Fig. 1

Mechanical impact testing setup. The patella holder can be seen to the right, positioned in the first load frame. For pure axial impactions, the cross-head of the servo-hydraulic load frame on the right delivered an orthogonal load to the patella facet. For impactions with elevated shear, the load frame on the right delivered an axial impaction, while the load frame to the left displaced the patella tangentially 10 mm to induce elevated shear forces. The stop bar in the inset image shows the stop bar (A) that limited tangential movement to 10 mm