Operative Dentistry

Effect of surface-penetrating sealant on wear resistance of luting agentsKoichi Shinkai* / Shiro Suzuki** / Karl F. Leinfelder*** / Yoshiroh Katoh****

The wear of composite resin is influenced by the presence of microcracks on its surface.Recentiy, it has been shown that surface-penetrating sealants are effective in reducing the wearof posterior composite resin by sealing these defects- The purpose of this study was toinvestigate the effects of one of these sealants on the wear of various luting agents inconjunction with composite resin inlays. The specimens were subjected to a three-body weartest for 400,000 cycles. The wear values of both composite resin and luting agents weredetermined by profiiometric tracings to the nearest 2 ^m. There were no statisticallysignificant differences between the treated and untreated groups as it related to the wear of theluting agenis. The wear valties of composite resin inlays treated with the sealant, however,were significantly lower than those without treatment, regardless of the type of kiting agentused for cementation. (Quintessence Int 799'í;25.-767-77/.)

Introduction

Microcracks are formed by polishing on the surfaceandsubsurface area of composite resin.'"-̂ These micro-cracks may propagate continuously in the subsurfacearea, causing a de bonding of the filler particles as wellas a weakening of the matrix itself. Such a condition iscommonly associated with generalized wear."*' Re-cently, it has been shown that the elimination of micro-cracks by placement of a surface-penetrating sealant(SPS) on a proprietary composite resin results in a sig-nificant reduction in the generalized wear.' Anotherexperiment confirmed that the SPS effectively reduces

the generalized wear, but only if the filler particle isgreater than 1 |j.m.' It has also been reported that amodified SPS system is effective in reducing the wearrate of glass-ionomer cement restorations."

Most elinieal studies indicate that the wear resistanceof the luting agent is less than that of the esthetic inlayswith which they are being used.'"" Because the lutingagent commonly is less structurally sound than the ad-jacent restorative material, it ii conjectured that theSPS would effectively reduce the wear of both the lut-ing agent and the inlay. The purpose of this study was toinvestigate the effects of an SPS (Fortify, Bisco Dental)on the wear of various luting agents in conjunction withcomposite resin inlays.

* Associate Professor, Department of Operative Dentistry. TheNippon Dental University, Sthool ofDentistry at Niigata, t-8Ham aura-cho, Niigata 951, Japan.

** Associate Professor, Department of Biom a te rials/Restora-tive Dentistry, University of Alabama at Birmingham, Schoolof Dentistry, SDB Box 49, UAB Station, Birmingham, Ala-batîia 35294.

*•** Ainmini Volker Professor of Clinical Dentistry, Director, Bi-om ate rials Clinical Research, and Chairman, Department ofBiomaterials. University of Alabama at Birmingham.

'**•* Professor and Chairman, Departmeni of Operative Dentistry,The Nippon Dental University, School of Dentistn,- at Niigata.

Address all correspondence to Dr S. Stizuki.

Method and materials

The luting agents tised for this study were Dual-Ce-ment (DC) (Vivadent), Twinlook cement (TC) (Kul-zer), Super-Bond C&B (SB) (Sun Medical), Fuji II LC(LC) (GC Dental), and Fuji BOND (FB) (GC Dental).P-50 (3M Dental) was selected for use as the compositeresin inlay and direct restorative material.

Human molars were fixed individtially into brasscups by means of an acrylic resin. The cusps of themounted tooth were eliminated to obtain a flat occlusalsurface by wet-grin din g with a 600-grit silicon carbide

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TC LC

Fig 1 Effect of SPS on wear of luting agents used for com-posite resin inlays.

Table I Mean (SD) wear values (pm) oflu ting agentsin eonjunction with composite resin inlays

Without SPS With SPS

DC S.OO (4,00)T C 9.44 (3,68)FB 13,19 (3,35)LC 22,75 (4.71)SB 26,13 (11,123)

T C 9,44(1,90)D C 11,31(2,55)FB 18.31(5.88)SB 1S,31 (3,56)L C 19,56(3.68)

Means connected by a vertical line are not significantly different

paper. Care was taken so that the ground surface con-sisted entirely of enamel, A slightly tapered cylindricalcavity was prepared on the llattened occlusal surface ofthe mounted tooth. The cavity was approximately4 mm in diarneter and 3 mm in depth.

Impressiotis of the cavities were taken with Rcprosil(Caulk/Dentsply), Die stone casts were fabricated withVeUMix (Kerr/Syhron), Composite resin inlays werethen made indirectly on those die stone replicas. Theinlays were cemented with the luting agents accordingto the respective manufacturer's directions. Direct res-torations were also ptepared as a control. A custom-made, hand-held device was used to finish and polishthe restored surface with a ftOfl-grit silicon carbidepaper in the presence of water. Four specimens wereprepared for each condition.

After the restored surface was finished, the entire oc-clusal area was etched with 37% phosphoric acid, thenwashed and dried. The surface-penetrating sealant(Fortify) was applied to two ofthe specimens for eachmaterial, Tlie SPS was applied with a small, fine brush.A thin layer of SPS was uniformly spread by a gentlestream of compressed air against the restored surface,

Tlie samples were subjected to a three-body weartest previously described by Leinfelder et al,'^ A loadof 17 lbs placed onto the surface of each restoration wasadjusted by means of a dial mtcrometer strain gauge.This load was continuously cycled at the rate of 120contacts per minute for 400,000 cycles.

After cyclic loading, the surface of eaeh restorationwas replicated with Reprosil |Caulk/Dentsply) atidHpoxy-Die (Ivoclar). Tlie replica was coated with gold-platinum in preparation for scanning electron micro-scopic (SEM) evaluation.

The generalized wear of each specimen was scannedand traced along four different planes at 45 degrees toone another using a profilometer (Surfanalyzer Systetn4000. Federal ), The mean depth of wear of the eompos-ite resin inlay on the respective tracings was calculatedby means of an image scanner and a computer pro-gram. The depth of wear of the luting agent on eachtracing was calculated by averaging the distance on thetracing between the bottom of the luting agent and thesurface of the composite resin. All data were statistic-ally analyzed with a / test, one-way analysis of variance,and Scheffe's test.

Results

The wear values of luting agents are shown in Fig 1,There were no significant differences between thetreated and untreated groups related to the wear oftheluting agents. Tlie wear values of the resin cements DCand TC. with or without SPS. were significantly lower(P < .05) than those of the other luting agents exceptFB without SPS (Table 1).

The wear values of composite resin inlays cementedwith different luting agents are shown in Fig 2, Thewear values of the composite resin inlays treated withSPS were significantly lower (P < ,05) than those of in-lays without treatment, regardless of the type of lutingagent used for cementation. The wear values of thecomposite resin inlays eemented with the resin cementsDC and TC were significantly lower (F < .05) thanthose of inlays cetnented with the other luting agentsand direct restorations, regardless of whether or notthe SPS was used (Table 2),

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Typical scanning electron micrographs of the mater-ials with SPS treatment are presented in Figs 3 to 8, Allpictures depict the cavosurface margin of the compos-ite resin inlay restoration, as well as the lutitig agentafter wear testing. An SEM of TC is shown in Fig 3,There was a loss of substance corresponding to a layerof TC between the enamel and the composite resin in-lay. Some filler particles of TC protruded from the gen-eralized worn surface of the cement layer.

An SEM of DC is shown in Fig 4. There was a loss ofsubstance at the interface of the cement layer and thecomposite resin inlay. Small interfacial gaps were alsodetected between the enamel and the cement layer.Overall, however, the generalized wear of DC was min-imal.

In general, the glass-ionomer cements were charac-terized by an exposed cavity wail and protruded fillerparticles (Figs 5 and 6). An SEM of SB is shown in Fig 7.The wear or vertical loss of SB was approximately thesame as that ofthe two glass-ionomer cements. In otherwords, the degree of exposed enamel surfaces was ap-proximately the same in both cases.

An SEM of direct restoration is shown in Fig 8. Twotypes of defects were observed near the cavosurfacemargin. One was a loss of substance between the en-amel and the composite resin, corresponding to a layerof bonding agent. The other was a microcrack in theenamel running nearly parallel to the cavosurface mar-sin.

Discussion

The indirect system for posterior composite resin res-torations was developed to minimize many ofthe prob-lems^^" associated with the direct restoration tech-nique. Although the indirect system has successfullyresolved numerous problems, several clinical investiga-tors have reported a reduced wear resistance of the lut-ing agents used with esthetic inlays.''" This study ex-amined the possibility of using surface-penetratingsealant to enhance the wear resistance of luting agentsused with composite resin inlays.

While the use of SPS seems to be an effective meansfor reducing the wear of posterior composite resinswith larger fillers, the results of this study showed thatthe SPS had no significant abihty to increase wear resis-tance of the luting agents. On the other hand, the use ofSPS was significantly effective in reducing the wear ofthe composite resin inlay P-50, regardless of the lutingagent used.

Direct

Fig 2 Effect of SPS on wear of composite resin iniays ce-mented with respective luting agents.

Table 2 Mean (SD) wear values (um) of compositeresin inlays cemented with respective luting agents

Without SPS

TCDCDirectLCFBSB

3.79(0.75)4,43 (0,72)9.67(3.16)9.92 (2.59)

10.88 (3.33)12,65 (3.56)

Vteans connected bv a verticaIP < .05)

With SPS

TCDCFBLC

1.85(0.28)2.40 (0.40)5.59(1.87)6.40(1.07)

Direct 6.65 (L81)SB 8,65 (1,99)

line are not significantly different

The effectiveness in reducing wear appears to be as-cribed to the sealanfs ability to penetrate and fill mi-crocracks generated on the surface of the restoration.ITie SPS can penetrate into microstructural defects assmall as 1 or 2 (j.m because of their low viscosity andhigh wettability."^ Polymerization of SPS in the defectswill cause the weakened surface to be more resistant towear. Accordingly, the ability to be effective in decreas-ing wear seems to be related to the generation of micro-cracks on the resin surface. Posterior composite resinswith larger fillers, such as P-50, tend to create micro-structural defects during the finishing process. Conse-quently, the use of SPS was significantly effective in re-ducing the wear of P-50,

The surface area of luting agents of restorations ismuch smaller than the surface dimensions of the inlay.

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Fig 3 Scanning eiectron micrograph of Twiniook cementwith SPS treatment showing typical wear pattern after400,000 cycies. (E) Enamel surface; (TC) Tviiinlook cement;(C) composite resin inlay, P-50. (Original magnificationx500.)

Fig 4 Scanning electron micrograph of Dual cement withSPS treatment showing typical wear pattern after 400,000cycles. Note the minimal wear compared with other iutingagents. (E) enamel surface; (DC) Dual cement; (C) compos-ite resin iniay, P-50. (Originai magnification x500.)

Fig 5 Scanning eiectrcn micrograph of Fuji II LC with SPStreatment showing typicai wear pattern after 400,000cycies. Note the exposed cavity waii and protruded parti-cles, (E) enamel surface; (LC) Fuji il LC; (C) composite resininlay, P-50. (Originai magnification x500.)

Fig 6 Scanning electron micrograph of Fuji Bond with SPStreatment showing typical wear pattern after 400,000cycles. (E) Enamei surface; (FB) Fuji Bond; (C) compositeresin iniay, P-50. (Originai magnification x500.)

Fig 7 Scanning eiectron micrograph ot Super Bond CâBwith SPS treatment showing typicai wear pattern after400,000 cycies. (E) enamei surface; (SB) Super Bond CSB;(C) composite resin iniay, P-5Q. (Originai magnificationX500.)

Fig 8 Scanning eiectron micrograph of direct-filied P-50restoration with SPS treatment showing marginal degrada-tion after 400,000 cycies. Note the microcrack in the enamelrunning neariyparaiiei to the cavosurface margin. (EJ enamelsurface; (C) composite resin. (Original magnification x50.)

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The horizontal gap width of luting agents in this studyranged fiom 20 to 110 |xm. with fí5 % ratiging from 30 to60 n.m. Tlie use of SPS appears not to be effective in re-ducing the wear of materials in such a narrow space.

Since the luting agent generally is less wear resistantthan the inlay, it is expected that the former would ex-hibit a gTeater loss of material. As the tnaterial beginsto wear away, the food bolus can no longer contact itssurface. Consequently the wear pTocess decreases aftera period of time. When the food bolus cannot eventu-ally abrade the surface of the cemeut. the wear processstops almost entirely. It is possible that during the earlystress periods or cycles the SPS increases wear resis-tance. However, after 400,000 cycles, the refortifiedstructure is also worn away. Perhaps in that regard,then, an intermittent application of the SPS may bemoreeffectiveinreducingthe wcarof the cement.

The wear value of P-50 depended on the kind of lut-ing agent with which it was used. The wear values of P-50 inlays cemented with resin cements were signific-antly lower than the values of those cemented withglass-ionomer cements, ff the composite resin inlay isstrongly bonded to the cavity wall with an appropriateadhesive system of resin cement, the masticatorystresses will not concentrate on the composite resinsurface. They instead will be transmitted to the adja-cent tooth structure through the bonded interface. Onthe other hand, if the inlay is not strongly bonded tocavity, for instance in the case of cementation withglass-ionomer cement, the masticatory stresses will notbe readily transmitted but will concentrate in the com-posite resin. Transmitting the masticatory stress will re-duce wear of the composite resin surface.

This concept has been carried even further by Krejciet al,'^ who have shown that even the hardness of thebase material between the floor of the preparation andthe restoration has an influence on wear resistance. Asthe hardness or compressive strength of the base in-creases, so does the wear resistance of the overlyingcomposite resm. This suggests that the more rigid therestoration, the greater the wear resistance. The com-posite resin inlay restorations used with resin cementare more rigid than those used with glass-ionomer ce-ment.

Summarj'

The results of this study indicated that the surface-pen-etrating sealant had no significant effect on wear of theluting agents. However, the wear resistance of the com-posite resin inlays was appreciably influenced by the

surface treatment. Additionally, the wear resistance ofthe composite resin inlays was dependent on the lutingagents with which they were used.

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12. Leinfelder KF Mirshahidi M, Curj' C. Q'Neal W. An in vitrowear device for determming wear of posterior composites [ab-stract 636]. J Dent Res 1991:70:345.

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