Clinical and Biomechanical Evaluation of Mid-Level Constrained and Posterior-Stabilized Polyethylene Inserts in Primary Total Knee Arthroplasty: An Analysis of 12,674 Cases

INTRODUCTION:Mid-level constraint polyethylene designs provide additional stability in total knee arthroplasty (TKA). The purposes of this study were to: 1) compare the survivorship and reason for revision between mid-level inserts and posterior-stabilized (PS) used in primary TKA, and 2) evaluate the biomechanical constraint characteristics of mid-level inserts. METHODS:We reviewed all cases of primary TKA performed at our institution from 2016 to 2019 using either PS or mid-level constrained inserts from one of six manufacturers. Data elements included patient demographics, implants, reasons for revision, and whether a manipulation under anesthesia (MUA) was performed. We performed finite element (FE) analyses to quantify the varus/valgus and axial-rotation constraint of each mid-level constrained insert. A 1-to-1 propensity score matching was conducted between the patients with mid-level and PS inserts to match for variables, which yielded two cohorts of 3,479 patients. RESULTS:For 9,163 PS and 3,511 mid-level TKAs, survivorship free from all-cause revision was estimated up to 5 years and was lower for mid-level than PS inserts (92.7 versus 94.1%, respectively, P=0.004). When comparing each company's mid-level insert to the same manufacturer's PS insert, we found no differences in all-cause revision rates (P≥0.91) or revisions for mechanical problems (P≥0.97). Using propensity score matching between mid-level and PS groups, no significant differences were found in rates of MUA (P=0.72), all-cause revision (P=0.12), revision for aseptic loosening (P=0.07), and revision for instability (P=0.45). Finite element modeling demonstrated a range in varus/valgus constraint from ±1.1 to >5 degrees, and a range in axial-rotation constraint from ±1.5 to ±11.5 degrees among mid-level inserts. CONCLUSION:Despite wide biomechanical variations in varus/valgus and axial-rotation constraint, we found minimal differences in early survivorship rates between PS and mid-level constrained knees.