Influence of Patellar Tilt Angle in Merchant View on Postoperative Range of Motion in Posterior Cruciate Ligament-Substituting Fixed-Bearing Total Knee Arthroplasty

Background: We investigated whether the patellar tilt angle influences the maximum knee flexion after total knee arthroplasty (TKA) performed by using a posterior cruciate ligament-substituting (PS) fixed-bearing prosthesis in patients with relatively loose or tight flexion gap. Methods: In this prospective cohort study, we followed up 169 patients for at least 2 years after TKA using PS fixed-bearing prosthesis. The patients were divided into two groups according to the flexion gap value-calculated by subtracting the thickness of the final bearing from the flexion gap measured intraoperatively under 200-cN-m force after patellar reduction and insertion of the final femoral and tibial components-into a relatively tight group (group T; 3-6.5 mm) and a relatively loose group (group 1; 7-11 mm). Patellar tilt angles and maximum non-weight-bearing active knee flexion angles were assessed postoperatively. Group T was further divided into subgroup Tn if the patellar tilt angle was < 5 degrees and subgroup Tw if the angle was >= 5 degrees. Pearson correlation test was used for the correlation analysis of the flexion gap, patellar tilt angle, and postoperative flexion range. Results: The mean postoperative flexion was 137.3 degrees in group T and 137.5 degrees in group L The mean patellar tilt angle was 6.5 degrees in group T and 6.9 degrees in group L In group T, a strong negative correlation (r = -0.78, p < 0.05) was observed between the patellar tilt and postoperative flexion range. However, further analysis revealed that only the subgroup Tw showed a strong negative correlation (r = -0.76, p < 0.05). Significant correlations were not found in the subgroup Tn and group L. Conclusions: In TKA where a relatively tight flexion gap (<= 6.5 mm) is created because of concerns about postoperative flexion instability due to a loose flexion gap, the patellar tilt angle should be < 5 degrees for maximal postoperative knee flexion.