The term tendinopathy refers to all diseases of the tendon.7 The suffix -pathie means disease or illness translated from the Greek. 8

Tendinitis

Tendinitis is defined as acute inflammation of tendon tissue due to small microtears.3 Typically, this occurs due to acute overuse of the musculotendinous unit.3 Previously, the term tendinitis was used to describe almost all tendon pain.2 Meanwhile, the term is most often used when swelling is present (indicating inflammation) after injury to the tendon.9 Other typical signs of tendinitis include hyperthermia and localized tenderness. The literature shows that often the cause of tendon disease is not inflammation but degenerative changes in the tendon, so the very often diagnosed tendinitis should actually be called tendinopathy or tendinosis. 3

A typical example of this is tennis elbow. The technical term for tennis elbow is lateral epicondylitis. The name epicondylitis suggests inflammation because of the ending. However, it is more commonly thought to be tendinosis, as no acute or chronic inflammation has been diagnosed in biopsies.3

Tendinosis

The term tendinosis refers to a degenerative change in the collagen of a tendon.3 Degeneration often results from repetitive loading of the tendon with insufficient recovery time.3 Pathologically, there is an increased presence of immature type III collagen in the tendon compared to healthy tendons with significantly more mature type I collagen. Type III collagen is always referred to as “immature collagen.” It is less stable and thinner than type I collagen. 10 Type I collagen is much more abundant in tendons than type III.11 Type III collagen is physiologically more abundant in the dermis and blood vessels,11 and pathologically in tendinopathies as discussed above.

In tendinosis, collagen fibers are also less continuously arranged and there is increased (neo- )vascularization, i.e., formation of new blood vessels.3 These vascular structures do not result in improved blood flow and thus are not associated with healing.3 One theory is that tendon pain is caused by this neovascularization, i.e., the new formation of nonfunctional “blood vessels.”

So what is the significance of the distinction between tendinitis and tendinosis? 

The goal of treatment for tendinitis is to reduce inflammation, typically with NSAIDs and cortisone. In the presence of tendinosis, however, NSAIDs and cortisone are contraindicated because of an association with reduced collagen regeneration. The distinction is also important in estimating recovery time. Tendinitis usually heals within 6 weeks whereas chronic tendinosis, in non-FQAD patients, takes 3-6 months, sometimes up to 9 months for regeneration. Tendons take 50-100 or even more days to form new collagen, depending on the source. 3,5 

Thus, it can be seen that even in non-fluoroquinolone-induced tendinopathy, regeneration can take a very long time.

In general, we can say that in tendinosis the goal is to stimulate collagen production and minimize the vicious cycle of repeated overuse, whereas in tendinitis the goal is to treat the inflammation and at best reduce scar tissue and adhesions.3 Overall, both terms tendinitis and tendinosis should be used only after proper histopathologic examination.4

In this context, it should be noted that the division into tendinitis and tendinosis is simplified. Often, both inflammatory and degenerative aspects are present in a tendinopathy. It is therefore to be understood rather as a spectrum. Also, it is thought that in tendinosis there is a “low grade inflammation”, but this is not manifested by the classic signs of inflammation such as hyperthermia, redness or swelling. 

Incidentally, it is interesting to note that tendinosis is often clinically silent, i.e., it does not cause pain.4,5 For example, in autopsies of 70-year-old individuals, tendinopathy is found in 30-50% of individuals.5 Frequently, the initial manifestation of tendinosis is a rupture. However, tendinosis may also occur together with painful paratendinopathy (that is, inflammation or damage to tissue surrounding the tendon).4 The question that naturally arises here in relation to FQAD is why the pain of, for example, the Achilles tendon is so severe after taking ciprofloxacin, levofloxacin, and the like, even though there is mostly no inflammation? Hint: Could it be more to do with the peripheral nervous system than the tendon itself?