Prosthesis position: UKA prosthesis Overhang will bring what consequences?

Unicompartmental knee arthroplasty (UKA) is a surgical alternative to total knee arthroplasty (TKA) for the treatment of unicompartmental osteoarthritis. However, data on the failure rate of UKA show a 7-year survival rate of 74%, which is significantly lower than that of TKA (92%). Although patients' own factors increase the risk of failure, such as younger patients and higher body mass index (BMI) may increase the risk of UKA failure, surgical technical errors have been considered as important risk factors for early failure. In UKA, it is challenging to achieve optimal prosthetic alignment and overhang (overhang). The tibial prosthesis should be sized and implanted in a manner that minimizes soft tissue irritation, and a medial tibial overhang of more than 3 mm has been shown to be a significant risk factor for decreased Oxford Knee Score (OKS) and increased pain. The primary objective of this study was to evaluate the overall clinical and imaging outcomes of UKA. The secondary objective was to assess the effect of prosthetic malalignment and overhang on implant survival.Unicompartmental knee arthroplasty (UKA) is a surgical alternative to total knee arthroplasty (TKA) for the treatment of unicompartmental osteoarthritis. However, data on the failure rate of UKA show a 7-year survival rate of 74%, which is significantly lower than that of TKA (92%). Although patients' own factors increase the risk of failure, such as younger patients and higher body mass index (BMI) may increase the risk of UKA failure, surgical technical errors have been considered as important risk factors for early failure. In UKA, it is challenging to achieve optimal prosthetic alignment and overhang (overhang). The tibial prosthesis should be sized and implanted in a manner that minimizes soft tissue irritation, and a medial tibial overhang of more than 3 mm has been shown to be a significant risk factor for decreased Oxford Knee Score (OKS) and increased pain. The primary objective of this study was to evaluate the overall clinical and imaging outcomes of UKA. The secondary objective was to assess the effect of prosthetic malalignment and overhang on implant survival.

Methods

This study was a single-center study that included male and female patients who underwent medial UKA at our institution from January 2008 to December 2017, and all patients underwent UKA using standard methods.Exclusion criteria for UKA included moderate to severe valgus deformity, patellofemoral compartment arthritis, posterior tibial displacement, and instability. Clinical and imaging results were obtained from a retrospective review of clinical records and institutional databases. Femoral coronal and sagittal angles (FCA and FSA) and tibial coronal and sagittal angles (TCA and TSA) were measured on radiographs for all fixed-platform and movable-platform UKAs, as shown.

Result

The mean follow-up was 4.9 years (0.03 to 10.2 years). We identified 36 revision surgeries with a mean of 3.7 years (0.03 to 8.7 years) postoperatively and a cumulative failure rate of 14.2%. 5-year survival was 88.0% (95% confidence interval [CI]=82.0% to 91.0%) and 10-year survival was 70.0% (95% CI=56.0% to 80.0%).

The most common reasons for revision were poor/unstable prosthesis alignment (31%), mechanical failure (22%), progression of osteoarthritis (14%), and unexplained pain (8%).

Only 11.9% of UKAs achieved all the desired alignment and prosthetic coverage. Anterior overhang of the prosthesis was not a significant risk factor for UKA failure (10.0% failure rate, p=0.090), and posterior overhang of the prosthesis (25.0%, p=0.006) and medial overhang of the prosthesis (38.2%, p<0.001) were significant risk factors for UKA failure.

Discuss

In this study, we evaluated the clinical and imaging outcomes of medial UKA and the impact of prosthetic alignment and overhang. 5- and 10-year survival rates were 88% and 70%, respectively, and were much lower than for TKA. the most common indications for revision were poor prosthetic alignment, mechanical failure, and progression of osteoarthritis. Poor prosthesis alignment and posterior and medial overhang are important risk factors for UKA revision.

In the published literature, the two main causes of UKA failure are aseptic loosening and osteoarthritic progression, both of which have uneven load distribution, suggesting that the above-mentioned causes of UKA failure may be mechanical in nature. This evidence suggests that poor prosthesis alignment is a potential cause of early UKA failure. Therefore, we believe that efforts should be made to improve surgical techniques to improve UKA prosthesis alignment and reduce the occurrence of overhang.

The current findings suggest that the strong association between poor prosthesis alignment/overhang and the risk of revision suggests a potential mechanism for the high failure rate observed in this study. The surgeon's ability to be consistently accurate in the placement of the prosthesis is an important factor in helping to improve UKA survival rates.

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