Subject-Specific 3D TMJ Muscle Attachment Morphometry Affects Joint Contact Forces

Objectives: Temporomandibular joint (TMJ) muscle morphometry is crucial in determining joint mechanics by altering the magnitude and direction of applied muscle forces. Large inter-subject variation (~20%) for 3D muscle attachment morphometry have been reported. The objective of this study was to quantify the effect of 3D TMJ muscle attachment size, location and orientation on applied muscle forces and joint contact forces using a multibody musculoskeletal model.
Methods: 5 healthy male subjects (age 30.5±8.4 years) participated in experiments collecting 3D jaw motions for three oral tasks while simultaneously recording temporalis and masseter EMG activity. Temporalis, masseter and pterygoid muscle attachment morphometry from 9 male cadavers were assigned to each subject based on similar anthropometry. 18 Hill-type muscle models for each subject were established to estimate muscle forces using recorded EMG signals and joint kinematics. Multibody models were optimized and validated to match net predicted muscle moments measured by inverse dynamics. Sensitivity of the multibody model on applied muscle forces and joint contact forces, was tested by varying temporalis muscle origin size, location and orientation.
Results: The mean relative errors in muscle moments between the multibody model prediction and inverse dynamics measurement were less than 4.3%. By varying temporalis origin size by +20%, contact forces during mandible max open-close increased 13.6% and temporalis muscle forces increased 8.2%. By elevating temporalis spatial location superiorly 20%, contact force increased 12.1% and muscle force increased 7.3%. By increasing the angle between the temporalis origin and sagittal plane by 20%, contact force decreased 10.5% and muscle force decreased 5.6%.
Conclusions: Variation of TMJ muscle attachment size, location and orientation significantly affects contact force and muscle force, highlighting the importance of 3D muscle morphometry and necessity of developing patient specific musculoskeletal models to understand joint mechanics.