The accessory carpal bone (ACB) is an important component of the equine carpus and a clinically relevant site of fracture, yet its detailed morphology and motion characteristics remain incompletely characterized. The objectives of this ex vivo study were to characterize the three-dimensional morphometry of the native ACB using computed tomography (CT), to assess its kinematics during carpal flexion and extension using dynamic 3D CT, and to evaluate fragment behavior following a standardized vertical fracture. Twelve equine forelimbs underwent CT imaging in the neutral carpal position, maximal flexion, and maximal extension before and after standardized osteotomy simulating a vertical plane fracture. Morphometric measurements included dorsopalmar width, lateromedial thickness, proximodistal length, and radii of medial concavity and lateral convexity. The ACB exhibited consistent geometry among specimens of comparable size. Dynamic imaging revealed independent physiological motion relative to the other carpal bones of the ACB, characterized by medial movement of its medial concave surface toward the caudal aspect of the radius during flexion. Following fracture creation, widening of the fracture gap and proximal displacement of the palmar fragment occurred predominantly in flexion, whereas the neutral carpal position and maximal extension promoted fracture gap closure and distal displacement. These findings improve understanding of ACB morphology and biomechanics and may have implications for fracture pathogenesis and surgical fixation strategies.
