Wear rates found for well-positioned conventional metal or ceramic-on-UHMWPE and ceramic-on-ceramic joints under standard testing conditions
Total hip replacement (THR) surgery has been available for several decades and is now a relatively common procedure. Since the introduction of the Charnley metal-on-ultra-high molecular weight polyethylene (UHMWPE) hip prosthesis, THR is seen as one of the most successful orthopaedic operations available today. There are currently over 80,000 hip replacement procedures carried out in England and Wales  each year and THR has now become more popular with the younger, more active patient. This is shown in the statistics reported in the National Joint Registry; 12% of the patients who undergo THR are under the age of 55 and 85% of these are recorded as being either fit and healthy (16%) or with mild disease that is not incapacitating (69%) . However, failure of these artificial joints does occur, leading to the need for revision surgery; approximately 10% of the THR procedures reported are revision operations .
Failure, in many cases, is due to aseptic loosening [1, 2]. With the conventional metal-on-UHMWPE joint this has been shown to be due to wear particle induced osteolyisis . Although 90% of these joints are operating well 15 years after implantation  this wear particle induced osteolysis may lead to repetitive revision requirements for the younger, more active patient.
An alternative to the conventional metal-on-UHMWPE type of hip joint is to use ceramic-on-ceramic joints. Ceramic-on-ceramic joints were first introduced in the early 1970s but often resulted in poor performance due to fixation problems, poor quality alumina as a result of inadequately controlled grain size and other material properties (leading to catastrophic wear), and sub optimal design parameters such as too large a clearance. The work of many people over the years, including material scientists and engineers, has improved the quality of these ceramics. Therefore, since the introduction of the standard for ceramic production (ISO 6474:1981 (second-generation ceramics), which was replaced with ISO 6474:1994 (third-generation ceramics) and has now been replaced by ISO 6474-1:2010 (fourth-generation ceramics)) the performance of these all ceramic bearings has been greatly improved [4, 5]. The third and fourth-generation alumina ceramics are manufactured using hot isostatic pressing. This produces a material that is highly pure with a small grain size (≤ 2.5 µm and many manufacturers produce ceramics with even smaller grain sizes) that provides material strength and minimises the risk of fracture. The majority of the ceramic-on-ceramic joints discussed in this chapter were produced using third-generation ceramics.
Ceramic-on-ceramic hip prosthesis performance will be reviewed in this chapter (
2. Ceramic-on-ceramic hip prosthesis performance
The performance of ceramic-on-ceramic hip joints has been evaluated using data obtained from joints operating well within the body, prostheses retrieved due to joint failure and also tests performed within the laboratory. Firstly, the
The majority of recent papers discussing the tribology (lubrication, friction and wear) of ceramic-on-ceramic hip joints detail tests done under ‘severe’ conditions such as malpositioning, edge-loading or microseparation. There are, however, some earlier studies that describe how these joints operate under ‘standard’ conditions.
A well positioned ceramic-on-ceramic hip, tested under the loads and motions expected during the standard walking cycle, performs exceptionally well in terms of friction, lubrication and wear [6-24]. Friction tests using different viscosities of carboxy-methyl cellulose (CMC) solution show that these joints operate close to full-fluid film lubrication with very low friction factors (0.002 at physiological viscosities) [6, 12]. These ceramic-on-ceramic joints have been shown to have very low surface roughness values that play a part in this low friction . Tests were also performed using different viscosities of bovine serum [6, 12]. Bovine serum is often used in the laboratory as a replacement for the body’s natural lubricating fluid, synovial fluid, as it contains proteins that act in a similar manner to those present in synovial fluid and although CMC fluids replicate the shear-thinning behaviour of synovial fluid, they do not contain any proteins. The introduction of these proteins into the lubricating fluid resulted in higher friction (0.03 at physiological viscosities) and mixed lubrication. These results are shown as Stribeck plots in Figure 1. A Stribeck plot shows the measured friction factor plotted against Sommerfeld number (a dimensionless parameter dependent on the lubricant viscosity, the entraining velocity of the bearing surfaces, the joint radius and the load applied). A rising trend of friction factor with increasing Sommerfeld number is indicative of a full-fluid film lubrication regime, whereas a falling trend is normally indicative of mixed lubrication. In full-fluid film lubrication, the surfaces are completely separated by the lubricant and the friction generated is due solely to the shearing of the lubricant film. In mixed lubrication, the load is carried in part by the contact between the asperities of the bearing surfaces and also by the pressure generated within the lubricant. Although the bovine serum tests suggested that these ceramic-on-ceramic joints were operating in mixed lubrication with some asperity contact, there was no surface damage evident on the joints after testing. It was speculated that the proteins that adhere to the ceramic surfaces when using protein-based lubricants produce a sufficiently thick layer to penetrate the fluid film and result in protein-to-protein contact and shearing. It is likely that the subsequent friction developed by the protein-to-protein contact is greater than that due to the shearing of the lubricant film alone. Therefore, although higher friction and mixed lubrication is encountered when testing under more physiological lubricating conditions, there is still little asperity contact during the normal walking cycle .
With this good lubrication and low surface roughness the wear volumes produced under ‘standard’ conditions in the laboratory have, inevitably, been shown to be very low (less than 0.4 mg/million cycles cf. approximately 35 mg/million cycles for conventional metal or ceramic-on-UHMWPE joints, see Table 1), and sometimes almost immeasurable. As these ceramic-on-ceramic joints are working close to full-fluid film lubrication there is little or no contacting of the asperities on the joint surfaces leading to this low wear and friction [6, 12]. Also, laboratory studies have shown that cup malpositioning and elevated swing phase load testing have not significantly affected the wear of these joints [21, 22, 25, 26], see Table 2. This combination should, therefore, lead to a very successful artificial joint.
|||Ceramic (zirconia)-on-UHMWPE||UHMWPE cup||37.9|
|||Ceramic (zirconia)-on-UHMWPE||UHMWPE cup||~29.0|
|||Ceramic (zirconia)-on-UHMWPE||UHMWPE cup||~29.5|
|||Ceramic-on-ceramic||Head and cup||~0.25|
|||Ceramic-on-ceramic||Head and cup||~0.16|
|||Ceramic-on-ceramic||Head and cup||< 0.04|
|||Ceramic-on-ceramic||Head and cup||0.09|
|||Ceramic-on-ceramic||Head and cup||~0.20|
More severe loading conditions have, however, given slightly different results. Microseparation was first introduced during the swing phase of the walking cycle on the Leeds hip wear simulator  to replicate the stripe wear sometimes seen on retrieved ceramic-on-ceramic joints. Microseparation was incorporated in the simulator studies at Leeds because a year earlier, it was suggested by Mallory
Using this microseparation technique, Nevelos
|||Standard||Cup only||< 0.02|
|||Standard||Head and cup||~0.16|
|||Standard||Head and cup||< 0.04|
|||Standard||Head and cup||0.09|
|||Standard||Head and cup||~0.20|
|||Elevated cup angle||Not stated||~0.20|
|||Elevated cup angle||Not stated||0.20|
|||Elevated swing phase load||Cup only||0.36|
|||Microseparation||Not stated||4.78 (after 800,000 cycles)|
|||Microseparation||Not stated||(mild) ~0.40|
|||Microseparation||Head and cup||1.55|
It is still unknown if it is this microseparation that causes the stripe wear that is observed on some retrievals or whether this type of wear is due simply to edge-loading of the head on the cup through a different mechanism. Higher rates of wear including the appearance of stripe wear may also be due to a steep acetabular cup implantation angle or repeated dislocations . Microseparation, or edge-loading, may occur during various different physical activities such as stair climbing, standing from squat position and deep flexion. It may, however, also occur during walking. These simulator studies incorporating microseparation of the head and cup into the walking cycle are therefore a severe testing method. The resulting rim contact and stripe wear does, however, replicate the more severe conditions that these joints may encounter thus producing the same effect that is seen on some retrievals but not necessarily through the correct corresponding actions.
If, and when, wear of these ceramic components occurs it is important to understand how the body is likely to react to these wear particles. It is well known that wear particle induced osteolysis is a major concern for conventional metal-on-UHMWPE joints  but is this the case for ceramic-on-ceramic prostheses? Promisingly, several workers have shown that the cellular response to ceramic particles is less severe than that due to polyethylene particles [35, 15]. An important point to note with the laboratory tests discussed above is that, even under extreme loading and motion conditions, these joints provide low wear with a minimal adverse tissue reaction to these wear particles. As stated by Fisher
As shown above, the laboratory test results for ceramic-on-ceramic joints are very promising. Is this mirrored by the
The short-term (mean follow-up of 50.4 months) performance of ceramic-on-ceramic joints was compared to that of metal-on-highly cross linked polyethylene (XLPE) joints in a study reported by Bascarevic
A comparative study was also performed by Amanatullah
Another short-term study comparing the results of 525 hips (421 ceramic-on-ceramic and 104 metal-on-UHMWPE) was reported by Johansson
A short-term study (60 month follow-up) reported by Nikolaou
Early to mid-term results were also reported by Stafford
Another 10 year follow-up was reported by Yeung
In a multicentre study performed by Capello
In the study reported by Lee
A long-term study on earlier generation ceramic joints was performed by Hernigou
Another longer-term study (mean 20.8 years), again with earlier generation ceramics, performed by Petsatodis
Although these results are promising, in the majority of cases, the follow-up period particularly for the third generation ceramics, was only short-term. It will, therefore be very interesting to evaluate the performance of these third and fourth-generation ceramic-on-ceramic joints, along with the metal and ceramic-on-highly XLPE, on a longer-term basis. These results are eagerly anticipated.
The papers discussed so far have shown exceptional performance of ceramic-on-ceramic joints and suggests that these joints may perform better than metal or ceramic-on UHMWPE. This, however, is not reflected in the data described in the National Joint Registry (NJR) of England and Wales (2011) . Promisingly though, the NJR states that there is ‘little substantive difference’ in the risk of revision for ceramic-on-ceramic, ceramic-on-polyethylene or metal-on-polyethylene joints. It is, however, not stated whether this polyethylene is UHMWPE or XLPE and the differences in performance between these two materials is not listed.
Although, as discussed above, many of these ceramic-on-ceramic hip joints perform exceptionally well, early dislocation is seen as a possible concern due to limited modular neck length and other factors. In the majority of the published literature referred to in this text [47, 36, 48, 43, 49, 50, 46, 51-57, 41, 40], the occurrence of dislocation is 0% - 2.3%. The number of dislocations was higher (6%) in a study reported by Chevillotte
Component fracture in ceramic-on-ceramic hips is another cause for concern for many surgeons and patients. The fracture rates of ceramic joints have been dramatically reduced since the introduction of third and fourth-generation ceramics with the new material processing methods. Ceramic-on-ceramic joints have strict regulations that must be abided by with regard to material properties such as burst strength; as discussed in work reported by Salih
For what other reasons does failure occur? Savarino
A case study was reported by Nam
3. The “squeaking” hip
Another concern with hard-on-hard bearing couples such as ceramic-on-ceramic and metal-on-metal is the incidence of noise or “squeaking” in these joints. Audible sounds such as squeaking, clicking, snapping, cracking, grinding, rustling, crunching and tinkling are all referred to in this text as “squeaking”. “Squeaking” can be present during different kinds of activity including stair climbing, bending forward, squatting, standing from a chair and walking. The occurrence of this “squeaking” has been reported by many workers to different degrees.
A study performed by Cogan
After 10 years of follow-up Chevillotte
The largest study to date (Sexton
There is great debate over the cause of “squeaking” in ceramic-on-ceramic THRs. Several possible causes of “squeaking” are edge-loading , component malpositioning [78-80] or component or stem design [74, 81, 70, 77]. Other workers have also related “squeaking” to patient weight, height or age [70, 78, 79]. It is possible that a number of factors need to be present to initiate the “squeaking” phenomenon. The occurrence of “squeaking” has not been found to compromise the results of ceramic-on-ceramic hip joints; however, some patients do request revision surgery in order to solve the “squeaking” issue .
Recently, laboratory studies have been performed in an attempt to re-create the conditions required to generate this “squeaking”. Some authors have observed “squeaking” with ceramic-on-ceramic hip joints lubricated under dry conditions [83, 84, 73]. However, such an extreme lubrication regime is not expected to occur
Although there are a few reports on poor ceramic-on-ceramic hip prosthesis performance, the majority of authors give good and optimistic results. There have been little or no fractures, dislocation, infection or osteolysis. Also, the few patients suffering from “squeaking” hips have, in the majority of cases, had no need for revision surgery. These are, therefore, excellent results, but Lee
4. The younger patient
As these artificial hip joints have been found to perform well in the younger patient (45 to 55 years), some surgeons have chosen to replace the diseased joints of even younger patients with this material combination.
A case study was reported by Capello and Feinberg (2009)  where ceramic-on-ceramic joints were implanted in a 13 year-old child with bilateral end-stage arthritis of the hip. Seven and eight years post-operatively the patient had no pain, no limp, and was able to walk long distances. The radiographs showed no implant loosening, osteolysis or wear. This is a very encouraging result, however, it was stated that the patient is still very young (20 years of age at the time of report) and, therefore, the need for revision surgery will be more than likely.
Other studies on younger patients have not had as good results as those reported by Capello though. Nizard
From the results reported here it is clear that ceramic-on-ceramic hip joints have good tribological results: low friction, good lubrication and very low wear