ORIGINAL ARTICLE Functional influence on sutural bone apposition in the growing rat Christos Katsaros, a Alexandros Zissis, b Andrea Bresin, c and Stavros Kiliaridis d Göteborg, Sweden, Nijmegen, The Netherlands, Saarbrücken, Germany, and Geneva, Switzerland Introduction: The aim of this study was to quantify the influence of reduced masticatory muscle function on sutural bone apposition in the growing rat. Methods: Twenty-six growing male albino rats were randomly divided into 2 equal groups; 1 (hard-diet group) received the ordinary diet of hard pellets, and the other (soft-diet group) received the powdered form of the ordinary diet mixed with water. The experimental period started when the rats were 4 weeks old and lasted 42 days. At days 0, 14, and 28, calcein was injected into all animals. At the end of the experiment, the animals were killed, and the heads were taken for preparation of undecalcified frontal sections, 120 m thick. Three representative homologous sections for each animal in both groups were selected and studied under a fluorescence microscope. The level of bone apposition at the time of calcein injection was marked with separate fluorescing lines. Because the lines from the first injection could not be seen in all areas, bone apposition in the internasal, naso-premaxillary, and inter-premaxillary sutures was quantified from day 14 to the end of the experimental period, by using an image analysis software. Results: In both groups, greater bone apposition was found between days 14 and 28 than between days 28 and 42 of the experimental period. Less bone apposition was found in the soft-diet group than in the hard-diet group in all sutures studied. Conclusions: The findings suggest that bone apposition in the studied facial sutures in the anterior facial skeleton of the growing rat is significantly affected by reduced masticatory function. (Am J Orthod Dentofacial Orthop 2006;129:352-7) A lthough various experimental and clinical studies have shown associations between masticatory muscle function and craniofacial growth, 1,2 the response of certain growth sites such as sutures is not fully elucidated. Mechanical loading seems to play an important role in the regulation of sutural growth. 3-5 Also, the devel- opment of interdigitations in the sutures seems to be related to the functional strains placed on a tissue. 6-13 Moreover, in histological studies, morphological differ- ences in the sutures in the anterior part of the snout and a number of synostoses in the internasal suture were observed in rats with reduced masticatory function, possibly because there was less tension on the premax- illa and the lateral part of the nasal bones. 14,15 The reduction of the loads applied to the skull during mastication by feeding the animals a soft diet was found to adversely influence the transverse growth of the premaxilla at areas under direct muscle influence near their attachments. 16 It is not fully elucidated whether the narrower internasal, naso-premaxillary, and inter-premaxillary sutures and the more parallel orientation of the bony surfaces of the naso-premaxil- lary suture under reduced functional demands reported in a previous study 17 are related to decreased sutural bone apposition and thus contribute to the above- mentioned reduction in the width of the premaxilla. The aim of this study was to quantify the influ- ence of reduced masticatory muscle function on sutural bone apposition in the anterior facial skeleton of the growing rat. MATERIAL AND METHODS Twenty-six growing male albino rats of the Sprague-Dawley strain, with a mean body weight of 100 g at the beginning of the experiment, were used. They were housed 4 or 5 in each cage and were fed and watered ad libitum. The experimental protocol was approved by the Ethics Committee of Göteborg Uni- versity. The animals were randomly separated into 2 equal groups (n = 13) according to diet; 1 (hard-diet a Professor, Department of Orthodontics and Oral Biology, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands; former research asso- ciate, Department of Orthodontics, Göteborg University, Göteborg, Sweden. b Private practice, Saarbrücken, Germany. c Former research associate, Department of Orthodontics, Göteborg University, Göteborg, Sweden. d Professor, Department of Orthodontics, University of Geneva, Geneva, Switzerland. Supported by grants from the Swedish Medical Research Council (05006). Reprint requests to: Professor C. Katsaros, Department of Orthodontics and Oral Biology, Radboud University Nijmegen Medical Center, 309 Tand- heelkunde, PO Box 9101, 6500 HB Nijmegen, The Netherlands; e-mail, C.Katsaros@dent.umcn.nl. Submitted, April 2003; revised and accepted, September 2004. 0889-5406/$32.00 Copyright © 2006 by the American Association of Orthodontists. doi:10.1016/j.ajodo.2004.09.031 352