Implant-based 3D superimposition of the growing rabbit mandible – A proof of concept study

Abstract

Introduction: The reliable assessment of craniofacial morphological changes during growth requires invariant regions for image registration. As these regions have not yet been identified in three dimensions, mandibular intra-osseous implants are required as fiducial markers for the reliable assessment of three-dimensional (3D) mandibular growth changes. The aim of this study was to develop an animal model for the assessment of the 3D morphological changes of the mandible during growth, using implants as fiducial markers.

Methods: Titanium implants were placed in the body of the mandible of six New Zealand White rabbits. Cone beam computed tomography (CBCT) scans were taken one-week following implant placement and after an additional eight-weeks of growth. Segmentations of CBCT images were exported into custom-made scripts, implant centroids were identified, implant stability during growth calculated, and the segmented mandibles were registered on the implant centroids. Semi-transparencies and surface distance colour maps were produced to enable visualisation and quantification of the growth changes. Results: All rabbits recovered well from surgery. A minimum of three implants were required to estimate implant stability during growth. The implant failure rate (defined as implant displacement during growth) was 4.5%. The buccal cortical bone of the body of the mandible was stable during growth and suitable for implant placement. Three or more stable implants were required for rigid registration. Bilateral implants resulted in more accurate rigid registration in the growing rabbit than only unilateral implants. This was partly due to transverse growth of the mandible, but was also likely to have been due to increased distance between contralateral implants. The distance between implants was likely to have had an affect on registration accuracy. Qualitative descriptions of bone modelling were achieved using semi-transparencies of 3D overlays, and growth changes were quantified using surface distance colour maps. Conclusions: This animal model appears to be feasible for the assessment of the 3D morphological changes occurring during mandibular growth. Although a minimum of three stable implants were required for accurate rigid registration, more than three implants should be placed, to allow for instability of some implants, and to improve the accuracy of rigid registration.