# Location of centers of rotation and existence of terminal hinge axes during jaw movements – a preliminary in vivo clarification

## TL;DR

This study shows that jaw opening involves both rotation and translation, challenging the assumption of pure rotation in mechanical articulators.

## Contribution

It provides in vivo evidence of combined rotational and translational condylar movement during jaw opening.

## Key findings

- The center of rotation during full mouth opening is located caudally and slightly posterior to the condyle center.
- Rotation-translation diagrams confirm combined condylar movement, not pure rotation.
- The findings challenge standard mechanical articulator concepts based on pure condyle rotation.

## Abstract

This study aimed to evaluate the behaviour of jaw opening movements in vivo, especially focusing on locating the center of rotation and extracting the translational and rotational components of mandibular movements respectively.

Jaw movements of healthy participants were recorded using an optoelectronic tracking device and kinematics registered to bony anatomy segmented from CT scans. An evaluation program was developed to support visualization and quantification of mandibular movements. Landmarks representing anatomic and arbitrary condylar positions were defined and their trajectories investigated. Center of rotation positions were calculated for the full opening paths, and rotation-translation diagrams were recorded to investigate condylar movements for different jaw opening angles.

Analyzing landmark trajectories in a virtual skull model explicitly demonstrated their dependency on the spatial 3D location relative to the condyle center. For the full mouth opening, the center of rotation averaged 32.4 (± 7.4) mm caudally and 0.2 (± 10.8) mm posteriorly to the condyle center. These values aligned well with a theoretical calculation assuming combined translational and rotational condyle movement. A combined condylar rotational and translational movement could also be demonstrated using the rotation-translation diagrams, indicating that no pure rotation around the condyle was observed for the initial mouth opening movement.

This in vivo study indicates that the assumption of a pure condyle rotation during initial mouth opening is not supported by the data. As this assumption underpins standard mechanical articulator concepts, these concepts need critical reevaluation.