thermoflex RPD

In vitro deformation of acetyl resin and metal alloy removable partial
denture direct retainers
Jean C. Wu, BDSc, MDSc, DDS,a George H. Latta, Jr, DDS, MS,b Russell A. Wicks, DDS, MS,c
Robert L. Swords, DDS,d and Mark Scarbecz, PhDe
College of Dentistry, University of Tennessee, Memphis, Tenn
Statement of the problem. Acetyl resin removable partial denture (RPD) direct retainers may provide an
esthetic alternative to conventional metal direct retainers. The effect of repeated stress on acetyl resin direct
retainers is unknown.
Purpose. This study compared deformation of acetyl resin and metal alloy RPD direct retainers after repeated
dislodgments over a test die.
Material and methods. Ten acetyl resin (Thermoflex) and 10 metal alloy (Ticonium Premium 100) RPD
direct retainers, fabricated to manufacturers’ specifications, were dislodged over a stainless steel die by means
of a laboratory test apparatus for a simulated 3-year period (5000 cycles). Occlusal and facial digital images
made before and after cycling were measured (mm) for direct retainer deformation by using computerimaging software (Scion Image 1.62). Student t tests (␣⫽.05) were performed for statistical comparisons.
Results. A significant difference in deformation between acetyl resin and metal alloy direct retainers
occurred in the occlusal view (P⫽.045), but not in the facial view (P⫽.832). Average deformation varied but
was greatest in the occlusal view: 0.09 ⫾ 0.8 mm for acetyl resin direct retainers compared with 0.01 ⫾ 0.9
mm for metal alloy direct retainers. Average facial view deformations revealed no significant differences:
0.039 ⫾ 0.6 mm for metal alloy and 0.033 ⫾ 0.7 mm for acetyl resin direct retainers.
Conclusion. Within the limitations of this in vitro study, significantly greater deformation resulted with
acetyl resin compared with metal alloy direct retainers after 3 years of simulated use. (J Prosthet Dent 2003;
90:586-90.)
CLINICAL IMPLICATIONS
Within the limitations of this in vitro study, deformation of acetyl resin direct retainers was
significantly greater than their metal alloy counterparts. Because deformation of RPD direct
retainers may adversely affect their clinical performance, it is likely that acetyl resin direct
retainers may lose some of their retentive characteristics.
A
n increased awareness of esthetics in dentistry has
resulted in the need for removable partial dentures
(RPDs) that reveal little or none of the metal supporting
structures or retentive elements. Krol and Finzen’s1 review of rotational path of insertion RPDs heightened
interest in RPD designs that avoid anterior direct retainers. Unfortunately, many clinical situations are not suitable for using these concepts, and conventional retainers
in the anterior region of the mouth are often necessary.
Direct retainers fabricated in a tooth-colored material,
Supported by a research grant from the University of Tennessee,
College of Dentistry Alumni Endowment Fund.
a
Private practice, San Diego, Calif.
b
Professor, Department of Restorative Dentistry.
c
Associate Professor, Department of Restorative Dentistry.
d
Associate Professor, Department of Restorative Dentistry.
e
Associate Professor, Department of Pediatric Dentistry and Community Oral Health.
586 THE JOURNAL OF PROSTHETIC DENTISTRY
such as acetyl resin (thermoplastic technopolymer), may
be more esthetic.
The function of an RPD is affected by the physical
properties of its direct retainers.2 Because acetyl resin has
a modulus of elasticity lower than that of metal alloys,3
the effectiveness of RPD direct retainers fabricated in
this material may be inadequate. In addition, it is known
that metals undergo deformation and fatigue after repeated stress,4 and studies of metal alloy direct retainers
have demonstrated this phenomenon.5-7 There have
been few investigations of thermoplastic direct retainers,
and a recent study did not address the problem of repeated stress.3
This study compared deformation of acetyl resin and
metal alloy RPD direct retainers after repeated dislodgments over a test die. The null hypothesis tested was that
there was no difference between deformation of acetyl
resin or metal alloy RPD direct retainers when subjected
to repeated dislodgments over a test die.
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WU ET AL
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 1. Occlusal view of metal alloy direct retainer (metal
specimen #8, pretest) and millimeter ruler in test apparatus.
Fig. 2. Facial view of acetyl resin (above) and metal alloy
specimens.
MATERIAL AND METHODS
An extracted molar tooth served as a model for production of a stainless steel test die with a Vickers hardness number of 613 (Personal communication; North
Central Tool (NCT), Collierville, Tenn). A vinyl polysiloxane impression material (Extrude Extra; Kerr Corp,
Orange, Calif) was used to make an impression of the
metal die and 2 identical definitive casts were produced
in dental stone (Microstone; Whip Mix Corp, Louisville,
Ky). Retentive assemblies that included a circumferential direct retainer incorporating a 0.254 mm undercut
were fabricated on these casts, 10 with a metal alloy
(Premium 100; Ticonium Co, Albany, NY) and 10 with
acetyl resin (Thermoflex; Austenal, Inc, York, Pa) (Figs.
1 and 2). The metal alloy has a Vickers hardness number
of 360,4 whereas the acetyl resin hardness number is
unknown but was assumed to be less than the stainless
steel test die. The selected sample size followed that of a
previous investigation.8
The direct retainers were fabricated by a commercial
laboratory and possessed different dimensions; the metal
alloy specimens exhibited conventional cast circumferential design, whereas the acetyl resin specimens were
more bulky. Specifically, the width of the acetyl resin
direct retainer (specimen 10) tapered from approximately 5.84 mm at the base to 2.01 mm near the tip; the
thickness varied from 2.01 mm to 1.40 mm. In contrast,
the width of the metal alloy direct retainer (specimen
10) tapered from approximately 2.73 mm to 1.82 mm,
and the thickness varied from 0.93 mm to 0.89 mm. The
dimensions of the acetyl resin direct retainers followed
manufacturer’s recommendations.
The test die formed a part of a test apparatus used in
a previous investigation (Fig. 3).8 Another component
of the apparatus was a precision-machined device that
held the direct retainer in a constant position during its
travel over the test die. This device moved forward and
DECEMBER 2003
backward in a keyway, simulating placement and removal of an RPD along its guide plane. Each retentive
assembly was subjected to 5000 placement and removal
dislodgments, simulating 3 years of clinical use.9
A digital camera (PhotoMed 1680; PhotoMed International, Van Nuys, Calif) mounted on the test apparatus in 2 positions (horizontal and vertical) recorded the
contour of each retainer before and immediately after
the 5000 test dislodgments. A focal length of 10 cm was
maintained in both dimensions throughout the study by
a camera-mounting device (University of Tennessee,
Health Science Center, Division of Biomechanical Instrumentation, Memphis, Tenn), and the direct retainers were positioned identically for each before and after
image. The horizontal photo images were considered to
be equivalent to occlusal views of the direct retainers,
whereas the vertical photo images recorded changes in
the facial views.
The photo images were analyzed by straight-line
measurements of identical before and after positions on
each retainer using computer imaging software (Scion
Image 1.62; Scion Corp, Frederick, Md). Two millimeter rulers were positioned on the test apparatus at the
same horizontal and vertical dimensions as the direct
retainers; the rulers were included in each of the photo
images and provided the baseline for software measurements. The linear measurements were subjected to an
independent sample t test (␣ ⫽.05) using statistical software (Microsoft Excel 2000; Microsoft Corp, Redmond, Wash) to determine significant differences between groups.
RESULTS
Statistical analysis of the data for the deformation of
the direct retainers in the occlusal view revealed a signif587
THE JOURNAL OF PROSTHETIC DENTISTRY
WU ET AL
Fig. 3. Test apparatus. D, Test die; R, retainer; K, keyway; RH, retainer holding device; T, natural tooth.
Fig. 4. T tests for occlusal (O-) and facial (F-) deformation of direct retainer types (95% CI about mean): occlusal view (P⫽.045),
facial view (P ⫽.832).
icant difference between the acetyl resin and metal alloy
direct retainers (P⫽.045). A second t test of the facial
view revealed no statistically significant difference
(P⫽.823).
Average deformation of the direct retainers after
5000 dislodgments occurred in varying amounts (Fig.
4), but was greatest in the occlusal view: 0.09 ⫾ 0.08
mm for the acetyl resin direct retainers compared with
0.01 ⫾ 0.09 for metal alloy direct retainers. The 95% CI
for the mean difference between retainer types was
⫺0.155 to ⫺0.002 mm. These results showed there was
588
a difference in deformation between the 2 retainer types.
This difference may be as small as 0.002 mm or as large
as 0.155 mm, or somewhere in between.
Average facial view deformations were similar: 0.039
⫾ 0.06 mm for metal alloy and 0.033 ⫾ 0.07 mm for
acetyl resin direct retainers. This was a mean difference
of 0.006 mm (95% CI ⫽ ⫺0.05255 to 0.06455 mm).
The range of deformation in the occlusal view for the
acetyl resin direct retainers (0.19 mm) was slightly less
than for the metal alloy direct retainers (0.22 mm). The
range of deformation in the facial view was identical but
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WU ET AL
reversed, 0.19 mm for metal alloy direct retainers to
0.22 mm for their acetyl resin counterparts.
There was no visual evidence of wear on the metal die
following any of the test cycles, nor after completion of
the investigation. This observation was expected because, as previously noted, the Vickers numbers of the
test specimens were much less than that of the stainless
steel test die.
DISCUSSION
The attractiveness of chromium-containing base
metal casting alloys for RPDs “stems from their corrosion resistance, high strength and modulus of elasticity,
low density, and low cost”; however, these attributes
also contribute to some disadvantages, including fatigue
under repeated load.4 In addition, the amount of retention a direct retainer is capable of generating is dependent in part upon flexibility, a quality that is the product
of several factors, including dimension and the material
from which the retainer is made.2
Matheson et al7 subjected wrought wire direct retainers to 1200 flexures (1 year simulation) and found that
they suffered permanent deformation, which the author
determined may be clinically significant. The influence
of direct retainer shape and pattern dimensions, as well
as material and fabrication procedures, have also been
shown to significantly affect their behavior.3,5,6 Repeated dislodgment of metal alloy direct retainers of
various designs cause wear of enamel and restorative
materials, and even the retainers themselves.8,9
VanderBrink et al3 compared certain physical properties of various RPD direct retainers, including thermoplastic materials (Flexite M.P. and Flexite II, Rapid Injection Systems Corp, Mineola, NY) injection-molded
into rectangular cross-section beams. The authors found
no significant differences in the stiffness or proportional
limit for direct retainers fabricated in these thermoplastic materials from their metal counterparts. That investigation differed from the present study as the direct
retainers were deflected only once in 0.125-mm increments up to and beyond the proportional limit, rather
than subjected to repeated stress.
As previously noted, the acetyl resin direct retainers
studied in the present investigation possessed different
dimensions than their metal alloy counterparts; in addition, their material content was different. Considering
the previously-described investigations, it is not surprising that these characteristics resulted in differences for
this study as well. Regardless of whether the observed
deformations were permanent in nature was not addressed, and the factor of resiliency is an area for future
research.
The range of deformation for retainers was either
0.19 mm or 0.22 mm, depending on whether viewed
from the facial or the occlusal. The difference of only
DECEMBER 2003
THE JOURNAL OF PROSTHETIC DENTISTRY
⫾0.03 mm is practically negligible, but the fact that
both direct retainer types exhibited this wide range of
deformation, in some instances deforming 0.10 mm,
may be the more important factor. Even though the
limitations of this investigation did not quantify the extent to which these amounts might affect clinical performance, some effect might be expected on the basis of a
previous report.7
This investigation revealed remarkable dimensional
stability after a simulated 3-year period in the occlusal
view for the metal alloy direct retainers (0.01 mm), but
much less stability for the acetyl resin direct retainers
(0.09 mm). In addition, a total deformation for acetyl
resin direct retainers of 0.17 mm (average ⫾ 1 standard
deviation) represents two thirds of the 0.254 mm of
undercut ordinarily used. Within the limitations of this
in vitro study, deformation of acetyl resin direct retainers
was significantly greater than their metal alloy counterparts, and therefore it might be inferred they may also
lose more of their retentive characteristics.
CONCLUSIONS
Within the limitations of this in vitro study, the following conclusions were drawn:
1. Deformation of the acetyl resin direct retainers was
significant in the occlusal view (P⫽.045), whereas no
significant differences between metal alloy and acetyl
resin retainers could be detected in the facial view
(P⫽.832).
2. Average deformation varied, but was greatest in
the occlusal view: 0.09 ⫾ 0.8 mm for acetyl resin direct
retainers compared to 0.01 ⫾ 0.9 mm for metal alloy
direct retainers; no significant differences between retainer types could be detected in the facial view: 0.039 ⫾
0.6 mm for metal alloy and 0.033 ⫾ 0.7 mm for acetyl
resin direct retainers.
3. The range of deformation was greater in the occlusal view for metal alloy direct retainers (metal alloy: 0.22
mm, acetyl resin: 0.19mm), but was reversed in the facial
view (acetyl resin: 0.22 mm, metal alloy: 0.19 mm).
The authors acknowledge the generous support of the following
faculty and staff, University of Tennessee, Health Science Center: Mr
Bob Gallik, Senior Design Technician, Department of Biomedical
Instrumentation, and Ms Katherine Troughton, Senior Research Associate, Department of Anatomy & Neurobiology.
REFERENCES
1. Krol AJ, Finzen FC. Rotational path partial dentures. Part 2. Replacement of
anterior teeth. Int J Prosthodont 1988;1:135-42.
2. McGivney GP, Carr AG, McCracken WL. McCracken’s removable partial
prosthodontics. 10th ed. St. Louis: Elsevier Science; 2000.
3. VandenBrink JP, Wolfaardt JF, Faulkner MG. A comparison of various
removable partial denture clasp materials and fabrication procedures for
placing clasps on canine and premolar teeth. J Prosthet Dent 1993;70:
180-8.
589
THE JOURNAL OF PROSTHETIC DENTISTRY
4. Mjor IA, O’Brien WJ. Dental materials and their selection. 3rd ed. Chicago:
Quintessence; 2002.
5. Brudvik JS, Morris HF. Stress-relaxation testing. Part III: Influence of wire
alloys, gauges, and lengths on clasp behavior. J Prosthet Dent 1981;46:
374-9.
6. Morris HF, Asgar K, Brudvik JS, Winkler S, Roberts EP. Stress-relaxation
testing. Part IV: Clasp pattern dimensions and their influence on clasp
behavior. J Prosthet Dent 1983;50:319-26.
7. Matheson GR, Brudvik JS, Nicholls JI. Behavior of wrought wire clasps after
repeated permanent deformation. J Prosthet Dent 1986;55:226-31.
8. Latta GH Jr, Wicks RA, Huget EF, Murray GA. Wear of visible light-cured
restorative materials and removable partial denture direct retainers. J
Prosthodont 1997;6:104-9.
9. Hebel KS, Graser GN, Featherstone JD. Abrasion of enamel and composite
resin by removable partial denture clasps. J Prosthet Dent 1984;52:389-97.
Noteworthy Abstracts
of the
Current Literature
WU ET AL
Reprint requests to:
DR GEORGE H. LATTA, JR
DEPARTMENT OF RESTORATIVE DENTISTRY
UNIVERSITY OF TENNESSEE
COLLEGE OF DENTISTRY
MEMPHIS, TN 38163
FAX: (901) 448-7104
E-MAIL: [email protected]
Copyright © 2003 by The Editorial Council of The Journal of Prosthetic
Dentistry.
0022-3913/2003/$30.00 ⫹ 0
doi:10.1016/j.prosdent.2003.09.020
Prosthodontic status among old adults in Pomerania related
to income, education level, and general health (results of
the Study of Health in Pomerania, (SHIP)
Mack F, Mundt T, Budtz-Jorgensen E, Mojon P, Schwahn C,
Bernhardt O, Gesch D, John U, Biffar R. Int J Prosthodont.
2003;16:313-8.
Purpose. The aim of the study was to evaluate associations among prosthetic status, socioeconomic
factors, and general health of subjects aged 55 to 79 years. The data were taken from the Study of
Health in Pomerania (SHIP).
Materials and Methods. Socioeconomic information (age, sex, education level), medical information (number of diseases), and details on smoking and alcohol consumption were obtained.
Prosthetic status in the maxilla and mandible was classified into complete denture (CD), removable
partial denture (RPD), ⱖ10 natural teeth or teeth replaced with fixed prosthodontics (10T⫹), and
ⱕ9 natural teeth including fixed prosthodontics (9T⫺).
Results. The data of 1877 subjects were evaluated. CDs in the maxilla were more frequent than in
the mandible. RPDs were more frequent in the mandible and in the group aged 65 to 74 years. Of
the individuals with a low education level, 47% had a CD in the maxilla, and only 21% had 10T⫹.
However, of subjects with a high education level, 22% had a CD in the maxilla, and 54% had 10T⫹.
The odds ratio of having a CD in the maxilla increased to 11.9 at the age of 75 to 79 years,
compared to 0.6 at the age of 55 to 59 years. Logistic regression analyses showed that the risk of
wearing a CD was significantly associated with old age, low education level, low income, smoking,
and alcohol abuse, whereas the number of diseases (used as an indicator of general health) was not.
Conclusion. Alcohol abuse, smoking, low education level, low income, and old age were significant predictors of wearing CDs.—Reprinted with permission of Quintessence Publishing.
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