Preoperative mental health predicts early satisfaction but not two-year knee-specific functional improvement after unicompartmental knee arthroplasty: an analysis of a robotic-assisted cohort

Abstract Background Preoperative mental health is an important determinant of patient-reported outcome measures (PROMs), but its influence in unicompartmental knee arthroplasty (UKA) remains poorly characterized. In this robotic-assisted UKA cohort, we determined the effect of poor preoperative mental health on PROMs at six months and two years, described the change in mental health after surgery, and whether preoperative mental health predicted satisfaction and expectation fulfilment. Methods Prospectively collected registry data on 241 consecutive robotic-assisted medial UKAs performed between 2017 and 2024 were reviewed. Of these, 121 formed the primary complete-case cohort, and patients were stratified by preoperative 36-item Short Form Health Survey (SF-36) Mental Component Summary (MCS) into poor mental health (MCS < 50, n = 51) and good mental health (MCS ≥ 50, n = 70). Outcomes included the Knee Society Function and Knee Score (KSFS & KSKS) and Oxford Knee Score (OKS), SF-36 Physical Component Summary (PCS), satisfaction, and expectation fulfillment. Multivariable logistic regression identified predictors of satisfaction, expectation fulfilment, and mental health recovery. Results Mean preoperative MCS was 41.2 in the low-MCS group and 58.2 in the high-MCS group ( p < 0.001), with the former achieving greater MCS improvement at six months and two years ( p < 0.05). Despite a lower preoperative KSKS and a higher (worse) OKS ( p < 0.05) in the low-MCS group, both groups demonstrated comparable improvements in KSFS, KSKS, and OKS. However, divergent global physical recovery was observed at two years, with the high-MCS group achieving greater improvements in the SF-36 PCS compared to the low-MCS group (Δ 17.0 versus 11.1, p = 0.006; MCID attainment 85.7% versus 66.7%, p = 0.024; ANCOVA-adjusted p = 0.002). At six months, satisfaction (82.4 versus 97.1%; p = 0.008) and expectation fulfillment (70.6 versus 90.0%; p = 0.008) were significantly lower in the low-MCS group, while no difference was detected at two years. In exploratory subgroup analysis, persistently low MCS patients had significantly worse two-year PROMs than the high-MCS group. Preoperative MCS independently predicted six-month satisfaction (OR 1.078; p = 0.005) and expectation fulfillment (OR 1.061; p = 0.005). Among low-MCS patients, six-month MCS was associated with two-year mental health normalization (OR 1.083; p = 0.008), with an internally derived cutoff of MCS ≥ 53 (area under the curve [AUC], 0.783). Conclusions Within this robotic-assisted UKA cohort, patients with poor preoperative mental health reported lower early satisfaction, but still achieved meaningful mental health recovery and knee-specific functional gains comparable to those with better preoperative mental health by two years, with no detectable difference in satisfaction at two years. Poor preoperative mental health should not preclude appropriately selected patients from undergoing UKA. The six-month MCS is an internally derived exploratory signal that may help identify patients warranting closer monitoring.