Onlay tibial implants appear to provide superior clinical results in robotic unicompartmental knee arthroplasty

Abstract

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.

Keywords: inlay; onlay; robotic surgery; tibial resurfacing; unicompartmental knee arthroplasty.

Fig. 1

Anteroposterior radiographs of a all-polyethylene inlay and b metal-backed onlay tibial components for unicompartmental knee arthroplasty.

Fig. 2

Computer-assisted design (CAD) models of the implanted components are virtually templated onto the patient’s computed tomography scan prior to surgery. Variables such as component rotation (a), polyethylene thickness (b), coronal alignment (c), and posterior slope (d) can be virtually templated and trialed with this software.

Fig. 3

Postoperative WOMAC subscores. Statistically significant differences (p ≤ 0.05) are indicated by an asterisk (*).

Fig. 4

Change in WOMAC scores after surgery. Statistically significant differences (p ≤ 0.05) are indicated by an asterisk (*). a Pain subscore, b stiffness subscore, c function subscore. Pre-Op preoperative, Post-Op postoperative.