Rotator cuff disease
Between the undersurface of the tip of the shoulder blade (the acromion) and the outer surface of the rotator cuff muscles is a filmy layer called the bursa. Because it is under the acromion it is known as the subacromial bursa, although it extends under the deltoid muscle quite a long way.
When the bursa becomes inflamed through rubbing or chemical irritation from the degenerating tissues, the lining membrane becomes thickened.
The body’s natural response to rubbing or inflammation is to produce fluid as a sort of buffer between the rubbing surfaces. With the collection of more fluid and increasing thickening of the membrane the space available for the moving surfaces diminishes, and the membrane starts to bend or buckle, causing all the nerve endings in the membrane to be stimulated: this is the sharp catching pain felt when the arm is moved away from the side of the body, or taken behind the body. This is called ‘impingement’ - not a great word for it, but one which has stuck in the literature of Rotator Cuff (RC) disease - and the phenomenon of buckling can be seen on an ultrasound scan (USS) of the shoulder very clearly.
Impingement is always caused by something - that is, impingement is not a diagnosis in itself - so we always look for the reason(s) behind the symptom of impingement (the sharp pain): most often it is due to the wear-and-tear of the RC itself. As the RC starts to weaken, it cannot cause the concavity compression (see above), so the humeral head starts to slide upwards on the glenoid, in turn pushing the RC tendons up under the part of the scapula which lies just above the RC and bursa (the acromion, or the point of the shoulder).