**2. Results**

#### *2.1. Collagen-induced arthritis (CIA) as a Model to Study Bone Injury in Inflammatory Conditions*

First, we established if the CIA model in the rat would be suitable to study the response to bone injury under inflammatory conditions by inducing it in otherwise healthy female Wistar rats and performing a critical size bone defect at day 21 after CIA induction (Figure 1).

Arthritis induction was effective in all animals, with macroscopic evidences of erythema and swelling in hind paws, and significant increases in swelling after day 14 when compared to the non-immunized/control animals (Figure S1A,B). The arthritis index score increased along the monitoring time until hitting a plateau between day 17 and 21 (Figure S1C).

At day 21 after immunization, a group of CIA animals with evident signals of arthritis were subjected to a cylindrical femoral bone defect (FD), as illustrated in the x-ray of Figure 1A, and followed up to three days post-injury. The combination of CIA with FD (CIA+FD) did not compromise animal welfare beyond the impact of CIA itself, with CIA+FD animals recovering well from surgery and keeping to a similar body weight as the control and CIA groups (Figure 1B). Hind paw swelling of CIA+FD animals showed a slight decrease at day 3 post-injury, albeit not significant (Figure 1C).

**Figure 1.** Combination of collagen-induced arthritis (CIA) model with femoral defect (FD). CIA was induced and allowed to develop for 21 days, before a cylindrical defect was performed in the femur of a half of the CIA animals to combine both animal models (CIA+FD) (**A**) X-ray of cylindrical defect performed on the right femur (white arrow indicates the defect site) of CIA animals at day 3 after surgery. (**B**) Body weight evaluation 3 days after bone defect. (**C**) Measurements of paw swelling in both paws (right and left) of control, CIA and CIA+FD, 3 days after bone injury. Box plots represent min-to-max distribution of *n* = 4 to 5 animals per group. \* *p* < 0.05, determined by Kruskal–Wallis test and Dunn's multiple comparisons test.
