Mid-Flexion Sagittal Stability of Total Knee Arthroplasty Implanted With Kinematic Alignment: A Quantitative Radiographic Laxity Study With Single-Radius Posterior-Stabilized and Condylar-Stabilized Implants

Background: Stability between full extension and 90 of flexion is crucial for successful outcomes in total knee arthroplasty (TKA).Thus, we compared the sagittal/anteroposterior (AP) stability of knees implanted with single-radius condylar-stabilized (CS) vs posterior-stabilized (PS) devices from a randomized, prospective study.We hypothesized that the implant group with superior sagittal/AP stability would exhibit better clinical outcomes.Methods: Sixty-five patients underwent primary TKA with a PS (n 28) or CS (n 37) device.Outcomes collected 5-7 years postoperatively included Knee Society (KS) Scores, range of motion (ROM), the Lower Extremity Activity Scale, the Forgotten Joint Score, and mechanical and anatomic alignment.Sagittal/AP stability was evaluated in 45 and 90 of flexion via stress radiographs.Results: The CS and PS groups averaged 85.8 and 77.3 months post-TKA, respectively, at evaluation.The CS group demonstrated greater mean AP laxity, especially in 45 mid-flexion.Alignment, ROM, and patient-reported outcome scores were similar.However, women in the CS group demonstrated inferior mean KS Function (P .02)and Total Composite scores (P < .01)compared to women in the PS group.These scores were not significantly different for men or the combined groups.Conclusions: Mid-flexion sagittal/AP laxity was substantial in both implant groups.With the limited population available, the greater laxity observed in patients with CS implants did not correspond to differences in Forgotten Joint Score, Lower Extremity Activity Scale, ROM, or alignment; however, the difference between the KS Function and Total Composite scores in women revealed an inverse relationship between sagittal laxity and clinical outcomes.Level of evidence: I.