The instantaneous centre of rotation of a fixed unilateral distal extension partial denture

A photographic technique was used to determine the instantaneous centre of rotation of a fixed unilateral distal extension partial denture and the teeth to which it was attached. The teeth involved were the lower canine and first premolar. When the two crowns were splinted together to provide support, both vertical and lateral saddle movement was reduced. Saddle support using only the first premolar produced a centre of rotation in the sagittal plane below the canine crown--which became located anterior to the canine when the two supporting teeth were splinted together. In the coronal plane, canine support produced an axis of rotation buccal to, and outside the confines of, the model; splinting of the canine and first premolar caused the centre of rotation in this plane to be located buccally and superior to the abutment tooth roots. The more posteriorly the saddle was loaded, the greater was the magnitude of the resultant movement.