MIA model of OA pain

Unilateral knee OA was induced by a single intra-articular injection of MIA (Sigma Aldrich) in the infrapatellar area of the right knee of male Wistar rats. A total of 30 animals were used (six rats per group). The experimental procedure was performed according to the method first proposed by Kalbhen,20 and later revised by Sagar et al.21 The dose of MIA was selected based on a combination of preliminary results and literature data. In particular, we noticed that 1 mg MIA was the minimum dose required to detect secondary allodynia and primary hyperalgesia associated with joint damage. Therefore, this dose was selected for the present study. On day 1, rats were anesthetized with 2% isoflurane in pure oxygen. The right leg skin was sterilized with 75% ethyl alcohol and the knee was located by palpation. A 26-gauge needle was inserted vertically to penetrate the skin, and turned distally for insertion into the articular cavity, until a distinct loss of resistance was detected; 1 mg of MIA in 50 µL saline was then delivered into the articular cavity. Control rats (sham) received 50 µL of saline solution. All animals were sacrificed 21 days after the intra-articular injection of either MIA or saline solution.