Passive resistance increases differentially in various jaw displacement directions

Area of Science:

• Biomechanics
• Human Physiology
• Dental Mechanics

Background:

• Understanding the passive resistance of the human jaw system is crucial for analyzing jaw movements and potential limitations.
• The Posselt-envelope defines the boundary of jaw movements, but passive resistance beyond this is less understood.

Purpose of the Study:

• To quantify the passive resistance of the human jaw system.
• To relate this resistance to three-dimensional jaw displacement and the Posselt-envelope.
• To compare in vivo measurements with computer simulations.

Main Methods:

• Passive jaw displacement in three orthogonal directions using step-wise increasing force in eight subjects.
• Electromyography (EMG) with visual feedback to monitor muscle relaxation.
• Development of a biomechanical model to analyze jaw muscle contributions.

Main Results:

• Vertical jaw displacement showed the largest excursion; protrusive and lateral directions were more restricted.
• The jaw could move beyond the Posselt-envelope in protrusive and lateral directions.
• Jaw stiffness increased with displacement, being higher in the protrusive direction than vertical or lateral; model predictions aligned with in vivo data.

Conclusions:

• The human jaw possesses a greater passive range of motion in protrusive and lateral directions than achievable through active muscle engagement.
• Jaw displacement stiffness is notably higher in the protrusive direction compared to vertical and lateral movements.