The degeneration of joints is common, and the shoulder complex is frequently affected. Arthrosis, also known as osteoarthritis or “wear-and-tear” arthritis, describes degenerative joint disease. This condition involves the gradual breakdown of the protective cartilage within a joint. When it occurs in the shoulder, it often targets the acromioclavicular (AC) joint, a small junction located at the very top of the shoulder.
The Anatomy of the AC Joint
The acromioclavicular joint is the connection point between two bones: the collarbone, or clavicle, and the highest part of the shoulder blade, known as the acromion. This joint is positioned directly on top of the shoulder girdle. Unlike the main ball-and-socket shoulder joint, the AC joint is a flat, planar joint designed to allow gliding and sliding movements between the two bone surfaces.
The AC joint allows for a significant portion of the shoulder’s full range of motion, particularly when the arm is raised overhead. The bony surfaces are normally covered and cushioned by a layer of tough fibrocartilage, which helps minimize friction during movement. Strong ligaments, including the acromioclavicular and coracoclavicular ligaments, surround and stabilize the joint, ensuring the clavicle remains correctly positioned relative to the acromion.
How Arthrosis Develops
AC joint arthrosis begins with the progressive deterioration of the protective fibrocartilage lining the joint surfaces. As this smooth, shock-absorbing tissue wears down, the exposed ends of the bones begin to rub directly against one another, which initiates the degenerative process. This mechanical friction causes the body to react by attempting to repair the damage, often leading to the formation of small, bony growths called osteophytes, or bone spurs, around the joint margins.
Repetitive stress is a primary mechanism that accelerates this wear and tear, especially from activities that involve frequent overhead or heavy lifting. Athletes who engage in weightlifting, particularly bench pressing, or those involved in throwing sports place high levels of strain on this joint. Over time, this chronic, excessive pressure contributes significantly to cartilage erosion.
A secondary and often rapid cause of degeneration is a previous traumatic injury, such as an AC joint separation or dislocation. Even if the injury was successfully treated, the damage to the ligaments and joint structures can change the joint’s mechanics, leading to post-traumatic arthrosis years later. Natural aging is also a major factor, as the prevalence of degenerative changes in the AC joint increases substantially in adults over the age of 40.
Recognizing the Signs of AC Joint Pain
The primary symptom of AC joint arthrosis is pain localized directly on the top of the shoulder. This discomfort is typically sharp or aching and focused on the bony prominence where the clavicle meets the acromion. The pain is aggravated by specific movements that compress or stress the joint.
Individuals commonly experience increased pain when reaching their arm across the chest, such as reaching for a seatbelt. Activities involving heavy lifting, pushing, or raising the arm above shoulder height also intensify the discomfort. Sometimes, a grinding, snapping, or popping sensation, medically termed crepitus, can be felt or heard as the roughened bone surfaces move against each other.
The pain may also radiate slightly into the neck, the trapezius muscle, or down the upper arm. Physicians confirm AC joint arthrosis through a physical examination that includes specific tests, such as the cross-body adduction test, which intentionally compresses the joint to elicit pain. Imaging, typically an X-ray, is then used to visualize signs of joint space narrowing and the presence of bone spurs, confirming the degenerative changes.
Managing and Treating the Condition
Management of AC joint arthrosis initially focuses on non-surgical methods to reduce pain and inflammation. Activity modification involves resting the joint and avoiding painful movements, such as dips, push-ups, and heavy overhead lifting. Non-Steroidal Anti-Inflammatory Drugs (NSAIDs), such as ibuprofen, are often used to reduce swelling and alleviate discomfort.
Physical therapy is recommended to improve overall shoulder function. Therapy focuses on strengthening surrounding muscles, such as the rotator cuff and scapular stabilizers, to provide better support and proper mechanics, rather than mobilizing the arthritic joint directly. For persistent pain that does not respond to initial measures, a corticosteroid injection directly into the AC joint can provide short-term relief from inflammation.
If chronic, debilitating pain persists despite conservative treatments, surgical intervention may be necessary. The standard surgical procedure is the Distal Clavicle Excision (DCE), also known as the Mumford procedure, which involves removing a small segment (typically 5 to 10 millimeters) from the end of the collarbone.
Removing this bone segment creates a gap between the clavicle and the acromion, eliminating the friction and painful rubbing of the damaged bone ends. The procedure is frequently performed using minimally invasive arthroscopic techniques, utilizing small incisions and a camera. Over time, the space created by the excision fills with scar tissue, allowing for movement without the direct, painful contact of the arthritic joint surfaces.