Many of us have experienced pain in the shoulder. It is often attributed to some deep-seated joint abnormality, and we wonder if it is the dreaded rotator cuff tear. However, one of the most common causes of shoulder pain is premature degeneration from overuse of the acromioclavicular (AC) joint, the so-called “joint on top”. If there is pain on top, with grinding or crunching, odds are it is the AC joint that is causing the problem.
Believe it or not, almost no one over age 20 has a normal AC joint. It seems that this joint was not meant to sustain the amount of activity that we humans undertake throughout our lives. In particular, the advent of backpacks, as well as modern sport, has put undue stress on this structure.
Since almost everyone over age 20 has an abnormal AC joint, this post will attempt to address (1) when this joint is the cause of the symptoms, (2) when it needs to be addressed, (3) what exacerbates disease in this joint, and (4) what imaging findings cinch the diagnosis.
You are shown three coronal images of the left shoulder of a 39-year-old male. He presents with pain, limited range of motion, and arm weakness.
Q1 – What symptoms should point you in the direction of the AC joint as the cause of discomfort?
Q2 – What activities in young athletes particularly aggravate this problem?
Q3 – In image 2, see if you can locate the swelling in the joint, edema of bone, and erosions. Since everyone has some of these, how can an imager know that this joint is actually the cause of the patient’s symptoms?
A1 – Pain on “top”; motion specific pain with the arms over the head; crunching or grinding noises that you can feel and sometimes hear.
A2 – Weightlifting (especially heavier weights), and performing the activities of bench pressing or military pressing. Most young men do this in the gym. A tip-off that this activity is occurring is the size of the shoulder muscles, especially the supraspinatus, trapezius and pectoralis major.
A3 – Check out image 5 for identification of some swelling (pink arrow), erosions and/or edema of bone (blue arrow), and further erosions (yellow arrow). Now look at image 6, the standard T2 sequence. In patients that have symptomatic acromioclavicular joint disease, there are often tip-offs on the less sensitive water weighted sequence:
(a) The swelling or high signal in the joint persists on the T2 weighted image as a sign of active disease (image 6, orange arrow).
(b) In younger patients there is often no other evidence of any other pathology or an alternative explanation.
(c) Finally, the patient’s muscularity suggests that weightlifting activity is occurring and exacerbating this condition, and corroborating your diagnosis.
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