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    Ceramic Femoral Heads in THA: Advantages and Disadvantages

    What does the literature say about ceramic heads versus cobalt chromium heads for total hip arthroplasty? Is there enough evidence to show that ceramic heads are definitively superior? Find out in this review article.

    Authors

    Christopher Scanlon, MD; Michael Sybert, MD; James Slover, MD, MSc; and Ran Schwarzkopf, MD, MSc

    Introduction

    Total hip arthroplasty (THA) is widely considered to be one of the most successful surgeries in orthopaedics. However, challenges remain with regard to implant longevity, which is largely related to wear of the bearing surfaces used for the femoral head and acetabular liner. [1] Cobalt chromium (CoCr) heads have the longest track record of use in THA, and pairing them with highly cross-linked polyethylene (XLPE) acetabular liners has resulted in excellent clinical survivorship. [2]

    Due to their mechanical properties, ceramic femoral heads used in conjunction with ceramic or XLPE liners have been proposed as an alternative bearing surface to help further reduce wear rates and improve implant longevity. [3] The recent development of ceramic femoral heads composed of zirconia-toughened alumina (ZTA; Biolox DELTA, CeramTec; Plochingen, Germany) has only increased interest in the role of ceramics in THA. Zirconia-toughened alumina combines the hardness of alumina with the crack resistance of zirconia, which is reported to help reduce the risk of head fracture. [1,3]

    RELATED: Bearing Surfaces in THA: Are We Ready for Ceramic on Ceramic?

    In this review, we evaluate the literature on the advantages and disadvantages of ceramic heads compared with CoCr heads and determine whether there is enough evidence to suggest that ceramic heads are definitively superior in THA.

    Advantages of Ceramic Femoral Heads

    The proposed advantages of ceramic heads, as compared with CoCr heads, include:

    • Lower wear rates [1]
    • Improved mechanical properties [3]
    • Decreased risk of trunnion corrosion [4]

    These proposed advantages, combined with the fact that wear particles from ceramic heads are biologically inert, [5] should theoretically lead to decreased osteolysis and loosening and longer implant survival in vivo. [6] Data supporting the proposed advantages are detailed below.

    Lower Wear Rates

    Multiple in vitro studies have demonstrated lower wear rates with the use of ceramic heads. A review of studies evaluating the wear rates of various bearing surfaces used in THA using hip simulator data demonstrated that wear rates were lowest with the use of ceramic-on-ceramic (CoC) bearings, including both alumina-on-alumina bearings (0.004 mm3/million cycles) and zirconia-on-zirconia bearings (0.013 mm3/million cycles), when compared with metal-on-standard polyethylene (PE) (0.119 mm3/million cycles) and metal-on-metal (MoM) (0.028 mm3/million cycles) bearings. [7] Even with the use of ceramic heads and PE liners, ceramic-on-standard PE bearings offered a 50% reduction in wear compared with metal-on-standard PE bearings. [7]

    These findings have been replicated in the context of XLPE as well. Galvin et al [8] used a hip simulator to measure and compare wear rates of 36-mm CoCr heads and XLPE liners versus 36-mm ceramic heads and XLPE liners. They found 40% lower steady-state wear rates with the use of ceramic heads. [8]

    Decreased Osteolysis and Improved Survivorship

    A growing body of literature suggests that the lower wear rates seen with ceramic heads translate to decreased rates of osteolysis and loosening in vivo and improved implant survivorship.

    Milosev et al [9] found that implant survival specifically attributable to aseptic loosening at 10-year follow-up was highest in CoC THA (98.4%), followed by metal-on-standard PE (95.6%) and MoM (87.9%). Clinical studies of CoC THA have shown 10-year survival rates of 86% to 99%, with most showing no or few (<2%) cases of osteolysis/loosening. [10]

    Studies of newer ZTA heads have demonstrated survival rates of 96.5% and 99% at 11.5- and 12.4-year follow-up, respectively, with no cases of osteolysis/loosening noted with CoC articulations. [11,12]

    Fewer studies have examined implant survivorship in ceramic-on-highly cross-linked polyethylene (CoXLPE) THA. Those that have been performed show survival rates of 95.7% to 99% at minimum 10-year follow-up, with a similarly low (0% to 1.7%) incidence of osteolysis/loosening. [13-17]

    Studies of metal-on-XLPE (MoXLPE) THA have shown implant survival rates of 93.5% to 100% at 10-year follow-up, with the incidence of osteolysis/loosening ranging from 0% to 25% in these studies. [10]

    Registry data from Australia, New Zealand, and the UK have corroborated the findings of these studies. [18-20] Data from the Australian Orthopaedic Association National Joint Replacement Registry showed the lowest revision rate with the use of CoXLPE bearings (4.6%), followed by MoXLPE (5.4%), CoC (6.5%), and MoM (7.7%) at 14-year follow-up. [18]

    Similar results were seen for overall revision rates in both the New Zealand Joint Registry at 14-year follow-up [19] and the National Joint Registry for Wales, England, and Northern Ireland at 11-year follow-up: [20]

    • New Zealand Joint Registry: CoXLPE, 1.8%; MoXLPE, 2.6%; CoC, 2.9%; MoM, 10.4%
    • National Joint Registry for Wales, England, and Northern Ireland: CoXLPE, 3.62%; CoC, 4.31%; MoXLPE, 5.32%; MoM, 20.18%

    Decreased Trunnion Fretting and Corrosion

    Adverse local tissue reaction (ALTR) was previously thought to be associated only with MoM bearings. [21] More recent studies, however, have shown that ALTR can also occur in patients with metal-on-PE bearings, likely due to fretting and corrosion at the trunnion-CoCr head junction. [22] The fretting and corrosion appear to be related to dissimilar alloy pairings, frequently seen with the use of titanium (Ti) stems and CoCr heads. [22]

    Multiple in vivo studies have confirmed that this is less of a concern with the use of ceramic heads. In a retrieval analysis, 50 head-stem pairs with ceramic heads were matched to 50 head-stem pairs with CoCr heads. Taper fretting and corrosion scores were significantly lower with the use of ceramic heads. [4] Another retrieval analysis of 52 ceramic head-stem pairs matched to 52 CoCr head-stem pairs showed significantly lower median fretting and corrosion scores in all zones in the ceramic group. [23]

    Eichler et al [24] used serum Ti level as a surrogate marker of trunnionosis. They followed 57 patients with large-diameter CoC THAs over 5 years and found a decrease in serum Ti levels, with no patients demonstrating a clinically significant elevation in Ti at any point during the study period. [24] In addition, no patients demonstrated radiographic signs of osteolysis or clinical signs of ALTRs, and no patients required reoperation. [24]

    Disadvantages

    The literature shows some disadvantages to using ceramic heads in THA based on their wear properties, biomechanics, and bioinert characteristics. Fourth-generation ceramics (ie, ZTA) are intended to address the concerns seen in earlier generation ceramics.

    Below, we review the comparative risk of revision in fourth-generation ceramics, residual fracture risks, theoretical concerns, and the financial considerations with the use of ceramics in THA.

    RELATED: Non-Traumatic Fracture of a Fourth-Generation Ceramic Femoral Head

    Risk of Revision

    As mentioned above, multiple in vitro studies have demonstrated improved wear properties and lower rates of trunnion-related metallosis with ZTA CoXLPE compared with CoCr MoXLPE. [7,8] How these data translate to clinically relevant information remains unclear.

    International registry data show a less than 1% difference in revision rates between CoXLPE and MoXLPE at up to 14 years. [18-20] A systematic review and meta-analysis of 18 RCTs by Wyles et al [25] showed no difference in implant survivorship and all-cause revision rates at 7 years among CoXLPE, MoXLPE, and CoC bearings.

    A retrospective review of registry data in the US by Cafri et al [26] evaluated all-cause revision rates and 1-year dislocation rates for MoXLPE, CoXLPE, and ZTA ceramic heads. Patients were stratified into groups based on head size. No differences were seen in all-cause revision rates for all head sizes between MoXLPE and CoXLPE. [26] The study authors noted a higher 1-year dislocation rate with CoXLPE compared with MoXLPE at head sizes less than 32 mm. [26] Extremely high-powered, long-term prospective studies will be needed to determine if there is a true clinical benefit to CoXLPE over MoXLPE.

    Ceramic Head Fracture

    Component fracture has historically been viewed as the major concern for using ceramic articulations in THA. Early-generation alumina ceramic articulations, popularized in the 1970s and 1980s, had failure rates as high as 13%. [1] Significant improvements in materials, component design, and manufacturing processes have led to manufacturers reporting a third-generation alumina fracture rate of 0.022%. [27]

    Zirconia ceramic heads were briefly introduced into the market in the 1980s due to reportedly increased fracture toughness, flexural strength, and resistance to crack propagation compared with alumina ceramics of the time. [1] However, the pure zirconia era was short-lived due to concerns of tetragonal to monoclinic zirconia phase transformation that reduces wear resistance and strength of the material significantly. [1] In addition, the phase transformation leads to volume expansion of the head that has been the proposed cause of the unexpectedly high rates of fracture in pure zirconia heads. [1]

    The goal with fourth-generation ceramics is to combine the wear properties of alumina ceramics with the fracture toughness of tetragonal zirconia. The manufacturer claims a 22-times decreased incidence of fracture in modern ZTA ceramics compared with older alumina ceramics, 0.001% vs 0.022%, respectively. [27] However, multiple large international registry studies suggest that the incidence of ceramic head fracture is an order of magnitude higher than previously reported, at 0.01% for ZTA and 0.13% for alumina. [28,29]

    Multiple statistically significant risk factors for fracture of modern alumina and ZTA ceramic heads were identified in a 2017 Norwegian Registry analysis: [28]

    • Male gender
    • Pure alumina heads (vs ZTA)
    • CoC articulations (vs CoXLPE)
    • Femoral head size less than 32 mm
    • Short head/neck length

    A systematic review/meta-analysis of 13 RCTs also demonstrated an increased risk of fracture with CoC compared with CoPE. [30]

    A biomechanical study by Lee et al [31] examined various loads placed on 28-mm alumina heads with varying neck lengths. They found that short neck tapers had a stress peak at the inner corner of the bore roof, corresponding with the thinnest area of the head. [31] A retrieval analysis of 4 of the fractured heads showed fracture initiation in the same area. All fractures were in short-neck, 28-mm alumina ceramic heads. [31] These data may also be relevant with regard to dual-mobility articulations, as case reports have shown fractures with 28-mm ZTA ceramic heads. [32]

    Ceramic head fracture poses a challenge in the revision setting due to the dissemination of large and microscopic ceramic fragments. If these fragments are not removed in their entirety, they can cause abrasive wear and osteolysis. [33,34] Outcomes after revision for ceramic head fracture have been historically poor, with survivorship of 63% at 7-year follow-up. [34] Modern studies have shown improved survivorship: 85% at 5 years with the use of CoC or MoXLPE articulations. [28]

    Risk factors for repeat revision with ceramic articulations have been identified as: [34,35]

    • Age over 50
    • Retained cup or stem
    • Revision with a stainless-steel head
    • Lack of synovectomy

    Recommended treatments for revision of ceramic head fracture include: [33]

    • Complete synovectomy
    • Cup revision if the locking mechanism is damaged
    • Stem revision for moderate or severe trunnion wear
    • Revision of any component for malalignment
    • Use of a fourth-generation ceramic head larger than 28 mm with a titanium sleeve

    There is currently no consensus on the use of a CoC bearing versus a CoXLPE bearing for revision of ceramic head fracture.

    Theoretical Concerns

    The major advantage of ceramic heads relates to the attractive material properties that lead to wear resistance. [7-8] However, scratches to the ceramic head that may occur during dislocation or reduction of the hip combined with increased local metal ion levels related to impingement or trunnionosis may result in metal transfer of Ti or CoCr to the ceramic head surface. [36-38]

    This metal transfer phenomenon has been shown to increase surface roughness and potential for abrasive wear. [39] A spectroscopy analysis of ZTA ceramic head retrievals showed evidence of phase transfer in all areas of metallization, as well as in areas of maximum stress. [40] The results of this study suggest that metallization may not only increase surface roughness, but it may also initiate the phase transformation cited as detrimental in previous pure zirconia ceramic heads. [1]

    Another biomechanical study examined burst strength of ZTA heads after simulated environmental degradation with hydrothermal aging. [41] The study authors found that moderately aged heads had a 6.22% reduction in burst strength, and severely aged heads had a 16.44% reduction. [41] They also found 10% to 15% relative increase of surface phase transformation in aged ZTA heads. [41]

    Cost Analysis

    A 2016 cost analysis compared CoXLPE with MoXPLE based on data from the Premier Research Database. [42] The study authors extrapolated data on more than 20,000 primary THAs to create nationally representative estimates, and then stratified patients into age groups by decade.

    Three cost differential price points were determined – $325, $600, and $1003 – based on the difference in hospitalization costs between CoXLPE and MoXLPE in the Premier database and the difference in reported pricing in the Orthopedic News Network. The cost per revision, according to the Premier database, was $23,244. The MoXLPE revision rate was determined to be 14.5 revisions per 100 patients over 20 years from the HealthEast registry. The CoXLPE revision rate was determined to be 10.2 revisions per 100 patients over 20 years from the UK registry. [42]

    Based on these variables, it was determined that at the $1003 cost differential, CoXLPE was not cost effective at any age group. At the $600 price point, CoXLPE was not cost effective in patients over 65 years. At the $325 price point, CoXLPE was not cost effective in patients over 85 years. [42]

    Conclusions

    Ceramic bearing surfaces for THA have evolved significantly since their introduction into the market. Zirconia-toughened alumina ceramic heads have shown great promise with regard to decreasing fracture rates while maintaining excellent wear properties seen with third-generation alumina ceramics. The use of ZTA CoXLPE has thus far shown excellent clinical results that rival and may surpass those of MoXLPE in registry data.

    Zirconia-toughened alumina has been shown to decrease trunnion-related metallosis compared with CoCr heads in multiple in vivo studies. Due to the advent of XLPE, it is unclear whether material differences between ZTA and CoCr translate to appreciable clinical differences with regards to polyethylene wear. The introduction of tetragonal zirconia to ZTA heads has decreased the fracture rate to about 1 in 10,000 cases; however, outcomes are still poor after revision for head fracture.

    Theoretical concerns about the longevity of ZTA ceramics as they pertain to induced material property changes due to metallization and phase transformation have been shown in vitro but have not yet been clinically correlated. Finally, understanding cost differentials with regard to ZTA ceramic heads and how these may evolve over time is important in determining the most appropriate bearing surface in a given age group.

    Author Information

    Christopher Scanlon, MD, and Michael Sybert, MD, are NYU/ISK Adult Reconstruction Fellows at NYU Langone Orthopedic Hospital, New York, New York.

    James Slover, MD, MSc, Professor of Orthopedic Surgery in the Department of Orthopedic Surgery at NYU Grossman School of Medicine, Director of Patient Reported Outcomes Research in the Department of Orthopedic Surgery, Director of the Adult Reconstruction Fellowship Program, and Chief of the Orthopedic Surgery Services at NYU Langone Orthopedic Hospital, New York, New York.

    Ran Schwarzkopf, MD, MSc, is Professor of Orthopedic Surgery in the Department of Orthopedic Surgery at NYU Grossman School of Medicine, Associate Chief of the Division of Adult Reconstruction, Director of the Adult Reconstruction Research Center, and Associate Director of the Orthopedic Surgery Residency Program at NYU Langone Orthopedic Hospital, New York, New York.

    Disclosures: The authors have no disclosures relevant to this article.

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