JBJS | Article

The Journal of Bone & Joint Surgery, Volume 84, Issue 1

Scientific Article | January 01, 2002

Effect of Acetabular Modularity on Polyethylene Wear and Osteolysis in Total Hip Arthroplasty

Anthony M. Young, BS; Christi J. Sychterz, MSE; Robert H. HopperJr., PhD; Charles A. Engh, MD

Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria, Virginia

Anthony M. Young, BS
Christi J. Sychterz, MSE
Robert H. Hopper Jr., PhD
Charles A. Engh, MD
Anderson Orthopaedic Research Institute, P.O. Box 7088, Alexandria, VA 22307

One or more of the authors has received or will receive benefits for personal or professional use from a commercial party (DePuy, Johnson and Johnson) related directly or indirectly to the subject of this article. No funds were received in support of this study.

The Journal of Bone & Joint Surgery. 2002; 84:58-63

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Abstract

Background: Debris from polyethylene wear causes osteolysis. In this study, we examined the effect of acetabular liner modularity on polyethylene wear and osteolysis.

Methods: We compared forty-one hips (thirty-nine patients) treated with a nonmodular, porous-coated acetabular component with a matched group of forty-one hips (forty patients) treated with a modular acetabular component. The groups were matched by patient gender and age, type of polyethylene material, method of polyethylene sterilization, femoral head size and manufacturer, and stem manufacturer. The mean follow-up period was 5.3 years (range, 3.8 to 6.8 years) for the nonmodular group and 5.5 years (range, 3.8 to 8.0 years) for the modular group. Using serial radiographs and a computer-assisted method, we measured two-dimensional head penetration into the polyethylene liner. Temporal head-penetration data and linear regression analysis were used to calculate the true wear rates.

Results: The nonmodular acetabular components demonstrated a lower, but not a significantly lower, mean true wear rate than did the modular components (0.11 compared with 0.16 mm/yr, p = 0.22), and they were associated with a significantly lower rate of osteolysis (2% compared with 22%, p = 0.01). In addition, the true wear rates of the nonmodular components were less variable than those of the modular components. The 95% confidence interval for the wear rates of the nonmodular components (0.08 to 0.13 mm/yr) was nearly half that of the modular group (0.11 to 0.20 mm/yr).

Conclusions: The lower and more consistent true wear rates of the nonmodular components could be attributed to the fact that these cups were designed to have greater liner-shell conformity, greater liner thickness, and less liner-shell micromotion than modular components. These design factors could have favorably altered the stress distribution throughout the liner and could have thereby decreased wear. Although nonmodular components may present a partial solution to the problems of wear and osteolysis, they pose a disadvantage when a failed liner in a bone-ingrown acetabular component needs to be revised.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | References

Osteolysis is a common complication associated with total hip arthroplasty^1-4. Several studies^5,6 have demonstrated a relationship between periprosthetic osteolysis and debris produced by wear of polyethylene liners. Different factors can affect polyethylene wear and the production of wear debris in vivo. Such factors include the roughness and material of the femoral head, the method of polyethylene sterilization, and the mechanical properties of the polyethylene itself⁷. One factor that has not been well studied is the effect of liner modularity on polyethylene wear in terms of the generation of wear debris and the subsequent development of osteolytic lesions.

Modular metal-backed acetabular cups are currently the most widely used uncemented acetabular components. Modular cups give surgeons the flexibility to change femoral head size, liner offset, and liner-lip buildup during hip arthroplasty as well as the ability to change the liner without removing a bone-ingrown shell during revision surgery. Although nonmodular acetabular components do not offer this flexibility, they have several advantages that might help to decrease polyethylene wear and the rate of osteolysis, which ultimately would increase the longevity of the component. Nonmodular acetabular components are designed to increase liner thickness and liner-shell conformity as well as to decrease liner-shell micromotion, a source of debris generation.

In this study, we examined the effect of liner modularity on polyethylene wear and osteolysis by comparing a group of nonmodular metal-backed acetabular components with a matched group of modular metal-backed acetabular components. We hypothesized that the nonmodular components would have lower true wear rates, have lower magnitudes of bedding-in, and be associated with less osteolysis than the matched group of modular components.

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Fig. 1:: Photographs showing the nonmodular cup (DePuy). The polyethylene liner is inserted at the factory with a shrink-fit process. This procedure increases liner-shell conformity by eliminating the need to manufacture the liner with radial clearance or an internal locking mechanism. The liner and shell are sterilized as a unit.

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Fig. 2:: Photographs showing the modular cup (Duraloc, DePuy). The polyethylene liner is held in place with a metal locking ring. The liner and shell are sterilized separately and then assembled during surgery.

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TABLE I: Osteolytic Lesions

*The zones were defined by DeLee and Charnley²⁹. †The zones were defined by Gruen et al.³⁰.

	Average Size of Lesion (mm²) [Number of Lesions in Zone]
Acetabular*			Femoral†
Zone 1	Zone 2	Zone 3	Zone 1	Zone 7
Nonmodular cup	?0 [0]	??0 [0]	0 [0]	??0 [0]	193.06 [1]
Modular cup	93.20 [2]	254.23 [2]	0 [0]	245.96 [5]	?78.71 [6]

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TABLE I: Osteolytic Lesions

*The zones were defined by DeLee and Charnley²⁹. †The zones were defined by Gruen et al.³⁰.

	Average Size of Lesion (mm²) [Number of Lesions in Zone]
Acetabular*			Femoral†
Zone 1	Zone 2	Zone 3	Zone 1	Zone 7
Nonmodular cup	?0 [0]	??0 [0]	0 [0]	??0 [0]	193.06 [1]
Modular cup	93.20 [2]	254.23 [2]	0 [0]	245.96 [5]	?78.71 [6]

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

We performed a retrospective review of our office charts and radiographs. Patient confidentiality was strictly maintained. We compared a group of forty-one hips (thirty-nine patients) treated with a nonmodular, porous-coated acetabular component (DePuy, Warsaw, Indiana) (Fig. 1) with a matched group of forty-one hips (forty patients) treated with a Duraloc modular acetabular component (DePuy) (Fig. 2). All patients included in this study had undergone primary total hip replacement at a single institution and had been treated with the same surgical technique. All also had been followed clinically for a minimum of 3.8 years (range, 3.8 to 8.0 years), and a time series of at least three standardized anteroposterior pelvic radiographs were available for analysis. All radiographs were made with a standard film-to-tube distance of 40 in (1 m) by the same group of technicians. Routine anteroposterior radiographs of the entire pelvis were made with the patient in the supine position with both lower limbs in maximal internal rotation.

The patients chosen for the nonmodular group had been treated with an acetabular component, an uncemented porous-coated femoral component (DePuy), and a 28-mm-diameter femoral head from the same manufacturer. The acetabular liners were manufactured from enhanced ultra-high molecular weight polyethylene (Hylamer; DePuy DuPont Orthopaedics, Newark, Delaware) and sterilized by gamma irradiation in air.

The group of forty-one modular acetabular components was selected from a large cohort of patients for comparison with the nonmodular group. The modular group was matched with the nonmodular group according to the type of polyethylene material (Hylamer), polyethylene sterilization method (gamma irradiation in air), femoral head size (28 mm) and manufacturer, manufacturer of the porous-coated femoral stem, and patient gender and age.

The modular Duraloc component (Fig. 2) had dome-loaded polyethylene and a minimum polyethylene liner thickness of 6.0 mm. Forty Duraloc-100 cups with a central dome hole and one Duraloc-1200 cup with a central dome hole and twelve screw holes were used in the modular group. The liner and metal shell of the modular component were sterilized separately with gamma irradiation in air and were assembled during surgery. The nonmodular components had a thinner metal shell and a slightly thicker polyethylene liner than the modular components. The polyethylene liners of the nonmodular components also had dome-loading of the polyethylene with a minimum thickness of 6.0 mm, but they had no holes in the metal shell. To increase conformity between the cup and the liner and to reduce liner micromotion, the liners of the nonmodular cups were inserted into the metal shell at the factory by means of a shrink-fit process. All of the nonmodular cups had a neutral liner, whereas thirty-one modular cups had a neutral liner and ten had a rim liner with 10Â° of elevation.

We obtained the manufacturer’s product numbers and lot numbers from the component boxes to confirm the method of sterilization and determine the shelf life of the polyethylene components. Shelf life was defined as the time between the date of product sterilization and the date of surgery.

Head-penetration data were obtained with use of a previously published and validated radiographic method^8,9. With this method, a computer system, a digitizer tablet (Numonics, Montgomeryville, Pennsylvania), and specially designed software are used to measure two-dimensional penetration by the femoral head into the polyethylene liner on anteroposterior pelvic radiographs. The system has a reported accuracy of ±0.19 mm for measurement of head penetration⁸. Movement of the femoral head perpendicular to the anteroposterior plane is not measured with this method. However, recent studies^10,11 have demonstrated that head penetration perpendicular to the anteroposterior plane is generally small, and thus two-dimensional measurements closely approximate three-dimensional measurements.

Linear regression analysis was used to model temporal head-penetration data for each patient. As previously described^5,9, the slope of the regression line represented the true wear rate of the polyethylene, and the intercept represented the amount of head penetration due to the so-called bedding-in process.

One orthopaedic surgeon (C.A.E.) examined all annual follow-up radiographs for the presence of osteolytic lesions and component fixation. Osteolysis of the pelvis was defined as a circumferential, expansile lytic area with a distinct sclerotic border. Osteolysis of the femur was defined by both the loss of trabecular osseous structure and endosteal cortical erosion. The size of the lesion was measured radiographically with a specialized computer program (Hip Analysis Suite; ARCH Development, Chicago, Illinois)¹⁰. The stability of the implant was assessed on serial radiographs and determined to be bone-ingrown, fibrous stable, or loose.

The activity level of the patient was assessed, at the final follow-up visit, on a scale of 1 to 5, with a score of 1 indicating an active lifestyle and a score of 5 indicating a sedentary lifestyle.

On the basis of the previous literature^8,9, we anticipated the standard deviation of the mean true wear rates to be approximately 0.1 mm/yr. A power analysis (SamplePower; SPSS, Chicago, Illinois) was performed on the basis of this anticipated standard deviation for the two groups of forty-one cases each. The analysis revealed that the difference in mean wear rates between the groups must be at least 0.06 mm/yr for the study to have a power of 80% to detect a significant difference.

Because patient age and component tilt were normally distributed, a Student t test was used to determine if there were significant differences between the modular and nonmodular groups with regard to these variables. A nonparametric Mann-Whitney U test was used to determine if there were significant differences between the two groups with regard to the true wear rate, amount of bedding-in, duration of follow-up, component anteversion, polyethylene thickness, body-mass index, and shelf life because none of these factors was normally distributed. A two-sided Fisher exact test was used to determine significant differences in femoral head material distribution and frequency of osteolysis between the two groups. A Pearson chi-square test was used to determine significant differences in Charnley class¹², activity level, and diagnosis between the two groups. Probability values of <0.05 were considered significant.

Results

Introduction | Materials and Methods | Results | Discussion | References

The nonmodular group was followed for a mean of 5.3 years (range, 3.8 to 6.8 years). The modular group was followed for a mean of 5.5 years (range, 3.8 to 8.0 years). The mean durations of follow-up were not significantly different (p = 0.62). The two groups also did not differ significantly with regard to mean patient age (p = 0.75), body-mass index (p = 0.78), Charnley class (p = 0.36), femoral head material distribution (p = 0.83), primary diagnosis (p = 0.64), or physician-assessed activity level (p = 0.86). However, there were significant differences in cup orientation and polyethylene thickness. Anteversion of the nonmodular cups (22.1Â°) was greater than that of the modular cups (15.7Â°, p = 0.005). Similarly, tilt of the nonmodular cups (43.4Â°) was greater than that of the modular cups (39.7Â°, p = 0.04). Although the mean outer diameters of the two groups were not significantly different (nonmodular cups, 56.2 mm, and modular cups, 55.7 mm; p = 0.40), the mean polyethylene thickness of the nonmodular cups was significantly greater than that of the modular cups (10.9 mm compared with 9.6 mm, p = 0.007). Additionally, the shelf life of the nonmodular components was shorter than that of the modular components (0.20 year compared with 0.59 year, p < 0.001).

Linear regression analysis of temporal head-penetration data showed that the mean true wear rate of the nonmodular group (0.11 mm/yr) was less than, but not significantly different from, that of the modular group (0.16 mm/yr, p = 0.22). Both the standard deviation (±0.08) and the 95% confidence interval (0.08 to 0.13 mm/yr) of the true wear rates of the nonmodular group were less than those of the modular group (±0.13 and 0.11 to 0.20 mm/yr). These results indicated greater consistency in wear behavior for the nonmodular components. There was no significant difference in the magnitude of bedding-in between the nonmodular and modular groups (0.11 compared with 0.14 mm, p = 0.53).

All cups in this study were determined radiographically to have undergone bone ingrowth. All femoral components also were stable: seventy-nine were considered to have undergone bone ingrowth, and three were considered to be stable through fibrous-tissue ingrowth.

In the nonmodular group, one hip (2%) had an osteolytic lesion in the lesser trochanter. The rate of osteolysis in this group was significantly lower than that of the modular group, in which nine hips (22%) had osteolytic lesions (p = 0.01). Two hips had acetabular lesions, and seven hips had femoral lesions (Table I). The mean durations of follow-up for hips with osteolysis (5.7 years) and hips without osteolysis (5.4 years) were not significantly different (p = 0.14). However, the patients with osteolysis were significantly younger than those without osteolysis (46.2 compared with 54.9 years, p = 0.02), and they had a higher true mean wear rate (0.24 compared with 0.12 mm/yr, p = 0.003).

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Debris generated by the wear of articulating components can trigger an osteolytic reaction, which can lead to osseous fracture or component loosening^6,13,14. Consequently, identification of variables that affect the wear process has become of paramount importance to the orthopaedic community.

This study focused on one variable that can affect polyethylene wear: modularity of the acetabular component. Nonmodular acetabular components demonstrated a lower mean true wear rate and a significantly lower rate of osteolysis (p = 0.01) than a matched series of modular components. In addition, the true wear rates of the nonmodular components were less variable than those of the modular components. A closer look at the differences between the two acetabular components can help explain our results.

One difference is liner-shell conformity. Modular cups have nonconformity between the polyethylene liner and the metal shell by design. Radial clearance between the polyethylene liner and the metal shell is necessary to ensure liner insertability. Nonconformity in modular components also can occur because of variations in tolerances allowed by the manufacturer or interference from locking mechanisms. Several finite-element studies^15-17 have shown that backside nonconformity due to manufacturing tolerances can increase the magnitude and location of contact stress in a polyethylene liner. With a nonmodular acetabular component, the polyethylene liner is inserted at the factory with use of a shrink-fit process. This step eliminates two factors likely to interfere with liner-shell conformity: the need to manufacture the liner with radial clearance and the need for an internal locking mechanism.

Another difference between the components is polyethylene thickness. The absence of an internal locking mechanism in a nonmodular component allows for a thinner metal shell and a correspondingly thicker polyethylene liner as compared with a modular component of the same size. As with backside nonconformity, polyethylene thickness has been shown¹⁶ to be related to the contact stress at the articulating and backside surfaces. Although not fully understood or investigated, increases in contact stress at the articulating surface of a polyethylene liner may result in increased wear.

The third difference is liner-shell micromotion. Previous studies have demonstrated that micromotion ranging from 5 to 300 m occurs between modular polyethylene liners and shells^18-20. Backside wear, resulting from micromotion between the liner and the shell in vivo, is an unintended source of debris generation. Although the rate of backside wear has been estimated to be orders of magnitude less than the rate of articular wear²¹, debris generated at the backside of the component has direct access to subchondral bone through screw holes and dome holes. This direct access may contribute to an increased prevalence of acetabular osteolysis around modular cups with holes²².

The significantly higher rate of osteolysis (p = 0.01) in the modular group was mainly due to the greater number of femoral osteolytic lesions in that group (seven) compared with the nonmodular group (one). The small number of acetabular lesions (none in the nonmodular group compared with two in the modular group) precludes our drawing conclusions about the relationship between dome holes and osteolysis. Instead, we attribute the greater prevalence of osteolysis to higher articular wear rates and backside wear of the modular components as well as to hydrodynamic forces. Because of the micromotion, it is possible for the liner in situ to act as a pump, moving joint fluid about the effective joint space. This pumping action disseminates wear debris in joint fluid to the periphery of the implant-bone interface, where osteolytic lesions can develop²³.

A final difference between the components was the sterilization process. Although all polyethylene liners were sterilized by gamma irradiation in air, the nonmodular liners were sterilized after they were inserted into the metal cups, whereas the modular liners and shells were sterilized separately. During gamma irradiation of the nonmodular components, the presence of a metal shell surrounding the polyethylene would have somewhat shielded the liner, effectively reducing the dose of radiation that the liner received. It has been reported²⁴ that lowering the dose of radiation to polyethylene can affect polyethylene properties and may decrease wear.

The ability of the aforementioned characteristics of nonmodular cups to reduce polyethylene wear remains largely theoretical. Polyethylene wear in vivo is a multifactorial process and, thus, it is difficult to determine the precise effect of any single variable. As a way of better isolating the effects of modularity on wear, we eliminated the influence of some confounding variables by matching groups according to patient gender, patient age, type of polyethylene material, method of polyethylene sterilization, femoral head size and manufacturer, and stem manufacturer. Moreover, we found no significant difference in body-mass index, head material distribution, activity level, or primary diagnosis. However, the groups were not matched according to component orientation or polyethylene shelf life. Several authors have reported that increased cup tilt is correlated with greater femoral head displacement²⁵ and a higher rate of osteolysis²⁶. However, our study showed that, although the tilt of the nonmodular cups was 3.7Â° greater than that of the modular cups, the former had a lower wear rate and were associated with a lower prevalence of osteolysis. The conflict between the findings in other studies and our result prevented us from attributing the differences in wear rates in our study to the differences in the angles of cup tilt.

The issue of shelf life of polyethylene sterilized by gamma irradiation in air also has been explored. Currier et al.²⁷ reported that gamma-irradiated liners with less than one year of shelf life had less oxidation and better in vivo performance than liners with a shelf life of greater than one year. In our study, although the mean shelf lives of the two groups were significantly different (0.20 year for the nonmodular group compared with 0.59 year for the modular group, p < 0.001), they were both considerably less than one year, indicating the likelihood of satisfactory in vivo performance. Furthermore, linear regression analysis showed a poor correlation between the true wear rate and shelf life (r² = 0.17). Therefore, we did not attribute the difference in true wear rates between the two groups to the small difference in shelf life.

Although nonmodular components may present a partial solution to the problem of wear, one obvious disadvantage is the relative difficulty of revising failed polyethylene liners in bone-ingrown nonmodular acetabular components. Despite this disadvantage, polyethylene liner exchange has been accomplished in nonmodular metal-backed components²⁸.

In conclusion, this analysis demonstrated that nonmodular components had less wear and associated osteolysis than a matched group of modular components. These findings emphasize the potential advantage of nonmodular components and underscore the need for continued clinical investigation of such components.

References

Introduction | Materials and Methods | Results | Discussion | References

Harris WH. The problem is osteolysis. Clin Orthop,1995;311: 46-53. 31146 1995 [PubMed]

Maloney WJ,Woolson ST. Increasing incidence of femoral osteolysis in association with uncemented Harris-Galante total hip arthroplasty. A follow-up report. J Arthroplasty,1996;11: 130-4. 11130 1996 [PubMed]

Maloney WJ, Peters P, Engh CA,Chandler H. Severe osteolysis of the pelvis in association with acetabular replacement without cement. J Bone Joint Surg Am,1993;75: 1627-35. 751627 1993 [PubMed]

Zicat B, Engh CA,Gokcen E. Patterns of osteolysis around total hip components inserted with and without cement. J Bone Joint Surg Am,1995;77: 432-9. 77432 1995 [PubMed]

Isaac GH, Dowson D,Wroblewski BM. An investigation into the origins of time-dependent variation in penetration rates with Charnley acetabular cups—wear, creep or degradation?. Proc Inst Mech Eng [H],1996;210: 209-16. 210209 1996 [PubMed]

Willert HG. Reactions of the articular capsule to wear products of artificial joint prostheses. J Biomed Mater Res,1977;11: 157-64. 11157 1977 [PubMed]

Scott DL, Campbell PA, McClung CD,Schmalzried TP. Factors contributing to rapid wear and osteolysis in hips with modular acetabular bearings made of hylamer. J Arthroplasty,2000;15: 35-46. 1535 2000 [PubMed]

Sychterz CJ, Engh CA Jr, Shah N,Engh CA Sr. Radiographic evaluation of penetration by the femoral head into the polyethylene liner over time. J Bone Joint Surg Am,1997;79: 1040-6. 791040 1997 [PubMed]

Sychterz CJ, Engh CA Jr, Yang AM,Engh CA. Analysis of temporal wear patterns of porous-coated acetabular components: distinguishing between true wear and so-called bedding-in. J Bone Joint Surg Am,1999;81: 821-30. 81821 1999 [PubMed]

Martell JM,Berdia S. Determination of polyethylene wear in total hip replacements with use of digital radiographs. J Bone Joint Surg Am,1997;79: 1635-41. 791635 1997 [PubMed]

Sychterz CJ, Yang AM, McAuley JP,Engh CA. Two-dimensional versus three-dimensional radiographic measurements of polyethylene wear. Clin Orthop,1999;365: 117-23. 365117 1999 [PubMed]

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Kurtz SM, Edidin AA,Bartel DL. The role of backside polishing, cup angle, and polyethylene thickness on the contact stresses in metal-backed acetabular components. J Biomech,1997;30: 639-42. 30639 1997 [PubMed]

Kurtz SM, Ochoa JA, White CV, Srivastav S,Cournoyer J. Backside nonconformity and locking restraints affect liner/shell load transfer mechanisms and relative motion in modular acetabular components for total hip replacement. J Biomech,1998;31: 431-7. 31431 1998 [PubMed]

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Huk OL, Bansal M, Betts F, Rimnac CM, Lieberman JR, Huo MH,Salvati EA. Polyethylene and metal debris generated by non-articulating surfaces of modular acetabular components. J Bone Joint Surg Br,1994;76: 568-74. 76568 1994 [PubMed]

Schmalzried TP, Guttmann D, Grecula M,Amstutz HC. The relationship between the design, position, and articular wear of acetabular components inserted without cement and the development of pelvic osteolysis. J Bone Joint Surg Am,1994;76: 677-88. 76677 1994 [PubMed]

Sutula LC, Collier JP, Saum KA, Currier BH, Currier JH, Sanford WM, Mayor MB, Wooding RE, Sperling DK, Williams IR,et al.. The Otto Aufranc Award. Impact of gamma sterilization on clinical performance of polyethylene in the hip. Clin Orthop,1995;319: 28-40. 31928 1995 [PubMed]

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DeLee JG,Charnley J. Radiological demarcation of cemented sockets in total hip replacement. Clin Orthop,1975;121: 20-32. 12120 1975

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TABLE I: Osteolytic Lesions

*The zones were defined by DeLee and Charnley²⁹. †The zones were defined by Gruen et al.³⁰.

	Average Size of Lesion (mm²) [Number of Lesions in Zone]
Acetabular*			Femoral†
Zone 1	Zone 2	Zone 3	Zone 1	Zone 7
Nonmodular cup	?0 [0]	??0 [0]	0 [0]	??0 [0]	193.06 [1]
Modular cup	93.20 [2]	254.23 [2]	0 [0]	245.96 [5]	?78.71 [6]

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TABLE I: Osteolytic Lesions

*The zones were defined by DeLee and Charnley²⁹. †The zones were defined by Gruen et al.³⁰.

	Average Size of Lesion (mm²) [Number of Lesions in Zone]
Acetabular*			Femoral†
Zone 1	Zone 2	Zone 3	Zone 1	Zone 7
Nonmodular cup	?0 [0]	??0 [0]	0 [0]	??0 [0]	193.06 [1]
Modular cup	93.20 [2]	254.23 [2]	0 [0]	245.96 [5]	?78.71 [6]