In patellofemoral instabilty the patella (kneecap) changes its relationship to the groove of the femur bone (thighbone) and may partially or fully de-rail.

Subluxation.

Dislocation.
Causes of patellofemoral instability
The patella may be unstable for a number of reasons. There may be local reasons related to the patella, such as patella alta, or related to the trochlear groove, such as trochlear dysplasia. There may also be remote causes in the hip or the foot - such as rotational deformities.
Synonyms:
patellar instability
kneecap instability
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FORUM DISCUSSIONS
Please Help - Patella Dislocation (Subluxation).
Constant maltracking/subluxation?
Opinions on Braces for Subluxation/Dislocation.
CLINICAL PUBLICATIONS
Modern management of patellar instability. Rhee S-J, Pavlou G, Oakley J, Barlow D and Haddad F. Int Orthop. 2012 Dec; 36(12): 2447–2456.
See also -
- J sign
- Patellar apprehension test
- Patella alta
- Patellar brace
- Patellar dislocation
- Patellar subluxation
- Trochlear dysplasia
- TT-TG Distance