Osteoarthritis, the most common degenerative joint disease, occurs more frequently in joints that have sustained injury. Currently, osteoarthritis is diagnosed with imaging that finds radiographic changes after the disease has already progressed to multiple tissues. The primary objective of this study was to compare potential metabolomic biomarkers of joint injury between the synovial fluid and serum in a mouse model of post-traumatic osteoarthritis. The secondary objective was to gain insight into the pathophysiology of osteoarthritis by examining metabolomic profiles after joint injury. 12-week-old adult female C57BL/6 mice (n=12) were randomly assigned to control, day 1 post injury, or day 8 post injury groups. Randomly selected stifle (i.e., knee) joints were placed into a non-invasive injury apparatus and subjected to a single dynamic axial compression causing anterior translation of the tibia relative to the femur to tear the anterior cruciate ligament. At days 1 and 8 post injury, serum was extracted then mice were immediately euthanized prior to synovial fluid collection. Metabolites were extracted and analyzed by liquid chromatography coupled to mass spectrometry. We detected ~2500 metabolites across serum and synovial fluid. Of these metabolites 179 were positively correlated and 51 were negatively correlated between synovial fluid and serum, indicating potential for the development of metabolomic biomarkers. Synovial fluid appeared to capture differences in metabolomic profiles between injured mice at both day 1 and 8 after injury whereas serum did not. However, synovial fluid and serum were distinct at both days 1 and 8 after injury. In the synovial fluid, pathways of interest across different time points mapped to amino acid synthesis and degradation, bupropion degradation, and the tRNA charging pathway. In the serum, notable pathways across time points were amino acid synthesis and degradation, the phospholipase pathway, and nicotine degradation. These results provide a rich picture of the injury response at early time points following traumatic joint injury. Furthermore, the correlations between synovial fluid and serum metabolites suggest that there is potential to gain insight into intra-articular pathophysiology through analysis of serum metabolites.

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Osteoarthritis, the most common degenerative joint disease, occurs more frequently in joints that have sustained injury. Currently, osteoarthritis is diagnosed with imaging that finds radiographic changes after the disease has already progressed to multiple tissues. The primary objective of this study was to compare potential metabolomic biomarkers of joint injury between the synovial fluid and serum in a mouse model of post-traumatic osteoarthritis. The secondary objective was to gain insight into the pathophysiology of osteoarthritis by examining metabolomic profiles after joint injury. 12-week-old adult female C57BL/6 mice (n=12) were randomly assigned to control, day 1 post injury, or day 8 post injury groups. Randomly selected stifle (i.e., knee) joints were placed into a non-invasive injury apparatus and subjected to a single dynamic axial compression causing anterior translation of the tibia relative to the femur to tear the anterior cruciate ligament. At days 1 and 8 post injury, serum was extracted then mice were immediately euthanized prior to synovial fluid collection. Metabolites were extracted and analyzed by liquid chromatography coupled to mass spectrometry. We detected ~2500 metabolites across serum and synovial fluid. Of these metabolites 179 were positively correlated and 51 were negatively correlated between synovial fluid and serum, indicating potential for the development of metabolomic biomarkers. Synovial fluid appeared to capture differences in metabolomic profiles between injured mice at both day 1 and 8 after injury whereas serum did not. However, synovial fluid and serum were distinct at both days 1 and 8 after injury. In the synovial fluid, pathways of interest across different time points mapped to amino acid synthesis and degradation, bupropion degradation, and the tRNA charging pathway. In the serum, notable pathways across time points were amino acid synthesis and degradation, the phospholipase pathway, and nicotine degradation. These results provide a rich picture of the injury response at early time points following traumatic joint injury. Furthermore, the correlations between synovial fluid and serum metabolites suggest that there is potential to gain insight into intra-articular pathophysiology through analysis of serum metabolites.

Objective: Post-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. Design: Female 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. Results: Epiphyseal 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. Conclusion: These 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.