Mechanical threshold changes (in grams) after surgery with or without intramuscular injection of fentanyl are presented as the mean ± SEM at the indicated time points post-surgery. A, B: abdominal and paw mechanical changes among normal, sham model and model groups after surgery. C, D: abdominal and paw mechanical changes among saline, fentanyl 5, 10 and 20 μg/kg groups. ##P<0.01 versus the sham model group, #P<0.05 versus the sham model group; *P<0.05, **P<0.01 versus the saline group. Short black lines represent the surgery duration. Fen: fentanyl, BL: baseline.