Effect of cross-linking on the microstructure and mechanical properties of ultra-high molecular weight polyethylene.
Ultra-high molecular weight polyethylene is a semicrystalline polymer, which means that a portion of the molecules is in a solid crystalline phase and the remaining portion is in a rubbery amorphous phase. Varying the polymer chemistry in the two phases can alter the mechanical properties of the material. When highly cross-linked polyethylene is formed, the cross-links occur in the amorphous but not the crystalline region. Remelting after irradiation-induced cross-linking neutralizes the free radicals that are caused by irradiation but also decreases the amount of crystallinity. Decreased crystallinity can contribute to a decrease in mechanical properties. Annealing below the melt temperature after irradiation retains a higher level of crystallinity. However, heating below the melt temperature does not neutralize irradiation-induced free radicals that can then react with oxygen, causing oxidative degradation. Newer "second-generation" highly cross-linked polyethylenes have been developed that are annealed below the melt temperature, but use either a pharmacologic antioxidant, mechanical deformation, or sequential low-dose irradiation and annealing treatments rather than heating above the melt point to neutralize residual free radicals. High-pressure treatment at elevated temperatures also can increase crystallinity. However, increased crystallinity is associated with an increase in modulus and contact stress, which can increase wear. Although cross-linking ultra-high molecular weight polyethylene can reduce wear, currently available highly cross-linked polyethylenes also decrease mechanical properties when compared with conventional ultra-high molecular weight polyethylene, so that use of these materials in total knee arthroplasty may contribute to mechanical failure of the bearing surface.
Ries
Michael D
MD
Department of Orthopaedic Surgery, University of California San Francisco, San Francisco, CA 94143, USA. riesm@orthosurg.ucsf.edu
Pruitt
Lisa
L
D019245:Compressive Strength; D003432:Cross-Linking Reagents; D003460:Crystallization; D005609:Free Radicals; D005720:Gamma Rays; D008422:Materials Testing; D019563:Mechanics; D011095:Polyethylenes / Q000737:chemistry / Q000528:radiation effects; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature; D013718:Tensile Strength / Q000528:radiation effects; D013816:Thermodynamics
D003432:Cross-Linking Reagents; D005609:Free Radicals; D011095:Polyethylenes; C111601:ultra-high molecular weight polyethylene