Association between the most superior point of patella-entrance of femoral trochlea distance ratio (SP-ET index) and chondromalacia patella: an investigation via magnetic resonance imaging

Purpose:
In this study, the effect of the most superior point of patella-entrance of femoral trochlea distance ratio (SP-ET index) on chondromalacia patella (CP) was investigated with 2 reviewers.

Material and methods:
A total of 348 knees of 308 patients were analysed retrospectively with magnetic resonance imaging (MRI). Patients with or without CP constituted the study and the control groups in this cross-sectional investigation. Two reviewers interpreted the dataset regarding the SP-ET index. This ratio was calculated as the distance between the most superior point of patella and the entrance of femoral trochlea (β) divided by the patellar articular surface length (α). The relationship between the SP-ET index and CP was presented using independent samples T-tests, and the intraclass correlation coefficient (ICC) was calculated to reveal the interobserver differences.

Results:
There was excellent agreement between the reviewers regarding α, β, and SP-ET values (ICC was 0.971, 0.964, and 0.943, respectively). Higher SP-ET values were obtained for patients with CP, in comparison with patients without any chondral lesion (p < 0.001). A significant, positive, and moderate level of correlation was revealed between SP-ET measurements and CP grades for the total study population.

Conclusions:
SP-ET index showed high interobserver agreement and indicated a significant difference between patients with and without CP. Both reviewers’ results indicated positive and significant correlation between the measured SP-ET values and different grades of CP for females, males, and the total study population.