What Is Trochlear Dysplasia?
The trochlea is the groove at the front of the femur (thighbone) that the patella (kneecap) slides through as the knee bends and straightens. In a normal knee, this groove is deep enough to keep the patella tracking smoothly in its channel throughout the full range of motion. Trochlear dysplasia is a condition in which the trochlear groove is abnormally shallow, flat, or even convex (dome-shaped), reducing the bony constraint that keeps the patella in place.
Trochlear dysplasia is one of the most important anatomic risk factors for patellar dislocation and recurrent patellar instability. When the groove is shallow or absent, the kneecap has less bony support to prevent it from sliding out of position, particularly during activities that involve knee flexion, pivoting, or sudden changes in direction.
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Dejour Classification: Types A Through D
Trochlear dysplasia is classified using the Dejour system, which grades the severity of the groove abnormality based on imaging findings. This classification helps guide treatment decisions and surgical planning.
| Type | Description | Crossing Sign | Clinical Significance |
|---|---|---|---|
| Type A | Shallow trochlear groove, relatively preserved shape | Present | Mildest form; groove is shallow but still concave. May contribute to instability when combined with other risk factors. |
| Type B | Flat or convex trochlea with a supratrochlear spur | Present | The groove is essentially absent. Significant contributor to patellar instability. |
| Type C | Asymmetry of the trochlear facets with a double contour sign | Present | The lateral facet is hypoplastic (underdeveloped), creating asymmetry. Less common. |
| Type D | Cliff pattern — vertical join between facets, with a prominent supratrochlear spur and double contour | Present | Most severe form. The trochlea has a prominent bump rather than a groove. Strongly associated with recurrent dislocation. |
The crossing sign is a key radiographic finding on lateral X-rays. In a normal knee, the line representing the floor of the trochlear groove does not cross the anterior contour of the lateral femoral condyle. When the groove is shallow or absent, these lines cross — this is the crossing sign, and it is present in all types of trochlear dysplasia.
Symptoms of Trochlear Dysplasia
Trochlear dysplasia itself does not always cause symptoms. Many people have mild dysplasia and are completely unaware of it. However, when the groove is significantly abnormal, it predisposes the knee to patellar instability, and it is the instability episodes that produce symptoms.
Symptoms associated with trochlear dysplasia and resultant patellar instability include:
- Recurrent episodes of the kneecap slipping out of place (patellar dislocation or subluxation)
- A sensation of the kneecap “shifting” or being unstable, especially during squatting, stairs, or pivoting
- Anterior knee pain (pain at the front of the knee), particularly with prolonged sitting, stairs, or loaded knee flexion
- Swelling after instability episodes
- Apprehension — a feeling of anxiety or guarding when the kneecap is pushed laterally during examination
- Crepitus (grinding or crunching sensation) behind the kneecap
- Activity limitation due to pain or instability
In patients with high-grade dysplasia (Types B, C, or D), the threshold for dislocation is much lower. These patients may dislocate with minimal trauma or even during everyday activities rather than requiring a significant sports injury.
Causes and Risk Factors
Trochlear dysplasia is a developmental anatomic variant, meaning it is determined by the way the knee joint forms during growth. It is not caused by injury, overuse, or lifestyle factors. Research suggests there is a genetic component, as trochlear dysplasia is more common in patients with a family history of patellar instability.
Trochlear dysplasia rarely exists in isolation. It is frequently found alongside other anatomic risk factors for patellar instability, including patella alta (a high-riding kneecap), an increased tibial tubercle-trochlear groove (TT-TG) distance, MPFL insufficiency, ligamentous laxity, and lower extremity malalignment such as femoral anteversion or genu valgum (knock knees). The combination of multiple risk factors significantly increases the likelihood of patellar dislocation and the risk of recurrence after an initial episode.
Diagnosis and Imaging
Trochlear dysplasia is identified through imaging studies, typically as part of a comprehensive workup for patellar instability.
Lateral X-ray: The crossing sign, supratrochlear spur, and double contour sign can all be identified on a true lateral radiograph of the knee. This is the initial screening tool for trochlear dysplasia.
CT scan: Axial CT images provide a detailed cross-sectional view of the trochlear groove and are used to measure the sulcus angle (the angle formed by the two trochlear facets), the trochlear depth, and the TT-TG distance. CT is particularly valuable for surgical planning.
MRI: In addition to evaluating trochlear morphology, MRI provides information about the MPFL (whether it is intact or torn), articular cartilage status, patellar height, and any associated bone bruising or loose bodies from prior dislocation episodes.
A complete assessment of patellar instability requires evaluation of multiple anatomic factors — not just the trochlea — including the TT-TG distance, patellar height index, and limb alignment. This comprehensive approach is central to Dr. Burnham’s philosophy on patellar instability management, as outlined in his 2021 editorial in Arthroscopy.
Trochlear Dysplasia and Patellar Instability
Trochlear dysplasia is found in up to 85% of patients with recurrent patellar dislocation, making it the single most prevalent anatomic abnormality in this population. In Dejour’s landmark 1994 study, 96% of patients with objective patellar instability had a positive crossing sign on lateral X-ray.
The trochlear groove normally provides the primary bony restraint to lateral patellar displacement during the first 20 to 30 degrees of knee flexion. When this groove is shallow or absent, the patella relies more heavily on soft tissue stabilizers, particularly the medial patellofemoral ligament (MPFL). Once the MPFL is torn or stretched — as occurs during a dislocation event — the kneecap has very little remaining restraint, and recurrent instability becomes highly likely.
This is why addressing only the soft tissue (through MPFL reconstruction alone) may not be sufficient in patients with significant trochlear dysplasia. If the underlying bony anatomy is not corrected, the reconstructed MPFL is placed under excessive stress and may eventually fail. The case presented at the 2026 ATPPS conference illustrates this point: a patient whose initial MPFL repair failed because the underlying bony abnormalities were not addressed in the first surgery.
Treatment
Non-Surgical Management
For patients with mild trochlear dysplasia who have experienced a first-time patellar dislocation, non-surgical management is often the initial approach. This includes physical therapy focused on strengthening the vastus medialis obliquus (VMO) and hip stabilizers, patellar stabilization bracing, and activity modification to avoid positions and movements that provoke instability. The goal is to optimize the dynamic (muscular) stabilizers of the patella to compensate for the deficient bony stability.
However, non-surgical treatment has important limitations in the setting of significant trochlear dysplasia. No amount of muscle strengthening can create a trochlear groove where one does not exist. In patients with high-grade dysplasia (Types B through D) and recurrent instability, conservative management alone has a high failure rate.
Surgical Management
Surgical treatment for patellar instability associated with trochlear dysplasia is individualized based on the specific combination of anatomic abnormalities present in each patient. Surgical options include:
- MPFL reconstruction — Restores the primary soft tissue restraint to lateral patellar displacement. This is the most commonly performed procedure for patellar instability but may not be sufficient alone in patients with significant bony abnormalities.
- Tibial tubercle transfer (osteotomy) — Moves the patellar tendon attachment point to improve patellar tracking and reduce the TT-TG distance. Often combined with MPFL reconstruction.
- Trochleoplasty — A procedure that reshapes the trochlear groove to create a more normal channel for the patella (discussed in detail below).
- Lateral release or lengthening — Addresses excessive lateral soft tissue tightness that contributes to lateral patellar tracking.
In many cases, a combination of procedures is needed to address all of the contributing factors simultaneously. The specific surgical plan is determined by the patient’s individual anatomy as evaluated through clinical exam, X-rays, CT, and MRI.
Trochleoplasty: Reshaping the Groove
Trochleoplasty is a surgical procedure that deepens and reshapes the trochlear groove to improve patellar tracking. It is considered in patients with high-grade trochlear dysplasia (Types B, C, or D) who have recurrent patellar instability that has failed or is unlikely to succeed with soft tissue procedures alone.
The most commonly described technique involves creating a new groove by elevating the articular cartilage, reshaping the underlying bone, and laying the cartilage back down into the newly created channel. This restores the bony constraint that is essential for stable patellar tracking in early knee flexion.
Trochleoplasty is a technically demanding procedure and is typically reserved for carefully selected patients with documented high-grade dysplasia and recurrent instability. It is often performed in combination with MPFL reconstruction and/or tibial tubercle transfer to address all contributing anatomic factors. Published results have demonstrated improved patellar stability, reduced redislocation rates, and high rates of return to sport in appropriately selected patients.
Trochlear Dysplasia and Patellar Instability Treatment in Baton Rouge, Louisiana
Patients in the Baton Rouge area with trochlear dysplasia and recurrent patellar instability have access to comprehensive evaluation and surgical treatment at Ochsner-Andrews Sports Medicine Institute. Dr. Burnham performs a full anatomic workup for every patellar instability patient — including TT-TG distance measurement, trochlear morphology assessment, patellar height evaluation, and limb alignment analysis — to determine whether MPFL reconstruction, tibial tubercle transfer, trochleoplasty, or a combination of procedures will provide the best long-term outcome. Dr. Burnham treats patients from across Louisiana and Mississippi, including those traveling from New Orleans, Lafayette, Hammond, Lake Charles, Gonzales, Prairieville, Denham Springs, Central, Zachary, and the Mississippi Gulf Coast.
FAQ: Frequently Asked Questions
What is trochlear dysplasia of the knee?
Trochlear dysplasia is a condition in which the groove at the front of the femur (thighbone) that the kneecap slides through is abnormally shallow, flat, or dome-shaped. This reduces the bony support for the patella and significantly increases the risk of the kneecap dislocating or subluxating (partially slipping out of place). It is one of the most common anatomic findings in patients with recurrent patellar instability.
Is trochlear dysplasia something you are born with?
Yes. Trochlear dysplasia is a developmental variant in how the knee joint forms during growth. It is not caused by injury, activity, or lifestyle. Research suggests a genetic component, as it is more common in families with a history of patellar instability. It is typically identified after a patellar dislocation event prompts imaging studies.
Can trochlear dysplasia be fixed without surgery?
Trochlear dysplasia is a structural bony abnormality, so it cannot be changed without surgery. However, not all patients with trochlear dysplasia need surgery. For patients with mild dysplasia and infrequent instability, physical therapy to strengthen the VMO and hip muscles, combined with patellar stabilization bracing, can help manage symptoms. Surgery is typically recommended when conservative treatment fails and the patient experiences recurrent patellar dislocations.
What is the crossing sign?
The crossing sign is a finding on a lateral (side-view) X-ray of the knee. In a normal knee, the line representing the bottom of the trochlear groove stays behind the anterior contour of the lateral femoral condyle. When the groove is shallow or absent (as in trochlear dysplasia), the trochlear floor line crosses over the condylar contour. This crossing sign is present in all grades of trochlear dysplasia and is considered a key diagnostic indicator.
What is trochleoplasty?
Trochleoplasty is a surgical procedure that reshapes the trochlear groove to create a deeper, more normal channel for the patella to track in. It is typically reserved for patients with high-grade trochlear dysplasia (Dejour Types B, C, or D) who have recurrent patellar instability. The procedure involves elevating the articular cartilage, recontouring the underlying bone, and repositioning the cartilage into the newly created groove. It is often performed alongside MPFL reconstruction and/or tibial tubercle transfer.
How does trochlear dysplasia cause kneecap dislocations?
The trochlear groove normally acts as a bony track that keeps the kneecap centered as the knee bends. This bony constraint is most critical in the first 20 to 30 degrees of knee flexion, when the kneecap is engaging the groove. When the groove is shallow or absent (as in trochlear dysplasia), the kneecap has less bony support and is more likely to slide laterally and dislocate, particularly during activities that involve knee bending, cutting, or pivoting.
Can MPFL reconstruction alone fix patellar instability caused by trochlear dysplasia?
In patients with mild trochlear dysplasia (Type A), MPFL reconstruction alone is often sufficient to restore patellar stability. However, in patients with more severe dysplasia (Types B through D), the reconstructed MPFL may be placed under excessive stress because the underlying bony architecture does not support normal patellar tracking. In these cases, additional procedures such as tibial tubercle transfer or trochleoplasty may be needed to address the bony contribution to instability. This is why a comprehensive anatomic assessment is critical before planning surgery.
Does trochlear dysplasia cause anterior knee pain?
Trochlear dysplasia can contribute to anterior knee pain (pain at the front of the knee) because abnormal patellar tracking increases contact stress on the articular cartilage of the kneecap and trochlea. Patients may experience pain with stairs, prolonged sitting, squatting, or any activity that loads the patellofemoral joint. However, anterior knee pain has many possible causes, and trochlear dysplasia is just one contributing factor. A thorough clinical and imaging evaluation is needed to determine the underlying cause.
References & Recommended Reading
- Burnham JM. Editorial Commentary: Treating Patients With Recurrent Patellar Instability With an Individualized, Anatomic Approach Is Needed: Learn From the Anterior Cruciate Ligament. Arthroscopy. 2021;37(5):1680-1682. doi:10.1016/j.arthro.2021.01.018. PMID: 33896516.
- Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surg Sports Traumatol Arthrosc. 1994;2(1):19-26. doi:10.1007/BF01552649. PMID: 7584171.
- Lippacher S, Dejour D, Elber A, et al. Observer Agreement on the Dejour Trochlear Dysplasia Classification: A Comparison of True Lateral Radiographs and Axial Magnetic Resonance Images. Am J Sports Med. 2012;40(4):837-843. doi:10.1177/0363546511433028. PMID: 22238057.