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Contamination of polyethylene cups with polymethyl methacrylate particles
The Journal of Arthroplasty Vol. 16 No. 7 2001
Contamination of Polyethylene Cups With Polymethyl Methacrylate Particles An Experimental Study Uldis Kesteris, MD, PhD,* Lennart Carlsson, PhD,† Conny Haraldsson, PhD,† ¨ nnerfa¨lt, MD, PhD,* Jukka Lausmaa, PhD,† Lars Lidgren, MD, PhD,* Rolf O and Hans Wingstrand, MD, PhD*
Abstract: The articulating surfaces of 6 ultra– high molecular weight polyethylene cups were exposed to curing polymethyl methacrylate (PMMA) bone– cement and examined with scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Three of the cups were exposed to blood and bone– cement, and the rest were exposed to bone– cement only. After removal of the bone– cement bulk, PMMA particles were found and identified in all 6 cups. The particles were verified by identifying zirconium with energy-dispersive x-ray fluorescence spectroscopy in 5 cups and with LA-ICPMS in 1 cup. The degree of surface contamination was estimated with LA-ICPMS. The number of zirconium-containing particles detected was on average 10 to 20/mm2. PMMA bone– cement left in polyethylene cups during polymerization can contaminate the articulating surface with adherent PMMA particles. Key words: polymethyl methacrylate, third body contamination, total hip arthroplasty.
One of the wear mechanisms in total hip arthroplasty is third-body abrasive wear [1,2,3,4]. It occurs when a third-body agent (debris of bone, cement, metal, or hydroxyapatite) causes direct damage on the articulating surfaces of the components, generates wear debris, and roughens the articulating surfaces. This wear may increase poly-
ethylene wear even if the third-body agent is no longer present between the articulating surfaces. Polymethyl methacrylate (PMMA) debris is one of these third-body agents. Embedded PMMA debris has been found in retrieved ultra– high molecular weight polyethylene (UHMWPE) cups, and increased surface roughness, caused by a third-body agent, has been found on retrieved femoral components [3,4]. The abrasive effect of particulate bone– cement causing roughening of articulating surfaces has been observed in pin-on-plate wear tests in vitro, in which particulate bone– cement was added to a tribologic system [3,5,6], showing that the presence of the PMMA debris between counterfaces can lead to third-body wear. Issac et al [5] suggested that clusters of barium or zirconium particles in the cement caused the abrasions. Analysis of the articulating surfaces of retrieved implants has shown that most of the PMMA particle ingress occurs during the early stage of the service
From the Department of Orthopaedics, University Hospital, Lund; and †SP, Swedish National Testing and Research Institute, Borås, Sweden. Submitted December 13, 1999; accepted April 12, 2001. Funds were received in partial or total support of the research material described in this article from Lunds sjukvårdsdistrikt; Science and Technology Procordia AB; Stiftelsen fo¨r Bistånd åt Vanfo¨ra i Skåne; Swedish Foundation for Strategic Research; Swedish Medical research Council project 09509; and The Medical Faculty, University of Lund, Sweden. Reprint requests: U. Kesteris, MD, Department of Orthopedics, Lund University Hospital, S-221 85 Lund, Sweden. Copyright © 2001 by Churchill Livingstone威 0883-5403/01/1607-0014$35.00/0 doi:10.1054/arth.2001.25554
905
906 The Journal of Arthroplasty Vol. 16 No. 7 October 2001 life [7] or in some cases already at the time of surgery [3]. The way in which the PMMA particles become entrapped between the articulating surfaces is still a matter of conjecture, however. We investigated to determine whether exposure of UHMWPE acetabular cups to curing PMMA bone– cement results in contamination of the articulating surface.
Materials and Methods PMMA bone– cement (Palacos, Shering-Plough) was mixed in the Optivac (ScandiMed, Sjo¨bo, Sweden) vacuum mixing system in a standardized manner and brought into 6 acetabular cups (ScandiMed). The cups were made from the same batch, consecutively machined from the same UHMWPE rod, and had the same size. Palacos bone– cement is PMMA with the following additives: radiopacifier (zirconium oxide; 15% of powder; particle size, 1–30 m); antibiotics (gentamicin sulfate; 2.1% of powder); color pigment (chlorophyllin; 0.02% of powder and 0.02% of liquid); and powder stabiliser (benzoyl peroxide; 0.8% of powder) [8]. To resemble perioperative conditions to some extent, 1 mL of blood was poured into 3 of the cups before cementing. The rest of the cups were exposed to bone– cement only. Each cup was marked with a number, and each set was marked with A or B. The cement was prechilled at 4°C and was brought into the cups 3 minutes after mixing. Slight thumb pressure was applied on the cement mass to obtain a close contact with the cup surfaces. The cups were placed in a warming cupboard at 37°C and were kept there for 15 minutes (ie, until the cement was completely hardened). Then the cement blocks were removed, and the cups were subjected to analysis, before which each cup was sawed into 3 parts. The specimen surfaces were blown with a dry air jet to remove loose particles, then sputter-coated with a conductive film of carbon a few hundred angstroms thick. At least 5 randomly chosen areas sized 5 mm ⫻ 5 mm on each specimen were investigated by scanning electron microscope (SEM) (SEM 5800 JEOL, Akishima, Tokyo, Japan). The SEM was operated in the secondary electron imaging mode at a beam voltage of 20 kV. The elemental composition of detected debris was analyzed with a Link ISIS energy-dispersive x-ray system (Oxford Instruments Analytical Ltd., High Wycombe, Bucks, England) in the SEM. The PMMA debris was verified by identifying the presence of zirconium in the respective particles. The SEM analysis was performed by 1 of the authors
(L. C.), without prior knowledge of which of the cups had been exposed to blood and which had not. The degree of surface contamination was estimated quantitatively by detection of zirconiumcontaining particles with laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Six randomly chosen areas, 1 mm2 in size on the cement-exposed inside surfaces and on 6 cutthrough unexposed polyethylene surfaces of the same samples, were analyzed. The cut-through unexposed surfaces were analyzed as controls to exclude the presence zirconium in the UHMWPE. Two cups, one with blood and one without blood, were chosen. Zirconium was detected by its main isotope at atomic mass 90. This mass was crosschecked against other isotopes of zirconium to exclude interference from other elements. The results obtained from the articulating surfaces of the cups were compared with results from the cut-through surfaces of the UHMWPE.
Results In all cases, the cement bulks were fixed firmly into the cups but could be removed without difficulties. Fig. 1 shows a typical SEM appearance of the articulating surface of a cup. The topography of the cup surfaces was dominated by regular machining grooves appearing with a periodicity of approximately 0.05 mm. Varying amounts of debris were found on all 6 cups. All cups had nearly the same visual appearance and degree of surface contamination.
Fig. 1. Scanning electron microscope image of the articulating surface of an ultra– high molecular weight polyethylene cup. Regular machining grooves with a large number of particles are seen.
Contamination of Polyethylene Cups •
Kesteris et al.
907
The SEM analysis with energy-dispersive x-ray system revealed that most of the particles were UHMWPE debris, and only few of the particles were of PMMA origin. Large amounts of particles that consisted of coagulated blood could be detected on the cups that were exposed to blood before cementing. The presence of zirconium was verified by energy-dispersive x-ray system in 5 cups. The size of the particles varied from ⬍1 m to almost 10 m (Fig. 2). The cup in which debris containing zirconium could not be verified with energy-dispersive x-ray system was 1 of the 3 exposed to blood before cementing. This cup was analyzed further with LAICPMS, which verified the presence of zirconium on the surface. Analysis of the cut-through unexposed UHMWPE surface with LA-ICPMS detected no zirconium (Fig. 3A). In contrast, numerous zirconium signal bursts were detected on the articulating surfaces of the 2 cups that were analyzed with LA-ICPMS (Fig. 3B). Assuming that each zirconium peak during the laser scan represents an individual PMMA particle, on average 10 to 20 particles per mm2 could be detected.
Discussion The results show that PMMA bone– cement bulk, if left in UHMWPE cups during curing, can contaminate the articulating surfaces with PMMA particles. In surgery, such situations can occur during cementing of either component. Superfluous cement can flow into the cup when pressing the cup into
Fig. 2. Scanning electron microscope image of a bonecement particle.
Fig. 3. (A) Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of a cut-through unexposed ultra– high molecular weight polyethylene surface. No zirconium (Zr) signal bursts are identified. (B) LA-ICPMS analysis of a contaminated ultra– high molecular weight polyethylene surface. Each peak corresponds to a zirconium particle. (Note the difference of the scale on vertical axis of the diagrams.)
acetabulum, a thin cement layer may become entrapped between the cup holder and the cup during curing, or a bigger bulk of cement can get into the cup when the femoral stem is pushed into the medullary cavity. It appears that in a situation when PMMA is in good contact with the polyethylene surface during curing, the risk of contamination is high whether blood is present in the cup or not. Surface contamination with great amounts of UHMWPE debris, which was observed on all samples, probably occurred during the preparation of the samples when cups were cut in pieces.
908 The Journal of Arthroplasty Vol. 16 No. 7 October 2001 In prosthetic hip joints, apart from direct surface contamination, there may be other possible ways in which the PMMA debris could get into the articulating surfaces of cups. Particles may be left in surrounding tissues during the removal of excessive cement, or they may become detached from the remaining bulk of cement in the course of time. The consequences of PMMA contamination may be early third-body wear and early damage (eg, roughening of the articulating surfaces), which may influence the total wear and survival of the implant. Cups that have been contaminated with PMMA during surgery may wear out faster because they have a poor start with early third-body wear. This could be an explanation for the wide range of polyethylene wear observed in vivo [9 –12]. Ranges of linear wear, from hundredths of a millimeter to one tenth of a millimeter per year, can be measured in matched patient groups and in patients operated on with the same type of prosthesis [11,13]. These ranges suggest that apart from the load, contact stress, and sliding distance there may be other factors influencing wear, especially in the initial postoperative stage. It is not known for how long PMMA particles can act as a third-body agent. The particles may be washed out from the joint space early or later during service when the cup is worn to the extent that allows more play between the head of the femoral component and the cup. PMMA particles have been found in the soft tissue surrounding implants [14], and obvious signs of third-body damage, but without presence of any embedded PMMA debris, have been found in retrieved cups [3]. Alternatively the PMMA particles may stay fixed firmly on the polyethylene surface and become embedded gradually by the pressure of the prosthetic head. McKellop et al [15] and Isaac et al [4] found embedded PMMA particles in retrieved UHMWPE cups. During surgery, the PMMA contamination should be avoided by preventing the bone– cement from getting into the cup.
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4.
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9. 10.
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References 15. 1. Davidson JA, Poggie RA, Mishra AK: Abrasive wear of ceramic, metal, and UHMWPE bearing surfaces
from third body bone, PMMA bone cement, and titanium debris. Biomed Mater Eng 4:213, 1994 McKellop H: Wear modes, mechanisms, damage and debris: separating cause from effect in the wear of total hip replacements. p. 21. In Galante JO, Rosenberg AG, Callaghan JJ (eds). Total hip revision surgery. Raven Press, New York, 1995 p. 21. Isaac GH, Atkinson JR, Dowson D, Wroblewski BM: The role of cement in long term performance and premature failure of Charnley low friction arthroplasties. Eng Med 15:19, 1986 Isaac GH, Wroblewski BM, Atkinson JR, Dowson D: A tribological study of retrieved hip prostheses. Clin Orthop 276:115, 1992 Isaac GH, Atkinson JR, Dowson D, et al: The causes of femoral head roughening in explanted Charnley hip prostheses. Eng Med 16:167, 1987 Caravia L, Dowson D, Fisher J, Jobbins B: The influence of bone and bone cement debris on counterface roughness in sliding wear tests of ultra-high molecular weight polyethylene on stainless steel. Proc Inst Mech Eng [H] 204:65, 1990 Hall RM, Unsworth A, Siney P, Wroblewski BM: Wear in retrieved Charnley acetabular sockets. Proc Inst Mech Eng [H] 210:197, 1996 Ku¨hn KD: Bone cements: up-to-date comparison of physical and chemical properties of commercial materials. Springer, Berlin, Heideberg, 2000 Charnley J, Halley DK: Rate of wear in total hip replacement. Clin Orthop 112:170, 1975 Livermore JT, IIstrup D, Morrey B: The effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am 72:518, 1990 ¨ nnerfa¨lt R: Kesteris U, IIchmann T, Wingstrand H, O Polyethylene wear in Scanhip arthroplasty with a 22 or 32 mm head. Acta Orthop Scand 67:125, 1996 Sochart DH: Relationship of acetabular wear to osteolysis and loosening in total hip arthroplasty. Clin Orthop 363:135, 1999 ¨ nsten I, Carlsson ÅS, Besjakov J: Wear in unceO mented porous and cemented polyethylene sockets: a randomised, radiostereometric study. J Bone Joint Surg Br 80:345, 1998 Willert HG, Semlitsch M: Reactions of the articular capsule to wear products of artifical joint prosthesis. J Biomed Mater Res 11:157, 1977 McKellop HA, Sarmiento A, Schwinn CP, Ebramzadeh E: In vivo wear of titanium-alloy hip prostheses. J Bone Joint Surg Am 72:512, 1990
Contamination of Polyethylene Cups With Polymethyl Methacrylate Particles An Experimental Study Uldis Kesteris, MD, PhD,* Lennart Carlsson, PhD,† Conny Haraldsson, PhD,† ¨ nnerfa¨lt, MD, PhD,* Jukka Lausmaa, PhD,† Lars Lidgren, MD, PhD,* Rolf O and Hans Wingstrand, MD, PhD*
Abstract: The articulating surfaces of 6 ultra– high molecular weight polyethylene cups were exposed to curing polymethyl methacrylate (PMMA) bone– cement and examined with scanning electron microscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Three of the cups were exposed to blood and bone– cement, and the rest were exposed to bone– cement only. After removal of the bone– cement bulk, PMMA particles were found and identified in all 6 cups. The particles were verified by identifying zirconium with energy-dispersive x-ray fluorescence spectroscopy in 5 cups and with LA-ICPMS in 1 cup. The degree of surface contamination was estimated with LA-ICPMS. The number of zirconium-containing particles detected was on average 10 to 20/mm2. PMMA bone– cement left in polyethylene cups during polymerization can contaminate the articulating surface with adherent PMMA particles. Key words: polymethyl methacrylate, third body contamination, total hip arthroplasty.
One of the wear mechanisms in total hip arthroplasty is third-body abrasive wear [1,2,3,4]. It occurs when a third-body agent (debris of bone, cement, metal, or hydroxyapatite) causes direct damage on the articulating surfaces of the components, generates wear debris, and roughens the articulating surfaces. This wear may increase poly-
ethylene wear even if the third-body agent is no longer present between the articulating surfaces. Polymethyl methacrylate (PMMA) debris is one of these third-body agents. Embedded PMMA debris has been found in retrieved ultra– high molecular weight polyethylene (UHMWPE) cups, and increased surface roughness, caused by a third-body agent, has been found on retrieved femoral components [3,4]. The abrasive effect of particulate bone– cement causing roughening of articulating surfaces has been observed in pin-on-plate wear tests in vitro, in which particulate bone– cement was added to a tribologic system [3,5,6], showing that the presence of the PMMA debris between counterfaces can lead to third-body wear. Issac et al [5] suggested that clusters of barium or zirconium particles in the cement caused the abrasions. Analysis of the articulating surfaces of retrieved implants has shown that most of the PMMA particle ingress occurs during the early stage of the service
From the Department of Orthopaedics, University Hospital, Lund; and †SP, Swedish National Testing and Research Institute, Borås, Sweden. Submitted December 13, 1999; accepted April 12, 2001. Funds were received in partial or total support of the research material described in this article from Lunds sjukvårdsdistrikt; Science and Technology Procordia AB; Stiftelsen fo¨r Bistånd åt Vanfo¨ra i Skåne; Swedish Foundation for Strategic Research; Swedish Medical research Council project 09509; and The Medical Faculty, University of Lund, Sweden. Reprint requests: U. Kesteris, MD, Department of Orthopedics, Lund University Hospital, S-221 85 Lund, Sweden. Copyright © 2001 by Churchill Livingstone威 0883-5403/01/1607-0014$35.00/0 doi:10.1054/arth.2001.25554
905
906 The Journal of Arthroplasty Vol. 16 No. 7 October 2001 life [7] or in some cases already at the time of surgery [3]. The way in which the PMMA particles become entrapped between the articulating surfaces is still a matter of conjecture, however. We investigated to determine whether exposure of UHMWPE acetabular cups to curing PMMA bone– cement results in contamination of the articulating surface.
Materials and Methods PMMA bone– cement (Palacos, Shering-Plough) was mixed in the Optivac (ScandiMed, Sjo¨bo, Sweden) vacuum mixing system in a standardized manner and brought into 6 acetabular cups (ScandiMed). The cups were made from the same batch, consecutively machined from the same UHMWPE rod, and had the same size. Palacos bone– cement is PMMA with the following additives: radiopacifier (zirconium oxide; 15% of powder; particle size, 1–30 m); antibiotics (gentamicin sulfate; 2.1% of powder); color pigment (chlorophyllin; 0.02% of powder and 0.02% of liquid); and powder stabiliser (benzoyl peroxide; 0.8% of powder) [8]. To resemble perioperative conditions to some extent, 1 mL of blood was poured into 3 of the cups before cementing. The rest of the cups were exposed to bone– cement only. Each cup was marked with a number, and each set was marked with A or B. The cement was prechilled at 4°C and was brought into the cups 3 minutes after mixing. Slight thumb pressure was applied on the cement mass to obtain a close contact with the cup surfaces. The cups were placed in a warming cupboard at 37°C and were kept there for 15 minutes (ie, until the cement was completely hardened). Then the cement blocks were removed, and the cups were subjected to analysis, before which each cup was sawed into 3 parts. The specimen surfaces were blown with a dry air jet to remove loose particles, then sputter-coated with a conductive film of carbon a few hundred angstroms thick. At least 5 randomly chosen areas sized 5 mm ⫻ 5 mm on each specimen were investigated by scanning electron microscope (SEM) (SEM 5800 JEOL, Akishima, Tokyo, Japan). The SEM was operated in the secondary electron imaging mode at a beam voltage of 20 kV. The elemental composition of detected debris was analyzed with a Link ISIS energy-dispersive x-ray system (Oxford Instruments Analytical Ltd., High Wycombe, Bucks, England) in the SEM. The PMMA debris was verified by identifying the presence of zirconium in the respective particles. The SEM analysis was performed by 1 of the authors
(L. C.), without prior knowledge of which of the cups had been exposed to blood and which had not. The degree of surface contamination was estimated quantitatively by detection of zirconiumcontaining particles with laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Six randomly chosen areas, 1 mm2 in size on the cement-exposed inside surfaces and on 6 cutthrough unexposed polyethylene surfaces of the same samples, were analyzed. The cut-through unexposed surfaces were analyzed as controls to exclude the presence zirconium in the UHMWPE. Two cups, one with blood and one without blood, were chosen. Zirconium was detected by its main isotope at atomic mass 90. This mass was crosschecked against other isotopes of zirconium to exclude interference from other elements. The results obtained from the articulating surfaces of the cups were compared with results from the cut-through surfaces of the UHMWPE.
Results In all cases, the cement bulks were fixed firmly into the cups but could be removed without difficulties. Fig. 1 shows a typical SEM appearance of the articulating surface of a cup. The topography of the cup surfaces was dominated by regular machining grooves appearing with a periodicity of approximately 0.05 mm. Varying amounts of debris were found on all 6 cups. All cups had nearly the same visual appearance and degree of surface contamination.
Fig. 1. Scanning electron microscope image of the articulating surface of an ultra– high molecular weight polyethylene cup. Regular machining grooves with a large number of particles are seen.
Contamination of Polyethylene Cups •
Kesteris et al.
907
The SEM analysis with energy-dispersive x-ray system revealed that most of the particles were UHMWPE debris, and only few of the particles were of PMMA origin. Large amounts of particles that consisted of coagulated blood could be detected on the cups that were exposed to blood before cementing. The presence of zirconium was verified by energy-dispersive x-ray system in 5 cups. The size of the particles varied from ⬍1 m to almost 10 m (Fig. 2). The cup in which debris containing zirconium could not be verified with energy-dispersive x-ray system was 1 of the 3 exposed to blood before cementing. This cup was analyzed further with LAICPMS, which verified the presence of zirconium on the surface. Analysis of the cut-through unexposed UHMWPE surface with LA-ICPMS detected no zirconium (Fig. 3A). In contrast, numerous zirconium signal bursts were detected on the articulating surfaces of the 2 cups that were analyzed with LA-ICPMS (Fig. 3B). Assuming that each zirconium peak during the laser scan represents an individual PMMA particle, on average 10 to 20 particles per mm2 could be detected.
Discussion The results show that PMMA bone– cement bulk, if left in UHMWPE cups during curing, can contaminate the articulating surfaces with PMMA particles. In surgery, such situations can occur during cementing of either component. Superfluous cement can flow into the cup when pressing the cup into
Fig. 2. Scanning electron microscope image of a bonecement particle.
Fig. 3. (A) Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis of a cut-through unexposed ultra– high molecular weight polyethylene surface. No zirconium (Zr) signal bursts are identified. (B) LA-ICPMS analysis of a contaminated ultra– high molecular weight polyethylene surface. Each peak corresponds to a zirconium particle. (Note the difference of the scale on vertical axis of the diagrams.)
acetabulum, a thin cement layer may become entrapped between the cup holder and the cup during curing, or a bigger bulk of cement can get into the cup when the femoral stem is pushed into the medullary cavity. It appears that in a situation when PMMA is in good contact with the polyethylene surface during curing, the risk of contamination is high whether blood is present in the cup or not. Surface contamination with great amounts of UHMWPE debris, which was observed on all samples, probably occurred during the preparation of the samples when cups were cut in pieces.
908 The Journal of Arthroplasty Vol. 16 No. 7 October 2001 In prosthetic hip joints, apart from direct surface contamination, there may be other possible ways in which the PMMA debris could get into the articulating surfaces of cups. Particles may be left in surrounding tissues during the removal of excessive cement, or they may become detached from the remaining bulk of cement in the course of time. The consequences of PMMA contamination may be early third-body wear and early damage (eg, roughening of the articulating surfaces), which may influence the total wear and survival of the implant. Cups that have been contaminated with PMMA during surgery may wear out faster because they have a poor start with early third-body wear. This could be an explanation for the wide range of polyethylene wear observed in vivo [9 –12]. Ranges of linear wear, from hundredths of a millimeter to one tenth of a millimeter per year, can be measured in matched patient groups and in patients operated on with the same type of prosthesis [11,13]. These ranges suggest that apart from the load, contact stress, and sliding distance there may be other factors influencing wear, especially in the initial postoperative stage. It is not known for how long PMMA particles can act as a third-body agent. The particles may be washed out from the joint space early or later during service when the cup is worn to the extent that allows more play between the head of the femoral component and the cup. PMMA particles have been found in the soft tissue surrounding implants [14], and obvious signs of third-body damage, but without presence of any embedded PMMA debris, have been found in retrieved cups [3]. Alternatively the PMMA particles may stay fixed firmly on the polyethylene surface and become embedded gradually by the pressure of the prosthetic head. McKellop et al [15] and Isaac et al [4] found embedded PMMA particles in retrieved UHMWPE cups. During surgery, the PMMA contamination should be avoided by preventing the bone– cement from getting into the cup.
2.
3.
4.
5.
6.
7.
8.
9. 10.
11.
12.
13.
14.
References 15. 1. Davidson JA, Poggie RA, Mishra AK: Abrasive wear of ceramic, metal, and UHMWPE bearing surfaces
from third body bone, PMMA bone cement, and titanium debris. Biomed Mater Eng 4:213, 1994 McKellop H: Wear modes, mechanisms, damage and debris: separating cause from effect in the wear of total hip replacements. p. 21. In Galante JO, Rosenberg AG, Callaghan JJ (eds). Total hip revision surgery. Raven Press, New York, 1995 p. 21. Isaac GH, Atkinson JR, Dowson D, Wroblewski BM: The role of cement in long term performance and premature failure of Charnley low friction arthroplasties. Eng Med 15:19, 1986 Isaac GH, Wroblewski BM, Atkinson JR, Dowson D: A tribological study of retrieved hip prostheses. Clin Orthop 276:115, 1992 Isaac GH, Atkinson JR, Dowson D, et al: The causes of femoral head roughening in explanted Charnley hip prostheses. Eng Med 16:167, 1987 Caravia L, Dowson D, Fisher J, Jobbins B: The influence of bone and bone cement debris on counterface roughness in sliding wear tests of ultra-high molecular weight polyethylene on stainless steel. Proc Inst Mech Eng [H] 204:65, 1990 Hall RM, Unsworth A, Siney P, Wroblewski BM: Wear in retrieved Charnley acetabular sockets. Proc Inst Mech Eng [H] 210:197, 1996 Ku¨hn KD: Bone cements: up-to-date comparison of physical and chemical properties of commercial materials. Springer, Berlin, Heideberg, 2000 Charnley J, Halley DK: Rate of wear in total hip replacement. Clin Orthop 112:170, 1975 Livermore JT, IIstrup D, Morrey B: The effect of femoral head size on wear of the polyethylene acetabular component. J Bone Joint Surg Am 72:518, 1990 ¨ nnerfa¨lt R: Kesteris U, IIchmann T, Wingstrand H, O Polyethylene wear in Scanhip arthroplasty with a 22 or 32 mm head. Acta Orthop Scand 67:125, 1996 Sochart DH: Relationship of acetabular wear to osteolysis and loosening in total hip arthroplasty. Clin Orthop 363:135, 1999 ¨ nsten I, Carlsson ÅS, Besjakov J: Wear in unceO mented porous and cemented polyethylene sockets: a randomised, radiostereometric study. J Bone Joint Surg Br 80:345, 1998 Willert HG, Semlitsch M: Reactions of the articular capsule to wear products of artifical joint prosthesis. J Biomed Mater Res 11:157, 1977 McKellop HA, Sarmiento A, Schwinn CP, Ebramzadeh E: In vivo wear of titanium-alloy hip prostheses. J Bone Joint Surg Am 72:512, 1990