Review Question - QID 217470

A Blackburne-Peel ratio (Figure B) > 1.0 is concerning for patella alta and patellar tendon rupture.

Patellar alta, suggestive of patellar tendon rupture can be quantified on a lateral radiograph in 30º of flexion by using one of multiple methods. The Insall-Salvati index, as seen in Figure A, is calculated by the length of the patellar tendon (Y)/length of the patella (X). A normal Insall-Salvati ratio is between 0.8-1.2. The Blackburne-Peel ratio, as seen in Figure B, is calculated by using the distance between a horizontal line adjacent to the tibial plateau and the inferior aspect of the patellar articular surface (Y)/length of the patellar articular surface (X). The normal Blackburn-Peel ratio falls between 0.5-1.0. Finally, the Caton Deschamps ratio, as seen in Figure C, is calculated using the distance from the tibial plateau to the inferior patellar articular surface (Y)/length of the patellar articular surface (X). Any of these calculations may provide a reasonable idea as to whether patellar alta is present and suggest rupture of the patellar tendon. MRI is the most sensitive imaging modality and can help detect partial vs. full tears in more equivocal cases.

Verhulst et al. explored the different ways of measuring patellar height, using radiographs, CT, and MRI. Interestingly, the Insall-Salvati ratio had the highest intra- and inter-observer reliability on all imaging modalities. They concluded that based on their results, the Insall-Salvati ratio could be reliably calculated on conventional radiography, CT, and MRI and was the most reliable method.

Seil et al. evaluated the reliability and variability of different techniques in measuring patellar height on lateral flexed radiographs. They noticed that between measurement techniques, there was variability in how patella alta or baja was classified. They ultimately recommended that reviewers use a ratio that measures the patellar articular surface, rather than the bony length of the patella and based on this and low interobserver variability, they concluded that the Blackburne-Peel ratio was most useful.

Figure A is a lateral knee diagram showing the Insall-Salvati index (Y/X). Figure B is a lateral knee diagram showing the Blackburne-Peel ratio (Y/X). Figure C is a lateral knee diagram showing the Caton Deschamps ratio (Y/X).

Incorrect Answers:
Answer 1: When calculating the Insall-Salvati index, X (patellar length) is divided by Y (patellar tendon length) and Y/X > 1.2 would indicate likely patellar tendon rupture. However, X/Y does not represent the desired calculation and X/Y =1.3 would not demonstrated patellar alta.
Answer 2: A Insall-Salvati ratio (Y/X) =0.95 is within the normal ranges and would not indicate patellar tendon rupture.
Answer 4: A Caton Deschamps ratio (Y/X) > 1.3 is concerning for patellar tendon rupture, however a ratio of 1.1 would be considered within the normal range.
Answer 5: The Caton Deschamps ratio (Y/X) is calculated using the length of the patellar articular surface (X) and its distance from the tibial plateau (Y). Therefore, X/Y = 1.45 does not represent the desired calculation and would not be indicative of patella alta or patellar tendon rupture.