Onlay Tibial Implants Appear to Provide Superior Clinical Results in Robotic Unicompartmental Knee Arthroplasty

Background: Unicompartmental knee arthroplasty (UKA) is an increasingly popular option for the treatment of single-compartment knee osteoarthritis (OA) in adults. Two options for tibial resurfacing during UKA are (1) all-polyethylene inlays and (2) metal-backed onlays. Questions/Purposes: The aim of this study was to determine whether there are any differences in clinical outcomes with inlay versus onlay tibial components. Patients and Methods: We identified 39 inlays and 45 onlays, with average 2.7- and 2.3-year follow-up, respectively, from a prospective robotic-assisted surgery database. The primary outcome was the Western Ontario and McMaster University Arthritis Index (WOMAC), subcategorized by the pain, stiffness, and function subscores, at 2 years postoperatively. The secondary outcome was the need for secondary or revision surgery. Results: Postoperative WOMAC pain score was 3.1 for inlays and 1.6 for onlays (p = 0.03). For 25 inlays and 30 onlays with both preoperative and postoperative WOMAC data, pain score improved from 8.3 to 4.0 for inlays versus from 9.2 to 1.7 for onlays (p = 0.01). Function score improved from 27.5 to 12.5 for inlays versus from 32.1 to 7.3 for onlays (p = 0.03). Four inlays and one onlay required a secondary or revision procedure (p = 0.18). Conclusions: We advise using metal-backed onlays during UKA to improve postoperative clinical outcomes.