The retention of a complete maxillary denture depends on a close fit of the baseplate to the mucosa and the correct border extensions. Amongst other factors, this means that the correct placement of the post dam is at the vibrating line, and coverage of the tuberosities is a must with prior surgical reduction is they are enlarged (see Completely Dentures Again).
However, from my experience, the principal reason that the posterior border is short of the vibrating line is that there is a very strong tendency amongst both dentists and their technicians to copy the existing border extensions without considering what might give a better patient outcome.
When preparing special trays from the primary impression, it is usually possible to identify the two foveae palatini, and these depressions mark the correct posterior border (Fig 1). Final adjustments for position should be made after an intra-oral check on the extension with the special tray seated and asking the patient to say ‘ah’. Underextensions can be corrected using moulding compounds or overextensions by reducing the excess with an acrylic bur. I have also found that a better result ensues if moulding compound is applied to the fit surface of the special tray in the region on the post dam, then seated in the mouth and left to cool. This corrects for the more displaceable tissues forming the side walls of the palate, and ultimately improves the posterior seal. A mucocompressive (ie not alginate) impression material for second impressions is mandatory
The second factor is the shape of the post dam. The vast majority of post dams consist of a single groove cut into the model before final processing of the denture in acrylic resin. Is this shape correct? I strongly believe this is not correct because it does not take into consideration the anatomy of the tissues in the area of the post dam. In the mid line there is the well marked raphe from the mid palatal bony suture line, and a similar mucoperiosteum tightly bound down to the underlying bone in the region of the hamular notches and the tuberosities. The walls between these two structures contain the greater palatine neurovascular bundle, mucous glands and displaceable connective tissues. It follows that, whilst the post dam of necessity from the underlying anatomy has to be linear at the midline and tuberosities, this is not the case for the side walls where it can be both deeper and wider to take account of tissue displaceability there. I advocate that the ideal shape of the post dam is that of a ‘cupid’s bow’ with the arches of the bow pointing anteriorly.
This better shape of post dam is made by marking out the cupid’s bow shape on the working model (Fig 2), then, using the discoid end of a le Cron carver, the area is scraped out. Then, the area is marked out in pencil and scraped again. This is usually sufficient to form a sufficiently deep post dam at the midline raphe and the tuberosities, but not the side walls. These are pencilled in and scraped a further twice. Even then, when the denture is fitted, the post dam may be insufficient. This can be added to at the chairside by the addition of a tissue friendly self- polymerising acrylic resin, for example Kerr’s ‘Kooliner’.
The problem of an insufficient post dam is even worse when a cobalt chromium based upper complete denture is prescribed. The linear post dam is usually incorrectly placed, and if this has to be removed, in whole or in part, for any reason, then denture base retention is seriously compromised. The solution is to add tagging to the posterior border, onto which adjustable acrylic resin can be processed (Tomlinson & Turner, 1984).
Further problems in achieving both retention and stability occur when an upper complete denture is opposed by natural mandibular teeth. That is another story and one solution has already been published in Dental Practice (Turner, 1985) and below entitled ‘On the Move’