What is rotator cuff tendinosis (tendinopathy)? What are the consequences of labeling it as “tendinitis"? What are the consequences of labeling rotator cuff tendinopathy as a “partial rotator cuff tear”? What are the consequences of labeling rotator cuff tendinopathy as “impingement syndrome”?

Rotator cuff tendinosis, also known as “shoulder bursitis” or “impingement syndrome”, is the most common cause of shoulder pain in adults. 

Rotator cuff tendinosis is characterized by tendon degeneration (tendinopathy), leading to pain with movement. The supraspinatus tendon is usually the focus of the condition, and thus overhead activity is usually the motion that is particularly painful. 

Rotator cuff tendinosis is produced by a combination of repetitive use, poor blood supply to the distal end of the tendon, and generalized senescence (age-related degenerative change). 

Rotator cuff tendinosis is also commonly known as rotator cuff tendinitis. It would be more factually accurate to describe the condition as tendonosis, as the “-itis” suffix misleadingly suggests that the tendon is damaged by an inflammatory process, which is not the case. 

Because there is usually some fraying of the tendon seen in the setting of degeneration, rotator cuff tendinopathy may be read on an MRI as a “partial tear” of the cuff. That nomenclature is sub-optimal, because it suggests a condition inflicted by trauma (“tearing”). Also, to some, the name connotes something in need of repair. ("If it’s torn, you fix it.") If one encounters a diagnosis of a “partial thickness rotator cuff tear” in the setting of degenerative tendinopathy, the word “tear” is best interpreted as it would be in the idiom “wear and tear.”  

Impingement comes from the Latin word meaning “to press upon” and indeed, years ago, it was thought that tendinitis was caused by pressure from the acromion. This ‘pressing upon’ theory makes intuitive sense: the pain experienced by patients with rotator cuff tendinopathy is worse with overhead arm movement, the arm position in which the supraspinatus is more apt to be, well, impinged, by the acromion. Moreover, there seems to be an association between larger acromial spurs and the risk of tendonitis.  It remains unclear to what degree this clinical condition is related to actual compression by the acromion or impingement, and the etiology is likely multifactorial.

Additional points to think about

Referring to rotator cuff tendinosis as an “-itis” may in turn compel treatment aimed at anti-inflammation, which is not needed in the rotator cuff itself. The associated bursitis does have an inflammatory component, however.  Thus, the ideal pharmaceutical treatment (and mode of focal delivery) has not yet been defined.

Attributing rotator cuff tears to impingement (Figure 1) might lead to unnecessary acromioplasty procedures (Figure 2), a very popular operation years ago. In a commentary in the Journal of Bone and Joint Surgery [J Bone Joint Surg Am. 2005;87:1399], Dr. Bert Zarins claimed that the hypothesis that impingement causes rotator cuff tears “does not appear to have withstood the test of time. It is more likely that rotator cuff dysfunction results in upward displacement of the humeral head and causes impingement of the humeral head against the acromion with shoulder use rather than the reverse.”  If Dr Zarins is correct, that acromial spurs are formed in response to the tendonitis, then removing the spur alone may not have the desired clinical effect. Likewise, to the extent improvement is seen, the benefit may not be produced by a relieve-the-bony-compression mechanism.

The implications of using medical terms that express a theory of pathogenesis or necessary treatment, especially when that theory is questionable, are discussed here: In the Beginning was the Word JBJS: February 2006 - Volume 88 - Issue 2 - p 442-445 https://pubmed.ncbi.nlm.nih.gov/16452759/  

Figure 1: Supraspinatus tendon path to insertion on humerus as seen on MRI: it sure does look like the acromion (noted by the red star) can “impinge” on this tendon. Note also the heterogeneous signal (mixed light and dark color) in the tendon, the region identified by the yellow arrow. This clearly shows that the damage is internal (modified from Radiopedia https://radiopaedia.org/cases/supraspinatus-tendinosis-1>)
Figure 2: Illustration example of acromioplasty. Shown on the left is a “Y” view (true lateral) of the shoulder. In a Y view, the body of the scapula forms the base of the Y, and the spine of the scapula and the coracoid form the left and right limbs, respectively, of the top of the Y. The acromion is a continuation of the spine of the scapula (shown on the left limb of the Y, highlighted by the red arrow). This particular acromion is slightly hooked (with an anterior spur). Removal of the spur is depicted on the radiograph to the right. (The x-ray image before annotation is borrowed from https://radiopaedia.org/cases/normal-shoulder)