Jaw Muscle Morphology at Jaw-opened Position in Skeletal Mandibular Asymmetry

Abstract

Objective: The objective of this study was to investigate the size and orientation of the masticatory muscles at jaw-opened position in patients with mandibular asymmetry. We hypothesized the differences of muscle morphology between deviated side and non-deviated side of each patient would be smaller at jaw opened-position than those at jaw-closed position.

Method:

The study was carried out in 20 patients with skeletal mandibular asymmetry without congenital diseases: 10 females and 10 males (16‑28 yrs). The deviation between the midline of upper and lower incisors ranged from 3 to 11 mm at jaw closed-position. Controls were 10 participants: 5 females and 5 males (20‑30 yrs). Both side of the masseter (MS), medial (MP) and lateral pterygoid (LP) muscles were evaluated by three-dimensional volume data of MRI at jaw closed- and opened-positions. The contours of muscles were traced and were then reconstructed using imaging software, showing the individual jaw muscles in any desired plane. Maximum cross-sectional-area (CSA), length, and orientation were examined. It should be noted that CSAs were measured at an angle perpendicular to the actual muscle’s long axis. Lengths were investigated along the actual muscle’s long axis. To know the orientation of muslces, the frontal angles of MS and MP relative to the Frankfort horizontal plane; and axial angle of LP were measured.

Result:

CSAs and frontal angles significantly changed after jaw-opening both in patients and controls (p<0.0001-0.0038, paired t-test). At at jaw-closed position, length of MS, frontal angles of MS and MP showed differences between deviated side and non-deviated side (p<0.0001-0.0012). However, at jaw-opened position, difference was shown only on length of MS (p=0.002). Further, most parameters showed no significant difference between patients and controls (t-tests with Bonferroni).

Conclusion:

The morphology of masticatory muscles in patients tend to be less asymmetry and close to controls at jaw-opened position.