Inlay Versus Onlay Tibial Implants in Robotic Unicondylar Knee Arthroplasty

Introduction: Two fixed bearing options exist for tibial resurfacing when performing unicompartmental knee arthroplasty (UKA). Inlay components are polyethylene-only implants inserted into a carved pocket on the tibial surface, relying upon the subchondral bone to support the implant. Onlay components have a metal base plate and are placed on top of a flat tibial cut, supported by a rim of cortical bone. To our knowledge, there is no published report that compares the clinical outcomes of these two implants using a robotically controlled surgical technique. We performed a retrospective review of a single surgeon9s experience with Inlay versus Onlay components, using a robotic-guided protocol. Methods: All surgeries were performed using the same planning software and robotic guidance for execution of the surgical plan (Mako Surgical, Fort Lauderdale, FL). The senior surgeon9s prospective database was reviewed to identify patients with 1) medial-sided UKA and 2) at least two years of clinical follow up. Eighty-six patients met these inclusion/exclusion criteria: 41 Inlays and 45 Onlays. Five patients underwent a secondary or revision procedure during the follow up period and were considered separately. Our primary outcome was the WOMAC score, subcategorized by the Pain, Stiffness, and Function sub-scores. The secondary outcome was need for secondary surgery. Continuous variables were analyzed using the two-tailed Student9s t-test; categorical variables were analyzed using Fisher9s exact test. Results: Average follow up was 2.7 years and 2.3 years in the Inlay and Onlay groups, respectively. The post-op WOMAC Pain score was 3.1 for Inlays and 1.6 for Onlays ( p = 0.03). The post-op Stiffness score was 1.8 for Inlays and 1.4 for Onlays ( p = 0.19). The post-op Function score was 10.3 for Inlays and 6.7 for Onlays ( p = 0.12). We identified a subgroup of 51 patients (23 Inlay, 28 Onlay) for whom there was both pre- and post-op WOMAC data available. There were no differences in the pre-op Pain, Stiffness, or Function scores between groups. In this subgroup, the Pain score improved from 8.3 to 4.0 for Inlays, versus an improvement from 9.2 to 1.7 for Onlays ( p = 0.01). The Stiffness score improved from 3.9 to 2.2 for Inlays, versus an improvement from 4.3 to 1.5 for Onlays ( p = 0.08). The Function score improved from 27.5 to 12.5 for Inlays, versus an improvement from 32.1 to 7.3 for Onlays ( p = 0.03). When all 86 patients in the study were considered, 4/41 Inlays (9.8%) underwent a secondary procedure during the two year follow up (two conversions to total knee replacement [TKR]; one conversion to an Onlay component; one internal fixation of a subchondral compression fracture). In the Onlay group, 1/45 patients (2.2%) underwent a secondary procedure, a conversion to TKR ( p = 0.20). Discussion: Onlay UKR demonstrated improved pain relief compared to inlay UKR at two years of follow up. There was a trend toward improved function and fewer secondary surgical procedures with the onlay device. We advise surgeons performing UKA to preferentially use Onlay implants in order to maximize clinical benefit.