The AC, or acromioclavicular, joint is the point on the top of the shoulder where the acromion process of the scapula, or shoulder blade, joins the lateral part of the clavicle, or collar bone. The joint allows the shoulder blade to slide on the back of the ribcage. This rotational component of the joint is what permits full range of motion of the arm overhead. Injury to the AC joint is common when there is a fall either on an outstretched arm or when the body weight lands directly on the joint itself, according to Dr. Beim in "Acromioclavicular Joint Injuries." A dislocation of the joint is the most common type of injury and is frequently seen in people playing contact sports.
Pain
Pain is the first and most common symptom of an AC joint injury. It is the body's signal that damage has occurred and cues the injured person to be cautious when using the affected arm. Pain is usually felt immediately after the injury, although for very minor sprains it may not begin until the following day. This symptom is present due to the tears in the ligaments that hold the joint together. Supporting the arm in a sling will typically reduce the strain on the ligaments and, in doing so, lessen the pain symptoms. Use of an ice pack for 20 minutes at a time is also an effective pain management technique.
Edema
Inflammation, or edema, in the joint is another of the body's initial responses to injury. Because the AC joint is fairly small, the amount of edema that is possible within the joint space is also minimal. Any increase in the amount of fluid in the joint space, however, adds to the pain in the shoulder and also limits the motion of the arm. Edema can be controlled by use of ice packs and with nonsteroidal anti-inflammatory medications, or NSAIDs.
Range of Motion Limitations
Damage to the acromioclavicular joint results in poor biomechanics of the joint and an inability to fully raise the arm over the head. The ligaments holding the joint together provide the stability that allows the shoulder blade to move on the fixed collar bone. Ligamentous tears degrade this stability and movement becomes abnormal. If the tears in the ligaments are minor, range of motion may be close to normal but may affect the ability of an athlete to use the extremity for throwing motions. Therapy is sometimes warranted to return full range and use of the limb.
Weakness
Depending on how significant the impact, injury to the AC joint may also involve damage to the musculature of the shoulder joint. Strains or tears of the shoulder muscles result in the inability to generate a normal muscle contraction, and the injured person notes weakness in the motions that that particular muscle performs. Significant tears may require surgery to repair, but less severe injuries usually improve with a regimen of rest and a gradual return to normal use of the arm. In some cases where the muscles themselves are not torn, shoulder weakness is the result of muscle atrophy from lack of use, as when a long-term immobilization period is necessary.
Joint Dislocation
The magnitude of the damage to the AC joint is often discussed in terms of grades of dislocation. The grading system ranges from Grade 1, which is very minor, through Grade 6, which requires surgical repair. The less damage to the joint, the more difficult it is to see the derangement to the joint without X-ray or MRI testing. The more serious the damage, the easier it is to see the dislocation with the naked eye. Accurate grading for grades two through six is always done via X-ray or MRI, no matter how obvious the damage is with visual examination, reports Weaver and Dunn in "Treatment of Acromioclavicular Injuries, Especially Complete Acromioclavicular Separation." Even after the joint has healed and returned to normal function, a slight abnormality in shape is typical of AC injuries.
References
- "Journal of Athletic Training;" Acromioclavicular Joint Injuries; G. M. Beim, MD; July-September 2000
- "Journal of Bone and Joint Surgery;" Treatment of Acromioclavicular Injuries, Especially Complete Acromioclavicular Separation; J. K. Weaver, H. K. Dunn; 1972
