JBJS | Commentary - Design Issues in Clinical Studies of the in Vivo Volumetric Wear Rate of Polyethylene Bearing Components*

The Journal of Bone & Joint Surgery, Volume 82, Issue 2

Commentary | February 01, 2000

Commentary - Design Issues in Clinical Studies of the in Vivo Volumetric Wear Rate of Polyethylene Bearing Components*

GLADIUS LEWIS, Ph.D.

The Journal of Bone & Joint Surgery. 2000; 82:281-87

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Introduction

Introduction | References

The issue of wear of ultra-high molecular weight polyethylene bearing surfaces in total hip and knee replacements is, without doubt, one of the dominant themes in contemporary orthopaedics. Thus, an array of relevant facets of this topic have been extensively and intensively researched, spawning, in the process, a very large volume of literature.

The main topics that have been studied are the place of wear in the hierarchy of degradation modes of polyethylene bearing surfaces, the role of polyethylene wear particles in the in vivo longevity of the prosthesis, the methods of determining the extent of wear of polyethylene bearing components (for example, the acetabular cup, acetabular cup liner, or tibial insert), the magnitude of the volumetric wear rate, the relationship between the results of in vitro wear-testing of polyethylene specimens or components and the clinical performance of polyethylene components, and the key factors influencing the clinical wear of polyethylene components.

It is widely acknowledged that wear is one of many degradation modes that affect polyethylene bearing surfaces in vivo (other modes include creep, delamination, and burnishing) and that, under certain circumstances, it is not the predominant one. For example, polyethylene creep of the liner predominates during the initial stages of penetration of the femoral head into the acetabular cup, but additional penetration after the first twelve to eighteen months in situ is due to wear²⁴.

A consensus has emerged that wear of polyethylene bearing components is implicated in the aseptic loosening of hip and knee implants, and in the development of osteolysis around those implants, through resorption of the periprosthetic bone initiated by a foreign-body-giant-cell granulomatous tissue reaction. This biological reaction arises in response to the polyethylene and other wear particles (such as acrylic cement and metal) that are released into the periprosthetic bone. Furthermore, in the case of hip implants, wear of the polyethylene acetabular component may have other consequences; for example, stress in the cup liner may increase as a result of liner eccentricity that, in turn, arises from gross movement of the femoral head into the liner.

The current so-called gold standard for determining the linear wear of retrieved polyethylene acetabular components is the method of Livermore et al.¹⁷, although newer methods, such as the three-dimensional method of Ramakrishnan et al.²² and the digital edge-detection method of Shaver et al.²⁶, have been presented. The method of Livermore et al. essentially involves six steps. First, the center of the femoral head is located on an anteroposterior radiograph of the pelvis, made two to four months postoperatively, with use of a transparent overlay with a series of concentric circles with very small increments in radial length (usually one millimeter). Second, a magnification factor is calculated as the ratio of the actual diameter of the femoral head to its apparent radiographic diameter. Third, on the most recent radiograph, a line indicating the shortest distance from the top of the cup to the center of the femoral head is drawn, which allows the so-called line of greatest wear (a unique distance between the top and bottom surfaces of the cup) to be determined. Fourth, all measurements are corrected by multiplying by the magnification factor. Fifth, the line of greatest wear is located on the initial postoperative radiograph and then measured and corrected for magnification (as before, with use of a transparent overlay of concentric circles over the femoral head). Sixth, the linear wear of the polyethylene is calculated as the difference between the corrected lengths of the line of greatest wear on the two radiographs. The methods of Livermore et al.¹⁷, Ramakrishnan et al.²², and Shaver et al.²⁶ share two main drawbacks. First, they use only plane radiographs and are thus affected by variability in the positioning of the patient. Second, a formula must be used in order to convert the linear wear (L) results to volumetric wear (v) values. The formula that is universally used is v = Lpr², in which r is the radius of the femoral head. The difficulty with this relationship is that it is predicated on the assumption that there is a single cylindrical wear track, which has been shown not to be the case³⁴. With the use of alternative approaches, such as the fluid-film-displacement¹³ and three-dimensional⁵ methods, the volumetric wear of the polyethylene acetabular component is determined directly.

In studies of hip implants, the volumetric wear rate of the polyethylene component has ranged from fifteen to 860 cubic millimeters per year, reflecting the wide variety of conditions (such as patient-related factors, design features, and polyethylene characteristics) reported in clinical studies²⁴.

A proliferation of in vitro studies of the wear of polyethylene specimens and components have been published^9,10,20,33. In some of those reports, such as that by Wroblewski et al.³³, the authors contended that the clinical results showed excellent agreement with the in vitro results. A matter for debate in the study by Wroblewski et al. is the assumption that one million cycles in a hip simulator is equivalent to one year of clinical service. (Although not explicitly stated in their report, it is clear, according to their Figure 3 and all other relevant discussion, that this assumption was made.) In a study of the walking activity of 111 patients who had had a total hip or knee arthroplasty, Schmalzried et al.²³ reported that, although the mean number of cycles per year was 900,000, there was a substantial difference in the number of cycles among various subsets of the study population; for example, the most active and least active subjects had a forty-five-fold difference in the number of steps per day, and men who were less than sixty years old and those who were sixty years old or more had a 40 percent difference in the amount of walking per day. The ages of the patients were not reported in the study by Wroblewski et al.³³. Nonetheless, in light of the findings of Schmalzried et al.²³, it is likely that the quantitative similarity in the results of the hip-simulator studies and clinical wear reported by Wroblewski et al. is in error. This observation is consistent with the widespread caution that has been expressed with regard to the utility of results of in vitro tests of wear²¹ and provides confirmation of the dissonance between the results of wear-testing with use of a joint simulator and measurements of clinical wear reported in other studies^18,19,32,35. This dissonance is not unexpected given the facts that (1) conditions in a joint simulator are controlled and those in vivo are variable²⁴ and (2) on occasion, these two types of studies have important differences with regard to some key factors, such as the oxidation state of the polyethylene^21,24. It should be noted that the use of a joint simulator for wear-testing is particularly attractive in the initial stages of research and development when the emphasis is on studying the comparative wear performance of a set of candidate polyethylenes or the influence of a large number of factors on the wear rate of one polyethylene, or both, under controlled conditions.

There is universal agreement that the wear of polyethylene bearing components in vivo is multifactorial in nature, involving a complex interplay between many endogenous and exogenous factors^4,16,25 (Table I). The orthopaedic literature is replete with reports of clinical studies in which the objective was to delineate the significance of each of these factors^{1-3,6-8,11-15,17,18,27-31}. Given the importance of this type of work, especially as it influences the research and development of arthroplasty components, it is worthwhile to examine these studies critically, with particular reference to the validity or plausibility of their conclusions, which is the subject of this Commentary.

A goal of this Commentary is to identify the important details that were omitted as well as the confounding factors in full-length reports of clinical studies on the impact of various factors on the in vivo wear of polyethylene bearing components, specifically acetabular cup liners and tibial inserts, published during the last nine years in peer-reviewed archival journals such as The Journal of Bone and Joint Surgery and Clinical Orthopaedics and Related Research (Tables I and II). Omitted details make it difficult to conduct meaningful comparisons of studies even when the purported objectives of the studies are the same. This difficulty is illustrated in a comparison of the reports by Devane et al.⁷ and Jasty et al.¹⁴ with regard to the effect of patient age on the volumetric wear rate of polyethylene acetabular components in total hip replacements. Given the differences between, and omission of, details in these studies, it is not surprising that their conclusions are diametrically opposed. Although Devane et al.⁷ found a significantly greater rate of volumetric wear of acetabular components implanted in younger patients (p = 0.020), Jasty et al.¹⁴ stated: "With the numbers available, the patient's age … did not have a significant relationship to the annual rate of volumetric wear."

Various parametric and nonparametric methods have been used for the statistical analysis of wear results vis-à-vis the independent variable or variables. The Student t test^3,6-8,15,27 and one-way analysis of variance^{2,6,7,13,15,17,18,30} are the parametric methods that have been used, whereas the Kruskal-Wallis¹³ and Mann-Whitney U²⁹ methods are among the nonparametric methods. However, with the exception of James et al.¹³, none of the investigators provided a rationale to support the use of a particular method. This is considered a serious omission because a parametric method should be used only if it is shown (or if there is a priori knowledge to indicate) that the data of interest are normally distributed; in contrast, a nonparametric method is used for the analysis of non-normally distributed data. It is gratifying to note that James et al.¹³ examined the wear-rate data for evidence of non-normality before applying a test to determine the significance of the difference in the means of the rates.

This combination of features—namely, the presence of confounding factors and the use of inappropriate statistical tests of significance—severely undermines confidence in the conclusions stated in the majority of the studies that were examined.

Most of the problems in these studies, as already discussed, are a consequence of the fact that they are retrospective. Thus, many key details were not recorded at the time of implantation of the prostheses, and the authors of subsequent reports were not able to control for details that were known.

In conclusion, three recommendations are offered for future researchers. First, the study should be performed in a prospective, randomized manner with long-term follow-up and it should be carried out in many centers (preferably in different parts of the world). Furthermore, all variables, other than the one being investigated, must be the same for each study group. Second, the groups must comprise large numbers of subjects and they must be similar in terms of size, all relevant patient characteristics, and the duration that the implants were in situ. Third, the investigators must demonstrate that the method or methods used for the statistical analysis of the wear rates are appropriate. Adoption of these recommendations would go a long way to ensure that sound, valid, and defensible conclusions are drawn from the study.

As an adjunct to this point, future work should include more replication studies (ten of the seventeen variables assessed in my review of the literature were addressed in only one report each) (Table II). Furthermore, a host of pertinent variables have not been investigated at all; some examples include the operative technique of implantation of the prosthesis, the packaging medium of the polyethylene during sterilization, and the shelf life of the polyethylene component before implantation.

I contend that if future work in the area of in vivo wear of polyethylene bearing components is guided by these recommendations, and if the gaps outlined here are filled, it will be possible to delineate the role of each endogenous or exogenous variable that was investigated. This information would help to identify the combination of these variables that are likely to result in minimal in vivo wear of these components and, in turn, might increase in vivo longevity of hip and knee arthroplasty components and patient satisfaction. Gladius Lewis, Ph.D. Department of Mechanical Engineering The University of Memphis Campus Box 526576 Memphis, Tennessee 38152-6576 E-mail address: glewis@memphis.edu

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this Commentary. No funds were received in support of this Commentary.

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TABLE I: FACTORS THAT INFLUENCE THE INVIVO WEAR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE BEARING COMPONENTS IN TOTAL HIP REPLACEMENTS

Factor	Information Stated in Report
Livermore et al.¹⁷ (1990)	Kabo et al.¹⁵ (1993)	Jasty et al.¹⁴ (1997)	James et al.¹³ (1999)
Patient characteristics
Age	No	Yes	Yes	Yes
Weight	No	Yes	Yes	Yes
Activity level	No	Yes	No	No
Gender	No	No	Yes	No
Characteristics of femoral head
Material	No	No	Yes	Yes
Diameter	Yes	No	Yes	Yes
Roughness	No	No	No	No
Modular or one-piece head- stem connection	No	No	No	No
Characteristics of acetabular cup or liner
Polyethylene starting resin	No	No	Yes	Yes
Liner fabrication method	Yes	No	Yes	Yes
Liner thickness	No	Yes	Yes	No
Liner packaging method	No	No	Yes	Yes
Liner sterilization method	No	No	Yes	Yes
Metal backing or all-polyethylene design	No	No	Yes	Yes
Operative technique (for example, straight lateral or posterolateral approach)	No	No	No	No
Number of surgeons who performed procedures	No	No	No	Yes
Fixation of femoral component with or without cement	Yes	No	Yes	Yes
Fixation of acetabular component with or without cement	Yes	No	Yes	No
Duration of implant in situ	No	Yes	Yes	Yes

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TABLE I: FACTORS THAT INFLUENCE THE INVIVO WEAR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE BEARING COMPONENTS IN TOTAL HIP REPLACEMENTS

Factor	Information Stated in Report
Livermore et al.¹⁷ (1990)	Kabo et al.¹⁵ (1993)	Jasty et al.¹⁴ (1997)	James et al.¹³ (1999)
Patient characteristics
Age	No	Yes	Yes	Yes
Weight	No	Yes	Yes	Yes
Activity level	No	Yes	No	No
Gender	No	No	Yes	No
Characteristics of femoral head
Material	No	No	Yes	Yes
Diameter	Yes	No	Yes	Yes
Roughness	No	No	No	No
Modular or one-piece head- stem connection	No	No	No	No
Characteristics of acetabular cup or liner
Polyethylene starting resin	No	No	Yes	Yes
Liner fabrication method	Yes	No	Yes	Yes
Liner thickness	No	Yes	Yes	No
Liner packaging method	No	No	Yes	Yes
Liner sterilization method	No	No	Yes	Yes
Metal backing or all-polyethylene design	No	No	Yes	Yes
Operative technique (for example, straight lateral or posterolateral approach)	No	No	No	No
Number of surgeons who performed procedures	No	No	No	Yes
Fixation of femoral component with or without cement	Yes	No	Yes	Yes
Fixation of acetabular component with or without cement	Yes	No	Yes	No
Duration of implant in situ	No	Yes	Yes	Yes

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TABLE II: SUMMARY OF DESIGN ISSUES IN CLINICAL STUDIES*

*THR = total hip replacement, and TKR = total knee replacement.

Study	Type of Study	Variable Studied	No. and Type of Implants	Important Features	Confounding Factors
Livermore et al.¹⁷ (1990)	Not stated	Femoral head diameter	227 THRs with 22-mm femoral head 98 THRs with 28-mm femoral head 60 THRs with 32-mm femoral head	All cup liners compression- molded and from 1 supplier Volumetric wear rates calculated	Different femoral head design in each group More men had 32-mm femoral head than had 22-mm femoral head
Cates et al.³ (1993)	Not stated	Metal backing of cup	134 THRs with metal- backed cup liner 99 THRs with all- polyethylene cup	All procedures performed by 1 surgeon All femoral heads same design All cups molded from same ultra-high molec. weight polyethylene resin Volumetric wear rates estimated
Kabo et al.¹⁵ (1993)	Not stated	Type of cup design	40 THRs with conven- tional cup 20 THRs with surface- replacement cup	Detailed expression used in calculating volumetric wear	Wide discrepancies in no. of implants, cup diameter, and thickness of polyethylene cup liner between 2 groups Duration of follow-up significantly different between 2 groups
Hernandez et al.¹² (1994)	Not stated	Method of fixation of femoral component	131 THRs with Bimetric stem and Universal cup design: 66 femoral components inserted with cement and 65 inserted without cement	Same stem and cup designs in all hips Same operative technique in all hips Volumetric wear rates calculated Patients matched for age, weight, diagnosis, gender, and duration of follow-up
Devane et al.⁶ (1995)	Not stated	Femoral head diameter	56 THRs, PCA design, 32-mm femoral head 85 THRs, PCA design, 26-mm femoral head	Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Wide range of age, weight, and duration of follow- up among patients in each group
Tanner et al.²⁷ (1995)	Not stated	Polyethylene quality	20 TKRs, Ortholoc-II design, no CP prefix in lot number 8 TKRs, Ortholoc-II design, CP prefix in lot number 1 TKR, Ortholoc-II design, no lot number	All procedures performed by 1 surgeon Same TKR design in all knees	Very small no. of implants in study Different polyethylene sterilization methods in each group Different polyethylene resin grades in each group Wear scores, rather than volumetric wear rates, determined Large difference in no. of implants between groups
Woolson and Murphy³¹ (1995)	Not stated	Patient age and activity level	80 THRs	All procedures performed by 1 surgeon Male-to-female ratio approx. 1 All cups same design	Some THRs primary procedures and others, revisions Some THRs inserted with cement and others, without cement Volumetric wear rates not determined
Bankston et al.¹ (1995)	Retrospective	Femoral head material	77 THRs, T28 design 77 THRs, TR-28 design 77 THRs, MOSC design	All procedures performed by 1 surgeon Patients matched for age, weight, and duration of follow-up All polyethylene cup liners compression-molded from same resin grade and of same thickness	Cementing technique not same for all hips Stems in each group made of different material Volumetric wear rates not determined
		Method of fabrication of polyethylene cup liner	54 THRs, Triad design 54 THRs, TR-28 design	Patients matched for age, weight, and duration of follow-up All polyethylene cup liners made from same resin grade	Procedures performed by 2 surgeons Cementing technique not same for all hips THR design not same for all hips Volumetric wear rates not determined
		Metal backing of cup	233 THRs, MOSC design	All prostheses same design All polyethylene cup liners compression-molded	Cementing technique not same for all hips Patients not matched for age, weight, or follow- up period
Callaghan et al.² (1995)	Not stated	Method of fabrication of polyethylene cup liner	23 THRs, Charnley design, machined polyethylene 61 THRs, Charnley design, molded polyethylene	Same THR design in both groups Same femoral head diameter in both groups All implants inserted with cement All procedures performed by 1 surgeon Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Metal backing of cup	20 THRs, Iowa design, all-polyethylene cup, no metal backing 43 THRs, Iowa design, polyethylene cup, metal backing	All procedures performed by 1 surgeon All implants inserted with cement and had same design and femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Method of anchoring femoral head	43 THRs, Iowa design, metal-backed molded- polyethylene cup inserted with cement 63 THRs, Iowa design, metal-backed milled- polyethylene cup inserted without cement	All procedures performed by 1 surgeon All implants had same design of cup-liner backing and same femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants beween groups Large difference in male- to-female ratio between groups Different methods used for fabrication of poly- ethylene cup liner
White et al.³⁰ (1996)	Not stated	Polyethylene sterilization method	26 TKRs, Ortholoc-II design: 18 gamma- radiation-sterilized and 8 ethylene- oxide-sterilized polyethylene inserts	All procedures performed by 1 surgeon Same TKR design in both groups	Very small no. of implants in study Different polyethylene resin grades in each group Time-weighted wear scores, rather than volumetric wear rates, determined Patient matching not done Large difference in no. of implants between groups
Devane et al.⁷ (1997)	Not stated	Patient age and activity level Femoral head diameter Orientation of cup	72 THRs	Same THR design in all hips Male-to-female ratio approx. 1.2 Volumetric wear rates determined directly	Procedures performed by 4 surgeons Wide range in thickness of polyethylene cup liner Very wide range in weight of patients
Hall et al.¹¹ (1997)	Not stated	Topography of surface of femoral head	35 THRs, Charnley design	Same THR design in all hips Volumetric wear rates calculated	Wide range in age and weight of patients and follow-up period
Jasty et al.¹⁴ (1997)	Not stated	Thickness of polyethylene cup liner	84 THRs, various designs, all-polyethylene cup 22 THRs, various designs, metal-backed cup	All components in both groups inserted with cement Volumetric wear rates determined directly	Different THR designs in each group Different type of cup in each group Different polyethylene sterilization methods, fabrication methods, and resin grades for cup liners in each group
Devane et al.⁸ (1997)	Prospective	Method of fixation of cup	69 THRs, Mallory-Head design, inserted with cement 70 THRs, Mallory-Head design, press-fitted	Same operative approach in all hips Randomized, prospective, double-blind study Almost equal nos. of THRs in each group Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Cup 1 piece in 1 group and modular in other group
Livingston et al.¹⁸ (1997)	Not stated	Polyethylene resin grade used in fabrication of cup liner Method of fixation of femoral stem	114 THRs, Osteonics design, stem inserted with cement, Hylamer cup liner 38 THRs, Osteonics design, stem inserted with cement, conven- tional polyethylene cup liner 24 THRs, Osteonics design, stem inserted without cement, Hylamer cup liner 12 THRs, Osteonics design, stem inserted without cement, conventional poly- ethylene cup liner	Same THR design in both groups (for a given variable) Same operative technique in all hips	Large difference in no. of implants between groups (for a given variable) Procedures performed by 5 surgeons Significant difference in mean age of patients in groups with components inserted with cement and those with compo- nents inserted without cement (for a given cup-liner material) Volumetric wear rates not determined
Urquhart et al.²⁸ (1998)	Prospective	Presence or absence of reinforced extension on femoral head	11 THRs, flange exten- sion on femoral head 55 THRs, no flange extension on femoral head	Prospective study Same cup design in both groups Femoral head made of same materials and of same diameter in both groups	Different designs of femoral component in each group Procedures performed by 2 surgeons Mixture of fixation methods for femoral stems in both groups Large difference in no. of implants between groups Volumetric wear rates not determined
van der Vis et al.²⁹ (1998)	Retrospective	Nature of head- neck-taper connection in modular prosthesis	34 THRs, Weber design, rotating bearing used in neck-head-taper connection 37 THRs, Weber design, fixed neck-head-taper connection	Same THR design in both groups Same operative technique in all hips Approx. same no. of implants in both groups	Significant difference in no. of surgeons who performed procedures between groups Volumetric wear rates not determined
James et al.¹³ (1999)	Not stated	Method of fabrication of polyethylene cup liner	19 THRs, T-Tap and Taperloc designs, polyethylene cup liner compression-molded from Himont resin 44 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 or 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Volumetric wear rates determined directly	Different polyethylene starting resins in each group Large difference in no. of implants between groups Difference in fixation of metal backing of acetab- ular component (some had porous coating and others did not)
		Polyethylene resin grade	20 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 resin 24 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Same method of liner fabrication in both groups Volumetric wear rates determined directly	Small no. of implants in each group

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TABLE II: SUMMARY OF DESIGN ISSUES IN CLINICAL STUDIES*

*THR = total hip replacement, and TKR = total knee replacement.

Study	Type of Study	Variable Studied	No. and Type of Implants	Important Features	Confounding Factors
Livermore et al.¹⁷ (1990)	Not stated	Femoral head diameter	227 THRs with 22-mm femoral head 98 THRs with 28-mm femoral head 60 THRs with 32-mm femoral head	All cup liners compression- molded and from 1 supplier Volumetric wear rates calculated	Different femoral head design in each group More men had 32-mm femoral head than had 22-mm femoral head
Cates et al.³ (1993)	Not stated	Metal backing of cup	134 THRs with metal- backed cup liner 99 THRs with all- polyethylene cup	All procedures performed by 1 surgeon All femoral heads same design All cups molded from same ultra-high molec. weight polyethylene resin Volumetric wear rates estimated
Kabo et al.¹⁵ (1993)	Not stated	Type of cup design	40 THRs with conven- tional cup 20 THRs with surface- replacement cup	Detailed expression used in calculating volumetric wear	Wide discrepancies in no. of implants, cup diameter, and thickness of polyethylene cup liner between 2 groups Duration of follow-up significantly different between 2 groups
Hernandez et al.¹² (1994)	Not stated	Method of fixation of femoral component	131 THRs with Bimetric stem and Universal cup design: 66 femoral components inserted with cement and 65 inserted without cement	Same stem and cup designs in all hips Same operative technique in all hips Volumetric wear rates calculated Patients matched for age, weight, diagnosis, gender, and duration of follow-up
Devane et al.⁶ (1995)	Not stated	Femoral head diameter	56 THRs, PCA design, 32-mm femoral head 85 THRs, PCA design, 26-mm femoral head	Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Wide range of age, weight, and duration of follow- up among patients in each group
Tanner et al.²⁷ (1995)	Not stated	Polyethylene quality	20 TKRs, Ortholoc-II design, no CP prefix in lot number 8 TKRs, Ortholoc-II design, CP prefix in lot number 1 TKR, Ortholoc-II design, no lot number	All procedures performed by 1 surgeon Same TKR design in all knees	Very small no. of implants in study Different polyethylene sterilization methods in each group Different polyethylene resin grades in each group Wear scores, rather than volumetric wear rates, determined Large difference in no. of implants between groups
Woolson and Murphy³¹ (1995)	Not stated	Patient age and activity level	80 THRs	All procedures performed by 1 surgeon Male-to-female ratio approx. 1 All cups same design	Some THRs primary procedures and others, revisions Some THRs inserted with cement and others, without cement Volumetric wear rates not determined
Bankston et al.¹ (1995)	Retrospective	Femoral head material	77 THRs, T28 design 77 THRs, TR-28 design 77 THRs, MOSC design	All procedures performed by 1 surgeon Patients matched for age, weight, and duration of follow-up All polyethylene cup liners compression-molded from same resin grade and of same thickness	Cementing technique not same for all hips Stems in each group made of different material Volumetric wear rates not determined
		Method of fabrication of polyethylene cup liner	54 THRs, Triad design 54 THRs, TR-28 design	Patients matched for age, weight, and duration of follow-up All polyethylene cup liners made from same resin grade	Procedures performed by 2 surgeons Cementing technique not same for all hips THR design not same for all hips Volumetric wear rates not determined
		Metal backing of cup	233 THRs, MOSC design	All prostheses same design All polyethylene cup liners compression-molded	Cementing technique not same for all hips Patients not matched for age, weight, or follow- up period
Callaghan et al.² (1995)	Not stated	Method of fabrication of polyethylene cup liner	23 THRs, Charnley design, machined polyethylene 61 THRs, Charnley design, molded polyethylene	Same THR design in both groups Same femoral head diameter in both groups All implants inserted with cement All procedures performed by 1 surgeon Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Metal backing of cup	20 THRs, Iowa design, all-polyethylene cup, no metal backing 43 THRs, Iowa design, polyethylene cup, metal backing	All procedures performed by 1 surgeon All implants inserted with cement and had same design and femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Method of anchoring femoral head	43 THRs, Iowa design, metal-backed molded- polyethylene cup inserted with cement 63 THRs, Iowa design, metal-backed milled- polyethylene cup inserted without cement	All procedures performed by 1 surgeon All implants had same design of cup-liner backing and same femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants beween groups Large difference in male- to-female ratio between groups Different methods used for fabrication of poly- ethylene cup liner
White et al.³⁰ (1996)	Not stated	Polyethylene sterilization method	26 TKRs, Ortholoc-II design: 18 gamma- radiation-sterilized and 8 ethylene- oxide-sterilized polyethylene inserts	All procedures performed by 1 surgeon Same TKR design in both groups	Very small no. of implants in study Different polyethylene resin grades in each group Time-weighted wear scores, rather than volumetric wear rates, determined Patient matching not done Large difference in no. of implants between groups
Devane et al.⁷ (1997)	Not stated	Patient age and activity level Femoral head diameter Orientation of cup	72 THRs	Same THR design in all hips Male-to-female ratio approx. 1.2 Volumetric wear rates determined directly	Procedures performed by 4 surgeons Wide range in thickness of polyethylene cup liner Very wide range in weight of patients
Hall et al.¹¹ (1997)	Not stated	Topography of surface of femoral head	35 THRs, Charnley design	Same THR design in all hips Volumetric wear rates calculated	Wide range in age and weight of patients and follow-up period
Jasty et al.¹⁴ (1997)	Not stated	Thickness of polyethylene cup liner	84 THRs, various designs, all-polyethylene cup 22 THRs, various designs, metal-backed cup	All components in both groups inserted with cement Volumetric wear rates determined directly	Different THR designs in each group Different type of cup in each group Different polyethylene sterilization methods, fabrication methods, and resin grades for cup liners in each group
Devane et al.⁸ (1997)	Prospective	Method of fixation of cup	69 THRs, Mallory-Head design, inserted with cement 70 THRs, Mallory-Head design, press-fitted	Same operative approach in all hips Randomized, prospective, double-blind study Almost equal nos. of THRs in each group Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Cup 1 piece in 1 group and modular in other group
Livingston et al.¹⁸ (1997)	Not stated	Polyethylene resin grade used in fabrication of cup liner Method of fixation of femoral stem	114 THRs, Osteonics design, stem inserted with cement, Hylamer cup liner 38 THRs, Osteonics design, stem inserted with cement, conven- tional polyethylene cup liner 24 THRs, Osteonics design, stem inserted without cement, Hylamer cup liner 12 THRs, Osteonics design, stem inserted without cement, conventional poly- ethylene cup liner	Same THR design in both groups (for a given variable) Same operative technique in all hips	Large difference in no. of implants between groups (for a given variable) Procedures performed by 5 surgeons Significant difference in mean age of patients in groups with components inserted with cement and those with compo- nents inserted without cement (for a given cup-liner material) Volumetric wear rates not determined
Urquhart et al.²⁸ (1998)	Prospective	Presence or absence of reinforced extension on femoral head	11 THRs, flange exten- sion on femoral head 55 THRs, no flange extension on femoral head	Prospective study Same cup design in both groups Femoral head made of same materials and of same diameter in both groups	Different designs of femoral component in each group Procedures performed by 2 surgeons Mixture of fixation methods for femoral stems in both groups Large difference in no. of implants between groups Volumetric wear rates not determined
van der Vis et al.²⁹ (1998)	Retrospective	Nature of head- neck-taper connection in modular prosthesis	34 THRs, Weber design, rotating bearing used in neck-head-taper connection 37 THRs, Weber design, fixed neck-head-taper connection	Same THR design in both groups Same operative technique in all hips Approx. same no. of implants in both groups	Significant difference in no. of surgeons who performed procedures between groups Volumetric wear rates not determined
James et al.¹³ (1999)	Not stated	Method of fabrication of polyethylene cup liner	19 THRs, T-Tap and Taperloc designs, polyethylene cup liner compression-molded from Himont resin 44 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 or 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Volumetric wear rates determined directly	Different polyethylene starting resins in each group Large difference in no. of implants between groups Difference in fixation of metal backing of acetab- ular component (some had porous coating and others did not)
		Polyethylene resin grade	20 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 resin 24 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Same method of liner fabrication in both groups Volumetric wear rates determined directly	Small no. of implants in each group

References

Introduction | References

Bankston, A. B.; Cates, H.; Ritter, M. A.; Keating, E. M.; and Faris, P. M.: Polyethylene wear in total hip arthroplasty. Clin. Orthop.,317: 7-13, 1995.3177 1995 [PubMed]

Callaghan, J. J.; Pedersen, D. R.; Olejniczak, J. P.; Goetz, D. D.; and Johnston, R. C.: Radiographic measurement of wear in 5 cohorts of patients observed for 5 to 22 years. Clin. Orthop.,317: 14-18, 1995.31714 1995 [PubMed]

Cates, H. E.; Faris, P. M.; Keating, E. M.; and Ritter, M. A.: Polyethylene wear in cemented metal-backed acetabular cups. J. Bone and Joint Surg.,75-B(2): 249-253, 1993.75-B(2)249 1993

Cuckler, J. M.: Factors influencing the limitations of polyethylene. Orthopedics,20: 761-762, 1997.20761 1997 [PubMed]

Devane, P. A.; Bourne, R. B.; Rorabeck, C. H.; Hardie, R. M.; and Horne, J. G.: Measurement of polyethylene wear in metal-backed acetabular cups. I. Three-dimensional technique. Clin. Orthop.,319: 303-316, 1995.319303 1995 [PubMed]

Devane, P. A.; Bourne, R. B.; Rorabeck, C. H.; MacDonald, S.; and Robinson, E. J.: Measurement of polyethylene wear in metal-backed acetabular cups. II. Clinical application. Clin. Orthop.,319: 317-326, 1995.319317 1995 [PubMed]

Devane, P. A.; Horne, J. G.; Martin, K.; Coldham, G.; and Krause, B.: Three-dimensional polyethylene wear of a press-fit titanium prosthesis. Factors influencing generation of polyethylene debris. J. Arthroplasty,12: 256-266, 1997.12256 1997 [PubMed]

Devane, P. A.; Robinson, E. J.; Bourne, R. B.; Rorabeck, C. H.; Nayak, N. N.; and Horne, J. G.: Measurement of polyethylene wear in acetabular components inserted with and without cement. A randomized trial. J. Bone and Joint Surg.,79-A: 682-689, May 1997.79-A682 1997

Fisher, J.; Reeves, E. A.; Isaac, G. H.; Saum, K. A.; and Sanford, W. M.: Comparison of the wear of aged and non-aged ultrahigh molecular weight polyethylene sterilized by gamma irradiation and by gas plasma. J. Mater. Sci. Mater. Med.,8: 375-378, 1997.8375 1997 [PubMed]

Goldman, M., and Pruitt, L.: Comparison of the effects of gamma radiation and low temperature hydrogen peroxide gas plasma sterilization on the molecular structure, fatigue resistance, and wear behavior of UHMWPE. J. Biomed. Mater. Res.,40: 378-384, 1998.40378 1998 [PubMed]

Hall, R. M.; Siney, P.; Unsworth, A.; and Wroblewski, B. M.: The effect of surface topography of retrieved femoral heads on the wear of UHMWPE sockets. Med. Eng. and Phys.,19: 711-719, 1997.19711 1997

Hernandez, J. R.; Keating, E. M.; Faris, P. M.; Meding, J. B.; and Ritter, M. A.: Polyethylene wear in uncemented acetabular components. J. Bone and Joint Surg.,76-B(2): 263-266, 1994.76-B(2)263 1994

James, S. P.; Lee, K. R.; Beauregard, G. P.; Rentfrow, E. D.; and McLaughlin, J. R.: Clinical wear of 63 ultrahigh molecular weight polyethylene acetabular components: effect of starting resin and forming method. J. Biomed. Mater. Res.,48: 374-384, 1999.48374 1999 [PubMed]

Jasty, M.; Goetz, D. D.; Bragdon, C. R.; Lee, K. R.; Hanson, A. E.; Elder, J. R.; and Harris, W. H.: Wear of polyethylene acetabular components in total hip arthroplasty. An analysis of one hundred and twenty-eight components retrieved at autopsy or revision operations. J. Bone and Joint Surg.,79-A: 349-358, March 1997.79-A349 1997

Kabo, J. M.; Gebhard, J. S.; Loren, G.; and Amstutz, H. C.: In vivo wear of polyethylene acetabular components. J. Bone and Joint Surg.,75-B(2): 254-258, 1993.75-B(2)254 1993

Lewis, G.: Polyethylene wear in total hip and knee arthroplasties. J. Biomed. Mater. Res.,38: 55-75, 1997.3855 1997 [PubMed]

Livermore, J.; Ilstrup, D.; and Morrey, B.: Effect of femoral head size on wear of the polyethylene acetabular component. J. Bone and Joint Surg.,72-A: 518-528, April 1990.72-A518 1990

Livingston, B. J.; Chmell, M. J.; Spector, M.; and Poss, R.: Complications of total hip arthroplasty associated with the use of an acetabular component with a Hylamer liner. J. Bone and Joint Surg.,79-A: 1529-1538, Oct. 1997.79-A1529 1997

McKellop, H.; Lu, B.; and Li, S.: Wear of acetabular cups of conventional and modified UHMW polyethylenes compared on a hip joint simulator. Trans. Orthop. Res. Soc.,17: 356, 1992.17356 1992

O'Connor, D. O.; Bragdon, C. R.; Lowenstein, J.; Jasty, M.; and Harris, W. H.: Wear and high cycle of a highly cross-linked UHMWPE. In Transactions of the Twenty-fifth Annual Meeting of the Society for Biomaterials, p. 508. Providence, Rhode Island, 1999.

Ohlin, A.: Tribological tests cannot replace clinical trials [editorial]. Acta Orthop. Scandinavica,69: 441-442, 1998.69441 1998

Ramakrishnan, H. K.; Kadaba, M. P.; Dumbleton, J.; and Green, K.: Validation of an improved polyethylene wear measurement technique in total hip arthroplasty. Trans. Orthop. Res. Soc.,23: 416, 1998.23416 1998

Schmalzried, T. P.; Szuszczewicz, E. S.; Northfield, M. R.; Akizuki, K. H.; Frankel, R. E.; Belcher, G.; and Amstutz, H. C.: Quantitative assessment of walking activity after total hip or knee replacement. J. Bone and Joint Surg.,80-A: 54-59, Jan. 1998.80-A54 1998

Schmalzried, T. P.; Dorey, F. J.; and McKellop, H.: Commentary. The multifactorial nature of polyethylene wear in vivo. J. Bone and Joint Surg.,80-A: 1234-1242, Aug. 1998.80-A1234 1998

Schmalzried, T. P., and Callaghan, J. J.: Current concepts review. Wear in total hip and knee replacements. J. Bone and Joint Surg.,81-A: 115-136, Jan. 1999.81-A115 1999

Shaver, S. M.; Brown, T. D.; Hillis, S. L.; and Callaghan, J. J.: Digital edge-detection measurement of polyethylene wear after total hip arthroplasty. J. Bone and Joint Surg.,79-A: 690-700, May 1997.79-A690 1997

Tanner, M. G.; Whiteside, L. A.; and White, S. E.: Effect of polyethylene quality on wear in total knee arthroplasty. Clin. Orthop.,317: 83-88, 1995.31783 1995 [PubMed]

Urquhart, A. G.; D'Lima, D. D.; Venn-Watson, E.; Colwell, C. W., Jr.; and Walker, R. H.: Polyethylene wear after total hip arthroplasty: the effect of a modular femoral head with an extended flange-reinforced neck. J. Bone and Joint Surg.,80-A: 1641-1647, Nov. 1998.80-A1641 1998

van der Vis, H. M.; Zwartelé, R.; Schuller, H. M.; Doets, H. K.; and Marti, R. K.: Socket wear in ceramic-on-polyethylene total hip arthroplasties: fixed versus rotating heads. Acta Orthop. Scandinavica,69: 248-252, 1998.69248 1998

White, S. E.; Paxson, R. D.; Tanner, M. G.; and Whiteside, L. A.: Effects of sterilization on wear in total knee arthroplasty. Clin. Orthop.,331: 164-171, 1996.331164 1996 [PubMed]

Woolson, S. T., and Murphy, M. G.: Wear of the polyethylene of Harris-Galante acetabular components inserted without cement. J. Bone and Joint Surg.,77-A: 1311-1314, Sept. 1995.77-A1311 1995

Wright, T. M.; Astion, D. J.; Bansal, M.; Rimnac, C. M.; Green, T.; Insall, J. N.; and Robinson, R. P.: Failure of carbon fiber-reinforced polyethylene total knee-replacement components. A report of two cases. J. Bone and Joint Surg.,70-A: 926-932, July 1988.70-A926 1988

Wroblewski, B. M.; Siney, P. D.; Dowson, B.; and Collins, S. N.: Prospective clinical and joint simulator studies of a new total hip arthroplasty using alumina ceramic heads and cross-linked polyethylene cups. J. Bone and Joint Surg.,78-B(2): 280-285, 1996.78-B(2)280 1996

Yamaguchi, M.; Bauer, T. W.; and Hashimoto, Y.: Three-dimensional analysis of multiple wear vectors in retrieved acetabular cups. J. Bone and Joint Surg.,79-A: 1539-1544, Oct. 1997.79-A1539 1997

Zichner, L. P., and Willert, H.-G.: Comparison of alumina-polyethylene and metal-polyethylene in clinical trials. Clin. Orthop.,282: 86-94, 1992.28286 1992 [PubMed]

Topics

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TABLE I: FACTORS THAT INFLUENCE THE INVIVO WEAR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE BEARING COMPONENTS IN TOTAL HIP REPLACEMENTS

Factor	Information Stated in Report
Livermore et al.¹⁷ (1990)	Kabo et al.¹⁵ (1993)	Jasty et al.¹⁴ (1997)	James et al.¹³ (1999)
Patient characteristics
Age	No	Yes	Yes	Yes
Weight	No	Yes	Yes	Yes
Activity level	No	Yes	No	No
Gender	No	No	Yes	No
Characteristics of femoral head
Material	No	No	Yes	Yes
Diameter	Yes	No	Yes	Yes
Roughness	No	No	No	No
Modular or one-piece head- stem connection	No	No	No	No
Characteristics of acetabular cup or liner
Polyethylene starting resin	No	No	Yes	Yes
Liner fabrication method	Yes	No	Yes	Yes
Liner thickness	No	Yes	Yes	No
Liner packaging method	No	No	Yes	Yes
Liner sterilization method	No	No	Yes	Yes
Metal backing or all-polyethylene design	No	No	Yes	Yes
Operative technique (for example, straight lateral or posterolateral approach)	No	No	No	No
Number of surgeons who performed procedures	No	No	No	Yes
Fixation of femoral component with or without cement	Yes	No	Yes	Yes
Fixation of acetabular component with or without cement	Yes	No	Yes	No
Duration of implant in situ	No	Yes	Yes	Yes

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TABLE I: FACTORS THAT INFLUENCE THE INVIVO WEAR OF ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE BEARING COMPONENTS IN TOTAL HIP REPLACEMENTS

Factor	Information Stated in Report
Livermore et al.¹⁷ (1990)	Kabo et al.¹⁵ (1993)	Jasty et al.¹⁴ (1997)	James et al.¹³ (1999)
Patient characteristics
Age	No	Yes	Yes	Yes
Weight	No	Yes	Yes	Yes
Activity level	No	Yes	No	No
Gender	No	No	Yes	No
Characteristics of femoral head
Material	No	No	Yes	Yes
Diameter	Yes	No	Yes	Yes
Roughness	No	No	No	No
Modular or one-piece head- stem connection	No	No	No	No
Characteristics of acetabular cup or liner
Polyethylene starting resin	No	No	Yes	Yes
Liner fabrication method	Yes	No	Yes	Yes
Liner thickness	No	Yes	Yes	No
Liner packaging method	No	No	Yes	Yes
Liner sterilization method	No	No	Yes	Yes
Metal backing or all-polyethylene design	No	No	Yes	Yes
Operative technique (for example, straight lateral or posterolateral approach)	No	No	No	No
Number of surgeons who performed procedures	No	No	No	Yes
Fixation of femoral component with or without cement	Yes	No	Yes	Yes
Fixation of acetabular component with or without cement	Yes	No	Yes	No
Duration of implant in situ	No	Yes	Yes	Yes

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TABLE II: SUMMARY OF DESIGN ISSUES IN CLINICAL STUDIES*

*THR = total hip replacement, and TKR = total knee replacement.

Study	Type of Study	Variable Studied	No. and Type of Implants	Important Features	Confounding Factors
Livermore et al.¹⁷ (1990)	Not stated	Femoral head diameter	227 THRs with 22-mm femoral head 98 THRs with 28-mm femoral head 60 THRs with 32-mm femoral head	All cup liners compression- molded and from 1 supplier Volumetric wear rates calculated	Different femoral head design in each group More men had 32-mm femoral head than had 22-mm femoral head
Cates et al.³ (1993)	Not stated	Metal backing of cup	134 THRs with metal- backed cup liner 99 THRs with all- polyethylene cup	All procedures performed by 1 surgeon All femoral heads same design All cups molded from same ultra-high molec. weight polyethylene resin Volumetric wear rates estimated
Kabo et al.¹⁵ (1993)	Not stated	Type of cup design	40 THRs with conven- tional cup 20 THRs with surface- replacement cup	Detailed expression used in calculating volumetric wear	Wide discrepancies in no. of implants, cup diameter, and thickness of polyethylene cup liner between 2 groups Duration of follow-up significantly different between 2 groups
Hernandez et al.¹² (1994)	Not stated	Method of fixation of femoral component	131 THRs with Bimetric stem and Universal cup design: 66 femoral components inserted with cement and 65 inserted without cement	Same stem and cup designs in all hips Same operative technique in all hips Volumetric wear rates calculated Patients matched for age, weight, diagnosis, gender, and duration of follow-up
Devane et al.⁶ (1995)	Not stated	Femoral head diameter	56 THRs, PCA design, 32-mm femoral head 85 THRs, PCA design, 26-mm femoral head	Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Wide range of age, weight, and duration of follow- up among patients in each group
Tanner et al.²⁷ (1995)	Not stated	Polyethylene quality	20 TKRs, Ortholoc-II design, no CP prefix in lot number 8 TKRs, Ortholoc-II design, CP prefix in lot number 1 TKR, Ortholoc-II design, no lot number	All procedures performed by 1 surgeon Same TKR design in all knees	Very small no. of implants in study Different polyethylene sterilization methods in each group Different polyethylene resin grades in each group Wear scores, rather than volumetric wear rates, determined Large difference in no. of implants between groups
Woolson and Murphy³¹ (1995)	Not stated	Patient age and activity level	80 THRs	All procedures performed by 1 surgeon Male-to-female ratio approx. 1 All cups same design	Some THRs primary procedures and others, revisions Some THRs inserted with cement and others, without cement Volumetric wear rates not determined
Bankston et al.¹ (1995)	Retrospective	Femoral head material	77 THRs, T28 design 77 THRs, TR-28 design 77 THRs, MOSC design	All procedures performed by 1 surgeon Patients matched for age, weight, and duration of follow-up All polyethylene cup liners compression-molded from same resin grade and of same thickness	Cementing technique not same for all hips Stems in each group made of different material Volumetric wear rates not determined
		Method of fabrication of polyethylene cup liner	54 THRs, Triad design 54 THRs, TR-28 design	Patients matched for age, weight, and duration of follow-up All polyethylene cup liners made from same resin grade	Procedures performed by 2 surgeons Cementing technique not same for all hips THR design not same for all hips Volumetric wear rates not determined
		Metal backing of cup	233 THRs, MOSC design	All prostheses same design All polyethylene cup liners compression-molded	Cementing technique not same for all hips Patients not matched for age, weight, or follow- up period
Callaghan et al.² (1995)	Not stated	Method of fabrication of polyethylene cup liner	23 THRs, Charnley design, machined polyethylene 61 THRs, Charnley design, molded polyethylene	Same THR design in both groups Same femoral head diameter in both groups All implants inserted with cement All procedures performed by 1 surgeon Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Metal backing of cup	20 THRs, Iowa design, all-polyethylene cup, no metal backing 43 THRs, Iowa design, polyethylene cup, metal backing	All procedures performed by 1 surgeon All implants inserted with cement and had same design and femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Method of anchoring femoral head	43 THRs, Iowa design, metal-backed molded- polyethylene cup inserted with cement 63 THRs, Iowa design, metal-backed milled- polyethylene cup inserted without cement	All procedures performed by 1 surgeon All implants had same design of cup-liner backing and same femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants beween groups Large difference in male- to-female ratio between groups Different methods used for fabrication of poly- ethylene cup liner
White et al.³⁰ (1996)	Not stated	Polyethylene sterilization method	26 TKRs, Ortholoc-II design: 18 gamma- radiation-sterilized and 8 ethylene- oxide-sterilized polyethylene inserts	All procedures performed by 1 surgeon Same TKR design in both groups	Very small no. of implants in study Different polyethylene resin grades in each group Time-weighted wear scores, rather than volumetric wear rates, determined Patient matching not done Large difference in no. of implants between groups
Devane et al.⁷ (1997)	Not stated	Patient age and activity level Femoral head diameter Orientation of cup	72 THRs	Same THR design in all hips Male-to-female ratio approx. 1.2 Volumetric wear rates determined directly	Procedures performed by 4 surgeons Wide range in thickness of polyethylene cup liner Very wide range in weight of patients
Hall et al.¹¹ (1997)	Not stated	Topography of surface of femoral head	35 THRs, Charnley design	Same THR design in all hips Volumetric wear rates calculated	Wide range in age and weight of patients and follow-up period
Jasty et al.¹⁴ (1997)	Not stated	Thickness of polyethylene cup liner	84 THRs, various designs, all-polyethylene cup 22 THRs, various designs, metal-backed cup	All components in both groups inserted with cement Volumetric wear rates determined directly	Different THR designs in each group Different type of cup in each group Different polyethylene sterilization methods, fabrication methods, and resin grades for cup liners in each group
Devane et al.⁸ (1997)	Prospective	Method of fixation of cup	69 THRs, Mallory-Head design, inserted with cement 70 THRs, Mallory-Head design, press-fitted	Same operative approach in all hips Randomized, prospective, double-blind study Almost equal nos. of THRs in each group Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Cup 1 piece in 1 group and modular in other group
Livingston et al.¹⁸ (1997)	Not stated	Polyethylene resin grade used in fabrication of cup liner Method of fixation of femoral stem	114 THRs, Osteonics design, stem inserted with cement, Hylamer cup liner 38 THRs, Osteonics design, stem inserted with cement, conven- tional polyethylene cup liner 24 THRs, Osteonics design, stem inserted without cement, Hylamer cup liner 12 THRs, Osteonics design, stem inserted without cement, conventional poly- ethylene cup liner	Same THR design in both groups (for a given variable) Same operative technique in all hips	Large difference in no. of implants between groups (for a given variable) Procedures performed by 5 surgeons Significant difference in mean age of patients in groups with components inserted with cement and those with compo- nents inserted without cement (for a given cup-liner material) Volumetric wear rates not determined
Urquhart et al.²⁸ (1998)	Prospective	Presence or absence of reinforced extension on femoral head	11 THRs, flange exten- sion on femoral head 55 THRs, no flange extension on femoral head	Prospective study Same cup design in both groups Femoral head made of same materials and of same diameter in both groups	Different designs of femoral component in each group Procedures performed by 2 surgeons Mixture of fixation methods for femoral stems in both groups Large difference in no. of implants between groups Volumetric wear rates not determined
van der Vis et al.²⁹ (1998)	Retrospective	Nature of head- neck-taper connection in modular prosthesis	34 THRs, Weber design, rotating bearing used in neck-head-taper connection 37 THRs, Weber design, fixed neck-head-taper connection	Same THR design in both groups Same operative technique in all hips Approx. same no. of implants in both groups	Significant difference in no. of surgeons who performed procedures between groups Volumetric wear rates not determined
James et al.¹³ (1999)	Not stated	Method of fabrication of polyethylene cup liner	19 THRs, T-Tap and Taperloc designs, polyethylene cup liner compression-molded from Himont resin 44 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 or 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Volumetric wear rates determined directly	Different polyethylene starting resins in each group Large difference in no. of implants between groups Difference in fixation of metal backing of acetab- ular component (some had porous coating and others did not)
		Polyethylene resin grade	20 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 resin 24 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Same method of liner fabrication in both groups Volumetric wear rates determined directly	Small no. of implants in each group

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TABLE II: SUMMARY OF DESIGN ISSUES IN CLINICAL STUDIES*

*THR = total hip replacement, and TKR = total knee replacement.

Study	Type of Study	Variable Studied	No. and Type of Implants	Important Features	Confounding Factors
Livermore et al.¹⁷ (1990)	Not stated	Femoral head diameter	227 THRs with 22-mm femoral head 98 THRs with 28-mm femoral head 60 THRs with 32-mm femoral head	All cup liners compression- molded and from 1 supplier Volumetric wear rates calculated	Different femoral head design in each group More men had 32-mm femoral head than had 22-mm femoral head
Cates et al.³ (1993)	Not stated	Metal backing of cup	134 THRs with metal- backed cup liner 99 THRs with all- polyethylene cup	All procedures performed by 1 surgeon All femoral heads same design All cups molded from same ultra-high molec. weight polyethylene resin Volumetric wear rates estimated
Kabo et al.¹⁵ (1993)	Not stated	Type of cup design	40 THRs with conven- tional cup 20 THRs with surface- replacement cup	Detailed expression used in calculating volumetric wear	Wide discrepancies in no. of implants, cup diameter, and thickness of polyethylene cup liner between 2 groups Duration of follow-up significantly different between 2 groups
Hernandez et al.¹² (1994)	Not stated	Method of fixation of femoral component	131 THRs with Bimetric stem and Universal cup design: 66 femoral components inserted with cement and 65 inserted without cement	Same stem and cup designs in all hips Same operative technique in all hips Volumetric wear rates calculated Patients matched for age, weight, diagnosis, gender, and duration of follow-up
Devane et al.⁶ (1995)	Not stated	Femoral head diameter	56 THRs, PCA design, 32-mm femoral head 85 THRs, PCA design, 26-mm femoral head	Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Wide range of age, weight, and duration of follow- up among patients in each group
Tanner et al.²⁷ (1995)	Not stated	Polyethylene quality	20 TKRs, Ortholoc-II design, no CP prefix in lot number 8 TKRs, Ortholoc-II design, CP prefix in lot number 1 TKR, Ortholoc-II design, no lot number	All procedures performed by 1 surgeon Same TKR design in all knees	Very small no. of implants in study Different polyethylene sterilization methods in each group Different polyethylene resin grades in each group Wear scores, rather than volumetric wear rates, determined Large difference in no. of implants between groups
Woolson and Murphy³¹ (1995)	Not stated	Patient age and activity level	80 THRs	All procedures performed by 1 surgeon Male-to-female ratio approx. 1 All cups same design	Some THRs primary procedures and others, revisions Some THRs inserted with cement and others, without cement Volumetric wear rates not determined
Bankston et al.¹ (1995)	Retrospective	Femoral head material	77 THRs, T28 design 77 THRs, TR-28 design 77 THRs, MOSC design	All procedures performed by 1 surgeon Patients matched for age, weight, and duration of follow-up All polyethylene cup liners compression-molded from same resin grade and of same thickness	Cementing technique not same for all hips Stems in each group made of different material Volumetric wear rates not determined
		Method of fabrication of polyethylene cup liner	54 THRs, Triad design 54 THRs, TR-28 design	Patients matched for age, weight, and duration of follow-up All polyethylene cup liners made from same resin grade	Procedures performed by 2 surgeons Cementing technique not same for all hips THR design not same for all hips Volumetric wear rates not determined
		Metal backing of cup	233 THRs, MOSC design	All prostheses same design All polyethylene cup liners compression-molded	Cementing technique not same for all hips Patients not matched for age, weight, or follow- up period
Callaghan et al.² (1995)	Not stated	Method of fabrication of polyethylene cup liner	23 THRs, Charnley design, machined polyethylene 61 THRs, Charnley design, molded polyethylene	Same THR design in both groups Same femoral head diameter in both groups All implants inserted with cement All procedures performed by 1 surgeon Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Metal backing of cup	20 THRs, Iowa design, all-polyethylene cup, no metal backing 43 THRs, Iowa design, polyethylene cup, metal backing	All procedures performed by 1 surgeon All implants inserted with cement and had same design and femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants between groups Large difference in male- to-female ratio between groups
		Method of anchoring femoral head	43 THRs, Iowa design, metal-backed molded- polyethylene cup inserted with cement 63 THRs, Iowa design, metal-backed milled- polyethylene cup inserted without cement	All procedures performed by 1 surgeon All implants had same design of cup-liner backing and same femoral head diameter Volumetric wear rates calculated	Large difference in no. of implants beween groups Large difference in male- to-female ratio between groups Different methods used for fabrication of poly- ethylene cup liner
White et al.³⁰ (1996)	Not stated	Polyethylene sterilization method	26 TKRs, Ortholoc-II design: 18 gamma- radiation-sterilized and 8 ethylene- oxide-sterilized polyethylene inserts	All procedures performed by 1 surgeon Same TKR design in both groups	Very small no. of implants in study Different polyethylene resin grades in each group Time-weighted wear scores, rather than volumetric wear rates, determined Patient matching not done Large difference in no. of implants between groups
Devane et al.⁷ (1997)	Not stated	Patient age and activity level Femoral head diameter Orientation of cup	72 THRs	Same THR design in all hips Male-to-female ratio approx. 1.2 Volumetric wear rates determined directly	Procedures performed by 4 surgeons Wide range in thickness of polyethylene cup liner Very wide range in weight of patients
Hall et al.¹¹ (1997)	Not stated	Topography of surface of femoral head	35 THRs, Charnley design	Same THR design in all hips Volumetric wear rates calculated	Wide range in age and weight of patients and follow-up period
Jasty et al.¹⁴ (1997)	Not stated	Thickness of polyethylene cup liner	84 THRs, various designs, all-polyethylene cup 22 THRs, various designs, metal-backed cup	All components in both groups inserted with cement Volumetric wear rates determined directly	Different THR designs in each group Different type of cup in each group Different polyethylene sterilization methods, fabrication methods, and resin grades for cup liners in each group
Devane et al.⁸ (1997)	Prospective	Method of fixation of cup	69 THRs, Mallory-Head design, inserted with cement 70 THRs, Mallory-Head design, press-fitted	Same operative approach in all hips Randomized, prospective, double-blind study Almost equal nos. of THRs in each group Same THR design in both groups Volumetric wear rates determined directly	Procedures performed by 2 surgeons Cup 1 piece in 1 group and modular in other group
Livingston et al.¹⁸ (1997)	Not stated	Polyethylene resin grade used in fabrication of cup liner Method of fixation of femoral stem	114 THRs, Osteonics design, stem inserted with cement, Hylamer cup liner 38 THRs, Osteonics design, stem inserted with cement, conven- tional polyethylene cup liner 24 THRs, Osteonics design, stem inserted without cement, Hylamer cup liner 12 THRs, Osteonics design, stem inserted without cement, conventional poly- ethylene cup liner	Same THR design in both groups (for a given variable) Same operative technique in all hips	Large difference in no. of implants between groups (for a given variable) Procedures performed by 5 surgeons Significant difference in mean age of patients in groups with components inserted with cement and those with compo- nents inserted without cement (for a given cup-liner material) Volumetric wear rates not determined
Urquhart et al.²⁸ (1998)	Prospective	Presence or absence of reinforced extension on femoral head	11 THRs, flange exten- sion on femoral head 55 THRs, no flange extension on femoral head	Prospective study Same cup design in both groups Femoral head made of same materials and of same diameter in both groups	Different designs of femoral component in each group Procedures performed by 2 surgeons Mixture of fixation methods for femoral stems in both groups Large difference in no. of implants between groups Volumetric wear rates not determined
van der Vis et al.²⁹ (1998)	Retrospective	Nature of head- neck-taper connection in modular prosthesis	34 THRs, Weber design, rotating bearing used in neck-head-taper connection 37 THRs, Weber design, fixed neck-head-taper connection	Same THR design in both groups Same operative technique in all hips Approx. same no. of implants in both groups	Significant difference in no. of surgeons who performed procedures between groups Volumetric wear rates not determined
James et al.¹³ (1999)	Not stated	Method of fabrication of polyethylene cup liner	19 THRs, T-Tap and Taperloc designs, polyethylene cup liner compression-molded from Himont resin 44 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 or 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Volumetric wear rates determined directly	Different polyethylene starting resins in each group Large difference in no. of implants between groups Difference in fixation of metal backing of acetab- ular component (some had porous coating and others did not)
		Polyethylene resin grade	20 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 412 resin 24 THRs, T-Tap and Taperloc designs, polyethylene cup liner machined from GUR 415 resin	All procedures performed by 1 surgeon Same THR design in both groups Same method of liner fabrication in both groups Volumetric wear rates determined directly	Small no. of implants in each group