Introduction

Since the first mobile-bearing total knee arthroplasty (MB-TKA) procedures have been performed in the 1980s [21], numerous scientific studies have compared MB-TKA with fixed-bearing total knee arthroplasty (FB-TKA) [3, 21, 55, 95, 100, 130, 133]. The mobile-bearing design was developed to allow rotation of the insert around the longitudinal axis (“rotating platform”) or to allow anterior–posterior translation between the insert and the tibial tray of the prosthesis (“meniscal bearing”). Due to the rotational and the translational properties between the insert and the tibial tray, the mobile-bearing insert can be modelled such that they have a better fit with the femoral component without compromising the natural rotation and translation between femur and tibia. This is contrary to the fixed inserts in FB-TKA, which are relatively flat and, therefore, allow some small rotations and translations, but much smaller compared to the MB-TKA [105].

MB-TKA has been theorised in the literature to result in four advantages over FB-TKA: reduced insert wear, less risk of loosening, fewer revisions and better clinical outcome. Firstly, MB-TKA is expected to result in less polyethylene wear because of a larger contact surface between the femoral component and the insert, induced by a more optimal fit of the femoral component and the insert [30]. In addition, the insert can rotate and translate relative to the tibial component, which means that the femoral component slides less on the surface of the insert, which also potentially results in less wear. Secondly, MB-TKA is hypothesised to reduce the chance of loosening of the prosthesis because of less osteolysis [30]. This is thought to be due to the movement of the insert on the tibial tray, resulting in less stress on the bone–cement interface of the tibial component, and less wear-induced osteolysis. The third advantage described in the literature is that less wear and loosening result in a lower number of revisions and, therefore, a better survivorship of the prosthesis [20]. The final theorised advantage of MB-TKA is better clinical outcome. The mobility and design of the insert are hypothesised to result in a more natural movement of the prosthetic knee in daily life. Several disadvantages of MB-TKA have also been described. A known complication of MB-TKA is dislocation of the insert [51, 130]. During surgery, a high level of precision in balancing of the flexion and extension gap is necessary to prevent dislocation or spin-off of the insert. Therefore, MB-TKA is acknowledged to be associated with a prolonged learning curve and an increased risk of soft tissue impingement [30]. Additionally, the fact that in MB-TKA there is a second articulating surface could be a risk for increasing wear as a larger surface of the insert is exposed to friction [30, 130].

In particular, in the last ten years, an increased number of high-quality articles have been published that have studied one or more of the four theorised advantages of MB-TKA. Reviews and meta-analyses have been performed to provide an overview of all the literature available, none finding any significant differences. However, these often include the same studies with a high level of evidence. Several studies have been performed that provide valuable information on MB-TKA and FB-TKA, but are omitted from these overviews because of their methodology. The goal of the present paper is to present an up-to-date overview of the scientific literature that includes studies of different levels of evidence that compare cemented MB-TKA with cemented FB-TKA with respect to insert wear, signs of loosening of the prosthesis, survivorship of the prosthesis and clinical outcome, and to arrive at an evidence-based advise with regard to the preferable type of insert.

Materials and methods

Search strategy

The PubMed MEDLINE database was searched for English language meta-analyses, (systematic) reviews, randomised controlled trials and comparative studies. The search terms used were: mobile bearing, rotating platform, meniscal bearing and anterior–posterior glide rotation. Fixed and total or TKA needed to be present as keywords. Unicompartmental and hemi were excluded in the search. The last search was performed on 17 February 2015. The complete search string can be found in Table 6 in “Appendix”

Eligibility criteria

Two independent reviewers (BF and DD) screened the results of the search, first using the title and abstract of the articles and second using the full text of the remaining articles to identify those eligible for inclusion. Studies comparing clinical, radiological and/or functional results of MB-TKA and FB-TKA were eligible for inclusion. The primary indication for TKA had to be osteoarthritis. In vitro studies, studies with kinematic results or studies that used biomechanical models, were excluded, as well as studies that focused on complete polyethylene tibial components or uncemented prostheses. In the current study, no differentiation was made between meniscal bearing and rotating platform subtypes of MB-TKA or between cruciate retaining and posterior stabilised prostheses.

Eligible articles of which the full text could be retrieved and which reported results on one or more of the four theorised advantages of MB-TKA were included. Wear of the insert had to be assessed by measuring the thickness of retrieved inserts, or using the Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System [35], or similar radiological measurement methods. The risk of loosening of the prosthesis had to be reported in the form of radiolucencies or osteolysis around the bone–cement interface. With respect to survivorship and the number of revisions, only those studies were included that reported survival as a result of aseptic loosening. Clinical outcome had to be compared by patient-reported outcome questionnaires looking at pain and functional impairment or by measuring the range of motion by looking at flexion and extension of the knee. In the case of disagreement about an article, this was resolved through a discussion between the reviewers.

Data collection

The following information was extracted from the included studies: author, year of publication, study design (meta-analysis, (systematic) review, randomised controlled trial, comparative study), level of evidence, type of prosthesis, number of prostheses, age of the MB group, age of the FB group and duration of follow-up. In addition, the main results of the articles were studied to see what results they reported for each of the four main outcome categories. For each outcome category that was reported in the article, the result was summarised as either MB = FB (no difference between MB-TKA and FB-TKA), MB (outcome favours MB-TKA) or FB (outcome favours FB-TKA). When several follow-up measurements were reported within a study, the final measurements were used for summarising the results. If a study did not perform a statistical analysis on an outcome category, no result was formulated for that specific category. If a study did not report consistent results within one of the four main outcome categories, there was no final conclusion made for that particular category. If a preference for a type of bearing was reported by patients who participated in bilateral comparative studies, this was also registered.

Data analysis

Final conclusions were based on the studies with the highest level of evidence (LoE), as determined by the reviewers using the criteria reported by several orthopaedic journals [108, 117, 140]. Bilateral comparative studies (where patients received a MB-TKA in one knee and a FB-TKA in the other knee) were considered as level 1 and non-systematic reviews as level 3. If information needed for determining the level of evidence was missing, the level of evidence was reported as one level lower. Because of the large amount of studies included in the current overview, conclusions on the four main categories were primarily drawn by analysing the studies with LoE 1, since these studies are considered to have the highest methodological quality. Afterwards, the results from studies with LoE 2 and 3 were analysed to see whether the results from those studies provided a different view.

Results

Search, Selection and Study characteristics

The PubMed search resulted in 258 articles, 121 of which were excluded based on abstract or title (Fig. 1). A full-text version was retrieved of the remaining 137 studies. After reading the full text, another 10 articles were excluded. Six of them did not report on the predetermined outcome variables, two articles had included uncemented prostheses in their analyses, and two articles were excluded because of comparing their own MB-TKA data with literature instead of their own FB-TKA data. All 127 studies are described in Table 1. The included studies consisted of 44 comparative studies (CS), 48 randomised controlled trials (RCT), 13 bilateral studies that compared MB-TKA in one knee and FB-TKA in the other (BiL), 10 reviews (R), three systematic reviews (SR) and nine meta-analyses (MA). No articles from before 2001 were found. Figure 2 shows the number of included papers that was published each year.

Fig. 1
figure 1

Study flow diagram [86]

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Table 1 Characteristics of included studies
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Fig. 2
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Number of publications per year

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Insert wear

Results of the LoE 1 studies that reported on insert wear are detailed in Table 2. All five studies, two of which were bilateral studies, did not find a difference between MB-TKA and FB-TKA when looking at the radiological signs of insert wear. When looking at the LoE 2 and 3 studies, all LoE 2 studies and four out of seven LoE 3 studies did not find a difference between MB-TKA and FB-TKA (Table 7 in “Appendix”). Three LoE 3 studies reported a significant difference in wear in favour of MB-TKA.

Table 2 LoE 1 insert wear results
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Signs of loosening of the prosthesis

Twenty-eight LoE 1 studies reported radiolucencies or osteolysis (Table 3). All studies except for 1 reported no difference for any of these variables. The exception was an RCT by Bailey et al. [5], who reported a significantly higher percentage of radiolucencies around the tibial component in MB-TKA. The LoE 2 and 3 studies did not find a difference between MB-TKA and FB-TKA (Table 8 in “Appendix”).

Table 3 LoE 1 signs of loosening of the prosthesis
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Survivorship

Table 4 shows all results on survival rate and number of revisions. Twenty-five LoE 1 studies were included, and none of these found a significant difference in either survival or revision rate between MB-TKA and FB-TKA. One LoE 2 study and three LoE 3 studies reported a significant difference in favour of FB-TKA for this parameter (Table 9 in “Appendix”).

Table 4 LoE 1 survival rate + revisions
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Clinical outcome

All clinical outcome results can be found in Table 5. Overall conclusion of the 50 LoE 1 studies was that there was no difference between MB-TKA and FB-TKA in almost all studies (n = 47), with 2 studies reporting results in favour of MB-TKA and 1 study reporting results in favour of FB-TKA. One LoE 2 study and four LoE 3 studies showed clinical outcome results in favour of MB-TKA, opposed to only one that showed more benefits of FB-TKA. However, the other 53 LoE 2 and 3 studies reported no differences (Table 10 in “Appendix”).

Table 5 LoE 1 clinical outcome parameters
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Table 6 Search string for PubMed/MEDLINE
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Table 7 LoE 2 + 3 insert wear results
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Table 8 LoE 2 + 3 signs of loosening of the prosthesis
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Table 9 LoE 2 + 3 survival rate + revisions
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Table 10 LoE 2 + 3 clinical outcome parameters
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Discussion

The most important finding of the present study was an absence in difference between MB-TKA and FB-TKA. When comprehensively reviewing all available literature, type of bearing in TKA did not appear to influence insert wear, signs of loosening, survival rate of the prosthesis and clinical outcome. Both the enlarged contact surface and the reduction in movement of the femoral component on the surface of the insert in MB-TKA were hypothesised to result in less polyethylene wear [30]. The studies with the highest LoE included in this overview did not show differences between MB-TKA and FB-TKA in insert wear. This could be explained by the fact that insert wear is rare altogether and occurs late in the life cycle of a prosthesis. Since the 15-year survival rate of TKA is known to be above 90 % [78], only a very small number of patients have revision surgery because of insert wear. With this in mind, studies with large numbers of patients and a very long follow-up are necessary to be able to determine a difference in insert wear between MB-TKA and FB-TKA. Since in vitro studies also have not been able to produce consistent results on insert wear [28, 39, 41, 82, 88], a possible decrease in insert wear does not appear to be an argument in choosing between MB-TKA and FB-TKA. However, when looking at studies included in this overview with a lower LoE, three out of seven LoE 3 studies showed results in favour of MB-TKA. Studies that include retrieved inserts are essential in assessing actual insert wear, but unfortunately this type of research is categorised in a lower LoE and, therefore, often overlooked. The fact that several LoE 3 studies find that MB-TKA appears to be associated with less insert wear is, therefore, noteworthy, but does not seem to be associated with differences in function, outcome or survival.

Taking the LoE of studies into account, the current study shows that radiolucencies and osteolysis around MB-TKA do not differ significantly from FB-TKA. The only LoE 1 study that found a higher percentage of tibial radiolucencies in MB-TKA also showed that this difference did not influence clinical outcome in their patients [5] and, therefore, this higher percentage seems not to be clinically relevant. It should be noted that the patients in this study were only evaluated at a maximum of 2-year follow-up. Whether the increase in tibial radiolucencies found in their MB-TKA group influences revision rates after 10 or 15 years is, therefore, unknown.

Several studies mentioned the number of revisions, but did not perform a statistical analysis to evaluate the differences. It was often unclear whether the revisions were caused by aseptic loosening or all causes. The LoE 1 studies did not show differences in the number of revisions. It is worth mentioning that three studies with a LoE of 2 or 3 showed a lower survival rate for MB-TKA compared to FB-TKA.

In this literature overview, patient-reported outcomes of questionnaires were included to quantify clinical outcome. The included literature showed that the experienced clinical outcome after undergoing MB-TKA did not differ from patients who underwent FB-TKA. The pain scores and ranges of motion of the knee also did not differ between both types of bearing. Besides, it has been shown that differences found in range of motion and questionnaires are hard to translate to clinically important differences, since these differences can fall within the variation of normal range of knee motion [105]. Furthermore, differences in objective measurements, like range of motion, do not appear to relate directly to the subjectively experienced quality of movement [115]. Based on this reasoning, in combination with the high amount of studies that did not find any differences in both the questionnaire results and the range of motion between MB-TKA and FB-TKA, it can be concluded that there is no difference between MB-TKA and FB-TKA in clinical outcome.

A strength of the current literature overview is the large number of included studies. In the first Cochrane review on this subject in 2004, only 2 articles were of sufficient methodological quality to be included [55]. Scientific research on MB-TKA has increased dramatically after this review, and out of the 127 studies included in the current study, 51 were LoE 1. The present paper provides an overview of both high and lower LoE studies that has not been presented earlier. Another strength is the fact that results were included on four different theorised advantages of MB-TKA, and, therefore, a more complete picture of the results of MB-TKA in comparison with FB-TKA is given.

There are also several limitations to the current study. Included studies were categorised according to their level of evidence [108, 117, 140]. Although this method has proven reliable and has been widely accepted for classifying methodological designs [13, 94], this classification does not fully address the methodological quality [99]. The results of several studies have been included twice, since several RCT and CS studies that were included are also used in the SR and MA studies. It is possible that a small number of studies have not been included because only the PubMed database was searched. However, the chance that these studies would alter the conclusions of this study is small, considering the large amount of included studies and their comparable results. It can be considered a limitation that all different brands of prostheses and the different types (e.g. posterior stabilised/cruciate retaining) in MB-TKA and FB-TKA groups were combined. Because of this heterogeneity, it is possible that better outcome of individual prostheses is not fully addressed. This is inherent to the design of this literature overview and to (systematic) literature studies in general. Based on the consensus amongst LoE 1 studies, it is not to be expected that further differentiation into different types of prostheses would change the conclusions of this literature overview. The number of studies published on MB-TKA and FB-TKA is large and still increasing. However, the recent increase in evidence does not seem to provide new insights. It can, therefore, be argued that the discussion concerning the differences between MB-TKA and FB-TKA is not furthered by additional studies on this subject.

Conclusion

An extensive literature review was performed on studies examining differences between MB-TKA and FB-TKA, including a large number of studies with a lower LoE that are generally overlooked in other reviews. No clear differences were found between MB-TKA and FB-TKA in insert wear, signs of loosening of the prosthesis, survival rate and clinical outcome. Because of this, surgeons deciding between MB-TKA and FB-TKA for use in their day-to-day practice should be guided by different arguments, like surgeon experience with a certain type of prosthesis and financial or logistic advantages of different prostheses.