Posterior Tibial Slope and Further Anterior Cruciate Ligament Injuries in the Anterior Cruciate Ligament–Reconstructed Patient

BACKGROUND: An injury to the anterior cruciate ligament (ACL) is a multifactorial event influenced by intrinsic and extrinsic risk factors. Recently, the geometry of the proximal tibia has come under focus as a possible risk factor for an ACL injury. HYPOTHESIS: An increased posterior tibial slope is associated with an increased risk of further ACL injuries in the previously ACL-reconstructed patient. STUDY DESIGN: Case-control study; Level of evidence, 3. METHODS: A total of 200 consecutive patients with isolated ACL ruptures who underwent primary reconstruction with hamstring autografts were enrolled in a prospective longitudinal study over 15 years. The posterior tibial slope was measured from a lateral knee radiograph by 2 blinded observers. The data were analyzed for the association between an increased posterior tibial slope and the incidence of further ACL injuries. Interobserver reliability of the posterior tibial slope measurements was assessed. RESULTS: Radiographs and follow-up were available for 181 of the 200 enrolled patients. Fifty patients had a further injury to either the ACL graft or the contralateral knee. The mean posterior tibial slope of those with a further ACL injury was 9.9° compared with 8.5° for those with no further injury (P = .001). The mean posterior tibial slope for those with both an ACL graft and contralateral ACL rupture was 12.9°. The odds of further ACL injuries after reconstruction were increased by a factor of 5, to an incidence of 59%, in those with a posterior tibial slope of ≥12°. CONCLUSION: An increased posterior tibial slope is associated with increased odds of a further ACL injury after ACL reconstruction. The increased risk is most pronounced in those with a posterior tibial slope of ≥12°.