Total hip arthroplasty (THA) is currently known as one of the most mature surgical procedures to treat diverse hip diseases including end-stage inflammatory or degenerative arthritis, displaced fractured neck of the femur, and end-stage avascular necrosis (1, 2). Up to date, more than 1 million THAs have been performed every year worldwide and the number in the United States alone was more than 5,00,000 (3). Despite the tremendous advances in prosthetic design and materials over the past decades, heated debates with respect to the choice of bearing surfaces remain controversial, especially about ceramic-on-ceramic (COC) components and ceramic-on-polyethylene (COP) components, both of which are most commonly used in clinical work (4). Since ceramic material was first introduced in THA by Boutin 50 years ago, it has attracted the attention of people owing to the typical feature of hardness and wettability with high resistance against wear (5). As the typical representative of “hard-on-hard” bearing surfaces, COC was successful in resolving the problem of wear and wear-related problems such as osteolysis (6), prosthetic loosening (7), and prosthetic joint infection (PJI) (8). As a result, the COC bearing surface is an optimal choice for younger patients with more activity. Compared with COC, COP was reported with higher wear rates and this may be the main concern in young patients and can be a choice for old patients and young patients with a possible contraindication of COC such as severe developmental dysplasia of the hip and posttraumatic acetabular deformity (9). However, patients often complained about COC due to obvious noise from clashing ceramics, which rarely occurs in COP (10). More importantly, given ceramic is more fragile than polyethylene (PE) in essence, both the patients and surgeries may concern fractures of the ceramic lining (11). Therefore, it is still hard to decide the optimal manner considering the complex factors such as the age of the patient (12), specific etiology (13), surgical approach (14), and follow-up time (15), all of which may affect the final result and interfere with the effect appraisal of prosthetic bearing surfaces. Therefore, the purpose of the present meta-analysis was to compare the outcomes of these two bearing surfaces in THA according to the selected randomized controlled trials (RCTs) and this can provide meaningful clinical guidance for the selection of prostheses.

This meta-analysis was performed based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (16).

Although many factors such as surgical technique, the status of the patient, and positioning of the prosthesis could affect the success of the surgery and the long-term survival of the artificial joint, prosthetic material, especially the selection of optimal bearing surface for the patient, must be considered by a surgeon due to different advantages and drawbacks of various types of bearing surfaces. Currently, the most commonly used bearing surfaces are COC and COP in clinical, both of which have shown reliable clinical results. COC bearing surface is attractive mainly owing to the low friction and low wear rates compared with the COP. Meanwhile, COC has material-related shortcomings such as the risk of fracture and the occurrence of audible noises. Despite previous clinical trials and meta-analyses having compared and evaluated these two kinds of bearing surfaces, the long-term follow-up data (≥10 years) are lacking. Therefore, we performed the meta-analysis to compare the COC with the COP according to included 15 RCTs and further subgroup analysis of the long-term follow-up studies.

The most important issue in the modern THA is the revision rate, which is mainly caused by wear debris-induced osteolysis, implant loosening, or deep infection. In this study, there were no significant differences between the two groups and the same result also appeared in the long-term follow-up studies. However, it is known that COC is with lower wear rate compared with COP due to the hardness and high wettability of the character of the material itself (31, 32). During the included RCTs, 5 studies reported the wear rate of bearing surface and all of them revealed that the mean liner wear rate in the COC group was lower than that in the COP group (9, 17, 18, 21, 22). In theory, a higher wear rate means more wear debris, which could induce osteolysis, implant loosening, or deep infection. Intriguingly, all of the three revision-related events were demonstrated, as no statistical difference in the two groups. Ceramic wear debris has shown to be less biologically active in laboratory studies (33). Therefore, the explanation that ceramic wear particles may be fewer in number, but more reactive than PE cannot be convincing. Considering the metal debris from the taper-trunnion junction in hip prostheses has been widely recognized (34), it is reasonable to speculate the COC with a lower wear rate can produce a considerable number of metal debris from the taper-trunnion junction, which may compromise the advantage of COC on wear rate. More importantly, except for the bearing surfaces, other reasons such as the diameter of the femoral head (4), the position of the acetabular cup (35), and the prosthesis from different manufacturers (36) can also affect the final result.

The COC bearing has two main drawbacks that hinder its widespread use. First, as a catastrophic complication, the brittleness of ceramics theoretically increases the risk of prosthesis fracture. Interestingly, ceramics fracture was mostly reported to appear in the liner with percentages between 0.13 and 1.1% and only occasionally occur in the head (37). In this study, COC bearing demonstrated a significantly higher prosthesis fracture rate compared with COP bearing and all the prosthesis fracture events were reported to happen in the liner (9, 17, 20, 21, 23–25, 27). Although it is easy to understand the fracture of the ceramic caused by exogenous violence, the liner fracture is rarely related to direct trauma. Instead, misalignment during insertion of the liner, metal back damage, or acetabular component malposition are the common reasons that lead to liner fracture (38, 39). Thus, it is reasonable to be careful in the preparation of the acetabulum and insertion of the liner when choosing the ceramic liner, while the PE liner seems to have a relatively higher “fault tolerance.” The second drawback of COC bearing is the audible noises (squeaking or clicking) and it is often complained about by patients after THA. In this meta-analysis, 4 studies reported the noise events and the positive rate was 14.6% in the COC group, while no hips were reported in the COP group. Most of the noises were caused by the separation of the head from the liner due to postoperative soft-tissue laxity and will subside automatically in some days, while only very few cases were caused by the real ceramic friction. More importantly, the surgeon needs to keep in mind that the occurrence of a delayed noise in a COC joint, especially accompanied by pain and malposition, can be caused by the fracture of ceramics (40).

Another issue after THA was the dislocation. Although the dislocation rate was not significantly different in the two groups, confounding factors could have interfered with the results. Both the femoral head size and liner design could affect the dislocation rate, except for the position of the acetabular cup. “Protective liners” including lipped liners (41) and constrained liners (42) could significantly reduce the risk of dislocation. However, to some extent, constrained liners will affect the range of joint motion and it is easier for impingement to happen compared with normal liners (43). More importantly, constrained liners should not be used as a remedial measure for defects in surgical technique or prosthesis placement. Although COC bearing was limited in the choice of the constrained liner because of the material properties, it was allowed to choose a larger head size. It was reported that a larger head could remarkably reduce dislocation by increasing impingement-free range of hip movement and jumping distance (44). However, larger diameter heads were theoretically associated with increased wear and revision rate, although long-term follow-up studies about this term were rare.

Despite the present meta-analysis including the most comprehensive RCTs and 4 of them reported long-term (≥10 years) follow-up results after THA, there are still some limitations. First of all, prostheses used in different trials were produced by different manufacturers and were of different sizes, all of which may result in bias. Second, outcomes parameters such as wear rate and hip function were different in different studies and cannot be pooled for further analysis. In addition, high-quality RCTs with a long-term follow-up are still urgently needed to compare the effect of these two bearing surfaces.

This meta-analysis indicated the comparable functional outcomes of COC and COP bearing surfaces following primary THA. There were statistically significant differences in component-related noise and ceramic component fracture between the two groups. Both the overall and subtype analysis showed similar dislocation rate, deep infection rate, osteolysis rate, prosthesis loosening rate, and revision rate. Considering the amount of included RCTs is limited, the generality of our conclusion is relatively restricted. Therefore, more high-quality RCTs are necessary to support the existing conclusion in the future.

Strengths: (1) The present meta-analysis included the most comprehensive randomized controlled trials (RCTs) on the “head-to-head” comparison of ceramic-on-ceramic (COC) components and ceramic-on-polyethylene (COP) components during total hip arthroplasty (THA); (2) Four of the trials reported long-term (≥10 years) follow-up results after THA, which increased the reliability of the results; and (3) This study could provide strong evidence for clinicians to make decisions on the choice of bearing surfaces during THA.

Limitations: (1) Prostheses used in different trials were produced by different manufacturers and were of different sizes, all of which may result in bias; (2) Outcome parameters such as wear rate and hip function were different in studies and cannot be pooled for further analysis; (3) High-quality RCTs with a long-term follow-up are still urgently needed to compare the effect of these two bearing surfaces.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author/s.

YF and XS: conceptualization, investigation, resources, and visualization. YF: writing—original draft preparation. XS: writing—review and editing. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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