Histological Study of the Normal Proximal Third Interosseus Muscle Enthesis in the Equine Pelvic Limb

In human medicine, the enthesis, or tendon-to-bone insertion, is generally considered a linked entity or 'enthesis organ' and plays a crucial biomechanical role. This study aimed to histologically assess the equine proximal third interosseus muscle (suspensory ligament) enthesis in 10 pelvic limbs from 7 horses. The area of the proximal third interosseus muscle enthesis was divided into 6 compartments based on post-mortem computed tomography and prepared for histology using haematoxylin/eosin and trichrome staining. The proximal third interosseus muscle enthesis showed a fibrocartilaginous nature. All adult entheses exhibited a characteristic four-layered structure consisting of ligament, uncalcified fibrocartilage, calcified fibrocartilage and bone. Juvenile specimens lacked clear layering, suggesting ongoing endochondral ossification. One or more smooth, regular tidemarks separated the uncalcified and calcified fibrocartilage, while the margins separating the ligament and bone from the fibrocartilage layer showed pronounced irregular interdigitations. Mean cellularity, fibrocartilage thickness and interdigitation size and shape varied among specimens. The fibrocartilaginous nature of the proximal third interosseus muscle enthesis in the equine pelvic limb supports the role of a stress dissipation mechanism and may have implications in the biomechanics and pathology of proximal third interosseus muscle enthesis injuries.