The Effect of Implant Design on Sagittal Plane Stability: A Randomized Trial of Medial- versus Posterior-Stabilized Total Knee Arthroplasty.

Up to 20% of total knee arthroplasty (TKA) patients report dissatisfaction with their outcome, especially with weight-bearing in flexion (WBiF) activities. Sagittal plane instability may contribute to dissatisfaction following TKA. We assessed the impact of implant design on TKA sagittal plane stability and clinical satisfaction. We randomized patients to receive one of two TKA implant designs: medial-stabilized (MS) or posterior-stabilized (PS). Sagittal stability was assessed using a KT-1000 arthrometer. Patient-reported outcome measures, including a custom bank of questions targeting patient satisfaction (0-100%) with WBiF activities, were administered to patients 2 years following surgery. The final analysis included 50 patients (25 MS, 25 PS). The MS group had greater sagittal plane stability than the PS group at 30-degree flexion (5.6 ± 1.9 vs. 10.2 ± 2.7 mm;  < 0.0001) but not at 90-degree flexion (4.1 ± 2 vs. 5.3 ± 3 mm;  = 0.14). Range of motion was not different (111.3 ± 10.4 vs. 114.7 ± 10.7 degrees;  = 0.31). There were no differences in the PROMIS (Patient-Reported Outcomes Measurement Information System) score, Oxford Knee Score, Knee Society Score, Forgotten Joint Score, or Veterans Rand. The MS group had no difference in satisfaction for WBiF activities versus non-WBiF activities (80.5 ± 18 vs. 88.3% ± 16.4%;  = 0.13), whereas the PS group had significantly worse satisfaction for WBiF versus non-WBiF activities (71.6 ± 24.6 vs. 87.8% ± 16.6%;  = 0.019). An MS prosthetic design was more stable in the sagittal plane in midflexion compared with a PS design. There was no difference in patient-reported outcomes, although custom survey data suggest improved satisfaction with MS design during WBiF activities.