Image-based robotic total knee arthroplasty preserves the knee joint line level even in advanced fixed flexion deformities when combined with functional alignment principles: A retrospective comparative cohort study

Purpose Fixed flexion deformity (FFD) is traditionally addressed in total knee arthroplasty (TKA) with extensive soft tissue release and distal femoral recut, which increases bone stock consumption and raises the knee joint line (JL). This study aimed to evaluate differences in the anatomical restoration of the JL and bone stock preservation between FFD knees and a control group during robotic-assisted (RA) TKA combined with functional alignment (FA). Methods A retrospective comparative cohort study examined 120 knees undergoing RA TKA. The knees were categorised into two groups: the study group, with FFD > 5 degrees, and the control group, without FFD. Further analysis stratified the study group based on the severity of the deformity: mild (5-9 degrees), intermediate (10-14 degrees) and advanced (>15 degrees). The Mann-Whitney U test was utilised to investigate the differences between the control and study groups. Results The study group comprised 64 knees, presenting an average flexion contracture and range of motion (ROM) of 11.3 +/- 4.7 degrees and 112.7 +/- 11.6 degrees, respectively. The control group comprised 56 knees, with an extension deficit and ROM of 1.6 +/- 2.1 degrees and 123.5 +/- 8.3 degrees, respectively. The JL was proximally displaced on average by 0.1 +/- 1.2 mm in the study group and lowered by 0.7 +/- 0.9 mm in the control group. Analysis of JL in the subgroups showed a lowering of 0.3 +/- 1.2 mm in the mild deformity subgroup and a rise of 0.08 +/- 1.3 mm and 0.8 +/- 0.8 mm in the intermediate and advanced FFD subgroups, respectively, showing no statistical significance. The combined thickness of tibial proximal and femoral distal bone cuts measured 12.3 +/- 1.6 mm in the study group and 11.4 +/- 1.4 mm in the control group. Conclusions FA in RA-assisted TKA can correct FFD, minimising bone cuts while preserving anatomical JL level.