The etiology of rotator cuff tendinitis is still under debate. The intrinsic, extrinsic, and overuse mechanisms have been proposed as etiological factors.
The degeneration in rotator cuff tendinitis occurs in response to chronic overuse of the tendon without giving it time to rest and heal, disrupting the balance between protective and regenerative mechanisms and overuse pathologic changes. The end result is weakness, pain, and loss of shoulder motion  .
The symptoms of rotator cuff tendinitis can be acute, occur after an injury to the shoulder, or be associated with overuse. Symptoms are often insidious in elderly patients. The pain is usually felt as a dull ache on the anterolateral part of the shoulder and is aggravated by overhead activities. It is worse on shoulder abduction or flexion between 60° and 120° and is frequently minimal or absent outside of this range. Passive abduction causes less pain, however, abduction against resistance may aggravate the pain. Feeling pain during the night is common, especially if the patient lies on the affected side.
Obtaining a detailed history is essential for ruling out other diagnoses such as referred pain from cervical spine or serious symptoms of cardiac origin. Shoulder examination should be systematical. The shoulder must be exposed entirely. Inspection, palpation, motor strength, the range of movement (ROM), and special tests should be performed.
Tenderness is often localized to the greater tuberosity and subacromial bursa. Active and passive ROM in all planes has to be evaluated as part of the physical examination. Strength assessment can help isolating the relevant muscles.
Two techniques have been described in the literature to test the supraspinatus muscle:
Although both of these techniques activate the supraspinatus muscle, neither of them can truly isolate it since other muscles are also active during the evaluation . Subtle weakness during either position can represent early degeneration in the rotator cuff.
Plain radiography of the shoulder in a true anteroposterior (AP) view, an axillary view, and supraspinatus outlet view is very helpful in the diagnosis of rotator cuff tendinitis. Magnetic resonance imaging (MRI) is extremely sensitive and specific while being non-invasive. It has the ability to detect the characteristics, size, and location of the pathology. Ultrasonography can also be employed to evaluate the rotator cuff muscles. it is less costly but as it is operator-dependent, the sensitivity and specificity of the results obtained may vary    .