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.¹³ 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.³⁶ 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.³⁴ 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.³⁶ 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.^9,13,16,37 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.^14,38 If the clearance between a ceramic femoral head and socket is not over 50 µm, then grain detachment and third-body wear will occur.³⁹ 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.⁴⁰ Fracture has also been reported after significant trauma such as a motor vehicle accident.³⁰ 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 mm³ per million cycles. The second phase is called the “steady-state” phase. During this period, volumetric wear rates decrease to less than 0.02 mm³ 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.^41,42

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 mm³ 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.^16,33

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.¹⁴ 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.^39,41–44 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.^19,21–24 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 colleagues⁴⁵ 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.

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