Although patient selection is thought to influence the success of UKA, controversy remains over which specific factors affect the outcome of this procedure. Patient age, gender, and weight have been examined in previous studies without conclusive findings. Other factors, such as pre-operative diagnosis Knee Society function scores and patellar arthritis, have rarely been studied in relation to failure of UKA implant. This study used prospectively collected data to examine seven factors that may be associated with failure of UKA implants. We followed 80 knees for an average of 60 months. The survivorship of the UKA implants was 84% at 10 years follow up which is comparable to those reported in the literature [9, 10]. Overall, we did not find any independent predictor of failure of UKA.
Traditionally, UKA was recommended for patients aged 60 years or over with a sedentary lifestyle . However, with a hazard ratio of 0.94, our results suggest that age is not a predictor of failure of UKA. Gioe et al. examined the survival of 1,047 knee arthroplasties in patients aged 55 years old or younger using a community registry and did not find an association between age groups and survival rate . Although the mean age in the failure group of the present study was 6 years younger than the non-failure group, young age was not found to be an independent predictor of failure. Several studies devote attention to younger patients (less than 60 years of age) treated with UKA, all of whom had excellent results. Schai et al. followed 28 knees in 28 patients who had a mean age of 52 years; only two knees were revised over a maximum of six years follow up . Similarly, Pennington et al. reported a survival rate of 92% at 10 years in a group of younger patients . Tabor and Tabor evaluated two patient cohorts to compare the survivorship and functional outcomes of UKA of patients aged 60 and over to those in a younger age group, and did not find a significant difference . However, there are also studies reporting a poor survival rate in younger patients [14–16]. Additionally, using a Cox proportional hazard model, two studies found a hazard ratio of failure that favors superior outcomes in older patients [14, 16]. The difference in these findings could be attributed to the age range of patients and the skills of the surgeons.
To date, gender has not been used as an inclusion/exclusion criterion for UKA, though some studies have found a difference in outcomes between male and female patients [10, 17, 18]. However, consistent with our findings, the majority of the studies did not find gender as a significant predictor of failure of UKA [11, 14–16].
Weight and obesity are other factors to consider when UKA is applied. A multi-center investigation by Heck et al. reported mean BMIs in the failure and non-failure groups of 33 kg/m2 and 25 kg/m2, respectively . However, many other studies have not found an association between weight and/or obesity and failure of UKA [10, 15]. One study even suggested that obese patients had a better survival rate when compared to their non-obese counterparts . In addition, excellent survival rates have been reported in studies that did not consider weight when qualifying patients for UKA . Despite some surgeons suggesting that patients over 80 kg or those who are clinically obese should not be treated with UKA [5, 20], such criteria do not seem to be supported by the majority of studies, including the findings in the present report.
Although most UKAs are performed to in patients with osteoarthritis, it is not the only indication for UKA. Osteonecrosis can be treated with UKA with good results. Parratte et al. studied 31 osteonecrotic knees receiving UKA with a minimum follow up of three years and reported the survival rate of 96.7% at 12 years . The authors noted that the outcomes of UKA were similar to those in primary osteoarthritis . Similarly, Gioe et al. reported that there is no difference in survival rate based on diagnosis .
Preoperative Knee Society objective and functional scores, and patellar osteophytes have rarely been studied as predictors for UKA failure. Although anterior knee pain is a relative contradiction for UKA based on conventional surgical criteria, a recent study found that it did not affect the success of UKA using the Oxford phase 3 device . Our findings indicate that pain and function of the affected knee are not related to failure of UKA. Patella osteophytes were also not a risk factor for UKA failure.
UKA is an effective treatment for unicompartmental knee disease. In addition to its clinical advantages, it may be more cost-effective when compared to TKA . Opponents of UKA cite the poor survival rate of UKA implant relative to TKA. However, several studies have reported excellent survival rates [19, 24]. Patient selection is a critical issue to success with this treatment modality. Conventional criteria suggest that patients should be over 60 years of age, weigh no more than 82 kg, and not perform heavy labor or be extremely physically active [20, 25]. Although careful selection of patients is a key to the success of UKA, excessive restrictions will discount the benefits of the procedure and underplay its importance in treating unicompartmental knee disease. Better outcomes may be achieved with expanded criteria as the surgical technique and devices continue to be developed. Improvement in our understanding of factors related to UKA failure will shed light on patient selection criteria and help improve surgical outcomes of UKA.
Several limitations are noted in this study. First, the sample size is relatively small. Certain patient factors, notably obesity, trended towards significance in our analysis of independent predictors of failure, and it is possible that a larger study group would provide additional power to better define the associations between the factors and risk of failure of UKA. Additionally, because of the small and diverse number of failures, we did not attempt to assess hazard ratios for each individual cause for revision. It is possible that such an analysis would reveal variability in independent associations for some modes of failure. Finally, the follow up time is relatively short compared to some other studies on UKA. The average length of follow up was five years, which affects the survival rate in this study. In addition, long-term outcomes could not be assessed.