Patellar instability

Written by Dr Sheila Strover on March 14, 2025

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.

Illustration looking down at one's knee, to show the position of the patella during a subluxation event.
Patellar subluxation is when the patella rides up the lip of the femoral groove, but does not actually dislocate.
Illustration looking down at the knee to show the position of the patella during a dislocation event.
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.

"....[Patellar instability] is a multifactorial problem. Many problems affecting the bone structure and muscles morphologically and functionally can cause it....[M]ain instability factors [include] trochlear dysplasia, patella alta, tibial tuberosity–trochlear groove (TT-TG) distance, and patellar tilt...."

Tugrul Ormeci et al

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Trochlear dysplasia as a cause of instability

Trochlear dysplasia 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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Patella alta as a cause of instability

Patella alta 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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Poor tendon alignment as a cause of instability

When the patellar tendon is tugged to one side of the knee by poor alignment of the trochlear groove and the tibial tubercle, patellar instability may result. The distance by which they are out of alignment is measured by the MRI calculation of TT-TG distance.

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Patellar tilt as a cause of instability

Patellar tilt - where the patella lists over usually to the lateral side - may lead to patellar instability.

This is usually associated with tight retinacular tissues on the same side.

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.

A combination of deformities may result in the condition known as miserable malalignment.

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A-Z Keywords -

(a) Proximal realignment (eg Medial reefing)

(b) Distal realignment (eg Elmslie-Trillat procedure, Maquet procedure, Galeazzi procedure, Fulkerson osteotomy, MPFL reconstruction)

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