CHARACTERIZATION OF THE MECHANICAL ENVIRONMENT IN MANDIBULAR DISTRACTION OSTEOGENESIS

Participants: D.G. Orton, M.J. Ignelzi, S. Buchman, S. Rhee, M. Longaker, S.A. Goldstein

Keywords: bone mechanics, distraction osteogenesis, finite element models, mechanical testing

Introduction

Distraction osteogenesis (DO) is a mechanical technique used to stimulate new bone formation in clinical disorders such as trauma, tumor resection, and congenital deformities. Although much is known about DO in long bones, very little is known about DO in the mandible. The long term objective of this research is to investigate how local and global mechanical conditions affect the newly generated bone by mandibular distraction osteogenesis. We are specifically focused on achieving a clearer understanding of how physical forces interact with precursor tissues at the distraction site in order to create an adaptive response resulting in osseous regeneration, and to differentiate these underlying mechanisms from the maladaptive response resulting in injury and fibrous union. This study is designed to determine the forces and strains applied to the tissue, the material properties of the tissue, and to relate these factors to the local tissue response using image-based finite element modeling.

Methods

Surgical Protocol: All animals undergo identical operations for the placement of the mandibular distraction device. The combination of latency, rate and frequency of distraction that lead to the adaptive response will be determined systematically using a combination of 4 different latency periods (0, 3, 5, and 7 days), 2 different distraction rates (0.125mm and 0.25mm per distraction), and 2 different distraction frequencies (once and twice per day)

Characterization of the in vivo boundary conditions: In DO animal models, the global boundary conditions on tensile strain are precisely determined by the distraction rate and frequency. The global boundary conditions on tensile force are measured by instrumenting the distraction device with miniaturized strain gages. During distraction, baseline and experimental force values will be determined by measuring strain gage data 1 minute before until 9 minutes after device activation.

Ex vivo biomechanical tissue characterization: Constrained torsion tests will be conducted on harvested distraction zones. In these mechanical tests, the distracted mandible is twisted while maintained at a constant length and the resulting torque and normal forces are measured as a function of twist. It has been shown that by assuming material isotropy and incompressibility, these tests yield the necessary data to establish a strain energy constitutive function for a hyperelastic material. The early stage tissue will be used to evaluate its viscoelastic behavior. Both the stress relaxation to imposed strain steps and the response to sinusoidal excitation will be measured. Each distraction zone will be tested in both step and frequency tests at any one level to reduce the effects of strain histories. The step levels proposed (50, 150, 250, 500, 750, 1000 microns) are representative of the range of distractions implemented.

Non-linear finite element analysis: Meshes for the finite element models will be developed based on average geometries of the rat models with "calibration" to the specific cross-sectional parameters measured by CT digital data sets and serial histologic sections. The boundary conditions and tissue characteristics (described above) will be applied to the model to determine the local tissue stresses and strains. Pattern correlations will be made between the predicted strain fields and the patterns of bone regeneration.