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Precision Plastic Partial Dentures By Kevin B. Scally
I have been involved with the design and fabrication of the "Every" partial denture system for some years. My experience has taught me, that unless the team of dentist and technician fully appreciate the subtleties of the design and the technical stages needed to achieve an accurate end result the denture's success is severely compromised. To develop this rapport it is vital that the dentist has had the technical discipline of making a partial denture.
When I first started making "Every" partial dentures in 1970, as a Dental House Surgeon at the Dental Department, Christchurch Hospital, New Zealand, I sent my cases to the Department's laboratory for processing. I got into awful strife with the technicians on two counts. Firstly, the denture's radical design elements were hard to reconcile with what was current teaching in partial denture design and construction; secondly, and probably more importantly, I had no practical training as an undergraduate at Otago University in the technical stages of making partial dentures. This meant I did not appreciate the difficulties and skill involved in producing good quality prostheses, irrespective of design. To my knowledge the situation has not changed. It was my conviction that I had to learn how to make the partials and sort out just what the difficulties were. That salvaged the situation between me and the department's dental technicians. It was only after making all my partial dentures, both plastic and metal, and doing all the designs that I got an insight into the technical subtleties of plastic and chrome-cobalt work.
It also became clear that the dental technicians did not have the advantage of the dentist's understanding of soft and hard tissues. They only sees a rigid monochromatic facsimile of the oral environment and cannot prescribe the degree of relief needed at the dento-gingival junction, or any other area for that matter, as they do not "know" the oral environment. The technician, therefore, constructs a prosthesis on purely mechanical criteria. So it is mandatory, in my experience, that the dentist prescribes the relief, clasp placement and post damming. Once a prosthesis is finished it is too late to discover something was over looked. The time to find out is at the intermediate stages - especially the waxed out stage before duplication. The importance of this is only appreciated by a handful of dentists who have done their own technical work.
Dr Everys designs are still largely unknown outside Christchurch, in spite of them having been published and presented at various places over the last three decades by Dr Every, Dr Reed, and myself. However, the concepts embodied in the designs developed by Dr Every are so different that they give dentists and technicians a chance to work as a team again. But the dentist needs to be prepared to learn the discipline of the technical stages if the team is to work again.
So that experiences gained at the Dental Department would not be lost, and patients I treated using the designs would be adequately managed, I decided some documentation was necessary. In 1974 I put together a comprehensive manual that detailed the technical steps involved in making partial dentures based on Dr Every's ideas. A copy of this was given to the Otago University Dental School. A chapter on the biomechanics and another on the theoretical argument for the design features was also included. This teaching set is based on this material.
The concepts developed in the " A New Perspective on Partial Denture Design" are implicit in these designs, especially contact-point-continuity, reciprocation, and periodontal protection.
Other features are introduced: atmospheric retention, and the tripping action clasp. This work-book sets out a series of technical controls to achieve a "precision" plastic partial denture. Without precision, the dentures inevitably fail
This precision is achieved primarily via a duplicate, modified, master cast. While most dental technicians and dentists argue that this is an unnecessary addition to the manufacture of a plastic partial denture the time saved in the clinic justifies the effort. In addition, the lessons learnt by doing this make the clinical fitting of plastic partial dentures, overdentures, and splints easy if the concepts are applied.
When doing this keep an eye on the soft tissue undercuts since this is often overlooked when planning a plastic partial denture. Having difficulty fitting a plastic partial denture with flanges that catch because of conflicting undercuts and paths of insertion is the most frustrating clinical experience in removable prosthodontics.
The next step is to mark how the partial will emerge from the inter-proximal area. The concept here is to mimic the shape of the tooth. Develop the denture outline after defining this emergence shape. The aim is to keep the acrylic at least 3 mm away from the gingival tissues (Figure 2 c).
In the finished prosthesis the aim is to have the plastic edges blend into the soft tissues, a little like chewing gum smeared on to the palate. A removable partial denture is a foreign body in the mouth and it should be as unobtrusive as possible.
To achieve this, the plastic margins are beaded into the tissues, rather like a post-damn. This beading is handled in two ways. Where there is only one tooth missing the beading skips across the pontic area. Where there is a larger edentulous space the beading goes over the ridge and it feathers out into the areolar mucosa (Figure 3 a and b). This beading substitutes as a finishing line in some places. This can be confusing for the technician since the usual practice is to grind back beyond the finishing line. If this is done at the polishing stage the blending-in feature is lost (Figure 8).
After the contact points and the outline has been defined and the post-damn drawn in the study model can be surveyed. At this stage the conflicts of various paths of insertion can be assessed.
Most surveyors do not have a rod long enough to evaluate the soft tissue undercuts but a modified four inch nail can be used in the surveyor. Since the waxing-out and undercut assessment in this system included developing a negative tooth form about the edentulous area there is far more opportunity to develop a path of insertion that is tolerant of some soft tissue undercuts.
Another concept to ensure precision is contact-point-continuity. The human dentition is a continuous arch. Open contacts as a consequence of orthodontic movement of teeth after the arch is broken following extractions needs to be restored. There are a number of ways to achieve contact-point-continuity. If the contacts are only slightly open, and if the teeth have amalgam restoration in them, it is easy to dove-tail an MO or DO into the existing restoration. If the teeth are unrestored, then an acid etch composite can be used to establish the continuity. If the gaps are large then a narrow tooth can be used.
Once the design exercise on the study model is complete it is important to articulate the models to see what interocclusal conflicts exist for the design. At this point it is useful to make a clinical check list of the modifications that are needed so that these can be done before the secondary impression. If the modifications to the existing teeth are unacceptable, the design needs to be reviewed. This process may need to be repeated several times. The advantage of these design rehearsals is the discipline they impose on the clinician and the technician. The exercise during the learning phase accelerates a facility in design that quickly spills over into clinical confidence in removable prosthodintics.
The working model is surveyed, contact-point areas defined, saddle flange extensions and palatal outline marked in. The peripheral beading is not to be confused with a finishing line even though it doubles as one in some areas of the partial denture.
Where the pontic area is broad the peripheral seal goes across the alveolar crest, avoiding the free gingivae by at least 3 mm and feathers out into the alveolar mucosa (Figure 4 a and b). In the finished denture, a full peripheral seal is achieved.
The beading is governed by the resiliency of the tissue. The depth of the inscription flows from thick to thin throughout the outline. This inscription should be done by the clinician or accurate detailing of its depth relayed to the technician
Once the seal's extent and depth has been determined it is inscribed as a well defined "V"
Check that the path of insertion is in harmony with the outline. However, the path of insertion can be a complex one not determined by surveying but rather by eye-balling the cast. Often a flange area can be placed into an undercut and the denture rotated into place, especially the anterior flange. This is determined by "feel"for the three dimensional path of insertion rather than measurement since a surveyor gives a plane of insertion only (Figure 5).
The interproximal area is waxed out, under the survey line, to present a negative tooth form. It is important that this wax out does not run into the inscribed peripheral seal (Figure 6).
The contact area can be scraped just a little to leave a ledge. This serves two purposes: it ensures a little extra material at the contact point if an amalgam is to be added and gives a finishing line to trim to. The aim is to place the denture into a patient's mouth utilising the flexibility and compressability of the mucoperiosteum, attached alveolar mucosa, and periodontal membrane. Once in place, the denture should blend into the oral tissues and be supported by them.
The surveyed, inscribed, waxed-out model is duplicated with alginate. This is best achieved by soaking the cast in warm water with a few drops of detergent added. This mimics the temperature of the oral environment, a temperature for which alginates are formulated. The detergent also lowers the surface tension, minimising bubble formation.
Once the impression has been poured up it is important to critically examine it for defects. This is difficult to do since all the guide lines and pencil marks are not present. Should any defects, or bubbles be present these can be carefully adjusted on the duplicate model using the master model as a guide. These areas are noted on the master cast so that seating difficulties after processing can be anticipated and these areas can be preferentially adjusted before placing the rough finished partial denture back onto the master cast. If the errors are too gross it is better to abandon the duplicate and redo it (Figure 7). The centric record, try-in, are routine.
The finishing stages
Without a reference cast it is impossible to assess this.
The success of the plastic partial denture is in large part determined by establishing contact point continuity and a durable tight prosthesis tooth contact.
The contact point is defined the wax-out stage. Since the contact area was adjusted on the master model there is some extra tolerance to finishing. Amalgam can be placed as a contact point if acrylic teeth are used. This produces a more durable contact. A class V amalgam can be quickly placed using a number 8 to 12 inverted cone bur (Figure 9). Highly polished accurately placed contact points can be positioned using the master cast as a reference (Figure 10).
If porcelain teeth are used this is not necessary.
Amalgam restorations can improve the aesthetics and further extend the life of the partial denture if placed as MOs, DOs, or MODs. These are done on the master cast using conventional clinical techniques; matrix bands, wedges, etc. These modifications and enhancements can be done in the laboratory or at the chairside (Figure 10).
Additional Retention Using a Tripping Action Clasp.
To enhance retention a distally placed tapered 9 mm stainless steel clasp wire is incorporated into the plastic partial denture. This is a standard feature of this partial denture system (Figure 10 & 11).
At rest the clasp is passive. Once incisive or incusive (crushing) forces are placed on the prosthesis, rotation about the alveolus inevitably occurs. Once this begins the end of the stainless wire engages the tooth surfaces in a tripping action. The angle the clasp makes with the tooth surface is critical and the clasp is adjusted clinically to achieve this tripping action.
At the Study Model design stage a choice of clasp retention influences the outline. The simplest solution is to have a distal contact point on the last maxillary molar. This stainless steel wire doubles as a tripping action clasp.
If an additional buccal clasp is contemplated it must be designed in such a way to encompass the tooth so the tooth will not move. This is especially so when clasping lower molars.
An additional solution is a plastic extension onto the palatal surface that acts as a reciprocating plate for a buccal stainless steel infra-bulge clasp.
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