The next tips continue with prosthodontics and pick up on areas of doubt in the minds of colleagues who have attended my ‘Basis of Occlusion’ courses. The title initials are in correct order and refer to the concept developed about 40 years ago for Kennedy Class I cobalt chromium based partial dentures by Krol. They refer to:
- R = a rest placed mesially on the last standing tooth for reason discussed last month in relation to surveying
- P = a plate connector
- I = a gingivally approaching ‘I’ shaped clasp for retention placed into a 0.5 mm undercut
It is also assumed that the line of insertion and withdrawal as determined by surveying is parallel to the last standing teeth, and that a shortened dental arch is contra-indicated.
There are real technical challenges with these cases not least because these dentures are both tooth and tissue borne. Whilst it is hoped that the remaining teeth are stable and will continue to function for the foreseeable future, the soft tissues under the saddle areas are not stable. Bone will continue to resorb, and the overlying soft tissues change contour too. The rate of this change is variable, with at least one report suggesting that when loaded for the first time, there will be some resorption within 3 months. I have seen this occasionally, but change within 6 months is much more likely. The challenge is how to diagnose this loss of tissue support!
Secondly, there is an implied assumption that when impressions are made, they will correctly record both solid teeth and give some mucodisplacement of the soft tissues to reproduce the conditions under load.
It is more logical to consider impressions first and with it the choice of tray design and impression material. Ideally, use a tray designed for the purpose (Fig 1) with a box section for the standing teeth anteriorly and a posterior area shaped for the saddle, when alginate would be a suitable material for the primary impression. The usual method of using a full arch box tray and alginate will always give an inadequate impression of the saddle area up to the pear shaped pad, unless the saddle area is modified first with impression compound – a time consuming process – prior to taking an alginate wash.
You can then construct a special tray, close fit to the saddle areas and spaced over the teeth. I found that a reasonable compromise second stage impression material was impregum which does provide some mucocompression over the saddles. Alginate as a second impression material is a definite NO, as it is mucostatic and therefore does not load the soft tissues and provides no support.
Once the chrome framework has been cast, purists might like to add close fit trays to the saddle areas, take a mucocompressive impression, section the original saddles from the working cast, refit the framework and pour up the saddle area in plaster– the Applegate split cast technique. I always found this quite fiddly, and it was not unusual to find that the framework had not been fully seated and further errors arose.
I advise making the best denture you can and warning your patient that tissue changes are likely during the first six months so please expect that your denture may need relining. You can decide whether to include the cost in the original provision of the denture, or charge separately, and this will depend on your own practice circumstances.
This leads on to how do you assess whether there is adequate tissue support? The most common suggestion I have received from colleagues attending my courses is to get the patient to bite together and look at the denture tooth position from each side. Sorry – this is a definite NO NO! If the denture base has moved you will never diagnose the movement this way.
We need to consider what happens when there is resorption under the saddle areas. Under occlusal load the denture can then rotate distally about an axis across the mesially placed occlusal rests, and as it does so, the plate connector moves to a greater or lesser extent away from the lingual surfaces of the remaining teeth. This then gives us out test. Place each index finger along the teeth on the denture saddle and gently press downwards. If the connector moves away from the teeth, as described above, then there must have been a loss of tissue support, and this is an indication for rebasing the denture.
As above I use impregum rather than zinc oxide impression paste because with paste, if it enters an undercut at the distal gingival; aspect of the last standing tooth, it will tear and the impression will be inaccurate. There is a further trick in taking the impression. Normally when taking such an impression the index and middle fingers will be placed on top of the saddles to hold the denture in place whilst the impression material sets. If you do this all you will achieve is a thin coating of impression material that has been pushed onto the ridge. The denture needs to be held in place with the index finger on the lingual aspect of the denture connector and the thumb under the chin. In this way the denture base is correctly seated and the true loss of alveolar bone and its soft tissue covering can be taken up by the impression material.
Your laboratory can then rebase the denture in acrylic resin. However, when you do come to the fit appointment you must allow extra time to adjust the occlusion. I have never yet had a case where adjustment was not required, and sometimes quite a lot, particularly if there are standing maxillary teeth which will have over-erupted down to the position of the saddle.
In summary, these dentures will always need checking for tissue support at every follow up appointment. If your patient is sufficiently dentally aware, then implants provide a much better solution, but these are expensive options when at least three are required in each quadrant. Another compromise I have used is to provide one implant in each quadrant with a stud retainer. This provides direct support and retention, and if the placement position is carefully chosen, more implants can be added to over time and the stud converted into a crown.