Why does tendon overuse sometimes evolve into a tendinitis and sometimes evolve into a tendinosis?

Question

From what I read, tendon overuse sometimes evolves into a tendinitis and sometimes it evolves into a tendinosis.

https://en.wikipedia.org/wiki/Tendinitis:

Individual variation in frequency and severity of tendinitis will vary depending on the type, frequency and severity of exercise or use.

https://en.wikipedia.org/wiki/Tendinosis:

The general opinion is that tennis elbow tendinosis is due to tendon overuse, and failed healing of the tendon. In addition, the extensor carpi radialis brevis muscle plays a key role.[4][5]

• [4] du Toit, C; Stieler, M; Saunders, R; Bisset, L; Vicenzino, B (2008). "Diagnostic accuracy of power Doppler ultrasound in patients with chronic tennis elbow". British Journal of Sports Medicine 42 (11): 572–576. doi:10.1136/bjsm.2007.043901. ISSN 0306-3674.
• [5] Nirschl RP, Ashman ES (2004). "Tennis elbow tendinosis (epicondylitis)". Instr Course Lect 53: 587–98. PMID 15116648.

How comes the same cause leads to two different injuries? What factors come into play?

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The incidence of tendinitis seems to be much lower than the incidence of tendinosis:

http://therxreview.com/really-tendonitis/:

Cases of tendonitis are actually very rare and pain goes away completely in several days to six weeks with simple treatments such as taking Advil (Ibuprofen) or Alleve (Naproxen).

http://www.bodybuilding.com/fun/drryan13.htm:

Tendonitis is actually very rare; the tendon is more likely to have a condition known as tendonosis

Answer 1

Here's my (non professional take), from reading and listening to Jill Cook and Steven Low:

TL;DR: Tendonitis/tendinosis are obsolete terms. If someone uses them, they are probably 10 or more years behind the times. The current one is tendinopathy. Tendinopathies provoke little or no inflammation, and we still have a long way to understand them. In particular, every person's tendons suffer disrepair and tears during their lifetime, but only a few feel pain --which can be present with little or no damage.

Steven Low (Overcoming Gravity, Bad Posture, and soon, Tendonitis)

http://stevenlow.org/overcoming-tendonitis/

• Connective tissue has less blood supplies than muscles, so it can't keep up if there's overuse.

• Tendinopathies provoke little or no inflammation. There's not really such a thing as tendinitis.

• An overloaded tendon goes into reactive tendinopathy: pain and microscopic degenerative damage that can be fully healed.

• If stress continues, normal regeneration processes can't keep up and tendon disrepair occurs, where collagen fibres are deposited out of alignment, leading to reactive on degenerative tendinopathy. An MRI can show small damage.

• Further damage produces a degenerative tendinopathy: the tendon fibers regenerate haphazardly which can't be reversed. Again, no inflammation, but large scale disruptions on tendon tissue, and the possibility of tearing under heavy loads.

  Jill Cook et all:

Current trends in tendinopathy management

Tendinopathy (pain and dysfunction in a tendon) is a prevalent clinical musculoskeletal presentation across the age spectrum, mostly in active and sporting people. Excess load above the tendon's usual capacity is the primary cause of clinical presentation. The propensity towards chronicity and the extended times for recovery and optimal function and the challenge of managing tendinopathy in a sporting competition season make this a difficult condition to treat. Tendinopathy is a heterogeneous condition in terms of its pathology and clinical presentation. Despite ongoing research, there is no consensus on tendon pathoetiology and the complex relationship between tendon pathology, pain and function is incompletely understood. The diagnosis of tendinopathy is primarily clinical, with imaging only useful in special circumstances. There has been a surge of tendinopathy treatments, most of which are poorly supported and warrant further exploration. The evidence supports a slowly progressive loading program, rather than complete rest, with other treatment modalities used as adjuncts mainly targeted at achieving pain relief.

Conference video: Jill Cook - Current concepts in tendinopathy rehabilitation