The trochlea is the groove of the femur at the knee joint, where the patella runs.

Page updated February 2024 by Dr Sheila Strover (Clinical Editor)

Illustration of the trochlea and patella as if looking down at the bent knee from the patient's perspective.


What is the importance of the trochlea?

The walls of the trochlea prevent the patella from subluxing or dislocating, and thus allow the lever action of the quads muscle group as it contracts, and thereby straightens and supports the knee.


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What is trochlear dysplasia?

severe trochlear dysplasia
Illustration of severe trochlear dysplasia, looking from the front of the knee, showing an abnormal bump instead of a groove.


In the condition of trochlear dysplasia, the groove may be flattened towards the upper part, or even heaped up into a bump, and that may make the patella unstable and prone to subluxation, or even dislocation.

The surgeon will evaluate the risk by analysing the X-rays.

  • Quote from peer-reviewed paper:

    "The Dejour classification is a well-known classification system of trochlear dysplasia evaluating the risk of patellar instability. However, many other methods have been described for evaluating trochlear dysplasia and patellar instability"

    Citation: Kazley JM, Banerjee S. Classifications in Brief: The Dejour Classification of Trochlear Dysplasia. Clin Orthop Relat Res. 2019 Oct;477(10):2380-2386. doi: 10.1097/CORR.0000000000000886. PMID: 31393338; PMCID: PMC6999944.

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How is subluxation identified?

When the patella subluxes, the patient may be aware of a 'catching' sensation as the knee straightens and goes into extension.

Subluxation occurs in extension because it is at the top of the trochlea that the groove is flatter.

The clinician may also be able to elicit a 'J-sign' as the knee goes into extension, when the patient lies on the coach with the leg over the side and actively bends and straightens the knee. Measurements from X-rays may also have indicated a high-riding patella, or patella alta possibly with some trochlear dysplasia.

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Can the trochlea become damaged in a dislocation event?

The bone of both the trochlea and the patella can become damaged during a patellar dislocation, and the medial patellofemoral ligament (MPFL) can become torn and incompetent, making the patient vulnerable to further dislocation events.

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Femoral groove
Trochlear groove
groove of femur

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

See biography...