Postoperative changes in CPAK‐classification do not significantly influence patient‐reported outcome measures following conventional or robotic‐assisted total knee arthroplasty: A randomised controlled trial

PURPOSE: The purpose of this study was to investigate the impact of pre- and postoperative changes in Coronal Plane Alignment of the Knee (CPAK) classification on patient-reported outcome measures (PROMs) and to determine the rate of CPAK classification change in patients undergoing either conventionally aligned total knee arthroplasty (CTKA) or imageless robotic-assisted total knee arthroplasty (RATKA). We hypothesised that PROM improvements would be comparable regardless of whether postoperative CPAK classification was preserved ("In-the-box") or altered ("Out-of-the-box") in both surgical techniques. METHODS: In this prospective randomised controlled trial, 180 patients were assigned to either CTKA or RATKA. CPAK classification was assessed pre- and postoperatively. PROMs were collected at baseline, 3, and 12 months postoperatively, including the Knee Society Score, Oxford Knee Score, visual analog scale (VAS) for daytime and night time pain, EuroQol-5D (index and VAS), and patient satisfaction. RESULTS: Baseline characteristics were similar between groups. Native CPAK classification was preserved in 31% of CTKA and 41% of RATKA patients. Postoperatively, CPAK II was most common (40% CTKA and 49% RATKA), followed by CPAK V. Both groups showed significant improvements in all PROMs at 3- and 12-months, regardless of CPAK classification change, and operative technique. CONCLUSIONS: Short-term functional outcomes following TKA were not significantly influenced by In-the-box or Out-of-the box CPAK classification. Both CTKA and RATKA yielded comparable PROM improvements. Although altered CPAK often required greater soft-tissue release, especially in CTKA, this did not affect outcomes, suggesting alignment strategy should prioritise individual anatomy and soft-tissue balance over strict CPAK matching. LEVEL OF EVIDENCE: Level I.