An investigation of central and peripheral factors affecting pressure pain thresholds of the human jaw muscles

Myogenous pain is frequently associated with tenderness of the involved muscle. One way of quantifying tenderness is to evaluate pressure pain thresholds [PPTs]. Pressure pain threshold meausurements, however, show marked inter-individual and intra-individual variations. We designed a series of experiments to investigate the effects of several factors on PPT threshold variations in human jaw muscle [masseter and temporalis] in healthy subjects and patients with myogenous pain. Short-term and long-term reliability of pressure algometry were evaluated in normal subjects; the inter-individual variability was found to be much higher than the intra-individual variability, a result suggesting that pressure algometers are particularly indicated for longitudinal within-subject design studies. A non-experimental emotional stressor [a difficult pre-graduate examination] in healthy subjects was found to decrease the mechanical pain thresholds of the masticatory muscles, The ovarian cycle in healthy fertile women also influenced muscle sensitivity, with PPTs being significantly lower in the periovulatory phase. Prolonged chewing activity in normal subjects did not modify pain thresholds, a result indicating that jaw muscles are extremely resistant to fatigue induced by prolonged dynamic contractions. In contrast, static endurance tasks [sustained bilateral biting at 7.5, 10, 15, 25 and 40% of the maximal voluntary contraction on bite force transducers in the molar regions until exhaustion] in healthy subjects significantly decreased PPTs. Short-term periodontal pain [provoked in healthy subjects by inserting orthodontic separators] significantly reduced PPTs. Pressure pain thresholds of jaw muscles in female myogenous patients [i.e., affected with myofascial pain] were significantly lower than in control subjects. In conclusion, in healthy subjects psychological stress, masticatory activity, heterotopic pain and ovarian cycle can affect PPT of the masseter and termporalis muscles, however, the contribution of each factor to the variations of PPT range from 0 to 25% at the most. These values are far below the differences in percentage found between muscle pain patients and control subjects [4-50%].