Subchondral bone and synovial fluid metabolomic profiles are altered in injured and contralateral limbs 7 days after non-invasive joint injury in skeletally-mature C57BL/6 mice

Abstract

AbstractObjectivePost-traumatic osteoarthritis (PTOA) is a common long-term outcome following ACL injury. However, early changes to bone and synovial fluid after ACL injury are not sufficiently understood. The objectives of this study were to (1) evaluate whether acute bone loss one week after ACL injury is accompanied by altered subchondral bone plate modulus, (2) determine if bone changes are localized to the injured limb or extend to the contralateral-to-injured limb compared with sham-loaded controls, and (3) identify shifts in synovial fluid metabolism unique to injured limbs.DesignFemale C57Bl\6N mice (19 weeks at injury) were subjected to either a single tibial compression overload to simulate ACL injury (n=8) or a small pre-load (n=8). Mice were euthanized 7 days after injury, and synovial fluid was immediately harvested for metabolomic profiling. Bone microarchitecture, bone formation, and subchondral bone modulus at the proximal tibia were studied using microCT, histomorphometry, and nanoindentation, respectively. Osteoclast number density was assessed at the distal femur. For each bone measure a mixed model ANOVA was generated to determine the effects of injury and loaded side.ResultsEpiphyseal and subchondral bone microarchitecture decreased while subchondral bone tissue modulus was unchanged after ACL injuries. Bone resorption increased but bone formation was not changed. Loss of bone microarchitecture also occurred for the contralateral-to-injured limb, demonstrating that the early response to ACL injury extended beyond the injured joint. While the metabolomic profiles of the injured and contralateral-to-injured limbs had many similarities, there were also distinct metabolic shifts present in only the injured limbs. The most prominent of the pathways was cysteine and methionine metabolism, which is associated with osteoclast activity.ConclusionThese results add to the understanding of early bone changes following ACL injury. Confirming prior reports, we observe a decline in epiphyseal and subchondral bone microarchitecture. We add the finding that subchondral bone modulus remains unchanged at one week after ACL injury. A potential biomarker of this initial bone catabolic response may be synovial fluid cysteine and methionine metabolism, which was only dysregulated in injured knees. Our results implicate a rapidly changing biological and mechanical environment within both the injured and contralateral joints that has the potential for influencing the progression to PTOA.