Over the last five decades, hard-on-soft bearings have been used in THA, especially cobalt-chrome femoral heads articulating with UHMWPE liners. Hard-on-soft bearings are widely used because of good long-term results. In addition, the use of ceramic heads on polyethylene liners is gradually gaining wider acceptance.  However, the main drawback of hard-on-soft bearings is the wear particle-related periprosthetic osteolysis, leading to THA failure. Periprosthetic osteolysis and aseptic loosening are severe problems in the durability of THA and are highly related to wear and the number of particulate debris. Wear and osteolysis occur mainly with standard polyethylene bearings associated with metalheads.[13, 14]
Highly cross-linked polyethylene (HXLPE) was developed to address the problem of wear and osteolysis associated with UHMWPE bearing surfaces. Many studies have shown that HXLPE has significantly reduced and is associated with a greater implant survival rate regardless of the head used than conventional UHMWPE.[7, 15]
Hard-on-hard bearings for total hip arthroplasty have improved dramatically over the past several decades. With the introduction of new material options on the bearing surface, the use of hard-on-hard bearings will likely continue to increase, particularly in active, young patients. In particular, ceramic bearings are attractive because of the hardness and high resistance of scratches, and thus, they are tolerant to wear debris. However, the fracture and the squeaking sound, as well as the high-cost problem, make the choice debatable.[16–18]
The use of materials with low production of wear debris has become an attractive alternative in active young patients. Ceramic bearings have far-reduced volumetric wear debris compared to other types of bearings. Many studies have reported that CoC-bearing surfaces decrease rates of wear, osteolysis, and aseptic loosening, which accounts for more than half of the revisions. However, the high cost, squeaking, component impingement, micro-separation, and prosthesis fracture make the choice difficult. A ceramic fracture can be a devastating complication that affects revision surgery employing the third body wears from ceramic debris. For these reasons, there has been concern regarding the increased use of CoC as an alternative to contemporary CoPs.
Previous studies have shown that the mean linear wear rate was 0.0043 mm/year in the CoC bearing, lower than that of the other bearings. Comparing CoP, the mean linear wear rate and volumetric wear were 0.2182 ± 13.7 mm/year and 136.2 ± 8.5 mm3/year, respectively, higher than CoC bearing.[21, 22] To improve the wear of polyethylene liners, some efforts have been made to improve the wear rates through design changes, material substitution, or polymer modification. Changes included varying the molecular weight of the polyethylene or changing the additives, and modification of the polyethylene material itself. Hence, newer materials significantly reduce the wear rate. HXLPE created by radiating and re-heating the polyethylene implants has been shown to have lower wear rates than conventional polyethylene (0.26 mm/year vs. 0.05 mm/year).
In our study, we found that the survival rates of CoP bearing at 15 years related to implant loosening and osteolysis did not differ significantly compared to previously known data with metal-on-polyethylene bearing. Furthermore, there were no severe complications such as fractures and squeaking of the CoC bearing. Potential problems with polyethylene bearing surfaces such as wear and osteolysis could be decreased as new materials were developed.
If there is no significant difference in the wear rate, ceramic fractures can also be an important factor influencing the determination of the bearing surface. Although THA with CoC bearings is expected to increase, ceramic brittleness remains a major concern for surgeons.[25, 26] Regarding the improvement of ceramic materials, there are few ceramic head fractures in the fourth-generation ceramic, but ceramic liner fractures are still a concern due to malseating. Previous studies have reported that the fracture rate of ceramic components is between 0.004% and 0.05% for femoral heads and between 0.013% and 1.1% for acetabular liners. On the other hand, there are negligible reports of ceramic head fractures with polyethylene liners. Furthermore, there were no cases of squeaking in the CoP group in this study, whereas CoC bearings were reported to have a squeak incidence of up to 20% in previous studies. Thus, CoP articulation is gradually spotlighted to reduce several problems in ceramic components. However, the problem with the choice of bearing surface remains controversial, although our results are relatively consistent with those of other previous reports.
Several limitations of this study were noted. First, our study was retrospective in nature, with a relatively small number of patients. The analysis was based on consecutive cases, with no randomization and no power analysis performed.
Second, we did not evaluate the clinical outcome because many patients were lost to follow-up. However, there have been few studies on the wear rate of CoP bearings in the long-term period because CoP bearings are not a mainstay. Therefore, the long-term result of the wear rate of CoP bearings is sufficient in this study.