Development of ceramic‐on‐ceramic implants for total hip arthroplasty

Abstract

Over the past three decades, alumina ceramic, now in its third/fourth generation, has been markedly improved in terms of its mechanical properties, including purity, grain microstructure, and burst strength. In the clinic, it is particularly suitable for young and for very active patients. This paper discusses the development and characteristics of different kinds of ceramics. In addition, ceramics in the third/fourth generation which are used in total hip arthroplasty clinically are reviewed in detail.

Introduction

A primary problem affecting total hip arthroplasty (THA) survivorship is wear debris, the resultant wear‐induced osteolysis, and eventual loss of fixation ¹ . At present, bearing surface combinations consisting of either moderate to highly cross‐linked polyethylene with metal or ceramic heads ² , ³ , metal‐on‐metal (MOM) ⁴ , and ceramic‐on‐ceramic (COC) ⁵ are currently available for use in the clinic. In general, the wear of polyethylene contact surfaces generates many polyethylene particles, and these wear particles can lead to aseptic loosening and osteolysis ² . On the other hand, metallic wear particles are produced on MOM surfaces. The wear from these small bodies releases active cobalt and chromium ions which are disseminated systemically before being excreted in the urine. Raised concentrations of serum cobalt and chromium are at present of unknown risk, but there are concerns with regard to carcinogenesis, hemopoietic cancers, and hypersensitivity reactions ⁶ , ⁷ . In younger, more active patients or women of reproductive age, this is of particular concern because they could be exposed to higher concentrations of ions for longer periods ⁸ , ⁹ . Because alumina COC couplings have lower wear rates than other bearing surfaces ¹⁰ , ¹¹ , they have become a promising choice for clinical use.

History of COC coupling

Boutin reported the first experience of COC THA in 1970 ¹² . Early and midterm COC THA clinical outcome reports from the initial experience in the USA have been encouraging ¹³ , ¹⁴ , ¹⁵ . At a minimum of 18.5 years follow‐up, Hamadouche et al. reported minimal wear, limited osteolysis, and a low rate of complication with COC THA ¹⁶ .

However, some important disadvantages of early‐generation COC articulations have been reported. Early ceramic component materials were of large grain size and contained a lot of impurities, resulting in unacceptable component fracture rates ¹⁰ , ¹⁷ . For instance, relatively high rates (up to 13%) of component fracture in first‐generation ceramics have been reported in the literature ¹⁸ . Although the second‐generation has been much improved, the reported fracture rate is still up to 0.014% ¹⁹ , ²⁰ , ²¹ , ²² . With the development of manufacturing processes such as hot isostatic pressing, laser etching and proof testing, the modern ceramic component material which is now produced has better characteristics (the third‐generation ceramic Biolox Forte [Ceram Tec Medical Products, Plochingen, Germany], approved by the US FDA in 2003) ²³ , ²⁴ , ²⁵ . The average size of the grains has been reduced from 3.2 µm to 1.8 µm and the bending strength hasbeen increased to 580 MPa for alumina ceramics ²⁶ . Table 1 summarizes the different mechanical characteristics and their evolution. The third‐generation alumina ceramics commercialized today benefit from all of these improvements, and the fracture rate has been reduced to 0.004% ¹⁹ . Compared to the old alumina ceramic in previous clinical studies, for which the 10‐year survival rate was 90.8%, third‐generation highly purified alumina has an almost 100% 10‐year survival rate ²⁷ .

Table 1.

Mechanical characteristics and the evolution of different ceramic materials

Properties of ceramics	Alumina ceramic ISO 6474	First and second generation alumina	Third generation alumina
4‐point bending strength (MPa)	400	500	580
Average grain size (µm)	About 4.5	<3.2	<1.8
Density (g/cm3)	3.94	3.96	3.98
Hot isostatic pressing	No	Yes	Yes

Characteristics of modern ceramics

The advantages of COC bearings are related to its tribologic properties, as mentioned above. Theoretically, alumina is 10 times harder than cobalt‐chrome ²⁰ . Alumina, with a hardness of approximately 2000 VH (Vickers Hardness), is one of the hardest materials ¹¹ . This hardness provides for improved scratch resistance. Another advantage is the improved lubrication due to its lower wetting angle. This permits better wettability, especially when coupled with the formation of a microfilm of lubrication on the surface of ceramics. In addition, strong bonding between the oxygen and aluminum ions provides extremely good corrosion resistance, leading to better biocompatibility, and because they are inert, there is no concern about allergic reactions ²⁰ , ²⁸ . As a result, modern ceramic has much less wear rate compared with other biomaterials (Table 2) ²⁹ , ³⁰ , ³¹ , ³² . These favorable qualities are particularly desirable for implants in high‐demand patients, such as young or more active patients ¹⁰ , ¹⁷ . However, ceramic has poor bending strength ³³ . Alumina ceramics are brittle and have no way to deform without breakage ³⁴ .

Table 2.

The wear rates among different total hip endoprostheses

Ball head	Cup/liner	Wear rate
Third‐generation ceramic	UHMWP	0.11 mm/year
Second‐generation metal	UHMWP	0.085 mm/year
Second‐generation metal	Metal	0.0063 mm/year
Third‐generation ceramic	Ceramic	0.004 mm/year

UHMWP, ultra‐high‐molecular‐weight polyethylene.

Clinical research

Improvements in ceramic components and design changes have reduced failure rates of THA over the past 30 years. Starting with a failure rate of 1:100 in the initial phase ³³ , ³⁵ , the incidence of ceramic fractures has been reduced nowadays to 1:25 000 ¹⁹ . Thus COC bearing surfaces should be preferentially considered for revision THA, especially in young patients ³⁶ .

Fractures

Fracture of a ceramic head and/or liner remains a major disadvantage for COC bearing combinations. However, all bearing surfaces, including polyethylene and metal, have the potential to break. At present, femoral head fractures with modern COC are rare. Early results of a Food and Drug Administration (FDA) multicenter study in the USA using third‐generation alumina‐on‐alumina ceramic revealed no ceramic fractures at 3 years follow‐up in 333 cases ¹⁴ . In a more recent multicenter Investigational Device Exemption (IDE) study using the Trident insert, Capello et al. ³⁷ reported a fracture rate of three in 1382 (0.2%). Using the same ceramic liner in a series of 301 patients, apart from one insertional chipped liner, no ceramic failures were observed ³⁸ .

In general, most liner fractures are caused by improper positioning and impaction, which generate uncontrolled peak stresses in the ceramic ³⁹ . There are reports in the literature of difficulty seating COC liners both with ⁴⁰ and without metal encasing, rates of improper positioning being as high as 16.4% ⁴¹ . In addition, titanium cups can deform as much as 0.16 mm at the rim, especially when impacting into hard bone ⁴² . Thus proper seating of the prosthesis at the time of surgery is critical. To reliably rule out ceramic liner failure, the soft tissues adjacent to the acetabular cup should also be examined for ceramic debris on postoperative radiographs.

On the other hand, many factors have been implicated in ceramic head fracture, including trauma, demanding physical activity, obesity, component mismatch, small ball diameter, defective component manufacture, and implantation error ⁴³ . As usual, management of ceramic head fracture should include excision of metalloid tissue, acetabular revision, and placement of ball head. There is debate about which type of femoral head is best used for revision of a ceramic component fracture. Future developments in the design of ceramic femoral heads may reduce hoop stresses and the incidence of trauma‐related fracture.

Dislocation

Although desirable in regard to prevention of wear debris, use of a COC THA system has raised concern regarding a potential adverse effect on dislocation prevalence. Dislocation occurs most frequently during the first year after THA ⁴⁴ , ⁴⁵ . During surgery, the following values are defined as risk factors for dislocation: cup anteversion less than 20°, a cup abduction angle greater than 50°, total postoperative anteversion less than 40° or greater than 60°, and a lowering of the postoperative hip rotation center greater than 2 mm compared with the preoperative position ⁴⁶ , ⁴⁷ , ⁴⁸ . For instance, the dislocation rate after a posterolateral surgical approach to the hip is much greater than that seen after an anterolateral or transtrochanteric approach ¹⁴ , ⁴⁹ . When utilizing a posterolateral approach, posterior capsular repair and reinforcement, can substantially reduce the incidence of dislocation after THA ⁵⁰ , ⁵¹ . Secondly, the height of the center of hip rotation is another prosthetic variable that may influence hip dislocation. Fackler and Poss demonstrated that a decreased height of the center of hip rotation was significantly more frequent in the group with dislocation ⁵² . Therefore, accurate optimal placement of the THA components has been recommended for decreasing the dislocation rate.

The greater range of options in regard to modular size and liner offset offered by the COC THA system are also helpful. Increasing head size can contribute to a lower dislocation rate, as shown by several clinical studies ⁵³ , ⁵⁴ , ⁵⁵ . For example, Khatod et al. reported a 2% dislocation rate for 28 mm heads and a 0.7% rate for 32 mm heads ⁵⁶ . With wear characteristics among 28, 32, and 36 mm COC THA systems equivalent at minimally detectable levels, head size may reasonably be increased proportionally with acetabular component size. It is noteworthy that a larger femoral head allows a wider range of movement without causing impingement, and is more effective in preventing dislocations. Recently, with the development of the Biolox Delta, the choice of femoral head has been increased to include the 36‐mm size.

Squeaking

Squeaking, another issue related to ceramic bearing couples, occurs at a reported frequency ranging from 0.7% to as high as 20.9% ³⁸ , ⁵⁷ , ⁵⁸ , ⁵⁹ , ⁶⁰ , ⁶¹ . Although most squeaking hips are pain‐ and symptom‐free, the squeak phenomenon can have a psychological impact on patients, sometimes leading to decreased satisfaction or revision. Walter et al. reported a 0.66% incidence of squeaking in a series of 2397 COC arthroplasties ⁶⁰ . They found that patients with squeaking hips were younger, heavier, and taller than patients without squeaking hips ⁶⁰ . Keurentjes et al. reported that neck length of the prosthetic design is a possible cause of squeak ⁵⁷ . Some studies have shown that patients with smaller neck geometry were 2.2 times more likely to experience squeaking than patients with larger necks ⁶² .

One possible explanation could be the increased joint laxity with shorter neck length. This may increase the amount of microseparation44, 63 and thus be a precursor of the squeaking sound. Another suggestion is that involvement of a roughened surface of the femoral head can contribute to production of metal deposits. Risk factors for metal deposition are microseparation, dislocation, abrasive neck‐socket impingement, or lack of lubrication fluid between the articulating surfaces attributable to third‐body wear ⁶⁴ , ⁶⁵ , head fracture or liner fracture ⁶⁵ , ⁶⁶ , ⁶⁷ . Others found that mismatch causes squeaking ⁶⁸ .

It is therefore probable that the causes are multifactorial, and related to a combination of implant, patient, and/or surgical factors. Taylor et al., in attempting to establish a potential cause of the squeak phenomenon, produced wear stripes on ceramic bearings in a laboratory setting ⁶⁵ . Squeaking was not observed with bearings in their pristine condition. They concluded that wear stripes caused by edge loading during either edge loading or normal articulation may be associated with bearing noise. Thus, acetabular component position and joint laxity may have a role in edge loading and also in the incidence of bearing noise.

The causes and implications of squeaking are yet to be determined. For future studies of this phenomenon, Capello et al. suggest the use of a grading scale as follows: I—occurs only rarely or occasionally (less than once per month) and is not reproducible; II—occurs occasionally (less than once per week) and is reproducible; III—occurs frequently (more often than once per week) and is reproducible; and IV—occurs with every step or position change ³⁷ . This grading scale may better quantify the incidence, as well as the severity, of this phenomenon.

Osteolysis

Periprosthetic osteolysis is regarded as an important factor in the long‐term durability of a total hip prosthesis, and is related to wear and the number of debris particles. COC materials have the lowest rates of in vivo wear on the surface bearings. Hernigou et al. retrospectively reviewed 28 patients (56 hips) with bilateral THA (one COC, and the contralateral ceramic‐on‐polyethylene [COP]) who had survived 20 years with neither revision nor loosening of either hip. They found that, while only three COC hips had evidence of pelvic osteolysis on plain radiographs, 20 COP hips had such evidence ⁶⁹ .

Current COC prostheses

Sandwich combination

In order to further reduce fracture of the ceramic‐on‐ceramic coupling and prevent impingement between the rim of the ceramic liner and the metal neck of the femoral stem, a modular acetabular component with a sandwich polyethylene insertion (alumina/polyethylene/titanium) has been investigated. However, Kircher et al. found that this new cup design resulted in a ceramic fracture rate of 9.7% overall and 15.2% for the alumina–alumina coupling ⁷⁰ . These findings are similar to those sporadic cases in which alumina liner fracture has been reported with use of a polyethylene‐alumina composite liner ⁷¹ , ⁷² . Many studies to explore the reasons for this very high fracture rate have been performed. It has been suggested that manufacturer‐specific factors such as design features may contribute to this high failure rate. First, the reduced thickness of the ceramic used in the sandwich in polyethylene ceramic inserts may increase the likelihood of a peripheral chip fracture and subsequent crack propagation under impingement conditions ⁷⁰ , ⁷³ . In addition, the second modular interface may unnecessarily introduce the potential for dissociation between the alumina and the polyethylene ⁷³ . Now the sandwich construction has been abandoned in favor of a directly fixed ceramic insert providing greater thickness and range of movement (ROM).

Alumina matrix composite

Alumina offers advantages like stability, biocompatibility and low wear; however, it has limited strength. Some applications are limited. To fulfill the increasingly exacting requirements of surgeons and patients, ceramists have developed a new ceramic composite, the alumina matrix composite (AMC). This material combines the best principles of the reinforcement of ceramics with their tribological qualities and provides better mechanical resistance. Examination of the tribological characteristics of AMC, especially under the challenging conditions of hydrothermal ageing, has demonstrated the aptitude of this material in wear applications ⁷⁴ .

Now various techniques in production have led to the newest generation of ceramics (named Biolox Delta), which incorporate zirconia into the alumina matrix. Nano‐sized, yttria‐stabilized tetragonal zirconia particles have improved the mechanical properties and reduced wear by preventing the initiation and propagation of cracks ⁷⁴ , ⁷⁵ , ⁷⁶ . Oxide additives produce platelet‐like crystals that dissipate energy by deflecting cracks. Chromium oxide (0.5%) has been added to improve the hardness and wear characteristics, and strontium crystals (0.5%) to enhance toughness and diffuse crack energy. The final AMC material consists of roughly 75% aluminum oxide, 25% zirconia, and less than 1% chromium oxide and strontium oxide. As a result, a longer lifespan has been predicted ⁷⁷ , ⁷⁸ . In a hip simulator wear test comparing 28‐mm heads composed of alumina‐on‐alumina versus AMC‐on‐AMC, these improved mechanical properties were found to have reduced the overall wear rates from 1.84 mm³ per million cycles to 0.16 mm³ per million cycles, respectively ⁷⁹ . The AMC material has a smaller grain size (less than 0.8 µm) compared with the grain size of third‐generation alumina (1–5 µm) (Table 3) ⁶³ , ⁸⁰ , ⁸¹ . This may lead to less disruption of the fluid film layer ⁶³ . A recent article reported no fractures at 6‐year follow‐up with more than 65 000 ball heads and 40 000 inserts of alumina matrix composite implanted ⁷⁶ . There is no doubt that these improved mechanical properties should decrease the ceramic fracture rate and allow the manufacturing of thinner liners and consequently the use of larger femoral heads. Furthermore, the material properties of AMC lead to a different wear response which may decrease or eliminate squeaking ⁷⁸ . For example, Hamilton et al. reported that, with minimum 2‐year follow‐up, no squeaking was reported in any of 264 hips replaced with Delta COC articulation ⁸² . The improved material properties of this ceramic combined with the ability to use larger‐diameter heads and liners to reduce instability and impingement make this bearing an attractive choice.

Table 3.

Differences between two generations of the COC prosthesis in the following crucial parameters

Parameter	Third generation (Biolox Forte)	Alumina matrix composite (Biolox Delta)
Al2O3(v/v)	>99.8	About 81.6
ZrO2(v/v)	No data	About 17
Other oxides (v/v)	Rest	About 1.4
Density (g/cm3)	3.98	4.37
4‐point bending strength (MPa)	580	1384
Fracture toughness (MPa m1/2)	3.2	6.5
Grain size (µm)	3	0.6
Wear volume (mm3/106 cycles)	1.84	0.16

Conclusion

Because of its improved wear characteristics and durability, COC technology has gained widespread popularity. Efforts are continually being made to improve implant design. Nowadays AMC ceramic material, with its superior mechanical properties and excellent wear behavior, is a promising new material for prostheses. Up until now, no complications have been reported in AMC THA. It is now possible to solve the existing longevity problems currently burdening many total joint systems in young and active patients.

Disclosure

No benefits of any type have been, or will be received, from any commercial party related to the subject of this manuscript.