musculo-aponeurotic architecture of the human masseter muscle An in vivo study of the musculo-aponeurotic architecture of the human masseter muscle Gheorghe TI1, Leekam R3, Lam EWN1,2, Perschbacher SE1,2, Liebgott B2,3, and AM Agur2,3 Oral and Maxillofacial Radiology Graduate Program1, Graduate Department of Dentistry2, and Division of Anatomy, Department of Surgery3, University of Toronto, Toronto, Canada

MM heads: Superficial (SH) and deep (DH) Background Masseter muscle (MM): Chewing Swallowing Speaking Clenching and grinding Pathosis: Masticatory muscle myalgia Temporomandibular joint disorders (TMDs) DH SH MM heads: Superficial (SH) and deep (DH) Agur and Dalley, 2012

Background: Laminae of SH Z Z L1 L1 DH DH L1 L3 L3 M L2 L1 M M L2 Fibre bundle: red line Laminae, L1-L3 Zygomatic arch, Z Mandible, M Deep head, DH

Gaps in Knowledge No in vivo ultrasound (US) studies of MM investigating the musculo-aponeurotic architecture volumetrically Asymptomatic MM architecture needs to be elucidated in vivo to understand pathological changes

Objective Investigate the in vivo musculo-aponeurotic architecture in asymptomatic participants using US in relaxed and maximally contracted states. Hypothesis The aponeurotic and fibre bundle components of the laminae differ significantly between the relaxed and maximally contracted states.

Methods Inclusion: Exclusion: 24 participants (48 MM); 12M/12F 20 - 40 years old Asymptomatic Score of 0 on TMD Pain Screener Exclusion: Positive TMD Pain Screener score (Gonzalez et al, 2011) Disc displacement, degenerative joint disease, or subluxation (Schiffman et al, 2014) Intramuscular pathosis: preliminary US scan

Methods In vivo US protocol based on “Musculo-Aponeurotic Architecture of the Human Masseter Muscle: a Three-Dimensional Cadaveric Study” L3 L2 L1 Ebrahimi, 2015 MSc Thesis

US scanning protocol for each lamina of SH in: Methods Equipment: GE Loqiq e (0.1mm axial resolution) US scanning protocol for each lamina of SH in: relaxed maximally contracted (maximum intercuspation) Electromyography (EMG) used to confirm muscle activation

Methods Images acquired from each lamina Axial: Superior, middle, inferior thirds Coronal: Panoramic Quadrants of SH Sup 1/3 Q3 Q1 Mid 1/3 Q4 Q2 Inf 1/3

Data Analysis Evaluation of SH: Evaluation of each lamina: Number of laminae Evaluation of each lamina: Fibre bundle length (FBL) Aponeurotic length Comparison of FBL and aponeurosis length: Paired t-test Wilcoxon signed-rank test

Results: Panoramic Coronal Relaxed Three laminae (L1-L3) Four laminae (L1-L4) n = 12 MM n = 8 MM Z Z S S L4 L3 L3 L1 L2 L2 L1 A A Zygomatic arch, Z; Subcutaneous tissue, S; Angle of mandible, A

FBL: length of red line; Aponeurosis: yellow Results: Coronal Scan Z Z S S R R S S Relaxed Contracted FBL: length of red line; Aponeurosis: yellow Zygomatic arch, Z; Subcutaneous tissue, S; Ramus, R

Results: Panoramic Coronal Z Z S S A A Relaxed Contracted Aponeurosis: yellow Zygomatic arch, Z; Subcutaneous tissue, S; Angle of mandible, A

Results: FBL SH: 4 laminae (n = 8 MM) Lamina (L) Relaxed Mean FBL (mm) Contracted Mean FBL (mm) L1 22.7±8.1 19.6±8.1 L2* 18.0±6.0 14.8±3.7 L3 12.6±2.6 12.3±3.8 L4 15.9±4.4 15.9±5.6 SH: 3 laminae (n = 12 MM) Lamina (L) Relaxed Mean FBL (mm) Contracted Mean FBL (mm) L1* 19.5 ± 5.8 17.9 ± 6.4 L2 13.0 ± 4.1 11.9 ± 3.4 L3 17.0 ± 4.7 16.6 ± 5.1 * Significant difference between relaxed and contracted (p<0.05)

Results: Aponeuroses SH: 3 laminae (n = 12 MM) Aponeurosis Located between Relaxed mean length of aponeurosis (mm) Contracted mean length of aponeurosis (mm) 1 L1-L2 33.8±6.4 36.5±4.9 2 L2-L3 33.6±12.1 32.4±13.1 SH: 4 laminae (n = 8 MM) Aponeurosis Located between Relaxed mean length of aponeurosis (mm) Contracted mean length of aponeurosis (mm) 1 L1-L2 30.8±16.2 36.1±12.8 2 L2-L3 35.1±6.5 42.7±10.5 3 L3-L4 30.1±14.4 26.0±14.0

Conclusions First in vivo US study investigating musculo- aponeurotic architecture volumetrically Successful development of US protocol to study MM based on 3D digitized data First report of varied changes throughout MM laminae upon maximal contraction

eg. protrusion, excursion, retrusion Future Directions Movement-based differential contraction in each lamina eg. protrusion, excursion, retrusion Musculo-aponeurotic parameters in pathologic MM

Thank you!