During the ODTP phase, partially edentulous patients can fall into one of the following three categories:
Stable partially edentulous situation where a new RPD is desired - remaining teeth have very favorable periodontal and restorative prognosis, and RPD design and fabrication is straightforward. The patient who has been using an RPD long-term who seeks a replacement RPD is an example.
Unstable situation which may eventually involve a new RPD. There may be poor or uncertain periodontal, endodontic, restorative or prosthetic prognosis for the remaining natural teeth. A common clinical presentation in the Pacific-Dugoni clinic is the patient with multiple teeth extracted long ago but never replaced, resulting in passive eruption, tipping of remaining natural teeth and slowly acquired occlusal plane mutilation. These cases require many inter-related clinical decisions, and there are always many treatment possibilities. A common approach for these patients is to simplify the prosthetic treatment by extracting problem teeth, so that the patient has a predictable prosthetic outcome like the category 1 patient described above. If immediate replacement is desired, a resin-base partial (stayplate) is indicated; metal framework immediate RPDs are not done at Pacific. Another approach is to salvage some or most of the problematic teeth by means of extensive treatment; in these cases the overall treatment objective remains to provide the patient with a stable and long-serving prosthetic result.
The third category of partially edentulous patients are those, with or without complications, who desire "permanent" replacement of missing teeth (by means of natural tooth or implant supported crowns and fixed partial dentures), and who are adamant about avoiding a removable solution. This category of diagnosis and planning also requires many inter-related decisions, and the treatment is more expensive and irreversible for patients (when compared with an RPD approach), but fixed reconstruction is the prosthetic solution closest to the original dentition. Removable prosthetic options are usually limited to immediate or remote acrylic base partial dentures (stay plates), which serve as transitional esthetic/functional replacements while definitive treatment is ongoing.
The following matters should be considered when planning treatment for category 2 and 3 patients above:
In patients whose primary dental pathology is periodontitis, the decision regarding tooth salvage are guided by the severity of the disease; generalized mild chronic periodontitis warrants effort to maintain the dentition, while severe periodontal disease is cause to extract most or all of the remaining teeth. There are few guidelines for patients with moderate periodontal disease and uncertain prognosis; a house-of-cards phenomenon is common with these patients, in which extraction of a few problematic teeth initiates arch collapse, or placement of a removable prosthesis complicates home care, accelerating failure of the natural dentition.
A patient with severe root caries, secondary to poor diet or pharmacologically induced xerostomia, has poor long-term prognosis with removable prosthesis use, because of home care complications and food/plaque trapping propensities of stayplates and RPDs.
Patients who present in our clinic with failure of extensively restored natural dentition may have difficulty affording a subsequent comprehensive rehabilitation. A less expensive but drastic alternative is to extract teeth that are not strategic for prosthesis support, and then replace them with an RPD.
Endodontically treated premolars or canines that are used as distal extension RPD abutments are unpredictable because of poor fatigue strength. A significant number of these teeth suffer root fracture, even when well-restored with posts and crowns. In descending order of preference, a posterior end tooth that has or requires endodontic treatment, when a distal extension RPD is planned, should be 1) extracted, 2) restored with fiber post & crown, 3) avoided by new RPD design. Occasionally, in order to avoid extraction, the root of an end tooth with existing root canal will be salvaged, after cutting off the coronal portion. Because of high incidence of caries and soft tissue inflammation in roots retained under denture bases, this approach should not be routine and can only be warranted in situations that argue against extraction (for instance to avoid risk of osseonecrosis in individuals with history of bisphosphonate) and situations that have low caries and periodontitis risk.
The first requirement of the try-in process is for the dentist to assess the accuracy and quality of the laboratory product before the patient appointment. Regarding RPD framework evaluation, the dentist should check that the laboratory prescription was followed, that the workmanship is acceptable, and that any corrections are made. Typical laboratory errors include extra or missing components, occlusal components (such as rests and minor connectors) in premature contact with opposing cast, incorrect clasp or clasp not engaging sufficient undercut, and excess metal thickness in areas of artificial tooth placement, such as proximal plates and buccal portions of plastic retention lattice. Mandibular distal extension frameworks have Triad saddles in place, to facilitate altered cast impressions and/or occlusal record support, and the dentist should check that these plastic extensions have appropriate fit and extension on the cast before the clinical try-in. At the clinical appointment, the framework is seated in the mouth with light pressure. It is advisable to never use firm seating pressure, nor to turn away from a patient with a framework partially seated (in the event that the patient might bite the framework into a locked position), since forcing the framework over tooth contours may result in the framework being locked in place. A well-fitting framework will seat completely with little resistance, the framework will be stable on all rests with no rocking, and the components that contact tooth structure (rests, minor connectors, lingual plate) will be in visibly close or flush contact. In the maxillary arch, proper fit also includes close contact between the palatal major connector and the tissue. In the event that the framework fails to seat, or if the framework appears to seat completely but is not stable, the first effort to reduce the number of potential interferences is to loosen the clasps, including bracing arms, by bending them gently outward from the tooth contour by means of three-prong pliers. If the problem persists, the interference(s) must be disclosed on the interior of the framework; white Fit-Checker is mixed and applied to all internal surfaces (dry, not wet) of the framework that touch the teeth, then the framework is seated while gently rocking against the prematurities. When the Fit-Checker has set, the framework is removed and the disclosing medium is inspected closely for metal show-through areas. These are marked with a pencil (red works best) then the Fit-Checker is removed and the marked areas of the framework are relieved with a medium grit diamond at high speed. The process is repeated until the framework seats fully and is stable on all its rests, at which point the abraded areas of the framework are smoothed with hi speed carbide finishing burs. If the framework adjustment process results in thinning RPD components, such as the junction between a rest and its minor connector, it is acceptable to conservatively adjust the corresponding tooth structure in order to preserve the integrity of the framework. It is also important to acknowledge that some framework discrepancies are impractical / impossible to correct, in which case another framework must be made from a new impression and new master cast.
Mandibular Distal Extension RPD
Because a distal extension prosthesis shares a substantial part of its support with the posterior residual ridge(s), and because impressions can distort the relationship between abutment teeth and distal extension soft tissue, particularly in the mandible, an additional impression is often required to stabilize the overall fit of mandibular Class 1 & 2 RPDs. This is the so-called "altered cast" impression, utilizing the plastic bases attached to the extension portions of the framework. The plastic bases permit border molding and a passive impression of the extension ridges simultaneously with complete seating of the framework on its abutments. This impression is indicated if the plastic base fit on the ridge is unstable at the framework try-in appointment (the base rocks or the indirect retainers lift when the base is loaded vertically), or if the anatomical capture of the original master cast is inadequate.
The plastic base is border molded in the customary fashion with Isofunctional compound to capture buccal shelf, lingual vestibule, and to cover the pear-shaped pad. The tray and compound are painted thinly with polyvinyl adhesive. The final impression is made by applying a mono-layer of medium body polyvinyl siloxane to all internal surfaces of the extension tray, drying the tissue, then quickly seating the framework completely on its rests. While the patient extends the tongue, the clinician holds the framework in place and molds the cheek upward. It is important to hold the framework completely still and to not allow the extension base to move or depress toward the ridge.
When the impression material sets, the framework is removed to inspect the impression. If adequate, the impression material is trimmed away from the teeth and to the internal metal- plastic finish line. The framework is placed back in the mouth and the impression is tested for stability by pressing firmly against the plastic base.
The distal extension impression is sent with the master cast to the laboratory for the altered cast fabrication.
Afterward, it is customary for the lab to remove the plastic tray material and fabricate a wax base on the framework, fitted to the altered cast, for record-making and subsequent artificial tooth arrangement.
Framework Occlusal Equilibration and Occlusal Records
When optimal framework fit is achieved, it should be evaluated for occlusal prematurities. If there are opposing natural teeth, the contact pattern between these teeth should be used as a gauge for equilibrating the framework; in particular, centric contact facets on natural anterior teeth are easy to view, and the objective is for the facet contacts to be the same whether or not the framework is in place in the mouth. Finding initial contacts on polished chrome alloy may be difficult because of its resistance to articulating film marks - abrading the metal with a diamond will improve mark visibility. Prematurities are adjusted on the metal, but attention must be paid to metal thickness. When occlusal framework components become less than 1.5mm thick, the prematurity should be adjusted (conservatively) on the opposing tooth surface.
When the framework no longer has occlusal prematurities, the clinician should decide whether the current mounting is correct or whether a new occlusal record is necessary, and if so, what type of record is appropriate in order to remount the case. The clinician should also grind the ppposing dentition if the teeth are extruded, malposed, or in any other way interfering with an optimal occlusal plane; occlusal plane correction is followed by a new impression of the modified arch.
For tooth bounded cases with multiple centric contacts forming at least a tripod or a highly stable quadrangle, no record is best. The casts are luted in the best fit (maximum intercuspal) position and the master cast is mounted with incisal guide pin at zero.
For tooth bounded cases with inadequate natural tooth centric stops, some type of interocclusal record is necessary. Aluwax or bite registration material used in the framework is straightforward; intraoral records should always be checked on the casts that are to be mounted before the patient appointment is over, in case the casts are not stable in the record, requiring a new record or a different approach.
Distal extension cases almost always require an interocclusal record, using the framework plastic retention lattice, with Triad or wax base attached, to support the registration material. Usually the desired jaw position for distal extension cases is centric relation, requiring cooperation from the patient to generate an accurate record.
Facebow (if not done diagnostically), artificial tooth selection, and gingival shade complete occlusal records for a partial denture case. In addition to tooth shade-matching, the mold selected should match contralateral natural teeth (if present), must allow the teeth to fit into the edentulous spaces mesiodistally, and in the appearance zone, the artificial teeth should also be similar length incisogingivally to the adjacent natural teeth. For anterior replacements, the facial midline should be indicated on the framework or master cast. In areas where the denture base will display, the dentist should also consider whether full flange, partial flange or no flange is indicated.
Mounting and Artificial Tooth Arrangement (Laboratory) Indexed mountings are performed for the casts and artificial teeth are arranged according to the following guidelines:
Existing natural tooth position generally guides placement of artificial teeth, including arch form, incisal length, horizontal and vertical components of overlap, and characterization.
Great care is advised when arranging teeth in the appearance zone, to create appropriate proximal contact between natural and artificial teeth, and to cover any visible part of the cast metal framework (plastic retention mesh, rests and proximal plates).
Centric occlusion on artificial teeth should be simultaneous with that of the natural teeth; it is usually necessary to grind the occlusal surfaces of posterior artificial teeth when opposing natural teeth, to optimize the contact patterns and occlusal "fit" of the set-up.
If it pre-exists and is optimal physiologically, natural tooth guidance should be preserved. If there are no guiding natural teeth in the opposing arch, artificial teeth may then be used to create shallow guidance. If the opposing arch is a complete denture, the RPD arrangement should provide excursive balanced occlusion.
Wax bases and flanges follow these guidelines:
Extension bases, whether anterior or posterior, tend to cover all available ridge and peripheral soft tissue structures, to provide maximum prosthetic support.
Tooth-supported edentulous replacements rarely utilize all of the available soft tissue surface areas as in extension bases, because nearby rests support the entire base. Instead, denture base coverage for tooth-supported areas is minimized to include ridge defect and to provide an esthetic soft tissue analog for the artificial teeth.
In the appearance zone, the base design should be esthetic. Where a plastic base is required, it should establish flush contours with neighboring gingiva and should cover all potentially visible metal framework components; occasionally the plastic base is custom tinted to create a visible shade match with adjacent natural soft tissue. Where a plastic base is not desired, that is, the artificial teeth create a pontic-type ridge lap fit with the tissue; consideration must include whether there is need for tissue-shade papillae, or whether the teeth should be processed in tooth-shade resin.