Chapter 56 · Conventional and Cross-Linked Polyethylene Properties – L.A. Pruitt 56 Conventional and Cross-Linked Polyethylene Properties L. A. Pruitt 56 Summary Highly cross-linked ultra high-molecular-weight poly- ethylene has shown great promise as an orthopedic bear- ing in total hip replacements. However, the enhanced re- sistance to plastic deformation that benefits wear behav- ior comes at the expense of other mechanical properties. Ultimate tensile strength, ductility, modulus, toughness, and crack propagation resistance are degraded at high cross-linking doses. The degradation in fracture proper- ties indicates that highly cross-linked polyethylene should not be used in applications where high stresses are expected. Lower degrees of cross-linking may be more appropriate when designing for both wear and fatigue in total joint replacements. Introduction Ultra high-molecular-weight polyethylene (UHMWPE) has been used as the bearing surface in total joint arthro- plasty for over four decades. UHMWPE was originally chosen for this application because it offers unique prop- erties including exceptional toughness, good wear resis- tance, a low friction coefficient, and biocompatibility. These properties are a result of the molecular structure of UHMWPE. In its conventional form the material is not cross-linked, but the polymer has exceptional mechani- cal integrity owing to its chain entanglements, high tie molecule density, moderate crystallinity, and very high molecular weight [1].A summary of the physical proper- ties of conventional UHMWPE is provided in ⊡ Table 56-1. Surface damage resulting from articulation and high con- tact stresses of UHWMPE remains the limiting factor for the longevity of the joint replacement. Over time in vivo, accumulated damage to the polymer implant results in debris formation. This polymer debris can lead to in- flammation, foreign-body response, and the need for re- vision surgery. Conventional UHMWPE is generated from the poly- merization of ethylene gas into a fine powder that is con- solidated under elevated pressure and above its melt tem- perature, using compression molding or ram-extrusion processes. Orthopedic components such as tibial plateaus or acetabular cups are typically machined from this stock material or molded directly into the final shape and subse- quently sterilized prior to implantation. Sterilization of conventional polyethylene is typically done with ethylene oxide gas, ionizing irradiation, or plasma treatment. Until 1995, the majority of orthopedic companies sterilized their devices with gamma irradiation in the presence of air at a dose of 25-40 kGy. By 1998, sterilization had shifted to non- ionizing techniques or irradiation in low oxygen environ- ments This change was prompted by scientific evidence that ionizing irradiation in the presence of oxygen results in a time-dependent process known as shelf aging that is accompanied by chain scission, loss of molecular weight, increased crystallinity, oxidation, and a concomitant de- crease in mechanical properties [2]. This shelf-aging prob- lem plagued the orthopedics community for a number years.Retrieved devices that had been sterilized by gamma irradiation in air showed evidence of pitting, delamina- tion, subsurface oxidation, and fatigue cracking. However, gamma-sterilized UHMWPE was used for many years and a plethora of literature is available for UHMWPE sterilized with ionizing irradiation; thus it remains a “gold standard” for comparison to material improvements to this polymer [3].“Conventional”UHMWPE can refer to UHMWPE ster- ⊡ Table 56-1. Physical properties of conventional (unirradiated) GUR 1050. Mechanical properties are taken from engineering stress- strain plots. (Adapted from [1]) Physical property GUR 1050 Molecular weight 3-6 million g/mol Crystallinity 45-50% Density 0.93-0.935 Ultimate tensile strength (21°C) 42-44 MPa Ultimate tensile strength (37°C) 36 MPa Yield strength (21°C) 20-23 MPa Yield strength (37°C) 21 MPa Elastic modulus (21°C) 1.0-1.39 GPa Elastic modulus (37°C) 0.67 GPa Elongation at break (21°C) 330% Elongation at Break (37°C) 375% Shore D Hardness (21°C) 60-65
Conventional and Cross-Linked Polyethylene Properties
Jun 21, 2023
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