Post Dam and its importance in a maxillary denture

• The relevance of the posterior palatal seal to the retention of the maxillary complete denture should be clearly understood.  Hardy and Kapur maintain that the retention and stability that is achieved from adhesion, cohesion and interfacial surface tension are able to resist only those dislodging forces that act perpendicular to the denture base.  
• Horizontal forces and lateral torquing of the maxillary denture can be resisted only by adequate border seal. 
• Terminating the denture borders on soft resilient tissues will allow the mucosa to move with the denture base during function and thereby maintain the denture seal. 
• The peripheral seal of the maxillary denture is the area of contact between the mucosa and the peripheral polished surfaces of the denture base; the seal prevents passage of air between the denture and the tissues. 
• This seal depends upon the proper extension of the denture borders, both in width and height, so that they fill the muco-buccal space and contact the cheek tissues laterally. 
• At the posterior aspect of the denture, in the area of the soft palate, there are no cheek tissues to seal the denture border.
• Therefore, the proper placement of the posterior palatal seal commands a definite clinical procedural protocol if one is to create an optimally retentive complete maxillary prosthesis. 
• The function of the posterior palatal seal in the completed maxillary prosthesis is to maintain contact with the anterior portion of the soft palate (the tissues that undergo shallow displacement) during functional movements of the stomatognathic system (that is, mastication, deglutition and phonation). 
• Therefore, the primary purpose of the posterior palatal seal is the retention of the maxillary denture. 
• The posterior palatal seal that has been correctly diagnosed and incorporated into prosthesis also will reduce patient awareness of this area with a subsequent reduction in the gag reflex, since there should be no separation of the denture base and soft palate during “normal” functional movements; reduce food accumulation beneath the posterior aspect of the denture, owing to proper utilization of tissue compressibility (palatal glandular tissue can withstand mild compressive forces); reduce patient discomfort when contact occurs between the dorsum of the tongue and the posterior end of the denture base, as the posterior denture border will closely approximate the soft palatal tissues, and compensate for the volumetric shrinkage that occurs during the polymerisation of methyl methacrylate resin. 
• The correctly placed posterior palatal seal will not impinge upon the non-displaceable tissues of the hard palate, nor will it limit the muscular movements of the soft palate.  It will, however, create a partial vacuum beneath the maxillary denture. 
• This partial vacuum is activated only when horizontal or tipping forces are directed against the denture base. 
• The duration of time that the partial vacuum acts on the tissues is extremely small and consequently little or no irreversible alterations to the underlying mucosa will take place.

(See S. Winkler)