The Biomechanical Effect of Loading Speed on Metal-on-UHMWPE Contact Mechanics

Ultra high molecular weight polyethylene (UHMWPE) is a material commonly used in total hip and knee joint replacements. Numerous studies have assessed the effect of its viscoelastic properties on phenomena such as creep, stress relaxation, and tensile stress. However, these investigations either use the complex 3D geometries of total hip and knee replacements or UHMWPE test objects on their own. No studies have directly measured the effect of vertical load application speed on the contact mechanics of a metal sphere indenting UHMWPE. To this end, a metal ball was used to apply vertical force to a series of UHMWPE flat plate specimens over a wide range of loading speeds, namely, 1, 20, 40, 60, 80, 100, and 120 mm/min. Pressure sensitive Fujifilm was placed at the interface to measure contact area. Experimental results showed that maximum contact force ranged from 3596 to 4520 N and was logarithmically related (R(2)=0.96) to loading speed. Average contact area ranged from 76.5 to 79.9 mm(2) and was linearly related (R(2)=0.56) to loading speed. Average contact stress ranged from 45.1 to 58.2 MPa and was logarithmically related (R(2)=0.95) to loading speed. All UHMWPE specimens displayed a circular area of permanent surface damage, which did not disappear with time. This study has practical implications for understanding the contact mechanics of hip and knee replacements for a variety of activities of daily living.