Quantification of the effect of cross-shear on the wear of conventional and highly cross-linked UHMWPE

Abstract

A computational model has been developed to quantify the degree of cross-shear of a polyethylene pin articulating against a metallic plate, based on the direct simulation of a multidirectional pin-onplate wear machine. The principal molecular orientation (PMO) was determined for each polymer site. The frictional work in the direction perpendicular to the PMO was assumed to produce the greatest orientation softening (Wang et al., 1997). The cross-shear ratio (CS) was defined as the frictional work perpendicular to the PMO direction, divided by the total frictional work. Cross-shear on the pin contact surface was location-specific, and of continuously changing magnitude because the direction of frictional force continuously changed due to pin rotation. The polymer pin motion was varied from a purely linear track (CS=0) up to a maximum rotation of ±55° (CS=0.254). The relationship between wear factors (K) measured experimentally and theoretically predicted CS was defined using logarithmic functions for both conventional and highly cross-linked UHMWPE. Crossshear increased the apparent wear factor for both polyethylenes by more than 5-fold compared to unidirectional wear.

2009 Knee Society Presidential Guest Lecture: Polyethylene Wear in Total Knees

“…Reduced cross shear ratio reduced the wear factor in the pin-on-plate studies for the two different stress levels studied [13,20] (Fig. 1).…”

“…The rapid decrease in the wear factor as the cross shear ratio decreased from 0.1 to 0 is of particular note. The experimental data were curved fitted over the cross shear range 0 to 0.25, and the logarithmic curve [20] extrapolated to cover the full range of cross shear possible up to 0.5. These results also indicated that the values of the wear factor were dependent on the contact conditions, the load, contact area, and stress level.…”

“…Standard wear testing methods were adopted that have been widely reported previously to determine the wear of polyethylene [13]. Material wear properties of GUR1050 polyethylene (taken from a single batch which was not irradiated to ensure time-independent properties) were determined using multidirectional pin-on-plate simulations [15,20]. The angle of rotation of the pin was varied from ± 10°t o ± 55°, as the pin reciprocated with a stroke length between 10 mm and 38 mm.…”

“…A continuous reciprocating movement forwards and backwards was applied, with cross shear introduced by rotation of the polymer pin. The cross shear ratio was defined as the frictional work transverse to the principal direction of motion divided by total frictional work [20], and the ratio had a range of values between 0 and 0.5 [13,20]. Studies were carried out with cross shear ratios between 0 and 0.254, hence extending beyond the range of kinematics found in the knee, which are less than 0.2.…”

How Does Wear Rate Compare in Well-functioning Total Hip and Knee Replacements? A Postmortem Polyethylene Liner Study

“…The 50% lower wear rate of the TKR inserts compared with the THR liners for regular wear (no delamination) may be a result of the lower degree of multidirectional motion in the knee [18,31]. This comparison entailed adjusting the wear rates to a patient age of 70 years to account for age dependence, and included only active patients.…”