Each zone was subjectively graded from a scale of 0 to 3 for seven different damage modes (burnishing, abrasion, cold flow, scratching, pitting, delamination, and embedded debris). The damage score was 0 for no damage, 1 for < 10% area affected (Fig. 3A), 2 for 10% to 50% area affected (Fig. 3B), and 3 for > 50% of area affected (Fig. 3C). The scoring system was based on previously established and validated protocols for damage scoring in retrieved polyethylene specimens [11, 22, 32]. The maximum possible damage score by this method was 105. Initially a single observer (KB) graded all the 233 polyethylene inserts based on the scoring system. To ensure the validity of the damage scoring results, rescoring was performed for Zone 1 in a sample of 46 polyethylene liners taken evenly from the four groups (12 each from PD-CPE and RW-CPE and 11 each from PD-XLPE and RW-XLPE). This sample size was based on established guidelines for achieving adequate power in reliability studies [37, 38]. Scoring by a second observer (MGT) was done to confirm the interobserver agreement and by the first observer again to confirm the intraobserver reliability. Both observers were blinded to previous results at the time of rescoring. For interobserver reliability, the weighted kappa score was 0.769, indicating substantial agreement. For intraobserver reliability, the weighted kappa score was 0.736, also indicating substantial agreement in the reproducibility of results for the primary observer. Because the weighted kappa scores showed high interobserver and intraobserver reliability for the 46 polyethylene liners, no further rescoring was performed.
The D'Agostino and Pearson omnibus normality test was used to assess the distribution of the data, and the appropriate parametric or nonparametric tests were applied. Demographics between implant groups were assessed using a one-way analysis of variance (age) or Kruskal-Wallis test (BMI and implantation time). Unpaired comparisons of damage were performed using an unpaired t-test or Mann-Whitney test. Paired comparisons of damage were performed using a paired t-test or Wilcoxon matched-pairs signed-rank test. Correlation between damage scores and age, BMI, and time in vivo was performed using Spearman correlation and linear regression. Because the CPE groups had a longer implantation time (described later in the Results section), the XLPE groups were matched in a pairwise fashion to the CPE groups based on implantation time and age. A p value < 0.05 was taken as statistically significant.
With CPE, there was more backside damage with the rough surface/wire lock socket design than with the polished surface/dovetail socket design. Total damage scores (Table 3) in the RW-CPE (19 ± 7) group were greater (p < 0.001) than the PD-CPE group (8 ± 4). The two most common damage modes were abrasion and burnishing, with scratching much more prevalent in the RW-CPE group.
Crosslinking reduced backside damage for the rough surface/wire lock socket design but not the polished surface/dovetail socket design. The total damage was reduced (p = 0.02) with crosslinked polyethylene for the RW-XLPE (9 ± 9) versus RW-CPE-type sockets (4 ± 4). However, there was no difference between CPE and XLPE liners in the PD-type sockets (p = 0.26). The three most common damage modes were abrasion, scratching, and pitting. The PD-XLPE group had more embedded debris and pitting than the RW-XLPE group.
When crosslinked polyethylene was used, there was no difference (p = 0.16) between the rough surface/wire lock socket design and the polished surface/dovetail socket. Age (r = −0.09, p = 0.16) and BMI (r = −0.02, p = 0.78) did not correlate with total damage scores for all the inserts (Fig. 4). However, a longer time in vivo was associated with higher total damage scores (r = 0.43, p < 0.001). After matching the RW-XLPE and PD-XLPE group demographics, the total damage score in the matched RW-CPE group (n = 13) was 15 ± 7 and 6 ± 5 in the matched PD-CPE group (n = 16).
The damage scores in the peripheral zone, which may reflect the effect of differences between locking mechanisms of the two socket designs, were higher for RW-CPE-type sockets as compared with the PD-CPE type (p < 0.001) and the RW-XLPE type (p = 0.02). The mean damage scores in the PR zones were 4 ± 3 for RW-CPE, 1.3 ± 1.3 for RW-XLPE, 1.4 ± 1.0 for PD-CPE, and 0.9 ± 1.1 for PD-XLPE. Within the other zones (Table 4), damage was greater (p < 0.001) within the superior zones (AS + PS, 7 ± 4) compared with the inferior zones (AI + PI, 3.0 ± 2.8). There was no evidence of rim fractures or cracking in any of the polyethylene inserts.
Backside wear in polyethylene liners continues to be controversial and there is a lacuna in the literature addressing the effect of cup design and polyethylene material (CPE or XLPE) on backside wear [4, 8, 15, 26, 28-30, 36, 44]. The Reflection and Duraloc sockets have been among the commonly implanted acetabular implants over the last two decades [9, 10, 18, 19, 34, 40, 41]. As such, surgeons performing revision THA will continue to deal with these sockets, especially in scenarios with the possibility of isolated liner revision where a well-fixed socket might be retained. We therefore asked the following: (1) With CPE, did the acetabular design make a difference in backside wear? (2) Is there a difference in backside damage between CPE and XLPE liners? (3) With XLPE, is the difference in backside wear between designs still present? (4) Is there any difference in backside damage in various zones on the backside of individual liners?
There are a few limitations of the current study. We did not perform correlations of our findings with articular wear in the current study. In addition, the cause of revision (especially infection and instability) may play a role in the amount of backside wear and this was not analyzed in the current study. Furthermore, visual backside damage (as measured in the current study) may not completely correspond with true volumetric wear. However, damage scoring is a valid and common method in retrieval studies [11, 22, 32] and we found substantial inter- and intraobserver reliability of this method. The two socket designs examined in this study were distinctly different with respect to both inner socket surface finish and locking mechanism; this makes it difficult to know the relative effect of each of these variables. Also, the alteration in wear properties can change the biological response to wear debris. Reactivity of the wear particles may depend on particle size and we did not study this. In addition, other factors that can potentially influence backside polyethylene wear in modular acetabular liners were not assessed, including differences in manufacturing and sterilization techniques, shell-liner conformity, the use of screws, the number of screw holes, the thickness and size of the liner used, evidence of neck impingement, radiographic parameters (ie, acetabular inclination/version), and clinical parameters (such as activity level of patient). Inclusion of all these factors would be challenging to evaluate in a study of this size, although we believe the large numbers (233 inserts) in the current study, in which only two acetabular designs were examined, were sufficient to support our conclusions with respect to backside polyethylene wear. As more crosslinked liners are revised over time, this will enable researchers in retrieval centers to examine such liners in greater numbers than the present study (n = 13 and 16 in the Duraloc and Reflection groups, respectively) and to examine additional factors.
The current study found lesser backside damage for CPE in Reflection sockets with polished inner surfaces and a more robust locking mechanism. Previously published studies have not fully established the effect of polishing of the inside surface of modular sockets. In their respective in vitro studies involving long-term simultaneous sinusoidal and static loading, Lieberman et al.  and Chen et al.  showed less backside polyethylene wear in modular sockets with polished inside surfaces. However, contrary to these findings, the in vitro studies by Shepard et al.  and Kurtz et al.  did not show any relation between backside wear and polishing of the inner surface of modular sockets. None of the acetabular liners had severe backside damage. The maximum possible backside damage score by the system used in the current study was 105; however, the mean damage scores in the study ranged from only from 4 for PD-XLPE to 19 for RW-CPE. This reflects minimal to very low backside damage, which is comparable to the retrieval studies by Akbari et al.  and Krieg et al. . However, the backside of the liner continues to be a source of particulate debris in THA and whether this backside wear is important enough to cause a strong osteolytic response in modern modular sockets remains to be answered.
We also found reduced backside damage in XLPE liners in comparison to the CPE liners. Although XLPE has been increasingly shown to be more resistant to articular-sided wear than the CPE [5-7, 13, 27, 31, 39], there have been no studies comparing backside wear in combination with surface treatment and locking mechanisms. We found similar backside damage for XLPE liners implanted in Duraloc sockets as compared with XLPE and CPE liners in Reflection sockets. Only the CPE liners in Duraloc sockets had higher backside damage scores. This finding suggests that XLPE seems to be more resistant to backside damage and tends to offset the deleterious effects of a roughened socket with a flexible locking mechanism. Although not proven by the current study, there may be a beneficial effect of XLPE on wear and potentially on osteolysis, which is reflected in the causes for revision of the retrieved implants in our study. Although polyethylene wear and aseptic loosening were important reasons for failure of CPE inserts, these were not seen with the XLPE inserts (Table 2). We found no evidence of an increased propensity for rim fracturing or cracking with the XLPE liners, which has been reported in some previous studies [16, 17, 35].
We also specifically evaluated the damage scores in the peripheral zone of the liner that contacts with the locking mechanism in the shell. Higher damage scores in the peripheral zones of Duraloc socket suggest higher in vivo micromotion of liner in this socket as compared with the Reflection socket. As far as the locking mechanism is concerned, Fehring et al.  and Williams et al.  reported less micromotion for liners with Reflection shells as compared with the Duraloc shells during in vitro testing. Decreased micromotion has also been shown to be associated with less backside polyethylene debris generation in laboratory testing [8, 44]. However, none of the studies have compared the in vivo backside damage for these two cups with differing locking mechanisms. The damage scores were also higher in the superior zones as compared with the inferior zones on the backside. This correlates with the direction of joint reaction forces in the hip and higher articular-sided polyethylene wear vector in vivo .
Considering the enormous number of modular cementless sockets that have been implanted worldwide, there are relatively few studies examining backside polyethylene wear. We have summarized these studies (Table 5). The relatively small number of retrieved liners reported on coupled with the lack of comparative groups within these studies limits their ability to draw solid conclusions. The current study, with much larger number of retrieved liners, shows higher backside damage in CPE liners, higher backside damage with rough/flexible wire-type sockets, and an apparent protective effect of XLPE on backside damage in any kind of acetabular socket. We also noted a correlation of backside damage scores with linear time in vivo for the inserts. The age of the patient, height, weight, or BMI had no correlation with damage scores. These results were in line with the previous retrieval study by Akbari et al. . Most of the other retrieval studies have not commented on the correlation of wear with patient demographics.
In conclusion, the current study notes the influence of both design and materials on backside polyethylene damage in modern modular acetabular sockets. Although the backside wear is, in general, low for retrieved acetabular liners, CPE liners have higher associated backside damage. This may be particularly relevant for CPE liners implanted in Duraloc-like roughened cups with a flexible wire locking mechanism; and this is a combination that should be carefully watched for the sequelae of backside damage. A socket with a polished inner surface and a rigid locking mechanism appears be a better design with in the setting of CPE. The use of XLPE liners, however, has been shown in the current study to protect the liner from backside damage in either cup design. With a competent, intact locking mechanism, and use of crosslinked polyethylene, revision arthroplasty surgeons may not need to worry about cup design when undertaking isolated liner revision.
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