Steeper posterior tibial slope markedly increases ACL force in both active gait and passive knee joint under compression

Changes in PTS moderately affected muscle forces and joint contact forces at mid-stance period. Both active (at mid-stance) and passive (at all flexion angles) models showed a substantial increase in the anterior tibial translation and ACL force as PTS increased with reverse trends as PTS decreased. In the active model of gait at mid-stance, ACL force increased by 75% (from 181 N to 317 N) in steeper PTS but decreased by 44% (to 102 N) in flatter PTS. The posterolateral bundle of ACL carried the load at smaller flexion angles with a shift to its anteromedial bundle as flexion increased. In accordance with earlier imaging studies, greater PTS is a major risk factor for ACL rupture especially in activities involving large compression forces.