Development of an ovine model to investigate orthodontic tooth movement in 3D

Abstract

INTRODUCTION Small animal models, including rats and mice, have been used in orthodontic research for over a century. Investigating orthodontic tooth movement (OTM) in small animals is very challenging and the translational significance of findings from rodents to humans is dubious. A sheep model may represent a suitable large animal model for investigating OTM. The pressure-tension theory of OTM is supported by animal research and has long been accepted. More recently, however, a new paradigm has been proposed. According to Frost’s mechanostat theory, the pattern of alveolar bone remodelling changes with the magnitude and distribution of stress within the periodontium, which in turn depends on the modality of tooth movement (e.g. tipping vs bodily). To the best of our knowledge, there is no available information relating tooth movements, as they actually occur in three dimensions (3D), with alveolar bone turnover and associated histological changes. AIMS: The primary aim of this thesis was to develop a model in the ovine mandible to investigate tooth movement. It was tested by moving teeth orthodontically and describing the outcomes in 3D. METHODS: In six Romney-cross ewes, the lower first and third premolars were extracted, and fiducial markers placed. Impressions were acquired for construction of customised orthodontic appliances. Two appliances were designed, to achieve a range of moment-to-force ratios and therefore tooth movement modalities. After six weeks, each sheep received one of each appliance, randomly allocated to the left and right sides. The lower second premolars were moved mesially, into the healed edentulous space. After twelve weeks, the sheep were euthanised and histological specimens obtained. Sequential CT scans were acquired and registered. Tooth movement between two time-points and associated changes to the alveolar bone were assessed in 3D. The registered teeth were segmented axially and matched with equivalent histological slides, enabling identification of stress vs strain surfaces and description of the surrounding periodontal tissues. RESULTS: Appliance retention was the greatest challenge, with a mean appliance survival of 8.8 weeks (SD 2.4). All appliances lasted a minimum of four weeks with two remaining in situ at the completion of the study. A variety of tooth movements were achieved with a mean crown movement of 6.4 mm (range 1.8-13.0 mm) and mean root apex movement of 2.7 mm (range 1.1-4.8 mm). The approach whereby the registered teeth were “matched” to the equivalent histological slides was successful. New bone was identified on the “pressure” surface on multiple slides. CONCLUSION: It is possible to move teeth orthodontically in the ovine mandible and describe the movement in 3D. Registered 3D imaging can be merged with equivalent histology, the pressure and tension surfaces identified, and site-specific bone remodelling compared.