Patellar alignment compared between kinematic and mechanical unicompartmental knee arthroplasties

Background

Surgical treatment of unicompartmental knee arthritis is typically treated by unicompartmental knee arthroplasty (UKA). UKA may be performed by more traditional mechanical alignment, or alternatively, kinematic alignment. The purpose of this study is to compare pre- and post-operative patellar tendon alignment between mechanical and kinematic UKA.

Methods

A retrospective study at Medstar Washington Hospital Center from 2015 to 2022 identified 156 cases of partial knee arthroplasties. Of these, 95 had mechanical alignments and 61 had kinematic alignments. Patients were evaluated pre and post-surgically for Insall-Salvati and Blackburne-Peel ratios. Analysis of joint imaging x-ray imaging was performed by a blinded MSK-trained radiologist.

Results

Pre-and-post-operative Insall-Salvati and Blackburne-Peel ratios were calculated for kinematic and mechanical knee replacement patients. The ratio differences for the kinematically aligned cohort demonstrated an average for Blackburne-Peele operative ratio difference of 0.089 ( ± 0.30) and an average Insall-Salvati ratio difference of 0.18 ( ± 0.34), whereas for mechanically aligned patients, the Blackburne-Peele ratio difference was 0.054 ( ± 0.47) and the Insall-Salvati ratio difference was 0.41 ( ± 0.31). There was a statistically significant difference in operative Insall-Salvati ratios between kinematic and mechanical patients. Additionally, a two-sample t-test found a significant difference between the postoperative Blackburne-Peel and Insall-Salvati ratios for the kinematically aligned knees with a p-value of 2.33 × 10⁻¹⁶ below the significance level of 0.05. A two-sample f-test found a significant difference in the standard deviation of the difference between mechanical and kinematically aligned knees with respect to the Blackburne-Peel ratio, with a p-value of 0.00183.

Conclusion

This study demonstrates that sagittal patella-femoral knee alignment when comparing Blackburne-Peel and Insall-Salvati ratios are more accurately reproduced with kinematically aligned knees, and a greater variability among the kinematically aligned knees.