Objective Traumatic joint injuries initiate a surge of inflammatory cytokines and proteases that may contribute to cartilage and subchondral bone degeneration. Detecting these early biological processes in animal models of post-traumatic osteoarthritis (PTOA) typically involves ex vivo analysis of blood serum or synovial fluid biomarkers, or destructive histological analysis of the joint. In this study, we used in vivo fluorescence reflectance imaging (FRI) to quantify protease activity, matrix metalloproteinase (MMP) activity, and Cathepsin K activity in mice following ACL rupture. We hypothesized that these processes would be elevated at early time points following traumatic joint injury (1–14 days), but would return to control levels at later time points (4–8 weeks). Design Mice were injured via tibial compression overload, and FRI imaging was performed at multiple time points from 1–56 days after injury using commercially available activatable fluorescent tracers to quantify protease activity, MMP activity, and cathepsin K activity in injured vs. uninjured knees. PTOA was assessed at 56 days post-injury using micro-computed tomography and whole-joint histology. Results Protease activity, MMP activity, and cathepsin K activity were all significantly increased in injured knees relative to uninjured knees at all time points, peaking at 1–7 days post-injury, then decreasing at later time points while still remaining elevated relative to controls. Conclusions This study establishes FRI imaging as a reliable method for in vivo quantification of early biological processes in a translatable mouse model of PTOA, and provides crucial information about the time course of inflammation and biological activity following joint injury. These data may inform future studies aimed at targeting early inflammation to reduce the development of PTOA.

Traumatic injuries can have systemic consequences, as the early inflammatory response after trauma can lead to tissue destruction at sites not affected by the initial injury. This systemic catabolism may occur in the skeleton following traumatic injuries such as anterior cruciate ligament (ACL) rupture. However, bone loss following injury at distant, unrelated skeletal sites has not yet been established. In the current study, we utilized a mouse knee injury model to determine whether acute knee injury causes a mechanically significant trabecular bone loss at a distant, unrelated skeletal site (L5 vertebral body). Knee injury was noninvasively induced using either high-speed (HS; 500 mm/s) or lowspeed (LS; 1 mm/s) tibial compression overload. HS injury creates an ACL rupture by midsubstance tear, while LS injury creates an ACL rupture with an associated avulsion bone fracture. At 10 days post-injury, vertebral trabecular bone structure was quantified using high-resolution microcomputed tomography (lCT), and differences in mechanical properties were determined using finite element modeling (FEM) and compressive mechanical testing. We hypothesized that knee injury would initiate a loss of trabecular bone structure and strength at the L5 vertebral body. Consistent with our hypothesis, we found significant decreases in trabecular bone volume fraction (BV/TV) and trabecular number at the L5 vertebral body in LS injured mice compared to sham (À8.8% and À5.0%, respectively), while HS injured mice exhibited a similar, but lower magnitude response (À5.1% and À2.5%, respectively). Contrary to our hypothesis, this decrease in trabecular bone structure did not translate to a significant deficit in compressive stiffness or ultimate load of the full trabecular body assessed by mechanical testing or FEM. However, we were able to detect significant decreases in compressive stiffness in both HS and LS injured specimens when FE models were loaded directly through the trabecular bone region (À9.9% and À8.1%, and 3, respectively). This finding may be particularly important for osteoporotic fracture risk, as damage within vertebral bodies has been shown to initiate within the trabecular bone compartment. Altogether, these data point to a systemic trabecular bone loss as a consequence of fracture or traumatic musculoskeletal injury, which may be an underlying mechanism contributing to increased risk of refracture following an initial injury. This finding may have consequences for treatment of acute musculoskeletal injuries and the prevention of future bone fragility.

Inter-observer reliability was also substantial for composite score (0.65, 0.41-0.89) and grade (0.64, 0.43-0.85). The least reliable feature was JSW, with an inter-observer weighted kappa of 0.34 (0.06-0.62). Percentage agreement across all grades was 88.3% for reviewer 1 and 81.7% between reviewers. Full results are given here: The prevalence of 'established' hip osteoarthritis in this cohort was 8.3% by individual and 4.8% by hip, comparing favourably to reported disease prevalence of 5-7% in women of a similar age range from other epidemiological studies. Conclusion: This new CT grading system shows substantial reliability in the assessment of hip osteoarthritis. The composite scoring system also shows substantial reliability and has the potential to offer greater sensitivity in disease assessment, as well as the ability to categorise atrophic, normotrophic and hypertrophic phenotypes according to score breakdown. Further testing will be performed in a prospective setting against the outcome of total hip replacement to test construct validity. 5 year follow-up imaging will also be used to test construct sensitivity.