Trochlear dysplasia

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.

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

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 and kneecap instability
• Classifying trochlear dysplasia
• Patellar instability near full extension
• Forum discussions

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.

Peer-reviewed paper

• Quote from peer-reviewed paper:

  "....the shape of the trochlea is largely maintained during growth and development and does not change significantly after birth....

  Citation: Onor GI Jr, Yalcin S, Kaar SG, Pace JL, Ferrua P, Farrow LD. The Evaluation of Trochlear Osseous Morphology: An Epidemiologic Study. Orthop J Sports Med. 2021 Apr 2;9(4):2325967121994548. doi: 10.1177/2325967121994548. PMID: 33869646; PMCID: PMC8020235.

End of paper

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Trochlear dysplasia and kneecap instability

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

Peer-reviewed paper

• Quote from peer-reviewed paper:

  "....trochlear dysplasia has been proposed to be one of the most significant factors implicated in recurrent patellar instability....

  Citation: Onor GI Jr, Yalcin S, Kaar SG, Pace JL, Ferrua P, Farrow LD. The Evaluation of Trochlear Osseous Morphology: An Epidemiologic Study. Orthop J Sports Med. 2021 Apr 2;9(4):2325967121994548. doi: 10.1177/2325967121994548. PMID: 33869646; PMCID: PMC8020235.

End of paper Quick links

• Patellofemoral subluxation

• Patellofemoral dislocation

End of links

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Classifying trochlear dysplasia

Classification systems exist to document the degree of abnormality.

Peer-reviewed paper

• Quote from peer-reviewed paper:

  "...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.

End of paper Quick links

• Patellar dysplasia

• Lateral trochlear inclination angle
• Patellotrochlear engagement index

End of links

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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

• "What's This Mean?"

  Patients tease out the issues of trochlear dysplasia.

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