Sterile water; a novel and promising human experimental craniofacial muscle pain model

Background The aim of this study was to investigate if intramuscular injection of sterile water can be used as a human experimental pain model that resembles clinical craniofacial muscle pain and to analyse if the effects differ between sexes. Methods This randomised, double‐blind, placebo‐controlled cross‐over study included 30 healthy age‐matched women and men (23.6 ± 2.4 years). At three sessions, with at least one week of washout in between, 0.2 mL of either sterile water (test‐substance), hypertonic saline (58.5 mg/mL; active control) or isotonic saline (0.9 mg/mL; passive control) was randomly injected into the right masseter muscle. Pain intensity (VAS) was continuously assessed during 5 min whereafter pain duration (s) and pain area (au) were calculated; pressure pain thresholds (PPT;kPa) were recorded every 5 minutes during 30 minutes. Results Sterile water evoked pain of similar intensity (74.5 ± 49.9) as hypertonic saline (74.0 ± 50.5); whereas, isotonic saline evoked low‐intensity pain (11.4 ± 23.4). The pain induced by sterile water and hypertonic saline had higher intensity (P