|ECR 2018 / C-0814|
|Myofascial pain: ultrasound amplitude > width of the masseter muscle|
According to the working hypothesis, the masticatory muscles of subjects with chronic MFP should be of greater width under resting conditions, and with a lesser increase in width under contraction, compared with the healthy controls. The present study identified no significant differences in either variable in the MMs of subjects suffering MFP with involvement of these muscles.
Only two publications in the literature have been found comparing a hypothesis similar to the present study. One of them detected statistically significant differences in MM width under resting conditions and in percentage width increase (equivalent to width at maximum contraction / UOC width ratio) between subjects with MFP and their controls.(9,10,11,12) The second article identified in the literature only reported significant differences in right MM thickness between subjects with MFP and their controls, though in contrast to what might be expected, thickness was found to be greater in the control group (11.16±1.37mm versus 10.07±1.45mm).(13)With the exception of the dimension of the right MM under resting or UOC conditions, the results of both studies are similar.
Positioning of the probe represents a potential source of bias in the ultrasound exploration. Emshoff and Bertram found that the maximum width is observed in the middle portion of the MM, and that measurements from various positions at one same level barely differ provided the ultrasound probe is kept perpendicular to the long axis of the muscle.(10,14) Both the present study and the two studies used to compare the resultsmade use of this ultrasound exploration procedure.
A considerable number of studies have examined the relationship between MM thickness as determined by ultrasound and facial growth pattern (15,16), malocclusions (17), dental condition (18,19), and even gender. (16,20,21) Not consider these variables in these study, since doing so would have fragmented the sample into too many subgroups, thereby adversely affecting the statistical power of the study.
Although the results obtained do not allow rejection of the null hypothesis, some of the data obtained suggest that rejection of the hypothesis cannot be ruled out entirely. In effect, with the exception of the left MM under contraction, all the values were higher in the MFP-subjects than in the controls (Table 2); the relative increase in muscle width under contraction was greater among the controls; and it is observed a nonsignificant tendency towards greater muscle width with a longer duration of MFP.
The present study study contributes new information on a subject that has been little investigated to date. To our knowledge, this is the first study involving standardized clinical diagnostic criteria, with reproducible UOC and maximum contraction positions, and with analysis of each side independently.
The relevance of this study is referred to the absence of changes in the dimensions of the MM in subjects presenting MFP with involvement of this muscle. This suggests the need to reconsider the hypothesis of an increase in muscle mass associated to muscle hyperactivity in the context of this disease condition.
In conclusion, no statistically significant differences in MM width between MFP subjects with involvement of these muscles and the controls as determined by ultrasound under both UOC conditions and at maximum contraction have been found.
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