CHAPTER OUTLINE
Basic Science 479
Manufacturing Overview 479
Mechanical Properties 479
Tribologic Properties 480
Wear Debris and Tissue Response 480
Ceramic Advantages 480
Ceramic Concerns 481
Clinical Studies 482
Summary 483
The standard bearing surface for total hip arthroplasty, a metal head articulating with a polyethylene socket, has provided pain relief and restoration of function to millions of patients with debilitating arthritis. However, long-term durability of this articulation has been limited by the generation of polyethylene wear debris and associated osteolysis. Osteolysis is a significant issue and is a leading cause of arthroplasty failure and the need for revision surgery. Attempts to address this problem have lead to efforts to improve the wear characteristics of the metal on polyethylene bearing surface. This bearing has been improved by cross-linking polyethylene and introducing ceramics to replace the metal head. Both of these improvements have substantially improved the wear characteristics of this articulation. Cross-linking has been in widespread clinical use since the late 1990s, and despite a lack of clinical studies to initially support its use, this technology has been quickly and widely accepted. Early clinical studies showed that there is a measurable improvement in wear with cross-linking; however, wear and osteolysis have not been eliminated. Most studies show that polyethylene wear, with either a metal or ceramic head, can be reduced by more than 50% with cross-linking. This clinical wear reduction is much more modest than the 90% reduction predicted by benchtop testing of this articulation. In addition, cross-linking has been shown to weaken the polyethylene, and cases of catastrophic polyethylene failure with cracking of the polyethylene liner or rim have been reported.
The alumina on alumina bearing is considered the standard ceramic on ceramic articulation. Alumina ceramic bearings have been in clinical use for more than three decades, and significant basic science and clinical research support their use. In vitro and clinical retrieval studies are available and show a significant reduction in liner wear rates with a ceramic on ceramic articulation—in fact, several thousand times less wear than with metal on polyethylene bearing surfaces. In addition, the small amount of ceramic particulate debris that is generated has been noted to be much less biologically reactive than metal or polyethylene particles. Most important, the incidence of osteolysis associated with use of ceramic on ceramic bearings appears to be minimal or nonexistent. In a clinical study of ceramic on ceramic bearings with a minimum follow-up of 18.5 years, Hamadouche and colleagues reported no cases of osteolysis with this articulation. There have now been reports of several clinical studies from the United States of ceramic on ceramic bearings with midterm follow-up. With clinical results now in the 5- to 8-year range, investigators have reported excellent clinical results with ceramic on ceramic bearings.
It is important to emphasize that osteolysis has not been identified with this type of articulation. However, it is equally essential to understand there are potential disadvantages to using ceramic on ceramic bearings in total hip arthroplasty. The primary concern about ceramics is their brittle nature and lack of fracture toughness. Although enhanced engineering and production methods have lessened the risk of component fracture, this complication has not yet been eliminated. In addition, there are other reported problems with ceramic on ceramic articulations, including stripe wear, limited available inventory, impingement, and motion-related noise.
This chapter reviews the basic science of ceramic on ceramic bearings for total hip arthroplasty. The chapter also includes a balanced review of the advantages and disadvantages of this articulation. Finally, clinical studies related to ceramic on ceramic total hip arthroplasty are summarized.
BASIC SCIENCE
Manufacturing Overview
Alumina ceramics are manufactured by a technologically demanding, complex process involving the performance of multiple steps with intense and optimal quality control. The mechanical properties of the final product are completely dependent on the proper performance of these manufacturing steps. Alumina component production begins with the mixing of alumina particulates with water and an organic binder. This mix is placed in a mold that is in the shape of the desired product. The formed piece is dried, evaporating the water, and the organic binder is removed by a thermal process. This product is sintered at a very high temperature, which causes the resultant part to become highly densified. The microstructure of the final product is very dependent on the use of a specific thermal process and the number of thermal steps plus the maximal temperature reached. All these factors determine the chemical structure and mechanical properties of the ceramic product. In addition, the mechanical and tribologic characteristics are further influenced by the grain size and purity of the powder used. Currently there are four companies producing medical grade ceramics, none of which are in the United States. These companies are Ceraver Osteal (France), CeramTec Ag (Germany), Morgan Matroc (United Kingdom), and Kyocera (Japan).
Catastrophic ceramic fracture is usually a result of a flaw in the manufacturing process. The flaw may be extremely small, perhaps the size of a few alumina grains. However, this flaw can lead to crack propagation and catastrophic fracture. Numerous improvements in manufacturing have diminished the risk of fracture. These improvements include the use of smaller grain sizes for fabricating components. Thirty years ago, when ceramics were first introduced, the average alumina grain size was 50 µm. Today, ceramics are produced with grain sizes of approximately 4 µm or less, and the risk of catastrophic fracture has dropped precipitously. Alumina is a standardized material (International Organization for Standardization [ISO] 6474) with very well-defined and specific characteristics. Alumina ceramics are classified as hard, stiff, and brittle materials.
Mechanical Properties
Because alumina ceramics are highly oxidized, the material is in a low state of energy and has a high state of thermodynamic stability. This oxidized chemical structure makes alumina biologically inert and resistant to further oxidation. The hardness of alumina creates a product with significant resistance to surface damage, and ceramics are much harder than other materials routinely used in orthopedic surgery. The hardness of alumina makes it very abrasive and wear resistant. In addition, the hardness of alumina increases its resistance to scratching, and it is much less likely to scratch than titanium or cobalt chromium alloys. In fact, the only material capable of scratching alumina is diamond. Clinically, this is important because alumina can resist third-body wear and is not scratched by retained cement particles or bone.
Although alumina has poor bending characteristics, it is extremely strong in compression. This lack of bending strength has currently limited its use in total hip arthroplasty to the femoral head and cup liner. Because alumina is very stiff, it does not deform under high loads. Therefore very precise production techniques are needed in order to ensure proper fit of the head within the socket. Polyethylene will mold around a femoral head if there is an initial, small incongruity. This is not true with ceramics, and poor manufacturing can lead to high wear rates. If the clearance between a ceramic femoral head and socket is not over 50 µm, then grain detachment and third-body wear will occur. The lack of ceramic deformation makes the contact areas between the head and socket smaller as compared with metal on polyethylene articulations. In order to maximize the contact surface area, clearance must be optimized. Since 1993, manufacturing techniques have been good enough that manufacturers have not had to factory match the sockets and heads, and exchangeable components are now available.
Alumina is more than 300 times stiffer than cancellous bone and almost 200 times stiffer than polymethlymethracrylate. Because of this significant modulus mismatch, cemented ceramic components have been found to be associated with higher cement fracture and loosening rates than metal or all-polyethylene components. Alumina is very brittle and under compression will deform linearly until fracture. No plastic deformation occurs before fracture. By definition, its fracture toughness is considered to be its resistance to fracture. The initial flaws in the material determine the risk of ceramic fracture, and flaws are related to the purity and density of the ceramic. A combination of improvements, including improved processing with smaller grain sizes, fewer impurities, laser etching, and proof testing, have led to a lower incidence of material fracture. The burst strength of alumina components improved from 38 kilonewtons in 1977 to 98 kilonewtons in 1998. (The U.S. Food and Drug Administration [FDA] recommendation is a burst strength of greater than or equal to 46 kilonewtons.)
Clinical issues such as applied load, use of small-diameter heads, and surgical technique have all been shown to affect the risk of fracture. There are case reports of fracture associated with small-diameter heads, cement or bone fragments trapped between the taper and head, and excessive hammering of the femoral head during impaction. Fracture has also been reported after significant trauma such as a motor vehicle accident. Today femoral heads of less than 26 mm and collared heads are not recommended for clinical use. Patients should also be advised about the risk of excessively vigorous activity after total joint arthroplasty.
Tribologic Properties
In vitro wear studies have proved that alumina on alumina is a very low friction couple, and wear is significantly reduced. The outstanding tribologic properties of the alumina articulation are due to its low surface roughness (secondary to small grain size), hardness, enhanced wetability, and fluid film lubrication. It has been shown that there are two wear phases during in vitro testing. The “run-in” phase is the first phase and involves the first million or so cycles. Volumetric wear rates for alumina against alumina bearings during this run-in phase measure 0.1 to 0.2 mm 3 per million cycles. The second phase is called the “steady-state” phase. During this period, volumetric wear rates decrease to less than 0.02 mm 3 per million cycles. Compared with metal on polyethylene couples, during both the run-in and steady-state phases, wear is reduced up to 5000-fold.
Under certain clinical conditions, accelerated wear can occur with alumina on alumina couples. One phenomenon called “stripe wear” occurs when accelerated wear is present over a discrete area. Stripe wear may be associated with separation of the ball from the socket such as during the swing phase of gait or when the ball is levered out of the socket by impingement. In vitro testing under the conditions of separation of the femoral head from the socket leads to increased volumetric wear. It was noted that wear as high as 1.24 mm 3 per million cycles could occur with separation and stripe wear results. A bimodal distribution of particle size was also noted in this study, with nanometer-sized particles (1 to 35 nm) probably associated with polishing of the articulation and micrometer-sized particles (0.05 to 10 µm) that likely originated from stripe wear and transgranular fracture of the alumina ceramic.
Numerous retrieval studies of ceramic bearings have been performed, and the results are interesting. One study examined retrieved alumina components associated with aseptic loosening of the socket at a mean of 11 years after implementation. Components were classified into three groups: (1) low wear with no visible signs of material loss; (2) stripe wear with a visible oblong worn area on the femoral head and a penetration rate below 10 µm/year; and (3) severe wear with visible loss of material on both components and maximum penetration higher than 150 µm. Evaluation of these 11 components revealed massive, severe wear on two devices. The remaining nine components had liner wear rates less than 15 µm/year. The authors concluded that two different types of wear are associated with ceramic on ceramic couples—one that is limited and has negligible effect on long-term performance of the implant, and a second type that catastrophically leads to rapid destruction of the bearing surface. Published wear rates examining clinical performance of the alumina on alumina bearing surface have reported wear to range from 0.3 µm/year to 5.0 mm/year. These variations may be related to implanted material, prosthetic, and design issues or surgical and patient factors. However, it is important to note that most catastrophic wear has been reported with products produced before 1990. In recent years, wear rates below 15 µm/year have been consistently reported. Many investigators believe that severe wear is related to clinically exceptional circumstances and that with properly implanted bearing surfaces, catastrophic wear is essentially nonexistent.
Wear Debris and Tissue Response
It has been shown, in vitro and in vivo, that alumina wear debris is biologically inert and well tolerated. Alumina particles induce very little cellular response and formation of granulomatous tissue. The small nature of most alumina on alumina wear particles and the low volume of particles generated leads to a low level of bioactivity. Giant cells have not been observed in contact with alumina wear debris. In contrast with polyethylene or metallic particles, foreign body reactions are routinely observed. Lerouge and colleagues compared 12 periprosthetic membranes obtained during revision for aseptic component loosening with an alumina on alumina couple. These were compared with a series of membranes obtained from revisions of a metal on polyethylene bearing. In the alumina on alumina group, the cellular reaction, which was generally mild, was determined to be in response to the zirconia ceramic particles used in the cement as an opacifying agent. No cellular reaction to the alumina particles was noted. This contrasted with the significant cellular activity noted in the metal on polyethylene group with reaction to the polyethylene debris.
Osteolysis associated with alumina on alumina total hip arthroplasty has been infrequently reported. In one study, when an implant made with large–grain-size ceramics, low density, and high porosity was used, large production of debris resulted and osteolysis occurred. Tissue obtained from failed hips with an alumina on alumina couple was shown to have significantly lower prostaglandin E 2 (PGE 2 ) levels compared with tissue obtained from hips with metal on polyethylene articulation. Both alumina and polyethylene debris stimulate cellular release of tumor necrosis factor (TNF)–α. However, polyethylene particles cause more release of TNF-α, and in fact the stimulation may be 8 to 10 times greater. Of importance, alumina particles induce macrophage apoptosis, which leads to decreased macrophage activity. This induced apoptosis explains the decreased levels of TNF-α associated with alumina and may also account for the paucity of ceramic-related osteolysis.
Ceramic Advantages
The potential advantages of using a ceramic on ceramic articulation for total hip arthroplasty can be quickly summarized as decreased wear and elimination of osteolysis. Osteolysis from wear debris is commonly viewed as the major obstacle blocking the development of a “lifetime” hip replacement. The need to eliminate wear and osteolysis has been magnified by the extension of indications for total hip arthroplasty to younger, more active, healthier patients with long life expectancies.
The potential for decreased wear is derived from the tribologic properties inherent to alumina. Alumina can be highly polished. As grain size has become smaller and polishing technology improved, the surface roughness of ceramic components has been greatly reduced. Alumina bearings are also very hard, and this characteristic increases their resistance to scratching and burnishing. The hardness minimizes third-body wear from entrapped bone, polymethylmethacrylate, or metal debris derived from surgical instruments or component fretting.
Alumina has ionic properties and therefore, in combination with body fluids, has better wetability than chrome cobalt. The fluid film that develops on ceramic surfaces decreases frictional drag and adhesive wear. Wear rates for modern ceramic on ceramic articulations have been shown to be as low as 4 µm/year or about the thickness of one crystallite of alumina. This low wear rate coupled with less alumina bioreactivity minimizes the likelihood of osteolysis. With currently used implant designs, osteolysis has not been reported with follow-up as long as 18.5 years.
Ceramic Concerns
Fracture
When ceramics were first introduced, technology limitations and lack of knowledge led to the production of aluminum oxide of inferior quality and association with a high incidence of component fracture. Improved material processing, smaller grain sizes, fewer impurities, laser etching, and proof testing have greatly diminished the risk of catastrophic in vivo fracture. The risk of ceramic fracture is estimated to have decreased nearly 100-fold in the last two decades. In 1990 the incidence of fracture was approximately 0.8%, and today is likely between 0.004% and 0.010%. Nonetheless, this complication is devastating and still occurs. The focus of this chapter is on alumina ceramics. However, it is important to be aware that device fracture has also been reported with zirconia ceramics. 31 In fact, the risk of fracture with zirconia may exceed that with alumina. Zirconia ceramics have been reported to undergo structural changes (phase change) in vivo, which alters mechanical properties significantly.
Even with proof testing, it is unlikely that failure by fracture will be eliminated. Although theoretically proof testing eliminates weaker components, flawed products that are likely to fail are not always eliminated. Proof testing theoretically is designed to be stringent enough to remove components with manufacturing flaws that are likely to clinically fail. However, the test must be nondestructive and not cause damage to the tested part. No proof test currently available is 100% effective.
In addition, it must be remembered that although the FDA carefully monitors the facilities of medical device manufacturers, production errors can and still do occur. In 1998 a manufacturing change resulted in a high fracture rate of ceramic balls. About one in three components clinically failed, and this was despite negative proof testing of all fractured devices. The production of ceramics is far more demanding than the manufacture of metal and polyethylene components, and the incidence of catastrophic failure for ceramics will always be higher than with other materials.
Ceramic component fracture may occur secondary to poor surgical technique. Improper component insertion predisposes the implant to fracture. Impaction of the femoral head on the trunnion should be performed only after ensuring it is concentrically placed. Placing the head nonconcentrically on the trunnion or not cleaning and drying it properly leads to stress concentrations in the femoral head. In addition, placing a ceramic head on a damaged trunnion also leads to stress concentration and a significantly reduced burst strength with the potential for fracture. It is also possible to nonconcentrically place the ceramic liner in the metal acetabular shell. However, the adverse effects and long-term consequences of this error have not been reported.
Ceramic component fracture is a double-edged sword. After fracture the patient is confronted with immediate debilitating pain and the need for emergency revision surgery. However, secondarily, revision of a fractured ceramic component carries the risk of a less than optimal outcome. Because of trunnion damage, revision with a ceramic head is usually not possible. Ceramic fracture debris embedded in the soft tissue can cause third-body wear and premature failure owing to accelerated wear if a metal and polyethylene articulation is used for the revision. During revision of fractured ceramic components, meticulous synovectomy and débridement is recommended to remove as much fracture debris as possible.
Stripe Wear
Separation of the ball from the socket in patients with a hip arthroplasty may occur during the swing phase of gait or with impingement of the trunnion on the acetabular rim levering the ball from the socket. When this separation occurs, the contact area of the femoral head on the acetabular liner becomes small and stripe wear can result. Stripe wear is concerning because volumetric wear associated with this phenomenon is high. In one study, stripe wear produced volumetric wear that averaged 1.24 mm 3 per million cycles. Equally concerning, a bimodal array of nanometer- and micrometer-sized particles was created with an enhanced profile of bioreactivity. Separation of the ceramic on ceramic hip articulation is most likely to occur in individuals with tissue laxity or excellent range of motion. Also, patients with vigorous lifestyles and those who perform activities that require placing the hip through a provocative range of motion may be prone to impingement and stripe wear. For patients with these risk factors, other articulation choices should be considered. Of course, malpositioned components, as with any articulation, increase the risk of impingement.
Motion-Related Noise
Hard on hard bearings can produce noise that can be disconcerting and annoying enough that revision surgery is requested by the patient. Specifically, with alumina on alumina bearings patients may describe this noise as “squeaking.” Ranawat and his colleagues reported that 10 of 159 ceramic on ceramic articulations squeaked and that the phenomenon was self-reported by the patient. The squeaking usually occurred in mid range of motion and was generally considered a significant issue of concern for the patient. Walter and colleagues noted that the characteristics of patients who reported squeaking after hip replacement was significantly different from the characteristics of those who did not ( Table 63-1 ).
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