Early Radiographic and Clinical Outcomes of Cementless Robot-Assisted Functional Alignment Total Knee Arthroplasty with Wide Boundary in Elderly Japanese Patients

Introduction: Functional alignment (FA) in total knee arthroplasty (TKA) aims to restore native limb alignment and soft-tissue balance, but the safety of wider coronal alignment boundaries in elderly patients with potentially reduced bone quality remains uncertain, particularly with cementless fixation. This study evaluated early clinical and radiographic outcomes of robot-assisted FA-TKA with cementless implants in elderly Japanese patients, focusing on the safety of allowing tibial varus up to 6° within an overall ±6° limb alignment boundary. Materials and Methods: This retrospective cohort study analyzed 99 cementless primary TKAs in 66 patients (mean age 74.3 ± 8.1 years) performed between July 2023 and July 2024 using a single-radius implant and a robot-assisted FA protocol. Preoperative computed tomography–based planning and intra-operative gap assessment were used to individualize component positioning within predefined coronal boundaries while minimizing soft-tissue release. Clinical outcomes, including the 2011 Knee Society Score (KSS) and Forgotten Joint Score-12 (FJS-12), range of motion, radiographic alignment, and complications were assessed at 12 months. Results: All 99 knees were available at 1 year. Extension and flexion range of motion, KSS domains, and FJS-12 improved significantly from preoperative to 12-month follow-up. Postoperative radiographs demonstrated alignment within the intended FA boundaries, with more horizontal joint-line obliquity and a shift toward Coronal Plane Alignment of the Knee (CPAK) Type I phenotypes. No progressive radiolucent lines or loosening were observed in 98 knees. One valgus knee treated with a lateral parapatellar approach developed medial tibial baseplate subsidence after lateralized tibial placement and required revision. Among 42 knees implanted with 6° tibial varus, no subsidence or revision occurred. Other complications included one patellar tendon rupture and two atraumatic patellar fractures. Discussion: Robot-assisted FA-TKA with cementless implants provided favorable early clinical and radiographic outcomes in elderly Japanese patients. Within 1 year, tibial varus up to 6° and overall limb alignment within ±6° were not associated with early tibial subsidence in varus knees, although meticulous planning and implantation remain essential, particularly in valgus knees