Mechanobehavior and mandibular ramus length in different facial phenotypes

Abstract

Objectives: To test the hypotheses that mechanobehavior scores (MBS) were correlated with mandibular ramus lengths (Co-Go) and differed between facial phenotypes.

Materials and methods: Subjects gave informed consent to participate. Co-Go (mm), mandibular plane angles (SN-GoGn, °), and three-dimensional anatomy were derived from cephalometric radiography or cone beam computed tomography. Temporomandibular joint (TMJ) energy densities (ED) (mJ/mm3) were measured using dynamic stereometry and duty factors (DF) (%) were measured from electromyography, to calculate MBS (= ED2 × DF,) for each TMJ. Polynomial regressions, K-means cluster analysis, and analysis of variance (ANOVA) with Tukey post-hoc tests were employed.

Results: Fifty females and 23 males produced replete data. Polynomial regressions showed MBS were correlated with Co-Go (females, R2 = 0.57; males, R2 = 0.81). Cluster analysis identified three groups (P < .001). Dolichofacial subjects, with shorter normalized Co-Go, clustered into two subgroups with low and high MBS compared to brachyfacial subjects with longer Co-Go. SN-GoGn was significantly larger (P < .03) in the dolichofacial subgroups combined (33.0 ± 5.9°) compared to the brachyfacial group (29.8 ± 5.5°).

Conclusions: MBS correlated with Co-Go within sexes and differed significantly between brachyfacial and dolichofacial subjects.

Keywords: Craniofacial form; EMG; Human; Jaw mechanics; Masticatory muscles; TMJ.

Figure 1.

Mandibular ramus length (Condylion-Gonion, mm) and Sella-Nasion-to-mandibular plane (SN-Gonion-Gnathion) angle (°) shown in example facial phenotypes.

Figure 2.

Three-dimensional anatomy for numerical modeling included force vectors for: TMJs (F_condyle; R = right, L = left), five muscle pairs (m_1,2 = masseter, m_3,4 = anterior temporalis, m_5,6 = lateral pterygoid, m_7,8 = medial pterygoid, m_9,10 = anterior digastric), and biting characterized by occlusal plane (θ_xz, 0–350°) and vertical (θ_y, 0–40°) angles. Modified with permission.

Figure 3.

(A) Custom occlusal registration appliance with head reference system and contrast spheres for magnetic resonance imaging and (1) light-emitting diodes (LED) for jaw tracking. LED also attached to (2) maxillary and (3) mandibular labial tooth surfaces via custom brackets and glass ionomer cement. Left condyle and temporal fossa and eminence (disc not shown) in superior coronal (B) and frontal (C) views showing path of stress-field centroid (red) during symmetrical jaw closing. Modified with permission.

Figure 4.

Ambulatory muscle activities recorded during one (A) day and (B) night. Laboratory muscle activities (top four rows) during (C) static and (D) dynamic bite forces (bottom row) over time (2-second intervals) produced (E) linear regression relations (root-mean-square [RMS] muscle activity [μvolts] per Newton of bite-force) for ambulatory EMG calibration by subject and muscle.

Figure 5.

(A) Mandibular ramus length (Co-Go) vs mechanobehavior score (MBS, ) in females and males. (B) Cluster analysis showed normalized Co-Go vs MBS centroid distances segregated a group with brachyfacial features from two subgroups with dolichofacial features.