Fractures of the proximal sesamoid bones are classified according to their location in the bone. The most common sesamoid fractures in Standardbreds and Thoroughbreds are apical. They are caused by overextension and often are associated with suspensory ligament damage. The lateral proximal sesamoid in the hindlimb of Standardbreds may be fractured as a result of torque forces induced by shoeing with a trailer-type shoe. Other fractures include mid-body, basilar, abaxial, axial, or comminuted, and they may involve one or both sesamoids. Most, apart from some abaxial and base fractures, are articular. Clinical signs include heat, pain, and acute lameness, which is exacerbated by flexion of the fetlock. There is often hemarthrosis and synovial effusion of the metacarpal/tarsophalangeal joint. Diagnosis is confirmed radiographically. Of Thoroughbred racehorses ≥2 yr old, 82% of horses with apical sesamoid fractures removed arthroscopically ran at the same or at an improved level. Horses with apical fractures of the forelimb medial proximal sesamoid are less likely to race after fragment removal. The presence of suspensory desmitis ( see Suspensory Desmitis in Horses Suspensory Desmitis in Horses Interosseous medius (suspensory ligament) of the horse. Injuries of the suspensory ligament (interosseous muscle) are common in forelimbs and hindlimbs of horses. Lesions are typically classified... read more ) in the affected limb also decreases the prognosis after surgery. Horses with apical sesamoid fractures in the forelimbs are less likely to race than those with fractures in the hindlimbs (55% vs 86%). Size and shape of apical sesamoid fractures does not appear to affect racing performance. Mid-body fractures typically require reduction using lag screw(s) fixation with a prognosis of 75% (44% in another study) to return to racing. The prognosis for basilar fractures involving a portion of the sesamoid is fair compared with that for fracture of most of the base or in horses with associated articular disease (poor prognosis). Complete disruption of the suspensory apparatus, including fractures of both sesamoid bones, is a catastrophic injury that may be accompanied by vascular compromise; however, some horses can be salvaged for breeding by surgical arthrodesis of the fetlock joint.