When a small tear extends either forwards or backwards it will extend from the supraspinatus tendon (SSP) into the infraspinatus tendon (ISP), and often into the upper part of the subscapularis tendon (SSC). Between the SSP and SSC is the biceps tendon. In the event that the front edge of the SSP tendon, or the top edge of the SSC tendon, tears the biceps tendon will start to change, in a fascinating example of how nature adapts to compensate for the loss of the main tendons. The biceps will often get broader, enlarging up to two or three times its original size. Unfortunately it also starts to split on its undersurface, and then it too starts to become painful.
If it then tears the biceps muscle drops into the arm (a “pop-eye” biceps), and the humeral head displaces upwards under the acromion into a new fixed location where it tends to jam, causing a loss of movement. At this stage the muscles of the RC are often very withered (“atrophic”), and replaced by fatty tissue (“fatty degeneration”): if this has happened then the tendon tears are nearly always irreparable, and other strategies for management need to be considered, including muscle transfers and special shoulder replacements.
The reader is referred to the associated reading material: “Rotator cuff disease”, an article first published in British Orthopaedic News, a publication of the British Orthopaedic Association.