Knee rotation in patients with and without patellofemoral instability and its correlation with patellar tendon trochlear groove distance

Background and study aims: There are limited studies assessing the role of “knee rotation” in patellofemoral instability (PFI). Knee rotation is the external rotation of the proximal tibia with respect to distal femur. The study was performed to compare knee rotation, patellar tendon lateralisation and patellar tendon trochlear groove (PTTG) distance measured on MRI between patients with and without PFI and to assess the relative association of these factors with PFI. Materials and methods: It was a retrospective cross-sectional study done on archival MRI films. A sample size of 20 for PFI group and 60 for control group was taken. PTTG distance, knee rotation and lateralisation of patellar tendon insertion were measured as described in literature. The knee rotation and patellar tendon lateralisation between PFI and non-PFI group were compared using t-test and their Pearson’s correlation (r) with PTTG distance was calculated. Relative associations of these factors with PFI were assessed using multivariate regression. Results: Lateralisation of patellar tendon insertion and external knee rotation were both found to be higher in PFI patients as compared to non-PFI patients. However,on taking PTTG distance into consideration, the associations of knee rotation as well as of patellar tendon lateralisation with PFI did not remain significant in generalised linear modeling. Overall, the PTTG distance showed moderate to strong correlation with the external knee rotation as well as with lateralisation of patellar tendon insertion. Conclusions: Knee rotation was found to be significantly higher in PFI patients as compared to controls. However, PTTG distance appears to be a composite measure that takes into account the influence of both the patellar tendon lateralisation as well as knee rotation on patellofemoral biomechanics.