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A hip wear simulator with 100 test stationsV SaikkoDepartment of Mechanical Engineering, Helsinki University of Technology, Laboratory of Machine Design, PO Box 4300,Helsinki, Fin-02015 HUT, Finland. email: [email protected] manuscript was received on 3 September 2004 and was accepted after revision for publication on 4 April 2005.

DOI: 10.1243/095441105X34301

Abstract: A novel high-capacity hip wear simulator of the pin-on-disc type was designed,built, and validated. This so-called Super-CTPOD (circularly translating pin-on-disc) device hasas many as 100 separate test stations, being an advanced version of the previously validated12-station CTPOD. A validity test was done so that in all stations the specimens and the testconditions were as similar as possible. Hence, for the first time in this field, an adequatenumber of similar tests was done for a proper statistical analysis of wear data. The pins wereconventional, gamma-sterilized ultra-high molecular weight polyethylene, and the discs werepolished CoCr. The lubricant was diluted calf serum and the test length 3 million cycles. Inthe course of the test, the pins became highly polished, whereas the discs remained practicallyunchanged. The majority of the polyethylene wear particles were rounded, with a meandiameter of 0.25 mm. The 100 wear factor values computed from the 100 steady state wear ratevalues of the pins were normally distributed, the mean ±95 per cent confidence interval being1.63±0.017×10−6 mm3/N m. The standard deviation was 5.4 per cent of the mean. Therewere no outliers. The wear mechanisms and the wear factor agreed well with clinical findings.Altogether, the Super-CTPOD test system was shown to be a unique combination of validity,low variation, capacity, efficiency, reliability, productivity, economy, ease of operation, andcompact size.

Keywords: hip wear simulation, ultra-high molecular weight polyethylene, wear datastatistics, wear test validation, high-capacity wear test

1 INTRODUCTION run, it can be stated that the scarcity of valid wear

testing capacity has been a true bottleneck in the

It is characteristic of wear tests that the results show

evolution of orthopaedic biomaterials.

a considerable variation, wear testing of orthopaedic In 1996, the present author designed and built abiomaterials being no exception. The variation is

12-station circularly translating pin-on-disc (CTPOD)

basically due to the fact that there are numerous device that was shown to produce wear highly similarfactors affecting the wear mechanisms, and the

to that occurring in total hip prostheses in vivo, being

phenomena lying behind the tribological behaviour the first truly simple design to reach this goal [1].of surfaces are complex. Some of the important

Hence, the CTPOD is called a hip wear simulator, in

variables, such as the degradation of the serum- distinction from hip joint simulators that are usedbased lubricant with time, are still poorly understood

for wear testing of actual hip prostheses. The crucial

and difficult to control. It is therefore most desirable factor was found to be the type of relative motion.to run a large number of similar tests for the statistical

In the circularly translating motion, the direction

analysis of test data and for the comparison ofof sliding changes continually, as is the case in thedifferences in means between materials, test con-

hip joint in walking. This type of sliding together

ditions, etc. Most of the existing wear test devices with a serum-based lubricant results in adhesivefor prosthetic joints and their materials are very

wear manifested as a polished appearance of the

expensive and have 12 test stations at the most. Since polyethylene wear surface and the production of poly-there are not too many of such devices, even world-

ethylene wear particles mostly in the 0.1–1 mm size

wide, and since one test takes at least 6–8 weeks to range. These observations are in good agreement with

H05704 © IMechE 2005 Proc. IMechE Vol. 219 Part H: J. Engineering in Medicine

310 V Saikko

clinical ones [2–4]. Moreover, the wear factor values against polished CoCr discs so that the specimensproduced by the CTPOD for conventional ultra-high and the test conditions in all 100 stations were

asmolecular weight polyethylene (UHMWPE) slidingsimilar as possible. This material combination wasagainst a polished CoCr counterface range from 1 to

chosen because it is the most commonly used com-

2×10−6 mm3/N m, being close to those measured bination in prosthetic joints, and there are plenty offor retrieved conventional metal/polyethylene total

scientific publications on its tribological behaviour.

hip prostheses [5]. All this is true despite the fact thatthe CTPOD test employs a flat-on-flat contact insteadof a ball-in-socket contact of the actual prosthetic 2 MATERIALS AND METHODShip. Interestingly, the CTPOD motion is analogous tothat of the most widely used hip joint simulator, the The Super-CTPOD design (Fig. 1) consists of threebiaxial rocking motion design, originally introduced

main modules: loading, motion, and pin guiding

by McKellop and Clarke [6]. The type of loading was(Fig. 2). The loading and pin guiding modules canfound to be clearly less important than the type of

be easily demounted, after which the specimens

motion [7]. (Fig. 3) can be disassembled with special tools (Fig. 4)In the present paper, the second generation offor cleaning, examination, and wear measureme

nt.the CTPOD design is introduced. This so-called The loading module is a rigid aluminium box thatSuper-CTPOD has 100 separate test stations to make

weighs 21 kg and contains 100 individual pneumatic

possible, for the first time, samples of sufficient size actuators. The maximum operating pressure of thefor a proper statistical analysis. The capacity of

actuator is 0.1 MPa, which generates a load of 165 N.

100 stations is unparalleled (Table 1). Despite the The motion module consists of a base plate, afact that the number of stations in so high andmotion plate, and the circular translation motio

nthe specimens are not miniaturized, the device ismechanism between them. The motion plate makesof very compact size, 384 mm×339 mm×333 mm the electromechanically driven circular translation in(w×d×h). With the set of special tools, the assemblythe horizontal plane. The motion plate is made ofand disassembly of the specimens is extremely quickand easy, although the test chambers are packedin the minimum of space. The motion, load, andspecimen size are identical to those of the 12-stationCTPOD. A new feature is the temperature controlsystem, which keeps the lubricant temperatureuniform within 1 °C in all 100 stations. The volumeof the lubricant in each test chamber is 16 ml, 33 percent higher than that in the 12-station CTPOD, toreduce the risk of excessive protein precipitation,which could compromise the validity of the wearsimulation. The only lacking feature of the Super-CTPOD compared with the 12-station CTPOD is thefriction measurement, which was dictated by spacelimitations.To see whether the device produces valid wear

results with reasonable variation, conventional Fig. 1 Super-CTPOD, 100-station hip wear simulatorgamma-sterilized polyethylene pins were tested with multidirectional motion

Table 1 Contemporary multidirectional motion pin-on-disc devices for hip wearsimulation

Type of device Motions Number of test stations Reference

Boston Two translations 1 with 6 specimens [8]Durham One translation, one rotation 4 [9]Winterthur Two rotations 4 [10]Ortho-POD Two rotations 6 [11]Leeds One translation, one rotation 6 [12]CTPOD Two translations 12 [1]Super-CTPOD Two translations 100 Present study

Proc. IMechE Vol. 219 Part H: J. Engineering in Medicine H05704 © IMechE 2005

A hip wear simulator with 100 test stations 311

Fig. 2 Super-CTPOD main modules (loading, motion, Fig. 4 Tools for assembly and disassembly of specimens,

and pin guiding) shown separated. Test chambers their holders, and test chambersfilled with serum-based lubricant and surroundedby coolant water are installed in the motionmodule. Pins are attached with press-fit sleevesto pin guiding shafts

test chamber and a piece of PVC (polyvinyl chloride)tube forms the brim with a height of 35 mm andinternal diameter of 28 mm (Fig. 3). The brim is fixedto the disc with a silicone O-ring. The chambers areplaced on the motion plate, locked horizontally bytwo guide pins per disc, and surrounded by the waterbath. The O-ring prevents the mixing of the lubricantand the circulating water. It was found in the systemtests that the lubricant temperature was one degreeabove the water temperature. For instance, if theincoming and outgoing water temperatures were18.5 and 19.5 °C respectively, the serum temperatureswere between 19.5 and 20.5 °C. The direction ofcirculation was regularly changed to reduce the tem-perature differences further between the chambers.The chambers are organized in 11 rows so that thenumber of chambers in the rows are 7, 8, 9, 10, 11,10, 11, 10, 9, 8, and 7. The rows are of unequal width

Fig. 3 One Super-CTPOD test chamber consisting ofthe test disc, PVC brim, and O-ring seal, andone test pin attached to guiding shaft with aplastic sleeve. Flanges of shafts protect the

testchambers against contamination from above.

in order to leave space for the pillars in the corners(Fig. 2). In this way, the dimensions of the devicewere minimized. The pillars carry the load and thefrictional force. The drive motor and the temperaturecontrol system are located beneath the installation

Note the diamond coating of the ends of stain- table shown in Figs 1 and 2.less steel shafts preventing crevice corrosion The pin-guiding module, with a total weight of

15 kg, is a thick polyacetal block having 100 precision-

stainless steel. Both of its sides are machined and

made bores for stainless steel precision shafts, the

ground. In addition, the upper side is polished and rotation of which is prevented by levers. A polyacetaldiamond coated. The motion plate is surrounded bysleeve is pressed around the lower end of the sha

fta brim forming a basin for the temperature controland the test pin is pressed into the lower end of the

system. The basin contains water circulated by a

sleeve (Fig. 3). By using this press-fit principle, the

pump through a heat exchanger. With the heat radial dimension of the pin holder is minimized.exchanger working by tap water run by a thermostat

The pin holder must be thin enough so that there is

valve, the water temperature within the basin can be enough space within the lubricant chamber with anaccurately controlled. There are 100 separate, similar

internal diameter of only 28 mm, considering the

test chambers. The test disc forms the bottom of the motion along a circular track of 10 mm diameter.


312 V Saikko

The advantages of the high shape of the chamber million cycles using a procedure published elsewhereare that the mixing of the lubricant by the pin [13]. During a weighing stop, the specimens, pinholder is efficient, preventing sedimentation, and the holders, lubricant chambers, and O-rings were care-surface area of the lubricant relative to its volume is fully cleaned. The pins were first vacuum desiccatedsmall, retarding evaporation. The pneumatic actuator for 30 min and then allowed to stabilize for 2 hourspushes the shaft downwards from its upper end.in the room atmosphere before the weighing, whichSince the shaft has only one degree of freedom in was done with a Mettler AT261 DeltaRange balancevertical translation, the contact force between the to a repeatability of ±0.03 mg. The original meanpin and the disc is equal to the force generated by weight of the pins was 717.11 mg (range 712.98–the pneumatic actuator. This is assisted by two 719.64 mg). After the weighing, the specimens werefactors: (a) there is a small clearance between the reassembled, and the test was continued with freshshaft and the bore, and (b) the frictional force with lubricant. The total length of the tests was 3 milliona continual change of direction tends to ‘whirl’ the cycles. The steady state wear rate, in mg per oneshaft within the bore, facilitating its axial movement million cycles, was taken to be the slope of the lineardownwards, as the pin shortens due to wear. regression line in the diagram of weight loss versusThe motion was such that the pin translated,number of cycles. The first half a million cycles

without rotation, along a circular track of 10 mm was omitted in the linear regression because it is adiameter on the disc. Therefore, the direction of transient phase dominated by the removal of thesliding relative to the pin changed continually. One original machining marks. Taking into account thecomplete revolution of the direction of sliding, density of polyethylene, the load, and the slidingwhich took 1.0 s, is called a cycle. The sliding speed distance per cycle, the wear factor k was calcu-was constant at 31.4 mm/s. The diameter of the lated so that the wear rate was divided by 1

a

06×cylindrical polyethylene pin was 9.0 mm and height 0.94 mg/mm3×70.7 N×0.0314 m. The wear factor is12.0 mm. The wear end of the pin was flat and a useful quantity because it makes possible the com-non-chamfered. The CoCr disc had a diameter ofparisons with in vitro and in vivo wear measure-28 mm and thickness of 10 mm. The contact was flat- ments for actual hip prostheses. All the 100 tests wereon-flat. The load was static 70.7 N and the nominal run simultaneously.contact pressure was therefore 1.1 MPa. Polyethylene wear particles were isolated fromThe pins and the discs were supplied by Center-samples of used serum lubricant by NaOH digestion,

pulse Orthopedics, a Zimmer Company. The pins HCl neutralization, and filtration on 0.05 mm pore sizewere made from conventional, calciumstearat-free polycarbonate filters, and examined with scanningultra-high molecular weight polyethylene GUR 1020, electron microscopy (SEM) and image analysis tools,‘Sulene-PE’, ISO 5834-1/-2. After the machiningas described elsewhere [14].from a compression moulded sheet, the pins weregamma-irradiated by 25–40 kGy in nitrogen. The discswere made from CoCrMo wrought alloy ‘Protasul-20’,ISO 5832-12. The wear faces of the discs were

3 RESULTS

machined, ground, and finally diamond-polishedto an arithmetical mean surface roughness value

In visual examination, all pins were highly polished

of 0.01–0.02 mm R . The surface roughness wasmeasured with a Taylor Hobson Surtronic 3+diamond stylus instrument using 0.8 mm cut-off and

(Fig. 5). Microscopy revealed a featureless, flat topo-graphy in every pin [Fig. 6(a)] with some mildcriss-cross scratches [Fig. 6(b)]. The discs were not

4 mm evaluation length. damaged, and no patches or layers formed on them

The lubricant was prepared so that triple 0.1 mm

during the tests. Only seldom some subtle, circular

sterile filtered HyClone Alpha Calf Fraction serum,

scratches were observed. The occurrence of these

Catalog number SH30076.03, lot number AFL5848,

rare, barely visible, scratches did not correlate with

was diluted 1:1 with Milli-Q-grade distilled water. the wear rate of the corresponding pins. The SEM

Hence, the protein content of the lubricant wasof the polycarbonate filters revealed that the over-

21 mg/ml. No additives were used. Each chamber

whelming majority of the wear particles (Fig. 7) were

contained 16 ml of lubricant. The lubricant tem-rounded and their mean equivalent circle diameter

perature during the test was maintained at 20 °C(range 19.5–20.5 °C).

(ECD)±SD (standard deviation) was 0.25±0.10 mm(n=125). Agglomerations, from which individual

The amount of wear was measured by stoppingparticles could not be distinguished with certainty,

the test and weighing the pins at intervals of half a were common.



Fig. 5 Polyethylene test pins on the handling tray after 3 million cycles. Topographical featuresappear pronounced due to polishing. Reflection of light varies because the pin height onthis handling tray varies slightly

Fig. 6 Optical micrographs from the polyethylenesurface worn for 3 million cycles

The dependence of wear on the number of cycleswas highly linear, the correlation coefficient R2 of the Fig. 7 Scanning electron micrographs of polyethylenelinear regression of the mean weight loss values being wear particles on the polycarbonate filter with0.9982, the slope of the regression line 3.40 mg per a 0.05 mm pore size


314 V Saikko

values lay within the mean ±1 SD, i.e. between 1.54

Fig. 10 Normal probability plot of the wear factor k

one million cycles, and the y axis intercept 0.70 mg(Fig. 8). The mean k value computed from theslope was 1.63×10−6 mm3/N m. The same value wasnaturally obtained by performing the linear regressionanalysis for each pin and computing the mean ofthe 100 values. The correlation coefficient R2 of thelinear regression analysis of the gravimetric wear ofindividual pins ranged from 0.9906 to 0.9998 with amean of 0.9973. The 100 k values, computed fromthe 100 steady state wear rate values, were normallydistributed (see Table 2 and Figs 9 and 10). Therewere no outliers. Normality tests (Shapiro-Wilk W,Anderson-Darling, Martinez-Iglewicz, Kolmogorov-Smirnov, D’Agostino skewness, D’Agostino kurtosis,and D’Agostino omnibus) could not reject normalityat the 5 per cent significance level. In a normal distri-bution, 68.3 per cent of the observed values lie within

Fig. 9 Frequency histogram and relative cumulative

frequency of the wear factor k

Fig. 8 Gravimetric wear (weight loss) of polyethylenepins. Each circle represents the mean value of100 pins. Vertical bars indicate the standarddeviation

of weight

loss

since

previous

weighing (10−6 mm3/N m) with a regression line passin

gthrough the first and third quartiles, and 95

perTable 2 Descriptive statistics of the wear factor cent confidence limits

k (10−6 mm3/N m) in the Super-CTPODtest

Statistic Value ±1 SD of the mean. In the present test, 71 of 100 k

Mean±95 per cent confidence interval 1.63±0.017Sample variance 0.0077 and 1.71×10−6 mm3/N m.Standard deviation 0.088Minimum 1.41First quartile (25th percentile) 1.57Median 1.63 4 DISCUSSIONThird quartile (75th percentile) 1.68Maximum 1.86Sample size 100 A novel pin-on-disc hip wear simulator, the Super

-Kurtosis 3.16 CTPOD, was introduced. The most useful andSkewness 0.05

2n−2 distribution that the lowest value of n

valuable characteristics of the device, validity, low



variation, capacity, efficiency, reliability, productivity,Low variation of the results indicated that the testeconomy, ease of operation, and compact size, are

method was of high quality. Achieving a 95 per cent

discussed below. confidence interval of the mean as small as that inValidity is the most important characteristic of a the present study, 1.63±0.017×10−6 mm3/N

m, iswear simulator. In the present case, the validity is very difficult with low n values, because the lengthjudged by the similarity of the wear produced by the of the interval is proportional to 1/√n. For the firstsimulator to that occurring in prosthetic hip jointstime in this field, it was shown that k is normallyin vivo. The crucial results, the polished appearance distributed. The comparison of the means of tw

oof the worn polyethylene surface, the undamaged samples using the t-test requires that the data arecounterface, the size and shape of the wear particles,

normally distributed. Consider a theoretical example:

and the mean wear factor value, agreed well with the wear properties of two types of polyethylene, Aclinical findings [2–5]. In the absence of severe

and B, are compared with the Super-CTPOD. The

femoral head damage, the main load-bearing zone mean k of type A is found to be 1.63×10−6 mm3/N mof retrieved polyethylene acetabular cups typically

and the mean k of type B is found to be 10 per cent

appears polished, indicating adhesion as the principal higher than that of type A, a difference not toowear mechanism [2, 3]. The majority of wear particles

dramatic but with potential clinical significance.

isolated from periprosthetic tissues are rounded and It is assumed that their wear factors are normallyrange in diameter from 0.1 to 1 mm with a mean of

distributed with SDs equal to that of Sulene-PE,

0.5–0.7 mm [3, 4]. In the present study, the mean ECD 0.088×10−6 mm3/N m. It is easy to calculate usingof wear particles was somewhat smaller, 0.25 mm, the Twhich may be at least partly due to the fact that the resulting in a 95 per cent confidence that type B reallyfilter pore size used was 0.05 mm, whereas the 0.2 mm

wears faster than type A is 4. This would leave an

pore size is the most widely used, and used also abundance of machine capacity for other materialsin the two studies of clinical particles [3, 4]. It has

to be run simultaneously. With new materials, the

been found that the filter pore size has a signifi-sample size must of course be decided before any-cant effect on the observed size distributions of the thing is known about the distributions, but thisparticles [15]. It is recommended that the 0.05 mm example gives an idea of the differentiation power of

pore size is used instead of 0.2 mm, because a signifi-

the Super-CTPOD. Note that in the present study,

cant proportion of particles are below 0.2 mm in (a) the run was successful at the first attempt, (b) alldiameter. The mean clinical wear factor measured

the 100 tests were run simultaneously, (c) there was

for 129 retrieved Charnley polyethylene acetabular not a single outlier pin with exceptionally low or highcups worn against 22 mm diameter stainless steel

k values (see Fig. 10, where all values are between

heads with varying abrasive damage was found to be the 95 per cent confidence limits), (d) none of the2.1×10−6 mm3/N m [5]. In the present study, there

results was rejected, and (e) there were no reruns.

was practically no damage on the discs, which may Moreover, the appearances of all 100 pins and of allpartly explain the slightly smaller mean wear factor

100 discs were highly uniform.

of 1.63×10−6 mm3/N m. Retrieved heads made of The capacity of 100 separate test stations isCoCr, which is harder than stainless steel, typically

superior to all previous devices (Table 1). The high

have retained their original mirror finish and show capacity is perhaps the single most original featureonly occasional scratches [3]. When scratching occurs,

of the Super-CTPOD. Wear testing of orthopaedic

it is often caused by abrasive acrylic particles, both in biomaterials is inevitably time-consuming. For athe cases of stainless steel [5] and CoCr heads [16]. thorough evaluation of, for example, the effect ofIn the present wear simulation, acrylic particles and different sterilization methods, different types ofall other possible contaminants were deliberately lubricants, or different types of counterface on theleft out, because first a valid, clean wear test that wear behaviour of a certain material, or for a com-will serve as a reference test for any future study

parison of the wear behaviour of a large number of

needed to be done. A scratched counterface has different materials, the capacity is of the utmostbeen shown to produce more large, elongated poly- importance. Typically, existing wear simulator an

dethylene particles [13]. The oxidation of polyethylene joint simulator units have 12 test stations at most,is another important issue, as it may seriously impair

making it impossible to implement large test pro-

the wear resistance in vivo [17]. The majority of grammes within a reasonable timescale. If miniaturelaboratory wear tests have been done with non-aged

specimens were used, it would not be too difficult to

material, as in the present study. pack even a much higher number of test stations


316 V Saikko

than 100 into one CTPOD device, but it has been polishing of the discs were done manually at the

shown that the wear behaviour of a miniaturized

Helsinki University of Technology (HUT), but the

contact surface is quite different from that of actual shape of the disc lends itself very well to modern,bearing surfaces in vivo [18]. Note that the Super- automatic polishing machines.CTPOD specimens are not miniaturized. On the Ease of operation is important from the point ofcontrary, 9.0 mm is a relatively large pin diameter.

view of efficiency, and special attention was paid to

Moreover, the wear face diameter can easily be this aspect in the design of the device. The Super-increased up to 12.5 mm, the outside diameter of the

CTPOD was the tenth wear test device designed,

pin holder sleeve. built, and operated by the present author, and so theEfficiency is, however, more than sheer capacity.

importance of this aspect was very clear from per-

The downtime for wear measurement, including sonal experience. For instance, all the parts that needdisassembly, cleaning, weighing, and reassembly,

to be cleaned (the specimens, pin holders, lubricant

must be as short as possible so that the entirechambers, and O-rings) are easily cleaned as batchestest can be performed in a minimum of time. Inin an ultrasonic cleaner. During the running, n

oSuper-CTPOD testing, the downtime proved to bemeasures by the operator are needed apart fromonly about 12 hours with one operator (V.S.). Withnormal supervision.a 1 Hz test frequency, a three million cycle test Its compact size, 384 mm×339 mm×333 mmfor 100 specimens with six periodic wear measure- (w×d×h, above the installation table shown inments can therefore be completed in just 38 days, as Figs 1 and 2), makes it easy to move the device andin the present study. No doubt, this is a mark of locate it, for example, in a clean air chamber,

ifefficiency. desired. The compact size makes the device and itsReliability, as an absence of technical malfunction,

modules light, but does not cause any inconvenience

proved to be as good as it can possibly be, since the for the operator thanks to the set of assembly andtest did not need to be stopped even once during disassembly tools for specimen handling (Fig. 4).the six 500 000 cycle test stages. It is essential for As the temperature was maintained at 20 °C, thereliability that the structure of the simulator isevaporation from the open lubricant chambers wa

ssound and robust from the machine design point of so minimal that the device did not need to beview, with all principles, functions, mechanisms,

stopped for the addition of water during the 139 hour

components, machine elements, and parts being periods that each 500 000 cycles took to run. Theoptimized and as simple as possible in relation tofact that the results were so similar to clinical fin

d-the objective, valid hip wear simulation with low ings proves that the lubricant temperature needvariation. not be kept at 37 °C. The lower temperature ha

sProductivity is naturally a result of the high the advantage that the microbial growth and the

capacity, efficiency, and reliability of the Super-

denaturation of the proteins is slower. Hence, the

CTPOD test system. Now it is possible, for the first wear mechanisms remain more stable. It is knowntime, to ‘mass-produce’ valid wear data without any

that the protein precipitation, which increases with

additional effort compared with existing low-capacity increasing temperature, can erroneously change thesimulators. Moreover, the Super-CTPOD design itself

wear mechanisms in a wear test so that precipitated

is ideal for serial manufacture. A set of, say, ten units protein slurry acts as an anti-wear agent [19], whichwith a massive total capacity of 1000 test stations

does not happen in vivo. Due to the microbial

would be a truly powerful hardware in ambitiousgrowth, the use of the 37 °C temperature necessitatesbiotribology research programmes. the use of antibiotics, such as sodium azide, th

atEconomy of the Super-CTPOD is based on its are not only toxic but also may affect the wear

sound design, consisting of simple parts made from

mechanisms. The present study showed that there is

common materials and of industrial componentsno need to use any additives whatsoever in the serum(pneumatics, bearings, electrical drive). The shapes

if the temperature is kept at 20 °C – not even EDTA

of the specimens are also simple, making them (ethylene diamine tetraacetic acid), since there waseconomical (Fig. 3). The pin is a simple cylinder and

no problem of calcium phosphate precipitation.

the only special features in the disc are the groove Insufficient lubricant volume is another possiblefor the O-ring, a recess for the O-ring removal tool,reason for the problematic protein slurry

formationand two 3 mm bores on the lower side for the locking [19]. Since no such problem was encountered inpins of the motion plate. No screws are needed inthe present tests, it can be deduced that 16 ml of

thetheir fixation. In the present study, the grinding and present lubricant, alpha calf serum diluted 1:1, is



sufficient with a load of 70.7 N and sliding velocity 5 CONCLUSIONSof 31.4 mm/s, at least with the clinically most widelyused combination, polyethylene against CoCr. The

For the first time, the number of similar tests done

average wear factor, 1.63×10−6 mm3/N m, cannot beconsidered too low in relation to clinical findings [5].

for the widely used conventional UHMWPE wassufficiently large (100) for obtaining a 95 per cent

The temperature control system of the Super-CTPODallows the use of a wide range of temperatures. The

confidence interval for a mean wear factor as low as±1.1 per cent of the mean value and for proving that

minimum lubricant temperature that was achieved

the wear factor is normally distributed. This was made

in system tests was 8 °C, as the cold tap water tem-perature of the laboratory was 7 °C. On the otherhand, temperatures above 40 °C proved problematic,as they led to excessive evaporation.

possible by the low variation of results without asingle outlier, which is indicative of a high-quality testmethod. The test conditions were shown to closelysimulate the clinical conditions in the prosthetic

With the 12-station CTPOD, a test has been donehip joint. Moreover, the results indicated that inso that the specimens and the test conditions in all12 stations were similar to those of the present study[20]. The only differences were that the lubricantvolume was 12 ml and the average lubricant bulktemperature was 28 °C. The mean wear factor wasfound to be 1.19×10−6 mm3/N m, 27 per cent lowerthan that of the present study, and the SD was 9.2 percent of the mean. It is logical to assume that thelower k value in the 12-station CTPOD test was dueto the smaller lubricant volume and higher lubricanttemperature, as the amount of precipitated proteininvolved in the wear mechanism was probablyhigher, resulting in a lower wear rate, in conformitywith hip joint simulator studies [19, 21].The nominal contact pressure used, 1.1 MPa, i

slower than the values used by most other researchers.

order to produce wear similar to that known to occurclinically the lubricant temperature of 20 °C is pre-ferable to higher values due to factors related toprotein degradation that is inevitable in vitro incontrast to in vivo conditions.Altogether, the Super-CTPOD wear test system

wasshown to be a unique and superior combination ofvalidity, low variation, capacity, efficiency, reliability,productivity, economy, ease of operation, and com-pact size, making it the most original and usefulinnovation among contemporary wear simulatorsfor prosthetic joint materials. The Super-CTPOD isthe solution to the capacity problem of valid wearsimulation, and has true potential to mark a newepoch in the biotribology research of orthopaedicbiomaterials.

However, as the average load value in a hip jointduring a gait cycle is 1000 N [22], the average contactpressure value on the contact zone with this load

ACKNOWLEDGEMENTS

is likely to within the range 1–2 MPa. A simplifiedformula assuming hemispherical contact, ellipsoidalcontact pressure distribution, and a 28 mm headdiameter yields 0.785×0.386×1000/142=1.5 MPafor the average pressure [23]. The average value is asensible choice for a pin-on-disc device. Excessivepressure is likely to lead to abnormal thermal pheno-mena [18]. Overheating of the tribocontact is a pitfallto be avoided in wear testing. After all, a simulator

must, for practical reasons, run 24 hours a day, which

21 Liao, Y.-S., McKellop, H., Lu, Z., Campbell, P., and

on the effect of rotation on the wear of metal-on- J. Tribol., 2003, 125, 638–642.9 Scholes, S. C. and Unsworth, A. Pin-on-plate studies

on the wear of ultra-high molecular weight poly-20 Saikko, V. Effect of lubricant protein concentration

p. 73.Society, Anaheim, California, 1–4 February 1999,45th Annual Meeting of the Orthopaedic Research

8 Bragdon, C. R., O’Connor, D. O., Lowenstein, J. D.,

degradation and its effect on the outcome of hip

J. Engineering in Medicine, 1999, 213(H), 301–310.19 Wang, A., Polineni, V. K., Essner, A., Stark, C., and

lubricant in wear simulation of polyethylene7 Saikko, V. and Ahlroos, T. Type of motion and

directional motion pin-on-disk device: effects of

1985, ASTM STP 859, pp. 351–368 (American Society

Second Symposium (Eds A. C. Fraker and C. D. Griffin),In Corrosion and degradation of implant materials:wear of ultra-high-molecular-weight polyethylene.

6 McKellop, H. A. and Clarke, I. C. Degradation and

Saikko, V., Lehto, M. U. K., and Ja¨rvinen, M.Pajama¨ki, K. J. J., Halonen, P. J., Nevalainen, J. K.,

17 Puolakka, T. J. S., Kera¨nen, J. T., Juhola, K. A.,

5 Hall, R. M., Unsworth, A., Siney, P., and Wroblewski,

475–483.J. Biomed. Mater. Res. (Appl. Biomater.), 2002, 63,

16 Sychterz, C. J., Engh Jr, C. A., Swope, S. W.,

Agrawal, C. M. Standardized analysis of UHMWPE

counterfaces. It has been shown to produce valid

limited to the study of soft materials against hard

The usefulness of the CTPOD principle is not

The study was supported by the Academy of Finlandthrough a Senior Research Fellowship and Grant43328. The author thanks Dr Claude Rieker (ZimmerGmbH, Switzerland) for providing the polyethylenepins and the CoCr discs, Mr Ilmari Pajama¨ki, Mr TeroKoskenneva, and Mr Kai Koski for manufacturing the

parts of the simulator, Ms Seija Suojakari for grindingand polishing the discs, and Mr Jaakko Kera¨nen forthe SEM of wear particles.

undeniably differs from the activity of a typicalarthroplasty patient. The present results indicatedthat static loading is sufficient. Dynamic loading didnot appear necessary, but it is a feature that can be

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