With a lot of shoulder movements especially during the tennis serve, the rotator cuff and the acromioclavicular joint is a vulnerable to injuries. Acromioclavicular joint injuries account for up to half of all sports related shoulder injuries, including tennis. This joint depends on the ligaments, capsule and the trapezius and deltoid muscle for stability. Injury often occurs during falls landing on the upper part of the shoulder with the arm adducted. In patients over 40 years of age, the pain can arise from non-traumatic causes like overuse or degeneration.
Your doctor will examine the joint and the shoulder for pain and instability. Bringing your arm across your body in adduction will reproduce the pain. An x-ray will be needed. And while type I and II injuries can be treated conservatively, types IV -VI will require surgery most of the time. The management of type III injuries remains controversial, with some opting to go for reconstruction after a period of conservative treatment. Surgery possibly involves putting a screw from the coracoid process to the clavicle, coracoclavicular ligament reconstruction and coracoacromnial ligament transfer. Acromioclavicular joint reconstruction remains the most biomechanically sound course of treatment.
An initial way of diagnosing and treating the pain is simply to do a steroid injection to the acromioclavicular joint. If the pain goes away, you will know that this is the likely cause. If the pain relief last for months, then all you may need is to repeat the injection from time to time. If the duration of the pain relief is not long, then surgery will probably be useful.
Unlike many other joint injuries, physiotherapy and rehabilitation may not be as useful. Resting it may help with the pain, but ultimately there may need to be a change in the activities that you participate in if you are not keen for surgery.