Compared to alumina, partially stabilised zirconia (PSZ) has higher flexural strength, fracture toughness and high Weibull modulus (which equates to better reliability), as well as lower Young's modulus and the ability to be polished to a superior surface finish. The higher fracture toughness is of importance in femoral heads due to the tensile stresses induced by the taper fit onto the femoral stem.
Wear Debris from Implants and Osteolysis
The production of particulate wear debris from implant materials and subsequent osteolysis has been recognized as the major cause of long-term failure in total hip replacement. The basic strategy to address the problem of osteolysis has been to reduce the number of polyethylene particles generated by improving the materials at the articulating surfaces. The use of a ceramic femoral head has been advocated especially in young active patients because it produces less polyethylene wear compared with a conventional metal femoral head. However, a number of attempts were made to eliminate the use of polyethylene through the use of metal-on-metal and ceramic-on-ceramic couples.
Number of Zirconia Implants Used and Types of Zirconia Used
Partially stabilized zirconia femoral heads made up about 25% of the total number of operations per year in Europe, and 8% of the hip implant procedures in USA. It has been reported that over 400,000 zirconia hip joint femoral heads have been implanted since 1985 until 2001. Most of the zirconia femoral heads (tetragonal zirconia polycrystal, TZP) consists of 97 mol%ZrO2 and 3 mol%Y2O3.
Wear Behaviour of Partially Stabilised Zirconia
Although not quite as hard as alumina, partially stabilised zirconia still possesses excellent wear resistance and has been used for similar orthopaedic applications as alumina. Wear rates of UHMWPE against partially stabilised zirconia have been found to be low enough such that tribological debris would not be a problem in clinical applications.
Published Results of Zirconia Implant Wear Rates
Published results indicate that the use of a ceramic-ceramic femoral head/acetabular cup system is advantageous over ceramic/UHMWPE systems as polymeric wear debris is avoided. In fact Chevalier and coworkers, in 1997 reported that the coefficient of friction between an alumina cup and zirconia head was much lower than ceramic against UHMWPE and the resultant wear between the two components was almost zero. Recent work by Clarke et al. (2000) on articulation of femoral heads in total hip replacement (THR) using hip simulators with alpha-calf serum as a lubricant, where alumina/alumina, zirconia/alumina, and zirconia/zirconia couples were investigated has shown that wear rates using zirconia/zirconia exhibit a mild run-in phase as opposed to a more evident run-in phase. While zirconia/alumina wear, where zirconia was the head and the liner alumina showed little weight loss and the alumina linear revealed a typical run-in phase followed by steady state weight loss. The study has revealed promise for hard/hard THR systems whereby wear rates were three times less in order of magnitude when compared to polyethylene cups. The study has employed alpha-calf serum at a 50% concentration whereby most other studies published were carried out using either water or saline solution, which can be quite detrimental to the performance of the ceramic in question.
The Effect of Testing Conditions for Simulated Implant Tests
Zirconia ceramic implants somehow have had a controversial history regarding their phase metastability, degradation in water lubricants in simulation studies and influence on friction and wear phenomena. At a Japanese orthopaedic meeting in 1988, the orthopaedic group from Bologna, Italy reported that the wear of zirconia-on-zirconia is 5,000 times worse than alumina-on-alumina. While the zirconia in such studies came from four different sources, the commonality in these studies appeared to be the use of water as the test lubricant.
The lubricant used in laboratory evaluation of total hip joint replacements (THR) has been shown to significantly influence the wear results. Various studies have been conducted successfully with water, saline, and serum lubrication of alumina-on-alumina bearings. However with zirconia-on-zirconia tests in water have consistently shown catastrophic results while serum has demonstrated good results. One such study by Oonishi and co-workers showed that Y-TZP balls transformed from tetragonal to monoclinic phase when run with saline in a hip simulator.
Yttrium Stabilised Tetragonal Zirconia Polycrystal
Yttrium stabilised tetragonal polycrystalline zirconia (Y-TZP) has a fine grain size and offers the best mechanical properties. Low temperature degradation of tetragonal zirconia polycrystal is known to occur as a result of the spontaneous phase transformation of the tetragonal zirconia to monoclinic phase during ageing at 130-300°C possibly within water environment. It has been reported that this degradation leads to a decrease in strength due to the formation of microcracks and accompanying phase transformation.
Studies of Zirconia-on-Zirconia Wear Rates in an Implant Simulator
There are also clinical studies available on zirconia balls combined with alumina cups with excellent short term result. However, contemporary clinical studies of zirconia on polyethylene have shown mixed results. Given these somewhat contradictory results between laboratory and clinical studies, Clarke et al. studied the wear of zirconia-on-zirconia bearings in both water and serum and analysed the debris and the zirconia implants using Raman Microprobe Spectroscopy. The water lubrication resulted in a wear about 10,000 times greater than with serum lubrication. The wear of both zirconia femoral head and cup in water lubrication showed a high weight loss of 28mg after only 6,100 cycles. In contrast, the zirconia wear with serum lubrication had a weight loss of only 0.74mg after 20 million cycles. This difference between the two lubricants was also distinct in the micro-wear of the ball surfaces. With serum, there were still some original machine tracks to be seen; with water there was total surface disruption. The size of the wear debris ranged from 0.38 to 16.8 microns averaging 1.80 microns. Analysis of the debris by Raman spectroscopy showed that the debris was almost exclusively monoclinic phase. Therefore, the simulator with water lubrication created extensive transformation of the zirconia. However, under serum lubricant conditions this transformation did not occur and the wear surface showed very little evidence of wear even at 20Mc. This illustrated that the appropriate choice of a joint analogue fluid was of paramount importance for satisfactory wear tests. The metastability of Y-TZP ceramic is greatly affected by moisture and high temperatures and can also be stress activated. Based on this difference in lubricant performance Clarke et al. postulated that the serum proteins either, formed an effective solid lubricant film between the zirconia surfaces or acted to trap an adequate supply of moisture that effectively reduced the friction between opposing zirconia surfaces. Their study revealed the dramatic effect that serum proteins had at the bearing surface greatly reducing the wear-rate of zirconia.
Interpretation of Wear Results
It has been widely accepted that to improve the longevity of total hip replacements, it is also necessary to solve the late complications associated with implant fixation and polyethylene wear. It was reported by Woolson et al. that the average wear rate of polyethylene against 28mm CoCr heads was 0.14mm/year at the mean follow-up of 5.7 years in cementless total hip arthroplasties (Harris-Galante prostheses). Although the present wear rate (0.140mm/year) is almost equal to the reported value, the present zirconia-on-polyethylene combination would be superior to the CoCr-on-polyethylene one as to the volumetric wear because of the different size of heads used. Since the wear rate of metal-backed polyethylene generally tends to be influenced greatly by the polyethylene creep in the acetabular metal shell particularly at the short follow-up duration, the present wear rate may become lower at the longer follow-up duration.
Bad Publicity for Zirconia Implants
In the history of zirconia implants, the most devastating historical event was the introduction of the so called “TH-zirconia” implants in 1998. This “TH-ball” fracture history while it represents a small (nine batches) but catastrophic experience of zirconia ball fractures in patients, it was unique to one ceramic company and one manufacturing process. Since the end of 2000, 317 head breakages have been reported in these implants. Since August 2001, the company in question has no longer been making implants for medical applications.
New Modified Zirconia Implants
In spite of early promising results, there has been major concerns regarding the above mentioned degradation phenomenon associated with the tetragonal-to-monoclinic phase transformation under the long-term aqueous conditions such as those encountered in vivo.
Improved Performance of Zirconia Implants
Most of the current manufacturers of zirconia femoral heads have been improving the conventional zirconia, to produce implants with increased strength and a much higher resistance to the phase transformation in physiological environment. It was reported by Nakamura et al. that in hip simulator studies the polyethylene wear against the improved zirconia head is much lower than that the Co-Cr femoral head. When articulated with highly cross linked polyethylene, not only zirconia heads but also Co-Cr heads showed very low wear rates. However, because zirconia is more scratch-resistant than Co-Cr, it was suggested that it would be a more suitable implant for the long-term clinical use.
New Zirconia-Based Formulations
Recently degradation-free new zirconia-alumina composites have been reported; TZP/alumina composite (80% TZP of [90 mol% ZrO2-6 Mol%Y2O3-4 mol%Nb2O5 composition] and 20% Al2O3). Another potential composite comprised of 70 vol%TZP (stabilized with 10mol%CeO2), and 30vol%Al2O3 and 0.05mol%TiO2 is currently being investigated in Japan.
Improving the Strength and Toughness of Tetragonal Zirconia Polycrystal
Kim et al. recently reported that the strength of zirconia ceramic (Y,Nb)-TZP was lower than that of 3Y-TZP. However, the addition of 10 vol% of Al2O3 increased both the strength and the toughness of the (Y,Nb)-TZP It was further reported that the improvement of the strength is mainly due to the increase in the toughness according to the linear elastic fracture mechanics and partially due to the grain size refinement of the (Y,Nb)-TZP since the alumina particles behave as an inhibitor for exaggerated grain growth of the zirconia grains during sintering.
The increase in the toughness arises from the contributions of the phase transformation toughening, enhanced by the residual stress as a result of the thermal expansion mismatch between the zirconia and the alumina particles, and the crack bridging toughening by the alumina particles.
Implant stability is critical in obtaining good long term success of total joint replacements. Loss of either biological or cement fixation can lead to accelerated wear, pain, loss of function, or even fracture of the implant, each of which could potentially necessitate revision surgery. Fixation strength can be increased by using macrotextured (porous or textured) surfaces, which enhance the potential for mechanical inter-lock at the interface.
Zirconia Implant Coatings and Macrotexturing
Oxidized zirconium, a material recently introduced for orthopaedic bearing applications (OxininiumTM, Smith & Nephew, Inc., Memphis, TN) and was reported to have beneficial wear and abrasion resistance. These new generation implants cannot be easily processed using traditional porous coating techniques. Therefore, an alternative chemical surface texturing method was utilised. Chemical texturing process has been used clinically on Ti6Al4V total hip replacement components to create a surface morphology suitable for bone ingrowth. This texturing method, known commercially as ChemTex ® 5-5-5 (CYCAM, Inc., Houston, PA), and a newly developed chemical texturing process, known commercially as Tecotex ® 1-103 (Tecomet, Woburn, MA), were selected to produce macro-textured surfaces (Rmax > 0.4 mm) on a zirconium alloy (Zr2.5Nb). These textured surfaces are subsequently oxidized to form a hard ceramic layer uniformly about 5 µm thick over the entire surface, which consists predominantly of monoclinic zirconia.
Note: A complete list of references is available by referring to the original text.