The Use of Optical Tracking to Characterize Fracture Gap Motions and Estimate Healing Potential in Comminuted Biomechanical Models of Surgical Repair

Fracture healing is stimulated by micromotion at the fracture site, whereby there exists an optimal amount of strain to promote secondary bone formation. Surgical plates used for fracture fixation are often evaluated for their biomechanical performance using benchtop studies, where success is based on overall construct stiffness and strength measures. Integration of fracture gap tracking to this assessment would provide crucial information about how plates support the various fragments present in comminuted fractures, to ensure there are appropriate levels of micromotion during early healing. The goal of this study was to configure an optical tracking system to quantify 3D interfragmentary motion to assess the stability (and corresponding healing potential) of comminuted fractures. An optical tracking system (OptiTrack, Natural Point Inc, Corvallis, OR) was mounted to a material testing machine (Instron 1567, Norwood, MA, USA), with an overall marker tracking accuracy of 0.05 mm. Marker clusters were constructed that could be affixed to individual bone fragments, and segment-fixed coordinate systems were developed. The interfragmentary motion was calculated by tracking the segments while under load and was resolved into compression-extraction and shear components. This technique was evaluated using two cadaveric distal tibia-fibula complexes with simulated intra-articular pilon fractures. Normal and shear strains were tracked during cyclic loading (for stiffness tests), and a wedge gap was also tracked to assess failure in an alternate clinically relevant mode. This technique will augment the utility of benchtop fracture studies by moving beyond total construct response and providing anatomically relevant data on interfragmentary motion, a valuable proxy for healing potential.