<p>Anterior cruciate ligament (ACL) ruptures are significant knee injuries. Many studies have analysed video footage of the mechanisms of injury; however, the three-dimensional biomechanics of ACL ruptures have not previously been captured and presented. This case series reports on two secondary non-contact ACL ruptures in adolescent females: one ipsilateral ACL re-rupture and one contralateral ACL rupture. The injuries occurred during unplanned 90° cuts, which were completed as part of the clinic’s standard of care return-to-play testing. Spatiotemporal parameters, joint and segment kinematics, ground reaction forces and joint moments and forces are presented during the approach, at the penultimate step and initial contact, and during the stance phase of the cutting step. These biomechanics are compared to planned 90° cuts and a pre-injury unplanned 90° cut. While there were slight differences in the exact injury mechanisms, common factors appeared to have contributed to the ACL ruptures. These potential mechanisms of injury included a fast approach speed without sufficient deceleration at the penultimate step; a larger touchdown distance; and a more outstretched leg in the anterior or lateral direction which, when combined with the rotation of the body towards the direction of the cut, may have limited knee flexion which likely contributed to large forces and moments acting at the knee. This case series highlights the need to develop training strategies aimed at reducing risky movement behaviours to reduce the risk of subsequent non-contact ACL injuries.</p>