Journal of Oral Rehahilitation. 1992, Volnmc 19, pages 115-122
Stresses generated by luting resins during cementation of composite and ceramic inlays J . S . R E E S (uul P . H . .1 A C O B S E N Depanment of Cotisetvative Dentistry, University of Wales College of Medicine. Cardiff, Wales. U.K.
Summary The present study used the finite element tnethod to model the stresses generated by a cotnposite luting cement around a class 1 composite restoration and a ceramic inlay. In many cases maximum tensile stresses of >20MPa were found, and failure of the restoration-dentine and r'estor'ation-glass ionomer interfaces was predicted. Introduction The main problem eneountered with direct placement of composite resins for' the restoration of posterior teeth is polymerization shrinkage. This leads to stresses at the restoration-tooth interface, which tends to pull the resin away frotn the cavity walls. The shrinkage stress has been estimated to be in excess of 20MPa (Asmussen & Munksgaar'd, 1985; Rees, 1988). Shrinkage stress may be counteracted by adhesion to tooth substance. Adhesion to enamel is sufficient to withstand these forees, but the shrinkage stress is dissipated by causing movement of the cusps (.lensen & Chan, 1985; Pearson & Hegarty, 1989). Adhesion of composite resin to dentine is much more tenuous, and almost invariably leads to marginal gap formation and microleakagc, even when using thir'd-generation bonding agents (Torstenson c*i. Brannstrom, 1988; Watts, 1990a). To overcome these problems, the use of an indirect polymeric inlay technique initially appears attractive, since the bulk of the polymerization takes place extraorally, and the thinner layer of composite used to cement the restoration shoirld produce lower shrinkage stresses and conseqtrently less cuspal llexure. Recently, there has been much debate about the tnagnittrde of the shrinkage stresses produced by thin layers of cor'rtposite resins. Watts (1990b) estimated that shrinkage stresses of 5-10 MPa were likely, while Mahler (1990) suggested that 4 MPa for a composite inlay and 7 MPa for a eeramic inlay were conceivable. However, as Feilzer, dc Gee and Davidson (1989) have pointed out, the stresses generated by thin resin layers can be quite considerable and, according to their calculalions (Feilzer, de Gee & Davidson, 1990), a shrinkage stress of 17-6MPa for a class I composite inlay and 27-6 MPa for a class I ceramic inlay would be likely. The aim of this study was to use the finite element method to model the shrinkage of a composite luting eemctit around a cotnposite and a ceramic class I itilay.
Correspotuletiee: Mr .I.S. Rees, Department of Conservative Dentistry, Detital Seliool, University of Wales College of Medieitie, Heath Park. Cardiff CF4 4XY, U.K.
115
116
.I.S. Rees and F.H. .htcobsen
Materials and methods The finite eletnent mesh (l^ig. 1), composed of 106 elements, was constritcted from a tracing of an upper first permanent ptetnolar restored wilh a class 1 inlay sectioned longitudinally thr'ough the centre of the tooth. An axisymmetric analysis was per'formed, sinee the stress profiles obtained from this type of analysis should closely approxitnate to a class 1 restoration. During this analysis the tnesh was rotated abottt the longitudinal (Y) axis. The physical properties of the tnaterials used in this atialysis are listed in Table 1. Three dilTerenl lute thicknesses, of 100, 200 and 300 jtm, were modelled since recetit work (Rees & .lacobseti, 1990) has suggested that composite inlays have cemctit lutes of 100 —300(t. The shritikagc stresses were gcticrated by utilizing the thermal
Enamel Inlay Composite lutitig cement Glass ionomer liner Dentine
Fig. 1. Axisymmetric tiicsh (note: circles show lixcd nodes).
Tiihle I. Physical properties of the materials used iti the analysis Material
Elastie modulus (MPa)
Poissoti's ratio
5,0tltl 12,162 15,t)0tl 2t),tlt)0 46,890 80,0t)0
tl-3 t)-3 tl-3 0-3 tl-3 tl-28
Cotnposite luting* cement Glass ionomer linert Composite inlay* Dentine* Enatiicl:|: Ccratiiic* * Feilzer et al., I99t), 'I' Rees. Williatiis & .lacobsett, 1989. t Yettratii, Wright & Pickard, 1976.
Stre.nes generated hv hi ting resins
117
loading option of the programme as reported previously (Rees, Williams & .lacobsen, 1989). The shrinkage of a bulk-packed direct placement composite was also modelled lor comparative purposes. This was achieved by modelling both the inlay and cement lute as a posterior composite and causing the whole of this material to shrink by 1%. Kesulls The results files were examined, and the average tensile nodal stresses along each of the following interlaces were evaluated: (i) enamel — composite cement; (ii) dentine — eornposite cement; (iii) glass ionomer liner — composite cement; (iv) composite/ceramic inlay — comj^osite cctnetit (cavity wall and lloor). The tetisile stresses for the eotnposite resin are listed in Table 2, and the tensile stresses for the ceramic inlay are shown in Table 3. The values for the direct placer'uent composite are shown in Table 4, and the associated movements of the tip of the cusp are given in Table 5, and are also illustratetl graphically in Fig. 2. Diseussion Feilzer el al. (1989) have reeently demonstrated that the stresses produced by thin layers of shrinking resins can be quite considerable. Tli present study tends to support these findings. The results of this analysis, assuming a cement lute shrinkage of 7% (as reported by pr"evious workers), yielded average values similar to the range of 4-10MPa suggested by Watts (1990b) and Mahler (1990). Tiihle 2. Tensile stress values (MPa) for the composite inlay Interface
\% shrinkage
3 % shrinkage
7% shrinkage
Itltl-micron lute Glass ion-ccnient Inlay-cemcnt (1) Enatiiel-cenient Dentine-ccmciit Inlay-cctiient (2)
t)-9 (0-3-3-2) tl-9 (0-2-2-5) 0-2(0-2-0-3) 0-9(0-3-2-5) tl-4 (0-1-1-6)
2 8 (1-4-7-6) 3-0(1-2-7-8) 1-4(1-0-1-8) 3-7(1-9-7-5) I-6 ((1-5-5-t))
6-5 (3-1-15-7) 7-t) (2-9-18-5) 3-3 (2-6-4-1) 7-2 ( 4 - 7 - 1 l-tl) 4-5 ( l - l - l t ) - , 5 )
2t)t)-micron lute Glass ion-ccmcnt Itrlay-ccmcnt ( I ) Enatnel-ectiient Dentitie-ccmcnt Inlay-cetnent (2)
t)-9 (0-5-2-5) 1-0 ( 0 - 4 - 2 - 6 ) t)-5 (0-3-0-6) 1-2(0-6-2-5) tl-6 (t)-2-1-7)
3-4 (2-t)-8-6) 3-6(1-6-8-6) 2-t) (1-3-2-4) 4-6 (2-8-9-tl) 2-6 (tl-4-5-3)
6-5 7-0 3-1 8-5 17-t)
3t)t)-micron Ittte Glass ioti-cement Itilay-eetnent (1) Enamel-ecment Dentine-cement Inlay-eement (2)
1-9(0-8-6-7) 1-3(0-6-2-9) 0-7(0-4-0-8) 1-9(0-4-7-7) 0-9 (tl-1 - 1-2)
3-1(0-7-9-5) 2-6(0-5-7-4) 7-3(7-1-7-4) 7-6(6-8-9-6) 7-4(6-5-8-1)
7-7 (3-7-20-0) 6-8 (2-5-16-4) 4-5 (3-t)- 5-7) lt)-7 (3-0-21-0) 5-7 (1-0-12-5)
(3-4-17-7) (2-9-l.S-O) (2-2-4-1) (4-3-17-4) (13-6-19-4)
Inlay-ccniciit (1) refers to the interfaee along the cavity wall anil inlayeetiietit (2) refers to the interfaee along the cavity floor. Mcati valttes are shown, with ratiges iti paretitlieses.
118
.J.S. Rees and P.H. .Jacobsen
Table 3. Tensile stress values (MPa) for the eeramie inlay Interfaee
1% shrinkage
3'X. shrinkage
1"/,, shrinkage
ltltl-micron Itite Glass ion-cement Inlay-cement (I) Enamel-eement Dentine-cement Inlay-eemcnt (2)
0-9(0-3-3-2) 0-9(0-2-2-5) 0-3 (0-1-tl-4) 1-t) (0-4-2-5) 0-6(0-2-1-6)
2-8(0-8-9-5) 2-7(0-7-7-4) tl-9 (tl-3-I-3) 3-0(1-2-7-6) 1-7(0-7-4-8)
6-5(1-9-22-1) 6-2(1-7-17-2) 2-tl (0-7-3-0) 6-8(2-6-17-4) 3-1 (tl-7-6-9)
2t)tl-mieron lute Glass ion-eement Inlay-eement (1) Enamel-cement Dentine-cement Inlay-cemenl (2)
1-0(0-5-2-5) 1-0(0-5-2-6) 0-6 (tl-3-0-8) 1-3(0-8-2-5) (1-8(0-3-1-7)
2-8(1-6-7-5) 3-1(1-4-7-9) 1-7(1-0-2-3) 4-t) (2-3-7-4) 2-5(1-0-5-0)
6-7(3-8-17-5) 7-2(3-3-18-2) 3-9(2-1-5-1) 9-2(5-2-17-3) 5-6(2-3-11-7)
300-mieron lute Glass ion-eement Inlay-ccment (I) Enatnel-eement Dentitie-ccment Inlay-cemcnt (2)
2-6 (2-3-3-tl) 2-4(1-7-2-7) 2-4(0-1-4-0) 1-6(0-1-3-0) 1-2(0-2-2-5)
3-3 (t)-2-8-5) 3-5(0-2-8-8) 2-3 (tl-2-2-9) 5-t) (3-2-9-t)) 3-2 (l-t)-5-3)
7-7 (5-tl-19-8) 8-5(4-11-20-3) 5-1(4-1-6-5) 11-3 (7-2-2t)-9) 7-2(2-2-12-4)
Inlay-eetiietit (1) refers to the interface alo[ig the eavity wall atid itilayeetnent (2) refers to the ititerfaee along the cavity floor. Mean values are shown, with ranges i[i paretitlieses.
Table 4. Tensile stress values (MPa) for the direct placctiicnt cotnposite resiti ( 1 % polynicrizatioti shrinkage) Interlace Enatnel-composite Dentine-cotnposite Glass ionomer-eotnposite
Tensile stress (MPa) 31-4 (30-8-.39-2) 42-9 (36-1-79-2) 34-3 (20-1—75-3)
Mean values av: shown, wivh ranges in paretitlieses.
The maximum tensile stresses observed were similar to the larger stresses of 18-28MPa suggested by Feilzer et al. (1990). By comparison, the stresses prodtrceil by the bulk-cured direct placetnent case were of the order of 40—50MPa, and are similar to the value of 53 MPa suggested by Feilzer (1990). The model used in the present analysis thus appears to be in agreement with previous calculations. Not surprisingly, the highest tensile stresses were found with the largest shrinkage value of 7% and the biggest lute width of 300 (.un. This is obviously related to the high polymerization shrinkage value and the larger volttme of shrinking resin. Comparison of the results for the eotnposite and ceramic inlays shows that the latter tended to produce slightly higher shrinkage stresses and cuspal movement. This is pr-esirmably due to the fact that the higher modulus ceramic did not undergo as much strain as the composite inlay, thereby plaeing more stress on the adjacent looth-eement interlace.
Stre.sses generated by luting resins
L19
5 . V a l u e s for the horizontal m o v e m e n t of t h e eusp tip ( m i e r o n s ) C u s p tip m o v e m e n t
Polymerization shrinkage
Composite inlay
Ceramie iiilay
U)t)-mieron lute 1% 3% 7%
1-1 3-2 7-4
0-3 0-9 2-0
2t)()-niieroti lute 1% 3% 7%
0-6 1-9 4-2
0-7 2-0 4-7
3t)0-mieroti lute 1% 3% 7%
0-2 0-6 0-8
2-2 3-5 7-7
Direet eure resin 1% shrinkage, movement = ,"i-.i microns: direct cure resin 3% shriiikage, movemetit = 15-2 niierotis.
O
2
3 4 5 6 Volumetric polymerization shrinkage (%)
8r
I
2 3 4 5 6 Volumetric polymerization shrinkage (%)
Fig 2. Cusp tip [liovetnetit vs. polytnerizatioti sliritikage for (a) composite atid (b) ceratnie inlay. ( • - • ) = 3t)tl-niicroti htte, ( • - • ) = 2t)t)-niicron Ittte, ( • • ) = 100-mieroti lute.
120
.I.S. Rees and P.H. .lacob.sen
It may be possible to predict failure ar'ound the tooth-restoration interface by comparison with adhesion test data. However, this must be done with some caution, since the results of laboratory test may be misleading (Van Noort et at., 1989). Furthermore, it is itnportant, when interpreting these results, to take note of the maximum tensile stress produced along each interface, because a high str'ess, even over a small area of an interface, will be sufficient to induce crack formation. This in itself will become an area of stress concentration, and is liable to induce further failure under' the stresses produced by the post-cure shrinkage of the inlay and/or the cement, or by occlusal loading. The maximum tensile stress that was placed on the enamel was 6-5 MPa. The bond strength of composite to etched enamel is approximately 15-20 MPa, so this interface is likely to remain intaet. However, the maximum stresses along the dentine-restoration interface were much higher. The maximum tensile stress measured was 21 MPa, and in most eases it was more than 10 MPa. If it is accepted that the bond strength of composite resin to dentine irsing curr'ently available adhesives is approximately JO MPa, then a cement with a shrinkage value of 3% or less is required in order to minimize gap formation. The reason for higher stresses along the dentine interface compared to enamel is that the enamel interface has a 'free' surface occlusally, from which resin can readily move. Along the dentine interfaee the cement is much tnore confined, and higher stresses r'esult. A similar i^attern was evident along the glass ionotner-restoration interface on the cavity lloor, maximum stresses of 10MPa or more being common. Since the maximum bond strength of composite resin to glass ionomer cement is 5-6MPa (Wilson & McLean, 1988), failure is only likely to be prevented with a luting cement with a polymerization shrinkage value of 1% or less. However, even with this low shrinkage value, a cement lute of 300^t may be suflicient to cause failure. Due to the relatively low bond strenglh of composite to glass ionomer, early failure at this interface is likely, and consequently most of the energy generated by the polymerization shrinkage will produce movement of the newly created 'free surface' of the cementing cotnposite, resulting in a large marginal gap. Indeed, this pattern of failure with integrity of the enamel interface, a small mar'ginal gap at the dentine interlace, and a larger marginal gap width along the glass ionomer liner interface has been observed //; vitro (Rees & .laeobsen, unpublished data). Failure between the inlay and the composite cetnent is more difficult to predict, since no data are available for the bond strength of composite cement to inlay composite. However, the values lor composite repaired with fresh composite are available, and these would appear to be r'epresentative. Values in the r'cgion of 15-17MPa have been reported and, since the highest tensile str'ess observed in the current analysis was 16-4MPa, integrity of the inlay-cement interface would seem probable. As far as the ceramic inlay is concerned, the bond strength of eornposite to silanized ceramic is 15-18MPa (Newburg & Pameijer, 1978; Calamia, 1986), so integrity of this interface is predicted. There have been lew reports about the cuspal llexure caused by eetnenting composite or ceramic inlays, but .lensen and Chan (1985) quote a figure of approximately 8 ^tm for composite inlays. This is similar to the ligur'cs suggested in the present analysis, the worst case, of 7% shrinkage combined with a 300|tm lute, giving a movement of 7-7|.,un.
Stresses generated bv Ititing resins
121
A similar trend to the tensile stress values were observed for the euspal flexure data. The tnagnitude of flexure decreased with a reduction in the marginal gap width and polymerization shrinkage values. For eornparative purposes, values in the range 5-5—15-2 |.im wer'e found with the dir'cct-j'ilaccd cotnposite r'esirt, which are similar to the values obtained by strain-gauge studies. A further finding was that the cuspal llexure values incr'eased markedly when the cement lute thickness reached 200|.tm. From a clinical viewpoint, it would appear' that the ideal inlay system vvotrld produce a restoration with a marginal gap of =£lOO^tm. It would also be useful if the manufacturers couki produce a composite luting agent with a polymerization shritikage value approaching 1%. Some of the current direct placement posterior composites have shrinkage values close to this figure (Rees & .lacobsen, 1989), suggesting that such materials could be used to cement these restorations. Obviously a chemical catalyst system would have to be added to the materials, but an additional advantage wotild be the presence of a harder-wearing polymer' on the occlusal aspect of the tooth-restoration interface. Conclusions (i) The pr'csent axisymmetric model of a composite or ceramic class 1 inlay appears to be in close agreement with other' experimental data. (ii) The values suggested for the irUcrlacial tensile stresses were very tntrch in line with current thinking on the str'esscs developed by thin composite films. (iii) Failure at the dentine- and glass iononier-restoratioti iriterlaccs was predicted. (iv) The authors suggest that dual-cure cotnposite cemctits with very low polymerization shrinkage values be used together with (.Icntinc bonding agents in an atternpt to ensure intcrlacial integrity. (v) Cuspal llexure valtres ar'c redttced using the inlay technique compared to direct placement teclinic|ues. References A s M u s s i i N , E . C . & MuNKS(iAAtit), I^.C. ( 1 9 8 5 ) A i l h e s i o n o f r e s t o r a t i v e r e s i n s t o d e n t i n a l tissties I n : Posterior Composite Resin Dentiil Restorative Materials (etis G. Vanlierte it D.C. Smith), pp. 2 1 7 - 2 2 9 . Peter Szule Publishing C o . , H o l l a n d . CAt.AMtA, .I.R. (1986) E t c h e d poreelaiti vetieers: the eurreiit state of the art. (Jiiinlesseine
International.
16, 5. Fiitt.zi-K, A.,I. Dt: Gt;t-:, A.,I &. DAVtDsoN, C . L . (1989) Inercasetl wall-to-wall euring c o n t r a c t i o n i[i thiiibondetl resiti layers. Jotirnal of Denial Research. 6S. 48. FiitL/.tiK, A.,I., mi Gtiti. A..I. & DAVtt)soN. C . L . (199tl) A n t h o r s reply to a letter from D.I.!. M a h l e r .
Journal of Dental Research. 69, 91.|. .lt;NSi:N, M . E . I'i CitAN, D . C N . (1985) P o h tnerisation shrinkage and tiiicrolcakage. In: Posterior Composite Dental Restorative Materials (etis G . V a n l i e r t e & D . C . S m i t h ) , p p . 243 — 262. Peter Szulc Publishitig C o . , Hollaiul. MAitLt:t(, D . U . (1990) L e t t e r to the Eilitor. Journal of Dental Research, 69, 913. Nt;wt!Lit(G, R. & PAMtit.n-;K, C M . (1978) C o t n p o s i t e resins bontleil to porcelain with silane soluti
Stresses generated by luting resins during cementation of composite and ceramic inlays J . S . R E E S (uul P . H . .1 A C O B S E N Depanment of Cotisetvative Dentistry, University of Wales College of Medicine. Cardiff, Wales. U.K.
Summary The present study used the finite element tnethod to model the stresses generated by a cotnposite luting cement around a class 1 composite restoration and a ceramic inlay. In many cases maximum tensile stresses of >20MPa were found, and failure of the restoration-dentine and r'estor'ation-glass ionomer interfaces was predicted. Introduction The main problem eneountered with direct placement of composite resins for' the restoration of posterior teeth is polymerization shrinkage. This leads to stresses at the restoration-tooth interface, which tends to pull the resin away frotn the cavity walls. The shrinkage stress has been estimated to be in excess of 20MPa (Asmussen & Munksgaar'd, 1985; Rees, 1988). Shrinkage stress may be counteracted by adhesion to tooth substance. Adhesion to enamel is sufficient to withstand these forees, but the shrinkage stress is dissipated by causing movement of the cusps (.lensen & Chan, 1985; Pearson & Hegarty, 1989). Adhesion of composite resin to dentine is much more tenuous, and almost invariably leads to marginal gap formation and microleakagc, even when using thir'd-generation bonding agents (Torstenson c*i. Brannstrom, 1988; Watts, 1990a). To overcome these problems, the use of an indirect polymeric inlay technique initially appears attractive, since the bulk of the polymerization takes place extraorally, and the thinner layer of composite used to cement the restoration shoirld produce lower shrinkage stresses and conseqtrently less cuspal llexure. Recently, there has been much debate about the tnagnittrde of the shrinkage stresses produced by thin layers of cor'rtposite resins. Watts (1990b) estimated that shrinkage stresses of 5-10 MPa were likely, while Mahler (1990) suggested that 4 MPa for a composite inlay and 7 MPa for a eeramic inlay were conceivable. However, as Feilzer, dc Gee and Davidson (1989) have pointed out, the stresses generated by thin resin layers can be quite considerable and, according to their calculalions (Feilzer, de Gee & Davidson, 1990), a shrinkage stress of 17-6MPa for a class I composite inlay and 27-6 MPa for a class I ceramic inlay would be likely. The aim of this study was to use the finite element method to model the shrinkage of a composite luting eemctit around a cotnposite and a ceramic class I itilay.
Correspotuletiee: Mr .I.S. Rees, Department of Conservative Dentistry, Detital Seliool, University of Wales College of Medieitie, Heath Park. Cardiff CF4 4XY, U.K.
115
116
.I.S. Rees and F.H. .htcobsen
Materials and methods The finite eletnent mesh (l^ig. 1), composed of 106 elements, was constritcted from a tracing of an upper first permanent ptetnolar restored wilh a class 1 inlay sectioned longitudinally thr'ough the centre of the tooth. An axisymmetric analysis was per'formed, sinee the stress profiles obtained from this type of analysis should closely approxitnate to a class 1 restoration. During this analysis the tnesh was rotated abottt the longitudinal (Y) axis. The physical properties of the tnaterials used in this atialysis are listed in Table 1. Three dilTerenl lute thicknesses, of 100, 200 and 300 jtm, were modelled since recetit work (Rees & .lacobseti, 1990) has suggested that composite inlays have cemctit lutes of 100 —300(t. The shritikagc stresses were gcticrated by utilizing the thermal
Enamel Inlay Composite lutitig cement Glass ionomer liner Dentine
Fig. 1. Axisymmetric tiicsh (note: circles show lixcd nodes).
Tiihle I. Physical properties of the materials used iti the analysis Material
Elastie modulus (MPa)
Poissoti's ratio
5,0tltl 12,162 15,t)0tl 2t),tlt)0 46,890 80,0t)0
tl-3 t)-3 tl-3 0-3 tl-3 tl-28
Cotnposite luting* cement Glass ionomer linert Composite inlay* Dentine* Enatiicl:|: Ccratiiic* * Feilzer et al., I99t), 'I' Rees. Williatiis & .lacobsett, 1989. t Yettratii, Wright & Pickard, 1976.
Stre.nes generated hv hi ting resins
117
loading option of the programme as reported previously (Rees, Williams & .lacobsen, 1989). The shrinkage of a bulk-packed direct placement composite was also modelled lor comparative purposes. This was achieved by modelling both the inlay and cement lute as a posterior composite and causing the whole of this material to shrink by 1%. Kesulls The results files were examined, and the average tensile nodal stresses along each of the following interlaces were evaluated: (i) enamel — composite cement; (ii) dentine — eornposite cement; (iii) glass ionomer liner — composite cement; (iv) composite/ceramic inlay — comj^osite cctnetit (cavity wall and lloor). The tetisile stresses for the eotnposite resin are listed in Table 2, and the tensile stresses for the ceramic inlay are shown in Table 3. The values for the direct placer'uent composite are shown in Table 4, and the associated movements of the tip of the cusp are given in Table 5, and are also illustratetl graphically in Fig. 2. Diseussion Feilzer el al. (1989) have reeently demonstrated that the stresses produced by thin layers of shrinking resins can be quite considerable. Tli present study tends to support these findings. The results of this analysis, assuming a cement lute shrinkage of 7% (as reported by pr"evious workers), yielded average values similar to the range of 4-10MPa suggested by Watts (1990b) and Mahler (1990). Tiihle 2. Tensile stress values (MPa) for the composite inlay Interface
\% shrinkage
3 % shrinkage
7% shrinkage
Itltl-micron lute Glass ion-ccnient Inlay-cemcnt (1) Enatiiel-cenient Dentine-ccmciit Inlay-cctiient (2)
t)-9 (0-3-3-2) tl-9 (0-2-2-5) 0-2(0-2-0-3) 0-9(0-3-2-5) tl-4 (0-1-1-6)
2 8 (1-4-7-6) 3-0(1-2-7-8) 1-4(1-0-1-8) 3-7(1-9-7-5) I-6 ((1-5-5-t))
6-5 (3-1-15-7) 7-t) (2-9-18-5) 3-3 (2-6-4-1) 7-2 ( 4 - 7 - 1 l-tl) 4-5 ( l - l - l t ) - , 5 )
2t)t)-micron lute Glass ion-ccmcnt Itrlay-ccmcnt ( I ) Enatnel-ectiient Dentitie-ccmcnt Inlay-cetnent (2)
t)-9 (0-5-2-5) 1-0 ( 0 - 4 - 2 - 6 ) t)-5 (0-3-0-6) 1-2(0-6-2-5) tl-6 (t)-2-1-7)
3-4 (2-t)-8-6) 3-6(1-6-8-6) 2-t) (1-3-2-4) 4-6 (2-8-9-tl) 2-6 (tl-4-5-3)
6-5 7-0 3-1 8-5 17-t)
3t)t)-micron Ittte Glass ioti-cement Itilay-eetnent (1) Enamel-ecment Dentine-cement Inlay-eement (2)
1-9(0-8-6-7) 1-3(0-6-2-9) 0-7(0-4-0-8) 1-9(0-4-7-7) 0-9 (tl-1 - 1-2)
3-1(0-7-9-5) 2-6(0-5-7-4) 7-3(7-1-7-4) 7-6(6-8-9-6) 7-4(6-5-8-1)
7-7 (3-7-20-0) 6-8 (2-5-16-4) 4-5 (3-t)- 5-7) lt)-7 (3-0-21-0) 5-7 (1-0-12-5)
(3-4-17-7) (2-9-l.S-O) (2-2-4-1) (4-3-17-4) (13-6-19-4)
Inlay-ccniciit (1) refers to the interfaee along the cavity wall anil inlayeetiietit (2) refers to the interfaee along the cavity floor. Mcati valttes are shown, with ratiges iti paretitlieses.
118
.J.S. Rees and P.H. .Jacobsen
Table 3. Tensile stress values (MPa) for the eeramie inlay Interfaee
1% shrinkage
3'X. shrinkage
1"/,, shrinkage
ltltl-micron Itite Glass ion-cement Inlay-cement (I) Enamel-eement Dentine-cement Inlay-eemcnt (2)
0-9(0-3-3-2) 0-9(0-2-2-5) 0-3 (0-1-tl-4) 1-t) (0-4-2-5) 0-6(0-2-1-6)
2-8(0-8-9-5) 2-7(0-7-7-4) tl-9 (tl-3-I-3) 3-0(1-2-7-6) 1-7(0-7-4-8)
6-5(1-9-22-1) 6-2(1-7-17-2) 2-tl (0-7-3-0) 6-8(2-6-17-4) 3-1 (tl-7-6-9)
2t)tl-mieron lute Glass ion-eement Inlay-eement (1) Enamel-cement Dentine-cement Inlay-cemenl (2)
1-0(0-5-2-5) 1-0(0-5-2-6) 0-6 (tl-3-0-8) 1-3(0-8-2-5) (1-8(0-3-1-7)
2-8(1-6-7-5) 3-1(1-4-7-9) 1-7(1-0-2-3) 4-t) (2-3-7-4) 2-5(1-0-5-0)
6-7(3-8-17-5) 7-2(3-3-18-2) 3-9(2-1-5-1) 9-2(5-2-17-3) 5-6(2-3-11-7)
300-mieron lute Glass ion-eement Inlay-ccment (I) Enatnel-eement Dentitie-ccment Inlay-cemcnt (2)
2-6 (2-3-3-tl) 2-4(1-7-2-7) 2-4(0-1-4-0) 1-6(0-1-3-0) 1-2(0-2-2-5)
3-3 (t)-2-8-5) 3-5(0-2-8-8) 2-3 (tl-2-2-9) 5-t) (3-2-9-t)) 3-2 (l-t)-5-3)
7-7 (5-tl-19-8) 8-5(4-11-20-3) 5-1(4-1-6-5) 11-3 (7-2-2t)-9) 7-2(2-2-12-4)
Inlay-eetiietit (1) refers to the interface alo[ig the eavity wall atid itilayeetnent (2) refers to the ititerfaee along the cavity floor. Mean values are shown, with ranges i[i paretitlieses.
Table 4. Tensile stress values (MPa) for the direct placctiicnt cotnposite resiti ( 1 % polynicrizatioti shrinkage) Interlace Enatnel-composite Dentine-cotnposite Glass ionomer-eotnposite
Tensile stress (MPa) 31-4 (30-8-.39-2) 42-9 (36-1-79-2) 34-3 (20-1—75-3)
Mean values av: shown, wivh ranges in paretitlieses.
The maximum tensile stresses observed were similar to the larger stresses of 18-28MPa suggested by Feilzer et al. (1990). By comparison, the stresses prodtrceil by the bulk-cured direct placetnent case were of the order of 40—50MPa, and are similar to the value of 53 MPa suggested by Feilzer (1990). The model used in the present analysis thus appears to be in agreement with previous calculations. Not surprisingly, the highest tensile stresses were found with the largest shrinkage value of 7% and the biggest lute width of 300 (.un. This is obviously related to the high polymerization shrinkage value and the larger volttme of shrinking resin. Comparison of the results for the eotnposite and ceramic inlays shows that the latter tended to produce slightly higher shrinkage stresses and cuspal movement. This is pr-esirmably due to the fact that the higher modulus ceramic did not undergo as much strain as the composite inlay, thereby plaeing more stress on the adjacent looth-eement interlace.
Stre.sses generated by luting resins
L19
5 . V a l u e s for the horizontal m o v e m e n t of t h e eusp tip ( m i e r o n s ) C u s p tip m o v e m e n t
Polymerization shrinkage
Composite inlay
Ceramie iiilay
U)t)-mieron lute 1% 3% 7%
1-1 3-2 7-4
0-3 0-9 2-0
2t)()-niieroti lute 1% 3% 7%
0-6 1-9 4-2
0-7 2-0 4-7
3t)0-mieroti lute 1% 3% 7%
0-2 0-6 0-8
2-2 3-5 7-7
Direet eure resin 1% shrinkage, movement = ,"i-.i microns: direct cure resin 3% shriiikage, movemetit = 15-2 niierotis.
O
2
3 4 5 6 Volumetric polymerization shrinkage (%)
8r
I
2 3 4 5 6 Volumetric polymerization shrinkage (%)
Fig 2. Cusp tip [liovetnetit vs. polytnerizatioti sliritikage for (a) composite atid (b) ceratnie inlay. ( • - • ) = 3t)tl-niicroti htte, ( • - • ) = 2t)t)-niicron Ittte, ( • • ) = 100-mieroti lute.
120
.I.S. Rees and P.H. .lacob.sen
It may be possible to predict failure ar'ound the tooth-restoration interface by comparison with adhesion test data. However, this must be done with some caution, since the results of laboratory test may be misleading (Van Noort et at., 1989). Furthermore, it is itnportant, when interpreting these results, to take note of the maximum tensile stress produced along each interface, because a high str'ess, even over a small area of an interface, will be sufficient to induce crack formation. This in itself will become an area of stress concentration, and is liable to induce further failure under' the stresses produced by the post-cure shrinkage of the inlay and/or the cement, or by occlusal loading. The maximum tensile stress that was placed on the enamel was 6-5 MPa. The bond strength of composite to etched enamel is approximately 15-20 MPa, so this interface is likely to remain intaet. However, the maximum stresses along the dentine-restoration interface were much higher. The maximum tensile stress measured was 21 MPa, and in most eases it was more than 10 MPa. If it is accepted that the bond strength of composite resin to dentine irsing curr'ently available adhesives is approximately JO MPa, then a cement with a shrinkage value of 3% or less is required in order to minimize gap formation. The reason for higher stresses along the dentine interface compared to enamel is that the enamel interface has a 'free' surface occlusally, from which resin can readily move. Along the dentine interfaee the cement is much tnore confined, and higher stresses r'esult. A similar i^attern was evident along the glass ionotner-restoration interface on the cavity lloor, maximum stresses of 10MPa or more being common. Since the maximum bond strength of composite resin to glass ionomer cement is 5-6MPa (Wilson & McLean, 1988), failure is only likely to be prevented with a luting cement with a polymerization shrinkage value of 1% or less. However, even with this low shrinkage value, a cement lute of 300^t may be suflicient to cause failure. Due to the relatively low bond strenglh of composite to glass ionomer, early failure at this interface is likely, and consequently most of the energy generated by the polymerization shrinkage will produce movement of the newly created 'free surface' of the cementing cotnposite, resulting in a large marginal gap. Indeed, this pattern of failure with integrity of the enamel interface, a small mar'ginal gap at the dentine interlace, and a larger marginal gap width along the glass ionomer liner interface has been observed //; vitro (Rees & .laeobsen, unpublished data). Failure between the inlay and the composite cetnent is more difficult to predict, since no data are available for the bond strength of composite cement to inlay composite. However, the values lor composite repaired with fresh composite are available, and these would appear to be r'epresentative. Values in the r'cgion of 15-17MPa have been reported and, since the highest tensile str'ess observed in the current analysis was 16-4MPa, integrity of the inlay-cement interface would seem probable. As far as the ceramic inlay is concerned, the bond strength of eornposite to silanized ceramic is 15-18MPa (Newburg & Pameijer, 1978; Calamia, 1986), so integrity of this interface is predicted. There have been lew reports about the cuspal llexure caused by eetnenting composite or ceramic inlays, but .lensen and Chan (1985) quote a figure of approximately 8 ^tm for composite inlays. This is similar to the ligur'cs suggested in the present analysis, the worst case, of 7% shrinkage combined with a 300|tm lute, giving a movement of 7-7|.,un.
Stresses generated bv Ititing resins
121
A similar trend to the tensile stress values were observed for the euspal flexure data. The tnagnitude of flexure decreased with a reduction in the marginal gap width and polymerization shrinkage values. For eornparative purposes, values in the range 5-5—15-2 |.im wer'e found with the dir'cct-j'ilaccd cotnposite r'esirt, which are similar to the values obtained by strain-gauge studies. A further finding was that the cuspal llexure values incr'eased markedly when the cement lute thickness reached 200|.tm. From a clinical viewpoint, it would appear' that the ideal inlay system vvotrld produce a restoration with a marginal gap of =£lOO^tm. It would also be useful if the manufacturers couki produce a composite luting agent with a polymerization shritikage value approaching 1%. Some of the current direct placement posterior composites have shrinkage values close to this figure (Rees & .lacobsen, 1989), suggesting that such materials could be used to cement these restorations. Obviously a chemical catalyst system would have to be added to the materials, but an additional advantage wotild be the presence of a harder-wearing polymer' on the occlusal aspect of the tooth-restoration interface. Conclusions (i) The pr'csent axisymmetric model of a composite or ceramic class 1 inlay appears to be in close agreement with other' experimental data. (ii) The values suggested for the irUcrlacial tensile stresses were very tntrch in line with current thinking on the str'esscs developed by thin composite films. (iii) Failure at the dentine- and glass iononier-restoratioti iriterlaccs was predicted. (iv) The authors suggest that dual-cure cotnposite cemctits with very low polymerization shrinkage values be used together with (.Icntinc bonding agents in an atternpt to ensure intcrlacial integrity. (v) Cuspal llexure valtres ar'c redttced using the inlay technique compared to direct placement teclinic|ues. References A s M u s s i i N , E . C . & MuNKS(iAAtit), I^.C. ( 1 9 8 5 ) A i l h e s i o n o f r e s t o r a t i v e r e s i n s t o d e n t i n a l tissties I n : Posterior Composite Resin Dentiil Restorative Materials (etis G. Vanlierte it D.C. Smith), pp. 2 1 7 - 2 2 9 . Peter Szule Publishing C o . , H o l l a n d . CAt.AMtA, .I.R. (1986) E t c h e d poreelaiti vetieers: the eurreiit state of the art. (Jiiinlesseine
International.
16, 5. Fiitt.zi-K, A.,I. Dt: Gt;t-:, A.,I &. DAVtDsoN, C . L . (1989) Inercasetl wall-to-wall euring c o n t r a c t i o n i[i thiiibondetl resiti layers. Jotirnal of Denial Research. 6S. 48. FiitL/.tiK, A.,I., mi Gtiti. A..I. & DAVtt)soN. C . L . (199tl) A n t h o r s reply to a letter from D.I.!. M a h l e r .
Journal of Dental Research. 69, 91.|. .lt;NSi:N, M . E . I'i CitAN, D . C N . (1985) P o h tnerisation shrinkage and tiiicrolcakage. In: Posterior Composite Dental Restorative Materials (etis G . V a n l i e r t e & D . C . S m i t h ) , p p . 243 — 262. Peter Szulc Publishitig C o . , Hollaiul. MAitLt:t(, D . U . (1990) L e t t e r to the Eilitor. Journal of Dental Research, 69, 913. Nt;wt!Lit(G, R. & PAMtit.n-;K, C M . (1978) C o t n p o s i t e resins bontleil to porcelain with silane soluti
