Sagittal Femoral Component Flexion in Primary Total Knee Arthroplasty: Is There a Clinically Relevant Inflection Point for Clinical Outcomes?

Abstract

The purpose of our study was to determine whether sagittal malalignment of the femoral component after primary total knee arthroplasty (TKA) has a flexion threshold beyond which postoperative clinical outcomes deteriorate. This retrospective cohort included 235 consecutive primary TKA patients operated on by a single surgeon with at least 12 months of follow-up. All procedures were performed using a conventional manual instrumentation technique. All knees received the same cemented, posterior-stabilized implant. Femoral component flexion was measured on standardized lateral radiographs as the angle between the long axis of the component and the distal femoral mechanical axis. Patients were categorized into low (0 to 2.5°), medium (2.6 to 5.5°), and high (5.6° or greater) flexion groups. At 12 months, pain, function, and patello-femoral symptoms were evaluated using the visual analog scale (VAS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Knee Society Score (KSS), Kujala score, and a four-point satisfaction scale. Group comparisons and regression analyses, including nonlinear and piecewise models with a knot at 6°, were performed. Medium flexion produced the most favorable outcomes, whereas excessive flexion was associated with significantly higher pain, worse WOMAC and KSS scores, and lower satisfaction. Spline regression demonstrated no meaningful change in outcomes between 0 to 6°, but greater than six degrees, each additional degree of flexion resulted in progressive deterioration, particularly in patello-femoral function. Flexion angle remained associated with anterior knee symptoms after adjustment for age, body mass index (BMI), and follow-up duration. Sagittal femoral component flexion was independently associated with patello-femoral symptoms and showed a significant nonlinear relationship with postoperative pain and functional outcomes after primary TKA in a single-surgeon series using a single posterior-stabilized implant design. A mid-range flexion of approximately 3 to 5° appears safest, while flexion greater than six degrees represents a clinically relevant inflection point beyond which clinical outcomes decline.