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?
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