Introduction                       

In the science of hip replacement development, one of the key areas of research is in the field of tribology. Tribology has been defined as the science and technology of interacting surfaces in relative motion and all particles related thereto. The study of wear, friction and lubrication in relation to the interacting surfaces all form part of tribology. In its most basic form, Tribology has been in existence since the earliest records of history. There are examples of how early civilizations used the principles of low friction to develop bearing surfaces. Tribology has also been studied scientifically for a long-time, however our understanding was limited and little progress was made for several centuries. The publication of Reynolds paper on hydrodynamic lubrication led to an explosion in research in to friction, wear and lubrication.1 Over the last few decades, in an attempt to understand and improve hip replacement technology, the tribological performance of several material combinations have been assessed. Improvements have been made in manufacturing processes, device designs and materials properties in order to minimize wear and friction and maximize component longevity in vivo (Fig. 2.1).

 

 

 

 

Figure 2.1: Scores of material combinations and designs have been considered as potential hip replacement options over the years, including ceramics, polymers and metals

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Wear                          

Total Hip Arthroplasty

 

The study of wear has proved to be complex and generalizations in this field can lead to component failure. The wear debris generated and the consequent tissue and immune response are key to the longevity of the orthopedic implant in vivo. The wear of the component through various mechanisms leads eventually to debris-induced osteolysis. Osteolysis is localized bone loss which causes deterioration in the fixation between implant and bone. The loss of material in the form of particles due to the relative motion between two surfaces is described as wear. The surface of engineering materials will always have asperities at the microscopic level which are also described as peaks and troughs. When two surfaces are put in contact with each other and a load is applied, the real area of contact is the sum of the area of the contacting asperities. When the two surfaces slide past each other, electro-repulsive and atomic binding interactions of the asperities cause the two surfaces to bind together. Asperities at the bound surface are then removed as the surfaces slide past each other to form particulate debris. Material and condition specific empirical equations have been derived in their hundreds as our understanding of the subject matter has increased. A widely used equation for dimensionless wear coefficient, k, was suggested by Archard (1953):

VH

 

k= Lx (1)

where, V is wear volume, x is the sliding distance, L is the normal load, and H is the hardness of the material. There are several wear mechanisms which need to be considered when characterising new materials for use in artificial hip replacements. Wear is defined as the progressive removal of material from contacting surfaces which are in relative motion to each other. The commonly considered mechanisms of wear in relation to hip replacements are as follows:

Adhesive wear: Describes the transfer of material from one surface to the counter surface it is in sliding contact with. Where the two surfaces are in contact, adhesion bonding takes place. These bonds result in the shearing of the material during sliding, which results in the detachment of material from either of the surfaces. According to Archard’s theory of sliding wear, shearing of the asperity junctions can occur in one of the two bodies depending on the relative magnitude of the interfacial adhesion strength and the breaking (shearing) strength of surrounding local regions.

Abrasive wear: Describes the removal of material from a surface as a result of either two-body or three-body interaction. The relatively softer surface is ploughed through by a harder material. In two-body wear, the abrasion occurs between the articulating surfaces, and in three-body wear, the abrasion results from rough hard particles trapped between the two sliding surfaces causing wear.

Corrosive wear occurs by the combination of mechanical and chemical reactions, when sliding motion occurs in a corrosive environment.

Different material types are affected by a combination of these wear mechanisms to varying degrees.

 

Lubrication                       

Component wear can be mitigated by employing lubrication principles effectively. The natural hip joint relies heavily on lubrication principles to reduce wear and improve joint mobility. The design of the natural hip joint, where cartilage, soaked in synovial fluid which is rich in macromolecules and proteinaceous matter, work synergistically to provide extremely low friction coefficients. There are five different types of mechanisms that facilitate shock absorption to disperse the body weight that is transmitted through the hip joint, they are

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Tribology and Bearing Materials

 

Figure 2.2: Stribeck curve relating friction coefficient to Sommerfeld number in terms of lubrication regime

 

 

hydrodynamic, squeeze film, boosting, hydrostatic, and boundary layer lubrication. Hydrodynamic lubrication is promoted by high relative sliding velocities of the two articulating surfaces normal to the load applied. The presence of a lubricant causes the two surfaces to be separated by a layer of fluid. Cartilage expresses synovial fluid through its porous structure when a load is applied, thus reducing friction. This is described as squeeze film lubrication. The natural hip joint is a finely tuned mechanical joint, with deceptively complicated bio-mechanical and chemical interactions between the bones and the synovial lubricant.

Depending on the effectiveness of the lubricant and the bearing conditions, the mode of lubrication may range from dry sliding conditions to complete fluid film lubrication, the intermediate being boundary or mixed lubrication where the components are partially separated (Fig. 2.2).

Similarly, whilst designing an artificial prosthesis, it is important to optimize the lubrication regime generated. Several factors influence the mode of lubrication that is achieved including, component clearance, radius and surface finish. Complete fluid film can be defined as the point when the peak asperities of the two surfaces are fully separated by the lubricant film. The equation that governs the thickness of the fluid film generated is as follows:2

 

0.65 -0.21

h  ηu   L 

min = 2.798    

 

(2)

x

 

Rx  E' Rx   E' R2 

where  is the lubricant viscosity, u is the entraining velocity, hmin is the minimum lubricant thickness and L is the load. E’ and Rx are as defined below:

 

 

Rx =

R1 R2 R1 – R2

 

(3)

where R1 and R2 is the radius of the cup and the head respectively (Fig. 2.3).

 

1 1 1 –  2 1 –  2 

=    1 +    2 

 

(4)

E' 2  E1

E2 

where E1 and E2 are the Young’s modulus of the cup and the head respectively and 1 and 2 are the Poisson’s ratio of the cup and the head respectively. As a simple rule, it is considered that if the film thickness is three times larger than the sum of the surface roughness of the two surfaces then there is complete fluid film lubrication. Lubrication regimes can be

described in three forms, boundary, mixed and fluid film lubrication.

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Total Hip Arthroplasty

 

Figure 2.3: Diametrical clearance is twice the radial clearance and is defined as the difference between the inner diameter of the cup and the diameter of the head, i.e. 2(R1–R2)

 

Experimental observations show that based on the following equation:3

 

 = hmin

S2 +S2

 

(5)

q1 q2

where Sq1 and Sq2 are the measured values of the root mean square of the roughness of the head and the cup respectively, if  >3 then the joint is operating under fluid film conditions, if 1 <  < 3 its mixed lubrication and if  <1 its boundary lubrication conditions.

 

Design                         

The designs of hip replacements play an important role in their long-term survivorship in vivo. History of hip arthroplasty has clearly shown, optimizing the design of components has led to significant improvements in component performance. There are several design parameters that need to be considered when assessing a material’s suitability as a hip prosthesis including, the clearance of the components, the diameter, the wall thickness and the fixation method used. These factors would be dependent on strength, hardness, Young’s modulus, surface finish, roundness, contact stresses and the lubrication regime that the component operates under.

Under dry conditions, the study of the effect of component diameter on wear has shown that the larger the diameter, the greater the volumetric wear. The wear equation states that:

V = K × L × X (6)

where V is the wear volume, K is the wear factor, L is the load and X is the sliding distance. Under the terms of this equation, wear volume (V) is directly proportional to the sliding distance (X) which is a function of the component diameter. However, there are other factors that confound this theory such as joint lubrication conditions. Keeping all else constant, components with larger diameters operate under better lubrication conditions which in turn will reduce the probability of a wear particle being generated (K), which then reduces the wear volume (V).

 

Patient                      Activity                      

The successful design of hip replacements requires an understanding of the hip anatomy as well as the kinetics and kinematics of the hip joint. Postoperative follow-up patient activity studies act as a useful indicator of the success of hip replacements. With conventional hip replacements, studies suggest that implant wear is related to patient activity more so than duration of the implant in situ. Sir John Charnley predicted that conventional replacements would perform poorly in young and active patients in the absence of other physical restraining factors.4

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STEP ACTIVITY MONITOR

Tribology and Bearing Materials

 

The step activity monitor (SAM, Cymatech, Seattle, Washington) is a small, unobtrusive apparatus designed to measure patient step counts continuously over the period the instrument is worn. The SAM is an accelerometer device, sensitive to the types of movement associated with a wide range of gait styles. It is designed with an electronic filter which acts to reject any extraneous signals.

Two studies were devised which looked at the activity levels of patients before and after receiving a total hip resurfacing to give an indication of the nature of patient ambulatory activity. The first, an ongoing prospective study in which 25 consecutive male patients who received a 50 mm Birmingham Hip Resurfacing (BHR, Smith and Nephew Orthopaedics Limited, Leamington Spa, U.K.) were recruited. The patients chosen had a mean age of 56 years (45 to 68). The same patients were followed up at one, two and four years postoperatively.

In the second group, a cross-sectional study of 183 patients postoperatively and 28 preoperative patients’ activity levels were assessed. The patients were followed up at various time points after their operation (1-10 years). The mean age of the patients in the cross-sectional study at the point of activity assessment was 54.5 years (32 to 65).

The patients were given the device and asked to wear it just above the lateral malleolus of the right leg or the medial malleolus of the left leg over a period of five to seven days throughout their waking hours. The temporal trend in change of step activity in these patients was recorded (Fig. 2.4). The device measures the activity on the leg it is fastened to; therefore on average every step measured on the device is equal to two step taken by the subject. The data is then sorted in to various measures of ambulatory activity as defined by the developers of the SAM. One such parameter is the peak index which is defined the 60 highest ranking step rates when looking at a window of one minute of step counting across the period measured. The maximum activity achieved over different designated time ‘windows’ of 1, 20 and 30 minutes is known as Max 1, 20 and 30 respectively, i.e. the maximum number of cycles of step activity over periods of one, 20 and 30 minutes of continuous measurement. The results of the prospective study showed that the 25 consecutive patients had a mean Max 1 of just under 60 cycles of a hip joint per minute (Fig. 2.5). This indicates that these patients, on average do not walk at a pace of 1 Hz (or 60 steps per minute) even over short periods. The Max 30 for this group of patients had an average just below 30 steps per minute and Max 20 was just above 30 steps per minute (Fig. 2.5).

In the cross-sectional study, the preoperative patients had a mean step activity rate of 3926 cycles per day, which is the equivalent of 1.4 million cycles per year (Fig. 2.6). The patients assessed postoperatively had a mean step activity rate of 5295 cycles per day, which can be extrapolated to approximately 1.9 million cycles per year. Each of the individual stage groups (1, 2, 4, 6 and 10 years) at follow-up had a higher step cycle rate than the preoperative cohort. In the follow-up patients there is an increasing trend till the 6 year follow-up, this is

 

 

 

 

Figure 2.4: A typical 24 hour activity profile, shows the number of cycles on one leg taken during every minute of the day. Total number of cycles is 5014, an approximation of the number of steps is 10,028 (twice the number of cycles)

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Total Hip Arthroplasty

 

Figure 2.5: Step activity data of 24 male patients at 1, 2 and 4 years follow-up showing Max 1, 20 and 30 and Peak index (±95% confidence intervals), the secondary y-axis shows the equivalent cycle frequency for those patients

 

Figure 2.6: Step activity monitor data of patient step cycles per day extrapolated to per year (±95% confidence limit)

 

followed by a marginal reduction at the 9-10 year follow-up patient group. However, the differences between the individual follow-up groups are not statistically significant.

 

Hip          Simulator          Studies                   

Hip simulators have been used increasingly for the last few decades, to determine the tribological performance of novel materials.5 Hip simulators have increased in their complexity and their physiological relevance, with testing protocols being fine-tuned to more closely replicate in vivo conditions (Figs 2.7A to D). However, accelerated wear studies on hip simulators have their limitations and it is often only with hindsight these have been realized. In an attempt to improve the physiological relevance of in vitro studies, the authors closely re-examined hip simulator protocols used for hard-on-hard material bearings. The mismatch between the kinetics and kinematics used in hip simulator studies and that observed through patient activity follow-up data was identified as a potential reason for the difference in wear measurements in vitro and in vivo. The use of the ISO standard (ISO 14242-1) has been justified in testing conventional metal-on-ultra-high-molecular weight polyethylene (MoUHMWPE) joints. Previous tribological studies have shown that MoUHMWPE components operate under boundary lubrication conditions due to their modulus of elasticity and surface roughness properties. The lubrication conditions and hence the wear of this bearing may be less sensitive to varying speeds of motion and the stop/start nature of normal ambulatory activity. However in the case of hard-on-hard bearings like metal-on-metal replacements, the success of the bearing is strongly related to the mode of

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Tribology and Bearing Materials

 

Figure 2.7: Hip simulator test set-up (a) anatomical loading of cup against head, (b) the femoral head fixture, (c) the cup fixture and (d) the hip simulator in operation with bovine serum as lubricant

 

lubrication components operate under. Modern MoM bearings rely on relatively low clearance components with excellent surface finish in order to achieve mixed and fluid film lubrication conditions, protecting the components from greater wear.

Under the ISO test conditions with walking cycles of 1 Hz (60 cycles/min) with uninterrupted and identical movements, MoM devices would generate an exaggerated lubrication regime, resulting in extremely low wear during the steady state phase in vitro. Sliding velocity between the two contacting surfaces is a significant factor that needs to be considered. As the sliding velocity increases, the fluid film thickness increases (Equation 2), therefore protecting the bearing surfaces and minimizing wear. For this reason it is important that the tests carried out in the hip simulators use physiologically relevant test frequencies and sliding velocity when using MoM components.

The first generation of MoM bearings which were successful were made of high carbon as-cast cobalt chromium molybdenum (CoCrMo) alloy. Many of the first generation implants lasted more than 30 years in vivo with no evidence of osteolysis. However, in an attempt to optimize bearing design, some manufacturers developed MoM bearings with a different CoCrMo microstructure by varying the carbon content and/or changing the processing methods. Thermal treatments are commonly used in engineering applications to alter the metallurgy of as-cast CoCrMo. Pin-on-disk/plate and hip simulator studies have been done to determine the effect of the presence of carbides on wear of high-carbon MoM components.6-10 However, the effect of material microstructure on wear is apparent only when the lubricant in the test is not completely separating the two articulating surfaces as is the case in pin-on-disk/plate studies and in vivo. A number of researchers have reported no significant difference between the wear of different microstructure CoCr alloys in hip simulator studies. However with MoM devices, the effect of material microstructure is far

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Total Hip Arthroplasty

 

less expressed in hip simulator studies due to the nature of repeated and noninterrupted motions of the joint according to the ISO test protocol.

 

Development   of   a   New   Test   Protocol            

HIP SIMULATORS

A new test protocol was developed to test MoM components,11 in order to improve the predictive value of hip simulator testing. The new protocol employed a test frequency of

0.5 Hz (30 hip cycles per minute) with stop/start motion added every 100 cycles.12 Two sets of kinematics and kinetics were employed in this protocol, and they were alternated every 100 cycles in order to emulate the variations in patients’ day-to-day activity. The first flexion/ extension range used was 30°/15° with an internal external rotation of ±10°. A Paul type stance phase loading was used, where maximum load applied was 3 kN and the minimum load during the swing phase was 0.3 kN.13 In the second set of kinetics and kinematics, flexion/extension range was ±22° and internal/external rotation was ±8°,14,15 maximum stance phase load of 2.2 kN and a swing phase load of 0.24 kN.16

Four double-heat-treated (DHT) and three as-cast (AC), 50 mm metal-on-metal devices were tested. The AC and DHT components used had similar clearances, mean clearance 234 μm (±5.7 μm) and 241 μm (±1.0 μm) respectively. The lubricant used in this study was newborn calf serum with 0.2% (weight/volume) sodium azide concentration diluted with de-ionized water to achieve an average protein concentration of 20 g/l. During the test, the lubricant was changed every 125 K cycles. Gravimetric wear of the components was measured at 0.5, 1.0, 1.5 and 2 million cycles following ISO standard (ISO 14242-20) using an analytical balance (Toledo xp504, Mettler, Leicester, United Kingdom) with an accuracy of 0.1 mg.

The lubricant used was collected from each of the hip simulator test stations and was stirred for an hour, then 1.5 ml of serum was collected. From the 1.5 ml sample, 0.5 ml was diluted in 9.5 ml (0.14 M) of nitric acid prior to analysis. Then the concentrations of cobalt, chromium and molybdenum ions were analyzed (Analytica AB Laboratories, Lulea, Sweden) using a high-resolution inductively coupled plasma mass spectrometer (ELEMENT, Thermo-Finnigan MAT, Bremen, Germany) under clean-room conditions (Figs 2.8A and B). The

 

 

 

Figures 2.8A and B: High resolution inductively coupled plasma mass spectroscopy (a) schematic of the test process, a source of ions in accelerated through a magnetic/electric field they are deflected to varying degrees determined by their mass-charge ratio (m/z) ratio. Assuming a single positive charge, larger massive ions will be deflected to a lesser degree than smaller lighter ions. These ions are aimed at the detector which determines the concentration of these ions. (b) Thermo-Finnigan Element HR-ICPMS

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Tribology and Bearing Materials

 

Figure 2.9: Running in (0-0.5 Mc) and steady state (0.5-2 Mc) wear rates of them as-cast and double-heat-treated components. (±95% confidence limit)

 

detection limits were 0.05 mg/l for cobalt and 0.2 mg/l for chromium and 0.3 mg/l for molybdenum. The ion concentration for all three were added together and converted into weight in milligrams using the following equation:

Mass = Volume × Concentration (7)

 

RESULTS

The wear results of the hip simulator study show a biphasic wear pattern, similar to that observed in vivo, showing the running-in phase (0 to 0.5 Mc)and the steady state (0.5 to 2 Mc) phases (Fig. 2.9) . The mean gravimetric wear rates during the running-in phase (0 to 0.5 Mc) of the double heat-treated and as-cast devices were 1.35 mm3/Mc (11.5 mg/Mc) and

0.99 mm3/Mc (8.4 mg/Mc), respectively. The difference in running-in wear for the as-cast and double heat-treated component groups were statistically significant (p = 0.014). The steady-state (0.5 to 2 Mc) wear of the as-cast components, 1.9 mg/Mc, was significantly lower (p = 0.002) than that of the double-heat-treated, 2.8 mg/Mc (Table 2.1).

The combined Cobalt, Chromium and Molybdenum metal ion levels showed a similar biphasic wear trend to the gravimetric wear measurements (Table 2.1). The trends are very similar, however, the wear measured through metal ions was lower than that measured using gravimetric measurements. This may be due to the incomplete ionization of all the particles in the lubricant used, during the measurement process. The mean wear rate during the running-in phase (0-0.5 Mc) for the ac-cast devices was 6.3 mg/Mc and 8.2 mg/Mc for the double-heat-treated devices. However, this difference in metal ions generated during the running in phase was not statistically significant (p = 0.159). The steady state metal ion levels for the as cast and the double-heat-treated components was 0.6 and 1.8 mg/Mc respectively. This metal ion levels of the double-heat-treated devices was three times greater than the as cast components, this difference during the steady state phase was statistically significant (p = 0.024).

The double-heat-treated devices showed an increase in wear rate during the steady state period, a similar increase was not observed in the as-cast components (Fig. 2.9). This increase may be explained by further analysis under the scanning electron microscope (SEM).

 

Table 2.1: Mean rate of release of metal ions using the as-cast and the double-heat-treated devices during the running-in and steady-state phases

 

Devices Mean metal ion rate (mg/Mc) (±95% confidence interval)

	Running-in phase	Steady-state phase
As-cast	6.3 ± (1.9)	0.6 ± (0.1)
Double-heat-treated	8.2 ± (1.4)	1.8 ± (0.6)

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Total Hip Arthroplasty

 

Figure 2.10: Carbide fragmentation and pull-out on the surface of the wear patch of the components

 

Images of the worn surface of the double-heat-treated devices showed evidence of fragmentation and pull-out of carbides (Fig. 2.10). The carbides are hard particles that can act as third bodies between surfaces of the joint leading to an increase in wear.

The entraining velocity between the articulating surfaces has also been shown to have a significant effect on the thickness of the lubrication film. An increase in entraining velocity would result in an increase in the film thickness generated. Hence it is important to consider walking speeds that most closely reflect the reality of in vivo conditions.

Volumetric wear rates as well as size of the particles generated have been measured using joint simulators for various bearing surfaces used in total hip arthroplasty (Table 2.2).

 

Table 2.2: Representative volumetric wear rates for various hip joint materials tested in joint simulators and the particle sizes measured
Material combinations	Particle sizes (nm)	Wear rates (mm3/million cycles) Running in Steady state	Study
Ceramic-on-ceramic17	10-100	n/a 0.1	Clarke et al. 2000
Ceramic-on-metal18	10-300	n/a 0.09-0.2	Williams et al. 2007
Metal/Ceramic-on-XLPE19	50-200	n/a 1-5	Essner et al. 2005
Metal-on-metal20,21	20-300	0.7-0.9 0.1-0.2	Vassiliou et al. 2007 Li et al. 2011
Oxinium-on-XPE22,23	20-200	n/a Undetectable	Good et al. 2003 Reiss et al. 2001

 

Materials Used in Hip Replacement Surgery

 

Over the last century, many different materials have been used to replace the diseased hip joint in an attempt to alleviate pain and improve the mobility for hip arthritis patients. The materials may be selected depending on age, sex and activity level (Table 2.3). The materials used can be catagorized into four major groups; natural materials, ceramics, metals and polymers. A myriad of materials that have previously been used have had varying levels of success. Each material used would have certain desirable characteristics; however all of them have had their limitations in vivo. In this section, some material types will be reviewed in terms of their physical properties, clinical survivorship and the reasons cited for component failure.

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Table 2.3: Suitability of various total hip replacement material combinations based on age, sex and activity levels

 

Tribology and Bearing Materials

 

Material combination Demand matched

 

Ceramic-on-ceramic All genders and ages, avoid high impact large athletes Ceramic-on-metal All genders, young to middle age, athletic to active lifestyles

Metal/Ceramic-on-XLPE All genders and ages, active patients but not high impact athletes Metal-on-metal All genders and all ages, athletic to active lifestyles; avoid in renal

compromised

Oxinium-on-XLPE All genders, all ages and all activity levels

 

 

METALS

Metals have been used in hip replacements for more than forty years. Initially due to the relatively simplistic manufacturing techniques used and the poor clinical results that ensued, the use of metal-on-metal was largely discouraged as this was at a time that conventional metal-on-polyethylene (MoPE) results were more consistent. However, the conventional MoPE components were not suitable for young and active patients, therefore initiating a renewed search for alternative bearings materials.

The excellent and repeatable tolerances made possible through modern manufacturing techniques and a better understanding of the tribology of MoM components, has led to their resurgence in hip arthroplasty in the last decade. It is an established fact that MoM components produce significantly lower volumetric wear than conventional MoPE components and offer the prospect of lower failure rates. Several centers have reported good medium term survivorship results for MoM components in vivo.24-26 A multi-surgeon study of 5000 Birmingham Hip resurfacing (Leamington Spa, Warwickshire, United Kingdom) operations, reported survivorship of 96.4% at mean follow up of 7.1 years.27

Alloys of cobalt, chromium and molybdenum (CoCrMo) have been used in orthopedics implants since the 1930’s when the Smith-Peterson hip surface arthroplasty was developed. CoCr alloys are biphasic materials with a primary cobalt alloy matrix phase and a secondary metal carbide phase. Chromium is added to the cobalt matrix to improve mechanical properties as well as promote the formation of a passive oxide layer and molybdenum provides corrosion resistance to the alloy (Fig. 2.11). The presence of carbides helps improve the mechanical properties of the alloy as well as increasing its material hardness (Table 2.4).

 

 

 

Figure 2.11: Pie chart showing the percentages of constituent elements present in CoCrMo alloys used in orthopedic implants

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Table 2.4: Mechanical properties of CoCrMo alloys used in hip replacements

 

Total Hip Arthroplasty

 

Property CoCrMo

 

Yield strength 450 MPa

Ultimate tensile strength 655 MPa

Yield strength ultimate elongation 8%

Hardness 500 HV

 

 

The properties of the CoCrMo alloy can also depend on the method of processing of the alloy. The CoCrMo alloy components have traditionally been fabricated using the investment casting process (also known as the lost wax process). The investment casting process requires a wax replica of the prosthesis to be constructed and coated in a silica slurry. Once set, the wax in the slurry mold is heated to above its melting point (100-150°C), to remove it from the mold. This remaining mold is then filled with the molten metal alloy and allowed to cool. Once the metal is set, the mold is then broken off. The as-cast component is then machined and polished to achieve low surface roughness values.

 

POLYMERS

Polymers have been used successfully in hip replacements for more than four decades, achieving good long-term survivorship when components are well placed in vivo.28,29 Polymers have been considered suitable due to their low coefficients of friction and low wear rates when articulated against metal components. However, since the adoption of hip replacements as a treatment for hip arthritis only a few polymer have been used including, polytetrafluoroethylene (PTFE), polyacetal, high density polyethylene (HDPE), polyesters, ultra high molecular weight polyethylene (UHMWPE) and carbon fiber reinforced polyethylene (CFRP). Sir John Charnley originally chose to use PTFE as a bearing material due to its general chemical inertness and low coefficient of friction. However these components failed within two years due to poor abrasive wear resistance and their low resistance to creep deformation. Soon after that Charnley used HDPE and UHMWPE. HDPE which was used in plastic gears was first used in a hip prosthesis in 1962.30 UHMWPE has better packing of linear chains; therefore it is more crystalline exhibiting improved mechanical properties which make it ideal for load bearing orthopaedic applications. The mechanical properties of HDPE and UHMWPE are compared in Table 2.5.

When Metal-on-polyethylene bearings showed good early results in total hip replacements, this combination was tried out in hip resurfacing devices in several countries including England,31 Italy,32 US33 and Japan.34 However, poor early results led to hip resurfacing being abandoned as a bad concept altogether.

 

Ultra-High-Molecular-Weight Polyethylene

Ultra-high-molecular-weight polyethylene (UHMWPE) has been used in hip joint replacements in the form of metal- or ceramics-on-polymer components. UHMWPE has good toughness,

 

 

Table 2.5: Mechanical properties of HDPE and UHMWPE

 

Material property	HDPE	UHMWPE
Molecular weight (million g/mole)	0.05-0.2	2-6
Density (g/cm3)	0.95-0.96	0.93-0.95
Tensile yield (MPa)	26.2-33.1	19.3-23.0
Impact strength – Izod (Ft-Lbs/in)	3	no break

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durability and is relatively biologically inert therefore making it an ideal bearing material (Table 2.3).

Tribology and Bearing Materials

 

UHMWPE belongs to the polyethylene family of polymers which has a repeat unit (monomer) of [C2H4]n, where n is an indication of the number of monomers. UHMWPE in orthopedic applications typically has a molecular weight of 2-6 million, where n is between 100-200 thousand. This non branching (linear) semi-crystalline polymer can be described as a two-phase composite of crystalline and amorphous phases. The crystalline phase has chains folded in to highly ordered lamellae, the crystals form orthorhombic structures.

Osteolysis has been linked to the wear of UHMWPE components when articulated against metal or ceramic counter-face through adhesive, abrasive, third body and fatigue wear mechanisms.35 This wear debris generated is likely to trigger an immune response which has the result of osteoclasts resorbing periprosthetic bone leading to osteolytic implant loosening. UHMWPE is also subject to oxidative degradation which is particularly accelerated by the use of gamma irradiation for sterilization. Gamma irradiation promotes the generation of free radicals in the hydrocarbon chains in UHMWPE, which can react with oxygen in the atmosphere forming hydroperoxides. Hydroperoxides can then degrade to form more free radicals in the backbone of the hydrocarbon, leading to a self catalyzing reaction producing several oxidation products without the continued gamma irradiation. This process is known as post-irradiation aging and was a problem when components were gamma irradiated in air and then packed in air permeable sleeves. Improvements in UHMWPE sterilization and component packaging were put in place to mitigate against oxidative degradation.

 

Cross-linked Polyethylene

In the 1970’s it was discovered that crosslinking polyethylene led to improvements in wear resistance and mechanical performance.36,37 Crosslinking is a process that is usually initiated by gamma irradiation. Gamma irradiation leads to hydrogen atoms being removed from the polyethylene chain backbone creating free radicals. The free radicals recombine by forming links with other neighboring chain free radicals. This process embrittles the polymer by forming cross-links between the hydrocarbon chains in the polymer as well as increasing the wear resistance and stiffness of the material. However, excessive stiffening of the polymer may result in reduced fatigue strength which leads to stress cracking in vivo. Not all the free radicals that are formed by gamma irradiation will recombine, these residual free radicals are highly reactive and can lead to oxidative aging of the polymer. Free radicals are removed by two post-irradiation heat treatments, annealing or remelting. Annealing maintains the strength of the polymer, whereas remelting has the effect of reducing the strength. Incorporating vitamin E into crosslinked polyethylene is another method of dealing with residual free radicals, as vitamin E is an antioxidant capable of consuming free radicals. Two methods of incorporating vitamin E into crosslinked polyethylene are currently being reported diffusion of vitamin E into bulk material and mixing of vitamin E powder into UHMWPE before consolidation. Further study of crosslinked polyethylene has the potential to be used as a bone conserving hip replacement option.

 

CERAMICS

Hard-on-hard materials have the theoretical advantage of low levels of wear and low friction as long as certain key parameter requirements are met such as clearance, geometry and fixation in vivo. Alumina and zirconia ceramic components have been used in orthopedics for the last 40 years. Alumina-on-alumina joints were first used in the 1970’s due to their superior wear resistance, low friction coefficient and biocompatibility compared to other materials used in this application.38

With ceramics, it is imperative that the bulk material has small and uniform grain size and full density in order to achieve the best mechanical properties. The presence of voids

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Table 2.6: Minimum requirements for mechanical properties of ceramics used in hip replacements

 

 

Material property Al2O3 ZrO2

Young’s modulus (GPa) 366 201

Ultimate tensile strength (MPa) 310 420

Total Hip Arthroplasty

 

Hardness (GPa) 20-30 12

 

 

increases the stress within the bulk material and degrades its mechanical properties, in particular the fracture toughness of the material. Advances in processing techniques of ceramics have resulted in a progressive improvement of their mechanical properties over the years (Table 2.6). Ceramic hip joints have shown good wear resistance and excellent survivorship in several studies,39,40 one study reported immeasurable wear and limited osteolysis at a minimum of 18.5 years in vivo.41 The particulate debris generated from ceramic components in the volumes measured has been shown to be chemically stable and biocompatible. Zirconia was first used in orthopedic applications in the 1980’s as an alternative option to Alumina due to its superior mechanical properties and has been gaining market share since then (Table 2.4).

 

Alumina

The initial reason for using alumina prosthesis was to reduce the incidence of osteolysis induced by polyethylene wear debris. Another reason for the use of hard-on-hard bearings was the relative poor success of metal-on-polyethylene in young and active patients. The quality and repeatable production of high-grade alumina has improved significantly in terms of higher density, reduced porosity and smaller grain size over the years. Alumina that is currently produced can achieve uniform grains with a mean grain size of 2 μm. Problems associated with fixing the femoral head to the stem lead to significant number of fractures and failures. The Morse taper was introduced in 1974, which improved the load transfer from the head to the stem.42 Improvements in roundness, clearance and surface finish have since been made, as well as introducing the concept of matched pairs of components. However, some studies have reported negative results of all ceramic components due to loosening and osteolysis.

The problems with ceramics included catastrophic failure of the components due to the brittle nature of the material. However, over the years, developments in manufacturing techniques and material science has helped to reduce the incidence of component fractures.

Oxidized Zirconium (OXINIUM)

Oxidized zirconium (OxZr) is formed by a patented process where oxygen is thermally diffused in to zirconium-2.5 wt% niobium (Zr-2.5Nb) metal which transforms the surface to an oxide without altering the bulk metal properties. OXINIUM has superior resistance to abrasion without the risk of brittle fracture, hence displaying synergistically beneficial traits of metals and ceramics. In vitro studies comparing OXINIUM and CoCr showed that when they are articulated against cross-linked polyethylene (XPLE), the OXINIUM-on-XLPE generated 45% less wear than the CoCr-on-XLPE.22 This study also showed 30% fewer wear particles were generated by the OXINIUM-on-XLPE joint couple. Another study which assessed the wear performance of OXINIUM under jogging conditions, showed a reduction in wear for the OXINIUM-on-XLPE compared to CoCr-on-XLPE.43 Several in vivo studies have also shown favorable results for OXINIUM components.44-47

 

Summary                        

Understanding the wear mechanisms of the myriad material combinations that have been used historically is essential in order to optimize component design. However, changes in

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parameters such as diameter, clearance, lubrication, surface roughness, amongst others, have varying implications on the complex tribological performance of those materials. Direct comparison of the performance of different materials in terms of wear can therefore be difficult. Specific material types perform better under different conditions; for example, metal-on-metal devices perform excellent in young and active patients who are otherwise unsuitable for conventional total hip replacements. Tables 2.2 and 2.3 are summaries of the wear performance of a cross-section of material types and our recommendations for their use in particular patient groups.

This chapter has focused on the current understanding of tribology and the various materials that have been used in hip replacements. Over the years various materials have been analyzed in terms of biocompatibility and tribology through in vitro and in vivo studies. Wear debris generated by these materials induces varying levels of inflammatory response based on the nature of the material used and the particles generated. There have been definite successes along the way and significant failures. However, engineers and clinicians have attempted to advance surgical techniques, component design and manufacturing processes to improve patient outcomes.

We have seen how improvements in hip simulators protocols based on patient activity need to be done in order to achieve physiological relevance and improved predictive value of the test. The treatment options for young and active patients have been limited; it wasn’t until the last decade that MoM hip resurfacing was considered a viable alternative to conventional hip replacements. In this chapter we have seen how the effect of material microstructure can impact the outcome of MoM components.

 

Tribology and Bearing Materials

 

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