330987835 a-selective-pressure-impression

prosthetics

1. A selective-pressure impression technique for the edentulous maxilla
Jacqueline P. Duncan, DMD, MDSc,
a
Sangeetha Raghavendra, DMD, MDSc,
b
and
Thomas D. Taylor, DDS, MSD
c
University of Connecticut School of Dental Medicine, Farmington, Conn
This article describes a selective-pressure impression technique for the edentulous maxilla that is intended
to compensate for the polymerization shrinkage of heat-polymerized polymethyl methacrylate resin and
provides improved palatal adaptation of the definitive denture base.(J Prosthet Dent 2004;92:299-301.)
There are several definitive impression techniques
for recording the edentulous maxilla. These techniques
may be categorized as functional, nonpressure, and
selective-pressure impressions. Unfortunately, the den-
tures made with these techniques rarely create the
pattern of tissue contact desired, owing to denture base
distortion caused by polymerization shrinkage that oc-
curs with heat-polymerized polymethyl methacrylate
(PMMA).
1,2
With the functional or ‘‘closed-mouth’’ technique,
the patient exerts masticatory force at the desired vertical
dimension of occlusion while the impression material is
setting/polymerizing. The custom tray in this technique
is fabricated with an occlusion rim that allows the patient
to occlude on either an opposing occlusion rim or natu-
ral dentition. The impression is designed to capture the
tissues in a functional state. It has been shown that teeth
are in contact for less than 30 minutes each day,
3
and
some suggest that it is difficult to rationalize a technique
that theoretically places the supporting tissues under
constant pressure when the mucosal tissues are in a func-
tional state for only minutes per day.
4,5
Dentures made
with a positive-pressure impression technique may
exhibit excellent initial retention, but alveolar ridge
resorption may be exacerbated by the pressure from
the denture, and the denture may loosen over a shorter
time period than would be anticipated with other tech-
niques.
4
The nonpressure or mucostatic technique records the
tissues in a nondisplaced, passive state.
6
This impression
technique captures only nonmovable tissues and relies
on interfacial surface tension for retention. A metal den-
ture base is recommended with this technique to ensure
intimate contact with the supporting tissues. The distor-
tion of heat-polymerized PMMA does not allow for the
intimate tissue contact required to achieve adequate in-
terfacial surface tension, yet this impression technique
remains popular.
The selective-pressure impression technique com-
bines aspects of both techniques, as pressure is applied
to certain tissues while other areas are captured with
minimal pressure. This impression philosophy is
credited to Boucher
5
and is based on a histologic under-
standing of the supporting tissues. Areas that are ana-
tomically favorable to withstanding pressure, such as
the 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. The
rugae, midline raphe, mandibular alveolar ridge, and
areas of movable tissue are relieved because they do
not provide the same favorable anatomic quality for
withstanding functional load.
Each of the above philosophies considers how much
pressure will result in the most retentive, stable, and well
functioning denture; however, as long as the denture
base is processed with heat-polymerized PMMA, distor-
tion can occur, resulting in a discrepancy between the
denture and palate. Denture bases fabricated from
heat-polymerized PMMA exhibit dimensional change
owing to volumetric shrinkage of as much as 6%.
7
The
shrinkage of the resin results in a space between the pal-
ate and definitive cast as well as heavy pressure on the
lateral flange area (Fig. 1). This results in a denture
Fig. 1.Poor palatal adaptation is obvious on this processed
denture. Posterior aspect of cast has been trimmed to expose
lack of adaptation of denture base to palate; denture has not
been removed from cast.
a
Assistant Professor, Department of Prosthodontics and Operative
Dentistry.
b
Assistant Professor, Department of Prosthodontics and Operative
Dentistry.
c
Professor and Chairman, Department of Prosthodontics and
Operative Dentistry.
SEPTEMBER 2004 THE JOURNAL OF PROSTHETIC DENTISTRY 299

2. base that does not contact the palate completely and thus
has less than ideal support, stability, and retention
(Fig. 2,A).
Various techniques have been described to minimize
or compensate for polymerization shrinkage of PMMA
through modification of the processing technique.
Some advocate modifying the definitive cast with holes
to anchor the acrylic resin during polymerization.
8,9
Others have described a technique using high-expansion
dental stone to compensate for PMMA shrinkage.
10
The
objective of this article is to describe a selective-pressure
impression technique that is intended to improve adap-
tation of the maxillary denture base by compensating for
polymerization shrinkage of the acrylic resin.
TECHNIQUE
1. Make a preliminary impression with irreversible
hydrocolloid and pour it in dental stone. Mark the
borders of the custom tray 2 to 3 mm from the
mucobuccal 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 and
space for impression material. Laterally, cover the
alveolar ridges with spacer wax up to and including
the borders of the custom tray. End the spacer wax
at the posterior limit of the rugae. Place a narrow
band of wax along the midpalatal suture (Fig. 3).
3. Cut four 535-mm tissue stops out of the wax bilat-
erally in the canine and first molar regions. Place the
tissue stops slightly labial or buccal to the crest of the
ridge to assist in accurately seating the tray. Do
not cover the remaining portion of the palate; this
includes half to two thirds of the alveolar ridge, as
this area should contact the palatal tissues during
the definitive impression (Fig. 3).
4. Fabricate the custom tray with the material of choice,
with consideration for polymerization shrinkage and
distortion. Avoid light-polymerized resins as they
are relatively accurate but have a tendency to rebound
or pull away from the cast during manipulation and
polymerization. Use autopolymerizing PMMA resin
for the tray material to maximize tray accuracy.
5. At the definitive impression appointment, border
mold the custom tray with modeling plastic im-
pression compound (Kerr Corp, Orange, Calif) or
other suitable material. Remove the spacer wax from
the tray before border molding is started. Remove
all wax residue to improve impression material
adhesion.
6. Make the definitive impression with a low-viscosity
impression material (Permlastic; Kerr Corp). No
vent holes are necessary in the tray but may be placed
over the ridge crest if desired. Use the 4 tissue stops
Fig. 2. A,Typical maxillary denture processed with heat-polymerized PMMA with poor palatal contact as demonstrated with
disclosing paste.B,Maximum palatal adaptation of denture base using modified selective-pressure technique as demonstrated
with disclosing paste.
Fig. 3.Spacer wax is placed over entire anatomic area of cast
except in areas outlined. No tray relief is placed in those
areas.
THE JOURNAL OF PROSTHETIC DENTISTRY DUNCAN, RAGHAVENDRA, AND TAYLOR
300 VOLUME 92 NUMBER 3

3. in repositioning the tray accurately. Seat the tray
completely and place moderately heavy pressure in
the first molar region of the tray while the impression
material polymerizes.
7. Remove the impression from the patient’s mouth
and verify the presence of show-through in the areas
where no spacer wax was placed (Fig. 2,B).
8. Process the denture with a standard heat-processing
technique,
11
finish, and polish.
9. Evaluate the denture intraorally, and note the adapta-
tion of the denture base with pressure-indicating
paste (PIP; Mizzy, Cherry Hill, NJ). Relieve areas of
heavy show-through, such as the tissue stops. Verify
excellent adaptation to all the supporting tissues,
particularly those of the palate (Fig. 2,B).
DISCUSSION
This technique provides many of the same advantages
as the posterior palatal seal; however, it affords a much
larger contact area with the supporting tissues than does
the posterior palatal seal. By displacing the tissues of
the palate and effectively creating a deeper vault on the
definitive cast, the technique compensates for the
shrinkage of the PMMA. The result is a denture that
has improved contact with the palatal tissues. There
are no significant disadvantages to this technique. If
the denture base is evaluated with PIP and is found
to have excessive pressure, these areas can be easily
adjusted.
As an alternative to this impression technique, the
definitive cast could be adjusted by arbitrarily scraping
stone in the palatal vault. This would create an artifi-
cially deepened vault to compensate for polymerization
shrinkage comparable to carving a posterior palatal seal.
However, the impression technique described above is
a more controlled method for creating a similar result.
SUMMARY
The selective-pressure impression technique de-
scribed provides the clinician with a method for im-
proving the palatal adaptation of maxillary complete
dentures fabricated with heat-polymerized PMMA.
REFERENCES
1. Latta GH, Bowles WF 3rd, Conkin JE. Three-dimensional stability of new
denture base resin systems. J Prosthet Dent 1990;63:654-61.
2. Lechner SK, Lautenschlager EP. Processing changes in maxillary complete
dentures. J Prosthet Dent 1984;52:20-4.
3. Graf H. Bruxism. Dent Clin North Am 1969;13:659-65.
4. el-Khodary NM, Shaaban NA, Abdel-Hakim AM. Effect of complete den-
ture impression technique on the oral mucosa. J Prosthet Dent 1985;53:
543-9.
5. Boucher C. Complete denture impressions based on the anatomy of the
mouth. J Am Dent Assoc 1944;31:17-24.
6. Addison I. Mucostatic impression. J Am Dent Assoc 1944;31:941-50.
7. Craig R. Restorative dental materials. 11th ed. St. Louis: Mosby; 2002.
p. 647.
8. Laughlin GA, Eick JD, Glaros AG, Young L, Moore DJ. A comparison of
palatal adaptation in acrylic resin denture bases using conventional and
anchored polymerization techniques. J Prosthodont 2001;10:204-11.
9. Polyzois GL. Improving the adaptation of denture bases by anchorage to
the casts: a comparative study. Quintessence Int 1990;21:185-90.
10. Sykora O, Sutow EJ. Posterior palatal seal adaptation: influence of high ex-
pansion stone. J Oral Rehabil 1996;23:342-5.
11. Zarb GA, Bolender CL, Carlsson G, Boucher CO. Boucher’s prosthodontic
treatment for edentulous patients. 11th ed. St. Louis: Elsevier; 1997. p.
332-46.
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UNIVERSITY OFCONNECTICUTHEALTHCENTER
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Copyright2004 by The Editorial Council ofThe Journal of Prosthetic
Dentistry
doi:10.1016/j.prosdent.2004.06.001
THE JOURNAL OF PROSTHETIC DENTISTRYDUNCAN, RAGHAVENDRA, AND TAYLOR
SEPTEMBER 2004 301