The trochlear groove is the concave surface where the patella (kneecap) makes contact with the femur (thighbone). Also called the 'trochlea'.

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

the trochlear groove from above


As the knee bends, the patella engages with the groove and is contained by its steep walls.

the trochlear groove from the side


Towards the top of the trochlear groove the walls are flatter. When the knee is in extension (straight) and the patella in the upper part of the groove, it is more easily able to disengage (sublux).



What is the purpose of the trochlear groove?

The side walls of the trochlear groove help to restrain the patella as the knee bends and straightens. The lower part of the patella is tethered to the tibia bone, while the upper part is continuous with the quadriceps muscle group. When the quadriceps (quads) contract and relax, the joint straightens and bends, and the patella is restrained within the groove during that excursion.

If the groove for some reason is not 'doing its job' then the patella may ride up, and even over, the walls of the groove - subluxation and dislocation.

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Trochlear dysplasia

Sometimes the groove is abnormal - this is called trochlear dysplasia. The abnormality may be a general flattening, or a flattening near the upper end, or it may even be convex at the upper end - and these abnormalities may result in patellar instability.

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What is patellar instability?

Patellar instability is when there are episodes of subluxation or diloacation of the patella.This may occur with activities or the examining practitioner may be able to'pop the kneecap out' when the knee goes into extension during an examination..

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Forum discussions

  • Please help a newbie!

    A patient with a shallow trochlea tries to understand the issues of potential surgery.

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Peer-reviewed papers

  • Quote:

    "Imaging is essential in the assessment of patellar instability..."

    Citation:Batailler C, Neyret P. Trochlear dysplasia: imaging and treatment options. EFORT Open Rev. 2018 May 21;3(5):240-247. doi: 10.1302/2058-5241.3.170058. PMID: 29951262; PMCID: PMC5994618.

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femoral groove
femoral trochlea

Explaining how the kneecap glides within the underlying groove of the femur bone.


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

See biography...