This manuscript describes a formulaic method to predict the severity of crossing motions experienced by ultra-high-molecular-weight polyethylene (UHMWPE) under sliding conditions across a bearing surface. A statistical model for evaluating the distribution of sliding about this orientation is described. This model compares favorably to published experimental data on UHMWPE under multidirectional pin-on-disk testing. These algorithms are applied to the tibial component of a total knee replacement using patient-specific kinematics for gait and stair-rise activities collected using fluoroscopy. A dynamic contact model is used to calculate contact pressures and slip velocities on individual surface elements from this kinematic data. The most significant degree of crossing motion intensity was observed in the lateral compartments for both gait and stair-rise activities. This coincided with the location of maximum tribological intensity. The maximum crossing motions are characteristic of 4° and 9° of bi-directional crossing motion for gait and stair respectively.