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

Sofia Louca Jounger,Helena Lindskog,Niklas Eriksson,Anna Oscarsson,Vivian Simonsson,Malin Ernberg,Nikolaos Christidis

doi:10.1038/s41598-019-51670-3

Abstract

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.

Highlights

This study investigated if repeated buffered acidic saline infusions into the masseter muscles induced muscle pain and mechanical sensitization
Standardized, easy, and safe human experimental pain models are needed in order to better understand the mechanisms behind M-temporomandibular disorders (TMD), any possible chewing and biting deficiencies caused by M-TMD, and to be able to improve diagnostics and treatment
We have shown that two repeated injections into the human masseter muscle, two days apart, with an un-buffered acidic saline solution with a pH of 3.3, resulted in mild short-lasting pain, but no mechanical allodynia[14,15]

Summary

Introduction

This study investigated if repeated buffered acidic saline infusions into the masseter muscles induced muscle pain and mechanical sensitization. A previous study has shown that two repeated acidic saline (pH 4) injections into rats gastrocnemius muscle, two to five days apart caused a mechanical hyperalgesia for 30 days. One can assume that there is a critical time-frame in which the repeated trauma of the muscle tissue post-infusion results in an exaggerated, more long-lasting hyperalgesia It is at present unclear whether the acidic saline model evoke similar effects in orofacial muscles as in limb muscle in rodents, since previous studies show conflicting results. In one study, repeated acidic saline (pH 4) injection into rat masseter muscle two days apart caused long-lasting mechanical allodynia[12], while another study with similar methodology failed to induce mechanical allodynia[13]

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