In osteoarthritis of the knee, recognition of pain pathophysiology and effective exercise therapy is an important issue for establishing effective treatment methods. Both animals and humans, the increase in mechanical stress due to joint instability induces joint deformity and pain. Improvement in joint instability may be effective in reducing joint deformity and pain. The purpose of this study is to understand a part of the pain mechanisms in the knee OA and to prove the pain relief effect by improving joint instability.

Forty-five Wistar male rats, aged 11 weeks, were divided into three groups, ACL transection (ACLT) group, controlled abnormal movement (CAM) group and sham group (each group n=5). At 4 weeks and 8 weeks after surgery, immunofluorescence staining of substance P, calcitonin gene-related peptide in spinal dorsal root ganglia (DRG) and safranin O staining of knee joint were performed. Furthermore, pain behavior evaluation was performed every week after surgery.

By immunofluorescent staining, the number of positive cells per DRG area of calcitonin gene-related peptid was significantly larger at 8 weeks than 4 weeks. Both the ACLT group and the CAM group showed significantly higher scores of cartilage damage than the sham group at 4 and 8 weeks. Pain behavior evaluation was significantly lower in the CAM group than in the other two groups.

From the results of immunofluorescent staining, it was suggested that inflammatory pain mainly appears until 8 weeks postoperatively. Therefore, it was inferred that this model may be early OA, which is the main symptom of intra-articular inflammation. We considered that inflammatory pain was not involved in lowering 50% paw withdrawal threshold of early knee OA. Intervention to improve joint instability from an early stage contributes to the reduction of inflammatory pain in early knee OA.