# Effect of Blood on Synovial Joint Tissues: Potential Role of Ferroptosis
## 5 In Vivo Models of Hemarthrosis
### Lapine/Rabbit  
Rabbit models are often utilized in musculoskeletal research due to being the largest \"small\" animal model and due to their cost-effectiveness [110]. One commonly used rabbit study induces joint bleeding by drilling bone holes in the femoral notch, where intra-articular bone injury reproducibly creates early joint inflammation with some chronic synovial changes and progressive cartilage damage consistent with OA in adult rabbits in 9 weeks [74; 81; 110; 111]. This [PERSON] study is the first to create a surgical model of OA in which the joint was not destabilized and its biomechanics were not affected [74]. This intra-articular bone injury creates reproducible early joint inflammation with some chronic synovial changes and progressive cartilage damage consistent with OA in 9 weeks that further degenerates to 52 weeks [81; 110; 111]. The drill holes in experimental animals progressed from early callus and fibrocartilaginous callus to bone in 52 weeks. The past literature has shown rabbit studies highlighting the impact of acute hemarthrosis on the histological, biomechanical, and biochemical characteristics of intact ACL [112]. Joint bleeding prior to or due to ACLR has been suggested to play a role in the clinical finding that patients \(\pm\) ACLR develop OA [119; 120]. In a separate much earlier study from the [PERSON] group, daily blood injection in the rabbit knee did not show a change to ACL architecture or structure-function [112]. Conversely, rabbit models have been proven to demonstrate a molecular response, such as mRNA expression of pathological markers, that results from induced hemarthrosis to the meniscus. This study followed up with a single intra-articular injection of dexamethasone (DEX) using the method developedby [PERSON] [81]. DEX administration led to events that are typically seen following induced hemarthrosis, including a decrease in key catabolic markers, normalization of the gene expression of inflammatory mediators, and suppression of the mRNA expression of molecules in the autophagy pathway [81]. Another study that models OA without joint instability was conducted by injection of interleukin-17 (IL-17) into the articular cavity of the knee. Blood exposure and cartilage degradation are commonly associated with increases in IL-1\(\beta\), TNF-\(\alpha\), IL-6, and IL-17 levels. While IL-1\(\beta\) is one of the most recognized pro-inflammatory cytokines, studies have shown that IL-17 similarly induces the degradation of the cartilage extracellular matrix [113]. Therefore, IL-17 injections were administered once every 2 days, for a total of 3 injections. Knees that were injected with IL-17 varied in concentrations and were compared with a preceding method of inducing OA, the Hulth method [113]. This study resulted in an identification of an optimal concentration of IL-17 that does not result in the joint destabilization caused by the Hulth method. The injection of 10 ng/mL of IL-17 led to morphological changes that are typical of late-stage OA. Additionally, the model led to an increase in pro-inflammatory factors that promote synovial hyperplasia and fibroblasts that lead to angiogenesis [113]. Overall, the similarity in the enzyme expression, radiological evaluations, and histological scores demonstrate that this group's dosage of IL-17 in a rabbit model produces similar effects to posttraumatic OA. Although Fer-1 has been used in mice/rat in vivo studies, the application to the rabbit is novel and represents a critical step toward translation to large preclinical animal models. The scale of the rabbit knee joint (and synovial fluid cavity volume) is more practical than the mouse or rat knee joint for intra-articular injections of therapeutic agents. The larger anatomy of the synovial joint ensures intra-articular placement and can better tolerate injections that induce bleeding and local joint damage that may impact the outcome of the proposed studies aimed at understanding joint bleeding-induced arthropathy.