Measurement of Molecular Orientation in Retrieved Ultra-high-molecular-weight Polyethylene (UHMWPE) Hip Sockets using Fourier-transform Infrared Spectroscopy

:  This paper describes the use of Fourier-transform infrared spectroscopy (FTIR) to measure the degree of orientation in retrieved ultra-high-molecular-weight polyethylene (UHMWPE) acetabular components of prosthetic hip joints quantitatively. Multidirectional shearing has been found to result in a wear rate that is several orders of magnitude higher than that for linear shear. This is because linear wear of UHMWPE is believed to induce orientation of the polymer lamellae. The FTIR technique described in this paper enables a direct comparison to be made between patient biomechanics and the molecular orientation of UHMWPE hip sockets. Patients were identified prior to revision surgery of the hip for loosening of components. Individual patient's hip-joint kinematics were quantified by the aspect ratio of movement loci developed from clinical gait analysis. It was found that patients with high aspect ratios and therefore more linear wear paths, exhibited measurable orientation of the UHMWPE polymer lamellae in their retrieved hip sockets. An aspect ratio of 5.36 resulted in 67% of the polymer lamellae being oriented in the a-axis direction. The technique was qualitatively validated by the use of transmission electron microscopy and it was found that an aspect ratio of 4.46 or greater resulted in the observed orientation of the polymer lamellae.