Normal Blackburne-Peel Index Size on Radiography
The Blackburne-Peel Index (BPI) is a radiographic ratio used to assess patellar height on a true lateral knee radiograph, calculated by dividing the perpendicular distance from the tibial plateau articular surface to the inferior patellar articular surface by the length of the patellar articular surface. Accurate assessment of patellar height is clinically important because abnormal positioning—either too high (patella alta) or too low (patella baja)—is associated with patellofemoral pain, instability, and post-surgical complications. The BPI offers an advantage over other patellar height indices as it is largely unaffected by tibial tuberosity position or skeletal maturity.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 0.5-1.0 |
Clinical Significance
A normal Blackburne-Peel Index falls between 0.5 and 1.0. A ratio exceeding 1.0 indicates patella alta, a superiorly displaced patella associated with reduced patellofemoral contact area, anterior knee pain, and increased risk of patellar instability and dislocation. Conversely, a BPI below 0.5 suggests patella baja (or infera), which may follow anterior cruciate ligament reconstruction, total knee arthroplasty, or tibial tubercle osteotomy, and can contribute to extensor mechanism dysfunction and reduced knee flexion.
Careful attention to technique is essential: the index is only valid on a true lateral radiograph with the knee in 30° of flexion. Rotational error or variation in flexion angle can introduce measurement error and lead to misclassification of patellar height.
- Patella alta — recurrent patellar instability or dislocation
- Patella baja — post-surgical complication (e.g., post-ACL reconstruction or tibial tubercle transfer)
- Patellofemoral pain syndrome — associated with borderline alta
- Cerebral palsy / neuromuscular conditions — frequently associated with patella alta
- Achondroplasia — may present with patella baja
Reference: Biedert RM. Patellofemoral Disorders, Diagnosis and Treatment. Wiley. (2005).
Imaging Notes
The Blackburne-Peel Index is measured exclusively on a lateral knee radiograph, ideally obtained with the knee flexed to 30° and the X-ray beam centered on the joint. The numerator is the perpendicular distance from the tibial plateau articular surface to the lower border of the patellar articular cartilage surface; the denominator is the length of the patellar articular surface. Because neither measurement involves the tibial tuberosity, the BPI remains reliable even when the tuberosity is surgically altered or not well visualized.
Ensure strict lateral positioning (superimposition of femoral condyles) to avoid parallax error. In skeletally immature patients, ossification centers may obscure landmarks, and caution is advised when interpreting borderline values.