Head, acetabular liner composition, and rate of revision and wear in total hip arthroplasty: a Bayesian network meta-analysis

Total hip arthroplasty (THA) is a common procedure for patients suffering from hip pain e.g. from osteoarthritis, osteonecrosis, or hip fractures. The satisfaction of patients undergoing THA is influenced by the choice of implant type and material, with one key factor being the selection of the appropriate material combination for the bearing surface. In this Bayesian network meta-analysis, we investigated the impact of material combinations for the bearing surface on the longevity of hip implants. The wear penetration rate per year and the total wear penetration in the liner resulting from different material combinations, as well as the survival rate at last follow-up, were examined. We analyzed a total of 663,038 THAs, with 55% of patients being women. Mean patient age was 59.0 ± 8.1 years and mean BMI 27.6 ± 2.6 kg/m2. The combination of an aluminium oxide (Al2O3) head and an Al2O3 liner demonstrated the lowest wear penetration at last follow-up and the lowest rate of wear penetration per year. Additionally, the combination of a crosslinked polyethylene (XLPE) liner and a zircon oxide (ZrO2) head demonstrated the lowest rate of revision at last follow-up. These findings underscore the importance of careful material selection for hip implant bearing surfaces to optimize their longevity and patient satisfaction after THA.

In September 2023, the following databases were accessed: PubMed, Scopus, Embase, Google Scholar, Cochrane.A time constraint was set from January 2000 to September 2023.The following matrix of keywords were used in each database to accomplish the search using the Boolean operator AND/OR: THA AND (hip OR arthroplasty OR replacement OR prosthesis) AND (metal OR ceramic OR alumina OR zirconia OR polyethylene OR steel) AND (wear OR revision).No additional filters were used in the databases search.

Selection and data collection
Two authors (F.M. and R.M.) independently performed the database search.All the resulting titles were screened by hand and, if suitable, the abstract was accessed.The full-text of the abstracts which matched the topic of interest were accessed.If the full-text was not accessible or available, the article was not considered for inclusion.A cross reference of the bibliography of the full-text articles was also performed for inclusion.Disagreements were debated and mutually solved by the authors.In case of further disagreements, a third senior author (J.E.) took the final decision.

Synthesis methods
The statistical analyses were performed by one author (F.M.) following the recommendations of the Cochrane Handbook for Systematic Reviews of Interventions 25 .For descriptive statistics, mean and standard deviation were used.For baseline comparability, the IBM SPSS software was used.Comparability was assessed through the Analysis of Variance (ANOVA), with P > 0.1 considered satisfactory.The network analyses were made through the STATA/MP software (Stata Corporation, College Station, Texas, USA).Only studies which stated clearly the nature of the material of the component (head and/ or liner) were included in the analyses.An overview of the material combinations of head and liner included in the present Bayesian network meta-analysis is shown in Table 1.
The analyses were performed through the Stata routine for Bayesian hierarchical random-effects model analysis.Continuous variables were analysed through the inverse variance method, with the standardized mean difference (SMD) effect measure.Binary data were analysed through the Mantel-Haenszel method, with the Log Odd Ratio (LOR) effect measure.Edge, interval, and funnel plots were performed and analysed.The overall transitivity, consistency, and heterogeneity, as well as the size of the treatment effect of interest within-study variance, were evaluated.The overall inconsistency was evaluated through the equation for global linearity via the Wald test.In P Wald values > 0.05, the null hypothesis could not be rejected, and the consistency assumption could be accepted at the overall level of each treatment.Confidence and percentile intervals (CI a d PrI, respectively) were each set at 95%.

Ethical approval
This study complies with ethical standards.

Study selection
The initial databases search resulted in 22,423 articles.Of these, 5567 duplicates were excluded.After screening titles and abstracts 16,443 articles were excluded because they did not match the following eligibility criteria: not comparing two or more bearing material combinations, not mentioning rate of revision surgery or wear related values, no matching study design, not focusing on THA.Of the remaining 413 articles, another 274 were excluded because they did not report quantitative data for wear penetration, or rate of revision surgery, or the follow up time was shorter than 12 months.Finally, 139 studies were included in this review.The results of the literature search are shown in Fig. 1.

Study characteristics
Data from 663,038 THAs were collected.55% of patients were women.The mean patient age was 59.0 ± 8.1 years, the mean BMI was 27.6 ± 2.6 kg/m 2 .The mean length of follow-up was 87.9 ± 46.3 months.At baseline, no statistically significant difference was found in mean age, BMI, and mean length of follow-up (P > 0.5).The generalities and demographic and further basic data of the included studies are shown in Table 2.

Synthesis of results
The combination of Al 2 O 3 head and Al 2 O 3 liner demonstrated the lowest wear penetration at last follow-up (Fig. 3) and the lowest rate of wear penetration per year (Fig. 4).The combination of HXLPE head and ZrO 2 demonstrated the lower rate of revision at last follow-up (Fig. 5).The equation of global linearity found no statistically significant inconsistency in all comparisons.

Discussion
The choice of the best hip implant design and material of the bearing surface is crucial for patient satisfaction and longevity of the prosthesis.Different factors must be taken into account when choosing the best material combination for each patient.An important factor for the choice of the bearing surface biomaterial is wear, which remains a major problem in the long run leading to potentially aseptic loosening, pseudotumor formation, and pain.This network meta-analysis was conducted to compare the currently used material combinations for hip implant bearing surface regarding wear penetration, yearly penetration rate and revision surgeries.
In this Bayesian network meta-analysis, the combination of Al 2 O 3 head and Al 2 O 3 liner demonstrated the lowest wear penetration at last follow-up as well as the lowest rate of wear penetration per year.On the other hand, the combination of a HXLPE liner and ZrO 2 head demonstrated the lowest rate of revision at last followup.Mean age, mean BMI, and mean length of the follow-up had no significant influence on wear behaviour and revision rate.
In general, bearing surfaces in hip implants can be distinguished in hard on soft bearings (with a polymeric material used for the liner and the hard femoral head) and in hard on hard (MoM or CoC) bearings.Given the hardness difference of the articulating partners, hard on hard bearings show lesser and smaller wear particles than hard on soft bearings 7,26,27 .
Wear modes in a tribological system depend on its structure, kinematic interactions, and the combination of wear phenomena.Wear modes are dynamic, and can change over time.Wear modes can be distinguished in normal wear (mode 1), wear occurring due to bearing surfaces articulating against non-bearing surfaces (mode 2), three-body wear (mode 3), and two non-bearing surfaces wearing against each other (mode 4).In the presence of hard wear particles, particularly, polyethylene wear increases.Harder materials result in a reduced contribution of third-body wear to overall wear 28,29 .
CoC bearings have been used in THA for a long time given their biocompatibility, high wear resistance and chemical durability 7 .Additionally, CoC bearing combinations have the advantage to produce smaller and inert debris compared to other bearing types, leading to harmless wear to the human body.For this reason, they are generally considered a good choice for young patients 30 .The first generations of alumina ceramics had a high risk of fracture, which was later reduced by improving their manufacturing process 31 .Despite further improvements, ceramics as bearing surfaces still have weaknesses such as bearing noise and reduced toughness, which led to the development of advanced material combinations, such as AMC/ZTA, for use as bearing materials.Biomechanical studies have shown that AMZ/ZTA ceramics exhibit lower wear rates under extreme conditions compared to Al 2 O 3 32, 33 .Nonetheless, our network meta-analysis found that Al 2 O 3 ceramics had the lowest wear penetration rate per year and the least amount of wear at last follow up.This could possibly be explained by the fact that the latest material such as AMZ/ZTA is newer on the market and the average study duration is thus potentially shorter.We only included studies with a minimum duration of 12 months in our analysis; nevertheless, shorter study durations may overestimate debris and wear given the influence of running-in effects 17 .
Despite its good wear resistance, the Al 2 O 3 -Al 2 O 3 combination did not exhibit the lowest revision rate in this meta-analysis.One major disadvantage of an Al 2 O 3 combination are the disturbing noises which are associated with vibrations of the femoral implant system 34,35 .Compared to MoP or MoM bearings, fracture of ceramic heads and liner still remains a major disadvantage for CoC bearings 34 .A study based on the Norwegian Arthroplasty Register found a 3.6 times higher occurrence of ceramic fracture in COC bearings compared to COP bearings.Furthermore, there was an elevated risk of fractures observed in Alumina ceramics compared to AMC heads 36 .Revision for ceramic fracture is of particular concern, as it can lead to catastrophic failures and severe complications because of third body wear caused by ceramic fragments 37,38 .Additionally, the use of CoC bearings is expensive and requires an exquisite surgical insertion technique to avoid chipping off from contact surfaces 39 .In this study, the combination of HXLPE liner and ZrO 2 head demonstrated the lowest rate of revision at last follow-up.National registries are an important tool to compare revision rates of different material combinations.In the Australian Orthopaedic Association National Joint Replacement Registry (AOA) in 2022 Ceramised Metal head on XLPE liner exibit the lowest 10-year revision rate followed by ceramic head on XLPE liner, which, however, has the lowest 20-year revision rate with 6.8%.20 year data for ceramised metal head on XLPE liner are not available yet 40 .The National Joint Registry (NJR) of England and Wales in 2022 reports ceramic on polyethylene to have the lowest 15 year revision rates for all fixation types 41 .The German Arthroplasty Registry (EPRD) registered the lowest 6-year-revision rate for CoC bearings for elective THA.Nevertheless, ceramic on HXLPE bearings were, with 49.2%, the most frequently used bearing type in Germany in 2021 42 .Regarding NJR data in England and Wales, MOP is still the most commonly used bearing with decreasing tendency, while the use of CoP bearings increases 41 .Crosslinked polyethylene is listed as the most commonly used polyethylene type, with 97.2% in 2021 in Australia 40 .In general, low revision rates for CoP and CoHXLPE are mentioned across all the registries.
The German registry classifies polyethylene into different degrees of crosslinking such as UHMWPE, MXLPE, and HXLPE, whereas the NJR only considers polyethylene as a single category.Similarly, the materials of the heads are divided only into broad categories of metal and ceramic or partly ceramised metals by the NJR.As a result, it is not possible to conduct a detailed analysis of the material properties in registry studies.Additionally, in registries, implant combinations are selected for patients based on individual characteristics, making comparisons between implant combinations highly susceptible to bias.Systematic reviews and meta-analyses have been conducted to overcome these limitations.A few exceptions aside 20 , most review studies only offer analyses of two or three material combinations 19,43 .
We performed a comprehensive Bayesian network meta-analysis investigating more than 600,000 THA with 23 different material combinations.As mentioned, in registry studies, CoP bearings exhibit low revision rates.Biomechanical studies found improved wear behaviour for HXLPE compared to PE, which should also entail a longer lifetime 15,44 .Zirconia as material for hip implants head has promising properties.In 2001, however, the largest manufacturer of zirconia femoral heads recalled their products for problems with thermal processing associated with some batches producing higher fracture rates, leading to a loss of confidence in zirconia as a reliable orthopaedic biomaterial 45,46 .ZrO 2 hip implant heads are also mentioned to be prone to aging 47 .Nevertheless, ZrO 2 is widely used in dental applications 48 .A registry study in 2012 stated that ZrO 2 heads are inferior to metal heads regarding revision rate at 12 years 49 .Of note, most studies evaluating ZrO 2 on HXLPE bearing surfaces included in this network meta-analysis were performed in Japan [50][51][52][53] .Demographic characteristics could thus influence the results of this study.Nevertheless, the positive results for ZrO 2 heads observed in the present network meta-analyses may prompt surgeons to rethink their attitude towards this material.However, only few studies investigated the survival rate of zirconia in the last few years.
The present study has several limitations that should be considered when interpreting the results.First, the influence of the head diameter, the fixation technique of stem and cup as well as the orientation of the cup and liner were not analysed.A high inclination angle can cause an increase in liner wear 54 .The head diameter of the prosthesis is an important factor that can affect the performance of the prosthesis, especially regarding the risk of dislocation 55,56 .A larger head diameter can lead to increased volumetric wear in polyethylene cups, while linear wear remains consistent [57][58][59] .From our analyses, we cannot tell whether certain materials were preferably used in specific sizes.Future studies should consider the influence of head diameter in their analyses.Second, other types of head designs such as dual mobility bearings or hip resurfacing were not explicitly described.Although we subdivided polyethylene into different categories based on the descriptions used in the studies (CPE/UHM-WPE, XLPE, HXLPE, MXLPE, HXLPE-VEPE), there could be differences arising from different manufacturing techniques such as annealing and remelting of the polyethylene or amount of crosslinking 60 .Currently, different treatments, including irradiation and melting, irradiation and annealing, sequential irradiation with annealing, irradiation followed by mechanical deformation, and irradiation and stabilization with vitamin E are available 61 .Irradiating UHMWPE results in cross-linking between the molecular chains, which improves the mechanical and tribological properties of this cross-linked PE 62 .The offset of that is that crosslinking affects the mechanical properties of UHMWPE, usually resulting in a decrease in toughness, stiffness, and hardness of the polymer 63 .Despite that effect, cross-linked UHMWPE is presently the standard of care.

Figure 1 .
Figure 1.PRISMA flow chart of the literature search.

Figure
Figure 2. Cochrane risk of bias tool.

Figure 3 .
Figure 3. From left to the right: edge, funnel and interval plots of the comparison: overall wear penetration.

Figure 4 .
Figure 4. From left to the right: edge, funnel and interval plots of the comparison: wear penetration per year.

Figure 5 .
Figure 5. From left to the right: edge, funnel and interval plots of the comparison: rate of revision at last follow-up.

Table 1 .
Material combinations of head and liner included in the present Bayesian network meta-analysis.

Author Year Design Head Liner Patients (n) Mean Age Mean BMI Women (%) Follow-up (months)
2. Cochrane risk of bias tool.

Table 2 .
Generalities and patient baseline data of the included studies.RCT randomised controlled trial; CoCr Cobalt-Chrome.