Effects of varying degrees of ligation in a neuropathic pain model induced by chronic constriction injury

Ligature tightness of chronic constriction injury (CCI) model remains inconsistent and controversial, presenting barriers for researchers. We summarized the different ligation criteria in literature and attempted to clarify their effects. To assess constriction under different criteria, we calculated the radial strain (εR) of ligated nerves from digital photographs. The mechanical withdrawal thresholds (MWT), thermal withdrawal latency (TWL) and sciatic functional index (SFI) were observed in rats of different groups to assess the state of model. Changes of myelin sheath were detected by pathological staining and immunohistochemistry. The median εR values in the Loose, Medium and Tight groups were 13.6%, 15.2% and 21.7%, respectively. Ligated groups had lower MWT than Sham group and the TWL of rats in the Loose approached to rats with sham operation, while that of the Tight group was higher than Medium group 14 days after surgery. Medium and Tight groups showed more abnormal in SFI, compared with the other two groups 14 days. Pathological staining revealed demyelination in three CCI groups, especially in the sciatic nerves. Myelin protein zero levels decreased in the sciatic nerves as the degree of constriction increased, but myelin basic protein of the Medium group was lowest abundant in the spinal cords of all rats. Our study demonstrated that the surrounding muscles briefly twitched when the diameter of the sciatic nerves was constricted by approximately 14–15%, which may provide a reference for other researchers for establishing CCI models.