In hip joint replacement, Highly Crosslinked Polyethylene (HXLPE) is introduced into clinical practice to reduce wear and the occurrence of revisions caused by osteolysis.
"The first generation of HXLPE"
The production process of the first-generation HXLPE involves three crucial steps: irradiation, post-irradiation annealing, and sterilization. Ideally, the reduction in wear is directly proportional to the absorbed radiation dose, and crosslinking typically saturates at around 100 kGy. The choice of irradiation source (gamma or electron beam) has no significant impact on the polymer's wear resistance, and both methods are employed in commercial materials.
While crosslinking is highly beneficial from a wear resistance perspective, it comes at the cost of reduced molecular mobility, decreased material ductility, and a decrease in fatigue and fracture resistance due to the crosslinking of molecular chains. Therefore, while crosslinking enhances specific properties, it sacrifices others. This means that orthopedic implant materials must strike a balance between crosslinking and maintaining mechanical properties and antioxidant capabilities.
The choice of heat treatment process significantly impacts crystallinity and mechanical properties and can also affect the material's antioxidant capabilities. Heat processing below the melting point (approximately 137°C) is referred to as annealing, while heat processing above the melting point is known as remelting.
At a dose of 100 kGy, the remelted material exhibits slightly lower values for elastic modulus, yield stress, and ultimate stress compared to the annealed material.
The annealed material has a crystallinity of 60%, whereas the remelted material has a crystallinity of 43%. Throughout the entire stress-strain curve, the higher crystallinity of the annealed material results in a greater ability to resist plastic deformation compared to the remelted material.
While both annealing and remelting processes have their advantages and disadvantages, the clinical performance of these two materials is generally successful.
"The second generation of HXLPE"
In addition, to wear, clinical failure modes such as fracture and impact due to malpositioning may become new limiting factors for the long-term clinical performance of HXLPE. Debates surrounding the importance of factors like oxidative resistance and edge cracking have driven the development of the second generation of HXLPE. The second generation of HXLPE refers to the use of methods other than irradiation to reduce material wear, such as multiple annealing cycles and the addition of vitamin E.
The goal of the second generation of HXLPE is to reduce material oxidation while preserving or even improving the mechanical properties required for high-stress applications.
(A) An accelerated aging combined with cyclic deformation test setup; (B) Standard flexural specimen tests under accelerated aging conditions (80°C, air) with cyclic deformation at an initial stress of 10 MPa; (C) 5-week testing of irradiated UHMWPE specimens with added vitamin E under the same conditions, showing no cracks; (D) Average oxidation levels of standard and irradiated UHMWPE with and without added vitamin E after 5 weeks of cyclic deformation and accelerated aging.
JUST Medical uses imported highly cross-linked polyethylene material to achieve a perfect balance between wear resistance, oxidation resistance, and mechanical performance.