Repeated buffered acidic saline infusion in the human masseter muscle as a putative experimental pain model

This study investigated if repeated buffered acidic saline infusions into the masseter muscles induced muscle pain and mechanical sensitization. Fourteen healthy men participated in this double-blind, randomized, and placebo-controlled study. Two repeated infusions (day 1 and 3) were given in the masseter muscles with either a buffered acidic saline solution (pH 5.2) or an isotonic saline solution (pH 6) as control. After 10 days of wash-out, the experiment was repeated with the other substance. Pressure pain thresholds (PPT), pain intensity, maximum unassisted mouth opening (MUO), and pain drawings were assessed before, directly following, and after each infusion at 5, 15, and 30 min and on day 4 and 7. Fatigue and pain intensity were assessed after a one-minute chewing test 30 min after infusions and day 4 and 7. Acidic saline induced higher pain intensity than control day 3 up to 5 min after infusions, but did not affect PPT. The chewing test did not evoke higher fatigue during chewing or MUO or after acidic saline infusion compared to control. Repeated acidic saline infusions in the masseter muscles induced a short-lasting muscle pain without mechanical hyperalgesia or functional pain. Hence, this model might not be superior to already existing experimental muscle pain models.