Blackburn and Peel ratio
The Blackburne-Peel ratio is used to assess patellar height, and determine presence of patella alta. A lateral radiograph of the knee with 30 degrees of flexion is obtained, and a horizontal line at the level of the tibial plateaux is drawn. Perpendicular to this line vertically and a measurement (B) made of the distance between the horizontal line and the inferior aspect of the patellar articular surface. A second measurement (A) is made along the patellar articular surface. 
• A/B is a measure of patellar height
Normal value = ratio of 0.8.
Patella alta = ratio >1.0
- Syed A. Ali, Robert Helmer and Michael R. Terk; Patella Alta: Lack of Correlation Between Patellotrochlear Cartilage Congruence and Commonly Used Patellar Height Ratios; AJR November 2009 vol. 193 no. 5 1361-1366. (Level of evidence: B)