An animal model of early-stage femoral head osteonecrosis induced by cryo-insult in small tailed Han sheep

This study investigated the ability of liquid nitrogen cryo-insults to induce early-stage osteonecrosis of the femoral head (ONFH) in small tail Han sheep. 16 animals were subjected to unilateral cryo-insult using cryogen equipment with a cryo-insult probe, followed 1, 3, and 6 months later by X-ray, CT scanning, micro-CT scanning, and histological evaluation. X-ray evaluation of operative femoral heads (Op-FHs) at each time point showed low density areas under the cartilage surface that paralleled sclerosis belts, and CT scans showed sclerosis and cyst areas in Op-FHs. Micro-CT analysis showed that the ratio of bone to total volume and mean trabecular thickness of regions of interest (ROIs) were lower in Op-FHs than in normal femoral heads (No-FHs) at each time point (n = 4, p < 0.05). Histological evaluation at 1 month showed that necrotic changes were dominant as evidenced by moderate empty lacunae, decreases in the number of hematopoietic cells, and moderate increases in the number of fibroblasts. At 3 and 6 months, fractured trabeculae, fibroblasts, and new blood capillaries were increased, indicating an absorption and repair process. Bone volume fraction of ROIs of Op-FHs was lower than in No-FHs at each time point (n = 4, p < 0.05) in histological evaluation. At 6 months, the maximum load of No-FHs was higher than Op-FHs (n = 4, p < 0.05). These findings indicate that early-stage ONFH can be induced in small tail Han sheep using cryogenic equipment. This animal model may be helpful in developing new treatment modalities for human ONFH.