The orbicular is orris closes the lips and, when contracting, its lateral insertions into the modioli are fixed by the contraction of the quadratus labii superioris and inferioris, zygomaticus, triangular is, and buccinator muscles. The incisal muscles also fix it to the sulci of the alveolar ridge.
During such function there is considerable inward pressure on the labial surfaces of the teeth and the labial sulci are reduced in depth. The upper denture, due to its better stability, can usually withstand such pressure provided its labial flanges have been designed correctly. The lower denture , however, particularly in cases presenting poorly formed alveolar ridges, is likely to be raised from the ridge in front and pushed backwards. Experienced denture wearers counteract this pressure of the lip by a forward pressure of the tongue but assistance can often be given to the tongue by carrying the heels of the lower denture up the ascending ramie of the mandible on such cases. The distal extension of the lower denture are often referred to as posterior stops.
During such function there is considerable inward pressure on the labial surfaces of the teeth and the labial sulci are reduced in depth. The upper denture, due to its better stability, can usually withstand such pressure provided its labial flanges have been designed correctly. The lower denture , however, particularly in cases presenting poorly formed alveolar ridges, is likely to be raised from the ridge in front and pushed backwards. Experienced denture wearers counteract this pressure of the lip by a forward pressure of the tongue but assistance can often be given to the tongue by carrying the heels of the lower denture up the ascending ramie of the mandible on such cases. The distal extension of the lower denture are often referred to as posterior stops.
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