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 May 2024 by Dr Sheila Strover (Clinical Editor)
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 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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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.
Trochlear dysplasia and kneecap instability
Trochlear dysplasia is commonly associated with an unstable patella (kneecap), and patellar subluxation or dislocation.
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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.
Classifying trochlear dysplasia
Classification systems exist to document the degree of abnormality.
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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.
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'.
Forum discussions
- "What's This Mean?"
Patients tease out the issues of trochlear dysplasia.
Relevant material -
- Trochlear groove
- J-sign
- Patellar instability
- Patellar subluxation
- Patellar dislocation
- Trochleoplasty
- Arthroscopic trochleoplasty
2008 - reviewed 2023 - Patellofemoral pain - by Dr Ronald Grelsamer (Knee Surgeon)
eBook - Factors contributing to an Unstable Patella
by Dr Lars Blønd (Knee Surgeon). Free to download
Journal interpretation - 2023 - Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode
- Authors: Maggioni DM et al. and summarised for readers by Dr Sheila Strover (Clinical Editor)