Normal Bernageau Length Size on Radiography
The Bernageau length is a radiographic measurement used to evaluate the relationship between the patella and the trochlear groove in the sagittal plane. It quantifies patellar height by assessing the position of the inferior patellar pole relative to the trochlear articular surface. Accurate measurement is essential in the workup of patellofemoral instability, anterior knee pain, and surgical planning for trochleoplasty or tibial tubercle osteotomy.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | -6-6 mm |
Clinical Significance
A Bernageau length within the range of −6 to +6 mm is considered normal. Positive values indicate that the inferior patellar pole sits above the trochlear articular surface (patella alta tendency), while negative values suggest it lies below (patella baja tendency). Values outside this range signal pathological patellar height and may predispose the joint to recurrent dislocation, chondral injury, or impingement.
Patella alta — where the patella engages the trochlea late in flexion — is one of the most significant risk factors for lateral patellar dislocation. Patella baja, conversely, may occur iatrogenically following anterior cruciate ligament reconstruction or tibial tubercle advancement, and can cause anterior pain and restricted flexion. Careful interpretation in the context of clinical findings and other patellar height indices (e.g., Caton-Deschamps, Insall-Salvati) is recommended.
- Recurrent lateral patellar dislocation (patella alta)
- Patellofemoral pain syndrome
- Trochlear dysplasia
- Iatrogenic patella baja post-surgery
- Osteochondral injury from abnormal engagement
Reference: Biedert RM. Patellofemoral Disorders, Diagnosis and Treatment. Wiley. (2005).
Imaging Notes
The Bernageau length is measured on a true lateral radiograph of the knee obtained in a standardized degree of flexion (typically 30°). The measurement is taken from the most distal articular cartilage margin of the trochlea to the inferior pole of the patella, with positive values recorded when the patellar pole is superior to this reference point. Strict positioning is critical — rotational obliquity causes superimposition of the femoral condyles and invalidates the measurement.
When lateral radiographs are unavailable or positioning is suboptimal, MRI can serve as an adjunct to assess patellar height and cartilage morphology, though the Bernageau method is specifically validated for plain radiography. Always correlate with the clinical presentation and additional patellofemoral indices for comprehensive assessment.