Dynamic Quantitative Imaging of the Masseter Muscles in Bruxism Patients with Myofascial Pain: Could It Be an Objective Biomarker?

Abstract

We aimed to investigate whether the collaboration of shear wave elastosonography (SWE) and B-mode ultrasonography (US) could be offered as diagnostic tools to assess the presence, severity, and progress of bruxism, as well as a biomarker for the effectiveness of treatment in daily clinical practice. The study was designed as a quantitative evaluation of the masseter muscles (MMs) of the clinically diagnosed bruxism patients suffering from myofascial pain and MMs of the healthy individuals. Clinical examinations were made according to the diagnostic criteria for temporomandibular disorders (DC/TMD), and pain was assessed using a visual analog scale (VAS). Painful MMs with VAS scores ≥ 4 were assigned to Group A, and healthy MMs were assigned to Group B. Also, the MMs of the painful bruxers were analyzed based on wearing occlusal splints. Group A was divided into two subgroups as splint users (Group AI) and non-users (Group AII). All the participants were scanned with dynamic US and SWE to quantify the size and stiffness of the MMs. Measurements of each muscle pair while the jaw is in a resting position (relaxation) and clenching position (contraction) were recorded. The significant differences in stiffness and thickness became visible in the relaxation state. Bruxism patients with myofascial pain had significantly harder and thinner MMs than healthy individuals. During the relaxation, the mean thickness and elasticity values were 9.17 ± 0.40 mm and 39.13 ± 4.52 kPa for Group A and 10.38 ± 0.27 and 27.73 ± 1.92 for Group B, respectively. Also, stiffer MMs were measured in Group AII (38.16 ± 3.61 kPa) than in Group AI (26.91 ± 2.13 kPa). In conclusion, the combination of SWE and US using a dynamic examination technique has the potential to be a valuable tool for the management of bruxism patients suffering from myofascial pain.

Keywords: bruxism; masseter muscle; myofascial pain; occlusal splint; shear wave elastosonography; stiffness; thickness; ultrasonography.

Figure 1

Scanning of the MMs. (a) The probe was placed on a line between the angle of the mouth and the lower implantation of the ear, crossing the bulky part (dashed line). (b) The subject was laid in supine position and her head was slightly turned in the opposite direction. A water-based transmission gel was applied. The transducer was held perpendicular and placed on the line.

Figure 2

Dynamic B-mode US imaging; thickness measurement of the left MM. Note that the length of the blue dashed lines indicates the thickness of the muscle, and the red dashed boxes show anterior–middle–posterior thirds of the MM in order from left to right. (a) Thickness during muscle relaxation: 7.8 mm, 8.2 mm, and 7.6 mm of anterior, middle, and posterior thirds, respectively. (b) Thickness during contraction: 10.3 mm, 10.9 mm, and 9.8 mm in order of anterior to posterior thirds. Under each condition, the measurements were repeated three times and averaged.

Figure 3

SWE of a 26-year-old woman with bruxism (VAS score is 5); dynamic imaging of the left MM. (a) The left MM in resting position on the shear wave propagation mode. Measurements were obtained from the ROI’s placed into regularly parallel contour lines. The E_max and E_mean values of the muscle on elasticity mode were 95.4 kPa and 79.2 kPa, respectively. (b) MM in clenching position with maximum force. The stiffness values were 145.7 kPa and 86.7 kPa in order of E_max and E_mean.

Figure 4

SWE on the propagation mode in the resting position. A 28-year-old woman with VAS scores of 4 in the right MM and 2 in the left MM. (a) The right MM. The E_max and E_mean values of the muscle on elasticity mode are 140.9 kPa and 79 kPa, respectively. (b) The left MM. The E_max is 91.5 kPa and E_mean is 45 kPa.