Difficulties with the 'N + 7 rule' in endoscopic anterior cruciate ligament reconstruction

In an effort to facilitate the use of interference screw fixation in the tibia, a method to orient the tibial tunnel termed the 'N + 7 Rule' was prospectively applied to 60 consecutive endoscopic anterior cruciate ligament reconstructions performed by a single surgeon. Four cases were excluded because a breach in the described protocol for application of the N + 7 rule was identified at the time of the operation. Of the remaining 56 cases, 28 (50%) were fixed with an interference screw on the tibia, whereas 28 (50%) required fixation with sutures tied over a post. The 28 cases not amenable to interference screw fixation included 17 cases in which the tibial tunnel was too long and 11 cases in which the tunnel was too short. It appears that the applicability of the N + 7 rule to an individual surgeon's practice may be altered by small variations in operative technique.