The acromioclavicular (AC) joint sits at the top of the shoulder, connecting two bones of the shoulder girdle. Because this joint is subjected to significant forces during movement, it is susceptible to degenerative changes over time. When abnormal stress or wear affects this area, the joint surfaces can react by thickening or enlarging, a process known as hypertrophy. This enlargement is a frequent and painful cause of shoulder complaints.
Understanding the AC Joint and Hypertrophy
The AC joint is the articulation point between the acromion (part of the shoulder blade) and the distal end of the clavicle (collarbone). It is a small, planar synovial joint that provides stability and allows for subtle gliding movements necessary for full shoulder range of motion. The joint is enclosed by a capsule and secured by several ligaments, which manage the significant forces transmitted through the shoulder.
Hypertrophy refers to the abnormal enlargement or thickening of the bone ends and the joint capsule. This biological response is typically a reaction to chronic friction or damage within the joint space. The enlargement often involves the formation of bony outgrowths, commonly called osteophytes or bone spurs. This condition is formally recognized as Acromioclavicular Joint Osteoarthritis or Arthropathy.
The physical consequence of hypertrophy is a reduction in the space between the acromion and the clavicle. This narrowing can cause the rough, arthritic surfaces of the bones to grind against each other during movement. The bony enlargement can also impinge on the soft tissues beneath the joint, contributing to pain and restricted movement.
Root Causes of AC Joint Enlargement
The development of AC joint hypertrophy is primarily driven by mechanical stress and joint degeneration over time. The most common underlying cause is chronic osteoarthritis, often referred to as “wear and tear” arthritis. As the protective cartilage within the joint erodes, the body attempts to stabilize the joint by producing new bone, leading to enlargement.
Repetitive, high-impact activities that place constant mechanical load on the shoulder are another contributing factor. Athletes involved in throwing sports or weightlifters who perform heavy bench presses or military presses subject the AC joint to excessive compressive forces. This chronic overload accelerates cartilage breakdown and stimulates the hypertrophic response.
Prior trauma to the shoulder, even if it occurred years earlier, can initiate this degenerative cycle, resulting in post-traumatic arthritis. For instance, a previous AC joint separation permanently alters the joint’s alignment and stability. This malalignment causes uneven force distribution, leading to chronic friction and inflammation that culminates in thickening.
How AC Joint Hypertrophy Presents
The most characteristic symptom of AC joint hypertrophy is localized pain situated directly over the top of the shoulder. This pain does not typically radiate down the arm but is concentrated specifically at the bony prominence of the AC joint. Patients can often pinpoint the exact location of the discomfort with one finger.
The pain is often aggravated by specific movements that compress the joint surfaces. Reaching the arm across the chest, known as cross-body adduction, is a particularly painful motion. Pain is also commonly experienced during overhead lifting or when raising the arm to its maximum height.
Physical examination may reveal tenderness directly at the joint line, and sometimes a mild swelling or prominence is present. Many individuals report that the pain disrupts their sleep, especially if they attempt to lie on the affected side. A clicking or grinding sensation, termed crepitus, may also be felt during shoulder movement as the irregular joint surfaces rub together.
Management and Treatment Options
Diagnosis of AC joint hypertrophy typically begins with a thorough physical examination, including provocative tests like the cross-body adduction maneuver. Imaging, such as an X-ray, is then used to confirm the diagnosis by revealing joint space narrowing, osteophytes, and other signs of degeneration. A localized injection of an anesthetic agent into the joint can serve as a diagnostic tool; if the pain is temporarily relieved, it confirms the AC joint as the source of the symptoms.
Initial treatment is non-operative, focusing on reducing inflammation and modifying activity. This approach includes rest and avoiding movements that compress the joint, such as push-ups or heavy overhead lifting. Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen are recommended to manage pain and swelling.
Physical therapy focuses on maintaining active range of motion and strengthening the muscles that stabilize the shoulder blade. If conservative measures fail to provide adequate relief, a corticosteroid injection directly into the joint can be administered. This anti-inflammatory medication can offer significant, though often temporary, pain relief by calming inflammation within the joint capsule.
For individuals whose pain persists despite comprehensive conservative treatment, surgical intervention may be considered. The most common procedure is a distal clavicle excision, also known as the Mumford procedure. During this surgery, a small segment of the outer end of the clavicle is removed. This creates a space between the acromion and the clavicle, eliminating the bony friction that causes the pain. The procedure is often performed arthroscopically, using small incisions.