Systematic Mapping of Pressure Pain Thresholds of the Masseter and Temporalis Muscles and Assessment of Their Diversity Through the Novel Application of Entropy.

Aims: To assess the diversity of pressure pain thresholds (PPTs) within the masseter and temporalis muscles by using the novel concept of entropy and to assess the differences in PPT scores between different sites of the masseter and temporalis muscles.

Methods: In this randomized, single-blinded study, the left and right masseter and temporalis muscles of 14 healthy volunteers were divided into 15 sites each, and the PPT was assessed for each of these sites. PPT assessments were performed in two different sessions. Entropy and center of gravity (COG) values were calculated for the PPTs of each muscle. Repeated measures analysis of variance was used to assess differences between muscles, sides, and sites for PPT, entropy, and COG scores.

Results: The main findings were: (1) PPT scores varied significantly between the masseter and temporalis muscles and within each of these muscles; (2) entropy values of PPT scores were not different between the masseter and temporalis muscles; and (3) COG values of PPT scores varied statistically, but these changes do not seem to be clinically relevant.

Conclusion: The results of this study suggest that the anatomical layout of the masseter and temporalis muscles has implications for mechanical pain sensitivity and that areas have different sensitivities within these muscles. Furthermore, reference values for the entropy of PPTs in healthy individuals have been estimated, and comparing these values with those of patients with muscle-related pain conditions can provide quantitative information about the spatial heterogeneity of mechanical pain sensitivity, which may be a valuable clinical outcome measure.