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Analysis of wear and friction of total knee replacements
Wear 265 (2008) 999–1008
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Analysis of wear and friction of total knee replacements Part I. Wear assessment on a three station wear simulator M. Flannery a , T. McGloughlin b , E. Jones c , C. Birkinshaw a,∗ a
Department of Materials Science and Technology, University of Limerick, Limerick, Ireland Materials and Surface Science Institute, University of Limerick, Limerick, Ireland c Stryker Orthopaedics, Raheen Industrial Estate, Limerick, Ireland b
a r t i c l e
i n f o
Article history: Received 21 November 2006 Received in revised form 14 January 2008 Accepted 18 February 2008 Available online 24 April 2008 Keywords: UHMWPE Cobalt chrome Interax® Total knee replacement Knee simulator Surface roughness
a b s t r a c t As part of a project to evaluate novel orthopaedic bearings, a simplified wear simulator has been evaluated and validated using conventional ultra high molecular weight polyethylene (UHMWPE)—cobalt chrome knee replacements. The simulator uses a constant load, applied pneumatically, and provides a combination of rolling and sliding motions. Wear rates measured gravimetrically correlated well with results from other studies. Adhesive wear was the dominant mechanism observed with the tibial inserts, manifested as burnishing of the contact area although features indicative of abrasive wear were also observed. Surface profilometry was used to assess the changes in the topography of the components throughout testing and the surface roughness parameters obtained were correlated to the observed wear. It was shown that the machined UHMWPE inserts were initially polished and, following this run-in phase, became rougher as particles of UHMWPE were continually being pulled from the tibial surface. The wear rates and observed wear features correlate with in vitro studies using simulators with more complex load-cycle profiles and also with in vivo retrievals and, therefore, it is considered that this apparatus is suitable for evaluation of novel bearing systems. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Total knee replacements (TKRs) have revolutionised the surgical treatment of arthritis with over 350,000 total knee arthroplasty procedures (TKAs) carried out annually in the U.S., compared to over 100,000 in the early 1990s [1]. The ever-increasing need for TKRs in younger patients under the age of 60, coupled with the increased longevity and activity of an ageing population is generating great interest in bearing surfaces and wear of TKRs. Ultra high molecular weight polyethylene (UHMWPE) tibial bearings articulating against metal femoral condylar components remain the materials of choice in TKA [2–6]. However, despite its success, UHMWPE wear has been one of the limiting factors to the long-term success of TKA [7]. UHMWPE is well tolerated in its bulk form, but when it is present as micronor submicron-sized particles it is capable of initiating a complex cascade of cellular events that ultimately results in osteolysis and aseptic loosening and failure of the TKR [8–13]. Particles in the 0.1–1.0 m size range have been shown to activate macrophages to release pro-inflammatory cytokines, which stimulate bone resorp-
∗ Corresponding author. Tel.: +353 61 202247; fax: +353 61 338172. E-mail address: [email protected] (C. Birkinshaw). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.024
tion, resulting in loosening and the eventual failure of the implant [14,15]. Clearly, the wear of tibial components is an important factor in the failure of TKRs in clinical use. In vitro wear testing of total joint replacements can be achieved through joint wear simulation allowing the researcher access to the TKR components at interval stoppages for the purposes of wear assessment. The work reported here forms part of a larger project to evaluate novel bearings, and is specifically concerned with validation of a simplified wear simulator. Screening tests, such as pin-on-disks, are limited because of their inability to create conditions of geometry and load similar to those occurring in vivo. Several knee wear simulators have been developed to test UHMWPE tibial inserts under variable conditions, such as artificial aging, and with different implant designs [16–21]. Specifically, joint simulators should be able to reproduce microscopic and macroscopic surface wear features similar to those found in vivo [22,23], by reproducing the forces and motions of the natural knee. However, definitive correlations between interlaboratory results can be compromised by the many possible variables. Results can be best compared via wear rates, wear factors and observed wear patterns on the UHMWPE tibial inserts. In particular, wear factors are useful in that they take into account the load and sliding distance of the TKR joints tested and therefore
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Nomenclature A–P anterior–posterior A–P–T anterior–posterior translation F/E flexion–extension I/E-R internal–external rotation M–L medial–lateral Ra average surface roughness Rp maximum peak height Rsk skewness maximum valley depth Rv SEM scanning electron microscopy TKA total knee arthroplasty TKR total knee replacement UHMWPE ultra high molecular weight polyethylene
makes comparisons between different experimental parameters clearer. McGloughlin et al. [24] developed a simplified three station wear simulator as an aid in the preliminary evaluation of TKR designs, which combined flexion/extension and variable anterior–posterior translation with a static constant load, to produce the rolling and sliding motion found in the natural joint. The principle idea of the simulator was to reproduce the correct wear phenomena, but avoid the complexity of commercial knee joint simulators, with the major difference between this simulator and conventionally accepted TKR wear simulators being the applied load. Other TKR simulators apply dynamic loads from 0 to 3.3 kN thereby simulating the full range of contact loads, which occur in the knee over one gait cycle. The load applied in this study is a static constant load equivalent to three times body weight, which is conventionally accepted as the maximum load during the walking cycle [25]. This is important, since it means that the two sliding surfaces are always in contact [26], and is a different situation from that found in vivo by a joint prosthesis where the load reduces almost to zero during the swing phase of locomotion. In Part I of this study, the wear simulator was used to evaluate the wear of six sets of standard TKR systems. In Part II the friction of the knee joints before, during, and after wear testing was assessed
to examine the lubrication mechanisms involved. Gravimetric wear rates determined from the weight change of the UHMWPE inserts over the duration of the tests, surface profilometry and qualitative observation were used to assess the suitability of the simulator in producing comparable wear rates and observed wear features to those seen in other in vitro and ex vivo studies, and thereby validate this simulator to study other bearing materials. 2. Materials and methods 2.1. Wear simulator The wear simulator used had three test stations, and has been described in detail elsewhere [24]. The simulator applies a cyclic variation of flexion/extension angle and a static constant force to the interface between the tibial and femoral components. Each station allows flexion/extension and adjustable anterior–posterior tibial displacement depending on the machine configuration. For the wear tests described here the loading and motion parameters were identical for each station with flexion–extension of 0–65◦ and anterior–posterior translation of 5 mm. To achieve a cost efficient design a pneumatic system was used to output a static load between the test components [24]. A static constant load of 2.2 kN was applied. Consequently, the articulating TKR components were constantly in the “stance phase” of the gait cycle and did not undergo an unloaded “swing phase” as is seen in the majority of other knee wear simulators [16,20,27–31]. The accuracy of the applied load was monitored over a 48 h period prior to wear testing using an s-type load cell and was found to be ±0.05 kN. Loading of the TKR test components was achieved using a pneumatic cylinder supported by a portal frame shown schematically in Fig. 1. The piston rod of the cylinder was attached to a series of intermediate elements that were connected to a machined holder for the femoral component. Motion of the UHMWPE tibial inserts was a reciprocating movement achieved by a series of linkages outputting simple harmonic motion and the frequency of the motion of the wear simulator was 1.1 Hz. A proximity sensor located in conjunction with the moving elements allowed the number of cycles to be observed. The TKR components were mounted in the anatomical position in the simulator. The femoral component was fixed to the
Fig. 1. Schematic diagram of the University of Limerick wear simulator.
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2.2. Total knee replacement components 10 mm thick UHMWPE Interax® Duration® tibial bearing inserts (Catalogue #: 6640-5-152) manufactured by Stryker Orthopaedics, Limerick were used in this wear study. These bearings were used as they are a standard off-the-shelf component and because they are of a similar design to the Kinemax® bearings used in other studies [20,27,35]. The UHMWPE tibial inserts were machined from medical grade GUR 1050 UHMWPE and had a surface roughness (Ra ), of between 0.45 and 0.54 m. The inserts were sterilised by 25 kGy gamma irradiation in an inert nitrogen atmosphere. Prior to wear testing the UHMWPE specimens were conditioned by immersing them in distilled water for a minimum period of 4 weeks to ensure equilibrium fluid absorption. The femoral components used were standard left side Interax® Midi 600 cobalt chrome (CoCr) femoral components (Catalogue #: 6640-0-361) manufactured also by Stryker Orthopaedics, Limerick and corresponded to the tibial inserts used. The contact area on the lateral tibial insert is larger than that on the medial insert owing to internal–external rotation in vivo during the kinematics. 2.3. Wear assessment
Fig. 2. Immersion bath and tibial component fixation.
oscillating shaft by means of a machined backing ensuring correct and repeatable alignment of the test components. The inserts were press-fitted into a machined recess in a plate, replicating the inner surface of the baseplate to be used in TKA, which was fixed into an immersion bath. A 2 mm thick stainless steel spacer was used to separate the medial and lateral tibial inserts (Fig. 2). The inserts were mounted in this way to replicate the snap-fit baseplate that would be used in vivo and yet avoid the possibility of material removal when snap-fitting and removing the polymer components. The tibial inserts were lubricated with ∼200 ml of 25% bovine calf serum (Sigma–Aldrich Ireland Ltd.), 75% deionised water and 0.2 wt.% of sodium azide preservative, which was prepared at each stoppage interval. A peristaltic pump system continuously replaced evaporated water from the immersion bath during testing with filtered deionised water. The lubricant was changed and the components and fixtures cleaned at each stoppage interval according to the standard procedures [32–34]. The lubricant was kept at 37 ± 1 ◦ C by a series of four heat lamps attached to the perspex chamber that surrounds each station. Loaded controls were also used to take account of fluid absorption. Station number 1 was used for the loaded non-articulating soak controls during wear testing and stations 2 and 3 were the actual wear test stations. Three series of tests were carried out, all comprising of a loaded soak control and two test components, A and B, C and D, and E and F.
2 million cycles is considered representative of 2 years service of the TKR joint in vivo [36,37]. Creep of UHMWPE inserts has been shown [38–40] to mostly occur in the first million cycles when the inserts are under a dynamic load. Therefore, it was assumed that creep and bedding-in would take place much faster in this study due to the constantly applied load and, the relatively short test duration of 2 million cycles was deemed appropriate. Wear of the TKR replacement components was monitored using optical microscopy, digital photography, scanning electron microscopy (SEM), gravimetric analysis and surface profilometry. 2.3.1. Gravimetric analysis The inserts and soak controls were weighed at each stoppage interval following the same cleaning protocol. This coincided with changing the lubricant at least twice a week. The lubricant deteriorated quickly at the operating temperature of 37 ◦ C, resulting in considerable precipitation, which occurred despite the use of sodium azide to retard biological degradation. The inserts were removed from the simulator and rinsed with tap water to remove any adhered serum or debris and cleaned for 30 min in an ultrasonic bath containing a 10% solution of Decon 90. The inserts were subsequently washed with tap water, distilled water and then in acetone before being dried initially in a jet of filtered air and then left to dry in air for 30 min. Each UHMWPE component was weighed three times on an analytical balance with an accuracy of ±0.01 mg, and the results averaged. The corrected weight loss, obtained after subtracting the weight gain of the controls, was plotted as a function of wear cycles and the wear rate was calculated by the method of least squares linear regression. The wear rate was also converted to a volumetric wear rate by dividing the mass loss by the density of the UHMWPE
Table 1 Wear rates and wear factors for the UHMWPE sets and for the medial and lateral inserts UHMWPE set
A C D E F Average S.D.
Wear factor (mm3 /N m × 106 )
Wear rates (mm3 /106 cycles) Medial
Lateral
Total
Medial
Lateral
Total
4.48 3.44 1.59 2.11 1.90 2.70 1.22
5.50 4.17 2.02 2.65 4.17 3.70 1.38
9.99 7.61 3.61 4.76 6.07 6.41 2.49
0.023 0.018 0.008 0.011 0.010 0.014 0.006
0.029 0.022 0.011 0.014 0.022 0.019 0.007
0.052 0.040 0.019 0.025 0.032 0.033 0.013
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Fig. 3. The average medial and lateral weight loss curves of the five sets of UHMWPE tibial inserts tested.
(0.935 g/cm3 ). To aid comparison with other wear test machines, wear performance is usually reported as a wear factor (k) defined as [34]
V = Nk
p dx
(1)
where V, volume of material removed (mm3 ); N, number of cycles; k, wear factor; p, load (N); x, sliding distance (mm). p dx is derived numerically from the load and motion profiles for the joint simulator and is essentially the area under the load translation curve. To calculate this term in this work two assumptions were made. First, the radius of curvature of the femoral and
Fig. 4. Lateral (a) and medial (b) tibial inserts following testing showing the worn/burnished areas and some slight A–P scratching and gouging. (c) Wear tracks and surface gouging of UHMWPE insert as viewed under optical microscope (5×). (d) Delamination on surface of UHMWPE set B following a period of dry-lubricated conditions.
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Fig. 5. Optical microscope image (5×) and digital photograph of some fine and heavy A–P scratches on the femoral components.
tibial components were the same throughout the range of motion of the joint and secondly, the anterior–posterior translation of the tibial inserts made a small contribution to the overall sliding distance [27].
125,000 cycles due to dry running of the joint as a result of a failure in the lubrication system.
2.3.2. Surface characterisation Surface profilometry was carried out using a Form Talysurf 120 stylus profiler (Taylor Hobson), with a cut-off length of 0.08 mm, at each stoppage interval to observe any surface topography alterations. Four traces 10 mm in length were carried out on each side of both the femoral and tibial components; two in the medial–lateral (M–L) direction and two in the anterior–posterior (A–P) direction. The four traces were taken in the wear areas of the components at interval stoppages and were averaged to eliminate or dilute any discrepancies in the measurements. Four basic parameters were taken from the profiles generated: average surface roughness (Ra ), the maximum height (Rp ) and depth (Rv ) of the peaks in the profile and the skewness of the profile (Rsk ). SEM and optical microscopy was carried out on one control specimen and all test specimens at the conclusion of testing. For SEM analysis the tibial inserts were prepared by cutting peripheral areas to the bearing surface to leave samples approximately 15 mm × 10 mm × 4 mm. Care was taken not to damage the bearing surface. These samples were gold coated using an Edward’s S150B sputter coater for approximately 5 min and the inserts examined under SEM (JSM-840 Scanning Electron Microscope, JEOL USA, Peabody, MA) to observe and photograph surface morphology.
The wear rates and wear factors for the five sets of tibial inserts are tabulated (Table 1) with the average weight loss curve of all TKR sets shown (Fig. 3). All results take into account water absorption of the loaded non-articulating soak controls used with each test. It can be seen in Fig. 3 that the average weight loss in the medial insert is less than the lateral. It is also clear that there is a steady weight loss in both tibial inserts of each set indicating that wear of the UHMWPE inserts was continuous for duration of the wear test.
3. Results Six sets (A–F) of standard (Interax® ) TKR systems were tested under the conditions described. Set B was discontinued after
3.1. Wear rates and wear factors
3.2. Surface characterisation Following visual examination, it was observed that throughout the wear testing, the UHMWPE inserts exhibited characteristics such as burnishing, gouging, embedded wear debris and scratching of the surface. The contact or wear areas were different on the medial and lateral inserts owing to the femoral design with the lateral contact area larger than the medial contact area. The wear area on the inserts appeared very smooth and shiny after 250,000 cycles indicating that the surface had been burnished (Fig. 4(a)) and also some fine scratching in the A–P direction was present (Fig. 4(b)). These were the dominant wear mechanisms of the inserts, although some gouging was observed using optical microscopy, as well as embedded debris as shown in Fig. 4(c). As expected for such a short test duration no fatigue related wear such as delamination, pitting, cracking or flaking of the UHMWPE occurred except for set B (Fig. 4(d)) which ran dry.
Table 2 The average surface roughness of the UHMWPE tibial inserts in the anterior–posterior and medial–lateral direction measured at various stoppage intervals Cycles (×106 )
Lateral Ra (m)
Medial Rp (m)
Rv (m)
Rsk
Ra (m)
Rp (m)
Rv (m)
Rsk
Average: anterior–posterior direction 0.00 0.50 0.25 0.17 1.00 0.10 2.00 0.09
2.27 0.78 0.56 0.67
1.64 1.62 0.53 0.59
0.19 −1.32 0.03 0.47
0.39 0.24 0.20 0.19
1.48 1.16 0.86 1.18
1.41 1.26 0.90 0.74
0.16 −0.28 0.38 0.95
Average: medial–lateral direction 0.00 0.63 0.25 0.30 1.00 0.31 2.00 0.60
3.33 1.21 1.33 2.60
2.17 2.54 2.63 4.00
0.45 −1.62 −0.55 −1.05
0.62 0.31 0.32 0.53
3.72 1.44 1.55 2.62
2.11 2.23 1.64 2.58
0.81 −0.99 −0.43 0.57
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Table 3 The average surface roughness of the CoCr femoral components in the anterior–posterior and medial–lateral direction measured at various stoppage intervals Cycles (×106 )
Lateral Ra (m)
Medial Rp (m)
Rv (m)
Rsk
Ra (m)
Rp (m)
Rv (m)
Rsk
Average: anterior–posterior direction 0.00 0.04 0.25 0.04 1.00 0.06 2.00 0.05
0.17 0.28 0.24 0.35
0.16 0.37 0.25 0.36
0.26 −0.38 0.04 0.44
0.05 0.05 0.07 0.08
0.23 0.19 0.32 1.06
0.30 0.32 0.36 0.39
0.32 −0.51 0.11 2.51
Average: medial–lateral direction 0.00 0.07 0.25 0.06 1.00 0.08 2.00 0.07
0.25 0.25 0.46 0.37
0.29 0.32 0.89 1.00
−0.53 0.17 −0.85 −2.78
0.06 0.07 0.10 0.12
0.35 0.24 0.65 1.23
0.38 0.57 1.16 1.10
−0.54 −0.97 −0.42 0.50
No wear was observed on the backside of the tibial components although impressions of the baseplate were visible in the backside of the inserts. The CoCr femoral components exhibited fine and heavy scratching at the first stoppage interval, and throughout the wear testing, as if they had been scored by some third body debris, possibly some proteinaceous precipitate from the serum. The scratches were in the A–P direction and were up to 100 m in width, as shown in Fig. 5. No UHMWPE transfer film was observed on any of the femoral components.
3.3. Surface profilometry The surface roughness measurements taken by the Talysurf were averaged for all five sets of components. Each roughness value shown in Table 2 represents the mean of 10 measurements, i.e. 2 Talysurf traces taken on all five UHMWPE sets tested. Table 2 shows the Ra , Rp , Rv and Rsk of the UHMWPE inserts. It should be noted that the surface roughness of sets E and F were taken at 1.1 × 106 cycles but were included in the average of the measurements of surface roughness at 1.0 × 106 cycles for ease of comparison.
Fig. 6. The average surface roughness (Ra ), over 2 million wear cycles, of each side of the (a) UHMWPE inserts and (b) CoCr femoral components in the medial–lateral and anterior–posterior directions. Each point taken for the graph represents the mean of 10 measurements, i.e. 2 Talysurf traces taken on all five UHMWPE sets tested.
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Fig. 7. Scanning electron micrographs of the tibial inserts showing some micro-delamination, wear tracks in the A–P direction and some UHMWPE flaking.
The data in Table 2 shows that the Ra , Rp and Rv of the tibial inserts decreased in the A–P direction, the dominant direction of motion. This decrease in Ra was significant in the initial run in to 250,000 cycles as the machining marks on the UHMWPE inserts are polished and then appear to level off. In the M–L direction there is an initial decrease in the early stages again due to the machining marks on the inserts surface being polished. However, thereafter the surface roughness of the UHMWPE inserts increases owing to the creation of pronounced wear tracks in the A–P direction. Overall, from Table 3 it appears that the surface roughness of the CoCr femoral components increased with the exception of the surface roughness in the medial–lateral direction of the lateral femoral
component. The data in Tables 2 and 3 are illustrated in Fig. 6(a) and (b) to show these effects. 3.4. Scanning electron microscopy Scanning electron micrographs revealed that the majority of the wear areas on the UHMWPE inserts were highly polished apart from some A–P scratching indicating adhesive and abrasive wear mechanisms. High magnification of some of the scratches and wear tracks exposed fatigue wear in the form of micro-delamination (Fig. 7). Scratches and wear tracks were also visible in the A–P direction on the CoCr femoral components as shown in Fig. 8.
Fig. 8. Scanning electron micrographs of the femoral components showing wear tracks in the anterior–posterior direction.
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Table 4 Comparison of UHMWPE volumetric wear rates and wear factors for different wear simulators Test conditions
Duration (106 cycles)
Wear rate (mm3 /106 )
Wear factor (mm3 /Nm 106 )
This study
Knee type: Interax® 37 ◦ C, static 2 kN load, 25% bovine serum, and ±65◦ F/E, 5 mm A–P–T Knee type: Stainless Steel Condyle V’s flat UHMWPE 37 ◦ C, static 1 kN load, 30% bovine serum, ±65◦ F/E, and various slide roll ratios ().
2
6.41
0.033
(1:0) 2.6
0.64
(1:0.25) 2.4 (1:0.5) 2.4 (1:0.75) 2.2 (1:1) 1.0
2.14 3.21 −0.11 0.11
(7 mm) 0.8*
(1)
(8.5 mm) 5.6 (8.5 mm) 5.6 (5.5 mm) 5.6 (5.5 mm) 5.6 (4.5 mm) 5.6*
(2)
McGloughlin et al. [24]
Ash et al. [27]
Burgess et al. [20] Muratoglu et al. [28]
Knee type: Kinematic(1,2,3) and Kinemax®(4,5,6) 37 ◦ C, 0–3 kN load, 30% bovine serum, ±65◦ F/E, various A–P–T (), passive A/A and I/E–R except (*where I/E–R = ±5◦ ).
Knee type: Kinemax® 37 ◦ C, 0–3 kN load, 30% bovine serum, ±65◦ F/E, various A–P–T, passive A/A and I/E–R Knee type: Durasul(1) and NKII(2,3) 0–3.3 kN load, undiluted bovine serum, ±55◦ F/E, 10 mm A–P–T, 10◦ I/E–R
4.72
0.095
1.33 2.96 (4) 4.74 (5) 1.70 (6) 4.89
0.027 0.059 0.095 0.034 0.098
8
2.86
0.053
10 5 5
(1)
(3)
(2) (3)
0.7 9.6 8.8
Saikko et al. [21]
Knee type: CoCr ball V’s UHMWPE discs Static 2 kN load, 50% bovine serum, ±21.2◦ F/E, ±5 mm A–P–T, 5◦ I/E–R
5 5
15.6 12.9
Treharne et al. [29]
Knee type: RMC(1) , Total Condylar(2) , Townley(3) , Anametric(4) and UCI(5) 37 ◦ C, 0–5.5 kN load, calf serum, no A–P–T,
(1)
(3)
1 1 1 (4) 1 (5) 1
1.59 3.71 13.8 13.9 32.6 14.35
(2)
0.390 0.320
Wang et al. [30]
Knee type: Duracon 0–2.7 kN load, 30% bovine serum, ±23◦ F/E, 12 mm A–P–T, ±14◦ I/E–R(*)
(*)
Wang et al. [16]
Knee type: Duracon irradiated with different doses 0–2.7 kN load, 50% bovine serum, ±23◦ F/E, 12 mm A–P–T, ±14◦ I/E–R(*)
(2.5 Mrad) 5
0.324
(5.0 Mrad) 5 (7.5 Mrad) 5 (10.0 Mrad) 5
0.300 0.181 0.148
2.5
2.5
Aurora et al. [31]
Knee type: NKII 37 ◦ C, 0–3.3 kN load, 50% bovine serum(1) and bovine serum and 1.5 g/l hyaluronic acid(2) , ±55◦ F/E, 10 mm A–P–T, 10◦ I/E–R
3.86
(1)
5
9.4
(2)
5
64.8
In some cases the gravimetric wear rates reported have been converted to volumetric wear rates taking the density to be 0.93 g/cm3 .
0.425 0.114
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4. Discussion In this study, a simplified wear simulator was used to evaluate the wear of six sets of UHMWPE inserts. Gravimetric wear rates and qualitative observation were used to assess the wear response of the components to in vivo like loading and lubrication conditions produced by the knee simulator. As expected the weight loss due to wear of the UHMWPE inserts increased with the number of wear cycles but the wear remained constant with the number of cycles, suggesting a steady state wear regime had been attained. The wear of the lateral inserts was greater than the medial inserts as a result of the larger contact area on the lateral side due to the geometry of the articulating femoral components. The variation in wear rates appears large, but it has to be remembered that a very small change in mass is being measured and compared with a much larger mass of the full insert (for example a 1 mg weight loss against a 15.4 g UHMWPE specimen). The potential errors arising from weighing, water absorption, etc. will be large relative to the quantities being measured. Typically the weight gain through water soakage will be several times the mass loss through wear and is partially dependent upon atmospheric pressure, temperature and handling procedures. Similar large variations in wear rate have been reported by Ash et al. [27] whose experimental conditions are shown in Table 4. It is therefore considered that in low wear bearings the largest possible number of samples should be evaluated to give a representative average result. The wear rates, wear factors and significant experimental variables of some in vitro knee wear tests are shown in Table 4. It can be seen that the wear rate and wear factor of this study correlates with those carried out by other research groups [16,20,21,24,27–31], some using commercially available knee wear simulators. Direct comparison is, however, difficult due to the difference in bearing design and wear measurement protocol as well as the differences in kinematic conditions and temperature. As a result of the constant 2.2 kN load, creep of the UHMWPE inserts is expected to occur more quickly, than if a dynamic load had been used as in other studies, resulting in a more rapid bedding-in of the CoCr in the UHMWPE. However, the constant load did not result in a significantly different wear rate of the UHMWPE inserts compared to other studies and also resulted in similar wear features to those observed in other in vitro and in vivo studies. Additionally, as Table 4 illustrates studies carried out by other research groups [16,21,28,30] at ambient temperatures had similar wear rates to those obtained here, suggesting that temperature does not significantly affect the wear of the UHMWPE inserts. However, it is considered that precipitation of proteinaceous precipitate, resulting from denaturing the BCS lubricant at 37 ◦ C was a significant factor in the scratching of the femoral components. It was observed that throughout the wear testing, the UHMWPE inserts exhibited wear characteristics such as burnishing, gouging, embedded wear debris and scratching or scoring of the surface as described in other published work on in vitro and ex vivo inserts [3,4,27,41–48]. Creep and plastic deformation, both of which include permanent deformation, also contributed to the observed scar on the articular surfaces of the UHMWPE inserts tested [42] although the extent of creep was not measured in this study. In general, the contact areas’ appearance showed that rather than a line or point contact the components had an “area contact” as noted in a study carried out by Pappas and Buechel [45]. No fatigue related wear such as delamination, pitting, cracking or flaking of the UHMWPE occurred, as has been seen in some ex vivo studies [48,49]. Delamination and other severe fatigue related phenomena were not expected since the sub-surface embrittlement induced by oxidation was absent as the inserts were not aged and not irradiated in air [21,50,51]. However, micro-delamination
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was observed using SEM in the M–L direction as raised flakes of UHMWPE similar to that seen in some other in vitro studies [30,52]. There was no non-articulating or backside wear of the tibial components as documented in other work [53–56]. The CoCr femoral components exhibited fine and heavy scratching at the first stoppage interval, and throughout the wear testing, as if they had been scored by some third body debris, possibly some denatured protein from serum. The scratches were in the A–P direction and were up to 100 m in width with fine grooves and dull edges, as shown in Fig. 5. Similar scratching of retrieved CoCr femoral condyles in the A–P direction was also seen in a study by Lakdawala et al. [57]. The CoCr femoral components exhibited fine and heavy scratching throughout wear testing. It is the authors’ belief that the scratching of the femoral components was caused by precipitation of calcified proteins from the lubricant into the articulation. During testing, particulate precipitates were formed in the lubricant in all cases, similar to that observed in work by Wang et al. [58,59]. It is believed that the elevated test temperature (37 ◦ C) cause the proteins present in the serum to degrade and subsequently proteinaceous precipitates are formed. These precipitates cause an increase in UHMWPE wear and scratch the CoCr counterface if they become trapped in the articulation. Surface profilometry illustrated that the machined UHMWPE inserts were initially polished and, following this run-in phase, became rougher in the M–L direction because of A–P scratching of the bearing surface. However, although it seems reasonable to assume that proteinaceous precipitate from the denatured serum, trapped between the articulating surfaces would have created these fine scratches, EDAX analysis failed to confirm any such precipitate adhering to the TKR components. The surface roughness of the UHMWPE inserts decreased in the A–P direction and in the M–L direction for both the medial and lateral inserts as the machining marks were polished. After a running in period up to 0.5 × 106 cycles the surface roughness increased in the M–L direction due to scratching and wear tracks in the A–P direction. Skewness indicated that grains of UHMWPE were continually being pulled from the tibial surface as there was no trend in skewness of the inserts. There was an overall increase in the surface roughness of the CoCr components due to scratching and scoring by third body wear particles. The counterface roughness is extremely important in controlling UHMWPE wear in vivo [3] and, therefore, it is interesting to see here that the roughness of the CoCr counterfaces increased over the testing period just as the weight loss of the UHMWPE increased, showing the close relationship between the two. In conclusion, the results obtained with the wear simulator were comparable with those obtained with other simulators. It is therefore considered that it is a valuable piece of equipment for work of this type and can be applied to other material combinations. Acknowledgements The authors would like to thank Stryker Orthopaedics (Limerick) and Enterprise Ireland for funding this work. References [1] P.G. Bailey, in: T.L. Group (Ed.), Advamed—The Medical Technology Industry at a Glance, Advanced Medical Technology Association, Falls Church, Virginia, 2004, p. 37. [2] D.A. Baker, R.S. Hastings, L. Pruitt, Study of fatigue resistance of chemical and radiation crosslinked medical grade ultrahigh molecular weight polyethylene, J. Biomed. Mater. Res. 46 (4) (1999) 573–581. [3] J.R. Cooper, D. Dowson, J. Fisher, Macroscopic and microscopic wear mechanisms in ultra-high molecular weight polyethylene, Wear 162–164 (Part 1) (1993) 378–384.
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[32] ASTM F732-00, Standard Test Method of Polymeric Materials Used in Total Joint Prostheses, American Society for Testing and Materials, 2004. [33] ASTM F1715-00, Standard Guide for Wear Assessment of Prosthetic Knee Designs in Simulator Devices, American Society for Testing and Materials, 2004. [34] BS-7251-8, Orthopaedic Joint Prosthesis. Part 8. Guide to Laboratory Evaluation of Form of Bearing Surfaces of Hip Joint Prosthesis, British Standard, 1990. [35] H.E. Ash, et al., The effect of bone cement particles on the friction of polyethylene and polyurethane knee bearings, Phys. Med. Biol. 49 (15) (2004) 3413–3425. [36] T.P. Schmalzried, et al., Quantitative assessment of walking activity after total hip or knee replacement, J. Bone Joint Surg. Ser. A 80 (1) (1998) 54–59. [37] C.A. Zahiri, et al., Assessing activity in joint replacement patients, J. Arthroplasty 13 (8) (1998) 890–895. [38] D.M. 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Fisher, et al., Wear of polyethylene in artificial knee joints, Curr. Orthop. 15 (6) (2001) 399–405. [48] G.W. Blunn, et al., Wear in retrieved condylar knee arthroplasties. A comparison of wear in different designs of 280 retrieved condylar knee prostheses, J. Arthroplasty 12 (3) (1997) 281–290. [49] K. Sugimoto, et al., Premature failure of the polyethylene tibial bearing surface of the Interax knee arthroplasty, J. Arthroplasty 20 (5) (2005) 658–663. [50] P.S. Walker, et al., Long-term durability testing of total knee replacements in a simulating machine, J. Biomech. 31 (Suppl. 1) (1998), p. 111. [51] G. Blunn, et al., Ultra high molecular-weight polyethylene (UHMWPE) in total knee replacement: fabrication, sterilisation and wear, J. Bone Joint Surg. Br. 84 (7) (2002) 946–949. [52] G.B. Cornwall, J.T. Bryant, C.M. 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Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Analysis of wear and friction of total knee replacements Part I. Wear assessment on a three station wear simulator M. Flannery a , T. McGloughlin b , E. Jones c , C. Birkinshaw a,∗ a
Department of Materials Science and Technology, University of Limerick, Limerick, Ireland Materials and Surface Science Institute, University of Limerick, Limerick, Ireland c Stryker Orthopaedics, Raheen Industrial Estate, Limerick, Ireland b
a r t i c l e
i n f o
Article history: Received 21 November 2006 Received in revised form 14 January 2008 Accepted 18 February 2008 Available online 24 April 2008 Keywords: UHMWPE Cobalt chrome Interax® Total knee replacement Knee simulator Surface roughness
a b s t r a c t As part of a project to evaluate novel orthopaedic bearings, a simplified wear simulator has been evaluated and validated using conventional ultra high molecular weight polyethylene (UHMWPE)—cobalt chrome knee replacements. The simulator uses a constant load, applied pneumatically, and provides a combination of rolling and sliding motions. Wear rates measured gravimetrically correlated well with results from other studies. Adhesive wear was the dominant mechanism observed with the tibial inserts, manifested as burnishing of the contact area although features indicative of abrasive wear were also observed. Surface profilometry was used to assess the changes in the topography of the components throughout testing and the surface roughness parameters obtained were correlated to the observed wear. It was shown that the machined UHMWPE inserts were initially polished and, following this run-in phase, became rougher as particles of UHMWPE were continually being pulled from the tibial surface. The wear rates and observed wear features correlate with in vitro studies using simulators with more complex load-cycle profiles and also with in vivo retrievals and, therefore, it is considered that this apparatus is suitable for evaluation of novel bearing systems. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Total knee replacements (TKRs) have revolutionised the surgical treatment of arthritis with over 350,000 total knee arthroplasty procedures (TKAs) carried out annually in the U.S., compared to over 100,000 in the early 1990s [1]. The ever-increasing need for TKRs in younger patients under the age of 60, coupled with the increased longevity and activity of an ageing population is generating great interest in bearing surfaces and wear of TKRs. Ultra high molecular weight polyethylene (UHMWPE) tibial bearings articulating against metal femoral condylar components remain the materials of choice in TKA [2–6]. However, despite its success, UHMWPE wear has been one of the limiting factors to the long-term success of TKA [7]. UHMWPE is well tolerated in its bulk form, but when it is present as micronor submicron-sized particles it is capable of initiating a complex cascade of cellular events that ultimately results in osteolysis and aseptic loosening and failure of the TKR [8–13]. Particles in the 0.1–1.0 m size range have been shown to activate macrophages to release pro-inflammatory cytokines, which stimulate bone resorp-
∗ Corresponding author. Tel.: +353 61 202247; fax: +353 61 338172. E-mail address: [email protected] (C. Birkinshaw). 0043-1648/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.02.024
tion, resulting in loosening and the eventual failure of the implant [14,15]. Clearly, the wear of tibial components is an important factor in the failure of TKRs in clinical use. In vitro wear testing of total joint replacements can be achieved through joint wear simulation allowing the researcher access to the TKR components at interval stoppages for the purposes of wear assessment. The work reported here forms part of a larger project to evaluate novel bearings, and is specifically concerned with validation of a simplified wear simulator. Screening tests, such as pin-on-disks, are limited because of their inability to create conditions of geometry and load similar to those occurring in vivo. Several knee wear simulators have been developed to test UHMWPE tibial inserts under variable conditions, such as artificial aging, and with different implant designs [16–21]. Specifically, joint simulators should be able to reproduce microscopic and macroscopic surface wear features similar to those found in vivo [22,23], by reproducing the forces and motions of the natural knee. However, definitive correlations between interlaboratory results can be compromised by the many possible variables. Results can be best compared via wear rates, wear factors and observed wear patterns on the UHMWPE tibial inserts. In particular, wear factors are useful in that they take into account the load and sliding distance of the TKR joints tested and therefore
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Nomenclature A–P anterior–posterior A–P–T anterior–posterior translation F/E flexion–extension I/E-R internal–external rotation M–L medial–lateral Ra average surface roughness Rp maximum peak height Rsk skewness maximum valley depth Rv SEM scanning electron microscopy TKA total knee arthroplasty TKR total knee replacement UHMWPE ultra high molecular weight polyethylene
makes comparisons between different experimental parameters clearer. McGloughlin et al. [24] developed a simplified three station wear simulator as an aid in the preliminary evaluation of TKR designs, which combined flexion/extension and variable anterior–posterior translation with a static constant load, to produce the rolling and sliding motion found in the natural joint. The principle idea of the simulator was to reproduce the correct wear phenomena, but avoid the complexity of commercial knee joint simulators, with the major difference between this simulator and conventionally accepted TKR wear simulators being the applied load. Other TKR simulators apply dynamic loads from 0 to 3.3 kN thereby simulating the full range of contact loads, which occur in the knee over one gait cycle. The load applied in this study is a static constant load equivalent to three times body weight, which is conventionally accepted as the maximum load during the walking cycle [25]. This is important, since it means that the two sliding surfaces are always in contact [26], and is a different situation from that found in vivo by a joint prosthesis where the load reduces almost to zero during the swing phase of locomotion. In Part I of this study, the wear simulator was used to evaluate the wear of six sets of standard TKR systems. In Part II the friction of the knee joints before, during, and after wear testing was assessed
to examine the lubrication mechanisms involved. Gravimetric wear rates determined from the weight change of the UHMWPE inserts over the duration of the tests, surface profilometry and qualitative observation were used to assess the suitability of the simulator in producing comparable wear rates and observed wear features to those seen in other in vitro and ex vivo studies, and thereby validate this simulator to study other bearing materials. 2. Materials and methods 2.1. Wear simulator The wear simulator used had three test stations, and has been described in detail elsewhere [24]. The simulator applies a cyclic variation of flexion/extension angle and a static constant force to the interface between the tibial and femoral components. Each station allows flexion/extension and adjustable anterior–posterior tibial displacement depending on the machine configuration. For the wear tests described here the loading and motion parameters were identical for each station with flexion–extension of 0–65◦ and anterior–posterior translation of 5 mm. To achieve a cost efficient design a pneumatic system was used to output a static load between the test components [24]. A static constant load of 2.2 kN was applied. Consequently, the articulating TKR components were constantly in the “stance phase” of the gait cycle and did not undergo an unloaded “swing phase” as is seen in the majority of other knee wear simulators [16,20,27–31]. The accuracy of the applied load was monitored over a 48 h period prior to wear testing using an s-type load cell and was found to be ±0.05 kN. Loading of the TKR test components was achieved using a pneumatic cylinder supported by a portal frame shown schematically in Fig. 1. The piston rod of the cylinder was attached to a series of intermediate elements that were connected to a machined holder for the femoral component. Motion of the UHMWPE tibial inserts was a reciprocating movement achieved by a series of linkages outputting simple harmonic motion and the frequency of the motion of the wear simulator was 1.1 Hz. A proximity sensor located in conjunction with the moving elements allowed the number of cycles to be observed. The TKR components were mounted in the anatomical position in the simulator. The femoral component was fixed to the
Fig. 1. Schematic diagram of the University of Limerick wear simulator.
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2.2. Total knee replacement components 10 mm thick UHMWPE Interax® Duration® tibial bearing inserts (Catalogue #: 6640-5-152) manufactured by Stryker Orthopaedics, Limerick were used in this wear study. These bearings were used as they are a standard off-the-shelf component and because they are of a similar design to the Kinemax® bearings used in other studies [20,27,35]. The UHMWPE tibial inserts were machined from medical grade GUR 1050 UHMWPE and had a surface roughness (Ra ), of between 0.45 and 0.54 m. The inserts were sterilised by 25 kGy gamma irradiation in an inert nitrogen atmosphere. Prior to wear testing the UHMWPE specimens were conditioned by immersing them in distilled water for a minimum period of 4 weeks to ensure equilibrium fluid absorption. The femoral components used were standard left side Interax® Midi 600 cobalt chrome (CoCr) femoral components (Catalogue #: 6640-0-361) manufactured also by Stryker Orthopaedics, Limerick and corresponded to the tibial inserts used. The contact area on the lateral tibial insert is larger than that on the medial insert owing to internal–external rotation in vivo during the kinematics. 2.3. Wear assessment
Fig. 2. Immersion bath and tibial component fixation.
oscillating shaft by means of a machined backing ensuring correct and repeatable alignment of the test components. The inserts were press-fitted into a machined recess in a plate, replicating the inner surface of the baseplate to be used in TKA, which was fixed into an immersion bath. A 2 mm thick stainless steel spacer was used to separate the medial and lateral tibial inserts (Fig. 2). The inserts were mounted in this way to replicate the snap-fit baseplate that would be used in vivo and yet avoid the possibility of material removal when snap-fitting and removing the polymer components. The tibial inserts were lubricated with ∼200 ml of 25% bovine calf serum (Sigma–Aldrich Ireland Ltd.), 75% deionised water and 0.2 wt.% of sodium azide preservative, which was prepared at each stoppage interval. A peristaltic pump system continuously replaced evaporated water from the immersion bath during testing with filtered deionised water. The lubricant was changed and the components and fixtures cleaned at each stoppage interval according to the standard procedures [32–34]. The lubricant was kept at 37 ± 1 ◦ C by a series of four heat lamps attached to the perspex chamber that surrounds each station. Loaded controls were also used to take account of fluid absorption. Station number 1 was used for the loaded non-articulating soak controls during wear testing and stations 2 and 3 were the actual wear test stations. Three series of tests were carried out, all comprising of a loaded soak control and two test components, A and B, C and D, and E and F.
2 million cycles is considered representative of 2 years service of the TKR joint in vivo [36,37]. Creep of UHMWPE inserts has been shown [38–40] to mostly occur in the first million cycles when the inserts are under a dynamic load. Therefore, it was assumed that creep and bedding-in would take place much faster in this study due to the constantly applied load and, the relatively short test duration of 2 million cycles was deemed appropriate. Wear of the TKR replacement components was monitored using optical microscopy, digital photography, scanning electron microscopy (SEM), gravimetric analysis and surface profilometry. 2.3.1. Gravimetric analysis The inserts and soak controls were weighed at each stoppage interval following the same cleaning protocol. This coincided with changing the lubricant at least twice a week. The lubricant deteriorated quickly at the operating temperature of 37 ◦ C, resulting in considerable precipitation, which occurred despite the use of sodium azide to retard biological degradation. The inserts were removed from the simulator and rinsed with tap water to remove any adhered serum or debris and cleaned for 30 min in an ultrasonic bath containing a 10% solution of Decon 90. The inserts were subsequently washed with tap water, distilled water and then in acetone before being dried initially in a jet of filtered air and then left to dry in air for 30 min. Each UHMWPE component was weighed three times on an analytical balance with an accuracy of ±0.01 mg, and the results averaged. The corrected weight loss, obtained after subtracting the weight gain of the controls, was plotted as a function of wear cycles and the wear rate was calculated by the method of least squares linear regression. The wear rate was also converted to a volumetric wear rate by dividing the mass loss by the density of the UHMWPE
Table 1 Wear rates and wear factors for the UHMWPE sets and for the medial and lateral inserts UHMWPE set
A C D E F Average S.D.
Wear factor (mm3 /N m × 106 )
Wear rates (mm3 /106 cycles) Medial
Lateral
Total
Medial
Lateral
Total
4.48 3.44 1.59 2.11 1.90 2.70 1.22
5.50 4.17 2.02 2.65 4.17 3.70 1.38
9.99 7.61 3.61 4.76 6.07 6.41 2.49
0.023 0.018 0.008 0.011 0.010 0.014 0.006
0.029 0.022 0.011 0.014 0.022 0.019 0.007
0.052 0.040 0.019 0.025 0.032 0.033 0.013
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Fig. 3. The average medial and lateral weight loss curves of the five sets of UHMWPE tibial inserts tested.
(0.935 g/cm3 ). To aid comparison with other wear test machines, wear performance is usually reported as a wear factor (k) defined as [34]
V = Nk
p dx
(1)
where V, volume of material removed (mm3 ); N, number of cycles; k, wear factor; p, load (N); x, sliding distance (mm). p dx is derived numerically from the load and motion profiles for the joint simulator and is essentially the area under the load translation curve. To calculate this term in this work two assumptions were made. First, the radius of curvature of the femoral and
Fig. 4. Lateral (a) and medial (b) tibial inserts following testing showing the worn/burnished areas and some slight A–P scratching and gouging. (c) Wear tracks and surface gouging of UHMWPE insert as viewed under optical microscope (5×). (d) Delamination on surface of UHMWPE set B following a period of dry-lubricated conditions.
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Fig. 5. Optical microscope image (5×) and digital photograph of some fine and heavy A–P scratches on the femoral components.
tibial components were the same throughout the range of motion of the joint and secondly, the anterior–posterior translation of the tibial inserts made a small contribution to the overall sliding distance [27].
125,000 cycles due to dry running of the joint as a result of a failure in the lubrication system.
2.3.2. Surface characterisation Surface profilometry was carried out using a Form Talysurf 120 stylus profiler (Taylor Hobson), with a cut-off length of 0.08 mm, at each stoppage interval to observe any surface topography alterations. Four traces 10 mm in length were carried out on each side of both the femoral and tibial components; two in the medial–lateral (M–L) direction and two in the anterior–posterior (A–P) direction. The four traces were taken in the wear areas of the components at interval stoppages and were averaged to eliminate or dilute any discrepancies in the measurements. Four basic parameters were taken from the profiles generated: average surface roughness (Ra ), the maximum height (Rp ) and depth (Rv ) of the peaks in the profile and the skewness of the profile (Rsk ). SEM and optical microscopy was carried out on one control specimen and all test specimens at the conclusion of testing. For SEM analysis the tibial inserts were prepared by cutting peripheral areas to the bearing surface to leave samples approximately 15 mm × 10 mm × 4 mm. Care was taken not to damage the bearing surface. These samples were gold coated using an Edward’s S150B sputter coater for approximately 5 min and the inserts examined under SEM (JSM-840 Scanning Electron Microscope, JEOL USA, Peabody, MA) to observe and photograph surface morphology.
The wear rates and wear factors for the five sets of tibial inserts are tabulated (Table 1) with the average weight loss curve of all TKR sets shown (Fig. 3). All results take into account water absorption of the loaded non-articulating soak controls used with each test. It can be seen in Fig. 3 that the average weight loss in the medial insert is less than the lateral. It is also clear that there is a steady weight loss in both tibial inserts of each set indicating that wear of the UHMWPE inserts was continuous for duration of the wear test.
3. Results Six sets (A–F) of standard (Interax® ) TKR systems were tested under the conditions described. Set B was discontinued after
3.1. Wear rates and wear factors
3.2. Surface characterisation Following visual examination, it was observed that throughout the wear testing, the UHMWPE inserts exhibited characteristics such as burnishing, gouging, embedded wear debris and scratching of the surface. The contact or wear areas were different on the medial and lateral inserts owing to the femoral design with the lateral contact area larger than the medial contact area. The wear area on the inserts appeared very smooth and shiny after 250,000 cycles indicating that the surface had been burnished (Fig. 4(a)) and also some fine scratching in the A–P direction was present (Fig. 4(b)). These were the dominant wear mechanisms of the inserts, although some gouging was observed using optical microscopy, as well as embedded debris as shown in Fig. 4(c). As expected for such a short test duration no fatigue related wear such as delamination, pitting, cracking or flaking of the UHMWPE occurred except for set B (Fig. 4(d)) which ran dry.
Table 2 The average surface roughness of the UHMWPE tibial inserts in the anterior–posterior and medial–lateral direction measured at various stoppage intervals Cycles (×106 )
Lateral Ra (m)
Medial Rp (m)
Rv (m)
Rsk
Ra (m)
Rp (m)
Rv (m)
Rsk
Average: anterior–posterior direction 0.00 0.50 0.25 0.17 1.00 0.10 2.00 0.09
2.27 0.78 0.56 0.67
1.64 1.62 0.53 0.59
0.19 −1.32 0.03 0.47
0.39 0.24 0.20 0.19
1.48 1.16 0.86 1.18
1.41 1.26 0.90 0.74
0.16 −0.28 0.38 0.95
Average: medial–lateral direction 0.00 0.63 0.25 0.30 1.00 0.31 2.00 0.60
3.33 1.21 1.33 2.60
2.17 2.54 2.63 4.00
0.45 −1.62 −0.55 −1.05
0.62 0.31 0.32 0.53
3.72 1.44 1.55 2.62
2.11 2.23 1.64 2.58
0.81 −0.99 −0.43 0.57
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Table 3 The average surface roughness of the CoCr femoral components in the anterior–posterior and medial–lateral direction measured at various stoppage intervals Cycles (×106 )
Lateral Ra (m)
Medial Rp (m)
Rv (m)
Rsk
Ra (m)
Rp (m)
Rv (m)
Rsk
Average: anterior–posterior direction 0.00 0.04 0.25 0.04 1.00 0.06 2.00 0.05
0.17 0.28 0.24 0.35
0.16 0.37 0.25 0.36
0.26 −0.38 0.04 0.44
0.05 0.05 0.07 0.08
0.23 0.19 0.32 1.06
0.30 0.32 0.36 0.39
0.32 −0.51 0.11 2.51
Average: medial–lateral direction 0.00 0.07 0.25 0.06 1.00 0.08 2.00 0.07
0.25 0.25 0.46 0.37
0.29 0.32 0.89 1.00
−0.53 0.17 −0.85 −2.78
0.06 0.07 0.10 0.12
0.35 0.24 0.65 1.23
0.38 0.57 1.16 1.10
−0.54 −0.97 −0.42 0.50
No wear was observed on the backside of the tibial components although impressions of the baseplate were visible in the backside of the inserts. The CoCr femoral components exhibited fine and heavy scratching at the first stoppage interval, and throughout the wear testing, as if they had been scored by some third body debris, possibly some proteinaceous precipitate from the serum. The scratches were in the A–P direction and were up to 100 m in width, as shown in Fig. 5. No UHMWPE transfer film was observed on any of the femoral components.
3.3. Surface profilometry The surface roughness measurements taken by the Talysurf were averaged for all five sets of components. Each roughness value shown in Table 2 represents the mean of 10 measurements, i.e. 2 Talysurf traces taken on all five UHMWPE sets tested. Table 2 shows the Ra , Rp , Rv and Rsk of the UHMWPE inserts. It should be noted that the surface roughness of sets E and F were taken at 1.1 × 106 cycles but were included in the average of the measurements of surface roughness at 1.0 × 106 cycles for ease of comparison.
Fig. 6. The average surface roughness (Ra ), over 2 million wear cycles, of each side of the (a) UHMWPE inserts and (b) CoCr femoral components in the medial–lateral and anterior–posterior directions. Each point taken for the graph represents the mean of 10 measurements, i.e. 2 Talysurf traces taken on all five UHMWPE sets tested.
M. Flannery et al. / Wear 265 (2008) 999–1008
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Fig. 7. Scanning electron micrographs of the tibial inserts showing some micro-delamination, wear tracks in the A–P direction and some UHMWPE flaking.
The data in Table 2 shows that the Ra , Rp and Rv of the tibial inserts decreased in the A–P direction, the dominant direction of motion. This decrease in Ra was significant in the initial run in to 250,000 cycles as the machining marks on the UHMWPE inserts are polished and then appear to level off. In the M–L direction there is an initial decrease in the early stages again due to the machining marks on the inserts surface being polished. However, thereafter the surface roughness of the UHMWPE inserts increases owing to the creation of pronounced wear tracks in the A–P direction. Overall, from Table 3 it appears that the surface roughness of the CoCr femoral components increased with the exception of the surface roughness in the medial–lateral direction of the lateral femoral
component. The data in Tables 2 and 3 are illustrated in Fig. 6(a) and (b) to show these effects. 3.4. Scanning electron microscopy Scanning electron micrographs revealed that the majority of the wear areas on the UHMWPE inserts were highly polished apart from some A–P scratching indicating adhesive and abrasive wear mechanisms. High magnification of some of the scratches and wear tracks exposed fatigue wear in the form of micro-delamination (Fig. 7). Scratches and wear tracks were also visible in the A–P direction on the CoCr femoral components as shown in Fig. 8.
Fig. 8. Scanning electron micrographs of the femoral components showing wear tracks in the anterior–posterior direction.
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Table 4 Comparison of UHMWPE volumetric wear rates and wear factors for different wear simulators Test conditions
Duration (106 cycles)
Wear rate (mm3 /106 )
Wear factor (mm3 /Nm 106 )
This study
Knee type: Interax® 37 ◦ C, static 2 kN load, 25% bovine serum, and ±65◦ F/E, 5 mm A–P–T Knee type: Stainless Steel Condyle V’s flat UHMWPE 37 ◦ C, static 1 kN load, 30% bovine serum, ±65◦ F/E, and various slide roll ratios ().
2
6.41
0.033
(1:0) 2.6
0.64
(1:0.25) 2.4 (1:0.5) 2.4 (1:0.75) 2.2 (1:1) 1.0
2.14 3.21 −0.11 0.11
(7 mm) 0.8*
(1)
(8.5 mm) 5.6 (8.5 mm) 5.6 (5.5 mm) 5.6 (5.5 mm) 5.6 (4.5 mm) 5.6*
(2)
McGloughlin et al. [24]
Ash et al. [27]
Burgess et al. [20] Muratoglu et al. [28]
Knee type: Kinematic(1,2,3) and Kinemax®(4,5,6) 37 ◦ C, 0–3 kN load, 30% bovine serum, ±65◦ F/E, various A–P–T (), passive A/A and I/E–R except (*where I/E–R = ±5◦ ).
Knee type: Kinemax® 37 ◦ C, 0–3 kN load, 30% bovine serum, ±65◦ F/E, various A–P–T, passive A/A and I/E–R Knee type: Durasul(1) and NKII(2,3) 0–3.3 kN load, undiluted bovine serum, ±55◦ F/E, 10 mm A–P–T, 10◦ I/E–R
4.72
0.095
1.33 2.96 (4) 4.74 (5) 1.70 (6) 4.89
0.027 0.059 0.095 0.034 0.098
8
2.86
0.053
10 5 5
(1)
(3)
(2) (3)
0.7 9.6 8.8
Saikko et al. [21]
Knee type: CoCr ball V’s UHMWPE discs Static 2 kN load, 50% bovine serum, ±21.2◦ F/E, ±5 mm A–P–T, 5◦ I/E–R
5 5
15.6 12.9
Treharne et al. [29]
Knee type: RMC(1) , Total Condylar(2) , Townley(3) , Anametric(4) and UCI(5) 37 ◦ C, 0–5.5 kN load, calf serum, no A–P–T,
(1)
(3)
1 1 1 (4) 1 (5) 1
1.59 3.71 13.8 13.9 32.6 14.35
(2)
0.390 0.320
Wang et al. [30]
Knee type: Duracon 0–2.7 kN load, 30% bovine serum, ±23◦ F/E, 12 mm A–P–T, ±14◦ I/E–R(*)
(*)
Wang et al. [16]
Knee type: Duracon irradiated with different doses 0–2.7 kN load, 50% bovine serum, ±23◦ F/E, 12 mm A–P–T, ±14◦ I/E–R(*)
(2.5 Mrad) 5
0.324
(5.0 Mrad) 5 (7.5 Mrad) 5 (10.0 Mrad) 5
0.300 0.181 0.148
2.5
2.5
Aurora et al. [31]
Knee type: NKII 37 ◦ C, 0–3.3 kN load, 50% bovine serum(1) and bovine serum and 1.5 g/l hyaluronic acid(2) , ±55◦ F/E, 10 mm A–P–T, 10◦ I/E–R
3.86
(1)
5
9.4
(2)
5
64.8
In some cases the gravimetric wear rates reported have been converted to volumetric wear rates taking the density to be 0.93 g/cm3 .
0.425 0.114
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M. Flannery et al. / Wear 265 (2008) 999–1008
4. Discussion In this study, a simplified wear simulator was used to evaluate the wear of six sets of UHMWPE inserts. Gravimetric wear rates and qualitative observation were used to assess the wear response of the components to in vivo like loading and lubrication conditions produced by the knee simulator. As expected the weight loss due to wear of the UHMWPE inserts increased with the number of wear cycles but the wear remained constant with the number of cycles, suggesting a steady state wear regime had been attained. The wear of the lateral inserts was greater than the medial inserts as a result of the larger contact area on the lateral side due to the geometry of the articulating femoral components. The variation in wear rates appears large, but it has to be remembered that a very small change in mass is being measured and compared with a much larger mass of the full insert (for example a 1 mg weight loss against a 15.4 g UHMWPE specimen). The potential errors arising from weighing, water absorption, etc. will be large relative to the quantities being measured. Typically the weight gain through water soakage will be several times the mass loss through wear and is partially dependent upon atmospheric pressure, temperature and handling procedures. Similar large variations in wear rate have been reported by Ash et al. [27] whose experimental conditions are shown in Table 4. It is therefore considered that in low wear bearings the largest possible number of samples should be evaluated to give a representative average result. The wear rates, wear factors and significant experimental variables of some in vitro knee wear tests are shown in Table 4. It can be seen that the wear rate and wear factor of this study correlates with those carried out by other research groups [16,20,21,24,27–31], some using commercially available knee wear simulators. Direct comparison is, however, difficult due to the difference in bearing design and wear measurement protocol as well as the differences in kinematic conditions and temperature. As a result of the constant 2.2 kN load, creep of the UHMWPE inserts is expected to occur more quickly, than if a dynamic load had been used as in other studies, resulting in a more rapid bedding-in of the CoCr in the UHMWPE. However, the constant load did not result in a significantly different wear rate of the UHMWPE inserts compared to other studies and also resulted in similar wear features to those observed in other in vitro and in vivo studies. Additionally, as Table 4 illustrates studies carried out by other research groups [16,21,28,30] at ambient temperatures had similar wear rates to those obtained here, suggesting that temperature does not significantly affect the wear of the UHMWPE inserts. However, it is considered that precipitation of proteinaceous precipitate, resulting from denaturing the BCS lubricant at 37 ◦ C was a significant factor in the scratching of the femoral components. It was observed that throughout the wear testing, the UHMWPE inserts exhibited wear characteristics such as burnishing, gouging, embedded wear debris and scratching or scoring of the surface as described in other published work on in vitro and ex vivo inserts [3,4,27,41–48]. Creep and plastic deformation, both of which include permanent deformation, also contributed to the observed scar on the articular surfaces of the UHMWPE inserts tested [42] although the extent of creep was not measured in this study. In general, the contact areas’ appearance showed that rather than a line or point contact the components had an “area contact” as noted in a study carried out by Pappas and Buechel [45]. No fatigue related wear such as delamination, pitting, cracking or flaking of the UHMWPE occurred, as has been seen in some ex vivo studies [48,49]. Delamination and other severe fatigue related phenomena were not expected since the sub-surface embrittlement induced by oxidation was absent as the inserts were not aged and not irradiated in air [21,50,51]. However, micro-delamination
1007
was observed using SEM in the M–L direction as raised flakes of UHMWPE similar to that seen in some other in vitro studies [30,52]. There was no non-articulating or backside wear of the tibial components as documented in other work [53–56]. The CoCr femoral components exhibited fine and heavy scratching at the first stoppage interval, and throughout the wear testing, as if they had been scored by some third body debris, possibly some denatured protein from serum. The scratches were in the A–P direction and were up to 100 m in width with fine grooves and dull edges, as shown in Fig. 5. Similar scratching of retrieved CoCr femoral condyles in the A–P direction was also seen in a study by Lakdawala et al. [57]. The CoCr femoral components exhibited fine and heavy scratching throughout wear testing. It is the authors’ belief that the scratching of the femoral components was caused by precipitation of calcified proteins from the lubricant into the articulation. During testing, particulate precipitates were formed in the lubricant in all cases, similar to that observed in work by Wang et al. [58,59]. It is believed that the elevated test temperature (37 ◦ C) cause the proteins present in the serum to degrade and subsequently proteinaceous precipitates are formed. These precipitates cause an increase in UHMWPE wear and scratch the CoCr counterface if they become trapped in the articulation. Surface profilometry illustrated that the machined UHMWPE inserts were initially polished and, following this run-in phase, became rougher in the M–L direction because of A–P scratching of the bearing surface. However, although it seems reasonable to assume that proteinaceous precipitate from the denatured serum, trapped between the articulating surfaces would have created these fine scratches, EDAX analysis failed to confirm any such precipitate adhering to the TKR components. The surface roughness of the UHMWPE inserts decreased in the A–P direction and in the M–L direction for both the medial and lateral inserts as the machining marks were polished. After a running in period up to 0.5 × 106 cycles the surface roughness increased in the M–L direction due to scratching and wear tracks in the A–P direction. Skewness indicated that grains of UHMWPE were continually being pulled from the tibial surface as there was no trend in skewness of the inserts. There was an overall increase in the surface roughness of the CoCr components due to scratching and scoring by third body wear particles. The counterface roughness is extremely important in controlling UHMWPE wear in vivo [3] and, therefore, it is interesting to see here that the roughness of the CoCr counterfaces increased over the testing period just as the weight loss of the UHMWPE increased, showing the close relationship between the two. In conclusion, the results obtained with the wear simulator were comparable with those obtained with other simulators. It is therefore considered that it is a valuable piece of equipment for work of this type and can be applied to other material combinations. Acknowledgements The authors would like to thank Stryker Orthopaedics (Limerick) and Enterprise Ireland for funding this work. References [1] P.G. Bailey, in: T.L. Group (Ed.), Advamed—The Medical Technology Industry at a Glance, Advanced Medical Technology Association, Falls Church, Virginia, 2004, p. 37. [2] D.A. Baker, R.S. Hastings, L. Pruitt, Study of fatigue resistance of chemical and radiation crosslinked medical grade ultrahigh molecular weight polyethylene, J. Biomed. Mater. Res. 46 (4) (1999) 573–581. [3] J.R. Cooper, D. Dowson, J. Fisher, Macroscopic and microscopic wear mechanisms in ultra-high molecular weight polyethylene, Wear 162–164 (Part 1) (1993) 378–384.
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