Abstract: BACKGROUND: Cobalt chloride solution is commonly used to induce osteoarthritis cell models in vitro. However, its ability to construct animal models of osteoarthritis by intra-articular injection remains unknown.
OBJECTIVE: To investigate the effect of intra-articular injection of different concentrations of cobalt chloride solution on cartilage degeneration in the knee joint.
METHODS: Thirty-six healthy adult male New Zealand rabbits were randomly divided into four groups: low, medium and high dose cobalt chloride groups and control group. The right hind knee was intra-articularly injected with 100, 200, and 300 μmol/(L·kg) of cobalt chloride, while the left hind knee served as the control knee and was injected with an equal amount of normal saline. At 4, 8 and 12 weeks after operation, four rats were killed respectively. The cartilage on the surface of the femur was exposed for gross morphological observation, and then the cartilage tissues were taken for hematoxylin-eosin staining, safranine O-fast green staining, the Osteoarthritis Research Society International scoring, and immunohistochemical staining of interleukin 1 and tumor necrosis factor α, to determine cartilage degeneration in various aspects.
RESULTS AND CONCLUSION: (1) Gross observation: At the same postoperative time point, with the increase of cobalt chloride solution concentration, cartilage degeneration tended to aggravate progressively. High doses of cobalt chloride even involved the deeper layers of cartilage and subchondral bone. At the same concentration of cobalt chloride solution, with the prolongation of the modeling time, the cartilage degeneration developed progressively. (2) Hematoxylin-eosin staining, safranine O-fast green staining, and the Osteoarthritis Research Society International score: At the same postoperative time point, as the cobalt chloride solution concentration increased, the cartilage surface became rough, the cartilage surface layer became thinner, and the destruction was aggravated, with a progressive increase in the International Osteoarthritis Research Association score (P < 0.05). At the same concentration of cobalt chloride solution, with the prolongation of the modeling time, the chondrocyte arrangement tended to be disordered, polarity was lost, and the destruction of the superficial cartilage and subchondral bone was progressively aggravated, and the International Society for the Study of Osteoarthritis (ISSOA) score was gradually increased (P < 0.05). (3) Immunohistochemistry: At the same postoperative time point, with the increase of cobalt chloride solution concentration, cartilage degeneration was aggravated, intracellular brown granules were increased, and the positive expression of interleukin 1 and tumor necrosis factor α was increased (P < 0.01). At the same concentration of cobalt chloride solution, with the prolongation of the modeling time, cartilage destruction and fissures were increased, and the positive expression of interleukin 1 and tumor necrosis factor α was increased (P < 0.01). As a result, a model of osteoarthritis induced in New Zealand rabbits by intra-articular injection of cobalt chloride solution was successfully established, the stability and reliability of the animal model was preliminarily verified, and it also proved that cartilage degeneration develops progressively with the increase of the modeling concentration and the prolongation of the modeling time.

Key words: osteoarthritis, animal model, articular cartilage, cobalt chloride, chemical induction, immunohistochemistry, New Zealand rabbit