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 of preoperative posterior tibial slope and postoperative posterior slope of the implant with outcomes following medial non-robotically assisted fixed-bearing medial UKA.

Methods

Patients who underwent medial UKA between 2002 to 2017 with a minimum 2-year follow-up were included. Preoperative posterior tibial slope and postoperative slope of the tibial implant were measured on radiographs. Failure was defined as conversion to total knee arthroplasty. Outcomes measures included KOOS, WOMAC, Lysholm, and VR12.

Results

One hundred thirty-one patients (70 females, 61 males; average age of 65 ± 10 years average BMI of 27.9±4) were included in the study. Four patients (3%) converted to TKA. The mean survival time was 15.2 years [95%CI: 14.6 – 15.7]. Survivorship was 98% at 5 years and 96% at 10 years. No patients who converted to TKA had a postoperative posterior slope of the implant of >7°. At a mean follow-up of 8 years (range 2 to 15), there were no significant correlations between preoperative posterior tibial slope or postoperative posterior slope of the implant and outcome scores. Increasing postoperative posterior slope of the implant was associated with worse pain. Postoperative posterior slope of the implant negatively correlated with KOOS pain scores (rho=-0.24;p=0.014). KOOS pain scores were lower in patient with =7° postoperative posterior slope of the implant. Seventy six percent of patients with =7° of postoperative posterior slope of the implant reached PASS for KOOS pain while only 59% of patients with =8° of postoperative posterior slope of the implant reached PASS for KOOS pain.(P=0.015)

Conclusion

Posterior tibial slope, preoperatively or postoperatively, was not associated with failure or most outcomes following non-robotically assisted, fixed-bearing medial UKA using modern implants with intramedullary rods, jigs and guides, similar to TKA. However, larger postoperative posterior slope of the implant (=8°) had significantly worse pain but function was similar.