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When An Shoulder Separation Isn't Just an AC Joint Injury

Many people, especially athletes, are familiar with the term shoulder separation. In medical terms, it's called an acromioclavicular or AC joint injury. The joint is located along the front of the shoulder where the clavicle (collar bone) meets the acromion (the bone that comes across the top of the shoulder from off the shoulder blade).

When the injury is severe enough, not only does the AC joint separate, but other damage can occur in and around the joint, too. That's the topic of this article. How often do these associated shoulder injuries occur? What type of injuries are they?

AC joint separation or dislocation is the result of direct trauma (usually from a fall) on the shoulder or arm when it is next to the body. Alternately, falling on an outstretched hand or elbow can cause the same injury. The mechanism is more indirect as force through the hand or elbow transfers to the shoulder. With enough force, the head of the humerus is pushed upward -- right into the acromion.

The majority of injuries occur during activities such as bike accidents, skiing or snowboarding, and other types of falls. Other activities leading to AC joint dislocation include soccer, motorcycle accidents, ice hockey, judo accidents, or horseback riding injuries.

It's not uncommon for the associated injuries to be missed at the time of the diagnosis for AC joint separation. The patient's shoulder is painful and range-of-motion is limited. So clinical testing is also limited. Without an arthroscopic exam, no one knows there are other problems. But once the AC joint has been treated, the patient with continued shoulder pain may need a second look.

Patients with severe AC joint separation may be spared the delay in recovery with early diagnosis of any associated injuries. In this study, preoperative testing for the presence of associated shoulder injuries was performed in cases of more severe AC separations (grade III through V). A special set of X-rays was taken called a trauma series. During the surgical procedure to repair the AC joint separation, the surgeon examined the joint carefully for other types of injuries.

Of the 77 patients tested, 18 per cent had intra-articular (inside the joint) injuries. Most of those injuries were superior labral anterior posterior (SLAP) lesions. Rotator cuff tears and fractures were the other most common types of intra-articular injuries.

A SLAP lesion refers to a tear along the entire upper surface of the labrum (fibrocartilage rim around the shoulder socket). Only the most severe type of AC joint dislocation (Type V) had a SLAP lesion. And only one of those SLAP lesions in this study was severe. All injuries (including the SLAP lesions) were repaired during the surgery to reconstruct the AC joint. The authors provide detailed descriptions of the surgeries performed.

The authors conclude that all patients with AC joint injuries should be evaluated carefully for additional injuries. Any force strong enough to dislocate the AC joint is often strong enough to damage other areas as well. MRIs may be useful but the most sensitive and specific test for this problem is arthroscopy.

Anyone with a high grade AC joint injury who continues to experience pain and loss of motion after surgical treatment should be further evaluated. The surgeon should look for the presence of other intra-articular injuries, especially labral tears, rotator cuff tears, and fractures. Post-operative MRIs using an intra-articular contrast dye give the best information. MRIs aren't needed when the surgeon assesses the entire shoulder complex arthroscopically in the acute phase.

References: Thomas Tischer MD, et al. Incidence of Associated Injuries with Acute Acromioclavicular Joint Dislocations Types III Through V. In The American Journal of Sports Medicine. January 2009. Vol. 37. No. 1. Pp. 136-139.

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