It begins as inflammation in the patellar tendon where it attaches to the patella and may progress by tearing or degenerating the tendon.
It is an overuse injury from repetitive overloading of the extensor mechansism of the knee. The microtears exceed the body's healing mechanism unless the activity is stopped.
The injury occurs in many athletes, but is most common in sports such as soccer, volleyball, or basketball which require explosive movements.
Early stages may be treated conservatively. Uncommonly it may require surgery to remove myxoid degeneration in the tendon. This is reserved for patients with debilitating pain for 6-12 months despite conservative measures.
Jumper's Knee by Bridget Mikysa, M.D., University of Washington Department of Radiology
Kaplan, Helms, Dussault, Anderson, and Major Musculoskeletal MRI 2001 .
Patellar tendinosis (jumper's knee): findings at histopathologic examination, US, and MR imaging. Victorian Institute of Sport Tendon Study Group. Radiology, 1996;200:821-827.
Long-term prognosis for Jumper's knee in male athletes: a prospective follow-up study American Journal of Sports Medicine, Sept-Oct, 2002 by Jyrki A. Kettunen, Martti Kvist, Erkki Alanen, Urho M. Kujala http://www.findarticles.com/p/articles/mi_m0918/is_5_30/ai_92519868