Anterior cruciate ligament (ACL) tears are activity-related knee injuries associated with an elevated risk of developing post-traumatic osteoarthritis 10-20 years post-injury. Immediately after the injury bone mass is lost. This is followed by a recovery period, though full recovery is not achieved even years later. Due to a lack of appropriate imaging modalities, no information is available on how the underlying bone microarchitecture is affected. In addition, the effects of concurrent soft-tissue injuries, such as meniscus tears or traumatic bone marrow lesions, on the underlying bone microarchitecture in human knees are not known.

In this thesis, a new method for in vivo assessment of bone microarchitecture of the human knee is introduced. Next, in two cross-sectional studies this technique is applied to populations that experienced unilateral ACL tears six to nine years earlier. Both studies revealed that bone is primarily affected in the femur of the injured knee. Trabecular bone mass is lower in the medial femur (-4.8% to -10.4%) while the subchondral bone plate is thicker in the lateral femur (9% to 29.6%) as compared to the contralateral knee. Further, the thicker subchondral bone plate is associated with surgical meniscus treatment (meniscectomy or repair) at the time of ligament reconstruction.

In a year-long longitudinal study, the new imaging technique is applied to a cohort with acute unilateral ACL tears to investigate how early injury-induced bone changes affect microstructure. Immediately following the injury, trabecular bone is lost throughout the injured knee (-4.9% to -15.8%), driven by a loss of trabecular elements and increased trabecular separation. Concurrently, the subchondral bone plate of the lateral femur thins (-9%). The trabecular bone changes are further accelerated in traumatic bone marrow lesions (-18.2% to -20.6%).

These findings show that while initial bone mass loss following the injury may recover six to nine years later (primarily in the tibia), the femur is affected long-term. The underlying structural changes are believed to be permanent, and while it is not known which individuals will develop osteoarthritis, limiting early injury-induced bone changes may reduce long-term risk of joint degradation.