The Impact of Free Radical Stabilization Techniques on in vivo Mechanical Changes in Highly Cross-Linked Polyethylene Acetabular Liners

Abstract

Introduction Numerous thermal free radical stabilization techniques are used in the production of highly cross-linked polyethylene (HXLPE) to improve oxidative stability. Little knowledge exists on the effects of in vivo time on the mechanical properties of HXLPE. The purpose of this study was to determine if free radical stabilization of HXLPE impacts mechanical properties as well as oxidative stability of acetabular liner rims after extended in vivo time. Methods Retrieved and control remelted, single annealed and sequentially annealed HXLPE liner rims were tested for mechanical properties. Oxidation was measured with FTIR spectroscopy and crystalline phase composition measured with Raman spectroscopy. Results No correlation was found between in vivo, ex vivo time and hardness for annealed groups. A statistically significant difference in hardness was identified between free radical stabilization groups. No correlation between maximum rim oxidation and in vivo time was found. Detectable levels of rim oxidation were present in 100% of single annealed, 75% of sequentially annealed, and 25% of remelted retrieved liners. Single and sequentially annealed liners demonstrated oxidation and increased crystallinity. Rim mechanical properties change in vivo for implant types. With in vivo time, retrieved remelted HXLPE demonstrated decreased mechanical properties, whereas retrieved single and sequentially annealed HXLPE properties remained stable. All liner cohorts demonstrated evidence of rim oxidation. Subsequent changes in crystallinity were only observed in oxidized annealed liners. Conclusion HXLPE acetabular liner rims show evidence of in vivo mechanical property degradation, notably in remelted HXLPE, which may be a risk factor in rim fracture and catastrophic implant failure.

The impact of free‐radical stabilization techniques on in vivo subsurface mechanical properties in highly cross‐linked polyethylene acetabular liners

“…These findings are consistent with current literature which supports remelted liners exhibiting a lower overall crystallinity, which is associated with lower yield strength and fatigue resistance but less susceptibility to in vivo oxidation. 13,39,40 Similar to prior work by Decker et al, 34 higher overall rim subsurface microhardness was shown to be present in both singleannealed and sequentially annealed liners, with the single-annealed group exhibiting the highest values among the three groups. Previous studies have demonstrated that changes in hardness correlate with the extent of oxidation.…”

“…Importantly, our study selected eight liners that exhibited the highest subsurface oxidation indices (OI) based on Fourier transform infrared spectroscopy (FTIR) testing in prior work. 34 These included two remelted, three single annealed, and three sequentially annealed liners. Each of these sample liners was sectioned with a Japanese Dozuki detail saw and polished by hand to maintain low processing temperatures to improve the accuracy of the microidentation interpretation.…”

“…These depths were chosen based on the peak rim subsurface OI as described in prior FTIR testing by Decker and colleagues. 34,[36][37][38]…”

“…Our previous research has examined the extent and location of oxidation at the implant rim for retrieved HXLPE liners after extended in vivo exposure time for various free-radical stabilization techniques and quantified the correlated changes in the surface mechanical properties. 34 The goal of this study was to look at the mechanical properties of retrieved HXLPE liners both at the implant rim and articular subsurface, which were assessed using a microindentation technique to assess hardness. It has been demonstrated that hardness correlates with the extent of oxidation; thus, an increase in the hardness would be expected for implants with a predilection towards oxidation and prolonged in vivo exposure.…”

A Multi-Attribute Decision-Making Model for the Selection of Polymer-Based Biomaterial for Orthopedic Industrial Applications

“…This is due to the formation of free radicals during the process of irradiation, which are responsible for continuing long-term oxidation reactions thus limiting the service life of UHMWPE. In order to stabilize the radiation-induced free radicals, number of methodologies are adopted including post irradiation annealing, melting, incorporation of bio-compatible of anti-oxidants, i.e., α-tocopherol (vitamin E) [ 9 , 10 , 11 ], irradiating the UHMWPE in the presence of predetermined amount of organo-silane [ 6 , 12 , 13 , 14 , 15 ], using the suitable silane-based clay [ 16 ], and preparing the UHMWPE nano composites with MWCNTs for quenching the radiation-induced free radicals [ 7 , 8 , 17 , 18 ].…”