Abstract
Introduction:
Correcting fixed flexion deformity (FFD) during total knee arthroplasty (TKA) is essential for achieving optimal outcomes in osteoarthritic patients. With mechanically aligned knees, correction of FFD can be difficult, and often requires increasing the distal femoral resection (Bellemans et al., 2006; Liu et al., 2016). Functional alignment (FA) aims to restore the patient’s native alignment and joint line obliquity while maintaining soft tissue balance through a range of motion. (MacDessi et al., 2023)
Objective:
This study investigates the distal femoral resection required to correct FFD in patients undergoing robotic-assisted TKA with a functional alignment approach.
Methods:
A single-centre retrospective study was conducted on consecutive patients undergoing unconstrained primary TKA between February 2024 and July 2025. All patients underwent rTKA (Stryker MAKO robot) with functional alignment. Baseline demographics, pre-resection range of motion, post-osteophyte clearance range of motion, resection values, implant sizes and final range of motion were evaluated. Subgroup analysis was performed based on the pre-resection fixed flexion deformity (hyperextension, 0-5 degrees, 6-10 degrees, >11 degrees). Pearson correlation coefficient was utilised to study the relationship between variables. A one-way ANOVA test between subgroups was performed.
Results:
A total of 253 patients were included. The mean pre-osteophyte clearance FFD was 4.68° (range -12° to 23°), improving to a mean of -1.55° post-operatively (range -10° to 5°). No significant correlation was found between pre-operative FFD and distal femur resection (r = 0.13), or between the amount of FFD correction and femoral resection (r = 0.09). There was no statistically significant difference in distal femur resection between all pre-osteophyte clearance FFD groups, F(3, 248) = 1.16, p = 0.33. When adjusting for polyethylene thickness implanted, there was a statistically significant difference in tibial resection between pre-osteophyte clearance FFD groups, F(3, 240) = 7.92, p < .001.
Conclusion:
In this single-institution series, patients undergoing robotic assisted TKA with functional alignment did not demonstrate an association between distal femur resection and the correction of a fixed flexion deformity. There was a small increase in tibial resection depth in those with a greater starting FFD, although this was still less than the planned resection. These findings confirm it is possible to correct a fixed flexion deformity in functionally aligned knees without the need for an increased distal femur resection. Historical dogma around increased distal femoral resection in FFD may not be applicable with modern alignment philosophy and technology assistance.