Leg pain behavior correlates fibrosis and angiogenesis of the infrapatellar fat pad induced by monoiodoacetic acid

Abstract

Purpose: Chronic knee pain is the major complaint of patients suffering from knee osteoarthritis (OA). Although, conservative therapy is usually applied in such cases, clinical outcomes are not always satisfactory to all the patients. One reason for that may be due to the complexity of pain etiology, which may differ from each patient. Severity of chronic knee pain usually associates with the degree of articular cartilage degeneration, however, non-negligible numbers of patients with relatively normal articular cartilage also have severe pain in clinical settings. Infrapatellar fat pad (IFP) is a cylindrical piece of fat situated behind the patella tendon. It is considered that the IFP plays significant roles in knee pain since it contains blood vessels and sensory nerves besides joint inflammation. In addition, we often observe inflammatory response within this tissue after joint injury or OA. To understand the IFP-related knee pain, we established a chemical induced IFP inflammation model in rats and examined the relation between the pain behavior and histological observations of IFP in this study. Methods: Male Wistar rats received injection of MIA (monoiodoacetic acid, 0.2 mg or 1.0 mg/10 μl) in the left IFP, and PBS (10 μl) in the right IFP. To prevent leakage of the solution into the knee joint as well as to avoid influence of the skin incision on corresponding lesions, a 5 mm small skin incision was placed on the medial side of the thigh was shifted toward the patellar tendon and injected into the IFP directly. To analyze the pain behavior quantitatively, incapacitance (weight bearing) test was performed before MIA injection and at days 1, 3, 5, 7, and 14 after MIA injection. Histochemical analyses of the knee joint were performed at days 7 and 14 after MIA injection. Results: Weight bearing of the MIA injected leg was significantly decreased between day 1 and 7 in both 0.2 and 1.0 mg groups. In the 0.2 mg group, this decrease was transient and significantly improved at day 7 and returned to the basal level at day 14. In contrast, weight bearing remained significantly decreased until day 14 in the 1.0 mg group (Fig. 1). Histological assessment indicated that significant numbers of the cells infiltrated around the blood vessels in the IFP at day 7 in both 0.2 and 1.0 groups. Thickening of the patellar tendon and extensive fibrosis and neovascularization in the IFP were observed more obviously at day 14 in the 1.0 mg group. Interestingly, little changes were observed in articular cartilage and synovial membrane in both groups (Fig. 2A and B). Immunohistochemical analyses indicated that the number of CGRP (Calcitonin gene related peptide) positive fibers was accumulated around the perivascular region and in the fibrous tissues in the IFP. At day 14, more numbers of the CGRP positive fibers were observed in the fibrotic region of the IFP in 1.0 mg group. Conclusions: Weight bearing was significantly reduced immediately after MIA injection. This suggests that acute inflammatory response of the IFP is important for inducing joint pain. Severe inflammatory response induced extensive neovascularization and fibrosis of the IFP after day 7. Since CGRP positive fibers were accumulated in these regions, we consider that neovascularization and fibrosis of the IFP induced by inflammatory response have critical roles in the prolongation of knee pain. Pain behavior correlates fibrosis and angiogenesis of the infrapatellar fat pad induced by monoiodoacetic acid.Fig. 2Histological examination of the knee stained with Hematoxylin and eosin (A), Safranin O (B).View Large Image Figure ViewerDownload Hi-res image Download (PPT)