WHAT IS THE ROTATOR CUFF AND WHAT GOES WRONG WITH IT?

There are 4 deep muscle tendon units that are interwoven to some degree and their collective job is to take the torque out of the shoulder to keep the humeral head centeredon the glenoid and also to compress the joint to provide stability.  These 4 deep muscle tendon units are collectively called the rotator cuff.  Tears most commonly occur in the tendon near the bony attachment on the humerus (upper arm bone).  See the illustration at the left.

Again, in my practice, the most common causes of cuff tears are 1. A fall onto the shoulder or arm, 2. Repeated overuse (like being a mechanic or swimmer), 3. Being yanked by a dog on a leash, 4. No identifiable cause.  It is amazing that up to 40% of people will have some degree of cuff tear in their life.