Blackburne and Peel reported a ratio for the patella based on measurements taken in a lateral X-ray of the knee with the patellar tendon under tension. The ratio measures the important components of the patello-femoral joint. It is somewhat similar to the Caton-Deschamps index.
Radiograph: lateral knee with knee flexed at 30 degrees (which places the patellar tendon under tension)
Measurements:
(1) length of the articular surface of the patella in cm (along its posterior surface)
(2) vertical distance from inferior edge of patellar articular surface to the horizontal tibial plateau line in cm
ratio =
= (distance from inferior edge of patellar articular surface to tibial plateau line) / (length of the patellar articular surface)
Interpretation:
• For a normal knee the ratio has a mean of 0.81, with standard deviation 0.13-0.14.
• In patellar alta (in Osgood-Schlatter disease) the ratio will be greater than 1.0.
• The lower limit of normal is 0.5.
Difference from Caton-Deschamps index:
(1) The Blackburne-Peel index measures the vertical distance to a horizontal line through the tibial plateaua, with the lines at 90 degrees to each other.
(2) The Caton-Deschamps index uses a distance running directly along the patella ligament.
Limitations:
• The method does not apply to children since cartilage is not visible on radiographs.
