A selective-pressure impression technique for the edentulous maxilla

Jacqueline P. Duncan, DMD, MDSc,a Sangeetha Raghavendra, DMD, MDSc,b andThomas D. Taylor, DDS, MSDc

University of Connecticut School of Dental Medicine, Farmington, Conn

This article describes a selective-pressure impression technique for the edentulous maxilla that is intendedto compensate for the polymerization shrinkage of heat-polymerized polymethyl methacrylate resin andprovides improved palatal adaptation of the definitive denture base. (J Prosthet Dent 2004;92:299-301.)

There are several definitive impression techniquesfor recording the edentulous maxilla. These techniquesmay be categorized as functional, nonpressure, andselective-pressure impressions. Unfortunately, the den-tures made with these techniques rarely create thepattern of tissue contact desired, owing to denture basedistortion caused by polymerization shrinkage that oc-curs with heat-polymerized polymethyl methacrylate(PMMA).1,2

With the functional or ‘‘closed-mouth’’ technique,the patient exertsmasticatory force at the desired verticaldimension of occlusion while the impression material issetting/polymerizing. The custom tray in this techniqueis fabricatedwith an occlusion rim that allows the patientto occlude on either an opposing occlusion rim or natu-ral dentition. The impression is designed to capture thetissues in a functional state. It has been shown that teethare in contact for less than 30 minutes each day,3 andsome suggest that it is difficult to rationalize a techniquethat theoretically places the supporting tissues underconstant pressure when themucosal tissues are in a func-tional state for only minutes per day.4,5 Dentures madewith a positive-pressure impression technique mayexhibit excellent initial retention, but alveolar ridgeresorption may be exacerbated by the pressure fromthe denture, and the denture may loosen over a shortertime period than would be anticipated with other tech-niques.4

The nonpressure ormucostatic technique records thetissues in a nondisplaced, passive state.6 This impressiontechnique captures only nonmovable tissues and relieson interfacial surface tension for retention. A metal den-ture base is recommended with this technique to ensureintimate contact with the supporting tissues. The distor-tion of heat-polymerized PMMA does not allow for theintimate tissue contact required to achieve adequate in-terfacial surface tension, yet this impression techniqueremains popular.

aAssistant Professor, Department of Prosthodontics and OperativeDentistry.

bAssistant Professor, Department of Prosthodontics and OperativeDentistry.

cProfessor and Chairman, Department of Prosthodontics andOperative Dentistry.

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The selective-pressure impression technique com-bines aspects of both techniques, as pressure is appliedto certain tissues while other areas are captured withminimal pressure. This impression philosophy iscredited to Boucher5 and is based on a histologic under-standing of the supporting tissues. Areas that are ana-tomically favorable to withstanding pressure, such asthe buccal surface of the maxillary alveolar process, lat-eral palate, or buccal shelf in the mandible, are loaded.These areas are supported by dense cortical bone. Therugae, midline raphe, mandibular alveolar ridge, andareas of movable tissue are relieved because they donot provide the same favorable anatomic quality forwithstanding functional load.

Each of the above philosophies considers how muchpressure will result in themost retentive, stable, and wellfunctioning denture; however, as long as the denturebase is processed with heat-polymerized PMMA, distor-tion can occur, resulting in a discrepancy between thedenture and palate. Denture bases fabricated fromheat-polymerized PMMA exhibit dimensional changeowing to volumetric shrinkage of as much as 6%.7 Theshrinkage of the resin results in a space between the pal-ate and definitive cast as well as heavy pressure on thelateral flange area (Fig. 1). This results in a denture

Fig. 1. Poor palatal adaptation is obvious on this processeddenture. Posterior aspect of cast has been trimmed to exposelack of adaptation of denture base to palate; denture has notbeen removed from cast.

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Fig. 2. A, Typical maxillary denture processed with heat-polymerized PMMA with poor palatal contact as demonstrated withdisclosing paste. B, Maximum palatal adaptation of denture base using modified selective-pressure technique as demonstratedwith disclosing paste.

base that does not contact the palate completely and thushas less than ideal support, stability, and retention(Fig. 2, A).

Various techniques have been described to minimizeor compensate for polymerization shrinkage of PMMAthrough modification of the processing technique.Some advocate modifying the definitive cast with holesto anchor the acrylic resin during polymerization.8,9

Others have described a technique using high-expansiondental stone to compensate for PMMA shrinkage.10 Theobjective of this article is to describe a selective-pressureimpression technique that is intended to improve adap-tation of themaxillary denture base by compensating forpolymerization shrinkage of the acrylic resin.

TECHNIQUE

1. Make a preliminary impression with irreversiblehydrocolloid and pour it in dental stone. Mark the

Fig. 3. Spacer wax is placed over entire anatomic area of castexcept in areas outlined. No tray relief is placed in thoseareas.

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borders of the custom tray 2 to 3 mm from themucobuccal fold to allow room for border molding.Determine the posterior border of the tray by mark-ing the vibrating line and hamular notches bilaterally.

2. Adapt 1 thickness of baseplate wax (Truwax;Dentsply, York, Pa) to the cast to provide relief andspace for impression material. Laterally, cover thealveolar ridges with spacer wax up to and includingthe borders of the custom tray. End the spacer waxat the posterior limit of the rugae. Place a narrowband of wax along the midpalatal suture (Fig. 3).

3. Cut four 53 5-mm tissue stops out of the wax bilat-erally in the canine and first molar regions. Place thetissue stops slightly labial or buccal to the crest of theridge to assist in accurately seating the tray. Donot cover the remaining portion of the palate; thisincludes half to two thirds of the alveolar ridge, asthis area should contact the palatal tissues duringthe definitive impression (Fig. 3).

4. Fabricate the custom tray with the material of choice,with consideration for polymerization shrinkage anddistortion. Avoid light-polymerized resins as theyare relatively accurate but have a tendency to reboundor pull away from the cast during manipulation andpolymerization. Use autopolymerizing PMMA resinfor the tray material to maximize tray accuracy.

5. At the definitive impression appointment, bordermold the custom tray with modeling plastic im-pression compound (Kerr Corp, Orange, Calif) orother suitable material. Remove the spacer wax fromthe tray before border molding is started. Removeall wax residue to improve impression materialadhesion.

6. Make the definitive impression with a low-viscosityimpression material (Permlastic; Kerr Corp). Novent holes are necessary in the tray but may be placedover the ridge crest if desired. Use the 4 tissue stops

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in repositioning the tray accurately. Seat the traycompletely and place moderately heavy pressure inthe first molar region of the tray while the impressionmaterial polymerizes.

7. Remove the impression from the patient’s mouthand verify the presence of show-through in the areaswhere no spacer wax was placed (Fig. 2, B).

8. Process the denture with a standard heat-processingtechnique,11 finish, and polish.

9. Evaluate the denture intraorally, and note the adapta-tion of the denture base with pressure-indicatingpaste (PIP; Mizzy, Cherry Hill, NJ). Relieve areas ofheavy show-through, such as the tissue stops. Verifyexcellent adaptation to all the supporting tissues,particularly those of the palate (Fig. 2, B).

DISCUSSION

This technique providesmany of the same advantagesas the posterior palatal seal; however, it affords a muchlarger contact area with the supporting tissues than doesthe posterior palatal seal. By displacing the tissues ofthe palate and effectively creating a deeper vault on thedefinitive cast, the technique compensates for theshrinkage of the PMMA. The result is a denture thathas improved contact with the palatal tissues. Thereare no significant disadvantages to this technique. Ifthe denture base is evaluated with PIP and is foundto have excessive pressure, these areas can be easilyadjusted.

As an alternative to this impression technique, thedefinitive cast could be adjusted by arbitrarily scrapingstone in the palatal vault. This would create an artifi-cially deepened vault to compensate for polymerizationshrinkage comparable to carving a posterior palatal seal.However, the impression technique described above isa more controlled method for creating a similar result.

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SUMMARY

The selective-pressure impression technique de-scribed provides the clinician with a method for im-proving the palatal adaptation of maxillary completedentures fabricated with heat-polymerized PMMA.

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Reprint requests to:

DR JACQUELINE P. DUNCAN

DEPARTMENT OF PROSTHODONTICS

UNIVERSITY OF CONNECTICUT HEALTH CENTER

FARMINGTON, CT 06030-1615

FAX: 860-679-1370

E-MAIL: jduncan@nso2.uchc.edu

0022-3913/$30.00

Copyright � 2004 by The Editorial Council of The Journal of Prosthetic

Dentistry

doi:10.1016/j.prosdent.2004.06.001

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