Figure 9.16 shows the thin posterior wall of the femoral tunnel, which indicates a weak posterior wall. The use of the interference screw would be contraindicated. It would not be strong enough to insert an interference screw. The screw would force the bone plug of the graft out the back of the tunnel, and loss of fixation would result.
When the back wall blowout is recognized, change from interference screw to Endo-button fixation. Another solution is to use the two-
incision technique, with the interference screw placed from outside in. It is also possible to advance the screw and graft farther up into the femoral tunnel. This is only possible when the blowout is more than a few millimeters.
Prevent the blowout by flexing the knee to 90°. Visualize posterior fringe and use a push/pull drill to make an initial footprint. If this is not blowing out the posterior wall or is not too anterior, then the drill bit can be advanced 35 mm up into the femur. The footprint of the femoral tunnel position is made before committing to drilling the femoral tunnel (Fig. 9.17).
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