Introduction
The mandible can move in endless ways, but every movement stays inside a fixed boundary set by the TMJs, ligaments and teeth. The edges of that boundary are the border movements; because ligaments limit them, they repeat reliably and can be copied in an articulator.
Posselt's envelope
Posselt was the first to describe the border movements, as a three-dimensional envelope traced by the lower incisor point. Seen from the side (sagittal plane) it has four parts:
- Posterior opening border: from centric relation the condyles first only rotate around the terminal hinge axis, for about the first 12 mm of incisor opening. Then the ligaments force them to translate forward down the eminence, until maximum opening.
- Anterior opening border: from maximum protrusion down to maximum opening.
- Superior contact border: along the teeth, from CR to the intercuspal position, edge-to-edge and maximum protrusion. The teeth shape this border.
- Functional movements (chewing, speaking) lie inside the envelope. They are not border movements: the neuromuscular system decides them.
The rest position also lies inside the envelope, a few millimetres below the intercuspal position.