Three-Point Bending Test as a Tool to Evaluate the Equine Proximal Interphalangeal Joint Biomechanics: Experimental Set-Up and Preliminary Results

Background/Objectives: The equine proximal interphalangeal joint (PIPJ), while often considered a low-motion joint, contributes to flexion/extension of the digit and accommodates multi-planar movements: the adduction/abduction and the internal/external rotation of the middle phalanx in comparison to the proximal phalanx. Its collateral ligaments are primary stabilizers, restricting movements of the equine digit outside the sagittal plane, and their injury often leads to joint instability and degenerative joint disease. End-stage degenerative joint disease in the PIPJ could be managed with surgical arthrodesis of the joint. After surgical arthrodesis of the PIPJ, joint fusion cause loss of PIPJ extension in late stance of the stride during locomotion, which is accommodated by increased extension of the distal interphalangeal joint (DIPJ). To validate joint-preserving repair techniques after a collateral ligament injury in replacement of arthrodesis, a reliable method for assessing PIPJ extrasagittal biomechanics is required. The objective of this study was to develop and validate a three-point bending test setup to quantitatively characterize the mechanical behaviour of the equine PIPJ during abduction/adduction, simulating loading conditions that challenge the collateral ligaments of the joint. Methods: Seven equine PIPJ specimens (five forelimb and two hindlimb) were tested. A customised 3D-printed system was used to stabilise the joints in an MTS testing system. A three-point bending test was performed to evaluate the joint's mechanical response to the mediolateral plane bending. A marker-based video analysis system with a custom MATLAB script was used to calculate the joint abduction/adduction bending angle. Results: The setup provided reproducible load-displacement and time-angle data. The use of a fixed loading rod and custom 3D-printed system minimized slippage and rotation. The forelimb PIPJs exhibited significantly greater maximum medio-lateral bending angles compared to hindlimb joints. No significant differences were observed between fresh and frozen-thawed specimens. Conclusions: The developed three-point bending test setup provides a reliable and reproducible method for the in vitro evaluation of the biomechanical of the equine PIPJ. This study establishes a baseline for healthy joints and is suitable for the future comparative assessment of novel surgical techniques for collateral ligament repair.