title: Entropy of Masseter Muscle Pain Sensitivity: A New Technique for Pain Assessment.
abstract: OBJECTIVE
  To test whether manipulation of mechanical pain sensitivity (MPS) of the masseter muscle is reflected in quantitative measures of entropy.
  
  
  METHODS
  In a randomized, single-blinded, placebo-controlled design, 20 healthy volunteers had glutamate, lidocaine, and isotonic saline injected into the masseter muscle. Self-assessed pain intensity on a numeric rating scale (NRS) was evaluated up to 10 minutes following the injection, and MPS was evaluated after application (at 5 minutes and 30 minutes) of three different forces (0.5 kg, 1 kg, and 2 kg) to 15 different sites of the masseter muscle. Finally, the entropy and center of gravity (COG) of the pain sensitivity scores were calculated. Analysis of variance was used to test differences in means of tested outcomes and Tukey post hoc tests were used to adjust for multiple comparisons.
  
  
  RESULTS
  The main findings were: (1) Compared with both lidocaine and isotonic saline, glutamate injections caused an increase in peak, duration, and area under the NRS pain curve (P < .01); (2) A pressure of 2 kg caused the highest NRS pain scores (P < .03) and entropy values (P < .02); (3) Glutamate injections caused increases in entropy values when assessed with 0.5 kg and 1.0 kg but not with 2.0 kg of pressure; and (4) COG coordinates revealed differences between the x coordinates for time (P < .01) and time and force for the y coordinates (P < .01).
  
  
  CONCLUSIONS
  These results suggest that manipulation of MPS of the masseter muscle with painful glutamate injections can increase the diversity of MPS, which is reflected in entropy measures. Entropy allows quantification of the diversity of MPS, which may be important in clinical assessment of pain states such as myofascial temporomandibular disorders.
authors.forename: Eduardo E
authors.initials: EE
authors.lastname: Castrillon
authors.forename: Fernando G
authors.initials: FG
authors.lastname: Exposto
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Sato
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Tanosoto
authors.forename: Taro
authors.initials: T
authors.lastname: Arima
authors.forename: Lene
authors.initials: L
authors.lastname: Baad-Hansen
authors.forename: Peter
authors.initials: P
authors.lastname: Svensson
mesh_terms: D000328:Adult; D019277:Entropy; D005157:Facial Pain / Q000503:physiopathology*; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D008406:Masseter Muscle / Q000503:physiopathology*; D010147:Pain Measurement / Q000379:methods*