Dislocation of the patella is when the kneecap (patella) comes right out of the groove in the thighbone (femoral groove) in which it normally rides.

kneecap well located in trochlear groove

Normal situation where the patella is well located in the underlying groove of the femur.

patellar dislocation

Patellar dislocation, where the patella has jumped completely out of the groove.

PetellardislocationChildMark

Dislocation of the patella is most common in young people (16-20 years) and most frequent in the female sex.

 

What allows the kneecap to dislocate?

Dislocation may follow a traumatic event, like a blow on the knee. Frequently, however, there is an underlying anatomical problem that predisposes the knee to patellar instability. In either case it is common to tear the medial patellofemoral ligament (MPFL) which then makes it easier for the patella to dislocate again. Most early surgery would be focused on addressing this problem and reconstructing the ligament. We have a Q&A looking into all the issues in more detail.

Synonyms: 
Kneecap dislocation
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The unstable patella

A discussion about patellar instability, the factors that may cause the patella to sublux or dislocate, and what can be done about it.

Editor's interpretation of a paper published in the medical literature.

A discussion of key concerns about dislocation of the patella.

This is the Editor's interpretation of a paper published in the orthopaedic literature in 2016.

An overview by Dr Lars Blond.

Easy-to-read interpretation of a 2012 medical article on primary traumatic patellar dislocation.

Patellar instability and dislocation

Discussing how the anatomy can affect the tendency to dislocate the patella.

Pages

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CLINICAL PUBLICATIONS

Primary traumatic patellar dislocation. Tsai C-H, Hsu C-J, Hung C-H and Hsu H-C. J Orthop Surg Res. 2012; 7: 21.

Current Concepts for Patellar Dislocation. Petri M, Ettinger M, Stuebig T, Brand S, Krettek C, Jagodzinski M and Omar M. Arch Trauma Res. 2015 Sep; 4(3): e29301.


See also -


eBook about the unstable patella

eBook - Factors contributing to an Unstable Patella

Easy to read ebook about kneecap dislocation.