A USAXS STUDY OF THE MORPHOLOGY OF POLYETHYLENE USED IN ORTHOPAEDIC IMPLANTS

1. A USAXS STUDY OF THE MORPHOLOGY OF POLYETHYLENE USED IN ORTHOPAEDIC IMPLANTS Mary Turell, Anuj Bellare Department of Orthopaedic Surgery, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA RESULTS AND DISCUSSION: USAXS scattering curves were obtained by plotting the scattered intensity I versus q where: q = (4π/λ)sinθ (1) such that θ equals one half of the scattering angle, and λ is the wavelength of x-rays (see Fig. 2). USAXS curves revealed a linear Porod region at ultra-low q values suggesting the presence of large micrometer size scatterers such as voids, regardless of thermal history. The scattering functions were converted to paired distance distribution functions (PDDF) or p(r) using the program ITP [2]. PDDF is related to the scattering function I(q) by the following equation: (2) where p(r) is the paired distance distribution function, A is the area of the lamella, I(q) is the experimental scattering function, q is the scattering vector, and r is the radial distance perpendicular to lamellar surfaces within a stack of lamellae. The USAXS long period for all samples was measured from the first maximum of p(r) (Figure 3). Together, the long period and DSC crystallinity were used to calculate the lamellar thickness (see Table 1) These results show that the chosen thermal histories strongly affect the morphological parameters such as inter-lamellar spacing and lamellar thickness. Preliminary wear tests have shown that these parameters strongly affect wear rates under conditions that simulate wear in the joint replacement prostheses. These results will be used to optimize wear performance of UHMWPE in these orthopedic implants. [2] Glatter, Acta Phys.Austriaca, 47, 83-102 (1977) ACKNOWLEDGEMENT We gratefully acknowledge Dr. P. Jemian and Dr. J. Ilavsky for their assistance with USAXS measurements. The UNI-CAT facility at the Advanced Photon Source (APS) is supported by the University of Illinois at Urbana Champaign, Materials Research Laboratory (U.S. Department of Energy, the State of Illinois-IBHE-HECA, and the National Science Foundation), the Oak Ridge National Laboratory (U.S. DOE), the National Institute of Standards and Technology (U.S. Department of Commerce), and UOP LLC. The APS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. W-31-109-ENG-38. • INTRODUCTION: • Ultra-high molecular weight polyethylene (UHMWPE) has attained worldwide acceptance as a bearing material used in total knee replacement prostheses. Each year, over 300,000 total knee replacement prostheses are implanted in the U.S. alone (Figure 1). • The lifetime of these implants is governed by UHMWPE tribological and fracture properties, specifically those that are related to fatigue failure. • The crystalline morphology of polymers have a strong influence on their mechanical and wear properties [1]. • The objective of this research is to investigate crystallization conditions that would optimize the relevant wear and mechanical properties of UHMWPE for application in total knee replacement prostheses. • In this study, UHMWPE was subjected to four different thermal histories with the objective of developing structure-property relationships that are relevant to their application as orthopedic bearing materials. The crystalline morphology resulting from these treatments was characterized using ultra small-angle x-ray scattering (USAXS) at the UNI-CAT beamline of the Advanced Photon Source (APS). • [1] R.W. Truss et al., Polym Eng Sci, 20, 747-755 (1980). • MATERIALS & METHODS: • A ram extruded rod stock (PolyHi Solidur Inc) of GUR 1050 UHMWPE resin (Ticona, Bayport, TX) was purchased and used as a control. • UHMWPE rod stock were melted and subjected to : (1) slow cooling to room temperature, (2) isothermal crystallization at 125C for a period of 48 hours, (3) quenching in liquid nitrogen, (4) quenching in liquid nitrogen followed by isothermal crystallization at 80C for a period of 48hrs. • Samples were characterized using a combination of differential scanning calorimetry (DSC) and ultra-small angle x-ray scattering. DSC was performed on a Perkin Elmer Pyris 1 to determine the degree of crystallinity in each sample. Percent crystallinity was calculated by normalizing the heat of fusion of each sample to the heat of fusion of polyethylene crystal (293 J/g). • USAXS was performed on each sample group using 10 keV x-rays using the USAXS camera at the UNICAT beamline of APS. FIGURE 1 FIGURE 2 FIGURE 3 TABLE 1