Wear of ultra high molecular weight polyethylene in total artificial joints

Biomechanics masterclass 3

Wear of Ultra High Molecular Weight Polyethylene in Total Artificial Joints

J. Fisher

Introduction In the last 3 years, the wear and generation of wear debris has been recognised as one of the major causes of failure in total joint replacement. In the vast majority of total joint replacements, one of the bearing surfaces consists of a hard very smooth metal or ceramic material, while the other surface is manufactured from ultra high molecular weight polyethylene (UHMWPE). The UHMWPE is used as the concave bearing surface; the acetabular cup in the hip and the tibia1 tray in the knee. UHMWPE was first introduced in the early 1960s and has, over the past 30 years, been one of the most widely used and perhaps successful biomaterials. Indeed, a review of the clinical orthopaedic literature published during the 1980s would indicate that wear of artificial joints and in particular wear of UHMWPE was not considered a major problem. During this period methods of fixation of implants, interfaces between biomaterials and bone, and loosening of implants were considered to be of major clinical importance. However. during the last 3 years it has been recognised that the generation of wear debris and in particular the wear of UHMWPE in artificial joints is a very important determinant of the long term clinical outcome of joint replacement.’ ’ The clinical problems associated with the wear of UHMWPE can be considered in three groups. Firstly, structural failure or fatigue of the bulk pothyethylene has been extensively reported in non-conforming knee joints with high contact stresses.” Secondly, high wear and penetration rates of UHMWPE in acetabular cups in Charnley joints causes adverse biomechanics.

impingement of the neck of the femoral stem on the rim of the cup which produces loosening of the acetabular cup.” Thirdly. and most importantly. UHMWPE wear particles generated at the articulating surfaces, are released into the tissues, surrounding the joint. cause adverse cellular reactions which lead to bone resorption and loosening.’ !’There is a clear indication that it is necessary to reduce the volume and number of UHMWPE wear particles in order to improve long-term clinical performance of total artificial joints. In this review. the fundamental processes that generate UHMWPE wear particles are introduced, clinical evidence of the contribution made by the different wear processes is presented. and the results of laboratory studies that provide insight into the tribological factors that control the wear processes are described. Finally, methods of reducing wear and the number of wear particles are discussed.

Tribological

Conditions and Wear Processes

The tribological conditions in total artificial joints are highly variable. Loads both static and dynamic of up to four times body weight are frequently applied to the joint and this produces contact stresses of up to 20 MPa in hip joints and in excess of 30 MPa in some non-conforming knee prostheses.“’ The velocity of sliding can vary from O-50 mm per second and the contact is lubricated with a pseudo-synovial fluid or bursal Ruid with a complex and variable biological composition. However, it is clear that this fluid does not have the rheological properties of healthy synovial fluid. Total artificial joints articulate with a mixed or boundary lubrication regime in that the Ruid fih~ls generated by elastohydrodynamic action (250 nm thick) are not sufficient to separate the surface

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asperities. Hence direct contact and wear of the bearing surfaces occur. However, under mixed lubrication conditions the role of the lubricant is extremely important as it can affect both the coefficient of friction and the wear processes.” Most joints used today comprise a hard metal or ceramic femoral component and an UHMWPE cup (in the case of hip prostheses) or an UHMWPE tibia1 tray in the case of a knee component. The tribotogicat principles behind the selection of these materials is that they provide tow friction and the hard femoral component is highly polished and provided it remains undamaged, it can product: tow wear rates of the UHMWPE surface. UHMWPE is chosen as the polymer matenat as Its high molecular weight and molecular structure produces a very high strain energq to failure Ynd much greater \vear resistance than other polymers quch as PTFE and acetat. Nevertheless the relatively tow Lvear volumes of polyethylene approximately 10 100 mm” per year’.“.” can produce very high numbers of particles (> IO”‘). which can cause adverse cellular reactions. bone resorption and loosening

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field increases fewer microscopic asperity interactions are required to remove a particle, and we,ir rate is greattv increased. This surface wear process produced by the microscopic asperities of the counterface has been widei! discussed in the wear literature and assumes that the polymer surface is extremely flat. In practice this is not the case and the second wear process is associated with the much larger roughness of the polymer surl‘ace.

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Wear iind the detachment of UHMWPE wear particles can be considered in the simplest sense as a failure of the polymer material due to cyclic stress fictds. It i,\ possible to identify different types of wear processes hk the size domain or scale of the cyclic stress fields that cause material failure. Within any type of process there may be several difTerent submechanisms depending on the nature of the stresses producing the wt’ar particles. Identification of wear processes on the basis of the dimensions of the cyclic stress field is particularly attractive as it may give some insight into the resulting size and morphology of the particles generated. In this review three different types of \vear process will be defined and results of clinical and laboratory wear studies wilt be discussed in terms of these fundamental wear processes. M‘~c/r,/)IVLV.C.\1

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scale of cyclic stress field in the polymer is produced by the microscopic asperities of the femoral counterface. The femoral counterface is extremely smooth with a surface roughness R:, of less than 0.03 /lrn and asperity height of the order of 0. I /lrn (Fig. 1). During sliding of the femoral counterface over the polymer the microscopic asperities repeatedly deform the polymer surface, both elastically and plastically. Any small element on the polymer surface sees many counterface asperity interactions prior to generatlon of a wear particle. The polymer wear particle is finally produced by fatigue failure.” For rougher femoral counterfaces with larger asperities. the process may change to a more abrasive or cutting action. 4s the amplitude of the cyclic stress nttv~/irw

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The macroscopic asperities of the polymer surface, typical height l-10 micrometres. are much greater than the microscopic asperities of the counterface (Fig. 2). On this scale the femoral counterface can be considered perfectly smooth. As the polymer is loaded. the macroscopic polymer asperities deform initially elastically and then plastically producing local stress concentrations above the nominal contact stress and frequently above the yield stress of the UHMWPE. During sliding the surface friction force also increases the local stress concentration in the macroscopic polymer asperity. For a constant load, the polymer asperity may fait by plastic def’ormation and rupture. However. under cychc loading. the macroscopic polymer asperity is cyclically deformed at the frequency of the loading cycle and this can produce crack propagation and surface fatigue within 10 /tm of the surface under the polymer asperity. This process has been termed macroscopic polymer ahperity wear.” but it may also be described ah microdelamination. As the dimension of the cyclic stress fields m this process is much larger than in process I. it is likely it wilt produce larger pieces of polymer wear debris, Both wear processes 1 and 3 are surface weal processes as they are produced by the interactions of the asperities of the two surfaces. and occur within 10 /ml of the surface.

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Metal Sphere \

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Fig. 3-A representation of structural cyclic stress fields (Scale 3).

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Process 3--structural ,failure ard wear. The third process which is the largest scale (Fig. 3) is associated with the overall structural stress field and as such is not fundamentally a surface wear process. The structural stress field is of the order of l-10 mm and varies with time as the load is applied, and varies spatially in the polymer as the contact area or force vector moves over the UHMWPE.’ On this large scale the polymer and metal surfaces are considered perfectly smooth and this stress field is determined by the geometry of the contact, the load and the elastic modulus of the materials.lO Although it would be normal to design engineering components with stress levels well below the yield stress and fatigue limit of the material, in a number of joint configurations the structural stress field in the UHMWPE can exceed both the fatigue limit and the yield stress of the polymer. Under cyclic loading conditions this can cause structural fatigue after relatively low number of loading cycles. Although this is not a surface wear process as such, the failed UHMWPE component can produce large amounts of polymer debris which is frequently termed wear debris. This structural failure has been widely described as delamination in knees. It is rarely found in appropriately designed acetabular cups which have lower levels of contact stress.

Clinical Studies of Wear There are now an increasing number of qualitative studies of the role of UHMWPE wear debris in prosthesis loosening.‘~*,’ One study has directly related loosening to measured wear volumes in hip prostheses.J It is necessary to consider polyethylene wear in hips and polyethylene wear in knees separately. There has been a number of studies of the penetration of the femoral head into the acetabular cup, and these reveal a great deal of variation. In larger studies average penetration rates of 6OGlOO pm per year have been recorded. but the variation is as

great as 10-500 prn.‘,13 The penetration of the head into the cup involves both creep and wear. However, creep primarily occurs in the first million cycles and often no more than 100 pm penetration.” If the penetration is greater than 1 mm or the clinical follow up is greater than 10 years then it may be reasonable to neglect the creep component. The penetration measurement does not accurately predict the wear volume for small penetrations as there are geometrical considerations associated with the radial clearance between the head and the cup. However, when the penetration is much greater than the radial clearance it is possible to estimate the wear volumes from the tunnelling equation where V = 71r*P where V is the volume, r is the radius of the femoral head and P is the penetration. For smaller penetrations this will over-predict the wear volume due to the geometrical effects and creep effects described above. For clinical studies of the Charnley prostheses a penetration to 60 pm per year corresponds to maximum wear volume of less than 20 mm”. Average values for the larger diameter heads have quoted wear volumes in the range IO-30 mm”/year. If particles generated have a mean dimension of less than 1 pm. this volume would generate more than 10’” particles per year. It is possible to normalise the wear volume with respect to load and sliding distance to produce a wear factor K such that K = V/Fx

mm”/Nm

where F is the load and x is the sliding distance. Wear volumes of 20 mm”/year correspond to wear factors of approximately 100 x 10 ’ mm”/Nm. Structural failure, wear process 3. is extremely rare in all polyethylene acetabular cups. where contact stresses can be relatively low, and the wear volumes measured can almost certainly be attributed to surface wear processes 1 and 2. Clinical studies have shown a correlation between surface roughness of the counterface and higher wear volumes.“’ This would be consistent with an increase in wear process 1. As the microscopic asperities of the counterface (femoral head) become much much larger, the wear rate and the mechanism moves from an increases” adhesive fatigue process towards an abrasive mechanism. Femoral heads are roughened by bone cement particles in the body,‘” and this third body damage has been confirmed in laboratory tests.‘” Most importantly in a recent study” where metallic femoral heads remained smooth over periods of between 17 and 23 years the average penetration rate was 0.022 mm/year corresponding to a volume change of less than 6 mm”/year, of which a significant proportion may be creep. In some of these cases. the wear factor was reduced to as low as 10 x 10~’ mm”/Nm. Ceramic heads which are more damage-resistant and can remain smooth in the body, have been reported to

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give lower long term wear rates in vivo than damaged metal heads. Studies in the late 1970s indicated a form of surface Fatigue in the polymer cups after long duration implants. Tt is not clear which of the wear processes creates this damage. It could be due to increased roughening of the femoral heads and the abrasive action of the microscopic counterface asperities. or alternatively it could be due to macroscopic polymer More recently direct asperity wear (process 2). evidence of macroscopic polymer asperity wear has been reported in explanted cups. with high residual plastic deformation under the polymer asperities. and approximately 5 /lm below the crack formation surface.“( This has been seen in prostheses where femoral heads have remained reasonably smooth. when explanted from the body. In contrast to rhe hip, uear process 3 structural failure has been widely reported in explanted nonconforming knees.“’ where contact stress levels are high.‘” These failures have been reported to occur as early as the first 5 years after implantation, and Imperfections in the polyethylene material. hot pressing as well a\ high contact stresses. have been implicated in these failures. These fatigue failures can occur as cracks initiating several millimetres below the surface in the arca of maximum shear stress and propagating to the surface. The term delamination has been used widely to describe this type of failure. It is important to recognisc that this process initiates in the bulk of the material and is not initially ;I surface wear process. High contact stresses and <,tructural failure can also occur in knees n%ich are iltcorrectly aligned in the body. uhcn the edge of Ihe femoral component produces ;I very small contact area of the polyethylene. Men&al knees which are more conforming are less like11 to suffer from structural failure and surface wear processes ( I and 3) similar to those in the hip”“.“’ arc more important. Generation of knee designs which eliminate structural tailure may allow better long-term performance. However. in the long-term all knees and hips will suffer from the generation of large numbers of wear particles b> wear processes I and 2. Clinical ebidencc iiiggests a large range of sizes of UHMWPES wear particles arc released into the tissues by the difleren t wear processes from sub-micrometre!’ up to IO iiiicrometres.~~~‘~’ This would be consistent with different particles produced by the different types of wear processes.

Laboratory

Wear Studies

.4lthough 2. number of studies of laboratory ivear have been carried out in full joint simulators.” which closely represent the physiological conditions, more extensive studies have been carried out in material wear testing systems such as pin on plate and pin on disc. The advantage of the simpler systems is that it is easier to control rhe tribological variables. and in

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addition It is possible to test material combinations prior to the manufacture of prostheses. The simplified motion and loading may well not simulate and generate all the wear processes present in prostheses. Howe\ er. the simplified polymer pin on plate tests do allow a number of tribological variables to be investigated. It has been shown that sliding velocity in the range 30~240 mm/s has little effect on the weal reciprocating motion\ of the rate. and similarly, polymc‘r pin cc)mpared to unidirectional motion onl! produces a small increase in the wear factor. Both these results might be expected when considering the origin or nature of the cyclic stress field In wear procea\ I, Tests run in deionised water hate been shown to modify and roughen the smooth counterfacc by transfer tilm formation and this can increase the wear produced. the wear rates were lower.” It is importan! to note that the wear factors for UHMWPE sliding on smooth countcrfaces in pin on plate and pin on disc tests are aboul 10 ’ mn?/Nm. which are one to two orders of‘ magnitude less than those reported for clinical Lvear \.olumex. If this very IOU ifear facto1 could be reproduced in the hod!. then Ices than half ;I cubic millimctrc of UHMWPE \bc;ir \+o!lld bc produc,ed e\ery year. The results currently available for hip joint simulators .Lrc highI> variable with wear factors from lesx than It1 Y IO ’ 10 greater than 100 x IO ’ mm”iUm. In a recent stud! nhen femoral heads remained qnooth we;1I iclor> a\ lo\% as IO )I 10 ’ mni71,~Nni Lverc’ recordtsd with 7irconia heads. ‘The ph!siologlcal hip joint 4mulatorh provide both cyclic loading and phyGologlca1 motions and this alloys both \vea~ proceshes I and I! to be simulated. and product ;i more realistic represcntarion of clinical wear

Discussion It is quite remarkable that after over 30 years In total artificial ioint<. a detailed understanding of‘ the

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fundamentals of wear of UHMWPE is still quite limited. Although our current knowledge allows us to accurately predict the effect of some tribological variables on the wear volumes, tribological design remains somewhat empirical. Perhaps one of the greatest concerns is the large variation in the wear factors produced in different laboratory tests, in joint simulators and in the body. This ranges from IO-’ mm”/Nm with a simple polymer wear test to 10 x IO-’ mm”/Nm for some simulators and to 100 x IO-’ mm”/Nm in the body. However. these differences do give us some insight into the accelerated wear rates that can occur. These wear factors correspond to a range of volumetric wear of less than half to greater than 20 mm3 per year. Figure 4 shows how the different tribological variables may control the wear processes and volumetric wear rate. It has been strongly suggested that the changes in UHMWPE in the body, following irradiation, such as oxidation, chain scission and reduced molecular weight may cause much higher wear rates in vivo. A recent pilot study in our laboratory has shown a marked increase in wear rate with 8-year-old aged irradiated material compared to freshly irradiated and non-irradiated material. This concern about increased wear with age following irradiation is becoming more widespread and it may well be partially responsible for higher wear rates in vivo. Alternative explanations for increased wear rates can be found by considering the tribological conditions in the body and in some hip joint simulators, which can lead to increased wear caused by the different wear processes. It is clear that many metallic femoral heads in hips are damaged in the body and consequently increase the wear of the UHMWPE by the microscopic counterface asperities (process l).la.15 This has been simulated in the laboratory tests with roughened and damaged femoral heads. Indeed, an analysis of changes in femoral head roughness during a hip joint simulator study, showed a strong correlation between head roughness and increased wear rates.” Ceramic heads which stayed smooth have shown wear factors of the order of IO x 10e8 mm”/Nm with volume changes of only a few mm” per year. Clinical studies with ceramic heads which are more damage resistant and undamaged metal heads have also shown low wear rates.” The use of a very smooth hard damageresistant femoral head, which remains smooth in the body throughout the lifetime of the joint is very important for reducing wear by this type of wear process. Consideration of type 2 wear processes may also give an explanation for the different wear rates found in the body and in tribological pin on plate tests.” In the body under time-dependent loading the macroscopic polymer asperities not only deform plastically with a high sub-surface shear stress within 5 micrometre of the surface, but the cyclic loading produces crack propagation in this region and the generation of the debris. In contrast in constant load

polymer pin on plate laboratory tests the plastic deformation and sub-surface shear is seen but cracks are not found. It is likely that cyclic loading found in the body will increase the macroscopic polymer asperity wear (wear process 2) by crack propagation and fatigue. The dynamic loading in hip joint simulators will also increase the macroscopic polymer asperity wear and although the wear factors are highly variable in the simulator tests, wear factors of the order of 10 x IO-’ mm3/Nm have been produced. This again supports the hypothesis that macroscopic polymer asperity wear (process 2) is important in the body, particularly when the femoral head remains smooth. Overall it is important to recognise the complex tribological conditions in the joint in the body, and in any prostheses that there may well be several competing wear processes. More simple tribological tests in the laboratory may not reproduce all these wear processes at the same rate. This would indicate a strong need to further develop laboratory simulation and test methods. However. it is essential to do this through an understanding of fundamental wear and to recognise more complex mechanisms, simulators and test machines may not necessarily produce a better representation of the in vivo conditions. The majority of tribological studies of artificial joints over the last 30 years have concentrated on determining the wear volumes. Although this is important, the parameter which controls loosening is in fact the number of wear particles. In simple terms the

Total

Number

of Particles

=

Total

Wear Volume

Volume

of a

Single Particle At present there is little knowledge of the size, morphology and volume of a single wear particle. It is clear that as the average size of a single particle is increased the total number of particles would decrease. Alternatively, a reduction in particle size for the same wear volume would cause an increased number of particles and probably increased incidence of loosening. There have been some suggestions that wear particles have a bimodal size distribution in hips and this would be consistent with two fundamentally different surface wear processes: wear caused by microscopic counterface asperities (process 1) and macroscopic polymer asperities (process 1).“’ It is important that the relationships between particle size and morphology and tribological conditions are established through an understanding of the wear processes. At present new material combinations are being introduced into artificial joints with expectations that wear volumes will be similar or less than existing devices and materials. However. it is not known if size of the wear particles will increase or decrease in the new materials and designs. and whether

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Acknowledgement Research on the wear of UHMWPE at Leeds University. this article. hsa been supported bq the 4RC and SERC

cited in

References H ( Campbell P. Kosso\sk\ N. Clarke 1C. Mcchamsm and clinical significance of wear debris-induced i~steol\\~j. Clan Orthop Rel Res I90 I : 276, - 1: 2 Ja\l\ %I. Smith F. Wear particles of total Jcvnt ireplacements .~ncl their role’ in periprosthetic osteol!\i\. Curl- OrI!’

I. -\m,tut/

Rht-umatol 191)‘. 4: 7OC7OY H (;. Semlitsch M. Reactlclns c>f the .IrtIculdI capsule to \‘,ear product\ c~i’artlficlal joint prosthese\. J B~r~med “vl;~:cr Kc\ 1’,7-‘: I I : 157-163

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An outhne MC’;II’ map indicating box the tribological control the \l\car procesre\ and volumetric wear rates

there will be an cj\erall reduction in the number of wear particles generated. Present studies with standard UHMWPE indicate that the number of wear particles will be reduced by using harder. smoother more damage-resistant femoral counterfaces. and secondly if the degradation of the UHMWPE in the body can be reduced. The recent recognition of the importance of the role of UHMWPE wear particles in long-term loosening of artificial joints, has demonstrated that it is extremely important to further develop the fundamental understanding of wear of UHMWPE in artificial joints so that future generations of prostheses can be developed to produce reduced number of wear particles.

Conclusion The generation of UHMWPE and the resulting adverse cellular reactions and bone resorption is now recognised as one of the major causes of loosening and failure of total joint replacements. Several diRerent wear processes, defined by the dimension of the cyclic stress field generating the wear particle, have been identified in clinical explants and laboratory tests. It is believed that theseprocesses producedifferent types and quantities of particles. Wear volumes are highly dependent on the roughness of the femoral counterface. and may also be increased by degradation of irradiated UHM WPE in the body. The use of hard. smooth damage-resistant femoral surfaces, such as alumina ceramic. which can retain a good surface tinish throughout the lifetime of the joint, can reduce the long-term wear rate in vivo. A reduction in the t-ate of degradation of the UHMWPE is also likely to reduce long-term wear rates.

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(; Ii:. W,rlkcr P S. Josh1 .A. Hardmge K. The dan~~nance ot cvclic sliding in producing wear 111total knee replxement:, C’hn Orthop Rel Re\ 1’391 : 27.3.2f? 260 6. WrohlcwsLI B cl I5 21 year results of the (‘harnltq IOU t’ricilon ,lrthroplast!. Clin Orthop Rel Res 19X6: 21 I : 30 35 7. Ho\iie D W. Ha!nc\ D R, Roger5 S D. Mc(kc M A. Pearq ‘VI J The Ire\pon\e to particulate debrl\ Orthop c‘lin North 4111 IYUi: 24 571 5s I x. Let M .I_ Sal\sr~ k 4. Betts F. DicarlL> E k. Dot\ 5 B. Bullough I’ <; Size of metallic and polyethylene dehrih partIck> 111failed cemented total hip replacementa JBJS I’)‘)‘: 74B: 3X0 3X-I Y. Shanbhalr A C. Giant T T. Gilbert J L. Black J. Galante J 0 ComposGion and morphology of’wear dehrk in falled uncemented total hip replacement. JBJS l9Y-l: 768 ho-67. IO. Walker P S. SathaGvam S. Optimisation of bearing \urthce geometry 01‘ total knees. J Biomech 1994: 1’: 255 264 I I. Fisher J. Dowson D. Tribology or artiticlal lolnt\. J Eng Med IYYI. 2115H. 7?~7Y 12 C‘I ;I residual ruh\urface \hear stralnh in ultrahigh molecular uelght pol!eth\lcnc .icet,lhular cups. J Mat Sci Marcr %fed !494: 5 52 -7 19. Engn 0 A. D\v,>yr K .4. Hawes C K. Polyethylene \rear ot mr1.d hxked ttblal component5 in total knee pro\thcse\ JBJS IY92. 7JB: Y I’ 20. HalIe! J L. Fisher .I. Dowson D. Sampath S A. Johnston R. Ell~l\ M ~2 trlboiogical study of retrieved .Iciord knee cupLint\ J ‘Lied Eng Phys 1994: (111prehsj 21. .4rgcnscm J >, O’Connor J J. Polyethylene biear III memscal hner, replacement JBJS 1992: 74B 22X 732 22. (‘oc‘pcr .I R. KIows~vl D. Fisher J. The eHt~t 01 tran\t’er tilm and surface r<>ughness on the wear of lubricated u!tr;l high mol~~culitr velsht p~~lyethylene. Clin Mater I YY.2 I I 2Yi ;(I? ‘3. Halie) J I.. Inghnm E. Fisher J. The IntluenLe (11 trlholoelcal condltlon\ Suhlnittcd

on the morphology ol’ polyethylene IO Ellropean Biomateriala Soclet\

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