It has been suggested that an increased posterior tibial slope (PTS) and a narrow notch width index (NWI) increase the risk of anterior cruciate ligament (ACL) injury. The aim of this study was to establish why there are conflicting reports on their significance. A total of fifty patients with a ruptured ACL and 50 patients with an intact ACL were included in the study. The group with ACL rupture had a statistically significantly increased PTS (p < 0.001) and a smaller NWI (p < 0.001) than the control group. When a high PTS and/or a narrow NWI were defined as risk factors for an ACL rupture, 80% of patients had at least one risk factor present; only 24% had both factors present. In both groups the PTS was negatively correlated to the NWI (correlation coefficient = -0.28, p = 0.0052). Using a univariate model, PTS and NWI appear to be correlated to rupture of the ACL. Using a logistic regression model, the PTS (p = 0.006) and the NWI (p < 0.0001) remain significant risk factors. From these results, either a steep PTS or a narrow NWI predisposes an individual to ACL injury. Future studies should consider these factors in combination rather than in isolation.