Bearing dislocation of mobile bearing unicompartmental knee arthroplasty in East Asian countries: a systematic review with meta-analysis

Unicompartmental knee arthroplasty (UKA) is an effective treatment for patients with unicompartmental knee osteoarthritis. Compared with total knee arthroplasty (TKA), patients who have undergone UKA have better functional recovery but a higher revision rate due to prosthesis failures, among which dislocation of the mobile bearing is considered the main cause [1‐3]. In a recently published systematic review of Oxford Phase III UKA, Ro et al. [4] found that Asian patients had a higher dislocation rate than Western patients because of their traditional lifestyle and religious practices, which involve the sitting kneel position or cross-leg sitting. However, whether this fact applies to all East Asian populations remains debatable [4‐9]. In this review, we focused only on patients from East Asia who shared similar lifestyle practices involving regular knee hyperflexion to identify differences among countries. Another unresolved issue of concern to orthopedic surgeons is the cause of bearing dislocation. Some researchers have recently suggested that bearing dislocation usually happens shortly after primary UKA because of errors in this procedure [4, 5, 10]. In contrast, some other researchers have shown that bearing dislocation can occur long after the primary operation, indicating that the direct cause might be bearing wear [6, 11]. Because few studies have investigated the mechanism of bearing dislocation, which is even more difficult to review by a meta-analysis, we evaluated the time interval between dislocation and the primary operation to accumulate more evidence. The treatment of bearing dislocation is another important issue for both joint surgeons and patients, and few studies have specifically focused on treatment. In this review, we also focused on the treatment strategy and outcome, especially with respect to the risk of re-dislocation.

The major aim of this meta-analysis was to determine whether the dislocation rates in East Asian countries were different. Second, we explored the epidemiology of this complication and aimed to determine the highest risk time period and most likely direction of dislocation in the Asian population. Third, we sought to identify the most suitable treatment and assessed the re-dislocation rate.

This systematic review and meta-analysis was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist. Ethical approval was not required.

Multiple researchers have agreed that Asian patients have a higher dislocation rate than Western patients after UKA [1, 4‐6]. The meta-analysis by Ro et al. [4] showed that the rate was three times higher in East Asian than Western countries and suggested that regular knee hyperflexion along with the anatomical features of the femoral condyle might contribute to this high rate. However, previous research has failed to explore differences among East Asian countries. Although their lifestyle and anatomical structure are similar, Korean patients had an even higher dislocation rate in the present review. In the subgroup of other East Asian countries (China and Japan), the incidence was only 0.74%, similar to the incidence of 0.7% among 1000 cases with a 15-year follow-up as reported by the Oxford group [21]. One possible explanation is that Korean surgeons might be more willing to share their failures with the public; this was also suggested by the funnel plot of publication bias. The high dislocation rate also drew attention, making Korean researchers more willing to take part in the discussion. Another important fact is that there was no artificial joint registry system in any of the East Asian countries, making multicenter studies with large sample sizes difficult to perform. It may be another cause of this difference because a small sample size would lead to selection bias. Nevertheless, the subgroup analysis of other countries suggested that the bearing dislocation rate was not as high as in Korean patients; thus, UKA is still an effective treatment for knee osteoarthritis in East Asian populations.

High heterogeneity was found in the Korean group (I² > 50%) when we included the study by Choy et al. [11], which involved 15 dislocations among 164 cases. After removing this study, the I² decreased to 0%. This might have been due to the long follow-up period. In their study, Choy et al. [11] evaluated a consecutive UKA case series for an average follow-up of 12.1 years; this is much longer than in the other six reports, in which the follow-up was no longer than 10 years. For this reason, the authors were able to include nine dislocations due to bearing wear, which was considered secondary dislocation and has been seldom seen in other reports. As these late-stage dislocations were added, the dislocation rate became even higher than that in other Korean reports [5, 6, 13]. The results of the present review also highlight the controversy regarding the dislocation onset time. Some researchers believe that most dislocations occur in the early postoperative period and that only a small number of dislocations combined with bearing wear or other complications (periprosthetic fractures, ligament ruptures, or aseptic loosening) occur in the late stage. A newly published multicenter retrospective study [5] showed that most dislocations occurred shortly after surgery, with an incidence of 55% within 2 years and 79% within 5 years. A meta-analysis by van der List et al. [2] also confirmed this. However, when we defined early-stage as 2 years after primary UKA, our meta-analysis showed no difference between these two subgroups. When we adjusted the threshold to 5 years, the difference became statistically significant. This finding might infer that the risk of bearing dislocation is higher during the first 5 years postoperatively. However, because the follow-up time of all included studies was around 6 to 7 years, information on the long-term result of mobile-bearing UKA is limited. Whether the dislocation risk peaks again after 10 years postoperatively remains unknown.

Several reports of the clinical outcomes of UKA have provided information on the direction of bearing dislocation. Most researchers agree that anterior and posterior dislocation is common while medial and lateral dislocation is rare. Our meta-analysis also conformed this, and to our knowledge, it is the first study to apply statistical methods to analyze this. Furthermore, we found no difference between anterior and posterior dislocations, which differs from the findings described in a previous report [22]. This could be explained by determining exactly how bearing dislocation occurs. We explored related research [5, 23‐25] of bearing dislocation mechanisms and identified five possible mechanisms: (1) bearing rotation due to malposition of the metallic component, (2) an imbalanced flexion–extension gap, (3) cam impingement, (4) medial collateral ligament (MCL) dysfunction or injury, and (5) an improper bearing size. In fact, these dislocation mechanisms may not exist alone, and the collaboration of multiple mechanisms may underlie the main process of bearing dislocation. The Oxford UKA system is anatomically designed to match the spherical design of the femoral component. The superior surface of the meniscal bearing is concave: thin in the middle with high surrounding rims. The anterior rim is 5 mm, the posterior rim is 3 mm, and medial and lateral rims are both 2 mm. Malposition of either the femoral or tibial component may lead to bearing rotation [25, 26], and in this situation, the lower rim of the medial and lateral part loses its restriction of the mutual movement between the femoral prosthesis and the bearing. As the knee suddenly flexes or extends, the osteophyte in the front or residual meniscus or bone cement in the back may force the bearing out of its normal position toward either the anterior or posterior direction [27]. An imbalanced joint gap, improper bearing size, or luxation of the MCL may all aggravate this phenomenon.

Various treatment options for bearing dislocations are available, including MUA, bearing change, revision to fixed-bearing UKA, and conversion to TKA. UKA metallic component revision has seldom been described in previous reports, possibly because of surgeons’ lack of confidence with this procedure. The treatment information in the articles of the present review is shown in Table 4. In the subgroup meta-analysis of the four Korean reports that provided treatment outcomes, the re-dislocation rate was unexpectedly much higher than the primary dislocation rate. The two most likely causes of this might be as follows. First, the surgeon can remove impingement factors such as osteophytes or redundant cement by exploring the possible causes of dislocation during the revision surgery. However, malposition or poor alignment of the metallic component are beyond the surgeon’s reach in either the bearing change procedure or MUA, in which the newly implanted meniscal bearing is still at high risk of dislocation [22]. Second, potential luxation of the MCL and imbalance of the flexion–extension gap may also contribute. The mechanism of dislocation should be considered before selecting the treatment strategy. Theoretically, re-dislocation will not occur if the underlying causes are corrected during the revision procedure. If not, bearing exchange may not be the most suitable treatment option. Because revision UKA to TKA has been proven effective with a complication rate equivalent to that of primary TKA [28‐30], conversion to TKA should definitely be considered.

This meta-analysis has four main limitations. First, we included only studies of East Asian patients to determine the difference in the incidence of bearing dislocation among countries. When analyzing the onset time, possible direction, and re-dislocation rate, inevitable selection bias was present because of the exclusion of Western studies. Second, all reports in the present review were case series with an evidence level of IV. Although there were no control groups, the design of the studies was still considered high-quality with a mean MCMS of 75.4 ± 4.7 (considered good quality). The assessment with the modified MINORS score showed that one article from China had a high risk, which may lead to selection bias. Third, the meta-analysis included few case series of UKA complications in China and Japan, which is also a source of bias. Additionally, the follow-up duration in most studies was < 10 years, which may not be long enough to analyze secondary dislocation. As suggested by Choy et al. [11], the timeline of the dislocation risk might have two peaks; however, we only identified the early-stage peak because of the limited follow-up time. Fourth, improvements in the surgical technique and instrumentation of UKA were not described specifically in this meta-analysis. The introduction of Microplasty® instrumentation in 2014 was considered an effective method to decrease the incidence of complications [6, 19, 25]. However, there are still no adequate clinical results available for analysis. During the next few years, we might be able to determine whether this instrument can decrease the dislocation rate by evidence-based methods.

In conclusion, our meta-analysis has demonstrated that Korea has a higher bearing dislocation rate among East Asian countries. Additionally, mobile bearings have a higher risk of dislocation in the first 5 years after UKA. Anterior and posterior dislocations are most common, but we found no difference between these two directions. The most important finding is that the re-dislocation rate is likely to be much higher than the initial dislocation rate if unsuitable management is applied.