Measurement technique for posterior tibial slope on radiographs can affect its relationship to the risk of anterior cruciate ligament rupture

Purpose: The purpose of this study was to determine the most reliable radiographic measurement method to evaluate PTS as a risk factor for ACL reconstruction failure. Methods: Patients who underwent ACL reconstruction or ACL revision reconstruction between January 2009 and December 2014 by a single surgeon were included. Fifty-two consecutive patients who underwent ACL revision reconstruction were compared to a random selection of 52 patients who underwent primary ACL reconstruction and a control group of 52 patients without ACL injury. ACL reconstruction was performed using either ipsilateral (primary) or contralateral (revision) quadrupled hamstring autograft. Lateral knee radiographs were evaluated using three methods: (1) longitudinal axis, (2) anterior tibial cortex axis, and (3) posterior tibial cortex. Results: A significant difference was found between subjects who underwent ACL reconstruction and control knees (6.79° vs. 5.31°, p = 0.046) using the posterior tibial cortex method. No other statistical significance was found between groups. A multiple linear regression analysis found that the PTS as measured by any method was not affected by the patient’s age, sex, height, weight, and BMI. All methods of measurement for PTS demonstrated excellent (ICC > 0.90) intra-rater and inter-rater reliability, but only the posterior tibial cortex method maintained excellent intra-rater and inter-rater reliability (ICC > 0.90) when evaluating patients with ACL revision reconstruction. Conclusions: The posterior tibial cortex measurement is the most reliable method for analyzing the PTS on lateral knee radiographs in patients undergoing ACL revision reconstruction.