Dimensional changes occurring in dentures during processing

the Journal of the American Dental A ss o cia tio n

D im e n s io n a l d u r in g

J u lia n and

T h e

r e la tiv e

c o m p le te te c h n ic )

m ade

om m ended

o f 12

d im e n s io n

d e n tu re c a s t.

w as

changed

upper

le s s t h a n

w hen fro m

th in

th e

c a lly

d iffe r e n t s ig n ific a n t.

w ith

re c­

o f re s in d e n tu r e s b y s p e c ia l m e th o d s o f

w as

(m e th y l and

m e t h a c r y la te ),

h a rd

de­

ru b b e r h a d

of

cu re d

gypsum le s s

lo w e r

th a n

d e n tu re s

ones. T h e

T h e

d iffe r ­ changes

not

c lin i­

s e lf-c u r in g

p o ly

th e

lo w

epoxy

c u r in g

r e s in ,

s h r in k ­

p r o c e s s in g , resin s,

or

of

th e

d e n tu re s

show ed

a

d is ­

t o r t i o n . 1C o n v e n t i o n a l a c r y l i c r e s i n s p r o c ­ essed

w ith

th e

c o m p r e s s io n in

d im e n s io n

te c h n ic s

usual

m o ld in g as

th e

in v e s t ig a t e d .

d e n ta l a re

te c h n ic

ju s t

s p e c ia l

as

of

s ta b le

r e s in s

and

by by

s p e c ia lly a

V in y l- a c r y lic

com pou nded

c o m b in a t io n

c o p o ly m e r s ,

of

b o th .

p o ly s t y r e n e ,

a n d e p o x y re sin s w i t h s p e c ia l p r o c e s s in g te c h n ic s h a v e of

been

p o ly (m e th y l

a d v o c a ted

in

m e th a c r y la te ).

p r o c e s s in g m e t h o d s a r e c la im e d

p la c e Som e

to p r o ­

d u c e a b s o lu t e ly a c c u r a te , s t r a in - fr e e d e n ­ tu re s w i t h e x c e lle n t s ta b ility o f fo r m . M any

d e n tis ts

b e lie v e

th a t

d e n tu re s

w i t h a n a c r y lic r e s in b a s e w a r p s e v e r e ly o n p r o c e s s in g a n d in s e r v ic e a n d t h a t th is s u p p o s e d w a r p a g e cau ses in a c c u r a t e a p ­ p o s it io n o f t h e d e n t u r e t o th e tissues. T h e p r im a r y

ages. A l l

D .D .S .,

C.

I n r e c e n t yea rs m a n y a tte m p ts h a v e b ee n

th e

a re

D .

m a d e t o i m p r o v e t h e a c c u r a c y o f th e f it

th e

m a te r ia ls

W a s h in g to n ,

186

changed and

C . P a ffe n b a rg e r, f

G eorg e

113

e n ce s a m o n g th e m o la r -t o -m o la r on

D .D .S .;

S w e e n e y ,% A . B . ,

th e p o s t e r io r s e c tio n

d e n tu re s

th ic k

d e n tu re s

of

te c h n ic s

o ccu rre d

in

and

fit

m a te r ia ls

re m o v e d

U p p er

lo w e r s ;

th e

T .

g r e a te s t lin e a r c h a n g e s in

a cro s s

d e n tu re s

B . W o e lfe l, *

c lin ic a l

p r o c e s s in g

te r m in e d . T h e

th e

of

(7 3

o c c u r r in g

p r o c e s s in g

W illia m .

a ccu ra cy

d e n tu re s

c h a n g e s

p u rp o se

of

th is

s tu d y

was

to

m e a s u r e a n d t o c o m p a r e t h e r e la t iv e a c ­ cu ra cy

of

th e

c lin ic a l a n d v a r io u s re s in s fe re n t n ics.

fit o f (T a b le

recom m en d ed The

186

d e n tu r e s

113 t e c h n ic ) 1)

(7 3

m a d e o f th e u s in g t h e d i f ­

p r o c e s s in g

m ea su rem en t o f

te c h ­

d im e n s io n a l

Acrylic resins, powder-liquid, cross-linked

Epoxy resin, slurry-liquid

Copolymer of

Duraflow

Epoxolon

Hydro-Cast

Dental rubber (pure para No. 37dark olive)

Polystyrene, precured bar

Hygienic Dental Rubber

Jectron

powder-liquid

acrylic resins,

Copolymer methacrylate, powderliquid, cross-linked, self-curing

Type of materia I

Aerolite 88

Brand

11-1-57

6-15-58 Processed in commercial laboratory

12-17-57

11-8-57

9-16-57

8-1-57

Date rec'd

75

06816

no batch no.

lot #78

0657E

190

Batch no.

Table 1 • Denture base materials and processing technics investigated

Jectron Co., Toledo, Ohio

Hygienic Dental Manufacturing Co. Akron, Ohio

Kay-See Dental Mfg. Co., Kansas City, Mo.

Surgident, ltd., los Angeles

Product Research laboratory, Inc. Cambridge, Mass.

Aerolite Co., Inc. New York

Manufacturer or distributor

Special equipment is needed. Special investment and sprueing are required. A precise, unalterable time schedule during preheating, injection and deflasking is mandatory. Investment is very hard, making deflasking difficult. In 3 hours a denture can be produced from the wax model.

liT ransinjection molding"

Conventional flasks and vulcanizer are needed.

Special equipment is needed. There is no trial closure. It is sometimes difficult to maintain water pressure during curing. Frequently molar teeth are depressed by hydraulic pressure during processing. In 3 hours denture can be produced from the wax model.

Specially prepared flasks and sprueing procedure are needed. Mold must be thoroughly dried in oven after wax removal. Denture resin is poured into preheated mold. Care must be taken to avoid trapped air. Investment is very strong, which makes deflasking difficult.

Conventional flasks are used. There is no trial closure. Technic is exceptionally simple.

Conventional flasks are used. Special tinfoil substitute is recommended. Care and speed are essential during trial closures before resin hardens.

Authors' comments on equipment and procedures

Vulcanization

Directional hydraulic compression (HydroCast technic!

Pouring (dry-low heat curel

Compression molding (no trial packing)

Compression molding (room temperature)

Processing method

z

0

:j

S;

0

0

V> V>

:>-

r-

>'

Z

m

0

z

:>-

'"0

m

;::

:>-

m

I

....

'T1

0

r-

:>-

Z

'"

c

0

t...

m

.... I

:;:

~

Injection-moist heat I"Pressure Cast" p rocessl

Compress ion molding

Compress ion molding

Injection- dry heat l"Unidirec tional Curing")

Compression molding

l'Electro-vacuum process" Ihigh temperatu re in vacuum)

Luxene, Inc. New York

American Consolida ted Manufactu ring Co. Philadelphia

American Consolida ted Manufactu ring Co. Philadelphia

Ticonium Div. of Consolida ted Metal Products Corp., Albany, N. Y.

Vernon-Benshoff Co. Pittsburgh, Po. Harold C. Emrick Laborator y Ft. Wayne, Ind.

no batch no.

no batch no.

A6A6

492

10-8-57

1-1-57

7-1-57

Copolyme rs of methyl and ethyl methacryl ate, uncross-Ii nked, gel

Acrylic resin, powderliquid, uncross-linked

Methyl methacryl ate, powder-li quid Ina catalyst in polymer or hydroquin one in monomer!

Vernonite

Vacalon

Acrylic resin powderliquid, cross-linked, containing 21% glass fibers by weight

Mystic 100

Tilon

Resin furnished and processed by manufactu rer July 1958 and August 1959

10-8-57

Acrylic resin powderliquid, cross-linked, containing 14% glass fibers by weight

Miracle 50

Processed by licensed commercial laborator y 1957 & 1958

Vinyl-acry lic copolyme r, gel

Luxene 44

Special equipment is needed. A specially prepared acrylic resin is recommended. There is no trial closure. IAII dentures were processed in manufacturer's laborator y.l

Conventio nal flasks and technic are used.

Special equipment is needed. Special tinfoil substitutes are recommended. Precise schedule is required. Investment is hard and bulky making def1asking difficult.

Conventio nal flasks are used. Powder and monomer are kneaded together in polyethyle ne bag. Packing consistency is stiff. Glass fibers make it difficult to trim off excess during trial packing. Glass fibers do not bond well to resin, making it diffi. cult to obtain highly polished smooth surface on the denture.

Special equipment is needed. Special sprueing technic, tinfoil substitute, investment and precise time schedule are necessary. DeAasking is difficult.

o

...

~

~ ....

'"

m

oco

-o o....

m

;::

r C

<

-<

Z

m m

m

~

f

(j)

'"

»

co

Z

m

» .,., .,.,

-0

'I

m

r .,.,

~

om

"

18/416 • T H E J O U R N A L O F T H E A M E R I C A N D E N T A L A S S O C IA T I O N

chan ges

o c c u r r in g

d e n tu r e s (s t o r e d

and in

in

in

b oth

th e

w a te r)

th e

c lin ic a l

t e c h n ic

d e n tu r e s

is c o n t in u in g .

du e to

w a t e r s o r p t io n

m ay

be

as h ig h

as 1 p e r c e n t .1

These

S evere

w a r p a g e o f a c r y lic re s in

den ­

re su lts w i l l b e r e p o r t e d w h e n th e d e n tu r e s

tu re s o n r e p e a t e d r e c u r in g 7 m a y h a v e le d

a r e a b o u t t w o y e a rs o ld .

t o th e p e rs is te n t b e l i e f t h a t a c r y lic re s in

D e n t is t s

have

b een

c r it ic a l

of

a v a il­

d e n tu r e s w a r p in a m o u n ts t h a t h a v e g r e a t

a b le o r g a n i c d e n t u r e b a s e m a te r ia ls s in c e

c lin ic a l s ig n ific a n c e . H o w e v e r , it h a s b e e n

th e

s h o w n 1,2' 4' 8 o n s e v e r a l h u n d r e d c lin ic a l

tu rn

o f th e

c e n tu r y .

H ard

ru b b er,

w i d e l y u s e d f o r o v e r 6 0 y e a rs , w a s c r i t i ­

and

c iz e d f o r its c o lo r , o p a c it y , p o r o s it y , in ­

c h a n g e s a r e u s u a lly f a r less t h a n

a b il i t y t o h o l d a p o lis h , a n d d e t e r io r a t io n

c e n t. S ix t e e n o f th e s e d e n tu r e s w e r e m e a s ­

w it h a g e . I t w a s a ls o d iff i c u l t t o p ro c e s s

u r e d a t in t e r v a ls d u r in g as l o n g a p e r io d

a n d r e p a ir . M a n y o r g a n ic d e n tu r e b a se

as s ix y e a r s .9 A c r o s s a 5 0 m m . m o la r - t o -

m a t e r ia ls

m o l a r s p a n , 1 p e r c e n t is o n ly 0 .5 m m .

w ere

t r ie d

as

s u b stitu tes

fo r

t e c h n ic

d e n tu r e s

th a t

th e

lin e a r 1 per

h a r d r u b b e r , c h ie f ly in a n e f f o r t t o i m ­

A

p r o v e e sth e tic s . N o n e o f th e se su b stitu tes,

t e r i o r s e c tio n

o f a p h e n o l- fo r m a ld e h y d e

re s in

h as b e e n

such

as

c e llu lo s e

fo r m a ld e h y d e v in y l

com pounds,

resin s, g l y p t a l

c o p o ly m e r s ,

A c r y lic

resin s,

p roved

fir s t

p h e n o l-

resin s

and

s a tis fa c to r y .

in t r o d u c e d

in

change

o f 0 .7 6 m m .

d e n tu re

o u t loss o f r e t e n t io n

acro s s th e

pos­

re p o rte d 2 w it h ­

o r c o m p la in t f r o m

t h e p a t ie n t .

th e

R a p i d tissu e c h a n g e s o f a m u c h la r g e r

U n i t e d S ta te s in 1937, s o o n d is p la c e d t h e

a m o u n t h a v e b e e n s h o w n b y c o m p a r is o n

o t h e r o r g a n ic d e n t u r e b a s e m a t e r ia ls t h e n

o f th e p r o file s o f a r t if ic ia l s to n e casts.10,11

in u se b e c a u s e o f a t t r a c t iv e a p p e a r a n c e ,

C a s ts w e r e p o u r e d in im p re s s io n s ( m a d e

a b il i t y t o r e t a in

a h ig h

p o lis h , a n d b e ­

o f f r e e f l o w i n g m a t e r ia ls )

o f u p p er and

c a u s e t h e p r o c e s s in g a n d r e p a i r i n g t e c h ­

lo w e r

(1 )

n ics w e r e s im p le . S in c e 1940, th e a c r y lic

f i t t i n g d e n tu r e s w e r e w o r n a n d

resin s h a v e d o m in a t e d th e o r g a n i c m a t e ­

e r a l d a y s a f t e r t h e i l l- f it t in g d e n tu r e s w e r e

r ia ls u s e d f o r d e n t u r e bases.

r e m o v e d . T h e c a s t p r o file s s h o w t h a t th e

L i n e a r c u r in g s h r in k a g e h a rd to

ru b b er ra n g ed

fro m

re p o rted

fo r

e d e n tu lo u s

o r a l tissu es c a n

arch es:

a f t e r ill(2 )

c h a n g e r a p id ly

sev­

and

in

0.3

p er cen t

s o m e in s ta n c e s as m u c h as 3 m m . S u c h

0 .5 p e r c e n t d e p e n d in g o n

th e ty p e

c h a n g e s a r e m a n y tim e s th a t r e p o r t e d f o r

o f r u b b e r a n d th e th ic k n e s s o f th e f la t

a c r y lic r e s in d e n tu re s .

r e c t a n g u la r s p e c im e n s . T h e s h r in k a g e o f s im ila r s p e c im e n s o f a c r y lic re s in r a n g e d

M ATER IALS AND M ETHODS

f r o m 0 .4 t o 0 .6 p e r c e n t .1 O t h e r r e p o r t s 2"6 g i v e s o m e w h a t d i f f e r e n t v a lu e s . T h e s m a ll

M a t e r ia ls a n d P r o c e s s in g M e t h o d s

d iffe r e n c e s

v a lu e s

t r a d e b r a n d o f m a t e r ia l, its g e n e r a l t y p e ,

a r e c a u s e d b y v a r i a t i o n in t h e size , s h a p e ,

a n d t h e p r o c e s s in g m e t h o d u s e d w i t h i t

a n d t r e a t m e n t o f th e s p e c im e n s a n d th e

are

c o m p o s it io n

ty p e s

p lo y e d .

am ong

of

th e

th e

rep o rted

v a r io u s

T h e s e d iffe r e n c e s

are

resin s

em ­

a c a d e m ic

show n of

liq u id

in

T a b le

a c r y lic

(b o t h

re sin s

1.

• The

The

d if f e r e n t

w ere:

p ow d er-

u n c r o s s -lin k e d

and

cro ss-

a n d a r e n o t la r g e e n o u g h to d e te c t c lin i­

lin k e d ),

c a lly .

fib e rs , p o w d e r - liq u id w it h o u t c a ta ly s t o r

A s p o i n t e d o u t b y s o m e in v e s tig a to r s ,

p o w d e r - liq u id

c o n t a in in g

gla ss

h y d r o q u in o n e , p o w d e r - liq u id s e lf- c u r in g ,

t h e e x p a n s io n o f h e a t - c u r in g a c r y lic resin s

and

c a u s e d b y w a t e r s o r p t io n c o m p e n s a te s in

a c r y lic

p a rt

re s in , a v in y l- a c r y lic c o p o ly m e r , a n d h a r d

fo r

th e

c u r in g

s h r in k a g e .

W h en

th e

gel

fo r m .

resin s,

a

In

a d d it io n

p o ly s t y r e n e ,

an

to

th e

epoxy

a c r y lic r e s in d e n tu r e s a r e c o n d it io n e d in

r u b b e r w e r e u s e d . T h e m a j o r p r o c e s s in g

a i r a t 3 2 p e r c e n t r e l a t i v e h u m id it y b e ­

m e th o d s

e m p lo y e d

f o r e im m e r s io n i n

m o ld in g

(w ith

w a t e r , th e e x p a n s io n

w ere:

c o m p r e s s io n

a n d w it h o u t t r i a l p a c k -

W O E L F E L — P A F F E N B A R G E R — S W E E N E Y . . . V O L U M E 61, O C T O B E R I960 • 19/417

Fig. 1 • A b o v e left: Flask and oven recom m ended for processing Epoxolon dentures A b o v e center: Equipm ent used in processing H y d ro -C a st dentures. A djustab le therm ostats (T) regulate tem perature of heating elements ( H ) . H yd rau lic ram ( R ) fo r closing flasks (F) is controlled by oil pum p and ga u g e assembly. W a te r under known pressure is supplied by w ater pum p and g a u g e assem bly ( W ) through large center hose to w ater m anifold ( M ) . H yd raulic force is applied through flasking gypsum of upper half of flasks and is directed only against tongue side of denture and a dia ph ragm o f resin extending from borde r of the denture to the e dge of the flasks (Fig. 2, above right). The two small side hoses entering the water m anifold ( M ) are used to circulate cold water fo r rapidly cooling of the cured dentures. Tim ing device is shown at (D ) A b o v e right: Injection apparatus and flask used in m aking Jectron dentures. Injection cylinder (1) contains preheated rod of Jectron. The flask assem bly (F) is seated on platform (P) so that the screw ram (R ) fits into end of injection cylinder ( I). A n electric m otor ( M ) drives ram down, fo rcing the preheated Jectron into the mold in the preheated flask (F) Below left: Cross-sectional view of equipm ent used in processing Luxene 44 dentures. C om p re ssed air fe d into cylinder ( C ) drives piston (P), which forces resin into mold ( M ) Below center: Battery o f three units used in processing Tilon dentures. C o m p re ssed air is fed into injector (I), fo rcin g Tilon into the m old in the flask ( F ) . Flask is heated by the platen (P) Below right: Equipm ent fo r processing Vacalon dentures. Flasks in press assem bly (F) and oven ( O ) with press asssem bly in muffle. R educed pressure is obtained in oven ( O ) with vacuum pum p ( P )

in g ),

d i r e c t io n a l h y d r a u lic

m o ld in g ,

in je c tio n

c o m p r e s s io n

m o ld in g

w it h

th e

c o m m e n t s in th e la s t c o lu m n o f T a b l e 1 are

based

on

th e

e x p e r ie n c e s

of

th e

flasks b e i n g h e a t e d in w a t e r o r b y p la te n s ,

a u th o rs . C o m p a r is o n s a r e m a d e w i t h t h e

t r a n s in je c t io n m o ld in g , h ig h te m p e r a t u r e -

c o n v e n t io n a l e q u ip m e n t a n d p r o c e d u r e s

vacuum

c u r e , f i l l i n g th e m o l d b y p o u r ­

in g

c o m b in e d

d ry

h eat

cu re,

w it h and

a

lo w

te m p e r a tu re -

v u lc a n iz a t io n .

The

n e e d e d t o fla s k , p ro c e s s , a n d fin is h c o m ­ p r e s s io n - m o ld e d a c r y lic r e s in d e n tu re s . F ig u r e

1, a b o v e le ft , s h o w s t h e o v e n

2 0/418 • THE J O U R N A L O F THE A M E R IC A N DENTAL A S S O C IA T IO N

Fig. 2 • A b o v e left: W a x d e n tu re in low er h alf o f m o d ifie d flask sh o w in g p o u r in g sp ru e a n d riser fo r v e n tin g m old in E p o x o lo n t e c h n ic A b o v e righ t: P artially d e fla sk e d H y d r o - C a s t d e n tu re fo rc e t o the t o n g u e sid e o f d e n tu re d u r in g c u r in g

sh o w in g

Below left: L o w e r h alv e s o f J e c t ro n sm all ve n ts

wax m o d e l d e n tu re s with

flasks c o n t a in in g

d ia p h r a g m

th a t

co n fin e s la rg e

h y d ra u lic

sp ru e s

and

Below righ t: W a x d e n tu re s a n d Y - s h a p e d sp ru e u se d in som e in jection te ch n ics

and hinged flask recommended for use with Epoxolon. The authors modified conventional flasks as suggested in the directions of the manufacturer. The lower half of one of these flasks with the wax denture and pouring sprue and riser is shown in Figure 2, above left. The epoxy resin was cured at from 43 to 52 °C. (110 to 125°F.) according to the manu­ facturer’ s instructions. The instrument for processing HydroCast dentures (Fig. 1, above center) uses hydraulic force on the tongue side of the dentures during curing. This hy­

draulic force is exerted through the gyp­ sum in the upper half of the flask and is confined to the tongue side of the den­ ture by the diaphragm that extends from the border o f the denture to the outside edge o f the flask (Fig. 2, above right). The platens (H ) were heated to 93°C. (200°F.) prior to insertion of the flasks, and were kept at this temperature for one hour. The flasks were then cooled in five minutes by circulating tap water through the manifold ( M ). Jectron dentures were made in the in­ jection apparatus and flask shown in

WOELFEL-PAFFENBARGER-SWEENEY . . . VOLUME 61, OCTOBER I960 • 21/419

Figure 1, above right. A precured rod o f Jectron is heated to 210°G. (410 °F.) in the injection cylinder ( I ) which is screwed into the preheated flask ( F ) . This flask assembly is bolted to the plat­ form ( P ) . T h e screw ram ( R ) engages in the top o f the charged injection cylin­ der ( I ) . T h e motor ( M ) drives the ram forcing the softened Jectron into the mold encased by the flask ( F ) . Th e lower halves o f Jectron flasks containing the wax model dentures with the ingress sprues and vents are illustrated in Figure 2, below left. A cross-sectional view o f the equip­ ment used in processing Luxene 44 den­ tures (Fig. 1, below le ft) explains this injection method. Compressed air is fed into the air gun cylinder ( C ) which drives the piston (P ) forcing Luxene 44 (gel form ) into the mold ( M ) . A single sprue was used for the upper denture and a three-way sprue for the lower denture. Th e flasks were at 96°C. (205 °F .) for 1J4 hours and were then bench cooled. Figure 1, below center, shows a bat­ tery o f three T ilon processing units. Com ­ pressed air is fed into the injector ( I ) forcing T ilo n (gel form ) into the mold within the flasks ( F ) . T h e components o f the flask are made o f alloys with d if­ ferent heat conductances so as to initiate the polymerization on the tissue side o f the denture. H eat is applied only to the bottom side o f the flask through the platen ( P ) which was preheated to 110°C. (2 3 0 °F .). Th e flasks were held at this temperature fo r at least 40 min­ utes and then bench cooled. A Y-shaped sprue was used fo r both upper and lower dentures except on the thick lower dentures where a four-way sprue was attached. A n example o f a Y-shaped sprue on an upper denture is shown in Figure 2, below right. Four o f the Vacalon technic dentures were processed in the manufacturer’s laboratory under reduced pressure using the equipment shown in Figure 1, below right. T h e flask assembly ( F ) was placed

in the muffle o f the oven ( O ) which was set at 66°C . (150 °F .) for one hour. T h e oven temperature was gradually raised to 193°C. (3 8 0 °F .) in 2/% hours and was held at 193°C. fo r one hour. D ur­ ing the heating and during cooling to 93°C. (200 °F .) a partial vacuum (737 mm. o f mercury) was maintained in the mufHe o f the oven by vacuum pump (P in Figure 1, below righ t). Approximately a year later, the manufacturer processed a set o f four technic dentures o f Vacalon in the dental research laboratories at the National Bureau of Standards. H e placed the flask assembly in the oven at 71 °C . (1 6 0 °F .) fo r 1*4 hours. Th e oven temperature was gradually raised to 138°C. (280 °F .) in a half hour and was held there for three quarters o f an hour. A partial vacuum (660 mm. o f mercury) was maintained. T h e other denture base materials were compression molded with conventional flasks in accordance with the manufac­ turers’ instructions. T es t D en tu r es • Four technic complete

dentures were made o f each material listed in T a b le 1. One set was thin; the other thick in cross section (Fig. 3 ). T h e average volume o f the base o f the dentures after polishing was 11 ml. for the thin uppers; 11 ml. for the thin lowers; 20 ml. fo r the thick uppers, and 25 ml. fo r the thick lowers. Th e sets o f thin dentures were nearly identical in shape, as were the sets o f the thick den­ tures. Trubyte Bioform combination technic teeth-mold 42F (w ith Pilkington-Tum er posterior teeth) were used for all the technic dentures. In addition to these technic dentures, eight clinical den­ tures (five complete upper and three complete lower dentures) were made o f each material (T a b le 1) with the fo l­ lowing exceptions: two dentures were made o f M iracle 50, Mystic 100, and hard rubber; no clinical dentures were made o f Vacalon. Vacuum-fired porce­ lain anterior and 33 degree posterior

22/420 • THE JOURNAL OF THE AM ERICAN DENTAL ASSOCIATION

teeth were used for all clinical dentures. The dentures were processed in the dental research laboratory at the National Bureau of Standards with the following exceptions : the Luxene 44 and hard rub­ ber dentures were processed by Rothstein Dental Laboratories, In c.; the Tilon dentures by the manufacturer in his labo­ ratory and by the investigators using equipment in the Naval Dental School, Bethesda, M d .; and the Vacalon den­ tures by the manufacturer in his labora­ tory and at the National Bureau of Standards. The same batches of denture bases, gypsum, and tinfoil substitutes were used for both the technic and clinical den­ tures. With few exceptions the clinical dentures were made for patients who are on the permanent staff of the National Bureau of Standards. With the exception of patients with very recent extractions or those needing immediate dentures,

Fig. 4 • D e v ic e u se d in m e a su rin g v e rtic a l d is ­ c re p a n c ie s in o c c lu sa l re la tio n s th a t o c c u rre d d u r in g p ro c e ss in g . P is in cisa l g u id e p in ; G , d ia l g a u g e ; B, b a se with s to p s ( S ) fo r a c c u ra te r e p o s itio n in g o f a rtic u la to r

none was refused. Thus, seemingly easy and difficult conditions were accepted without selection as the patients came. In all but two instances the patients had worn dentures previously. Impression and Jaw Relation Technics • The following impression technic was used: an individual preformed tray of self-curing poly(methyl methacrylate) was m ade; border molding and postdam were accomplished with “ green” model­ ing com pound; and the final impression was a corrective wash of zinc oxideeugenol paste. Face-bow transfer and beeswax (centric and protrusive) interocclusal records were used in mounting the gypsum casts on a Hanau M odel H articulator. All dentures were remounted on the articulator and corrections were made from a new centric interocclusal record taken at the time of insertion. VERTICAL DISCREPANCIES IN OCCLUSAL RELATIONS

c.

,

~

,.

x t n L - j i .

r ig . 3 • C r o s s se c tio n s ot sets o f te c h n ic d e n tu re s m a d e d istal to first m o la rs

M ethod . of' Measurement • Metal split r remounting plates (Hanau) were used

WOELFEL—PAFFENBARGER—SWEENEY . . . VOLUME 61, OCTOBER i960 • 23/421

T a b le 2 • Increase in ve rtic a l occlusal relations Sin g le clinicai d e n tu re sf

C o m p le te u p p e r and lo w e r d e n tures* M a te r ia l

Num ber of sets o f dentures

A v e r a g e v e rtica l in cre a se in height o f Incisal g u id e pin

Num ber of dentures

mm.

mm. H y d r o -C a s t E p o x o lo n lu x e n e 44 H a rd rubber Jectron D u ra flo w V e rn on ite p o w d e r-liq u id A cra lite 88 M ir a c le 50 & M y stic 100 THon V a c a lo n

4 5 5 5 1 6 8 5 4 6 7 4

O.OOt 0.33 0.33 0.62 0.65 0.65 0.68 0.68 0.75 1.10 1.29 1.49

A v e r a g e vertical in crease in height o f incisal g u id e pin

1 1 2 2 0 2 2 2 3 0 0 0

o.oof 0.10 0.33 0.36 — 0.28 0.41 0.41 0.65 — — —

*ln clude d both technic and clinical dentures. fVafues fo r a sin gle denture will, o f co u rse , be less than for a set of dentures, so d ata in the two columns g iv in g the a verage increase are not directly related. JThere was an interocclusal space between the upper and tower m olar teeth o f from % to f/2 mm. Differences in the rate o f a p p ly in g the hydraulic force, differences in the proportionate am ount of the surface of the teeth in contact with the wax and with the gypsum , and differences in the amount of resin between the surface of the cast and the rid g e la p of the teeth a ll have their effect and m ay account for the space between the m olar teeth in these H y d ro -C ast dentures.

in form ing all gypsum casts. Then the gypsum casts could always be repositioned accurately on the articulator. T h e artic­ ulator, with the wax dentures mounted in the desired relationship, was positioned by stops on a heavy metal base to which was fastened a rigid upright arm holding a dial gauge (Fig. 4 ). T h e reliability o f the method was tested by removing the articulator from the device, followed by removing and replacing the casts on the articulator and then by repositioning the articulator in the device. T h e discrepancy in the prior and subsequent readings was never greater than ± 0.04 mm. T h e initial reading on the dial gauge (G in Figure 4 ) o f the vertical position of the incisal guide pin ( P ) was taken with the completely form ed w ax dentures on the articulator immediately prior to flasking. T h e next reading was taken with the processed dentures (unremoved from their respective casts) replaced on their mounting and with the articulator accurately repositioned on the base ( B ) . T h e difference between these two read­

ings showed the discrepancy in the oc­ clusal relationship caused by processing. A ll readings were made with the casts in centric relation with each other. (O n the Hanau M odel H articulator this would be represented by the balls in the condyle slots being forward.) T h e data are shown in Table 2. C h a n g es in V er tica l O cclu sa l R ela tion s • Four sets and a single denture o f HydroCast showed no vertical increase in the height o f the incisal guide pin (P in Fig­ ure 4) (T a b le 2) but did show a de­ pression o f the molar teeth. A ll o f the bicuspids, because o f their shape and size, were not similarly depressed. Otherwise there would have been a decrease in the height o f the pin since the anterior teeth were not set in contact in centric occlu­ sion. This retraction o f the molar teeth toward the casts is caused by the hy­ draulic pressure o f 400 to 600 psi against their occlusal surfaces. A n error of this type is more difficult to correct than an error o f similar size caused by interceptive

24/422 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

T ab le 3 • Measurement schedule for dentures* M o la r - to -m o la r m easurem ents on ly W a x denture in lo w e r h a lf o f flask W a x d enture flasked to o c clu sa l a n d incisal su rfa ce s o f teeth P rocessed resin denture with investment re m o v e d to o c c lu sa l a n d incisal surfaces o f teeth Resin denture on ca st with flasking investment re m o v e d f M o la r - t o -m o la r a n d fla n g e -to -fla n g e m easurem ents D en ture re m o v e d from ca st a n d immersed in w a te r fo r Z% hou r Polished a n d sto re d fo r 24 hours in w a t e r j * A II measurements and water storage were at 22 ± l° C . (72 ± 2°F.). fin 66 dentures reference pins were inserted in the border of the buccal flan ges before the cured denture was rem oved from the gypsum cast. This was done b y cutting away a sm all section o f the cast overlying the border of the flan ge a b ove the second molar. ^Periodic measurements are b ein g continued after stora ge in water (technic dentures) and after use (clin ical dentures). These d ata will be presented in another report.

molar or bicuspid contact. Five sets and a single denture o f Hydro-Cast had small increases (an average of 0.33 mm. and 0.10 mm., respectively) in the pin height (T a b le 2 ). It would appear that the flask design, the preformed flash or diaphragm, and the hydraulic pressure used in the Hydro-Cast technic minimize the increase in the vertical occlusal relations during processing. Epoxolon dentures had a similar small increase (0.33 m m .). This is understand­ able since the flasks are fastened together when the thin fluid is poured into the mold. L ittle or no force is exerted against the resin and the curing temperature (43 to 52°C. [110 to 125°F.]) is very low. Th ere is no significant difference (0.65 to 0.68 m m .) among the next five mate­ rials (Luxene 44, hard rubber, Jectron, Duraflow, and V ern on ite). Acralite 88 (a self-curing acrylic resin) showed a slightly larger vertical opening o f the incisal guide pin (0.75 m m .). T h e slight difference between this value and those o f the previous five materials is probably due to a slight hardening o f the resin before the final closure o f the flask could be accomplished. T h e resins containing glass fibers (M ir ­ acle 50 and Mystic 100) averaged slightly over 1 mm. pin opening for six seta of

dentures. These resins are somewhat stiff at packing consistency, and the glass fibers make it difficult to trim o ff the flash perfectly while trial packing. Th e T ilo n dentures averaged 1.29 mm. for seven sets o f dentures. Some o f this change can be attributed to movement o f the biscuspids adjacent to the sprue attachment. T h e four sets o f Vacalon technic dentures with an average increase in pin opening (1.49 m m .) were proc­ essed by the manufacturer. T h e data in Table 2 show that the acrylic resins processed with conventional compression m olding reproduce occlusal relationships as satisfactorily as special resins and special processing technics. T h e metal-to-metal contact o f the halves o f the flask does not in itself pre­ vent an increase in height o f the incisal guide pin. A ny force that distorts the investment or causes the movement of teeth into it increases the height. Such forces may be produced by packing, in­ jecting, polymerizing, and heating meth­ ods. T h e average increase in the height of the incisal guide pin fo r 31 sets o f technic dentures (in which Pilkington-Tum er posterior teeth were used) was 0.89 mm. with a standard deviation about the mean o f 0.61 mm. In contrast the avei^

WOELFEL— PAFFENBARGER—SWEENEY . . . VOLUME 61, OCTOBER I960 • 25/423

age increase for 28 sets o f clinical den­ tures (using 33 degree posterior teeth) was 0.61 mm. with a standard deviation about the mean o f 0.46 mm. T h e greater value for the technic dentures is probably due to the sharper and more definite anatomy of the Pilkington-Turner pos­ terior teeth which were selected to make the conditions in the technic dentures as critical as possible.

M01AR-T0-H0LARDISTANCE45? LOW ERDENTURE(CH)

MONTHS IN CLINICAL SERVICE C HAN GES IN M O L A R -T O -M O L A R

Fig. 6

Effect o f m e a su re m e n t e rro r on d a ta

D IM E N S IO N

M eth od, o f M ea s u r e m e n t • Linear meas­ urements on each denture (molar-tom olar) were made at various times (T a b le 3) at 22 ± 1°C. (72 ± 2 °F .) with a toolmaker’s microscope and were recorded to the nearest 0.0025 mm. (0.0001 in c h ). Th e reference marks were fine cross lines ruled on polished stainless steel pins cemented in the second molars as shown in Figure 5. Posterior linear measurements instead o f contour meas­ urements were made, as a previous study8 had shown that linear changes occurring across the posterior portion of a denture are o f significantly greater magnitude than in any other area. Also the changes occurring in the posterior region are the

Fig. 5 • D raw ing of low er denture showing lo c a ­ tion o f stainless steel reference pins in molars and in b uccal flan g e borders. Molar-to-molar and flange-to-flange distances here given v a ry with individual denture

most important in the retention o f den­ tures because o f the anatomy of the mouth and the shape o f the dentures. Finally, contour measurements with the available instrument,12 or with modifica­ tions thereof,13,14 are much less accurate than linear measurements. T h e necessity o f having the measure­ ments as precise and as accurate as pos­ sible is shown in Figure 6. W ith an error o f ± 0.13 mm., recorded data would fall on either one o f the outer dotted lines. M any o f the linear changes occurring in a denture are smaller than this error. Errors in measurement o f such magni­ tude would seriously affect comparisons among materials and technics. W ith a plus error for one denture and a negative error for another, the true picture could be obliterated. H owever, a skilful ob­ server using a calibrated microscope and good reference lines can hold the error in measurement to within ± 0.0025 mm. Differences among various observers may increase this error two to three times. W ith an error o f ± 0.0075 mm., recorded data would fall in the solid line (Fig. 6 ). Because the denture base resins (w ith ­ out filler) have a high coefficient o f thermal expansion [(7 5 ± 15) X 1 0 '6 per °C .] it is necessary to have good tempera­ ture control. As stated in Table 3, meas­ urements were made at 22 ± 1°C. Even this latitude allows an error o f ± 0.008 per cent (0.004 m m .) on a 50 mm. molar-to-molar span.

26/424 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

0 a: —O.l -

z£-0.3

-

o 5- 0.6 — 1-0.7 -

-0.8

I-----1 -

. . . . . .

Fig. 7 • Relationship o f bulk and shape of denture to ave ra g e molar-to-molar shrinkage during p roc­ essing. Fid ucial measurement was made on wax denture flasked to occlusal surfaces o f the teeth and final m easurement was m ade a fte r polished denture had been stored in w a ter fo r one d ay

T h e high coefficient o f thermal ex­ pansion o f these denture resins must be considered in interpreting the data ob­ tained at 22 ± 1°C. (72 ± 2 °F .). I f the dentures were heated to mouth tem­ perature (37°G. [98.6°F.]) an expected approximate increase of 0.12 per cent should be added to all o f the measure­ ments made at room temperature. T h e experimental design permitted the accurate measurement o f linear changes in both technic and clinical dentures dur­ ing processing and subsequently, using a method employed formerly.1’ 8’ 15 In ad­ dition, the effects o f the size and shape o f the dentures on the dimensional changes during processing, on storage in water, and during clinical service could be determined. Then, too, the precise difference in the behavior of technic dentures stored in water and o f clinical dentures in service could be learned. T h e detailed report o f these observations will be given in a subsequent paper. M o la r -to -M o la r D im en sion a l C h an ges •

T h e relationship of the bulk and shape o f a denture to average molar-to-molar shrinkage is shown in Figure 7. T h e thick upper and lower technic dentures had the same general pattern o f dimensional change as the thin upper and lower tech­ nic dentures, but the average change for the thick dentures was smaller (Fig. 7 ).

T h e four bars in Figure 7, representing data on technic dentures, show the aver­ age molar-to-molar change on 12 den­ tures o f identical shape (Fig. 3 ), each made o f a different organic denture base material (T a b le 1). Th e thin upper technic dentures shrank 2.1 times as much on the average as the thick upper ones; the thin lower technic dentures, over 2.2 times as much on the average as the thick lower ones (Fig. 7 ). These findings were contrary to expectations as the dentures containing the most resin should have the greatest volumetric shrinkage on polymerization. W hy then did the bulky dentures show less molar-to-molar change than the thin dentures? Probably the dentures with the thicker cross section were stiff enough to prevent release o f some o f the strain when the cured dentures were removed from the casts. Th e average shrinkage o f the lower technic dentures was 1.5 times as much as the average shrinkage o f the upper technic dentures. This re­ lationship is probably due to gross differ­ ence in the shape o f upper and lower dentures. T h e average molar-to-molar changes o f the upper and lower clinical dentures fall between those o f the thick and thin technic dentures (Fig. 7 ). This general relationship has continued after the clini­ cal dentures have been in service fo r 1/> years and the technic dentures have been immersed in water for 1J/2 years. I f this pattern continues for several years, the construction and measurement o f a tech­ nic denture in the laboratory made from the materials listed in Table 1 would probably constitute a valid test fo r di­ mensional changes o f denture base resin in service. Figure 8 shows graphically the average molar-to-molar changes after flasking, curing, removal from the cast and polish­ ing, for the four technic dentures o f each material (Fig. 3 ). T h e changes after flasking and curing are small as compared with the changes occurring when the den­

WOELFEL— PAFFENBARGER—SWEENEY . . . VOLUME 61, OCTOBER I960 • 27/425

tures were removed from their gypsum casts. T h e comparatively large shrink­ ages in the dentures, caused by their removal from the gypsum casts, include the shrinkages which always occurred on removing the sprues on Luxene 44 and Tilo n dentures (Fig. 2, below right) and the diaphragms on Hydro-Cast dentures (Fig. 2, above righ t). T h e additional change after polishing and storage in water fo r 24 hours was usually small and probably indicates the release o f addi­ tional strain.16 Th e amount o f shrinkage that occurred when the dentures were removed from

their casts was close to 0.2 per cent for Acralite 88 and Epoxolon, and between 0.3 and 0.5 per cent for the other ma­ terials. This shrinkage was probably caused by the release o f elastic strain when the denture was removed from the cast. T h e strain was probably placed in the denture by the greater contraction o f the resin than o f the gypsum mold during cooling. T h e two resins showing the least change, Acralite 88 and Epoxolon, were processed at 22°C. (7 2 °F .), and 45°C. (1 1 3 °F .), respectively, whereas the other materials had to be cured at much higher temperatures. Th e increase in curing

+ 0.2

° —0.2 -

-0.3 -

- 1-0.4

-

EPOXOLON HARDRUBBER ACRALITE 88 VERNONITE P-L DURAFL0W HYDRO-CAST LUXENE 4 4 JECTR0N TILON MIRACLE 50 VACAL0N MYSTIC100

-0.6 -

-0.7 — WAX DENTURE I---WAX DENTURE -CURED DENTURE -CURED DENTURE •-CURED DENTURE UNFLASKED TO REMOVED FROM FLASKED TO AFTER POLISHING ON GYPSUM CAST CAST PLUS ONE DAY OCCLUSALSURFACES OCCLUSALSURFACES IN WATER F ig . 8 • A v e r a g e m o la r -t o -m o la r a n d o n e thin , u p p e r a n d lo w e r)

changes

on

fo u r

t e c h n ic

d en tu re s

d u r in g

p r o c e s s in g

(o n e

th ic k

28/426 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION

AVERA6ESH RINKAGEINPERCENT

>.02 -0.23 -0.

H ARDRU BBER

VERN ONITEPOW DEK-UOW O

HTDBO-CAST

F ig. 9 • M o la r -t o -m o la r c h a n g e on fo u r te c h n ic d e n tu re s d u r in g p ro c e ssin g . (T o ta l c h a n g e fro m wax d e n tu re t o p o lish e d d e n tu re sto re d f o r on e d a y in w ater)

temperatures caused by exothermic heat given o ff during polymerization was not measured. Th e high temperature and pressure during vulcanization weaken the gypsum, and it is possible that in this state the gypsum is not able to exert a strong restraining force on the hard rub­ ber during cooling. This may account for the low value o f approximately 0.3 per cent, when the hard rubber dentures were removed from the casts. T h e molar-to-molar changes that oc­ curred while fabricating each type and shape o f technic denture o f each base material (T a b le 1) are shown in Figure 9 together with a bar chart giving aver­ age changes. T h e Vacalon dentures cured in the manufacturer’s laboratory at a maximum temperature o f 193°C. (3 8 0 °F .) had an average shrinkage of 0.56 per cent. Th e four Vacalon den­ tures processed by the manufacturer in the dental research laboratory at the National Bureau of Standards had a lower maximum processing temperature ( 138°C, [280°F.]) and consequently had a lower average shrinkage (0.42 per cent).

T h e differences in the average molar-tomolar shrinkages among the nine resins having the highest shrinkages processed at comparatively high temperatures (71° to 210°G. [160° to 410°F.]) are small, barely over 0.1 per cent (0.05 mm. on a 50 mm. molar-to-molar sp a n ). D if­ ferences of this order are largely aca­ demic and certainly have no practical significance. Thus the choice o f a resin or a processing method among these nine would have to be made on some other basis than dimensional accuracy during processing. In fact, even the greatest differences in shrinkage among the clini­ cal dentures made o f the materials shown in Figures 8 and 9 could not be detected clinically by either the patient or the dentist at the time o f insertion. Figures 8 and 9 show that Epoxolon dentures have a very low shrinkage on polymerization. Unfortunately this low shrinkage on curing is offset by poor color stability, a tendency to collect stain rapidly, and a high expansion in water (technic dentures) and in service (clin i­ cal dentures). T h e expansion o f the clinical dentures is so great that the por­ celain teeth are loosening and many re­ placements have been made in the first 18 months o f service. Dimensional changes in service w ill be reported in a subsequent paper. D im en sion a l C h an ges S h ow n by C rossS e ctio n in g U p p er D en tu r es in th e Flash­ ing G yp su m • Gypsum molds containing

processed technic upper dentures were removed intact from the metal flasks and cross-sectioned. Inspection o f these cross sections showed how well the different materials and technics filled the molds and how well the processed dentures fit­ ted the gypsum casts prior to deflasking. T h e best adaptations o f the thin and thick dentures to the gypsum casts were seen in the cross sections of Epoxolon, Hydro-Cast, Jectron, Luxene 44, T ilo n and Vacalon dentures. Th e others had poorer adaptation than the foregoing but

W O E L F E L - P A F F E N B A R G E R - S W E E N E Y . . . V O L U M E 61, O C T O B E R I960 • 29/427

Fig. 10 • C r o s s se ctio n o f p ro c e ss e d th ic k u p p e r d e n tu re s illu stra tin g g o o d ( A ) a n d p o o r { B ) a d a p ta t io n t o g y p s u m casts

the differences were small as shown in Figure 10. T h e use o f hydraulic force directed toward the tongue-cheek side o f the den­ ture during polymerization does force the resin unidirectionally as shown by the cross section o f the thin upper technic denture ( A in Figure 11). I f the hy­ draulic force is applied on the tissue-

borne side o f the denture, then the resin is not adapted to the cast (B in Figure 11). O f course, this is not done in prac­ tice but is included here as additional evidence o f the effect o f the unidirec­ tional hydraulic force. H owever, when the conventional compression molding technic was used (G in Figure 11), the adaptation to the cast was just as good as

Fig. I I • C r o s s se ctio n o f thin u p p e r t e c h n ic d e n tu re p ro c e sse d : ( A ) w ith h y d ra u lic fo rc e (F ) d ire c te d to w a rd t o n g u e -c h e e k sid e o f d enture, ( B ) with the h y d ra u lic fo rc e (F ) d ire c te d to w a rd t iss u e -b e a rin g sid e o f den tu re, ( C ) c o n v e n tio n a l c o m p re ssio n m o ld in g . A , p ro c e ss e d b y m a n u fa c tu re r; B, reverse d ire c tio n o f h y d ra u lic fo rc e ; C , c o m p re s s io n m o ld e d in H y d r o - C a s t flask

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Fig. 12 • R e p la c e m e n t o f c lin ica l u p p e r d e n tu re on g y p s u m c a st o n w h ich it w as cu red . M o la r -to m o la r a n d fla n g e -t o -fla n g e sh rin k a g e o f this d e n ­ tu re w as 0.4 p e r ce n t on rem oval fro m this g y p s u m cast. W h e n d e n tu re w as re p la c e d on c a st lightly, th e a d a p t a t io n w as as show n in A . A d a p t a ­ tio n w as so m e w h a t b e tte r a n d m o la r-to -m o la r a n d fla n g e m e a su re m e n ts w ere stre tch e d (0 .4 p e r c e n t) t o t h e ir fo rm e r d im e n sio n s w hen d e n tu re w as fo rc e d on c a s t as sho w n in B, b u t d is c r e p a n ­ cie s in th e fit are still a p p a re n t

when the hydraulic force was used ( A in Figure 11). In fact, there seems to be no advantage in using the hydraulic force as far as the fit o f the denture to the gypsum cast is concerned either before or after removal from the cast. There is little or no relationship be­ tween the adaptations o f the dentures to the gypsum casts as seen in the cross sections and the molar-to-molar shrink­ ages that occur when the dentures are deflasked and removed from the casts (Fig. 8 ). These shrinkages prevent the dentures from seating properly on the gypsum casts on which they were cured. D IS T O R T IO N

OF DENTURES

T h e relative deformation o f the poste­ rior section o f an artificial denture can be

judged by the molar-to-molar and flangeto-flange shrinkages. As stated before, a large percentage o f the linear shrinkage, from the w ax denture to the polished denture, occurs when the denture is re­ moved from the gypsum cast on which it was cured. When the denture is re­ placed lightly on the cast it w ill not fit ( A in Figure 12). In this instance both molar-to-molar and flange-to-flange shrinkages were 0.40 per cent when the denture was removed from the cast. Even when the denture was forced back onto the cast it would not fit (B in Figure 12) exactly. These discrepancies appear large when a hard denture is seated on a hard cast. But in the mouth such a change had no apparent effect on the fit o f the denture. This denture was comfortable when inserted and has re­ mained so fo r one year (August 195859). M ore distortion should occur if there were large differences between the molarto-molar and flange-to-flange shrinkages. Generally the flange-to-flange shrinkage in upper dentures was greater proportion­ ally than the molar-to-molar shrinkage when the denture was removed from the cast. T h e reverse was true in the lower dentures where the molar-to-molar shrinkages were generally proportionally greater than the flange-to-flange shrink­ ages. Th e differences were small, amount­ ing to an average o f 0.12 per cent with a standard deviation about the mean o f 0.10 per cent on 66 dentures. SUM M ARY

T h e relative accuracy o f the fit o f 186 complete dentures (73 clinical and 113 technic) was determined: (1 ) by com­ paring the position o f the incisal guide pin o f a M odel H Hanau articulator be­ fore and after curing the dentures; (2 ) by measuring the molar-to-molar shrink­ age from the wax model denture to the polished denture; (3 ) by examining cross sections o f cured dentures surrounded by

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the flasking gypsum; (4 ) by noting the fit o f polished dentures on the casts on which they were cured and by correlating the fit with molar-to-molar and flangeto-flange shrinkages; and (5 ) by observ­ ing the relative fit o f the dentures in the mouth. These dentures were made with eight different types o f acrylic resins, a poly­ styrene, a vinyl-acrylic copolymer, an epoxy resin, and hard rubber (T a b le 1). These 12 different organic denture base materials were processed with the equip­ ment and the method recommended by the manufacturer. Four technic complete dentures (thick and thin uppers and low­ ers) and usually eight complete clinical dentures were made o f each material. T h e changes in the occlusal relation­ ships, as evidenced by the increase in the height o f the incisal guide pin before and after processing, ran from 0.00 to 1.49 mm. T h e least average change in height o f the pin (0.00 to 0.33 mm .) was on the dentures made of an acrylic resin proc­ essed by directional hydraulic compres­ sion molding or o f an epoxy resin where the resin was poured into the mold. T h e highest average change (1.49 m m .) was on dentures made with an acrylic resin processed with a high temperaturevacuum cure. T h e severity o f the changes o f the other materials and methods fell in between the foregoing. In some in­ stances there was an interocclusal space between the upper and lower teeth o f from 0.25 to 0.5 mm. in the dentures of acrylic resin processed by the directional hydraulic compression molding. T h e data in Table 2 show that special resins and special methods of processing them do not make fo r significantly greater accuracy o f reproducing occlusal relationships than do the acrylic resins with conventional compression molding. Molar-to-m olar changes across the posterior section o f the dentures were measured on a toolmaker’s microscope under adequate temperature control when: (1 ) the wax denture was on the

gypsum cast; (2 ) the w ax denture was flasked to the occlusal surfaces; (3 ) the denture was cured and the occlusal index was rem oved; (4 ) the denture was re­ moved from the cast on which it was cured; and (5 ) the denture was cleaned, polished, and stored one day in water. T h e average molar-to-molar shrinkage of the thin technic dentures o f all m a­ terials was approximately two times that o f the thick dentures. Probably the den­ tures with the thicker cross section were stiff enough to prevent release o f some o f the strain when the cured dentures were removed from the casts. Th e average molar-to-molar shrinkage o f the lower technic dentures was 1.5 times as much as that o f the upper tech­ nic dentures. This relationship is probably due to the gross difference in the shape o f the upper and lower dentures. Th e molar-to-molar changes on the clinical dentures followed the general pattern o f the technic dentures. T h e largest part o f the molar-to-molar processing shrinkage occurs when the cured denture is removed from the gyp­ sum cast. This is apparently caused by the release o f elastic strain produced when' the high-expanding resin is cooled in the low-expanding gypsum mold. M a ­ terials processed at low temperatures have in general less molar-to-molar shrinkage when the denture is removed from the cast. T h e differences among the molar-tomolar changes on the different materials appear large on the chart (Fig. 8 and 9) but they are not clinically significant since they could not be detected by either the patient or the dentist at the time o f insertion. Th e molar-to-molar changes during curing o f dentures made with an epoxy resin were lowest. This good property is offset by the poor color stability, a tendency to collect stain rapidly, and such a high expansion in service that the por­ celain teeth are separating from the base. T h e largest average molar-to-molar

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change (0.7 per cent) amounts to 0.35 mm. on a 50 mm. molar-to-molar span. T h e differences in average molar-tomolar shrinkages among the nine resins processed at comparatively high tempera­ tures are small, barely over 0.1 per cent (0.05 mm. on a 50 mm. molar-to-molar span). Thus, the choice o f a resin or a processing method within this group o f nine resins would have to be on some other basis than dimensional accuracy during processing. Th e gypsum molds containing the processed technic dentures were removed intact from the metal flasks and cross­ sectioned. T h e best adaptation o f the dentures occurred in the poured epoxy resin, an acrylic resin processed by direc­ tional hydraulic compression molding, a polystyrene resin using transinjection molding, a vinyl-acrylic copolymer injec­ tion molded, and an acrylic resin gel in­ jection molded in a special flask intended to propagate the polymerization directionally. Th e other resins and methods showed poorer adaptation. However, there is little or no relationship between the adaptation of the dentures to the gypsum casts as shown in the cross sec­ tions and the molar-to-molar shrinkages that occur when the dentures are deflasked and removed from the casts. Once a denture is removed from the gypsum cast on which it has been cured, the denture, when replaced on the cast, will not fit it. Thus, all o f the dentures show a dis­ tortion. This distortion is least when the percentage changes in the molar-to-molar and the flange-to-flange distances are small and approximately equal. O n 66 dentures reference pins were inserted in the flanges (after removing a small sec­ tion of the cast to expose the flange in the second molar area) before the cured denture was removed from the cast. Th e changes on these dentures were small, that is, 0.12 ± 0.10 per cent, show­ ing that no large and clinically detectable deformations were occurring.

T h e conventional acrylic resins proc­ essed with the usual dental technic o f compression m olding are just as stable in dimension as the special resins and technics investigated.

This in v e s tig a tio n was su p p o rte d in p a rt b y research g ra n t D-605, E va luatio n o f Resin Denture Base M a te ria ls , to th e A m e ric a n D e nta l A s s o c ia tio n fro m th e N a tio n a l In s titu te fo r D ental Research, and by th e U n ite d States N a val D e nta l S chool, who assigned M r. John C . C a rlson, a d e n ta l la b o ra to ry te c h n ic ia n , to assist in th is in v e s ti­ g a tio n . This r e p o rt is a p a rt o f the d e n ta l research p ro g ra m c o n d u c te d a t th e N a tio n a l Bureau o f Standards In c o o p e ra tio n w ith th e A m e ric a n D ental A s s o c ia tio n , the A rm y D ental C o rps, th e D ental Sciences D ivision o f th e School o f A v ia tio n M e d ic in e , USAF, th e N aval Dental C o rp s , and th e Veterans A d m in is tra tio n . •R esearch associate, A m e ric a n D ental A s s o c ia tio n , N a tio n a l Bureau o f S tanda rds, on leave fro m the fa c u lty o f th e C o lle g e o f D e ntistry, O h io S tate U n iversity, C o lum bus, O h io . t S e nior research associate, A m e ric a n D ental A s s o c ia ­ tio n , N a tio n a l Bureau o f S tandards. | C h ie f, d e n ta l research section , N a tio n a l Bureau o f Standa rds. 1. Sweeney, W . T. Denture base resin. J .A .D .A . 26:1863 N o v. 1939.

m a te ria l:

a c ry lic

2. Sweeney, W . T.* P a ffenbarger, G . C ., and Beall, J . R. A c r y lic resin f o r dentures. J .A .D .A . 29:7 J a n . 1942. 3. Peyton, F. A ., and M ann, W . R. A c ry lic and a c ry lic siyrene resins* th e ir p ro p e rtie s in re la tio n to th e ir uses as re s to ra tiv e m a te ria ls . J .A .D .A . 29:1852 O c t. 1942. 4. Skinner, E. W ., and C o o p e r, E. N. Physical p ro p e r­ tie s o f d e n tu re resins: P art I. C u rin g shrinkage and w a te r s o rp tio n . J .A .D .A . 30:1845 Dec. 1943. 5. Skinner, E. W M and Jones, P. M . D im ensional s ta b ility o f s e lf-c u rin g d e n tu re base a c ry lic resin. J .A .D .A . 51:426 O c t. 1955. 6. G rü n e w a ld , A . H .; P a ffenbarger, G . C ., and D ic k ­ son, G . The e ffe c t o f m o ld in g processes on some p ro p e rtie s o f d e n tu re resins. J .A .D .A . 44:269 M arch 1952. 7. T u c k fie ld , W . J., and W o rn e r, H . K. A c r y lic resins in d e n tis try . Part I I. T heir use fo r d e n tu re c o n s tru c tio n . A u s tra l. J . Den. 47:1 M arch 1943. 8. V ie ira , D io ra c y , F. In flu e n c ia d e m a te ria ls e técnicas s o b re a p o s i$ a o re la tiv a dos dentes, na construca o d e uma base de d e n ta d u ra . Tese. Facu ldad e de Farm acia e O d o n to lo g ía da U n iversíd ade d e Sao Paulo, 1958. 9. Sweeney, W . T. A c r y lic resins ín p ro s th e tic d e n ­ tis try . D. C lin . N . A m e ric a , N o v. 1958, p . 593. 10. L ytle, R o b e rt B. M a n age m ent o f abused o ra l tissues in c o m p le te d e n tu re c o n s tru c tio n . J . Pros. Den. 7:27 Jan. 1957. 11. L ytle, R o b e rt B. C o m p le te d e n tu re c o n s tru c tio n based on a study o f d e fo rm a tio n o f u n d e rly in g soft tissues. J . Pros. Den. 9:539 J u ly -A u g . 1959. 12. Rupp, N . W ., and others. A m eth o d fo r m easuring th e m ucosal surface contours o f im pressions, casts, ana dentures. J .A .D .A . 54:24 J a n . 1957. 13. A n th o n y , D. H ., and Peyton, F. H . E va luatin g dim e n s io n a l a c c u ra c y o f d e n tu re bases w ith a m o d ifie d c o m p a ra to r. J . Pros. Den. 9:683 J u ly -A u g . 1959. 14. Ryge, G unnar, and F airhurst, C . W . The c o n to u r m e te r: an a p p a ra tu s fo r com p a ris o n o f m ucosal surface c o n to u r o f im pressions, m odels, and den tures. J . Pros. Den. 9:676 J u ly -A u g . 1959. 15. M ow ery, W . E., and others. D im ensional s ta b ility o f d e n tu re base resins. J .A .D .A . 57:345 Sept. 1958. 16. D a h l, G . E. D im ensional s ta b ility o f s m all c ylinders o f a c ry lic resins. M aste r's thesis, N o rthw e stern U n iv e r­ sity, 1946.