Patella alta: lack of correlation between patellotrochlear cartilage congruence and commonly used patellar height ratios.
OBJECTIVE
The purposes of this study were to assess the degree of patellotrochlear chondral overlap (patellotrochlear index), correlate it with the Insall-Salvati and modified Insall-Salvati indexes, and determine the association between these measurements and patellofemoral chondral defects.


METHODS
Sagittal 1.5-T and 3-T MR images of 100 consecutively registered patients with symptoms were analyzed, and the Insall-Salvati index, modified Insall-Salvati index, patellotrochlear index, and patellophyseal index (ratio of the height of patella above the physeal line to the length of the patellar articular cartilage) were calculated. The upper and lower limits of 2 SDs were used to define patella alta and baja, and the correlation coefficient curves were plotted to compare techniques. The indexes in normal knees were compared with those in knees with severe chondral defects.


RESULTS
The mean patellotrochlear index was 0.49 +/- 0.15 (SD) (range, 0-0.88). On the basis of calculation of 2 SDs, patella alta was determined to have a patellotrochlear index less than 0.18 and patella baja, an index greater than 0.80. Weak correlation was found between the measured patellotrochlear index and Insall-Salvati index (r = -0.224) and between the patellotrochlear index and modified Insall-Salvati index (r = -0.073). A strong correlation was found between the patellotrochlear index and patellophyseal index (r = -0.813). A statistically significant (p < 0.05) difference in the modified Insall-Salvati index and patellophyseal index was found between knees with normal and those with severe cartilage defects.


CONCLUSIONS
Our results indicate that the commonly used Insall-Salvati and modified Insall-Salvati indexes do not correlate with patellotrochlear articular cartilage congruence. We did find an association between the modified Insall-Salvati and patellophyseal indexes and the presence of severe chondral defects.
Department of Radiology, Emory University, Atlanta, GA 30329, USA. saali4@emory.edu
Syed A
SA
Ali
Robert
R
Helmer
Michael R
MR
Terk
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002358:Cartilage, Articular / Q000033:anatomy & histology* / Q000473:pathology; D002648:Child; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D010329:Patella / Q000033:anatomy & histology* / Q000473:pathology