splint was designed to shift the mandible laterally (left) during closure. Groups of 5 Wistar rats were killed at 5, 9, 15, 21, 30, and 40 weeks (n = 30), with an equal number of controls. The experimental animals developed shorter, asymmetrical mandibles compared with the control animals. The left condyle became larger and thicker than the right condyle. Microcomputed tomography assessment of the left and right condylar trabecular bone indicated that both had less bone volume than the control condyle. The right masseter muscle significantly lost fiber size and type IIA oxidative fibers, suggesting that the right masseter muscle was used with less tension development. In contrast, the left masseter maintained its fiber size and was similar to the control masseter fiber diameters. Comparison in the sequence of changes indicated that the morphologic changes occurred first in the ramus (age, 5 weeks), before the corpus (age, 15 weeks), and before changes in masseter fiber size and composition (age, 9 weeks). This study showed that both the mandible and the condyle modified their shape and size, as well as the trabecular bone of the condyle, during shifting of the mandible to one side as it closed. (Am J Orthod Dentofacial Orthop 2004;126:410-20) O rthodontists use physical forces to modify both the dentition and the relationship of the max- illa and mandible. Much of the treatment presumably affects the forces developed on the dentoal- veolar region, the mandibular and maxillary bones, and the condyles of the 2 temporomandibular joints (TMJ). Physical force is an important physiological factor that modifies bone. 1-5 Muscles attaching to a bone can alter its shape, mineralization, and mass. 6-8 Most studies of the effect of muscles on bone did not evaluate the changes over time or compare the effects between the bone and the attached muscle. 9-16 The craniofacial system provides an integrated muscle-bone complex in which changes can be followed in detail. A common clinical problem involving the cranio- facial region is an asymmetrical mandible. 17-21 An asymmetrical mandible can be induced in a rapidly growing experimental animal. At least 2 experimental models have been developed. The mandibular muscles can be weakened unilaterally with electrolytic lesioning of motoneurons to these muscles, and this induces an asymmetrical mandible and craniofacial skeleton. 22,23 A second approach alters the occlusal surfaces so that the animal must shift its mandible in 1 direction during closing. 24-32 This study provides a method to evaluate the bilateral changes in bone under the 2 TMJ and compare them with changes in a primary jaw muscle, the masseter, during development of the asymmetrical mandible. A relatively recent technique, high-resolution mi- crocomputed tomography (micro-CT), allows evalua- tion of the condylar trabecular bone as the animal develops the asymmetry. 33-38 This approach allows determining the 3-dimensional (3D) changes in bone morphology and bone density. 39 Detailed analysis of the trabecular pattern can be quantified with several a Lecturer, Department of Orthodontics, Showa University, Tokyo, Japan. b Professor, Department of Orthodontics, Showa University, Tokyo, Japan. c Professor, Division of Orthodontics, School of Dentistry, University of California at San Francisco. Reprint requests to: Arthur J. Miller, PhD, Professor, Division of Orthodontics, School of Dentistry, University of California at San Francisco, San Francisco, CA 94143-0438; e-mail, amiller@itsa.ucsf.edu. Submitted, September 2003; revised and accepted, April 2004. This work was supported by the Japanese Ministry of Education and conducted in the collaboration with the Department of Orthodontics at Showa University and the Division of Orthodontics at the University of California at San Francisco. 0889-5406/$30.00 Copyright © 2004 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2004.04.016 410