Functionally aligned total knee arthroplasty: A lateral flexion laxity up to 6 mm is safe!

Purpose: Loose flexion gaps are associated with poor functional outcomes and instability in total knee arthroplasty (TKA). The effect of a trapezoidal flexion gap in a functionally aligned TKA remains unknown. The aim of this study was to investigate the effect of a larger lateral flexion gap in a robotic-assisted (RA), functionally aligned (FA) and cruciate-retaining (CR) TKA on clinical outcomes. Methods: Data from 527 TKA in 478 patients from 2018 to 2020 were collected. All patients underwent an RA (MAKO, Stryker), FA and CR TKA. Gap measurements were collected intraoperatively. Patient-reported outcome measures (PROMs), pain Visual analogue score (VAS) and range of motion were collected postoperatively. Patients were also asked about the ease of stair ascent and descent and kneeling on a 5-point scale. The minimum follow-up was 2 years. Patients were stratified into three groups based on lateral flexion laxity. Results: At 2 years postoperatively, the group with a looser gap (3–6 mm) had higher mean PROMs when compared with the group with a gap of 2–3 mm. There were no differences detected in any other outcomes at 2 years. A total of 70.9% of patients in the group with a 3–6 mm gap reported being able to walk down a flight of stairs 'easily', compared with 56.7% in the 2–3 mm group and 54% in the <2 mm group (p = 0.04). Conclusion: The study shows that a loose lateral flexion gap in functionally aligned CR TKA does not adversely affect outcomes in the short term. Level of Evidence: Level III, retrospective cohort study.