Trochlear dysplasia is a condition of the knee where the  trochlear groove is abnormal, usually at its upper end where it is abnormally flat or even heaped into a 'bump'.

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

mild trochlear dysplasia with shallow trochlea

Illustration looking at the knee from the front, with the trochlear groove in profile at the top of the condyles. Shows mild trochlear dysplasia - with a shallow trochlear groove.

trochlear dysplasia with a flat upper trochlea with no groove

Same view illustrating a more severe situation where the upper trochlea has no groove at all.

trochlear dysplasia with a trochlear bump

Same view showing severe trochlear dysplasia. Instead of being convex, the trochlear at its upper end forms a concave 'bump', and the patella finds no comfortable groove with which to engage.


What causes trochlear dysplasia?

Trochlear dysplasia is simply something one is born with. Very rarely it can be attributed to damage to the patella in a young child.

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Trochlear dysplasia and kneecap instability

Trochlear dysplasia is commonly associated with an unstable patella (kneecap), and patellar subluxation or dislocation.

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Patellar instability near full extension

Classically with severe trochlear dysplasia the patient experiences there may be a visible subluxation near full extension, where the patella suddenly slips laterally. This is called the 'J-sign'.

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Forum discussions

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Peer-reviewed papers

  • Quote:

    "...the Dejour classification is unreliable and should not be used routinely in clinical practice...MRI can be used to evaluate...the lateral trochlear inclination...- found to be 93% sensitive and 87% specific for identifying patellar instability." Other valid measurements include "trochlear depth,...the lateral condyle index and patellotrochlear index.

    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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dysplastic trochlea
trochlear bump
shallow trochlea

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

See biography...