Patellar instability is when the kneecap does not ride smoothly throughout its excursion in the underlying groove of the femur, popping over to one side, or even de-railing.

Page updated September 2023 by Dr Sheila Strover (Clinical Editor)

patellar subluxation


Patellar subluxation is when the patella rides up the lip of the femoral groove, but does not actually dislocate.

dislocation of the patella


Patellar dislocation is when the patella jumps right out of the underlying groove, creating an unsightly bulge at the front of the knee. It may reduce by itself, or the clinician may need to reduce it.


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 - when the kneecap is higher than normal and may fail to engage in the underlying groove when the knee is bent, or related to the trochlear groove, such as trochlear dysplasia - when the groove in the femur underlying the patella may be shallow or malformed. There may also be remote causes in the hip or the foot - such as rotational deformities of tibia or femur.

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Patella alta

This is a developmental anomaly when a patient is simply born with the kneecap too high, so that sometimes it does not properly engage with the underlying groove when the knee is bent.

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Trochlear dysplasia

This is also a developmental anomaly when a patient is born with the top of the femoral groove too shallow, totally flat, or very rarely even convex in shape. This makes it difficult for the patella to easily engage, and it may sublux or even dislocate.

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Rotational deformity of the long bones

Either or both of the long bones - tibia and femur - may have abnormal twist, which creates stress for the patella and its tendons. If there is already another underlying issue making the patellar unstable then this will aggravate the issue. The conditions are called tibial torsion and femoral anteversion.

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Forum discussions

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Peer-reviewed papers

  • Patellar Instability - a book outline

    Citation: Wolfe S, Varacallo M, Thomas JD, Carroll JJ, Kahwaji CI. Patellar Instability. 2023 Aug 4. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan–. PMID: 29494034.

    Quote - "most patients with patellar instability are aged 10 to 16 years old and female."....."A previous patellar dislocation is associated with the highest risk of persistent patellar instability later in life."

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Patellofemoral instability

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Dr Sheila Strover (Editor)
BSc (Hons), MB BCh, MBA

See biography...

Symptoms -

Causes of patellar instability -

Signs of underlying problems -

Surgery -

Video -

Dr Lars Blønd2017 - The J-Sign - by Dr Lars Blønd (Knee Surgeon)



Expert views -

Editor's interpretations of published papers -