The strength of multilayer and repaired composite resin D. B. Boyer, D.D.S., Ph.D.,* K. C. Chan, D.D.S., M.S.,** and D. L. Torney, D.D.S., M.S.* College of Dentistry, University of Iowa, Iowa City, Iowa

I t is often difficult to place large composite resin restorations with a single increment of material because of the necessarily short working time. If there were adequate bonding of unpolymerized resin to partially polymerized resin, restorations could be built up in a stepwise manner. Similarly, defects in a composite resin restoration discovered at the time of placement could be repaired without replacing the entire restoration. Old composite resin restorations often become stained on the surface and at the margins or become otherwise superficially defective. If there were adequate bonding between the old resin and freshly added resin, the surfaces of restorations could be repaired without replacing the entire restoration. Ibsen and Neville' recommended inserting composite resin in several layers to minimize the effect of shrinkage on the restoration. T h e y noted the absence of reports in the literature concerning the multilayer technique and suggested using a thin mix of a liquidpowder-formulated composite resin. One manufacturer states that its composite resinJ" can be placed in stepwise increments but does not recommend additions after 30 minutes. Another company has suggested that its enamel bonding agent,~ an unfilled resin, m a y be used in conjunction with a composite resin to resurface stained restorations. The ultraviolet-light-initiated productsw must be added in steps to properly cure the resin. There have been several in vitro studies concerning the repair of composite resins. Forsten and Valiaho 2 measured the transverse strength of *Assistant Professor of Operative Dentistry. **Professor of Operative Dentistry. ~'Adaptic, Johnson & Johnson, East Windsor, N. J. *Enamel Bond, 3M Co., Dental Products Div., St. Paul, Minn. w and Nuva Seal, The L. D. Caulk Co., Milford, Del.

THE JOURNALOF PROSTHETICDENTISTRY

composite resin specimens with ground surfaces to which new resin had been added. The strengths of the repaired resins ranged from one fifth to one half of the strength of the unrepaired composite resins, depending on the commercial product used. The authors concluded that such repair m a y be clinically satisfactory although mechanical retention may be needed where there is exposure to biting forces. Reisbick and Brodsky 3 obtained similar results when testing bonding to composite resin surfaces cured against an aluminum spacer. The transverse strength of samples that had polymerized for 10 minutes before the additions were made was about one half of the control value. Additions at 94 hours resulted in bonds with strengths of about one third of the control group's strength. Reisbick and Brodsky concluded that the materials tested could not be effectively repaired by relying on bond strength alone. Causton ~ measured the shear strength of samples of several commercial products that had been repaired by adding new resin to surfaces that had been abraded with emery paper. The range of strengths found was 15 to 30 Meganewtons per square meter (2,200 to 4,400 psi), with the lowest strengths obtained with the most viscous products. The authors concluded that these values represented clinically acceptable strengths since acid-etch-retained composite resins with tensile bond strengths of 18 Meganewtons per square meter have proved to be clinically acceptable. The purposes of the present study were to (1) determine the tensile strengths of muhilayer composite resin samples, (2) determine the effect of preparation of the adherend surface on bond strength, and (3) investigate methods of improving bonding to composite surfaces.

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MATERIALS AND METHODS 1

A single batch of a commercial composite resin* was used throughout the study. T h e composite resin was a two-paste system with a c o m p a n i o n unfilled resin for acid-etch retention. A split die made of technique metal was used to prepare the samples for measurement of tensile Strength with an Instron universal testing instrum e n t . t A diagram of the die is shown in Fig. 1. T h e neck of the die was 2 by 2 m m and the overall length was 15 mm. T h e resin was wiped into the die resting on a glass slab, and a glass slide and weight were placed on top. W h o l e samples were used as the controls (procedure 1) and were tested 24 hours after being made. H a l f samples were prepared with the following three types of surfaces to simulate different clinical situations: (1) surfaces cured against a plastic matrix, (2) surfaces cured while exposed to air, and (3) cut surfaces. T h e second half of the resin was added after varying periods of time had elapsed and after varying surface treatments had been performed. T h e procedures that were carried out for each condition are shown in Table I. All of the procedures were carried out at room temperature (21 ~ C) except for No. 7, which was done at 37 ~ C. T h e n u m b e r of samples tested for each variable ranged between six and ten. Samples with voids were not used. Care was taken to produce samples without voids at the interface of the two halves of resin. T h e crosshead speed of the Instron was 3 centimeters per minute. *Concise, 3 M Co., D e n t a l Products Div., St. Paul, M i n n . t I n s t r o n Corp., Canton, Mass,

64

2

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5

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Fig. 2. Tensile strengths of composite resin bonded to uncut surfaces. In procedures 2 to 4 substrate surfaces were formed against a plastic matrix. Resin was added at times shown. In procedures 5 to 8 substrate surfaces polymerized while exposed to air. In procedure 7 the substrate was cured at 37 ~ C for 30 minutes before the addition of resin; otherwise all procedures were done at 21 ~ C. All samples were tested 24 hours after the addition of resin.

RESULTS T h e tensile strengths of composite resins added to uncut composite surfaces are shown in Fig. 2. T h e controls tested at 24 hours had an average strength of 4,040 + 261 psi. A group of controls tested at 7 days demonstrated no significant difference in strength according to the analysis of variance (p = ,601) and the D u n c a n multiple range test at the .05 level of significance. Strength of b o n d i n g to surfaces cured against a plastic matrix ranged from 84% to 95% of the cohesive strength of the resin. T h e analysis of variance (p = 0.389) and the D u n c a n multipl e range test (p > .05) showed no difference in b o n d i n g strength of samples Nos. 2, 3, and 4 at the three times studied. There was no difference between the control and the 7 and 30 minute samples. However, there was a statistically significant difference between the control and the 24 h o u r samples. M a n y of the samples broke at sites other than the interface. Bonding strength to surfaces which were cured while exposed to air decreased with the age of the surface, although there were no statistical differences

JANUARY 1978

VOLUME 39

NUMBER 1

STRENGTH OF COMPOSITE RESINS

T a b l e I. E x p e r i m e n t a l c o n d i t i o n s Elapsed time before addition of second resin

Description of procedure

1. 2. 3. 4. 5. 6. *7. 8. 9. 10. 11. 12. 13. 14. 15. ~16. 17. 18. 19. ~20. 21. 22.

Controls-whole samples Surface cured against plastic Surface cured against plastic Surface cured against plastic Surface cured while exposed to air Surface cured while exposed to air Surface cured while exposed to air at 37 ~ C Surface cured while exposed to air Surface cut with Carborundum disk and cleaned with air Surface cut with Carborundum disk and cleaned with air Surface cut with crosscut fissure bur and cleaned with air Surface cut with crosscut fissure bur and cleaned with air and H20 Surface cut with bur, etched with 37% H3PO~ for 2 minutes, and cleaned with H~O and air Surface cut with bur, polished with fine cuttle disk and chalk with rubber cup Unfilled liquid resin]" blotted with cotton pellet Unfilled liquid resin blotted with cotton pellet Unfilled liquid resin not blotted, filled in a thick layer Unfilled liquid resin,], blotted Unfilled catalyst resin,w blotted Unfilled catalyst resin, blotted Unfilled catalyst resin blotted after 5 minutes Unfilled universal resin,w blotted

7 min 30 min 24 hr 7 rain 30 min 30 rain 24 hr 15 rain 30 rain 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr 24 hr

*All experiments were done at room temperature (21 ~ C) except No. 7. ].Adaptic bonding agent, Johnson & Johnson, East Windsor, N. J. ~All samples were tested 24 hours after the addition except for Nos. 16 and 20, where the samples were tested at 7 days. w Bond, 3M Co., Dental Products Div., St. Paul, Minn.

a m o n g t h e 7 a n d 30 m i n u t e s a m p l e s at t h e .05 level of significance. T h e 30 m i n u t e r o o m - t e m p e r a t u r e s a m p l e a n d t h e 24 h o u r s a m p l e w e r e s i g n i f i c a n t l y different f r o m the c o n t r o l at t h e .05 level of signific a n c e ; t h e o t h e r s w e r e not. M a n y o f t h e 7 m i n u t e samples did n o t b r e a k at the i n t e r f a c e as did g r e a t e r n u m b e r s o f t h e 30 m i n u t e a n d 24 h o u r samples. T h e tensile s t r e n g t h s o f s a m p l e s p r e p a r e d b y a d d i n g c o m p o s i t e resin to u n t r e a t e d c u t surfaces a r e s h o w n in Fig. 3. R e g a r d l e s s of the m a n n e r in w h i c h the surface was c u t (by disk or fissure b u r ) or c l e a n e d , t h e b o n d s t r e n g t h was a b o u t o n e h a l f o f t h e cohesive s t r e n g t h o f t h e controls. E t c h i n g t h e s u r f a c e di~t n o t s i g n i f i c a n t l y i n c r e a s e r e t e n t i o n a t t h e .05 level o f significance. All of t h e s a m p l e s w e r e s i m i l a r at the .05 level e x c e p t for t h e p o l i s h e d s a m p l e s , in w h i c h a d h e r e n c e was e v e n f u r t h e r r e d u c e d . T h e results o f t r e a t i n g t h e c u t surfaces w i t h unfilled resin b e f o r e a d d i n g t h e n e w c o m p o s i t e are s h o w n in Fig. 4. C o m p a r i s o n o f t h e s a m p l e s w i t h t h e D u n c a n m u l t i p l e r a n g e test at t h e .05 level is s h o w n in T a b l e II. T h e use o f C o n c i s e E n a m e l B o n d or A d a p t i c b o n d i n g a g e n t to j o i n i n c r e m e n t s o f C o n c i s e

THE JOURNALOF PROSTHETIC DENTISTRY

c o m p o s i t e resin c a u s e d o n l y a u n i f o r m l y slight increase in b o n d s t r e n g t h w h e n tested at 24 hours. T h e s t r e n g t h was n o t s i g n i f i c a n t l y d i f f e r e n t (p = .05) t h a n t h a t o b t a i n e d w i t h a n u n t r e a t e d c u t surface. H o w e v e r , w h e n t h e a d d i t i o n s w e r e tested a f t e r 7 d a y s the s t r e n g t h i n c r e a s e d to 75% o f t h e c o n t r o l v a l u e and was s t a t i s t i c a l l y d i f f e r e n t f r o m s t r e n g t h e v i d e n c e d in i n c r e m e n t s to t h e u n t r e a t e d c u t surface. W h e n t h e u n f i l l e d c a t a l y s t resin was used b y itself as the b o n d i n g a g e n t s its 24 h o u r s t r e n g t h was g r e a t e r t h a n t h a t o b t a i n e d w i t h t h e m i x t u r e o f the c a t a l y s t a n d t h e u n i v e r s a l u n f i l l e d resins. G r e a t e r 24 h o u r s t r e n g t h w a s o b t a i n e d w h e n t h e c a t a l y s t resin r e m a i n e d in c o n t a c t w i t h the c o m p o s i t e resin surface for 5 m i n u t e s b e f o r e t h e n e w l a y e r was a d d e d . T h e 7 day strength of the samples treated with the catalyst resin was t h e o n l y v a l u e t h a t was s i m i l a r to t h e c o n t r o l v a l u e at t h e .05 level. T h e use of t h e u n i v e r s a l resin a l o n e as a b o n d i n g a g e n t c a u s e d a d e c r e a s e in s t r e n g t h , as d i d t h e use o f a thick layer of b o n d i n g agent. H o w e v e r , these strengths w e r e n o t s i g n i f i c a n t l y d i f f e r e n t (p = .50)

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Fig. 3. Tensile strengths of composite resin bonded to cut surfaces. In procedures 9 and 10 the surfaces were cut with a Carborundum disk. In procedures 11 to 14 the surfaces were cut with a crosscut fissure bur. The surfaces were cleaned with either a blast of air or a water spray followed by air drying. II. B o n d i n g of composite resin to cut surfaces treated with unfilled resin

Table

Duncan

Procedure

multiple range test (p -- .05)*

12 Untreated cut surface 22. Universal resin

1,860 • 270"~ 1,250 • 220 "-1

17. 15. EnamelBond, EnamelBond, thick 24 hr

1,7502250 2,320 • 260

18. Adaptic bonding agent, 24 hr 19. Catalyst resin, 24 hr 16. Enamelbond, 7 days 21. Catalyst resin, 5 rain, 24 hr 20. Catalyst resin, 7 days 1. Control

2,420 • 360 2,790 • 120 3,000 • 360 3,230 • 200 3,360 • 290 4,040 • 260

_l 1 1 l

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Fig. 4. Tensile strengths of composite resin bonded to cut surfaces with unfilled resins as interlayers. All samples were tested after 24 hours except in procedures 16 and 20 where 7 days elapsed9 The procedural conditions are described in Table I and the text.

T h e j u n c t i o n was visible as a dark line w h e n a thick layer of unfilled resin was used. T h e a d d i t i o n of composite resin to a n u n c u t surface produced a less visible j u n c t i o n . T h e j u n c t i o n was least visible w h e n composite resin was a d d e d to a n u n t r e a t e d cut surface or a cut surface was coated with a t h i n layer of unfilled resin. DISCUSSION

-]

J 7 -3

*Values connected by brackets are not significantly different at p < .05. "~Standard error. from those o b t a i n e d with u n t r e a t e d cut surfaces. Essentially all cut samples failed at the interface of the new a n d old resin regardless of the m e t h o d of treatment. T h e visibility of the interface of the old a n d new resin d e p e n d e d u p o n the m a n i p u l a t i v e variables.

66

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Tensile strength (psi)

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T h e r e is a marked variance in the b o n d strength of composite resin j o i n e d to cured resin; that strength depends u p o n whether or not the surface receiving the a d d i t i o n has been cut. T h e strength of the b o n d i n g of resin to a surface that was formed against a plastic matrix a n d has not been cut is nearly equal to the cohesive tensile strength of the resin itself. T h e surface that forms against a m a t r i x has been shown to be pure resin a n d very smooth. 5 This surface a p p a r e n t l y is ideal for b o n d i n g of the new layer. However, the surface of a resin wears in the oral environment6; a d d i t i o n to this type of surface would be limited to the time of initial placement. It is possible to add to composite resin that has polymerized while exposed to air. However, the b o n d s t r e n g t h of samples declines w h e n resin is added at later times. T h e surface formed w h e n the resin polymerizes while exposed to air is rough a n d covered with a liquid film; this film is most certainly a m o n o m e r , the polymerization of which has been

JANUARY1978 VOLUME39

NUMBER1

STRENGTH OF COMPOSITE RESINS

i n h i b i t e d b y c o n t a c t with air. T h e film a p p a r e n t l y acts as a b o n d i n g a g e n t b u t its effectiveness decreases as time elapses a n d the film polymerizes. It w o u l d seem t h a t good clinical results could be o b t a i n e d with u n c u t surfaces a n d stepwise p r o c e d u r e s d o n e d u r i n g p r a c t i c a l working times of u p to l/z hour. T h e b o n d strength o f a c o m p o s i t e resin a n d a c u t composite resin surface is a b o u t one h a l f of the tensile strength of the resin regardless of the t i m e of a d d i t i o n or the m e t h o d o f cutting. This finding m a y be correlated with the fact t h a t a b o u t one h a l f of the cut surface is filler. Because resin has been f o u n d to a d h e r e p o o r l y to ceramic filler the m a n u f a c t u r e r uses a silane c o u p l i n g a g e n t to p r o m o t e b o n d i n g . 7 T h e cut surface of the filler has no layer of silane a n d is p r o b a b l y p o o r l y wetted b y the resin. B o n d i n g to cut composite resin surfaces was p r o m o t e d b y using unfilled liquid resin as a b o n d i n g agent. T h e greatest strength, over 80% of the strength of the control specimen, was o b t a i n e d using a thin layer of the unfilled catalyst resin. S t r e n g t h also d e v e l o p e d m o r e r a p i d l y when using the c a t a l y s t resin r a t h e r t h a n the m i x t u r e of the catalyst a n d universal liquid resins. It is i m p o r t a n t to use a very thin layer of the liquid resin; a thick layer of liquid a p p a r e n t l y prevents good a d a p t a t i o n of the new to the old c o m p o s i t e resin d u r i n g p l a c e m e n t . T h e liquid resin p r o b a b l y wets the micropores of the cut surface as well as the filler. T h e j u n c t i o n s o f old a n d new resin o f carefully p r e p a r e d samples were invisible. T h e mixes of resin must be identically p r o p o r t i o n e d , however, or one h a l f m a y a p p e a r d a r k e r t h a n the other. It w o u l d be best to design a m u l t i l a y e r restoration or to r e p a i r a restoration so t h a t only a single layer is visible.

THE JOURNAL OF PROSTHETIC DENTISTRY

CONCLUSIONS C o m p o s i t e resin surfaces t h a t f o r m e d against a plastic m a t r i x or t h a t p o l y m e r i z e d while exposed to air were excellent substrates for the a d h e s i o n of new resin. S a m p l e s formed by the a d d i t i o n of c o m p o s i t e resin to a cut surface h a d a tensile strength of one h a l f of the cohesive strength of the resin. C o a t i n g the cut surface with a thin layer of unfilled resin e n h a n c e d b o n d i n g of the second composite resin layer. T h e use of a thin layer of unfilled catalyst resin as a b o n d i n g a g e n t caused the most r a p i d d e v e l o p m e n t of strength a n d the greatest strength in samples tested after 7 days. REFERENCES 1. Ibsen, R. L., and Neville, K.: Adhesive Restorative Dentistry. Philadelphia, 1974, W. B. Saunders Company, p 55. 2. Forsten L., and Valiaho, M.-L.: Transverse and bond strength of restorative resin. Aeta Odontol Scand 29:527, 1971. 3. Reisbick, M. H., and Brodsky, J. F.: Strength parameters of composite resins. J PROSTHETDENT 26:178, 1971. 4. Causton, B. E.: Repair of abraded composite fillings. Br Dent J 139:286, 1975. 5. Johnson, L. N., Jordan, R. E., and Lynn, J. A.: Effects of various finishing devices on resin surface. J Am Dent Assoc 83:321, 1971. 6. Eames, W. B., Ward, R. B., Jr., and Butler, M. F.: The effect of brushing on the surface of filled resins. J Ala Dent Assoc 55:22, 1971. 7. Phillips, R. W.: Skinner's Science of Dental Materials, ed 7. Philadelphia, 1973, W. B. Saunders Company, p 234. Reprint requests to: DR. D. B. BOYER DEPARTMENT OF OPERATIVE DENTISTRy UNIVERSITY OF IOWA COLLEOE OF DENTISTRY IOWA CITY, IOWA 52242

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The strength of multilayer and repaired composite resin.

The strength of multilayer and repaired composite resin D. B. Boyer, D.D.S., Ph.D.,* K. C. Chan, D.D.S., M.S.,** and D. L. Torney, D.D.S., M.S.* Colle...
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