The wear behavior of ultrahigh molecular weight polyethylene

The wear behavior of ultrahigh molecular weight polyethylene

279 Wear, 37 (1976) 279 - 289 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands THE WEAR BEHAVIOR OF ULTRAHIGH MOLECULAR WEIGHT POLYETH...

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279

Wear, 37 (1976) 279 - 289 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

THE WEAR BEHAVIOR OF ULTRAHIGH MOLECULAR WEIGHT POLYETHYLENE J. H. DUMBLETON* and C. SHEN department of ~a~e~.a~ Science and Metallur~.cal En~‘neering, ~in~i~~ati, Ohio 45221 (U.S.A.)

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of ~inctnnati,

(Received October 4,1975)

Summary The wear behaviour of ultrahigh molecular weight polyethylene has been studied using a thrust-washer testing machine. The data are examined in the light of findings of other workers and of clinical results. The wear factor exhibits a m~imum at about 500 lb ine2 surface pressure for polyethylene samples tested in water. The use of plasma in place of water does not change the wear factor and this indicates that the wear of polyethylene is not sensitive to the type of liquid environment likely to be encountered in a total joint prosthesis.

Introduction Ultrahigh molecular weight polyethylene ~UHMWPE~is widely used in total joint prostheses due to its properties of chemical inertness, biocompatibility, low friction and high wear resistance. However, concern that the wear of the UHMWPE component might limit the lifetime of prostheses has led to several investigations of the wear of this polymer. Some of these studies have been summarised by Dumbleton et al. [l] , who concluded that wearing-out of the polyethylene component should not be a problem. The present work deals with investigations published later, which are to some extent contradictory, and further data on the wear of UHMWPE are presented. Experiment Hifax 1900** of viscosity average molecular weight 3 X lo6 was used in the form of a bar in this investigation. The wear samples. were flat plates about l/2 in square and l/16 in thick. The faces were abraded with 600 grade sandpaper in water. Samples were rinsed and air dried following polishing. They were not irradiated.

*Now at Howmedica International, **Supplied by Zimmer, U.S.A.

Inc., Limerick, Ireland.

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Wear tests were carried out at room temperature using a thrust-washer wear tester [I]. The counterface was 316L stainless steel polished in the final stage with 0.05 pm aiumina grit to a finish of about 5 pin. An oscillatory motion of 110 o was used at constant load, the load being chosen to give a nominal surface pressure of up to about 1600 lb inp2. A few tests were carried out with no liquid present and some tests were done using blood plasma. Most testing was done with the samples immersed in distilled water. All testing was made at 200 oscillations min- ’ corresponding to a surface speed of 1.5 in s-l. Wear was measured with a Dektak ,profilometer at appropriate intervals during the test. Figure 1 shows a graph of change in thickness of a polyethylene sample uersus sliding distance. The initial part of the curve represents creep and wearing-in. After about lo5 in of sliding the wear rate decreases and becomes constant. In this steady state wear region a wear factor k may be defined based on the equation &= kPx

(1)

where h is the thickness worn, P the nominal pressure and x the sliding distance. Figure 1 shows how the creep component may be subtracted from the total curve to give a graph of thickness worn h versus distance slid x from which the wear factor may be calculated. Studies of the wear samples and counterfaces were made using scanning electron microscopy to identify wear mechanisms and to characterise wear debris. Results Figure 2 shows curves of thickness change against distance slid for four UH~WPE samples from the same bar tested at 500 lb in-’ in water.

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Fig. 2. Illustration of the reproducibility of the wear results.

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Fig. 3. The effect of pressure on wear of UHMWPE tested in water.

Although the initial parts of the curves (creep plus wearing-in) are quite variable, all curves show similar slopes at longer sliding distances. The h values found are 1.12,1.17,1.13 and 1.18 X 10-l’ in2 lb-’ (1.15 + 0.03 X lo‘-‘2 in” lb- ’ ). Thus the wear factor values are very reproducible. The effect of pressure on wear is shown in Figs. 3 and 4 for UH~~P~ samples tested in water, For these conditions the initial part of the curve, wear and creep, generally increases with increasing pressure and the distance before the linear part of the curve is reached is 0.5 - 1 X lo5 in. Some tests were carried out at 500 lb in-’ with no liquid present and some with blood plasma in place of water. The k values found are 3.45 X lo-r2 in2 lb-’ and 1.14 X lo-l2 in2 lb-’ respectively. SEM photographs of the wear tracks are shown in Fig. 5 for samples tested at 500 lb inV2 dry, in water and in plasma. Figure 6 shows a micrograph of the st~nless-s~el counterface used for the dry test.

Fig. 4. The effect of pressure on wear of UHMWPE tested in water.

Discussion There is still some argument about the ideal method of testing the wear resistance of UHMWPE especially with respect to the relevance of the data obtained to in uiuo conditions. It may be said at the outset that there is no ideal method. Each method has its own advantages and disadvantages and the important thing is that the individual investigator should realise the shortcomings of his test. The conditions in an artificial joint prosthesis are not precisely defined. The load varies during the walking cycle and depends on patient weight and degree of activity. A fluctuating load is,used on hip joint simulators [2 - 41 but not on bench wear testers [ 1, 5 - 71. The pressure developed depends on the area of contact. For a conformal bearing the area would be about 1 in2 in a typical total hip prosthesis. However, examination of components tested on a hip joint simulator in the authors’ laboratory has shown that the worn area is usually no more than 0.3 - 0.5 in’. For a peak load of 500 lb, which is by no means unreasonable, the pressure is from 1000 to 1500 lb in2 if the worn area is taken as a direct measure of the contact area. The velocity at the joint surfaces is fairly well defined with a fast rate of walk giving 2 steps s’l, corresponding to an average speed of 1.5 in s-l, for a Mueller prosthesis. The velocity varies during the walking cycle and is not a sinusoidal function of time. Some hip joint simulators allow for this variation [ 41 but bench wear testers do not [l, 5 - 71. It is not known what kind of fluid exists between the surfaces of an artificial joint prosthesis. This has led to testing in water, Ringer’s solution, mineral oil, bovine serum and blood plasma. From the above it may be gathered that the test method is a compromise between faithful simulation of the in uiuo conditions, assuming these to be known and well defined, and the gathering of data in a reasonable time. Despite these reservations the different evaluations of the wear resistance of UHMWPE in connection with total hip replacement generally agree that

283

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(b)

Cc) Fig. 5. SEM photographs of water tracks water; (c) in blood plasma.

on UHMWPE:

(a) under dry conditions;

(b) in

wearing-out of the polymer component is not a problem. For example, Galante and Rostoker [ 51 find a k value of 2 X lo- l2 in2 lb- ’ for RCH 1000 in water using a disc-on-plate configuration. Dumbleton et al. [l] presented a k value of 1 X lo-l2 in2 lb-’ for Hifax 1900 in blood plasma using a thrustwasher configuration. Both tests were carried out at similar pressures (around 500 lb ine2). Seedhom et al. [7] with a tri-pin-on-disc arrangement studied irradiated solid phase formed UHMWPE and reported k values in the range

284

Fig. 6. SEM photograph

of 316L

stainless-steel

counterface

following

a dry test.

8 X lo-l3 - 4 X 10-l’ in2 lb-l. Bovine serum was used and the pressure was somewhat greater than 2000 lb in- ‘. In a later publication, Dowson et al. [8] report h values of 6 X lo- l4 - 2 X lo- l3 in2 lb-l for UHMWPE in water. Other differences between the test procedures include testing temperature, speed and type of motion. Dumbleton et al. [l] tested at room temperature, with oscillatory motion at 1.5 in s--l. Galante and Rostoker [ 51 tested at 37 “C in rotation at about 4 in s- ‘, while Dowson et al. [8] tested at 42 “C! in rotation at 9.4 in s- i . In one year the sliding distance is some 2 X lo6 in for a total hip prosthesis. Taking the pressure P as 500 lb in 2 and the k value as 1 X lo-i2 in2 lb-’ gives the thickness worn per year h as 1 X lop3 in. This is somewhat lower than the value of 0.003 in per year found on simulator studies for a Mueller total hip prosthesis and 0.006 in per year found for a Charnley prosthesis [4]. Based on a nine to ten year follow-up of patients, Charnley and Cupic [9] found an average of 0.005 in yearr’ for acetabular cup wear of Charnley prostheses. Some cups showed no wear while others showed wear up to three times the average. These differences may lie in variations in the polymer, differences in insertion, in patient activity or uncertainties in the wear measurement. It cannot be denied that the measurements are subject to large errors. However, this is a disadvantage outweighed by the fact that the findings are on patients. The present results reinforce the conclusion that the k value is around 1 x lo-- i2. Furthermore, it may be seen that the wear rate is the same whether water or plasma is used. This is important since it means that it is not necessary to know precisely the nature of the joint fluid. This is not so for other polymers. For example, polyoxymethylene shows a much higher rate of wear in water than in blood plasma [lo]. The SEM photographs (Fig. 5) reveal that the wear track has a similar appearance for tests in water or blood plasma, showing a rippled appearance with the ripples perpendicular to the direction of motion. Similar patterns have been observed by Walker and Gold [ll] on the polyethylene cup of a

285

six month removed Charnley prosthesis; the pattern was ascribed to a fatigue process and cracks were observed. Cracking and rippling have been reported for polymer components by Weightman et al. [ 41. In the present work no cracks were seen accompanying the ripples. The wear of polyethylene is higher if the testing is carried out with no liquid present and the wear mechanism is different. The wear track shows no rippling but there are cracks which run below the surface and act to separate sheets of polymer from the track. This is very similar to the delamination wear seen by Suh and coworkers for a variety of materials [ 1 Z] . The counterfaces were also examined using the SEM. There are no noteworthy features for the counterfaces used in the plasma or water tests. The surface is smooth with only small quantities of adherent polymer. In the dry test, however, there is a large amount of transfer, the metal is scarred and some of the transfer is blackened, which is indicative of high temperatures and degradation of the polyethylene (Fig. 6). The behaviour of the polyethylene under dry conditions brings up the question of the PV limit of the polymer. The product of pressure and velocity, or PV, is commonly used in bearing design to define the allowed conditions under which a polymer bearing may operate. The PV limit is that product of pressure and velocity above which excessive wear or instabilities in the friction and wear behaviour occur [ 131. For UHMWPE the limiting PV given by the vendor* is 3000 lb in2 ft min- 1 for dry conditions. Shen and Dumbleton 1141 found a PV of 5400 lb ine2 ft min- ’ for RCH 1000 of viscosity average molecular weight 1.25 X 106. Under dry conditions the PV limit of polyethylene could be exceeded under normal operating conditions in a total hip prosthesis. If a liquid is present the allowed PV is much higher. Dowson et al. [8] actually carried out testing at PV values around 100 000 lb in- 2 ft min-’ without catastrophic wear occurring. The other point in connection with the operating conditions of a bearing concerns the effect of pressure and velocity on the wear factor. Often, the wear factor will be found to be constant until the PV limit is approached and then to increase steeply. Figure 7 shows the effect of PV on the wear factor for UHMWPE samples in water at constant oscillation frequency (PV of 1150 - 12 000 lb in-’ ft min-‘). The k value is not constant but passes through a maximum in the region of 500 lb in- 2. k values obtained by other workers may also be plotted and are found generally to fit the curve although there is some degree of scatter. It appears that the k value decreases as PV increases. The scatter about the curve is due to differences in molecular weight, method of preparation of the samples and testing methods and conditions. Thus the results reported by Rostoker and Galante [15] are for moulded UHMWPE of different molecular weights. The samples with the higher molecular weights show the lower wear factors. It would appear that testing at high PV

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1900TM, Cadillac

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286

Fig. 7. Wear factor

us. PV for UHMWPE tested

in water.

values gives low h values not representative of the k values to be found in a total hip prosthesis. Although the k values reported by different workers are basically in agreement, especially when allowance is made for differing PV conditions, there are some discrepancies. Rostoker and Galante [ 151 report that a titanium alloy counterface gives high rates of wear on UHMWPE with the generation of a black deposit embedded in the polyethylene. Such behaviour has not been observed in the present work. It may be that the explanation lies in the difference in testing method and conditions employed by Rostoker and Galante. Haward [ 161 has analysed the metal content of Hifax 1900 and has found 155 ppm of titanium catalyst residual. It is possible that this plays a role in the surface interaction with a titanium counterface leading to a severe interaction. Incidentally, Hostalen GUR has only 9 ppm of titanium and therefore should not show such severe interaction if this hypothesis is correct. It may be pointed out that the sliding distance necessary for running-in varies greatly between the different tests. With the thrust-washer configuration a distance of 0.5 - 1 X lo5 in was needed while the disc-on-plate arrangement required 1 X lo6 in and 6 X lo6 in was needed for the pin-on-disc method. One drawback of the present bench testing methods is the short-term nature of the test. This disadvantage was dramatically illustrated recently. Galante and Rostoker [ 51 in 1973 advocated the use of graphite-filled UHMWPE for joint prostheses based on wear tests of less thaTi ! O5 in I )f sliding. Recently, Rostoker and Galante [ 151 have reported long-term wear tests on this material and show that there is a sharp upturn in the wear rate after 4 X lo6 in of sliding. A similar upturn was found after about 1 X lo6 in of sliding in a hip joint simulator. The reason for the upturn may be due to the abrasiveness of the wear debris, especially if it cannot escape from between the bearing surfaces. A possibility is that large wear particles are created by a fatigue mechanism that takes a certain number of stress cycles to become active. Sliding experiments up to 6 X lo6 in did not show any departure from steady state wear for UHMWPE. However, Dowson el al. [8] have reported an upturn in the wear of UHMWPE after 4 X 10’ in of

sliding. These authors maintain that a certain degree of transfer is needed to smooth the counterface and promote wearing-in but that excessive transfer roughens the counterface and is responsible for the upturn in the wear rate. Accompanying lumpy transfer cracks were seen on the polyethylene surface, again suggesting a fatigue mechanism. It is certainly disturbing if the wear rate of UHMWPE components increases after a certain distance of sliding. It is not known whether the behaviour is due to the high PV employed or whether such an effect could occur in a joint prosthesis. Experiments by the present authors have shown no upturn in the wear rate up to sliding distances of 5 X lo6 in. It should be remembered that 4 X 10’ in represents some 20 years of sliding. However, this aspect of the wear of polyethylene deserves further study. The influence of transfer on wearing-in and in promoting an upturn in the wear rate brings up the subject of transfer films. It is well known in connection with the frictional behaviour of polymers that transfer films are beneficial and Pooley and Tabor [17] explain the low frictional behaviour of PTFE and high density polyethylene in terms of the formation of a thin coherent oriented transfer film on the counter-face. Efforts have been made to detect transfer films on removed total hip prostheses but the search is made difficult because of the handling processes before the prosthesis reaches the investigator and the small amount of transfer needed to form the film, which may be only 100 A thick and need not be continuous over the surface [17]. One point that should be emphasised is that transfer and transfer films are not identical. Transfer indicates that material, in this case polymer, is transferred from one surface to the other. The transfer need not be in the form of a coherent oriented layer but may be lumpy. It is generally agreed that formation of a transfer film is beneficial but transfer may or may not be beneficial. Lumpy transfer may increase the wear rate. It seems that the ability to form a transfer film depends on the molecular architecture of the polymer and polyethylene satisfies the criteria in this respect. Whether or not a transfer film will form, however, depends upon the sliding situation and may be affected by load, speed, environment and counterface material and topography among other fact,ors [8,17]. There is conflict in this area of transfer film formation and clarification is needed on several points. Finally, there is need for comment on the statement that increase in molecular weight increases the wear resistance of polyethylene. This statement is true but the precise form of the relationship must be in doubt due to the difficulty in measuring molecular weights of UHMWPE. Generally, absolute molecular weight determinations have only been made on polymers with molecular weights a factor of ten lower than the polyethylenes considered here. The viscosity method is used with UHMWPE but there are difficulties with gel and with degradation of the polymer at the high solution temperatures employed. Even more serious are the problems of converting viscosities into molecular weights. Depending on the equation used the results can be different by a factor of 4 [16]. Thus , the molecular weight numbers quoted in the literature must be regarded with care. Even if an accurate molecular weight

288

average is obtained the molecular weight distribution in the polymer is still unknown; the nature of the distribution may well have an effect on the wear performance of the polymer. Conclusions The present work does not change the earlier conclusion that wearing out of UHMWPE should not be a problem. There are differences between the wear results provided by different workers and there is a problem of deciding which phenomena are specific to polyethylene and which are due to the method of test. The possibility of an upturn in the wear rate at large sliding distances certainly is worthy of further investigation. The performance of total hip prostheses is satisfactory but the same cannot be said for total knee prostheses. The authors have studied several removed UHMWPE tibial components. Each component had a similar appearance in which the surface was torn, scuffed, pitted and abraded. A similar behaviour has been seen on other removed knee prostheses of this type and has been reported in the literature [X4]. At present, loosening of the component presents a more immediate problem but the severe wear problem must be treated once a secure fixation method is achieved. The cause of the wear problem is not fully understood but may be related to the fact that escape of wear debris is difficult in this type of total knee prosthesis and illustrates the point that the satisfactory performance of a material at one joint location does not ensure satisfactory performance for other joints. References J. H. Dumbleton, C. Shen and E. H. Miller, A study of the wear of some materials in connection with total hip replacement, Wear, 29 (1974) 163 - 171. D. Dowson, P. S. Walker, M. D. Longfield and V. Wright, A joint simulating machine for load-bearing joints, Med. Biol. Eng., 8 (1970) 37 - 43. J. H. Dumbleton, D. A. Miller and E. H. Miller, A simulator for load bearing joints, Wear, 20 (1972) 165 - 174. B. 0. Weightman, I. L. Paul, R. M. Rose, S. R. Simon and E. L. Radin, A comparative study of total hip replacement prostheses, J. Biomech., 6 (1973) 299 - 311. J. 0. Galante and W. Rostoker, Wear in total hip prostheses, Acta Orthop. Scan., 145 (1973) 1 - 46. P. S. Walker and E. Salvati, The measurement and effects of friction and wear in artificial hip joints, J. Biomed. Mater. Res. Symp., 4 (1973) 327 - 342. B. B. Seedhom, D. Dowson and V. Wright, Wear of solid phase formed high density polyethylene in relation to the life of artificial hips and knees, Wear, 24 (1973) 35 - 51. D. Dowson, J. R. Atkinson and K. Brown, The wear of high molecular weight polyethylene with particular reference to its use in artificial human joints, Preprints of Am. Chem. Sot. Meeting on Friction and Wear of Polymers, Los Angeles, 1974, pp. 354 - 363. 9 J. Charnley and Z. Cupic, The nine and ten-year results of the low-friction arthroplasty of the hip, Clin. Orthop., 95 (1973) 9 - 25. 10 J. H. Dumbleton, C. Shen and G. H. Miller, Friction and wear of Delrin in connection with total hip replacement, Paper presented at the 7th Int. Biomaterials Symp., Clemson, s. c.

289 11 P. S. Walker and B. Gold, Comparison of the bearing performance of normal and artificial human joints, J. Lubr. Technol., 95 (1973) 333 - 341. 12 S. Jahanmir, N. P. Suh and E. P. Abrahamson, II, The delamination theory of wear and the wear of a composite surface, Wear, 32 (1975) 33 - 49. 13 B. Ricour and R. A. Scherer, Designing plastic bearings, SPE J., 28 (1972) 41 - 45. 14 C. Shen and J. H. Dumbleton, The friction and wear behaviour of irradiated very high molecular weight polyethylene, Wear, 30 (1974) 349 - 364. 15 W. Rostoker and J. 0. Galante, Some new studies of the wear behaviour of ultrahigh molecular weight polyethylene, to be submitted for publication. 16 R. N. Haward, Personal communication. 17 C. M. Pooley and D. Tabor, Friction and molecular structure: the behaviour of some thermoplastics, Proc. R. Sot. London, Ser. A 329 (1972) 251 - 274. 18 F. W. Reckling, M. A. Asher, F. A. Mantz and D. 0. Helton, Performance analysis of an ex-uiuo geometric total knee prosthesis, J. Bone Jt. Surg., 57 A (1975) 108 - 112.