A new method to determine graft angles after knee ligament reconstruction

In posterior cruciate ligament (PCL) reconstruction, the acute angle of the graft at the tibial tunnel ('the killer curve') has been implicated as a cause of graft failure. Similar acute angles also occur at the entrance to the femoral tunnel. We describe a method to determine the true graft angle, based on measurements taken from two orthogonal radiographs in a cadaveric model, analysed using three-dimensional geometry. The true angles for the tibial and femoral tunnels are presented for a variety of tunnel positions. The effect on these angles of knee flexion and extension is analysed. Resultant graft-tunnel forces at a constant ligament tension are described. Based on these results, drilling the femoral tunnel from outside-in and lateral placement of the tibial tunnel are theoretically advantageous.