Relevance of the Tibial Slope on Functional Outcomes in ACL-Deficient and ACL Intact Fixed-Bearing Medial Unicompartmental Knee Arthroplasty

Abstract Background Excessive posterior tibial slope in medial unicompartmental knee arthroplasty (UKA) has been implicated in early failure. The purpose of this study was to evaluate the relationship between preoperative posterior tibial slope (PTS) and postoperative slope of the implant (PSI) on outcomes in patients with ACL intact and ACL-deficient knees following fixed-bearing medial UKA. Methods Patients who underwent a medial UKA between 2002-2017 with a minimum 3-year follow-up were included. Preoperative PTS and postoperative PSI were measured. Outcomes measures included KOOS subscales, Lysholm and VR-12. Failure was defined as conversion to total knee arthroplasty. Results Of 241 knees undergoing UKA, 131 patients (70 females, 61 males; average age of 65±10 years (average BMI of 27.9±4) were included. For all patients, survivorship was 98% at 5 years and 96% at 10 years with a mean survival time for UKA was 15.2 years [95%CI: 14.6 – 15.7]. No failure had a PSI>7°. There were no superficial or deep infections. There were no significant differences in outcome scores between the ACL intact and the ACL-deficient group; therefore, the data were combined for analysis. At mean 8-year follow-up, KOOS pain scores were better in patients with PSI≤7° (87±16) compared to those PSI>7° (81±15). 76% of patients with PSI≤7° reached PASS for KOOS pain while 59% with PSI>7° reached PASS for KOOS pain (p=0.015). Conclusion Patients with postoperative posterior slope of the tibial implant >7° had significantly worse postoperative pain, without conversion to TKA, and with maintenance of high function. In ACL deficient and intact knees, non-robotically assisted, fixed-bearing medial UKA had a 96% survivorship at 10 years.