Ceramic-on-Ceramic Bearings

Key Points

Ceramic-on-ceramic bearings in total hip arthroplasty were first implemented as a solution to wear debris and subsequent osteolysis seen in traditional bearing surfaces.
Ceramics are hard, scratch resistant, wear resistant, low friction, thermodynamically stable, chemically inert, biocompatible, and resistant to corrosion.
Concerns about ceramics include risk of fracture, the phenomenon of stripe wear, motion-related noise, impingement, limitations in component options, and increased cost relative to metal-on-polyethylene bearings.
Early ceramics experienced a high fracture rate; however, owing to manufacturing and design advances, modern ceramics have shown promising clinical results.
Ceramics are best used in a young active population in whom wear and osteolysis are concerns.

Currently, the most frequently used bearing surfaces in total hip arthroplasty (THA) are metal or ceramic heads articulating with an ultra-high-molecular-weight polyethylene (UHMWPE) socket. As the indications for THA are extending to a younger and more active population, the focus has been moving toward increasing the longevity of these implants. Wear debris from polyethylene leading to periprosthetic osteolysis is considered the major long-term complication resulting in the need for revision. The extent of osteolysis is of major importance not only in terms of the likelihood of failure but also in terms of the complexities of obtaining fixation of new components during revision surgery and the long-term success of the revision surgery itself. The need for improved bearing surfaces has led to cross-linking of polyethylene via a variety of methods. These advancements have greatly improved the wear characteristics of polyethylene. Studies have shown that cross-linking polyethylene in an articulation with a metal or ceramic head can reduce wear by more than 50%. However, cross-linking polyethylene may weaken the polyethylene, causing catastrophic failure with cracking of the liner.

Ceramics were originally introduced as a solution to the problems of friction and wear seen in metal-on-polyethylene and metal-on-metal configurations. The intended goal of the ceramic-on-ceramic bearing is to reduce biologically active wear debris, thereby minimizing the occurrence of osteolysis and aseptic loosening. The standard ceramic-on-ceramic articulation is the third-generation alumina-on-alumina bearing. Both in vitro and clinical retrieval studies have demonstrated a significant reduction in wear and particle production when compared with metal-on-polyethylene articulations. Osteolysis in these bearings appears to be minimal. Although osteolysis has only rarely been identified with the ceramic-on-ceramic articulation, potential disadvantages have been identified. Ceramics are hard, brittle materials that lack fracture toughness. Improvements in processing and machining of ceramics have reduced fracture risk; however, this complication has not yet been eliminated. Other disadvantages of ceramic-on-ceramic articulations include stripe wear, motion-related noise, impingement, and limited head and liner options.

This chapter will review the basic science of ceramic-on-ceramic bearings, including mechanical properties, advantages, and disadvantages of this articulation. Clinical studies related to ceramic-on-ceramic bearings will be summarized, and future directions will be discussed.

Basic Science

History

The ceramic-on-ceramic articulation was first introduced by Pierre Boutin and associates in 1970 as an alternative to the conventional metal-on-polyethylene total hip. Early results for this first prosthesis showed promise; however, ceramic fracture was a matter of concern. High fracture rates were attributable to the large grain sizes, low density, and impurity of the alumina. Prior fixation methods involved gluing the head to the stem with a resin or screwing it in place. As the production of surgical-grade dense alumina ceramic evolved, strong fixation of the ceramic head to the metal stem was achieved with the introduction of the Morse taper in 1977, significantly reducing the risk of fracture to 2%. Current alumina production techniques have brought the fracture rate to 0.004%. With the reduction in the risk of fracture, acetabular component fixation leading to loosening and subsequent revision became the major long-term problem for these devices. Various methods of fixation were explored, finally leading to the use of porous-coated titanium shells with modular acetabular inserts press-fit into bone. Despite their use in Europe, alumina femoral heads only became available in the early 1980s in the United States, with ceramic-on-ceramic alumina bearing surfaces available by the early 2000s. Alumina has been a standardized material since 1984 (International Standard Organization [ISO] 6474).

General Statements About Ceramics

The two ceramic materials currently in clinical use as bearing surfaces are aluminum oxide (alumina) and zirconium oxide (zirconia). The ceramic-on-ceramic bearing surface is an alumina-on-alumina bearing surface because zirconia produces high wear when articulating against itself. These materials exist in their highest oxidation state, allowing excellent biocompatibility, thermodynamic stability, chemical inertness, and resistance to corrosion. Ceramics are water insoluble and have excellent compression strength but poor bending strength. Because of their mechanical properties, ceramics are considered hard and brittle in nature.

Use of proprietary ceramic processing methods by each manufacturer reflects the fact that ceramics are not all alike and subtle differences exist between ceramics manufactured by each company.

Manufacturing Process

Alumina ceramics are manufactured through a complex process involving multiple steps under intense and optimal quality control. The mechanical properties of the final product rely heavily on proper performance of these manufacturing steps.

The third generation of ceramic processing consists of mixing aluminum oxide powder with organic bonding agents, water, and lubricants. The mixture is isostatically pressed into a mold that will give it its final shape. The formed piece is then dried while the water is evaporated, and a thermal process removes the organic binder. The product is then sintered at a very high temperature (between 1600°C and 1800°C) under high pressure. The quality and purity of the initial powder and control of precision over the thermal process applied affect the final microstructure of the ceramic. Mechanical strength and tribological characteristics are determined by the purity, porosity, and grain size through the ceramic. When ceramics were first produced, longer sintering times were necessary to achieve full or nearly full density. However, larger grain size resulted, thus reducing overall strength and contributing to early failures. The addition of materials such as CaO or MgO prevented grain growth, allowing manufacturers to achieve smaller grain sizes and thus higher strength and reliability. These additions in the late 1980s and early 1990s would be considered second-generation ceramics, which would later give rise to the third-generation ceramics in use today. The third-generation ceramics were developed in 1994. They constitute ceramics that employ hot isostatic pressing, further resulting in smaller grain size, minimal grain boundaries and inclusions, and increased burst strength and wear resistance.

Alumina Matrix Composite (AMC)—Delta Ceramic

As of 2019, fourth-generation “Delta” ceramic is the standard material used for ceramic-on-ceramic THA. Fourth-generation ceramics include the addition of zirconium, which further enhances toughness and fracture resistance in the laboratory setting. Composite materials were first discovered when industrial manufacturers began investigating the method of transformation toughening as a means to strengthen alumina. Alumina matrix composite (AMC) is composed of small zirconia grains incorporated into the alumina matrix. These new mixed-oxide ceramics seem to provide a better ceramic in terms of fracture toughness without decreasing the sliding properties. Composite ceramics also have the luxury of providing more component options (the offer of few component options is a limitation of alumina-on-alumina bearings). Benchtop testing in one study demonstrated that AMC on alumina and AMC on AMC produce significantly lower wear rates than hot isostatically pressed (HIPed) alumina. The same study also noted that AMC showed similar wear mechanisms and wear debris in previous alumina retrieval studies. AMC was first clinically used in the United States in June 2000 as a ceramic-on-polyethylene bearing.

According to the manufacturer, CeramTec (Plochingen, Germany), the complication rate for this material was 0.002% among 3.6 million ceramic heads implanted from 2000 to 2010. That said, authors have reported that the clinical failure rate—although small—is somewhat higher than that reported by the manufacturer.

Currently, four major companies in the world meet the technologically demanding and complex process requirements needed to produce medical grade ceramics, including Ceraver Osteal (Roissy, France); CeramTec AG (Stuttgart, Germany), the producer of Biolox ceramic implants; the Technical Ceramics business of Morgan Advanced Materials (Rugby, UK); and Kyocera (Kyoto, Japan). Currently, the majority of major arthroplasty manufacturers offer ceramic heads in the United States.

Flaws in the manufacturing process can lead to catastrophic ceramic fracture. Crack propagation and subsequent fracture can result from flaws as small as the size of a few alumina grains. Improvements in the manufacturing process, such as diminishing the size of grains used for fabricating components, have reduced the risk of fracture. When ceramics were first introduced, the average grain size was 50 µm. Modern alumina has an average grain size of 2 µm, contributing to the substantial decrease in risk of catastrophic fracture.

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