Impression and Model Techniques for Complex Prosthodontics

With the median age of the population rising and the growing benefits of preventive dental care, patients are retaining more teeth longer.1 Education of the public has increased patients’ awareness that removable prosthodontics are not their best choice for long-term care and that fixed prosthodontics are preferable. This rapidly growing trend in the direction of fixed prosthodontics has created the need for a more predictable, accurate impression technique that is readily attainable.

This article will illustrate a controlled, consistent method for fixed prosthodontic impressions utilizing an articulated custom impression with a layered impression technique that produces accurate dies.

Once basic dental care has been completed and the patient has a stable and comfortable occlusion in centric relation, the planning phase for fixed prosthodontics can proceed. Study casts should be mounted with a face bow and appropriate registrations in centric relation. Custom trays are the key component of fixed restorative dentistry.2,3 The many advantages of custom trays far outweigh the minimal time and expense of their fabrication: less impression material is required; there is better coverage of anatomic variables; and gagging can be minimized by control of palatal coverage. Extension of the tray into critical areas can enhance the capture of important details thereby reducing the need for costly retakes. Utilizing full, accurate casts of occlusions adjusted to centric relation will minimize the deflective contacts, which are the major source of occlusal inaccuracy in mounted casts, and hence reduce the clinical time required to adjust the final prosthesis. One additional major advantage is that the dentist’s hands are free while the impression material sets.

IMPRESSION TECHNIQUE

 

 
Figure 1. Mounted casts.

The preoperative technique should proceed as follows. Mount the study casts (containing good anatomic detail) on an articulator with a face bow in centric relation (Figure 1). Cover the cast of the arch to be restored with 2 layers of baseplate wax to within 2 mm of the mucobuccal fold. One additional layer of baseplate wax may be added in the area to be restored. Make 3 to 4 internal stops by cutting away wax in the supporting areas. Using light-cured tray material (Denplus Photobase), cover the waxed area. On the maxillary cast, leave the palate open unless a removable appliance is anticipated. To the external occlusal surface of the tray add 3 occlusal stops. The 2 external bilateral posterior stops are made by adding an additional layer of tray material in the molar areas, and the anterior stop can utilize an extension of the tray handle.

 

   
Figure 2. Circled areas preoperatively of occlusal stops on tray. Figure 3. Articulated impression tray preoperatively.

Place the cast with tray on the articulator and close until there is light contact on the external stops. Care should be taken to keep the external stops as flat as possible to avoid the tray shifting on sloping contacts (Figure 2). Cure the tray on the model using a laboratory light-curing unit (Triad, DENTSPLY). Remove the tray from the model, and after removing the wax from within the tray, cure the underside of the tray as well. Trim and smooth the periphery of the tray as required. Perforate the tray for mechanical retention, and recheck the tray on the cast to ensure stable seating internally and balanced occlusal contacts externally (Figure 3).

 

 

   
Figure 4. Preformed impression on cast with Saran Wrap. Figure 5. Impression side of preformed impression with Saran Wrap.

Intraorally, proceed as follows. To gain maximum adhesion of the impression material to the tray, coat the interior of the tray with adhesive within 30 minutes of the initial impression. The following technique works well with any of the polyvinyl siloxane impression materials.4 Preoperatively, load the tray with heavy-bodied putty impression material (eg, Star VPS Heavy Body, Danville Materials) and cover the putty with a polyethylene sheet. Immediately, seat the aforementioned tray over the cast of the arch to be prepared and close the articulator on the external occlusal stops until the internal tray stops are in contact (Figure 4). Ensure that the tray is centered and remove all peripheral excess, especially on the palate. Quickly transfer this preformed impression, covered with the polyethylene sheet, to the mouth (Figure 5) and recheck orientation, occlusion, and fit. Ensure there is adequate clearance of the putty in the areas to be prepared. If in doubt, displace the putty sufficiently to avoid contact with preparations. Then, proceed to the preparation of the teeth or implant abutments. On completion of preparations and retraction of gingiva, block out undercuts (if required) and isolate the region. To maximize detail of the impression, ensure that the preparations are cleaned of all debris and dried.5

 

 

Figure 6. Detail of preformed impression without Saran Wrap. Figure 7. Assistant injecting light body in preformed impression.

 

Figure 8. Extralight body injected intraorally. Figure 9. Examining final impression detail.


When ready for the final impression, the dental assistant will remove the polyethelene sheet from the preformed impression (Figure 6). He or she will then inject light-bodied polyvinyl siloxane impression material (Figure 7, eg, Lite Body/Reg-ular Splash, Discus Dental) into the prepared areas and lightly over the remaining occlusal areas in the preformed impression. Meanwhile, the dentist injects extralight-bodied polyvinyl siloxane impression material6 (eg, Extra Lite Bodied/Regular Splash, Discus Dental) around the preparations (Figure 8). The extralight impression material should be aggressively dispersed with air to form a thin film on the preparations and to provide flow subgingivally. A second layer should then be injected over the preparations. The preformed impression, with light-body impression material added, is now seated in the mouth with care as to orientation. It is now almost self-centering because of the preformed putty internal matrix. The patient is instructed to gradually squeeze up on the occlusal stops until excess light and extralight impression materials are seen in the mucobuccal fold.7 Once the impression is set (4 to 5 minutes), remove the tray and wash and dry the impression. Examine the impression with magnification and high-powered light for detail, completeness, and accuracy (Figure 9).

This method overcomes many of the concerns of more conventional impression techniques. By means of positional control through the self-locating, preformed impression and ensuring adequate relief in the critical preparation areas, there is minimal risk of heavy-bodied putty showing through the light body in the final impression. Control of excess bulk of putty is easily managed by preforming it extraorally on the cast. Control of excess flow of light-bodied impression material is readily limited by the dental assistant’s experience in placement of the light-bodied material in the preformed putty impression. Concerns about excessive hydraulic pressure causing distortion are dramatically reduced by the rigidity of the combined tray/putty preformed impression and also by the use of lower-viscosity and higher-flow characteristics of the light and extralight injectable impression materials. Finally, by having the putty and impression material set first in the tray, its distortion, however small, occurs prior to the final impression and therefore is not contributing to overall distortion in a cumulative manner.

 

THE ACCURATE AND AESTHETIC PINNED DENTAL MODEL

 

It is imperative that dental laboratories understand the importance of the manufacture of accurate and aesthetic plaster models from which the foundation can be developed for precise fabrication of prosthetic restorations. Unfortunately, too many laboratories give insufficient emphasis to the quality of their model work, which may compromise the fabrication of the final restoration. In addition, model technicians should be given sufficient support both financially and psychologically for a job that, when done well, is both extremely important to the laboratory and the successful outcome of the final prosthetic restoration.

Numerous studies have shown that wet trimming of plaster/stone models causes wear of the surfaces due to the constant surface abrasion by the water spray, which has plaster particles in the suspension. It is these minute particles that literally abrade the surface of the model. Fine, delicate details such as preparation margins can be easily worn, causing inaccuracies to the model and the eventual restoration.

Careful dry trimming of the dried poured model removes this problem by eliminating the abrasion of the model surface and by keeping the model completely dry until after the margins have been trimmed and sealed with model sealant. Following is a technique that will give good, consistent results for crown and bridge models.

 

MODEL TECHNIQUE

 

There are many excellent materials from which to choose. The materials illustrated within this article are being used on a daily basis within our laboratory and were selected with the following considerations in mind: quality, handling capabilities, availability, and cost. The armamentarium used with this technique is as follows:

(1) die stone—Garreco/Excelsior Resin Die Stone

(2) base stone—Garreco/Buff Cast In Stone

(3) die pins and sleeves—Great Lakes Orthodontics

(4)model separator—Bredent/Gipsisolierung Gis

(5)base former withmagnet—Gamundia Dental Produkts

(6) model hardener—DVA Die Stone Conditioner

(7) die spacer—Yeti Die Space

(8) dry model trimmer—KaVo

(9) laboratory handpiece—KaVo

 

 

Figure 10. Dry model, trimmed dry. 

First, all impressions are disinfected using a recommended commercial antibacterial cleaning product in order to destroy any bacteria or viruses. Mechanical debris is then cleaned in order to ensure accurate setting of the stone material. All saliva may not be removed at this stage due to its high viscosity. If left, it may inhibit the surface setting of the stone material. The saliva can be removed from the impression by sprinkling it with plaster powder and rinsing the powder out with running water. Any remaining saliva will be re-moved along with the plaster (Figure 10).

After pouring and inspection, the models are left overnight in a warm, dry environment for an ideal drying time of 18 to 24 hours. Once the model is dry, it can be dry-trimmed, with special attention being given to the following:

(1) the occlusal plane of the teeth/preps being parallel to the trimmed base (horizontal);

(2) the vertical height of the trimmed model being kept to a minimum; and

(3) all visible landmarks being maintained.

 

 

Figure 11. Placing bevel with handpiece. Figure 12. Noncorrosive pins.

Using decreasing grit size of sandpaper belts ensures a smooth finish to the trimmed model. Using a handpiece, trim the external of the model with a bevel to allow it to seat passively into the base (Figure 11).

Numerous types of die pins are available, therefore it is important to select pins and sleeves that fulfill the following requirements:

(1) consistent in size

(2) narrow in diameter

(3) accurate fit of sleeve onto pin—not overly retentive

(4) the sleeve fitting flush to model is an advantage, as no model stone from base will enter the sleeve and prevent seating

(5) noncorrosive pins (Figure 12).

 

 

Figure 13a. First base ready to be poured with magnet. Figure 13b. First base poured.

 

 
Figure 14a. Second base without retentive disc. Figure 14b. Second base with retentive disc.

 

Figure 15. Bases ready to be trimmed. Figure 16. Trimmed bases separated.


After coating the base of the model with separator, the pinned model is then placed into a base former, which holds the magnet in place. The base is then poured with buff stone (Figures 13a and 13b). After being poured, the base is inverted, a retentive disc is added to the magnet, and with separator on the model a second base is poured (Figures 14a and 14b). A drying period of 6 hours in a drying oven is recommended before once again trimming the bases with the dry trimmer (Figures 15 and 16).

 

 

Figure 17. Dies sectioned. Figure 18. Dies trimmed.

The base having been trimmed, the model is ready for sectioning and die trimming (Figures 17 and 18). The use of the magnet will aid in removing the model from the articulator during the fabrication of the restoration, thus saving time.

With this technique, the accurate integrity of the margin is maintained. At no time has it been abraded with water or particles of stone. It can be die-trimmed under magnification and treated with model hardener, and die spacer can be applied accordingly. To preserve the finish of the base stone and to prevent staining and discoloration, fine sandpaper the base and coat with clear polish.

 

CONCLUSION

An impression technique that provides many advantages over more traditional methods has been provided. While providing good control, extension, and orientation of the impression, less impression material is required and fewer retakes should occur. Patient acceptance is very high, and there are fewer concerns over gagging. Causes of distortion are reduced while accurate detail is recorded. In this age of dental teamwork, the kindest courtesy one can provide to a dental technician is a high-quality, complete, and accurate impression. A technique for producing accurate and aesthetic pinned models has also been presented. When consistent, high-quality models complement excellent impressions, the dentist/technician team can function in harmony to produce quality prostheses.


References

1. Rempfer RK. Changing issues and demographics affecting periodontal and implant therapy. J Calif Dent Assoc. 2002;30:351-354.

2. Christensen G. Complex fixed and implant prosthodontics: making nearly foolproof impressions. J Am Dent Assoc. 1992;123:69-70.

3. Chee WW, Donovan TE. Polyvinyl siloxane impression materials: a review of properties and techniques. J Prosthet Dent. 1992;68:728-732.

4. Impression Materials. In: Miller, MB, ed. Reality. Vol 17. Houston, Tex: Reality Pub Co; 2003:465.

5. Petrie CS, Walker MP, O’mahony AM, et al. Dimensional accuracy and surface detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under dry, moist, and wet conditions. J Prosthet Dent. 2003;90:365-372.

6. Wyman RJ. New choices for crown and bridge impressions: the phase/ three impression technique. Ontario Dentist. 2003;93:30-32.

7. Schoenrock GA. The laminar impression technique. Dent Today. Apr 2002;21:56-63.


Dr. Pepper maintains a private practice in Dundas, Ontario, and has completed Level VI at the Pankey Institute. He can be reached at (905) 628-3070.

Ms. Toth is the clinical manager of Dr. Pepper's practice and received her CDA II in 1998. She can be reached at (905) 628-3070.

Mr. Laingchild was educated in London, has managed a dental laboratory in Germany, served as chief technician for an American hospital in Saudi Arabia, and expanded his technical skills in Norway. He currently owns and operates The Burlington Dental Studio in Ontario, Canada. He is a member of the American Academy of Cosmetic Dentistry, teaches dental technology at George Brown College in Toronto, and is on the advisory board of Spectrum Magazine. He also conducts hands-on courses at his facility in Burlington, Ontario. He can be reached at (800) 342-1508.



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