Repetitive trauma or stress from training and lifting causes tiny fractures of the distal end of the clavicular bone. Excessive traction on the AC joint from bench presses or chest fly exercises occurs when the elbows drop below or behind the body. This places the shoulders in a position of excess extension. Because the bone doesn't have a chance to heal before the next training session begins, the bone actually starts to dissolve.
There may be a history of an acute injury of the AC joint. But the condition can occur without any known trauma. In most cases, there is repetitive stress to the affected upper extremity. Weight training, intensive lifting, and operating an air hammer are examples of the activities leading to acute distal clavicular osteolysis.
There is evidence that the body tries to heal itself but the bone dissolves or is resorbed by the body instead. A network of blood vessels forms in the area during the attempted healing process. Chronic inflammation with scar tissue called fibrosis is commonly found when tissue from the area is examined under a microscope.
The synovial lining of the joint starts to overproduce itself. Invasion of the underlying bone begins. Degenerative joint disease occurs as an end-result of the pathologic process.
