Serious trauma can result in fracture to any part of the shoulder region. However, the main sites affected are the supraglenoid tuberosity of the scapula (which serves as the origin of the biceps brachii muscle), the mid to distal scapula, and the proximal humeral metaphysis.
Supraglenoid tuberosity fractures, if complete, invariably displace in a craniodistal direction, because of the pull of the biceps. Large fractures can be surgically repaired. However, this is not easy, because the fragments are often difficult to reduce and the implants are both difficult to place and prone to failure during recovery from anesthesia or during convalescence. Smaller fragments can be removed, but the involvement of the biceps tendon of origin has to be resected. Very large fragments can involve the joint surface. Cases usually present with severe lameness and a history of trauma (eg, a fall during jumping or collision with a fixed object). On manipulation, there is often a sense of disarticulation between the lower limb and shoulder as the biceps is disrupted. Crepitus may be felt. In most cases, because the inciting cause is significant trauma, other signs such as soft-tissue abrasions or swelling will pinpoint the shoulder as the site of pain. Radiographs will reveal the fracture, and ultrasonography can be very useful to assess the biceps tendon. Management varies and depends on intended use, age, size of fragment, size of horse, etc. The prognosis for restoration of normal function is guarded. The size of the fragment, degree of displacement, presence/absence of articular involvement, degree of biceps disruption, and intended use of the horse are probably the most important prognostic factors.
Mid to distal scapula fractures occur through trauma or, in racehorses, as stress fractures associated with cumulative cyclical fatigue. Trauma can result in complete or (presumably because of the flexibility of the bone, especially in foals) incomplete fractures. Radiographs rarely help, because of the difficulty in obtaining diagnostic images of the area. Ultrasonography can accurately assess the integrity of the bone surface and is the technique of choice. Scintigraphy can also detect the injuries. Comminuted fractures can occur, and the prognosis worsens with increased complexity of the fracture. Simple, nondisplaced, or minimally displaced fractures usually heal well with rest alone.
Ultrasonography can be used to monitor healing. Stress fractures are almost always incomplete and heal very well, carrying an excellent prognosis for return to training. Very rarely, scapula fractures manifest as severe, unstable, comminuted injuries necessitating euthanasia on humane grounds. The clinical appearance is key to making a decision in these circumstances; although painful, scapula fractures with a good prognosis cause no observable limb instability.
Deltoid tuberosity fractures are seen infrequently. They are usually the result of trauma, often a kick from another horse, and may involve a wound and variable amounts of infection within the injury. Lesion-oriented oblique radiographs and ultrasonography define these injuries, and affected horses usually recover fully with rest alone, although surgical debridement of infected bone may be required in rare cases.
Stress fractures affect the proximal humerus also, almost exclusively in the caudal metaphyseal region. They are an uncommon but important cause of lameness in racehorses (the craniodistal metaphysis of the humerus is also affected, see Fractures of the Shoulder in Horses). The typical history is one of sudden onset, often moderate to marked lameness closely associated with recent exercise, in an animal usually but not always in faster work. Lameness is usually transient, and the horse generally becomes sound within a short time (days to a week). If exercise resumes, lameness recurs. Localization is difficult; many are detected after the lower limb has been eliminated as the source of pain, or with scintigraphy. Radiographs can identify periosteal and endosteal new bone at the site of injury. Recovery is usually uncomplicated and complete with a few weeks’ rest. Prolonged confinement may be counterproductive, and light exercise (walking only) may be introduced surprisingly quickly once the initial painful period has subsided. The injury remains evident on radiographs long after the bone is strong enough to withstand exercise, but a gradual smoothing and resolution of the callus will be seen as remodelling proceeds. Undetected humeral stress fractures can result in failure of the bone during exercise and complete breakdown, necessitating euthanasia.