The second group, followed for 3.1 to 9.5 years, consisted of 127 hips that had been treated with a Tri-Lock cup (DePuy) (Table I). This cup has a porous-coated titanium shell with a hemispherical outer geometry and a cylindrical inner geometry (Fig. 2). The polyethylene liner is not of uniform thickness; it is thicker at the dome than at the cup face. Only two sizes of liner, either 4.7 or 6.9 millimeters in thickness at the dome, were used in this group. All Tri-Lock components were sterilized with use of gamma irradiation in air.
The third group, which was followed for 3.2 to 7.9 years, consisted of thirty-seven hips in which a Harris-Galante-I cup (Zimmer, Warsaw, Indiana) had been inserted (Table I). Both the cup and the liner are hemispherical. The locking mechanism is exterior to the fiber-mesh-coated titanium shell and does not interfere with the seating of the liner (Fig. 3). Unlike the other first-generation cups in the study, the polyethylene thickness of the Harris-Galante cup increased as the cup's diameter increased. The polyethylene liner ranged in thickness from 5.3 to 11.3 millimeters. All Harris-Galante components were sterilized with use of gamma irradiation in air.
A fourth group, followed for 3.0 to 6.0 years, consisted of forty-seven hips that had had insertion of a Duraloc cup (DePuy) (Table I). The Duraloc cup was the second modular cup design manufactured by DePuy and was a replacement for the first-generation Tri-Lock cup. It is considered a second-generation component because it has an improved locking mechanism, dome-loading of the polyethylene, polyethylene from a known source, and a required minimum polyethylene thickness of 6.0 millimeters. The cup has a porous-coated titanium shell and a hemispherical polyethylene liner; the liner is held in the shell with a metal locking ring that is designed not to interfere with liner-shell conformity (Fig. 4). The thickness of the liner, which increases as the outer diameter of the cup increases, ranged from 6.0 to 12.2 millimeters in this study. All Duraloc components were sterilized with use of gamma irradiation in air.
The mean ages and weights of the patients were not significantly different among the three groups that received a first-generation component, nor were there any significant differences with regard to gender distribution (Table I). The mean age of the patients who had a Duraloc component was significantly greater than that of those who had a Tri-Lock or Arthropor component (p < 0.05), but there was no difference in the mean weight or the gender distribution between the patients who received a first-generation component and those who were managed with a second-generation component (Table I).
We used anteroposterior radiographs of the pelvis to determine two-dimensional penetration by the femoral head into the polyethylene liner at annual intervals, according to a previously reported protocol12. To reduce interobserver error, only two individuals (engineering students employed by our laboratory) performed the measurements. The positioning of the patients and the radiographic technique were consistent over time as the same group of technicians at the same institution made all of the radiographs. Only patients who had had a primary total hip arthroplasty were included in the study; if a patient later had a revision operation, the annual radiographs made before the revision were digitized and included in the analysis. This was done in order not to exclude patients who might have had a high rate of wear.
This method of analysis involved use of a computer system, a digitizer tablet, and software specially designed to measure penetration by the femoral head into the polyethylene liner. The computer operator aligned an anteroposterior radiograph of the pelvis on the digitizer tablet so that the interteardrop line was parallel to the edge of the tablet. The operator then digitized at least five points around the circumference of the femoral head and at least five points around the circumference of the acetabular cup. The computer software, after correcting for magnification and distance from the center of the x-ray beam, fit circles to these points and determined their centers. The movement of the center of the head relative to the center of the cup then was used to calculate the amount and direction of penetration by the femoral head into the polyethylene liner in the plane of the radiograph. The amount of penetration on the immediate postoperative radiograph was assigned the zero position; penetration by the head was calculated as subsequent motion from this initial point.
Validation of this measurement system with use of polyethylene liners machined to a known amount of wear has been documented12. That study revealed that the calculated amount of penetration by the head tended to differ from the true amount of penetration by a mean of 0.19 millimeter and that the operator error was smaller than the computer error. The operator's validation measurements never exceeded a standard deviation of 0.11 millimeter. The operator was so consistent in the choice of digitized points that the results of the penetration measurements were extremely reproducible. The main limitation was related to the accuracy of the computer software, but because the accuracy was constant for all groups it had little effect on the comparisons made in that study.
Analysis of the Data
We used multiple linear regression analysis to determine the best-fit line for data on penetration over time in each of the four groups. Each best-fit line was characterized by its own slope and intercept. The slope of the best-fit line described the true rate of wear in that group, and the intercept estimated the amount of penetration due to the bedding-in process.
There is a substantial difference between measurements of wear based on only the most recent follow-up radiograph and those based on a series of radiographs made over time. With the former method, the total penetration by the head is divided by the duration that the component has been in situ. If, for example, two millimeters of penetration was measured on the eight-year follow-up radiograph (Fig. 5, point D), the rate of wear would be 0.25 millimeter per year (Fig. 5, slope of solid black line). However, the latter method—analysis of a series of radiographs made over time—allows penetration by the head to be measured at multiple intervals (for example, during the second, fourth, sixth, and eighth postoperative years; Fig. 5, points A through D). The rate of penetration by the head between the second and the eighth postoperative year is calculated as the slope of the best-fit line connecting those points (Fig. 5, dashed black line between points A and D). In this example, the slope of the line between the second, fourth, sixth, and eighth years is 0.08 millimeter per year. Since the effects of bedding-in are negligible after the second postoperative year3,6,8,15,16, this rate of penetration (from the second to the eighth year) must represent movement of the head due solely to the removal of polyethylene particles, or the true rate of polyethylene wear. Thus, the rate of wear that is calculated on the basis of multiple radiographs is much lower than that calculated on the basis of the eight-year radiograph only (0.08 compared with 0.25 millimeter per year).
To continue with this example, it also is possible to estimate the amount of penetration of the head attributable to the bedding-in process6. If it is assumed that polyethylene particles are removed from the liner at a constant rate during the initial years that the component is in vivo, then the line that connects the points representing movement of the head can be extended backward to estimate the removal of polyethylene particles during the initial postoperative period (Fig. 5, dashed gray line). The point where this line crosses the y axis (1.3 millimeters in the current example) thus represents the amount of penetration that cannot be attributed to the removal of polyethylene particles. This point (the intercept of the best-fit line) represents movement of the head due to the bedding-in process.
Coded variables in the linear regression analysis were used to determine whether the slopes and intercepts differed among the groups. One-way analysis of variance with post hoc (Scheffé) tests was used to determine statistical differences in age and weight between the patients who received a first-generation cup and those who received a second-generation cup.
Temporal Patterns of Penetration by the Femoral Head and True Wear
To illustrate the interactions among the amount of penetration by the head, the rate of penetration, bedding-in, and true polyethylene wear, we constructed graphs of the temporal penetration patterns for the three groups of first-generation acetabular components. As expected, the amount of penetration increased in each postoperative year (Fig. 6). During the first few years, this movement was due to both bedding-in and polyethylene wear. With time, however, bedding-in diminished and penetration resulted mainly from the removal of polyethylene particles from the liner3,6,8,15,16. The large standard deviations were an indication of the wide variety of individual wear patterns.
The rate of penetration by the femoral head was highest in the first two postoperative years and then decreased with time (Fig. 7). This rate was calculated as the total amount of penetration divided by the duration that the component had been in situ. A high initial rate of penetration reflected bedding-in. Although bedding-in occurred only during the initial postoperative years, its effects were reflected in the calculations of the rate of penetration throughout the life of the implant; in other words, the inclusion of initial bedding-in in subsequent calculations caused all subsequent annual rates of penetration to increase.
The data on penetration in the groups managed with the first-generation components revealed wide variations in individual responses. Predictably, however, the mean amount of penetration by the femoral head increased steadily with time in all three groups. Therefore, we used multiple linear regression analysis to model the data on penetration as a separate line for each type of cup and to determine the slopes and intercepts for these lines (Fig. 8). This relationship between time and penetration by the head was significant (p < 0.001 and r2 = 0.33). The r2 value indicated that the linear model was able to account for 33 percent of the variation in the data on penetration.
Analysis of true wear (slopes) and bedding-in (intercepts) indicated that the Harris-Galante cups performed better than the other two components (Fig. 8). The slope of 0.13 millimeter per year for the Arthropor components was significantly greater (p < 0.05) than the slope of 0.08 millimeter per year for both the Tri-Lock and the Harris-Galante components. The larger slope for the Arthropor components suggests that the true mean rate of wear in this group was greater than that in the other two groups of first-generation components. The components also had different intercept values; the intercept for the Tri-Lock group (0.33 millimeter) was greater than that for the Arthropor group (0.26 millimeter) and significantly greater than that for the Harris-Galante group (0.22 millimeter; p < 0.05).
The lower value for true wear of the Harris-Galante cups corresponds well with the excellent clinical results and low rates of osteolysis associated with these components that have been reported in the literature9,14.
Similar to the first-generation acetabular components, the second-generation (Duraloc) components had a wide variation in individual responses and an increase in the mean amount of penetration by the head over time. Inclusion of the data on the Duraloc components in the linear regression analysis of the first-generation components increased the r2 value incrementally to 0.36. This r2 value meant that the analysis was able to account for 36 percent of the variation in the data on penetration.
Despite the introduction of design modifications that were intended to decrease the rate of polyethylene wear, the true mean rate of wear of the Duraloc components (0.12 millimeter per year) was actually higher than that of two of the first-generation designs (0.08 millimeter per year) (Fig. 9). These modifications, which included an improved locking mechanism and increased liner-shell conformity, did, however, decrease the liner's bedding-in period, as reflected by the fact that the intercept for the Duraloc cups (0.09 millimeter) was significantly less than the intercepts for the Arthropor and Tri-Lock cups (p < 0.05) and was nearly significantly less than that for the Harris-Galante cups (p = 0.08).
The current literature suggests that the rate of penetration by the femoral head associated with metal-backed acetabular components is greater than that associated with all-polyethylene acetabular components inserted with cement5,7,11. The literature also documents a high prevalence of osteolysis associated with some metal-backed components1,2,4,10,13. In the hope of decreasing polyethylene wear and reducing complications associated with long-term wear, surgeons have turned to newer designs of modular acetabular components, different materials for the femoral head, and newer types of polyethylene.
Using radiographic measurements of penetration by the femoral head, clinicians have attempted to assess the effects of these changes in design and material on polyethylene wear. Typically, this process involves measuring the difference between the center of the head and the center of the metal shell on the most recent postoperative radiograph. This measurement, divided by the duration that the component has been in situ, represents the rate of polyethylene wear for that particular component.
The problem with this method of measurement is that penetration by the head as assessed radiographically is a combination of true wear and bedding-in of the polyethylene liner. In all-polyethylene components inserted with cement, the major component of bedding-in is creep, whereas in metal-backed acetabular components, there are several contributing factors, including an unsupported or noncongruent polyethylene liner, a poor locking mechanism, and creep. Consequently, modular acetabular components with different designs may have different magnitudes of bedding-in. Since bedding-in does not generate particles of polyethylene, this process must be distinguished from true wear in order to determine whether new designs of components actually decrease wear-related complications.
Radiographic monitoring of penetration by the head over time allows temporal wear curves to be constructed for individual patients, and these curves can be used to distinguish bedding-in from true wear of the liner. Isaac et al. developed this concept for the evaluation of polyethylene wear of acetabular components inserted with cement6. They proposed that penetration be viewed in terms of an initial, relatively rapid, irrecoverable plastic deformation followed by a longer-term, steady-state rate of wear. Although polyethylene wear occurs throughout the entire duration of implantation, Isaac et al. stated that the rate of creep decreases to nearly zero within the first one to two years. Consequently, temporal wear patterns can be modeled simply as a line with the slope representing the true rate of wear and the intercept representing the initial bedding-in period.
By analyzing the temporal wear patterns of polyethylene components in this way, we were able to compare the true rates of wear (the slopes) and the initial bedding-in (the intercepts) among four different types of acetabular cups (three first-generation components and one second-generation component). Of the first-generation components, the Harris-Galante cups had the least penetration by the femoral head into the polyethylene liner. The Arthropor cups had greater penetration than the Harris-Galante cups because of a higher true rate of wear, and the Tri-Lock cups had greater penetration than the Harris-Galante cups because of significantly greater bedding-in.
Our results can be explained when they are considered together with the clinical observations of the implants. A recent study of Arthropor implants showed radiographic evidence of separation of the beads from the porous surface of the metal cup in eleven (11 percent) of 104 hips13. If loose beads from the cup were to gain access to the articular interface, this third-body debris would affect polyethylene wear dramatically; this could explain why the Arthropor cups in the current study had a greater rate of wear than the other first-generation cups. As for the Tri-Lock cups, the significantly greater bedding-in could be explained by their poor design. Because of the geometry and lip of the Tri-Lock liner (Fig. 2), the polyethylene often did not seat properly in the metal shell and there often was a gap between the bottom of the liner and the shell1. This gap caused increased stress and deformation of the lip, contributing to rapid initial penetration by the head. Therefore, it is not surprising that the design of this cup greatly increased the bedding-in of the liner. The so-called gap problem with the Tri-Lock cup clearly illustrates the need to quantify the bedding-in process. The inclusion of this design in our analysis allowed us to quantify the true rate of wear of this component, something that has not been possible with use of previous techniques.
With regard to the performance of the second-generation Duraloc component, we found that, despite modifications in design, the true rate of wear was not lower than that of two of the first-generation acetabular components that were studied. Surprisingly, these modifications (which included thicker polyethylene, an improved locking mechanism, and better liner-shell conformity compared with those of the first-generation Tri-Lock component) merely decreased the bedding-in of the liner. Thus, our findings indicate that it is unlikely that within the next five years the Duraloc components will have fewer wear-related complications than the earlier designs of components in our study.
We acknowledge that polyethylene wear is a multifactorial process and that differences that were detected among the groups in this study might not be attributable solely to differences in the designs of the cups. The large standard deviations in the wear data (Figs. 6 and 7) demonstrate the wide variation in individual responses; these differences could be due to a number of unquantifiable, confounding variables. For instance, third-body wear debris and the roughness of the femoral head are both known to affect wear; however, as long as the component remains in situ, neither the roughness of the component nor the amount of debris in the joint can be assessed. Another factor that is difficult to quantify objectively or to include in any analysis of wear is the patient's level of activity. Although this factor can strongly influence polyethylene wear, we were unable to account for it, and this was a weakness of the current study. However, there was no bias toward the selection of the design of the cup according to the perceived activity level of the patient; the cup simply was chosen according to the design being used at that time.
We attempted to eliminate the influence of some confounding variables by including only patients who had had a primary hip replacement with insertion of an AML femoral component, a thirty-two-millimeter-diameter femoral head, and a standard ultra-high molecular weight polyethylene liner. However, femoral heads made of two different types of materials were used in our series. Although the material of the femoral head can affect the wear of the liner over time, it has little effect on the bedding-in of the liner, which depends largely on the design of the cup and the polyethylene. We therefore believe that our conclusions are valid.
Our findings and our technique of analysis have clinical relevance for surgeons who evaluate polyethylene wear with use of annual postoperative radiographs. First, penetration by the femoral head in the early postoperative years may not be caused by the removal of polyethylene particles from the liner but, rather, may be a result of a change in the position of the head due to the bedding-in process. Wear calculations that include bedding-in artificially inflate the rate of wear and therefore misrepresent the potential risk of wear-related complications. This is especially true with regard to comparisons of different designs of modular cups, in which conformity and tolerances between the polyethylene liner and the metal shell can vary greatly. Second, analysis of penetration by the head at multiple time-intervals can be used to distinguish true abrasive wear from bedding-in. Such an analysis allows more accurate determination of the true rates of wear of different designs of modular cups and therefore a more accurate assessment of the potential risk of wear-related complications. Although the prevalence of wear-related complications (such as osteolysis) was not specifically analyzed in the current study, the relationship between true rates of wear and wear-related complications is an important area for future investigation.
Finally, we recognize that, although the present analysis improves upon previous analyses, it could be an oversimplification of the wear process. First, our data on penetration by the femoral head are strictly two-dimensional. Because two-dimensional measurements exclude penetration perpendicular to the plane of the anteroposterior radiograph, they may represent an underestimation of three-dimensional movement of the head. Second, the r2 value for our regression analysis was 0.36, indicating that this linear model accounted for only 36 percent of the variation in the data on penetration. Although this percentage might seem low, it is impossible to control the many variables affecting wear. As the study of polyethylene wear progresses, orthopaedic researchers will be able to identify and control more of the variables that truly affect wear. The inclusion of these variables in increasingly sophisticated models of wear should lead to a gradual increase in the r2 values. The current study represents a small step toward understanding a complex phenomenon. It is hoped that other investigators will build on our data and, with use of more sophisticated models, will further define the true wear process and reveal the variables that affect it.
*One or more of the authors has received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
Investigation performed at the Anderson Orthopaedic Research Institute, Alexandria
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