Trabecular anisotropy and collagen fibre orien- tation in the mandibular condyle following experimental functional appliance treatment using sheep B. Ma 1,6 , W. J. Sampson 1,6 , O. W. Wiebkin 2,6 , D. F. Wilson 3,6 , N. L. Fazzalari 4–6 ¹Orthodontic Unit, Dental School; 2 Department of Medicine, Royal Adelaide Hospital; 3 Oral Pathology, Dental School; 5 Department of Pathology; 6 Australian Jaw Joint Project, Faculty of Health Sciences, The University of Adelaide, Adelaide, Australia 4 Division of Tissue Pathology, Institute of Medical and Veterinary Science, Adelaide, Australia Summary In order to study the modifying effects of functional ap- pliances on the mechanical environment of the tempo- romandibular joint (TMJ), we characterised the struc- ture of the mandibular condyle subsequent to an ex- perimental functional appliance intervention. Eight, four-month-old, castrated male Merino sheep, were randomly allocated to experimental and control groups (n = 4 in each group). Forward mandibular displace- ment was induced with an intraoral appliance. The study period was 15 weeks, during which time fluor- ochromes were administered to all of the animals. Mid- sagittal sections of the TMJ were selected for analysis and trabecular anisotropy was estimated using bone histomorphometry. Only the experimental group dem- onstrated that the trabecular bone in the central condy- lar region was less anisotropic when compared to the subchondral region. Also, the variation in trabecular anisotropy of the central condylar region was found to be smaller in the experimental group. The collagen fibre orientation was analysed under polarised light as the proportion of the dark or bright fibres observed in regions which existed before, and regions which formed during the experiment, as determined by the fluoroch- rome labels. In the experimental group, more bright collagen fibres were found in the most superior region of the mandibular condyle when compared with the controls. These results suggested that the experimental functional appliances changed the orientation and pat- tern of the mechanical forces acting on the mandibular condyle, and possibly increased the magnitude of the lateral functional forces applied to the most superior part of the condyle during such treatments. Keywords Sheep mandibular condyle, growth modification, collagen fibre orientation, trabecular anisotropy Vet Comp Orthop Traumatol 2006; 19: 35–42 Received November 20, 2004 Accepted July 20, 2005 Vet Comp Orthop Traumatol 1/2006 Original Research 35 © 2006 Schattauer GmbH Introduction Growth modification in the region of the human temporomandibular joint (TMJ) by the use of functional appliances is of great interest to orthodontists for the treatment of class II malocclusion. Functional ap- pliances have been reported to achieve cor- rection of class II discrepancy through in- creasing the mandibular length (17, 29, 34) and rotating the mandible (6, 35). These changes have been suggested to result both from stimulation of mandibular condylar growth beyond that which would normally occur in growing children, and redirection of condylar growth from an upward and for- ward vector to a more posterior orientation (36). However, the mandibular trabecular bone structural orientation and the ortho- paedic mechanical stress acting on bone formation in the TMJ has not been deter- mined. The mandibular condyle is a growth site within the TMJ but current knowledge of the responses of the mandibular condylar tissue to functional appliances comes mainly from animal experiments. In order to mimic orthodontic treatment procedures, specific functional appliances have been used to prompt the mandible to a protrusive position in various animal species (9, 22, 26). The overall results of the subsequent bone adaptation have shown increased bone-forming activity of the mandibular condylar tissue. For example: this increased bone-forming activity has been manifested as elevated alkaline phosphatase activity and 45 Ca uptake (30) and as newly-formed trabecular bone being deposited on the pos- terior border of the condyle and mandibular ramus (26, 32). In addition, the increased bone forming activity is not uniformly dis- tributed among regions within the mandibu- lar condyle (23). The bone structure is an indicator of its mechanical properties having adapted to its mechanical environment. In trabecular bone, the anisotropy and bone volume (BV) to total tissue volume (TV) fraction (BV/ TV) are of special interest, since a large pro- portion of the variance in the mechanical properties can be explained by these par- ameters (24). However, previous reports on the trabecular architecture of the mandibu- lar condyle have not provided sufficient in- formation on how trabecular bone structural orientation adapts to orthopaedic mech- anical stress altered by a functional ap- pliance. After experimental functional ap- pliance treatment, a posteriorly directed ro- tation of the trabeculae in the mandibular condyle has been found (9). More recently, the BV/TV has been found to be lower in the mandibular condyle after the treatment des- pite an increased bone forming activity (23). Based on available data, functional ap- pliance treatment seems to change the orientation of the functional forces acting on the TMJ whereas the change of the mag- nitude of the functional force is still uncer- tain. The same uncertainty regarding changes in the muscle forces following functional appliance treatment has also For personal or educational use only. No other uses without permission. All rights reserved. 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