The peel-back sign refers to what is seen using an arthroscope to diagnose a SLAP lesion of the shoulder. A SLAP lesion is a tear of the riangular-shaped piece of cartilage on the rim of the shoulder socket. The rim is called the labrum.
The biceps muscle is divided into two main parts. The long head of the biceps tendon comes up the arm from the elbow and over the top of the head of the humerus. It attaches just above the labrum and helps hold the labrum in place. This is called the biceps
anchor.
This area is at risk from injuries related to overuse and traumatic injury. If the labrum is torn but the biceps anchor isn’t, then repetitive overhead throwing puts force on the labrum through the biceps. The added load can keep the injury from healing.
The peel-back mechanism is a likely cause of one type of SLAP lesion. As the shoulder is rotated outward (like a pitcher reaching back to throw the ball), the base of the biceps twists. An increased load goes to the labrum.
The actual peel-back sign can only be seen visibly during the arthroscopic exam. The arm is placed into 70-90° of abduction (away from the body) and then turned outward or externally rotated more and more. During this motion, the base of the biceps twists.
A torsional load is transmitted to the superior labrum. If there’s a tear in the back half of the labrum, this twisting of the biceps actually peels back the labrum. Surgery is needed to stitch the labrum back in place. The patient can’t externally rotate the arm for three weeks while the labrum heals.