Comparison of In-Vivo Performance Characteristics of First-Generation and Second-Generation Cross-Linked and Conventional Explants

Abstract

Background: Implants made of ultra high molecular weight polyethylene (UHMWPE) has been used for almost 60 years in hip joint arthroplasty as articulating surface. UHMWPE implants have evolved over time from conventional to cross-linked implants. Chemical, morphologic, and micromechanical characteristics play important roles in overall in vivo performance.

Methods: This study aimed at comparing chemical, morphologic, and micromechanical in vivo performance characteristics of conventional and cross-linked explants. Optical damage scoring, Fourier transform infrared spectroscopy, differential scanning calorimetry, and depth sensing indentation techniques were used for the measurements. The measurement results were used to correlate oxidation index, degree of crystallinity, E-Modulus, and hardness.

Results: Different manufacturing processes directly affect implants' in vivo performance. Conventional explants are more susceptible to oxidation in the absence of thermal treatment and air ambient gamma irradiation sterilization, whereas they have higher degree of crystallinity and E-Modulus values. Introduction of a thermal treatment step to first-generation cross-linked explants has decreased the oxidation susceptibility but degraded the crystalline structure however, such explants manufactured with moderate total cross-linking irradiation dose and a combination of both remelting and annealing thermal treatment methods are the exceptions. The second-generation cross-linked explants in general have much better E-Modulus, that is, hardness values over all other types of explants and are least susceptible to oxidation.

Conclusion: The results suggest that in vivo performances of explants could benefit from the hybrid UHMWPE implant manufacturing techniques, such as moderate cross-linking irradiation doses, remelting, followed by annealing and ethylene oxide or gas plasma sterilization.

Keywords: UHMWPE; cross-linking; crystallinity; damage scoring; hip explants; morphology.