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

Myogenous pain is frequently associated with tenderness of the involved mus cle. One way of quantifying tenderness is to evaluate pressure pain thresho lds [PPTs]. Pressure pain threshold meausurements, however, show marked int er-individual and intra-individual variations. We designed a series of expe riments to investigate the effects of several factors on PPT threshold vari ations in human jaw muscle [masseter and temporalis] in healthy subjects an d patients with myogenous pain. Short-term and long-term reliability of pre ssure algometry were evaluated in normal subjects; the inter-individual var iability was found to be much higher than the intra-individual variability, a result suggesting that pressure algometers are particularly indicated fo r longitudinal within-subject design studies. A non-experimental emotional stressor [a difficult pre-graduate examination] in healthy subjects was fou nd to decrease the mechanical pain thresholds of the masticatory muscles, T he ovarian cycle in healthy fertile women also influenced muscle sensitivit y, with PPTs being significantly lower in the periovulatory phase. Prolonge d chewing activity in normal subjects did not modify pain thresholds, a res ult indicating that jaw muscles are extremely resistant to fatigue induced by prolonged dynamic contractions. In contrast, static endurance tasks [sus tained bilateral biting at 7.5, 10, 15, 25 and 40% of the maximal voluntary contraction on bite force transducers in the molar regions until exhaustio n] in healthy subjects significantly decreased PPTs. Short-term periodontal pain [provoked in healthy subjects by inserting orthodontic separators] si gnificantly reduced PPTs. Pressure pain thresholds of jaw muscles in female myogenous patients [i.e., affected with myofascial pain] were significantl y lower than in control subjects. In conclusion, in healthy subjects psycho logical stress, masticatory activity, heterotopic pain and ovarian cycle ca n affect PPT of the masseter and termporalis muscles, however, the contribu tion of each factor to the variations of PPT range from 0 to 25% at the mos t. These values are far below the differences in percentage found between m uscle pain patients and control subjects [4-50%].