Background: Osteoarthritis (OA) is a complex disease of entire joint, affecting cartilage, bone and synovial membranes. Inflammation of articular cavity is accompanied to pathological changes that have a critical impact on neuropathic pain and locomotion disorders. The sodium hyaluronate is similar to a substance that occurs naturally in the joint. It may work by acting as a lubricant and shock absorber in the joint, helping the knee to move smoothly, thereby lessening pain. However, current studies have limited information about mechanisms of this sodium hyaluronate. Therefore, we investigated the effects of sodium hyaluronate on locomotor function, hyperalgesia degree, and expressions of inflammation and collagen fibers (type I and II) in the articular cavity following osteoarthritis rats. 
Methods: OA-induced rats received an intra-articular injection of monosodium iodoacetate (MIA; 3 mg/kg, volume: 60 ul) into a right knee joint. Fourteen days after the MIA injection, sodium hyaluronate in 50 ul containing respective dose (10, 15, and 20 mg/ml) was directly applied to the injured intra-articular only once. Functional recovery in each rat was repeatedly assessed using the weight load test and walking track analysis at 14, 28, and 42 days after staring of the MIA injection. In 43 days since the MIA injection, plantar test for pain threshold was also conducted. For histological and inflammatory analysis, hematoxylin & eosin and masson’s trichrome staining were performed. The immunohistochemistry for collagen fiber type I, II in the articular cavity was also conducted. 
Results: In the present results, OA increased pain susceptibility and suppressed leg strength due to arthrostenosis and inflammation in the articular cavity. Treatment with sodium hyaluronate improved leg strength and decreased pain susceptibility. Also, it suppressed arthrostenosis and inflammation in the articular cavity. In addition, sodium hyaluronate treatment increased collagen type I expression, whereas, decreased expression of collagen type II in the articular cavity.
Conclusion: These results showed that sodium hyaluronate facilitates functional recovery and inhibits pain susceptibility by suppressing inflammation and modulating collagen fibers in OA. We suggest that sodium hyaluronate might be considered as a new therapeutic intervention for functional recovery and pain control in OA.

Study in the sodium hyaluronate attenuating the functional recovery and thermal hyperalgesia by altering expressions of inflammation and collagen fibers in the osteoarthritis rats