Dental Research

Effect of a new resin inlay/onlay restorative material on cuspalreinforcementLuis M, P. Lopes* / Jorge G, M, Leitao** / William H, Douglas***

Nine maxillary premolars were restored with composite resin inlays involving large in-tracoronal cavity preparations. Buccal and lingual bonded strain gauges mea.swed thecuspal flexure under a carefully controlled application of occiusal force. The intacttooth was compared with the corresponding preparation and final restoration. The prep-aration itself greatly reduced the coronal rigidity, but this was completely recovered inthe restored tooth, within the functional force of 111 N. A stiffness ratio showed a 97%recovery. From the point of view of cu.spat strength, this may mean that larger intraco-ronal restorations are feasible with this type of re.'itoration. However, other factors, suchas ehairside time and complexity, and material properties, such as occiusal wear, have tobe taken into consideration. (Quintessence Int 1991,22:641-645.)

Introduction

Reduced resistance to fracture of prepared teeth hasbeen reported in the literature; the weakening of theteeth has been related directly to the extension of theprepared cavity,'"^ Materials, such as amalgam or goldinlay, that simply obturate the cavity preparation donot necessarily strengthen the remaining tooth struc-ture. Only with partial or full cuspal coverage is thestrength of the tooth increased.^ Measurement of cus-pal flexure with strain gauges showed that a bondedrestoration has the potential to recover tooth stiffnessequal to that of a sound tooth,' However, this recovery-does not apply when the preparations are large, ex-

* Lecturer, Dental Materials Deparltnent, Cidade Universitaria,School of Dentat Medicine, 1600 Lisbon, Portugal,

"" Associate Professor and Chairman, Dental Materials Depart-tnent, Cidade Universitaria,

*** Harvey L, Anderson Endowed Professor, Director of Bjonia-terials Research Center, Department of Oral Scietice, Univer-sity of Minnesota, School of Dentistry', 16-212 Moos Tower,515 Delaware Street SE, Minneapolis, Minnesota 55455,

Address all correspondence 10 Dr W, H. Douglas,

ceeding one third the intercuspa! distance. It isthought that in large restorations, an increased de-mand would be placed on the material properties ofthe restoration.^-''

Currently there is substantial interest in fabricatingcomposite resin inlays and onlays by the direct tech-nique. The physical and mechanical properties of thecured composite resin are further improved throughheat treatment in an oven. The restoration is hondedto the hard tissues after acid etching of enamel, useof a dentinal bonding resin, and fmal cementationwith a dual-curing cement.

The purpose of this study was to evaluate to whatdegree this indirect resin restoration method wouldcontribute to the stiffness recovery of the tooth whenlarge cavity preparations with weak eusps were in-volved.

Method and materiuls

The basic design of this experiment was an adaptationof the methodology previously described by Morinet a l '

Nine freshly extracted human maxillary premolarsfree of carious lesions and/or restorations, were se-lected. The teeth had been stored in deionized water

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at 4°C and were first inspected under a stereomiero-scope to exclude those with cracks in the enamel sur-faces. During the preparation and testing phases ofthe experiment, care was taken to prevent dehydra-tion.

Strain gauges (CEA-0^032 UW-I20, MeasurementsGroup) with lead wires soldered to them were trimmedto conform to lhe tooth contour. Gauges were thenbonded to the buccal and lingual surfaces just abovethe cementoenamel junctions using an acid-etchingtechnique. The site for bonding was etched with 37%orthophosphoric acid for 1 minute, rinsed with water,and dried with a stream of air. The backing of thegauge was cleaned with chloroform and primed witha catalyst before it was attached to the tooth withadhesive (M Bond 200, Measurements Group), Thegauge, the solder contact, and the immediate toothstructure were then covered with three layers ofScotchbond 2 (3M Dental Products Div}.

Two teeth with attached strain gauges were mountedin close proximity to each other in a nylon mountingring using clear acrylic resin. The teeth were mountedso that 2 mm of root surface was exposed. The leadwires were partially embedded in the resin to immo-bilize them. One tooth was used as the experimentaltooth and the other tooth served as a compensator forany strain resulting from temperature fluctuations.The gauges were eonnected to the strain measurementsystem (Measurements Group) with the strain readingdirectly proportional to the deformation of the toothstructure. The resin ring with the mounted teeth wasattached to the lower platen of a servohydraulic testingmachine (MTS 812, MTS Systems). A steel sphere,6.3 mm in diameter, was rigidly fastened to the uppermember of the MTS machine. The lower platen withthe teeth was brought into contact with the steelsphere, which served as a maxillary clement. Smallgrooves were cut into the buccal and linguai cusps witha flame-shaped bur to prevent lateral deflection of thesphere. The points of contact of the steel sphere onthe tooth were far enough up the cuspal incline toavoid any contact of the steel sphere with the subse-quent restoration.

The MTS machine was used in a load control con-figuration with a ramped loading of 37 N/sec for 3seconds and ramped unloading at rate of 37 N/sec for3 seconds. This provided a maximal load of 111 N. Aforce-strain curve was recorded in a MTS X-Y Re-corder (MTS Systems) from the buccal and lingualgauges prior to any alteration of the tooth and aftereach sequential procedure. Eaeh tooth was loaded and

unloaded five times prior to alteration of the toothand then provided with a mesio-occiusodistal (MOD)preparation and an indireet resin restoration. Ihestrain was measured up to lhe maximal load of 111 Nin each case.

Cavity preparation

Mesio-occlusodistal cavities were prepared on the nineexperimental teeth with a No. 703 diamond point ina high-speed handpiece with water spray. The widthof the isthmus of the occlusal preparation was onehalf the intercuspal width. The mean cavity depth was3.0 mm. The buccoiingual width of each proximal boxwas one half the width of the tooth. The boxes me-siodistally were made to place the gingivoaxial lineangle 2.0 mm axial to the cervical cavosurface margin,which was plaeed 1.0 mm above the cementoenameljunction. Walls were divergent between 10 and 20 de-grees and internal box forms were rounded. A longbevel of 1 mm was prepared with a G82 diamond onall cavosurface margins of the boxes. Occlusal marginshad butt joints. Maximal strain at the maximal loadfor the preparation was recorded in a stress-straincurve as described previously.

Dentin at the axial and pulpal floors was protectedwith glass-ionomer cement (Vitrebond, 3M DentaiProducts Div). The enamel was etched for 15 seconds,rinsed with water for 20 seconds, and dried thorough-ly. Scotchbond 2 primer and resin were placed ac-cording to the manufacturer's directions. Followingcuring, the Scotchbond 2 layer was wiped with analcohol swab to remove the oxygen-inhibited layer.

Inlay fabrication

The strain gauges and attached wires were coveredwith wax to prevent their damage during impressiontaking. Impressions were made using poly(vinyl si!-o.xane)material (Express, 3M Dental Products Div) ina metal tray. Impressions were poured up with epoxydie material (experimental, 3M Dental Products Div).After 4 minutes, the cast was removed, and minordefects were corrected with composite resin. A thin,uniform layer of die release (experimental, 3M DentalProducts Div) was applied, cautiously, to minimize thethiekness of ihis material on the margins. Followinga 2-minute wait, a restoration of P-50 composite resin(3M Denial Products Div) was plaeed und shaped.Placement was done in a series of small increments ofmaterial; each increment was light cured for 40 sec-

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onds (Visilux curing light, 3M Dental Products Div),The final occlusal cure was for 60 seconds. Fine dia-monds and Sofiex disks (3M Dental Products Div)were used to refine the marginal fit with the restorationon the die. After removal from the die, postcuring wascarried out at 120 ^C for 5 minutes in an oven (UnitekCorp),

Inlcn placement

The prepared cavity was rinsed with water and thesurface was swabbed with alcohol. Next, Scotchbond2 primer was applied for 30 seconds. Internal surfacesof the resin inlay were cleaned with alcohol. The res-toration was cemented with a tooth-colored, dual-cur-ing resin luting agent (experimental, 3M Dental Prod-ucts Div), Equal parts of paste A and paste B weremixed to obtain a utiiform consistency, A uniformcoating of the resin was applied to the restorationbefore it was placed into the prepared tooth. Excessmaterial was removed from the margins of the occlusalsurface with an explorer. Each area (buccal, lingual,interproximal, etc) was then light cured for 30 seconds.Final finishing of the tooth-rcsin interface was donewith conventional disks, fine diamond points, and ab-rasive strips.

As described previously, stress-strain curves weregenerated, and the strain was recorded with maximalstress of 111 N, To obtain maximal curing strength,the strain measurements were made 2 hours after ovencuring.

Results

The raw strain data for the buccal and lingual cuspsare presented in Table 1, The decision was made topresent the sum of the mean strains of the buccal andlingual cusps since small premature contacts of thesteel sphere could occur from condition to conditionin the same sample tooth. Furthermore, in some ofthe indirect resin restoration samples, both cusps de-formed in the same direction instead of opposite di-rections as would be expected {Figs 1 and 2), Themean strain of both cusps can be used to study thetotal tooth behavior in the different conditions withmore precision.

The strain data are also presented in Table 1 in theform of the mean relative deformafion (RD) and themean relative stiffness (RS), The sound tooth was giv-en the value of 1. In accordance with Morin et al,' the

Table I Strain values,* relative deformation, andrelative stiffness of the buccal and lingual cusps

Buccal cuspLingual cuspBuccal and

lingual cusps

Mean relativedeformation

Mean relativestiffness

Mean % stiffness

Soundtooth

76 (66)125 (66)

201 (76)

1,00

1,00 ( - )—

MOD Inlaypreparation preparation

297 (212)281 (222)

578 (226)

2,98

0.33 (0.11)—

94 (102)111 (98)

205 (93)

1,02

0,98 (0.28)97%

* In mcrostraÎD.Parentheses indicate standard deviation.

relative stiffness of the tooth cusps was calculatedfrom the following equation:

Max strain in sound toothRS = :—: ——,

Max strain in test condition

The relative deformation is simply the inverse of therelative stiffness:

R D -Max strain in test condition 1Max strain in sound tooth RS

Normalizing the data to the sound tooth facilitatescomparison of the effects of the different treatmentconditions by minimizing the experimental and struc-tural differences among the nine sample teeth,'

The other important parameter presented in Table 1is the stiffness ratio (C):

RS restored-RS MOD

RS sound tooth - RS MOD

Besides allowing the evaluation of the percent ofrecovery of stiffness of the restored tooth in compar-ison with the original sound tooth, this parameter hasthe additional advantage of being insensitive to vari-ations in tooth morphology and strain gauge orien-tation. It can be used to compare results from differentteeth,'

A one-way analysis of variance demonstrated thatthere was a highly significant difference in stiffnessamong the three conditions. Multirange testingshowed a significant difference (P < ,001) between thesound tooth and the MOD-prepared tooth; and

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MOD Prepared Tooth

Lingual CuspCusp

nicrostrain

Fig 1 f4^ Force-strain curve; fflj the buc-cal and lingual cusps flex in opposite di-rections.

Resin Inlay Restored Tooth

Fig 2 (A) Force-strain curve; (B) the buc-cal cusp of the inlay-restored tooth de-flects in the same direction as the lingualcusp.

between the MOD-prepared tooth and the indirectresin-restored tooth (P < .001). Between the soundtooth and the indirect resin-restored tooth, there wasno statistically significant difference.

Discussion

The large MOD preparation, as expected, left the re-maining cusps flexing outward to a greater degree un-der a given force, implying that the relative stiffnessof the tooth decreased substantially. In the large MODpreparations used in this study, the prepared tooth hadonly 33% of the stiffness of the original tooth. Con-versely, this means that the remaining cusps flexed

outward almost three times as much on average as thesound tooth at 111 N of occlnsal force. If cuspal flex-ure were sustained at this level, it could lead to earlierfatigue fracture of the tooth. It is for this reason thatthe study of tooth deformation is clinically significant.

What was surprising was the ability of the indirectresin restoration to recover the tooth stiffness to anamount not signiftcantly different from that of thesound tooth, up to a force of Ul N. This may beexplained not only by the high bond strength achievedbetween the restoration and the hard tissues, but alsoby the high degree of stiffness of the restorative ma-terial following the postcure procedures. It should benoted that 111 N is an acceptable functional occlusalload on a tooth.'""

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It appears that the ultimate crushing strength of atooth is unlikely to be reached in the oral cavity. Thetooth fracture mechanism seems to be, in most eases,irelated to fatigue. Weakened teeth with cusps that liesoutward to a greater degree under masticatory forcesdemonstrate high hysteresis (see Fig 1), because thetooth returns from its stressed slate to its original stateat slow rate. Consequently, energy Is lost to the tooth,leading to crack formation and propagation alongfailure planes and ultimately to clinical cusp fractureunder quite low biologic force.̂ "'' A bonded restorationlinking both remaining cusps together may extend thefatigue hfe of the toolh. This point is demonstratedindirectly in Fig 2, where the restored tooth is shownto be more rigid and to demonstrate less hysteresisthan the prepared tooth. This point has also beenconfirmed clinically by Hansen'- in endodonticallytreated teeth.

The indirect resin inlay restoration is at this timethe unique intracoronal restoration capable of restor-ing the initial stiffness at low biologic forces, even inlarge preparations. Based on this in vitro study, its usewould appear to be recommended where an entirelyintracoronal teehnique is required to restore large cav-ities with weak cusps.

There are other points to be considered. Althoughthe present report is entirely an in vitro study, thedentist has two options in restoring a tooth with anindirect composite resin tnlay. The technique may beentirely chairside, or the inlay may be fabricated inthe laboratory and delivered to the operatory. In theentirely chairside technique, there will be a learningcurve before the dentist reaches proficiency. At theoutset, more chairside time wtll be requtred to com-plete the restoration. However, these early constraintswill not apply if a laboratory option is chosen.

In addition to considerations of strength, the indi-rect technique makes some of the problems of com-posite resin restorations more manageable. Chiefamong these are the ability to establish better andmore functional interproximal contacts, and the op-portunity to complete macrofinishing before place-ment of the restoration. As with any restoration, thepros and cons are a question of balance, in which eachindividual clinician has to decide.

References

1. Vale WA: Cavity preparation, triah Denl Rev I956;2:33^1.2. Vale WA: Cavity preparation and further thoughts on high

speed. Sr Denl J 1<I59; 107:333-346.3. Mondelli J, Steagall L, Ishikiriama A, et al: Fractun; strength

of human teeth with cavity preparations / Proslhet Dent19S0;43:419-422.

4. Larson TD, Douglas WH, Geistfeld RE: Effeci of preparedcavities on the strength of teeth. Oper Dent 1981:6:2-5.

5. Blaser PK, Lund MR, Coehran MA: Effects of designs of ClassII preparations on resistance ol' tooth to fracttire. Oper Dent1983;8:6-lü.

6. Reeh ES, Messer HH, Douglas WH: Reduction in tooth stiff-ness as a result of endodontic and restorative procedures.

7. Morin D, DeLong R, Douglas WH: Cusp reinforcement by theacid-etch technique, J Dent Res «84:63:1075-1078.

8. Morin D, Douglas WH, Cross M, et al: Biophysical stress anal-ysis of restored teeth: experimental strain measurements. DentMater 1988;4:41-4S.

9. Bell JG, Smith MC, de Pont JJ: Cuspal failures of MOD re-stored teeth. Ausi Dent J 1982;5:283-287.

10. De Long R, Douglas WH: Development of an artificial oralenvironnient for the testing of dentai restoratives: biaxial forceand movement control. J Dent Res 1983:62:32-36,

11. DeLong R, Douglas WH: An artificial oral environment fortesting dental materials. ¡EEE Trans Biomed Eng Dent 1991;38:334-338.

12. Hansen EK: In vivo cusp fracture of endodontically treatedpremolars restored with MOD amalgam or MOD resin fillingsDent Mater 1988:4:169-173. •

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