Patellofemoral interactions in walking, stair ascent, and stair descent using a virtual patella model.

Restoration of normal patella kinematics is an important clinical outcome of total knee arthroplasty. Failure of the patella within total knee systems has been documented and, upon occurrence, often necessitates revision surgery. It is thus important to understand patella mechanics following implantation, subject to load states that are typically realized during walking and other gaits. Here, a computational model of the patella is developed and used to examine the effects of walking, stair ascent, and stair descent on the development of stress and contact pressure in the patella throughout the gait cycle. Motion of the patella was governed by a combination of kinematic and force control, based on knee flexion and patellofemoral joint reaction force data from the literature. Unlike most previous analyses of full gait, quasi-static equilibrium was enforced throughout the cycle. Results indicate that, though peak forces vary greatly between the three gaits, maximum contact pressure and von Mises stress are roughly equivalent. However, contact area is larger in stair ascent and descent than walking, as patellofemoral loading, implant geometry, and polyethylene yield increase conformity between the femoral component and patella. Additionally, maximum contact pressure does not coincide with maximum load except for the case of walking. Though specific to the implant design considered here, this result has important ramifications for patella testing and emphasizes the need to characterize patella mechanics throughout gait.