Influence of microarchitecture on stressed volume and mechanical fatigue behaviour of equine subchondral bone

Fractures of the equine metacarpophalangeal (MCP) joint are among the most common and fatal injuries experienced by racehorses. These bone injuries are a direct result of repetitive, high intensity loading of the skeleton during racing and training and there is consensus that they represent a mechanical fatigue phenomenon. Existing work has found the fatigue life of bone to be strongly determined by bone microarchitecture and the resulting stressed volume (i.e., the volume of bone stressed above assumed yield). The purpose of this study was to quantify the influence of bone microarchitecture on the mechanical fatigue behaviour of equine subchondral bone from the MCP joint across a wide variety of sample types. Forty-eight subchondral bone samples were prepared from the third metacarpal (MC3) and proximal phalanx (P1) of 8 horses and subsequently imaged using high resolution micro-computed tomography (μCT) to quantify microarchitectural features of interest, including bone volume fraction, tissue mineral density, pore size, pore spacing, and pore number. Samples were cyclically loaded in compression to a stress of 70 MPa, and fatigue life was defined as the number of cycles until failure. Finite element models were created from the μCT images and used to quantify stressed volume. Based on the expected log point-wise predictive density, stressed volume was a strong predictor of fatigue life in both the MC3 and P1. A regional analysis indicated fatigue life was more strongly associated with bone volume fraction in the superficial (r2 = 0.32, p < 0.001) and middle (r2 = 0.70, p < 0.001) regions of the subchondral bone, indicating the prominent role that the cortical plate played in the fatigue resistance of equine subchondral bone. By improving our understanding of the variance in fatigue life measurements, this research helps clarify the underlying mechanisms of the mechanical fatigue process and provides a basic understanding of subchondral bone injuries in the equine fetlock joint.

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.