Five-Year Survival of 20,946 Unicondylar Knee Replacements and Patient Risk Factors for Failure

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Abstract
Background: Improvements in implant design and surgical technique of unicondylar knee arthroplasty have led to reduced revision rates, but patient selection seems to be crucial for success of such arthroplasties. The purpose of the present study was to analyze the 5-year implant survival rate of unicondylar knee replacements in Germany and to identify patient factors associated with an increased risk of revision, including >30 comorbid conditions. Methods: Using nationwide billing data of the largest German health-care insurance for inpatient hospital treatment, we identified patients who underwent unicondylar knee arthroplasty between 2006 and 2012. Kaplan-Meier survival curves with revision as the end point and log-rank tests were used to evaluate 5-year implant survival. A multivariable Cox regression model was used to determine factors associated with revision. The risk factors of age, sex, diagnosis, comorbidities, type of implant fixation, and hospital volume were analyzed. Hazard ratios (HRs) and 95% confidence intervals (95% CIs) were calculated. Results: During the study period, a total of 20,946 unicondylar knee arthroplasties were included. The number of unicondylar knee arthroplasties per year increased during the study period from 2,527 in 2006 to 4,036 in 2012. The median patient age was 64 years (interquartile range, 56 to 72 years), and 60.4% of patients were female. During the time evaluated, the 1-year revision rate decreased from 14.3% in 2006 to 8.7% in 2011. The 5-year survival rate was 87.8% (95% CI, 87.3% to 88.3%). Significant risk factors (p < 0.05) for unicondylar knee arthroplasty revision were younger age (the HR was 2.93 [95% CI, 2.48 to 3.46] for patient age of 74 years was used as the reference); female sex (HR, 1.18 [95% CI, 1.07 to 1.29]); complicated diabetes (HR, 1.47 [95% CI, 1.03 to 2.12]); depression (HR, 1.29 [95% CI, 1.06 to 1.57]); obesity, defined as a body mass index of ≥30 kg/m2 (HR, 1.13 [95% CI, 1.02 to 1.26]); and low-volume hospitals, denoted as an annual hospital volume of ≤10 cases (HR, 1.60 [95% CI, 1.39 to 1.84]), 11 to 20 cases (HR, 1.47 [95% CI, 1.27 to 1.70]), and 21 to 40 cases (HR, 1.31 [95% CI, 1.14 to 1.51]) (>40 cases was used as the reference). Conclusions: Apart from known risk factors, this study showed a significant negative influence of obesity, depression, and complicated diabetes on the 5-year unicondylar knee replacement survival rate. Surgical indications and preoperative patient counseling should consider these findings. Level of Evidence: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.

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