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Wear of polyethylene acetabular cups against zirconia femoral heads studied with a hip joint simulator
‘WEAR Wear
ELSEVIER
176 (1994) 207-212
Wear of polyethylene acetabular cups against zirconia femoral heads studied with a hip joint simulator V. Saikko” Helsinki
University of Tecimology,
Laboratory
of Machine Design, Puumiehenkuja
Received 13 December
1993; accepted
5, FIN-02150
Espoo, Finland
14 April 1994
Abstract Five zirconia (ZrO,) femoral heads 32 mm in diameter were articulated against ultra-high molecular weight polyethylene (UHMWPE) acetabular cups for five million walking cycles in a five-head hip joint simulator designed for wear studies on five total replaceme& hip joints at once. The test environment was at body temperature and distilled water was used as lubricant. The wear was measured by weighing the cups. The average wear rate of the cups was found to be very low, 4.1 mg per one million cycles (range from 0.4 to 7.1 mg per one million cycles). It was estimated to correspond to a clinical wear rate of 5 pm year-‘. The average wear factor was calculated to be 8 X 10m8 mm3 N-’ m-l. Notable amounts of UHMWPE wear debris were nevertheless produced. Low wear rate is important in total replacement hip joints to avoid adverse tissue reactions that cause loosening of the prosthesis fixation. A wear rate of 5 Frn year-’ would hardly cause problems clinically. Keywords: Polyethylene; Hip joint wear; Zirconia
1. Introduction Zirconia (ZrO,) ceramic has been introduced as an alternative to alumina (Al,03) ceramic as the material of the modular femoral head in total replacement hip joints. Zirconia has higher fracture toughness, which is important in modular femoral heads because of the high tensile stresses caused by the taper-fit attachment. Alumina femoral heads have fractured in the human body. One way of circumventing the problem of femoral head fractures is to find a more ductile material. Against alumina femoral heads, the wear of ultrahigh molecular weight polyethylene (UHMWPE) cups has been measured from X-ray photographs to be significantly lower than that against stainless steel and Co-G-Mo femoral heads [1,2]. An alternative femoral head material must not compromise this advantage gained by the use of alumina. The low wear rate of the acetabular cup is most desirable because UHMWPE wear particles are likely to cause adverse tissue reactions leading to the loosening of the femoral and acetabular component fixation [3,4]. In Charnley prostheses (stainless steel on UHMWPE), occasional rapid penetration of the femoral head into the acetabular cup precludes *Tel. 358 0 451 3562; Fax 358 0 451 3454; Telex 125161 htkk sf.
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a long service life [5]. The average penetration rate in Charnley prostheses is about 0.1 mm year-’ [6]. Penetration rates exceeding 0.2 mm year-’ have been found to contribute to loosening in Miiller prostheses (Co-Cr-Mo on UHMWPE) [7]. Little is known about the wear of UHMWPE acetabular cups against zirconia femoral heads. The present study evaluates the wear of UHMWPE cups articulating against experimental zirconia femoral heads in a hip joint simulator especially designed for wear studies of total replacement hip joints [8]. The apparatus has previously been used in extensive studies of UHMWPE cups articulating against stainless steel, Co-G-MO, ionimplanted Ti-6Al-4V and alumina femoral heads [g-11].
2. Materials and methods The experimental zirconia femoral heads were made by Viking Chemicals (Denmark) and the Danish Technological Institute within a Scandinavian research project “Ceramics for Tribological Applications”, financed by the Nordic Industrial Fund. The zirconia was yttria stabilized. After sintering, the heads were hot isostatically pressed. Then they were ground using two disks with 126 pm and 46 I.crndiamond particles, respectively.
208
Table 1 Diameter, Specimen
I/
roundness
and surface Diameter (mm)
roughness
Suikko
of zirconia
I Wear 176 (1994)
femoral
Deviation roundness
207-212
heads
.._______
from max.
Original surface roughness R,
(km)
Surface roughness H, of load-bearing area after test
(wm)
(w) 1 2 3 4 5
31.97 31.98 31.96 31.96 31.98
2.8 3.1 3.4 3.3 6.4
Lastly they were polished with diamond paste with progressively finer particles, from 50 pm to 1 pm. The diameter, roundness and surface roughness measurements are presented in Table 1. The diameter was measured with a micrometer and the roundness with an RTH Talyrond apparatus. The surface roughness was measured on five traces with an RTH Form Talysurf apparatus, the stylus tip radius being 2 pm and cutoff length 0.25 mm. The diameter and surface roughness of the zirconia femoral heads were comparable with those of commercially available alumina Biolox heads, but the deviations from roundness of the zirconia heads were roughly an order of magnitude higher than the very small values of the Biolox heads [9,10,12]. As the zirconia heads were experimental and finished by hand, it was not possible to achieve values as low as those of Biolox heads. The values are nevertheless relatively low. IS0 7206-2 standard [13] allows a maximum 5 pm deviation from roundness for stainless steel and cobalt-based alloy heads, and a maximum 8 pm for titanium-based alloy heads. In a hip joint simulator test, the attachment of the specimens to the apparatus is crucial. Since the present zirconia femoral head is not a part of any total hip system, no mating femoral stem was available to attach it to the cradle of the simulator, as was done in previous studies [all], so a special femoral head holder had to be developed. The five holders for the five spheres were machined from stainless steel 329. In the neutral position of the flexion-extension cradle, the neck of the holder formed an angle of 30” relative to the axis of loading. The conical (1:lO) interface between the femoral head and the holder was covered by silicone sealant, because previous studies had showed that wear and corrosion particles from the taper joint between sphere and femoral stem would become trapped between the sphere and the acetabular cup, and have a major effect on the wear of the cup. Because the present holders were made from a non-surgical alloy, corrosion of the taper-fit interface and the escape of wear particles from the taper-fit interface had to be prevented, since such damage would not have been representative of the behaviour of a complete prosthesis.
0.014 0.007 0.007 0.007 0.008
0.012 0.009 0.009 0.009 0.008
How medica’s P.C.A. acetabular insert was selected to be the UHMWPE cup because no wear was observed in it when articulating against alumina Biolox femoral heads [9], even in the extended test of 35 million walking cycles’ duration [lo]. That combination is thus a sensible (and stringent!) standard of comparison for any alternative femoral head material. The five inserts (cat. no. 6285-2-525) were from the same batch and they had been manufactured for implantation. The insert was backed by an Co-Cr-Mo P.C.A. acetabular shell (cat. no. 6289-5-052) which gives virtually rigid support. The shell was attached to an acrylic cup holder that functioned also as a lubricant receptacle. The internal radii of cups are presented in Table 2. The radii were determined after the simulator test using the simple three-ball method described in ref. 12. The average original relative clearance of the joints was about 1.5%, as it was in the previous tests with alumina heads. By engineering standards, this value may seem high, but it is obviously an empirical optimum and typical of total replacement hip joints. Small clearance is known to increase frictional torque and to impair lubrication, resulting in overheating. Note that the deviations from roundness in UHMWPE cups are usually high compared with those of femoral heads. This is generally thought to be tribologically unimportant because of the low creep resistance of UHMWPE. The thickness of the cups at the point of the loading and in its direction was 5.6 mm.
Table 2 Internal radii Specimen
of UHMWPE Radius, original (mm)
cups
surface
Radius, middle of load-bearing area after simulator test (mm)
1 2 3 4 5
16.30 16.23 16.20 16.29 16.15
16.26 16.39 16.47 16.18 16.40
209
K Saikko I Wear I76 (1994) 207-212
a (DEGREES) 30
Id+
t (s)
0
-30
1
2
1
2
4 f= (kN) 3.5
t (s) 0
m--
HEEL STdlKE
TO&
Fig. 1. Schematic illustration of centre station of hip joint simulator, and two cycles of motion and load waveforms: (1) femoral head, (2) acetabular cup, (3) flexion-extension cradle, (4) support bearing, (5) crank mechanism, (6) electric motor and gear, (7) loading arm, (8) pneumatic load actuator, (9) femoral stem, or a special femoral head holder, (10) control cup. Symbol (Y represents flexion-extension angle, F load, and t time. In the present study, the control cup and its head were replaced with a Hooke (universal) joint.
Fig. 1 is a schematic illustration of the centre station of the five-station hip joint simulator. The motion and load cycles are also specified in Fig. 1. The apparatus simulates the flexion-extension motion and the vertical component of the joint contact force in walking. To make sure that the prosthesis was not loaded during the swing phase of the gait cycle, the cup was slightly separated from the head during the swing phase, the height of the gap being by estimation 0.05 mm at most. The test frequency was 65 walking cycles per minute. The development of the simulator, and the test methodology have been described in detail elsewhere [8,9,11]. The five joints were tested simultaneously in identical conditions. The temperature of the test environment was 37 + 1 “C and distilled water was used as a lubricant. The length of the test was five million walking cycles, corresponding to 5 years of use in the human body. The test took 8 weeks, and ran virtually continuously, the only stops being due to the wear measurements which required the removal of the cups. The cups were weighed at every millionth cycle. Prior to the weighings, the cups were vacuum desiccated, and the amount of residual water was taken from the soak data of six similar P.C.A. cups in a previous study [9], in which it was found that the variation of weight gain (mg) due to water absorption with the number of cycles was linear, 2.6 mg+ 1.0 mg per one million cycles (r= 0.87). In the present study, a Hooke (universal) joint replaced the control cup and its head in the loading arrangement, rendering the cup self-centring on the femoral head, which is essential for controlled loading conditions.
3. Results The wear, i.e. the weight loss of the cups as the number of cycles increases, is shown in Fig. 2. The
30 -
20
0 --
--
--
--,
-10 0
1
2
3
4
5
Cycles (million) Fig. 2. Variation of wear of acetabular cups with number of walking cycles. Symbols refer to cups as follows: n , cup 1; A, cup 2; 0, cup 3; 0, cup 4; +, cup 5. The broken line indicates average weight gain measured in six similar cups in a previous study. It has been used for a water absorption correction in the present study.
negative signs of the first three values of cups 4 and 5 indicate net weight gain and show that their wear was non-existent and that their weight gain due to absorption of water was higher than the average weight gain of the control cups. The wear was calculated by subtracting the weight of the test cup from the initial weight, and by adding to this figure the mean weight gain of the control cups at the number of cycles in question. The difficulty that the test cup absorbs slightly more water than the control cup was also observed in previous experiments [9,10]. An alternative method of gravimetrically measuring the wear would be the weighing of wear particles. However, a part of the debris is easily lost. The wear rates and wear factors are presented in Table 3. The wear rates were determined by linear
210
1/
Soikko
I Wwr
Table 3 Wear rates and wear factors Specimen
1 2 3 4 5
Wear rate (mg per one million cycles)
Clinical wear rate (Km year-‘)
7.1 5.7 5.8 0.4 1.5
9.4 7.6 7.7 0.6 2.0
Wear factor (X lo-’ mm” w’
176 (1994)
207-212
However, the wear on the outer surface of the cups was so minimal that it hardly affected the weighings, unlike substantial wear would naturally have done.
m-‘)
1.4 1.1 1.1 0.1 0.3
regression. The mean wear rate is 4.1 mg per one million cycles, the sample standard deviation being 2.9 mg per one million cycles. The wear rate is converted into the so-called clinical wear rate by assuming that one million cycles corresponds to one year in the human body [14], that the density of UHMWPE is 0.94 mg mme3 [15], and that the penetration is the volumetric wear divided by the projected area of the femoral head, 803 mm2 [16]. In the beginning of the penetration, however, the true contact area- shown by the wear trace - in the present, relatively thin, metal-backed cups was much less than hemispherical, and consequently the true penetration rate could naturally be higher. The average wear rate, 4.1 mg per one million cycles, corresponds to a clinical wear rate of 4.1 mg year-l/ (0.94 mg mm-3 X 803 mm*) = 5 pm year - ‘. The average wear factor was 4.1 mg/(O.94 mg mmM3X 106X 55.8 N m) = 8 X lo-’ mm3 N-’ m-l, i.e. 8 x lo-’ mm3 of UHMWPE was worn off for every newton of load and metre of rubbing distance. No UHMWPE transfer to the zirconia surface or any other change on the femoral heads could be detected in a visual examination, and the surface roughness after the wear test was not different from the initial roughness (Table 1). In cups 1, 2 and 3, the production of UHMWPE wear particles started soon after the start of the test, and continued throughout the test. In cup 5, the production of wear particles started only after three million cycles. From cup 4, virtually no particles were removed. The typical UHMWPE wear debris had the form of a thread or a thin streamer that could be as long as 2 cm. The threads and streamers were brittle and broke up easily when fingered. The fact that they were not entrapped between the head and the cup and broken up in the simulator was due to the arrangement of the apparatus: the articulation was loaded during the extension phase only and so the UHMWPE shreds were projected from the anterior edge of the contact area, and the shreds, being lighter than water, tended to rise to the surface, away from the contact area, since the prosthesis was upside-down. There were also slight wear traces on the outer surface of the cups, i.e. on the lower surface of the flange that had been rubbed against the rim of the Co-Cr-Mo acetabular shell.
4. Discussion Conditions in a simulator represent only an approximation of the very complex conditions in the human body. Therefore, the wear rate obtained with a simulator is not necessarily an accurate prediction of clinical behaviour. The main simplification in the present simulator is that continuous walking only is simulated. The wear rate obtained for Charnley prostheses (stainless steel on UHMWPE) in a previous study [9], 0.15 mm year-l, is consistent with clinical observations [5,6,16], which suggests that the simulation is realistic. Hip joint simulator studies are important because without them information on the tribological behaviour of total replacement hip joints could be obtained only by implanting in patients. Synovial fluid from prosthetic hip joints is not available in sufficient amounts to be used as a lubricant in simulator wear tests. Some investigators use blood serum as a lubricant, because they have found that serum, unlike water, prevents the formation of an UHMWPE transfer layer on a metallic counterface, i.e. the adhesive wear mechanism, and that in a visual examination such a layer cannot be detected on metallic femoral heads removed from patients [17]. Other investigators, however, have found that the adhesive wear is the dominating clinical wear mechanism [18], and that water is almost as good as other lubricants [16]. Little is known about the effect of the unavoidable degradation of serum, and about the effect of the additives used to retard degradation, on the lubricating properties of serum. In another hip joint simulator study, the wear rate of UHMWPE cups against zirconia femoral heads in serum was about the same as that against alumina heads [19]. However, the wear rate against Co-Cr-Mo femoral heads in that study was lower than that against ceramic heads, the two being about 20 and 30 mg per one million cycles, respectively. This ranking is in glaring contrast to clinical observations. The wear against ceramic heads was apparently strongly affected by a dull, tenacious calcium phosphate layer that formed on the ceramic heads and was attributed to the use of serum lubricant. Such a layer is not seen on ceramic femoral heads removed from patients, so the suitability of serum as a lubricant in long-term hip joint simulator wear tests appears questionable. On the other hand, in a pin-on-flat study, in which UHMWPE pins were tested against zirconia and alumina plates in water, saline and serum, the UHMWPE wear did not prove sensitive to the type of lubricant, and no UHMWPE transfer layer was formed on zirconia and alumina counterfaces in
V. Saikko I Wear 176 (1994) 207-212
any lubricant [20]. No transfer was observed in the present study, either. Hence, distilled water seems a good choice for lubricant. The average wear rate of P.C.A. cups against zirconia heads in the present study, 5 pm year-l, is equal to that against Co-Cr-Mo heads (Howmedica’s Vitallium) [9]. The direct mechanical consequences of such a low penetration rate would be negligible, but notable amounts of UHMWPE wear debris were produced in four of the five cups in the present study, and wear debris may cause adverse tissue reactions. Neither wear nor wear debris was observed in three out of three P.C.A. cups that articulated against alumina Biolox heads, two cups for three million cycles and one for 35 million cycles [9,10]. However, this difference is not necessarily due to the head materials, but it may be due to the better roundness of the Biolox heads. The sphericity of the femoral head has been found to affect the wear of the UHMWPE cup, prot~ding deviations from spheric@ being detrimental and concave deviations advantageous 1211. So far as the author knows, clinical wear studies on the Biolox head on P.C.A. cup prostheses are not available, but in other alumina on UHMWPE designs, wear of the cup does occur clinically, although it is much lower than that against metallic heads [1,2]. That cup 4 was the only one from which no wear particles were removed may be because head 4, unlike the other four heads, had a sintering defect, a circumferential equatorial groove approximately 1 pm deep. The groove may have improved lubricant entrapment during the non-loaded swing phase of the gait cycle, and consequently lubrication during the stance phase. Intentional grooves or dimples may be worth studying in future. The subject is of present interest as metal-on-metal articulation is being re-examined in many laboratories. Such articulation requires good lubrication to perform properly. The amount of wear debris needed to cause adverse tissue reactions is not known. Probably the average production rate of UHMWPE debris measured in the present study, 4 mg year-‘, would not be detrimental in the human body, because in a radiographic study on the penetration of femoral heads into acetabular cups in Mtiller prostheses, only penetration rates exceeding 0.2 mm year-’ contributed to loosening [7]. A penetration rate of 0.2 mm year-’ can be converted into a production rate of UHMWPE debris of 150 mg year-‘. So far as the author knows, there are no studies available yet on the penetration of zirconia femoral heads into UHMWPE cups in the human body, or on the internal volume changes of UHMWPE cups worn against zirconia heads and removed from patients, that could be compared with the simulator data. In a water-lubricated, reciprocating pin-on-flat study done by the author [22], the wear rate of UHMWPE
211
pins was extremely low against both alumina (Vitox) and zirconia (Zyranox) plates, both manufactured by Morgan Matroc Ltd, but clearly higher against Co-Cr-Mo plates (Ho~edi~a’s Vitallium). In that study, the coefficient of friction of UHMWPE against zirconia was about 20% higher than that against alumina. However, the pin-on-flat study and the present hip joint simulator study are not readily comparable, mainly because different alumina and zirconia materials were used, and test conditions also differed. In the other re~ipr~ating and unidirectional pin-on-flat study 1201, the wear rate of UHM~E was significantly lower against zirconia (manufactured by Kyocera Corp.) than against alumina, which is in contrast to the pin-on-flat and hip joint simulator observations of the present author. In a frictional study done by the author, the zirconia femoral head in question generated slightly higher frictional torque against an UHMWPE cup than an alumina head [12]. The scatter in the wear results is substantial (Fig. 2, Table 3). Care was naturally taken to perform five identical tests and so no unequivocal explanation for the scatter can be presented. There was slight variation in original clearance, and in deviation from roundness and surface roughness of the heads, but their correlation with wear was poor. In general, considerable scatter, even over several orders of magnitude, is not uncommon in wear tests, which is due not only to environmental and material property variations, but also to the inherent stochastic feature of the wear process [23]. In clinical penetration rates, scatter over two orders of magnitude is common [5,16]. The scatter in clinical penetration rates is not likely to be attributable to the differences in conditions between patients, because in the study of prostheses removed from patients, poor correlation was found between penetration rate, and femoral head roughness, patient mass and age [5]. The upside-down position of the prosthesis is necessary to avoid the drying out of the articulation that trapped air bubbles would cause if the prosthesis were not inverted. In friction tests done with the singlestation simulator [12], in which the cup is located above the head, air bubbles had to be removed prior to measurements. The removal was not too difficult, but long-term wear tests are much more demanding in this respect, because air bubbles tend to gather in the air pocket during the articulation, if the cup is above the head. The risk of drying out must be eliminated, because drying out would result in rapid overheating which would ruin the specimens and the test. This was learned in early system tests of the present simulator: drying out happened once as the water supply failed inadvertently. However, inversion of the prosthesis decreases the possibility of entrapment of particles with density less than that of water, and increases the possibility of entrapment of particles with density higher than
212
I*!
Suikko 1 Wear 176 (1994) 207-212
that of water. This may somewhat reduce the veracity of the simulation.
5. Conclusions The wear rate of UHMWPE acetabular cups that articulated against experimental zirconia femoral heads in a hip joint simulator proved very low, the mean value being 4.1 mg per one million cycles, which can be estimated to correspond to a clinical wear rate of 5 ,um year-‘, the wear factor being 8 x lo-’ mm3 N-’ m -I. Therefore, zirconia seems to be tribologically suitable for the modular femoral head of the total replacement hip joints.
Acknowledgments
The author thanks the Academy of Finland, Nordic Industrial Fund, Instrumentarium Science Foundation, and Yrjii and Senja Koivunen Foundation for financial support, Howmedica - a Division of Pfizer Oy Finland for the donation of the acetabular cups, and Mr I. Pajamski and Mr T. Koskenneva for the skilful machining of the specimen holders.
References 111 H. Oonishi, Y. Takayaka, I.C. Clarke and H. Jung, Cornparative wear studies of 28-mm ceramic and stainless steel total hip joints over 2 to 7 year period, J. Long-Term Eficts of Medical Implants, 2 (1992) 37-47. VI H.M. Schiiller and R.K. Marti, Ten-year socket wear in 66 hip arthroplasties. Ceramic versus metal heads,Acta Orthop. &and., 61 (1990) 240-243. [31 H.C. Amstutz, P. Campbell, N. Kossovsky and I.C. Clarke, Mechanism and clinical significance of wear debris-induced osteolysis, Clin. Orthop., 276 (1992) 7-18. I41 K.R. St. John (ed.), Particulate debris from medical implants: mechanisms of formation and biological consequences, ASTM Spec. Tech. Publ. 1144, ASTM, Philadelphia, PA, 1992. [51 G.H. Isaac, B.M. Wroblewski, J.R. Atkinson and D. Dowson, A tribological study of retrieved hip prostheses, Clin. Orthop., 276 (1992) 115-125. [61 B.M. Wroblewski, 15-Zl-year results of the Charnley lowfriction arthroplasty, Clin. Orthop., 211 (1986) 30-35. [71 U. Buchhorn, H.-G. Willert, M. Semlitsch and H. Weber, Changes in the dimensions of polyethylene acetabuli in total hip replacements after Miiller,Z. Orthop., 122 (1984) 127-135 (in German). PI V. Saikko, P. Paavolainen, M. Kleimola and P. Slstis, A five-station hip joint simulator for wear rate studies, J. Eng. Med., 206 (1992) 195-200.
and P. Slitis, Wear of the [91 V. Saikko, P. Paavolainen polyethylene acetabular cup. Metallic and ceramic heads compared in a hip simulator, Acra Orthop. &and.. 64 (1993) 391-402. WI V. Saikko, Wear of polyethylene acetabular cups against alumina femoral heads. 5 prostheses compared in a hip simulator for 35 million walking cycles, Acta Orthop. Stand., 64 (1993) 507-512. [111 V. Saikko, Tribology of total replacement hip joints studied with new hip joint simulators and a materials-screening apparatus, Acta Polytech. Stand., Mech. Eng. Series No. 110, Helsinki 1993, 44 pp (Doctoral dissertation). [=I V. Saikko, A simulator study of friction in total replacement hip joints, L Eng. Med., 206 (1992) 201-211. Standard IS0 7206-2, Implants for surgery 1131 International -Partial and total hip joint prostheses-Part 2: Bearing surfaces made of metallic and plastics materials, Intemational Organization for Standardization, 1987. P41 B.B. Seedhom and N.C. Wallbridge, Walking activities and wear of prostheses, Ann. Rheum. Dk., 44 (1985) 838-843. Standard IS0 5834, Implants for sur1151 International gery-ultra-high molecular weight polyethylene-Part 2: Moulded forms, International Organization for Standardization, 1985. J.R. Atkinson, D. Dowson, G.H. Isaac and B.M. Wroblewski, PI Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints III: The measurement of internal volume changes in explanted Charnley sockets after 2-16 years in viva and the determination of wear factors, Wear, 104 (1985) 225-244. P71 H.A. McKellop and I.C. Clarke, Degradation and wear of ultra-high-molecular-weight polyethylene, in A.C. Fraker and C.D. Griffin (eds.), Corrosion and degradation of implant materials: second symposium, ASTM Spec. Tech. Publ. 859, ASTM, Philadelphia, PA, 1985, pp. 351-368. WI J.R. Atkinson, D. Dowson, G.H. Isaac and B.M. Wroblewski, Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints II: A microscopical study of the surfaces of Charnley polyethylene acetabular sockets, Wear, 104 (1985) 217-224. P91 H. McKellop, B. Lu and P. Benya, Friction, lubrication and wear of cobalt+hromium, alumina and zirconia hip prostheses compared on a joint simulator, 38th Ann. Meet., Orthopaedic Research Society, February 17-20, 1992, Washington, DC. PO1 P. Kumar, M. Oka, K. Ikeuchi, K. Shimizu, T. Yamamuro, H. Okumura and Y. Kotoura, Low wear rate of UHMWPE against zirconia ceramic (Y-PSZ) in comparison to alumina ceramic and SUS 316L alloy, J. Biomed. Mater. Res., 25 (1991) 813-828. VI H. Oonishi, Studies on the shape and contour of the metal prosthetic head in total hip prosthesis, Proc. Congr. Inf. Society of Orthopaedic Surger)’ and Traumatology, 1972. pp. 107-123. V. Saikko, Wear and friction properties of prosthetic joint PI materials evaluated on a reciprocating pin-on-flat apparatus, Wear, 166 (1993) 169-178. v31 N.C. Wallbridge and D. Dowson, Distribution of wear rate data and a statistical approach to sliding wear theory, Wear, 119 (1987) 295-312.
ELSEVIER
176 (1994) 207-212
Wear of polyethylene acetabular cups against zirconia femoral heads studied with a hip joint simulator V. Saikko” Helsinki
University of Tecimology,
Laboratory
of Machine Design, Puumiehenkuja
Received 13 December
1993; accepted
5, FIN-02150
Espoo, Finland
14 April 1994
Abstract Five zirconia (ZrO,) femoral heads 32 mm in diameter were articulated against ultra-high molecular weight polyethylene (UHMWPE) acetabular cups for five million walking cycles in a five-head hip joint simulator designed for wear studies on five total replaceme& hip joints at once. The test environment was at body temperature and distilled water was used as lubricant. The wear was measured by weighing the cups. The average wear rate of the cups was found to be very low, 4.1 mg per one million cycles (range from 0.4 to 7.1 mg per one million cycles). It was estimated to correspond to a clinical wear rate of 5 pm year-‘. The average wear factor was calculated to be 8 X 10m8 mm3 N-’ m-l. Notable amounts of UHMWPE wear debris were nevertheless produced. Low wear rate is important in total replacement hip joints to avoid adverse tissue reactions that cause loosening of the prosthesis fixation. A wear rate of 5 Frn year-’ would hardly cause problems clinically. Keywords: Polyethylene; Hip joint wear; Zirconia
1. Introduction Zirconia (ZrO,) ceramic has been introduced as an alternative to alumina (Al,03) ceramic as the material of the modular femoral head in total replacement hip joints. Zirconia has higher fracture toughness, which is important in modular femoral heads because of the high tensile stresses caused by the taper-fit attachment. Alumina femoral heads have fractured in the human body. One way of circumventing the problem of femoral head fractures is to find a more ductile material. Against alumina femoral heads, the wear of ultrahigh molecular weight polyethylene (UHMWPE) cups has been measured from X-ray photographs to be significantly lower than that against stainless steel and Co-G-Mo femoral heads [1,2]. An alternative femoral head material must not compromise this advantage gained by the use of alumina. The low wear rate of the acetabular cup is most desirable because UHMWPE wear particles are likely to cause adverse tissue reactions leading to the loosening of the femoral and acetabular component fixation [3,4]. In Charnley prostheses (stainless steel on UHMWPE), occasional rapid penetration of the femoral head into the acetabular cup precludes *Tel. 358 0 451 3562; Fax 358 0 451 3454; Telex 125161 htkk sf.
0043-1648/94/$07.00 0 1994 Elsevier Science S.A. Al1 rights reserved SSDI 0043-1648(94)06462-Q
a long service life [5]. The average penetration rate in Charnley prostheses is about 0.1 mm year-’ [6]. Penetration rates exceeding 0.2 mm year-’ have been found to contribute to loosening in Miiller prostheses (Co-Cr-Mo on UHMWPE) [7]. Little is known about the wear of UHMWPE acetabular cups against zirconia femoral heads. The present study evaluates the wear of UHMWPE cups articulating against experimental zirconia femoral heads in a hip joint simulator especially designed for wear studies of total replacement hip joints [8]. The apparatus has previously been used in extensive studies of UHMWPE cups articulating against stainless steel, Co-G-MO, ionimplanted Ti-6Al-4V and alumina femoral heads [g-11].
2. Materials and methods The experimental zirconia femoral heads were made by Viking Chemicals (Denmark) and the Danish Technological Institute within a Scandinavian research project “Ceramics for Tribological Applications”, financed by the Nordic Industrial Fund. The zirconia was yttria stabilized. After sintering, the heads were hot isostatically pressed. Then they were ground using two disks with 126 pm and 46 I.crndiamond particles, respectively.
208
Table 1 Diameter, Specimen
I/
roundness
and surface Diameter (mm)
roughness
Suikko
of zirconia
I Wear 176 (1994)
femoral
Deviation roundness
207-212
heads
.._______
from max.
Original surface roughness R,
(km)
Surface roughness H, of load-bearing area after test
(wm)
(w) 1 2 3 4 5
31.97 31.98 31.96 31.96 31.98
2.8 3.1 3.4 3.3 6.4
Lastly they were polished with diamond paste with progressively finer particles, from 50 pm to 1 pm. The diameter, roundness and surface roughness measurements are presented in Table 1. The diameter was measured with a micrometer and the roundness with an RTH Talyrond apparatus. The surface roughness was measured on five traces with an RTH Form Talysurf apparatus, the stylus tip radius being 2 pm and cutoff length 0.25 mm. The diameter and surface roughness of the zirconia femoral heads were comparable with those of commercially available alumina Biolox heads, but the deviations from roundness of the zirconia heads were roughly an order of magnitude higher than the very small values of the Biolox heads [9,10,12]. As the zirconia heads were experimental and finished by hand, it was not possible to achieve values as low as those of Biolox heads. The values are nevertheless relatively low. IS0 7206-2 standard [13] allows a maximum 5 pm deviation from roundness for stainless steel and cobalt-based alloy heads, and a maximum 8 pm for titanium-based alloy heads. In a hip joint simulator test, the attachment of the specimens to the apparatus is crucial. Since the present zirconia femoral head is not a part of any total hip system, no mating femoral stem was available to attach it to the cradle of the simulator, as was done in previous studies [all], so a special femoral head holder had to be developed. The five holders for the five spheres were machined from stainless steel 329. In the neutral position of the flexion-extension cradle, the neck of the holder formed an angle of 30” relative to the axis of loading. The conical (1:lO) interface between the femoral head and the holder was covered by silicone sealant, because previous studies had showed that wear and corrosion particles from the taper joint between sphere and femoral stem would become trapped between the sphere and the acetabular cup, and have a major effect on the wear of the cup. Because the present holders were made from a non-surgical alloy, corrosion of the taper-fit interface and the escape of wear particles from the taper-fit interface had to be prevented, since such damage would not have been representative of the behaviour of a complete prosthesis.
0.014 0.007 0.007 0.007 0.008
0.012 0.009 0.009 0.009 0.008
How medica’s P.C.A. acetabular insert was selected to be the UHMWPE cup because no wear was observed in it when articulating against alumina Biolox femoral heads [9], even in the extended test of 35 million walking cycles’ duration [lo]. That combination is thus a sensible (and stringent!) standard of comparison for any alternative femoral head material. The five inserts (cat. no. 6285-2-525) were from the same batch and they had been manufactured for implantation. The insert was backed by an Co-Cr-Mo P.C.A. acetabular shell (cat. no. 6289-5-052) which gives virtually rigid support. The shell was attached to an acrylic cup holder that functioned also as a lubricant receptacle. The internal radii of cups are presented in Table 2. The radii were determined after the simulator test using the simple three-ball method described in ref. 12. The average original relative clearance of the joints was about 1.5%, as it was in the previous tests with alumina heads. By engineering standards, this value may seem high, but it is obviously an empirical optimum and typical of total replacement hip joints. Small clearance is known to increase frictional torque and to impair lubrication, resulting in overheating. Note that the deviations from roundness in UHMWPE cups are usually high compared with those of femoral heads. This is generally thought to be tribologically unimportant because of the low creep resistance of UHMWPE. The thickness of the cups at the point of the loading and in its direction was 5.6 mm.
Table 2 Internal radii Specimen
of UHMWPE Radius, original (mm)
cups
surface
Radius, middle of load-bearing area after simulator test (mm)
1 2 3 4 5
16.30 16.23 16.20 16.29 16.15
16.26 16.39 16.47 16.18 16.40
209
K Saikko I Wear I76 (1994) 207-212
a (DEGREES) 30
Id+
t (s)
0
-30
1
2
1
2
4 f= (kN) 3.5
t (s) 0
m--
HEEL STdlKE
TO&
Fig. 1. Schematic illustration of centre station of hip joint simulator, and two cycles of motion and load waveforms: (1) femoral head, (2) acetabular cup, (3) flexion-extension cradle, (4) support bearing, (5) crank mechanism, (6) electric motor and gear, (7) loading arm, (8) pneumatic load actuator, (9) femoral stem, or a special femoral head holder, (10) control cup. Symbol (Y represents flexion-extension angle, F load, and t time. In the present study, the control cup and its head were replaced with a Hooke (universal) joint.
Fig. 1 is a schematic illustration of the centre station of the five-station hip joint simulator. The motion and load cycles are also specified in Fig. 1. The apparatus simulates the flexion-extension motion and the vertical component of the joint contact force in walking. To make sure that the prosthesis was not loaded during the swing phase of the gait cycle, the cup was slightly separated from the head during the swing phase, the height of the gap being by estimation 0.05 mm at most. The test frequency was 65 walking cycles per minute. The development of the simulator, and the test methodology have been described in detail elsewhere [8,9,11]. The five joints were tested simultaneously in identical conditions. The temperature of the test environment was 37 + 1 “C and distilled water was used as a lubricant. The length of the test was five million walking cycles, corresponding to 5 years of use in the human body. The test took 8 weeks, and ran virtually continuously, the only stops being due to the wear measurements which required the removal of the cups. The cups were weighed at every millionth cycle. Prior to the weighings, the cups were vacuum desiccated, and the amount of residual water was taken from the soak data of six similar P.C.A. cups in a previous study [9], in which it was found that the variation of weight gain (mg) due to water absorption with the number of cycles was linear, 2.6 mg+ 1.0 mg per one million cycles (r= 0.87). In the present study, a Hooke (universal) joint replaced the control cup and its head in the loading arrangement, rendering the cup self-centring on the femoral head, which is essential for controlled loading conditions.
3. Results The wear, i.e. the weight loss of the cups as the number of cycles increases, is shown in Fig. 2. The
30 -
20
0 --
--
--
--,
-10 0
1
2
3
4
5
Cycles (million) Fig. 2. Variation of wear of acetabular cups with number of walking cycles. Symbols refer to cups as follows: n , cup 1; A, cup 2; 0, cup 3; 0, cup 4; +, cup 5. The broken line indicates average weight gain measured in six similar cups in a previous study. It has been used for a water absorption correction in the present study.
negative signs of the first three values of cups 4 and 5 indicate net weight gain and show that their wear was non-existent and that their weight gain due to absorption of water was higher than the average weight gain of the control cups. The wear was calculated by subtracting the weight of the test cup from the initial weight, and by adding to this figure the mean weight gain of the control cups at the number of cycles in question. The difficulty that the test cup absorbs slightly more water than the control cup was also observed in previous experiments [9,10]. An alternative method of gravimetrically measuring the wear would be the weighing of wear particles. However, a part of the debris is easily lost. The wear rates and wear factors are presented in Table 3. The wear rates were determined by linear
210
1/
Soikko
I Wwr
Table 3 Wear rates and wear factors Specimen
1 2 3 4 5
Wear rate (mg per one million cycles)
Clinical wear rate (Km year-‘)
7.1 5.7 5.8 0.4 1.5
9.4 7.6 7.7 0.6 2.0
Wear factor (X lo-’ mm” w’
176 (1994)
207-212
However, the wear on the outer surface of the cups was so minimal that it hardly affected the weighings, unlike substantial wear would naturally have done.
m-‘)
1.4 1.1 1.1 0.1 0.3
regression. The mean wear rate is 4.1 mg per one million cycles, the sample standard deviation being 2.9 mg per one million cycles. The wear rate is converted into the so-called clinical wear rate by assuming that one million cycles corresponds to one year in the human body [14], that the density of UHMWPE is 0.94 mg mme3 [15], and that the penetration is the volumetric wear divided by the projected area of the femoral head, 803 mm2 [16]. In the beginning of the penetration, however, the true contact area- shown by the wear trace - in the present, relatively thin, metal-backed cups was much less than hemispherical, and consequently the true penetration rate could naturally be higher. The average wear rate, 4.1 mg per one million cycles, corresponds to a clinical wear rate of 4.1 mg year-l/ (0.94 mg mm-3 X 803 mm*) = 5 pm year - ‘. The average wear factor was 4.1 mg/(O.94 mg mmM3X 106X 55.8 N m) = 8 X lo-’ mm3 N-’ m-l, i.e. 8 x lo-’ mm3 of UHMWPE was worn off for every newton of load and metre of rubbing distance. No UHMWPE transfer to the zirconia surface or any other change on the femoral heads could be detected in a visual examination, and the surface roughness after the wear test was not different from the initial roughness (Table 1). In cups 1, 2 and 3, the production of UHMWPE wear particles started soon after the start of the test, and continued throughout the test. In cup 5, the production of wear particles started only after three million cycles. From cup 4, virtually no particles were removed. The typical UHMWPE wear debris had the form of a thread or a thin streamer that could be as long as 2 cm. The threads and streamers were brittle and broke up easily when fingered. The fact that they were not entrapped between the head and the cup and broken up in the simulator was due to the arrangement of the apparatus: the articulation was loaded during the extension phase only and so the UHMWPE shreds were projected from the anterior edge of the contact area, and the shreds, being lighter than water, tended to rise to the surface, away from the contact area, since the prosthesis was upside-down. There were also slight wear traces on the outer surface of the cups, i.e. on the lower surface of the flange that had been rubbed against the rim of the Co-Cr-Mo acetabular shell.
4. Discussion Conditions in a simulator represent only an approximation of the very complex conditions in the human body. Therefore, the wear rate obtained with a simulator is not necessarily an accurate prediction of clinical behaviour. The main simplification in the present simulator is that continuous walking only is simulated. The wear rate obtained for Charnley prostheses (stainless steel on UHMWPE) in a previous study [9], 0.15 mm year-l, is consistent with clinical observations [5,6,16], which suggests that the simulation is realistic. Hip joint simulator studies are important because without them information on the tribological behaviour of total replacement hip joints could be obtained only by implanting in patients. Synovial fluid from prosthetic hip joints is not available in sufficient amounts to be used as a lubricant in simulator wear tests. Some investigators use blood serum as a lubricant, because they have found that serum, unlike water, prevents the formation of an UHMWPE transfer layer on a metallic counterface, i.e. the adhesive wear mechanism, and that in a visual examination such a layer cannot be detected on metallic femoral heads removed from patients [17]. Other investigators, however, have found that the adhesive wear is the dominating clinical wear mechanism [18], and that water is almost as good as other lubricants [16]. Little is known about the effect of the unavoidable degradation of serum, and about the effect of the additives used to retard degradation, on the lubricating properties of serum. In another hip joint simulator study, the wear rate of UHMWPE cups against zirconia femoral heads in serum was about the same as that against alumina heads [19]. However, the wear rate against Co-Cr-Mo femoral heads in that study was lower than that against ceramic heads, the two being about 20 and 30 mg per one million cycles, respectively. This ranking is in glaring contrast to clinical observations. The wear against ceramic heads was apparently strongly affected by a dull, tenacious calcium phosphate layer that formed on the ceramic heads and was attributed to the use of serum lubricant. Such a layer is not seen on ceramic femoral heads removed from patients, so the suitability of serum as a lubricant in long-term hip joint simulator wear tests appears questionable. On the other hand, in a pin-on-flat study, in which UHMWPE pins were tested against zirconia and alumina plates in water, saline and serum, the UHMWPE wear did not prove sensitive to the type of lubricant, and no UHMWPE transfer layer was formed on zirconia and alumina counterfaces in
V. Saikko I Wear 176 (1994) 207-212
any lubricant [20]. No transfer was observed in the present study, either. Hence, distilled water seems a good choice for lubricant. The average wear rate of P.C.A. cups against zirconia heads in the present study, 5 pm year-l, is equal to that against Co-Cr-Mo heads (Howmedica’s Vitallium) [9]. The direct mechanical consequences of such a low penetration rate would be negligible, but notable amounts of UHMWPE wear debris were produced in four of the five cups in the present study, and wear debris may cause adverse tissue reactions. Neither wear nor wear debris was observed in three out of three P.C.A. cups that articulated against alumina Biolox heads, two cups for three million cycles and one for 35 million cycles [9,10]. However, this difference is not necessarily due to the head materials, but it may be due to the better roundness of the Biolox heads. The sphericity of the femoral head has been found to affect the wear of the UHMWPE cup, prot~ding deviations from spheric@ being detrimental and concave deviations advantageous 1211. So far as the author knows, clinical wear studies on the Biolox head on P.C.A. cup prostheses are not available, but in other alumina on UHMWPE designs, wear of the cup does occur clinically, although it is much lower than that against metallic heads [1,2]. That cup 4 was the only one from which no wear particles were removed may be because head 4, unlike the other four heads, had a sintering defect, a circumferential equatorial groove approximately 1 pm deep. The groove may have improved lubricant entrapment during the non-loaded swing phase of the gait cycle, and consequently lubrication during the stance phase. Intentional grooves or dimples may be worth studying in future. The subject is of present interest as metal-on-metal articulation is being re-examined in many laboratories. Such articulation requires good lubrication to perform properly. The amount of wear debris needed to cause adverse tissue reactions is not known. Probably the average production rate of UHMWPE debris measured in the present study, 4 mg year-‘, would not be detrimental in the human body, because in a radiographic study on the penetration of femoral heads into acetabular cups in Mtiller prostheses, only penetration rates exceeding 0.2 mm year-’ contributed to loosening [7]. A penetration rate of 0.2 mm year-’ can be converted into a production rate of UHMWPE debris of 150 mg year-‘. So far as the author knows, there are no studies available yet on the penetration of zirconia femoral heads into UHMWPE cups in the human body, or on the internal volume changes of UHMWPE cups worn against zirconia heads and removed from patients, that could be compared with the simulator data. In a water-lubricated, reciprocating pin-on-flat study done by the author [22], the wear rate of UHMWPE
211
pins was extremely low against both alumina (Vitox) and zirconia (Zyranox) plates, both manufactured by Morgan Matroc Ltd, but clearly higher against Co-Cr-Mo plates (Ho~edi~a’s Vitallium). In that study, the coefficient of friction of UHMWPE against zirconia was about 20% higher than that against alumina. However, the pin-on-flat study and the present hip joint simulator study are not readily comparable, mainly because different alumina and zirconia materials were used, and test conditions also differed. In the other re~ipr~ating and unidirectional pin-on-flat study 1201, the wear rate of UHM~E was significantly lower against zirconia (manufactured by Kyocera Corp.) than against alumina, which is in contrast to the pin-on-flat and hip joint simulator observations of the present author. In a frictional study done by the author, the zirconia femoral head in question generated slightly higher frictional torque against an UHMWPE cup than an alumina head [12]. The scatter in the wear results is substantial (Fig. 2, Table 3). Care was naturally taken to perform five identical tests and so no unequivocal explanation for the scatter can be presented. There was slight variation in original clearance, and in deviation from roundness and surface roughness of the heads, but their correlation with wear was poor. In general, considerable scatter, even over several orders of magnitude, is not uncommon in wear tests, which is due not only to environmental and material property variations, but also to the inherent stochastic feature of the wear process [23]. In clinical penetration rates, scatter over two orders of magnitude is common [5,16]. The scatter in clinical penetration rates is not likely to be attributable to the differences in conditions between patients, because in the study of prostheses removed from patients, poor correlation was found between penetration rate, and femoral head roughness, patient mass and age [5]. The upside-down position of the prosthesis is necessary to avoid the drying out of the articulation that trapped air bubbles would cause if the prosthesis were not inverted. In friction tests done with the singlestation simulator [12], in which the cup is located above the head, air bubbles had to be removed prior to measurements. The removal was not too difficult, but long-term wear tests are much more demanding in this respect, because air bubbles tend to gather in the air pocket during the articulation, if the cup is above the head. The risk of drying out must be eliminated, because drying out would result in rapid overheating which would ruin the specimens and the test. This was learned in early system tests of the present simulator: drying out happened once as the water supply failed inadvertently. However, inversion of the prosthesis decreases the possibility of entrapment of particles with density less than that of water, and increases the possibility of entrapment of particles with density higher than
212
I*!
Suikko 1 Wear 176 (1994) 207-212
that of water. This may somewhat reduce the veracity of the simulation.
5. Conclusions The wear rate of UHMWPE acetabular cups that articulated against experimental zirconia femoral heads in a hip joint simulator proved very low, the mean value being 4.1 mg per one million cycles, which can be estimated to correspond to a clinical wear rate of 5 ,um year-‘, the wear factor being 8 x lo-’ mm3 N-’ m -I. Therefore, zirconia seems to be tribologically suitable for the modular femoral head of the total replacement hip joints.
Acknowledgments
The author thanks the Academy of Finland, Nordic Industrial Fund, Instrumentarium Science Foundation, and Yrjii and Senja Koivunen Foundation for financial support, Howmedica - a Division of Pfizer Oy Finland for the donation of the acetabular cups, and Mr I. Pajamski and Mr T. Koskenneva for the skilful machining of the specimen holders.
References 111 H. Oonishi, Y. Takayaka, I.C. Clarke and H. Jung, Cornparative wear studies of 28-mm ceramic and stainless steel total hip joints over 2 to 7 year period, J. Long-Term Eficts of Medical Implants, 2 (1992) 37-47. VI H.M. Schiiller and R.K. Marti, Ten-year socket wear in 66 hip arthroplasties. Ceramic versus metal heads,Acta Orthop. &and., 61 (1990) 240-243. [31 H.C. Amstutz, P. Campbell, N. Kossovsky and I.C. Clarke, Mechanism and clinical significance of wear debris-induced osteolysis, Clin. Orthop., 276 (1992) 7-18. I41 K.R. St. John (ed.), Particulate debris from medical implants: mechanisms of formation and biological consequences, ASTM Spec. Tech. Publ. 1144, ASTM, Philadelphia, PA, 1992. [51 G.H. Isaac, B.M. Wroblewski, J.R. Atkinson and D. Dowson, A tribological study of retrieved hip prostheses, Clin. Orthop., 276 (1992) 115-125. [61 B.M. Wroblewski, 15-Zl-year results of the Charnley lowfriction arthroplasty, Clin. Orthop., 211 (1986) 30-35. [71 U. Buchhorn, H.-G. Willert, M. Semlitsch and H. Weber, Changes in the dimensions of polyethylene acetabuli in total hip replacements after Miiller,Z. Orthop., 122 (1984) 127-135 (in German). PI V. Saikko, P. Paavolainen, M. Kleimola and P. Slstis, A five-station hip joint simulator for wear rate studies, J. Eng. Med., 206 (1992) 195-200.
and P. Slitis, Wear of the [91 V. Saikko, P. Paavolainen polyethylene acetabular cup. Metallic and ceramic heads compared in a hip simulator, Acra Orthop. &and.. 64 (1993) 391-402. WI V. Saikko, Wear of polyethylene acetabular cups against alumina femoral heads. 5 prostheses compared in a hip simulator for 35 million walking cycles, Acta Orthop. Stand., 64 (1993) 507-512. [111 V. Saikko, Tribology of total replacement hip joints studied with new hip joint simulators and a materials-screening apparatus, Acta Polytech. Stand., Mech. Eng. Series No. 110, Helsinki 1993, 44 pp (Doctoral dissertation). [=I V. Saikko, A simulator study of friction in total replacement hip joints, L Eng. Med., 206 (1992) 201-211. Standard IS0 7206-2, Implants for surgery 1131 International -Partial and total hip joint prostheses-Part 2: Bearing surfaces made of metallic and plastics materials, Intemational Organization for Standardization, 1987. P41 B.B. Seedhom and N.C. Wallbridge, Walking activities and wear of prostheses, Ann. Rheum. Dk., 44 (1985) 838-843. Standard IS0 5834, Implants for sur1151 International gery-ultra-high molecular weight polyethylene-Part 2: Moulded forms, International Organization for Standardization, 1985. J.R. Atkinson, D. Dowson, G.H. Isaac and B.M. Wroblewski, PI Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints III: The measurement of internal volume changes in explanted Charnley sockets after 2-16 years in viva and the determination of wear factors, Wear, 104 (1985) 225-244. P71 H.A. McKellop and I.C. Clarke, Degradation and wear of ultra-high-molecular-weight polyethylene, in A.C. Fraker and C.D. Griffin (eds.), Corrosion and degradation of implant materials: second symposium, ASTM Spec. Tech. Publ. 859, ASTM, Philadelphia, PA, 1985, pp. 351-368. WI J.R. Atkinson, D. Dowson, G.H. Isaac and B.M. Wroblewski, Laboratory wear tests and clinical observations of the penetration of femoral heads into acetabular cups in total replacement hip joints II: A microscopical study of the surfaces of Charnley polyethylene acetabular sockets, Wear, 104 (1985) 217-224. P91 H. McKellop, B. Lu and P. Benya, Friction, lubrication and wear of cobalt+hromium, alumina and zirconia hip prostheses compared on a joint simulator, 38th Ann. Meet., Orthopaedic Research Society, February 17-20, 1992, Washington, DC. PO1 P. Kumar, M. Oka, K. Ikeuchi, K. Shimizu, T. Yamamuro, H. Okumura and Y. Kotoura, Low wear rate of UHMWPE against zirconia ceramic (Y-PSZ) in comparison to alumina ceramic and SUS 316L alloy, J. Biomed. Mater. Res., 25 (1991) 813-828. VI H. Oonishi, Studies on the shape and contour of the metal prosthetic head in total hip prosthesis, Proc. Congr. Inf. Society of Orthopaedic Surger)’ and Traumatology, 1972. pp. 107-123. V. Saikko, Wear and friction properties of prosthetic joint PI materials evaluated on a reciprocating pin-on-flat apparatus, Wear, 166 (1993) 169-178. v31 N.C. Wallbridge and D. Dowson, Distribution of wear rate data and a statistical approach to sliding wear theory, Wear, 119 (1987) 295-312.