The mechanisms of knee fracture were studied experimentally using cadaveric knees and analytically by computer simulation. Ten 90 degree flexed knees were impacted frontally by a 20 kg pendulum with: a rigid surface, a 450 psi (3.103 Mpa) crush strength and a 100 psi (0.689 MPa) crush strength aluminium honeycomb padding and with a 50 psi (0.345 MPa) crush strength paper honeycomb padding at a velocity of about five m/s. A three-dimensional finite element (FE) human knee model was developed to try to understand the internal stress distribution during knee impacts. The model included a femur, patella, tibia, associated ligaments modeled as bar elements, and a pendulum. The calculated pendulum force for a rigid impact and impacts using the 100 psi and 450 psi honeycomb matched that obtained from the tests, but it did not for the 50 psi honeycomb case. The calculated stress concentration location in the femur coincided clearly with the fracture location in the rigid impact test. The ultimate crush strength of the honeycomb padding needed to prevent knee injury was presumed to be 90 psi (0.620 MPa), based on linear interpolation of this limited series of experiments.
Abstract