Dental Research

Experimental investigation of the shear strengths of teeth in the regionof the dentinoenamei junction

Thomas Pioch*/Hans Jörg Staehle**

Abstract The shear strengths ofhuman incisors, canines, premolars, and molars and bovineincisors in the region ofthe dentinoenamei junction were tested. The median sliearstrength of all human teeth, 38.99 MPa, was not .significantly different fmm that ofbovine teeth, 37.40 MPa. Among the groups of human teet¡i, the highest medianshear strength was obtained from mandibular premolars (46.15 MPa), and thelowest from mandibular canines (32.63 MPa). Physical treatments (cooling ordrying) led to a significant reduction in shear strengths. A similar effect was foundafter exposure to amlno butyrate (a caries-removing agent) but not after exposure tosodium hypochlorite (used in endodontic treatment), ¡n all Investigated groups ofteeth, the fracture areas were mainly in dentin and never exactly at thedentinoenamei junction; Indicating that the cohesion of dentin, not the adhesionbetween dentin and enamel, is tlie limiting factor in the shear strength.(Quintessence Int ¡996;27:7¡¡-7¡4.)

Clinical relevance

The development of dentinal bonding systems istheoretically limited by the cohesive strength ofdentin, which may be reduced by different physicalor chemical treatments.

Introduction

The adhesive strength between enamel and dentaltnaterials has been found to be up to 20 MPa when theshear test is used. With commerciaUy available dentinalbonding systetns, sitnilar shear bond values can befound under experimental conditions,' The cohesivestrength of untreated dentin has been foimd to besomewhere in the region of 36 MPa,^ In the literature,

* Clinical Assistant, Physicist, Department of Restorative Dentistr>',University of Heidelberg, Heidelberg, Germany,

" Professur and Director of the Department of Restorative Dentistry,University of Heidelberg, Heidelberg. Germany,

Reprint requests: Dr Thomas Pioch, Poliklinik für Zahnerhaltungs-ktmde, University of Heidelberg, Im Neuenheimer Feld 400, 69120Heidelberg, Germany,

it is reported that cohesive fractures can be observedwhen new dentinal bonding systems are tested andfound to have values of around 20 MPa.^'' However, ithas not yet been clarified whether this is the result ofconditioning of the tooth surface and/or due todifferent aspects, such as over-drying of the dentin.

In this context, how does the bond strength ofthenatural adhesive zone between dentin and enamelcompare with the cohesive strength of untreated dentinin the region ofthe dentinoenamei junction? Further-more, it would be useful to investigate whether thereare differences in cohesive strengths between dentinfound in teeth from different locations of the mouth.

For testing the cohesive strength of dentin, so-calledpunch tests have been performed, '̂̂ Through thismethod, cohesive strengths of 68 to 147 MPa havebeen found, depending on the distance from the pulp;however, the shear-testing method is far more popular,Gwinnett-foundameanvalueof36.18 ± 6.81 MPa byusing a special shear strength-testing method.

The first aim of this study was to determine the shearstrengths of tooth substances in the dentinoenameiregion from different groups of teeth. The other goalwas to test the effects of physical and chemicaltreatments on shear strength values.

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DENTIN

LU

2.0 mm

lA

FORCE

Fig 1 (Left) Conditions for lesling the shear bond '^"^^strengths of Ihe enamel cylinders '^^ ,

Fig 2 Scanning eiectron microscopic ¡mage of a siiearedcylinder The fracfured area is al fhe top,

Fig 3 Scanning eiedron microscopic image of Ihe frac-fure area. Typicaliy, iaiiure occurs cohesively in dentinrather than in enamel or at fhe denfinoenamel junction.

Method and materials

Investigations were carried out on 120 extractedhuman teeth and 105 extracted bovine teeth. Acylinder-shaped diamond borer with constant watercooling was used to remove a wide area of labialenamel around a marked circle (diameter of 2.0 mm).The result of this procedure was the creation of anenamel cylinder with its axis perpendicular to thedentinoenamel junction. During preparation, the dia-meter was continuously controlled with a slide gauge.This limited the divergence to a maximum value of

2.0 + 0.1 mm. The preparation stopped shortlybeyond the dentinoenamel junction.

The 120 human teeth were divided into 60 maxillaryand 60 mandibular teeth. In each group, there were 15incisors, 15 canines, 15 premolars, and 15 molars.

The bovine teeth were divided into groups ( 15 each)and treated physically or chemically (see Table 2). Onechemical agent used was Caridex solution (aminobutyrate) (Caridex system, Dental Products). TheCaridex treatment was carried out by dipping thewhole specimen into the liquid for a period of either 30or 60 minutes.

After each of the groups had been individuallytreated, the teeth were exposed to water for 24 hoursand fixed in the material-testing apparatus. Thecohesive strengths of the enamel cylinders weremeasured in a shear experiment with a defined speedof 0.75 mm/min (Fig 1).

Statistical evaluations were performed with thenonparametric U test. The sheared cylinders weremounted on specimen holders that are conventionallyused in scanning electron microscopy (SEM), andthen they were coated with gold. With the help of SEMpictures, the failure modes of the fractured surfaceswere analyzed (Figs 2 and 3).

Results

The results from testing of human teeth are shown inTable 1. The only statistically significant difference( /*= .05 ) was found between maxillary and mandibular

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incisors. The cobesive strengths in the region of thedentinoenamel junction of the mandibuiar incisorswere higher than values for the maxillary incisors.There was no statistically significant difference amongincisors, canines, premolars, and moiars.

The mean cohesive strength for all human teeth was39.53 ± 8.90 MPa, This value proved not to bestatistically significantly different from the value foruntreated bovine teeth (36,49 + 7,67 MPa),

The physical and chemical treatments performed onthe bovine teeth led to a significant reduction in shearstrength, except in the case of exposure to liquidnitrogen for 3 minutes or sodium hypochlorite (NaOCl )for 1 hour (Table 2),

Scanning electron microscopic investigation showedthat in 87% ofthe human specimens and in 80% ofthebovine specimens, fractures were located exclusively indentin. In 16% of specimens, fractures were observedin both enamel and dentin. There was no incidence ofÖBCture directly coincident with tbe dentinoenameljunction.

Discussion

The SEM investigation showed that fractures appearedmainly in dentin. This indicates that it was not theadhesion between enamel and dentin that was mea-sured, but the cohesive strength ofthe dentin close tothe enamel junction. With this experimental method,we had to work on the assumption that the notchtension was not homogeneous, because there was theadditional effect of a certahi amount of lateral pres-sure.' This explains the modes of fracture observed,which probably resulted from the testing methodchosen and not from the treatment ofthe teeth. Using asimilar method for testing the cohesive strength ofdentin, Gwinnett- obtained vaines of 36,18 + 6,81MPa from human molars. In tbe present study, similarmeanvaiues of 38,63 ± 8,03 MPa were obtained fromhuman molars.

There is no direct clinical evidence to support theidea that maxiliary incisors have a lower stability thando mandibular incisors. This notion, however, cor-relates with observations by Drake,^ who foiind ahigher rate of failure among restorations in maxillaryincisors than among those in mandibular incisors. Inthe ease of molars, this divergence does not exist.Among other factors, such as size, existing dentinaldamage, type of dentin, location within the mouth,exposure to saliva, functional load, esthetics, and soon, the cohesive strength of dentin can also influencethe quality of restorations.

Table ! Median shear strength of human teeth in theregion ofthe dentinoenamel junction

Teeth

Mandibuiar incisorsMaxillary incisorsMandibular caninesMaxillary caninesMandibular premolarsMaxillary premolarsMandibular molarsMaxillary molars

n

1515151515151515

Median shearstrength (MPa)

44.56*35,01*32,6342,9746,1538,9939-7938,20

* Statistically significantly differenl .05).

Table 2 Median shear strength of bovine teeth in theregion of the dentinoenamel junction

Treatment group

None (control group)Cooling in liquid nitrogen

for 1 minCooling in liquid nitrogen

for 3 minDrying in a vacuum

(20 hPa) for 6 dExposure to NaOC!

(5%) for 60 minExposure to Caridex

for 30 minExposure to Caridex

for 60 min

n

15

15

15

15

15

15

15

Median shearstrength (MPa)

37,40

27,06*

26,66

29,92*

39-79

22-28*

27-85*

• statistically significantly different from the control group {P= ,05),

In many investigations, an analogy between humanteeth and bovine teeth is presumed. In tbe case of shearstrengths, this presnmption can be confirmed, becausethere were no statistically significant differences- Thereduction of shear bond strength after coohng in liquidnitrogen is probably a result of tbe different linearcoefficients of thermal expansion of enamel (11 xlO-^K) and dentin (8 x lO'^K),^ Temperaturegradients probably lead to the setting up of mechanicalforces within the tooth substance, resulting in a ioss ofstrength.

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Drying of tooth substance causes dimensionalchanges,'" which are more exaggerated in dentin thanin enamel. Therefore, mechanical forces are likely to becreated, especially at the dentinoenamel junction. Aloss of strength resulting from these drying effects thatis similar to the cooling effects already described mayappear. The dr>'ing and cooling extremes used in thisstudy would not occur during clinical treatment pro-cedures, but the results indicate that physical treat-ments can change the properties of dentin. Further-more, for the correct interpretation of experimentalinvestigations, it is important to be aware of theseeffects to prevent artificial drying.

The Caridex caries-removal system is a procedure ofcaries excavation based on a che mo mechanical meth-od developed by Goldman and Kronman. ' ' The activesubstance of the Caridex solution is a 0.08% N-monochlor-dl-2-amino buiyrate solution with a pHvalue of 10.5 to 11,4.'-•'•' This substance destroys thecollagen fibers in dentin. Collagen fibers are reportedto be responsible for the physical adhesion betweenenamel and dentin.''' Therefore, the destruction ofcollagen by the Caridex system may decrease both thebonding between enamel and dentin and the cohesivestrength of dentin, which contains about 30% collagenfibers.'^ The reduction in shear bond strength ob-served after application of the Caridex solution wasprobably a result of the destruction of collagen fibers;however, evaluation of the fracture modes revealed novisible differences between the Caridex-treated spec-imens and the control group.

Sodium hypochlorite is used for chemomechanicalendodontic treatment. The results of this study showedthat the application of NaOC 1 did not change theshear strength value under the given conditions. Boththe concentration of the NaOC 1 and the exposuretime tised were higher than are used in clinicalsituations. Therefore, the use of NaOCl most likelydoes not reduce the stability of dentin after endodontictreatment.

Different physical and chemical treatments canchange the biophysical properties of dentin. The shearstrength of the adhesive bond between dentin and adental material is about 11 to 18 MPa,' clearly lessthan the cohesive strength of dentin, about 36 MPa,-or 39 MPa'* close to the enamel junction. Therefore,

the development of dentinal bonding systems istheoretically limited by the cohesive strength of dentin;however, the existing bonding systems do not approachthis limit. Further studies should clariiy whether thecomponents of denrina! bonding systems are likely tochange the physical properties, especially the cohesivestrength, of dentin.

References

1. Perdigao J, Swift EJ. Denehy GE, Wefei JS, Doniy KJ. In vitro bondstrength and SEM evaluation of dentin bondinK syslems to differentdentin substrates. J Dent Res 1994;73:44-55

2. Gwinnett AJ. A new method to test the cohesive strength of dentin.Quintessence Itil Í994:25;215-218.

3. Eick JD, Robinson SJ, Chappell RP. Cobb CM, Spencer P. Thedentinal burface: Its influence on dentinal adbesion. Part Hi.Quintessence Im 1993;;4:;7I-582.

4. Swift El, Perdigaol, Heymann HO. Bonding to enamel and dentin:A brief hÍ5tor>' and state of the an. 1995. Quinlessence Inl1995;25 95-110,

5. Carter JM. Sorensen SE, Johnson RR, Teiteibaum RL. Levine MS.Punch shear testing of extracted vital and endodontically trealedteeth. J Biomech 1983:16:841-848.

6. Roydhotise R. Punch shear test for dental purposes. J Dent Res1970:49:131-136.

7. Van Noort R, Norooii S. Howard [C. Catdew G. A critique ofbondstrength measurements. J Dent 1989;17:61-67.

S. Drake CW. A comparison of restoration longevity in maxillary andmandibular teeth. J Atn Dent Assoc 1988:116:651-654.

9. Dickson G. Barton J. Anisotropie and Time Dependent Variation inthe Thermal Expansion of Dentine. Report 10889. National Bureauof Standards, 1972.

10. Ten Cate JM, Nyvad B, van de Pia ssc he. Simon s YM, FejerskovO. Aquantitative analysis of mineral loss and shrinkage of in vitrode mineralized human root surfaces. J Dent Res 1991:70:1371-1374.

11. Goldman M, Kronman JH. A preliminary report on a cheinûme-chanicalmeansofremovingcanes.J Am Dent Assoc 1976;93:1149.

12. Kronman JH, Goldman M. Habib CM, Mengel L. Electronmicroscopic evaluation of altered collagen structure after treatmentwith N-monochloro-DL 2 aminobutyrate (GK 101 E), J Dent Res1979i58:1914,

13. Scheul^el P. Möglichkeiten und Grenzen des Caridex-Systems alsAlternative zur herkömmlichen Kariesentfemung. Dtsch ZahnärztlZ Í989;44;6I2-514.

14. Janda R. Die Schmelz/Dentin-Grenze—eine natürliche Verbund-ione. Phillip J 1991:8:57-64.

15. Schroeder HE. Orale Strtikturbiologie. Stuttgart: Thieme. 1992.

16. Pioch T, Golfeis D. Staehle HJ. An experimental study of thestability of irradiated teeth in the region of the dentinoenameljunction. Endod Dent Traumatol 1992^8:341-244. D

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