Correlation between trochlear dysplasia and the notch index.

PURPOSE To evaluate the correlation between trochlear dysplasia and the notch index. METHODS Magnetic resonance images (MRI) of 95 knees in 54 male and 36 female patients aged 4 to 74 (mean, 28) years were reviewed by 2 musculoskeletal radiologists. Standard MRI sequences were used. Based on the Dejour classification of trochlear dysplasia, the knees were classified into normal or types A, B, C, and D. A notch index of <0.2 was considered narrow. Normal knees and knees with trochlear dysplasia were compared. RESULTS 60 of the 95 knees had trochlear dysplasia, of which 39 were Dejour type A, 13 were type B, 7 were type C, and one was type D. Dejour types B, C, and D were combined as non-type A. Inter-observer agreement in assessing the notch index was good (Kappa=0.6). The mean notch indices in normal knees and knees with trochlear dysplasia were comparable (0.161 vs. 0.157, p=0.18), as were in Dejour type A and non-type A knees (0.154 vs. 0.160, p=0.54) and in Dejour types A, B, C, and D knees (0.154 vs. 0.165 vs 0.153 vs. 0.2, p=0.17-0.7). The rate of ACL injuries was similar in patients with normal knees and those with type-A trochlear dysplasia. A low notch index (narrow notch) was not associated with ACL injury. CONCLUSION The notch index and trochlear morphology are 2 independent entities. A narrow notch does not imply a shallow trochlear grove.