| Abstract / Summary | Background
Aseptic component loosening, a consequence of progressive osteolysis caused by wear of the ultra high molecular weight polyethylene (UHMWPE), has been the leading cause of failure in total hip arthroplasty (THA) since the introduction of this material for use as a bearing surface. Thus, researchers and clinicians alike have worked to improve the polyethylene articulating surface with the main goal of reducing wear. Highly cross-linked UHMWPE (HXLPE) was introduced in 1998 as a potential solution. The increased cross-link density reduced deformation that lead to wear. However, residual free radicals left the material prone to oxidation. A post-irradiation heating step eliminates free radicals, but compromises the fatigue strength of the material. Subsequently, vitamin E diffused HXLPE was developed to eliminate free radicals without reducing strength, while simultaneously maintaining the low wear observed in HXLPE. Since HXLPE and vitamin E diffused HXLPE deform and wear at dramatically reduced rates compared to UHMWPE, it is necessary to use the most sensitive and accurate tools, such as radiostereometric analysis (RSA), to monitor the performance of these materials over time.
Objective
The aim of the thesis was to use RSA to evaluate current generations of polyethylene to accurately and precisely determine their performance. Since osteolysis adversely affects THA survival, it is necessary to closely monitor wear, particularly in the mid-term and beginning stages of the long-term, to ensure that wear-related failure will not be a concern into the long-term.
Methods
In study I, the 13 year performance of HXLPE articulating with 28mm and 36mm femoral heads was assessed using RSA, plain radiographs, and computerized tomography (CT) scans. Study II was a five year randomized controlled trial comparing a medium cross-linked polyethylene (ArComXL) to a vitamin E diffused HXLPE (E1) using markerless RSA (shell only) and patient-reported outcome measures (PROMs). Study III utilized RSA and PROMs to assess the five year performance of E1 using the gold-standard RSA (shell combined with liner beads) method. Finally, Study IV was a methodological evaluation of the three measurement methods available in marker-based RSA to monitor polyethylene wear and acetabular cup migration.
Results
The 13 year mean ± standard error of the mean (SEM) proximal femoral head penetration into the HXLPE was 0.06 ± 0.03mm, with no significant differences within the whole cohort after two years, and no significant differences between the two head sizes at any interval. No patient showed any signs of osteolysis on the plain radiographs or CT scans over the 13 years. In study II, the five year median (interquartile range [IQR]) penetration into the E1 liners was -0.04mm (-0.13 to 0) with significant differences between 6 weeks compared to three and five years (p<0.001 for both analyses) and between one year compared to three and five years (p=0.005 and p=0.001, respectively). The ArComXL group had 0.07mm (-0.03 to 0.16) of penetration at five years with no significant differences over time. Significant differences in penetration between E1 and ArComXL were observed at three and five years ((p=0.029 and p=0.019, respectively). In study III, the mid-term mean SEM penetration into the E1 liners was 0.06 0.01mm, with no significant differences after two years. All patient-reported outcomes in studies II and III (PROs) improved significantly from the preoperative follow-up compared to all postoperative intervals (p<0.001), with no differences between the two polyethylene groups in study II. Finally, study IV indicated that the shell + liner method was most desirable for measuring polyethylene wear and acetabular cup migration.
Conclusion
The current generations of polyethylene used in THA are performing safely at the mid- and long-term intervals. The medium cross-linked polyethylene, HXLPE, and vitamin E diffused HXLPE all demonstrated dramatically reduced wear compared to previously reported UHWMPE wear values. Additionally, the long-term HXLPE cohort showed no evidence of osteolysis, regardless of femoral head size. Patients in studies II and III all continued to report favorable PROs at the mid-term. The unexpected result of negative penetration values of the E1 cohort in study II could be due to the markerless RSA measurement method. The precision analysis of the three measurement methods done in study IV indicated that the shell only method was the least precise. Perhaps the negative values are a reflection of the software’s attempt at measuring nearly zero penetration. Furthermore, the same material was used in study III, and the penetration was low and positive throughout the five years. Despite this discrepancy, the magnitude of all penetration values across the body of work was considerably lower than the reported wear of UHMWPE. Continued monitoring of all patients analyzed in this work is necessary to determine if the low penetration and safety of these current generations of polyethylene is maintained into the true long-term.
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