Retrofitting a New Crown to an Existing RPD
When retrofitting crowns to an existing RPD using a dual-arch impression technique, proper relation of the clasp assembly to the crown preparation is critical to success.
Clinicians and dental technicians know how difficult and time consuming it may be to fabricate a new crown to fit an existing removable partial denture (RPD). There have been numerous techniques described in the literature for more than 50 years.1-13 Many opportunities exist for an error to be introduced by the clinician or the technician that will necessitate re-fabrication of the crown, re-fabrication of the RPD, or an extended clinical period to adjust the crown and/or the RPD to ensure proper fit. One of the most critical aspects of retrofitting a new crown to an existing RPD is to properly orient the clasp assembly to the crown preparation. The following technique allows the impression, and thus the clasp assembly, to be properly related to the preparation while ensuring the RPD is fully seated. A well-interdigitated occlusion is paramount for this technique to provide an acceptable retrofitted restoration. The dual-arch impression technique for a crown is well documented in the literature14-17 as being a viable and accurate technique if used appropriately.
The first case involves a 62-year-old woman who presented with recurrent caries and a fractured lingual cusp and amalgam on tooth No. 28 (Figure 1). The patient was advised that the best treatment to restore the tooth was a porcelain-fused-to-metal (PFM) crown. This tooth was one of the retainers for a mandibular RPD. It was explained to the patient that the best treatment would involve fabrication of the PFM crown followed by a new RPD. The patient expressed great concern about the finances involved with this treatment plan. It was then proposed that a PFM crown be fabricated with the intent to retrofit her existing RPD. It was explained to her in detail the possible complications and resulting fit of her RPD to the new crown if she selected this treatment option. She understood that a new RPD may ultimately be required but wished to attempt the retrofitting of a PFM crown to her existing RPD.
After tooth preparation, the RPD was placed in the mouth to ensure adequate reduction (Figure 2). After verification of adequate reduction in all planes, the RPD was removed and the tooth was prepared for the impression using a double-cord tissue management technique.
For the impression, a metal dual-arch tray (Temrex Bite Relator Wide, Freeport, NY) was used. One-half of a disposable Rite Bite Tray (Temrex Bite Relator Wide, Freeport, NY) was inserted into the Bite Relator followed by an intact Rite Bite Tray. The paper corner of the insert was removed to prevent interference with the anterior teeth and soft tissue (Figure 3). This extended tray modification provided more surface area of the RPD to be captured in the impression. A rigid fullmouth dual-arch tray could have been substituted for this quadrant method. The RPD was reinserted into the mouth and the tray placed in the mouth to verify that the tray would not interfere with the patient’s ability to fully close into maximum intercuspation (Figure 4). It is suggested that the procedure for the impression be fully explained to the patient and several “trial runs” be performed so that the patient understands the importance of closing their teeth in maximum intercuspation.
For the master impression, the syringe material was injected around the preparation followed by the insertion of the RPD. The dual-arch tray was loaded with heavy-bodied polyvinyl siloxane (PVS) impression material and then placed over the preparation and the RPD. The patient was then instructed to fully close. The adjoining teeth were evaluated to verify that the patient was in maximum intercuspation. The patient was instructed to maintain closure with a normal, evenly distributed biting force for the duration of the setting time of the material (Figure 5).
After the material had fully set, the impression, along with the RPD, was removed in one piece (Figure 6). Provisionals were fabricated and cemented. The impression with the attached RPD was sent to the production laboratory for cast and die work. The laboratory can generally return the RPD the next business day and it can be given back to the patient. When the laboratory has completed the die preparation and is ready to finalize the fabrication of the crown, the patient returns to the dental office and the removable prosthesis is returned to the laboratory.
In the laboratory, any exposed intaglio surface of the RPD is lubricated lightly with petroleum jelly and the impression poured, pinned, and mounted in a conventional dual-arch die technique (Figure 7). As much of the intaglio surface of the RPD as possible is included in the pour. This will aid the RPD to be securely and accurately placed back on the working cast during the fabrication of the crown. The laboratory technician now has an impression of the crown preparation with the RPD under function. This provides a stable RPD that is accurately related to the crown preparation. No auxiliary template (pattern resin, etc) is needed for fabrication. The crown’s opposing occlusion, under a normal biting force, is also accurately represented. The laboratory can now fabricate a casting for the PFM crown in a traditional laboratory technique. After the RPD is returned to the laboratory, the porcelain can be applied to fit the existing RPD rest, guide planes, and retentive area. A restoration is shown with the proper rest support, reciprocation, and retention after fabrication to fit the existing RPD (Figure 8).
At the delivery appointment, the crown was seated with minimal adjustment and the RPD was seated to verify the fit and retention of the prosthesis. The crown was then cemented and the patient was dismissed with a well-fitting RPD (Figure 9).
This case involved an 82-year-old woman who presented with both teeth Nos. 22 and 27 requiring crown restorations due to the breakdown of old composite restorations and recurrent caries. Finances were a major concern, as the patient did not have the means to restore both teeth with crowns and refabricate a RPD. The teeth were badly broken down so that her existing RPD had no retention from either tooth. Caries control and build-ups were completed before PFM crown preparations. The patient’s existing RPD was many years old and the occlusion was very well interdigitated with the opposing denture. The existing partial denture was stable on the residual ridge and presented with no observable instability other than a lack of retention. Without this type of occlusion, the fabrication of these two crowns would not have been possible.
After preparation of both teeth, the RPD was inserted in the mouth and the reductions verified (Figure 10). A full-arch NeoTray (Premier Dental Products, Inc, Plymouth Meeting, PA) was tried into the patient’s mouth to ensure proper size. The RPD was removed from the mouth and the tissue was managed to prepare for the impression. The syringe material was placed around the prepared teeth and the RPD was seated into the mouth. The full dual-arch tray loaded with medium-body PVS impression material was placed over the RPD and the prepared teeth. Next, the patient was instructed to close fully (Figure 11). The patient was instructed to maintain closure with a normal biting force (as would be used for a denture reline procedure) for the duration of the setting time of the material. After the material was fully set, the impression along with the RPD was removed in one piece. Provisionals were fabricated and cemented.
With this full-arch type of impression, it is very difficult to verify that the teeth were in maximum intercuspation during the impression. After removal of the impression, the occlusion may be evaluated to some degree by observing the translucency of the impression material over the RPD and the opposing denture. As shown in Figure 12, the opposing maxillary denture and the mandibular RPD appear to have been in intimate contact. If the metal of the partial denture is visible at the preparation through the impression material, this may indicate that additional preparation reduction may be required to allow adequate space for the restorative materials to be fabricated by the laboratory. If, upon the disassembly of the RPD and the working cast, the mounted casts are obviously not in proper occlusion, the impression would have to be remade. Even if an interocclusal record was made, hand articulation would not be acceptable because the full seating of the RPD could not be ensured. The impression would have to be remade.
The impression with the attached RPD was sent to the production laboratory for cast and die work. The RPD was returned to the patient the following day. She returned the RPD when the laboratory was to the point of fabricating the crowns. The crowns were fabricated in the same manner as described in the first case. The mounting and resulting working casts are shown in Figure 13 and Figure 14.
The technician fabricated the two crowns in the same manner as described for the single crown previously described in Case 1. Both crowns were seated with minimal adjustment and the RPD was seated to verify the fit and retention of the prosthesis (Figure 15 and Figure 16). Both crowns were then cemented and the patient dismissed with a well-fitting RPD.
Although this technique may not be considered the most optimum treatment for restoring a tooth that will be an abutment for a RPD, it should be considered when financial circumstances prevent the practitioner from fabricating a new crown and new RPD.
This technique does have the negative aspect of the necessity of the patient having to leave the office without their RPD (unless the practitioner has the laboratory capabilities to pour, pin, and mount the case). This may be an esthetic concern if the RPD involves replacement of anterior teeth. The patient must decide if the inconvenience of not having the RPD during the laboratory phase is worth the cost savings of a new RPD.
The patient must have the type of occlusion that will allow the prosthesis to be held firmly in place during the impression phase. The technique provides the laboratory technician a working cast with the RPD properly oriented to the tooth preparation. The chance of the RPD not being fully seated or for movement to occur during the impression phase is virtually eliminated.
This technique has been used by the author in private practice and student doctors at our university to retrofit numerous crowns to existing RPDs. The technique has resulted in the seating of the crown and existing RPD with minimal chairside adjustments needed to the crown or the RPD.
Thanks to Cindy Deaton, CDT, for the laboratory work and Dr. Lew Powell for the intraoral photographs.
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About the Author
Michael A. McBride, DDS
Associate Professor Director
Division of Prosthodontics Department of Restorative Dentistry
College of Dentistry
University of Tennessee