Proximal sesamoid bone fractures in horses

The proximal sesamoid bones are encased within the elastic suspensory ligam ent proximally and the nonelastic distal sesamoidean ligaments distally on the palmar/plantar surface of the fetlock, collectively forming the suspens ory apparatus. This system functions as an energy-storing device in horses and minimizes hyperextension of the fetlock during the stance phase of the stride. Exercise fatigues the proximal portion of the suspensory apparatus, and fetlock extension may become maximal such that the tensile forces exer ted exceed the biomechanical tolerance of the structures, leading to failur e of the bone or soft tissues. It appears that training strengthens the sus pensory ligament so that the weakest component of the apparatus becomes the proximal sesamoid bones. Chronic sesamoiditis has also been implicated as a factor in the development of proximal sesamoid bone fractures owing to ch anges in the material properties of the bone. Studies of the microvasculatu re of the proximal sesamoid bones have demonstrated that the bones have a s ubstantial intraosseous vascular supply through multiple perforating vessel s. The degree of lameness, pain upon manipulation, and synovial effusion pr esent with proximal sesamoid fractures are highly variable. Radiographic st udies of the affected limb are diagnostic for a fractured sesamoid bone. Tr eatment and prognosis vary depending on the type of fracture and extent of concurrent soft-tissue injuries.