Normal Trochlear Depth Size on MRI: Knee Reference
The femoral trochlear depth is a quantitative MRI measurement reflecting the concavity of the trochlear groove, which guides patellar tracking throughout knee flexion. Accurate assessment of trochlear depth is essential because a shallow or convex trochlea is a primary anatomical risk factor for recurrent patellar dislocation and patellofemoral instability.
Normal Reference Values
| Measurement |
|---|
| >3 mm |
Clinical Significance
A trochlear depth of greater than 3 mm is considered normal, indicating adequate bony restraint for the patella. When depth falls at or below this threshold, femoral trochlear dysplasia is diagnosed, predisposing the patient to lateral patellar subluxation and dislocation. Trochlear dysplasia is one of the most significant anatomical contributors to patellofemoral instability syndrome and is graded using the Dejour classification (Types A–D) based on additional morphological features including the crossing sign, trochlear spur, and supratrochlear bump.
Clinicians should interpret trochlear depth alongside other patellofemoral parameters — including the tibial tubercle–trochlear groove (TT-TG) distance, patellar tilt, and patellar height indices — as isolated trochlear dysplasia may have variable clinical expression. Surgical interventions such as trochleoplasty are considered in symptomatic patients with severe dysplasia refractory to conservative management.
- Recurrent patellar dislocation / lateral patellar instability
- Patellofemoral pain syndrome
- Dejour Type A–D trochlear dysplasia
- Combined trochlear dysplasia with elevated TT-TG distance
- Post-traumatic trochlear flattening
Reference: Pfirrmann CW, Zanetti M, Romero J et al. Femoral trochlear dysplasia: MR findings. Radiology. 2000;216(3):858-64.
Imaging Notes
On MRI, trochlear depth is measured on an axial image at the level of the proximal trochlea, typically on a proton-density or T1-weighted sequence with the knee in slight flexion or full extension. The measurement is obtained by drawing a line connecting the highest points of the medial and lateral trochlear facets, then measuring the perpendicular distance from this line to the deepest point of the trochlear groove. Use the most superior axial slice where both femoral condyles are visible and continuous with the trochlea to ensure reproducibility.
Fat-suppressed sequences can improve cartilage visualization but bony landmarks for depth measurement are best assessed on non-fat-suppressed sequences. Ensure adequate slice thickness (≤3 mm) and in-plane resolution to avoid partial-volume averaging, which can artificially reduce apparent trochlear depth and lead to overcalling dysplasia.