The shoulder joint of the dog functions like a ball-and-socket. The head of the humerus (the bone extending from the shoulder to the elbow) is rounded and acts as the ball fitting inside of the socket created by the glenoid fossa at the bottom of the scapula (shoulder blade). A dislocation occurs when the ball is displaced from the socket.
Shoulder dislocations in dogs can result from congenital malformations, degenerative changes, or traumatic injuries to the joint. When a dislocation occurs, the ligaments, tendons, and muscles of the joint, as well as the joint capsule which surrounds the joint, are damaged and the shoulder becomes painful.
Perform a physical examination on the injured joint. Observe the dog at a walk. Manipulate the injured joint while watching for evidence of pain. Note whether crepitus (grating, popping, or crackling noises or sensations) is present within the joint. Lay the dog on his side with the injured leg uppermost and palpate the joint for instability.
Radiograph the injured joint by taking two views: one medio-lateral (from side to side) and the other cranio-caudal (from front to back). Examine the radiographs for bone injuries such as fractures and to access joint damage.
If the radiographs are not diagnostic, perform an arthroscopic examination of the shoulder joint by introducing the arthroscope into the joint through a small incision through the skin, underlying tissue, and joint capsule.
Shoulder dislocations will usually make the dog lame or non-weight-bearing on the injured leg. Determine whether there is instability within the shoulder joint by stabilising the scapula with one hand and attempting to abduct the limb (move the limb away from the body). The ability to abduct the leg more than 30 degrees indicates instability within the joint and supports a diagnosis of a dislocated shoulder. The stability of the joint in a cranial to caudal (front to back) plane can be evaluated as well by stabilising the scapula with one hand and attempting to move the head of the humerus frontward and backward. Comparing the injured leg to the normal leg can be helpful in determining whether the joint is unstable. A dislocation will usually be easier to see radiographically on the cranio-caudal view of the joint. However, in rare instances, it may be more easily visualised on the medio-lateral view. In some cases, changes within the joint may be too subtle to see well and the radiographs may not be diagnostic. Arthroscopy is minimally invasive. Tears in the ligaments and damage to the muscles supporting the joint which are not visible on a radiograph may be seen arthroscopically. These types of injuries may result in shoulder instabilities which cause chronic lameness.