36 Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001, 36-40.

ENAMELHARDNESS AND CARIESSUSCEPTIBILITY IN HUMAN TEETH,

M. P. Gutíérrez-Salazar' and J. Reyes-Gasga'

1.Facultad de Odontología, UNAM Circuito de la Investigación Científica s/n.Cd. Universitaria, 04510 Coyoacán,México.

2. Instituto de Física, UNAM Apartado Postal 20-364, 01000 México D.F, México

Abstraet.

This paper presents a comparative study of enamel hardness in teeth ofhuman beings with different susceptibility to cariesdisease. Six orthodontic patients were classified as high or low caries risk, on the basis of caries experience. Then by takingtransversal ground sections of premolar from classified patients hardness measurements were made. The hardness valuesfound for sound enamel VHN 268 - 375 are in agreementwith published data. The lower hardness values corresponded to highcaries risk patients. The mean ofVHN between the high and low caries risk groups differ significantly. The differences found inthis study show a relationship of hardness enamel with high susceptibility to caries disease.

Keywords. Enamel, microhardness, caries susceptibility.

Resumen.

Se presentan los resultados de un estudio comparativo sobre la dureza del esmalte entre sujetos con diferente susceptibilidada la caries dental. Se emplearon seis premolares, extraídos por razones ortodoncicas, de pacientes clasificados como de alto obajo riesgo, en base a la experiencia de caries que presentaron. Las mediciones de dureza se hicieron sobre cortes transversalespulidos. Los valores de dureza Vickers para esmalte sano, obtenidos en este estudio, concuerdan con otros reportados en laliteratura. Los sujetos clasificados como de alto riesgo a la caries presentaron los valores de dureza más bajos, comparados con .los sujetos de bajo riesgo. Las diferencias en los valores de dureza fueron altamente significativas. Los resultados muestran unaasociación de la dureza del esmalte con la mayor susceptibilidad a caries.

Palabras Clave: Enamel, microdureza, suceptibilidad a las caries.

1. Introductión.

K.nowledge of the mechanical properties.of dental tissueis important to understand how mastication strain is distrib-uted throughout a tooth, and for predicting how stress andstrain are altered by dental restorative procedures, age andmainly by caries disease.

Between the mechanical properties the hardness of enamelis the most widely studied and has greater theoretical sup-port. The enamel hardness values have been reported eitherby K.noop hardness number (K.HN) or Vicker hardness num-ber (VHN), depending on the method used. However, thevalues obtained with both methods showed no importantdifferences, according to Riges study [1). In Craigs study[2] enamel hardness values were K.HN 292 - 390. Collys [3]reported a value ofK.HN 354.9, Stephen [4] found a valueK.HN 27l. Graspersic [5], in occlusal enamel, found VHN

359.5 -424.3. Ryge [lll] VHN254-348. Reyes-Gasga [6]reported a range ofVHN 254 - 383.

Microhardness indentation provides a relatively simple,nondestructive and rapid method for classification of mate-rials and for comparative studies of their properties. How-ever the measurement can be affected by different factors.The large standard deviations in hardness value reported inearly studies [2] show that very different values can be ob-tained in distinct sections of a tooth. In a recent enamelhardness study [6] it was found that factors as sample prepa-ration, indenter position, and reading error in indentationlength can influence the hardness measurements. Then it isimportant to control these factors to obtain reliably mea-surements. Other important factor is the selection of the areafor hardness test. According to literature [6] in transversal

M P Gutiérrez and J R. Gasga /Revista Latinoamericana de Metalurgia y Materiales 37

sections well shaped indentations are obtained more easilythan along the length sections.

In dental research, microhardness indentation measure-ments have been employed inthe study of'reminerakzationphenomena [4]. Eeatherstone [8] reporten a relationship ofenamel hardness values with mineral content ofthe tissue ina weight basis. More recently, Attin [9] found a significantcorrelation between initial enameL hardness and abrasiondegree. Moreover, the hardness can influence the caries sus-ceptibility because of the exposition of enamel to environ-ment oral factors.

Caries is a multifactorial disease and enamel representsan important factor in the caries risk condition.

Table 1.Caries experience in a sample ofpatients.

Dental caries is a dynamic. de-or remineralizing processresulting from microbial metabolism on the tooth surfacewhich may result in a net loss of mineral s, and subsequentlycavitatien.j l O] The probability te reach cavitation level inlesions is variable fór people, then low and high caries canbe observed in the population. The high caries risk personsshow a very fast process.

In this study we investigate the enamel hardness in teethof human beings with different susceptibility to caries dis-ease. The indentation lengths were measured on SEM im-ages for least error in measurements.

Patient Age Caries Experience Ri sk qualifi cationDecay Missing Filled DMFT

l:MR 13 O O O O Low susceptible2DS 13 O O O O Low susceptible3 :MM: 14 3 O 6 9 High susceptible4LP 14 12 O O 12 High susceptible

5DC 24 O O 8 8 Low sus ceptible6BB 24 O 3 8 11 High susceptible

2. Method.

The study selected six premolars extracted from six orth-odontic patients who had been chosen on the basis of theircaries experience as low or high caries forming. Four .of thepatients were 13 - 14 and two 24 years old. Caries experiencewas.recorded and the magnitude of caries les ion was mea-sured according to known criteria [11].

Premolars in younger patients of both groups presentedno caries lesions. The teeth from adults patients in low andhigh caries risk groups were filled, but the hardness testswere made in healthy enamel on a transversal section of thebuccal cusp.

The premolars were embedded in acrylic resin, then weresectioned parallel to the oclusal surface. The first cut just below the buccal cusp was used for hardness tests. On trans-versal ground sections from each tooth fifteen hardnessmeasurement were made. A Matzuzawa microhardness-tester

MHT2 with a Vickers diamond at a load of25 g, for 15 s. wasused. The indentation was observed and analyzed usingboth a light microscope (LM) and scanning electron micro-scope (SEM). The criteria for accepting a correct indenta-tion were sharpness of diagonal edges, uniformity of diago-nal shape(geometry) and freedom of intérference from ir-regularities in the testing area [1]. The/¡ndentations werenevér closer to the edges of the specimen or to anotherindentation of less than two times the length of the diago-nal. The measurements, were done only by one observer.The reliability in reading indentation length was Kappa 0.90.Then indentation length (IL) and Vickers hardness number(VHN) were obtained.

The statistic analysis was done using the SPSS statisti-cal programo One way analysis of variance ANOVA andScheffe post hoc test were employed to test the

38 Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001.

Fig. l. Well-shaped indentation on transversal section.

Fig. 2. Indentationwith irregular diagonal.

The structural features of tested surface were observedby SEM. Phosphoric acid was used during 60 seconds inorder to develop the prism structure.

The chemical composition of enamel was analysed forpatients 1 and 4, using the EDS method.

3. ResultsIn this study only well-shaped indentations were consid-

ered and special attention was given to sampIe preparationand reading error ofthe indentation length. Figure 1 shows awell-shaped indentation. Because ofthe structural featuresof the ename1 tissue, no well-shaped indentations can be

"óbtained. Ano well-shaped indentation is showed in Figure2. The indentation can be easi1y observed by SEM and im-mediately accepted o rejected according with the shape theyshowed.

Caries experience of selected patients is presented in table1. Younger patients classified as low risk (patients 1,2) donot show clinically carious lesions. The adult patient (pa-tient 5) presented eight filled teeth, with no teeth loss orbigger fills. The fills only covered occlusal grave, becausethe caries lesions presented before the dental treatmentmust not have been extensive. Figure 3 shows the longitudi-nal section of tooth from this patient, the fill includes onlyenamel tissue.This patient did not have fast caries process.The oral condition is very different for the adult patient inthe high caries risk graup. She had loss the three first molarsand had eight filled teeth.

The younger patients in this group (patients 3, 4) pre-sented 9 and 12 affected teeth. For patient 3, three decayteeth and six teeth were filled, in two ofthese, the fill coveredtwo thirds of the crown. Patient 4 presented 12 decayedteeth; all these affecting enamel and dentin, and four withextended lesions.

Fig. 3. SEM image oflongitudinal section oftooth from patient5. The fill include only enamel tissue. D dentin, E enamel, F fill.

Table 2 presents the results of the hardness tests for sixsamples from patients. The mean and standard deviation forboth indentation length and Vickers hardness number areindicated. The lower hardness values were for high cariesrisk patients, the mean ofVHN between the groups differingsignificantly (p < .0001).

The hardness is a very sensitive scale. In table 2 for val-ues of IL the standard deviations are < 1fl and these are inthe hardness value up to 34 units. The difference of IL forpatients 1 and 5 is 0.80 u, (11.93 - 11.13), and for hardnessvalues the difference is so great (VHN 375.93_and 327.20respectively) that it reaches statistic significance (p< 0.1).

M P Gutiérrez and J R. Gasga /Revista Latinoamericana de Metalurgia y Materiales 39

Table2.HardnessEnamelValuesIndentationLength(IL)andVickersHardness Number (VHN) for the patiet s samples studied.

Patient Mean IL (SD) Mean VHN (SD)

lMR 11.93 (0.56) 327.20 (29.77)2DS 11.97 (0.40) 324.67 (2l.24)3MM 12.83 (0.72) 283.07 (33.09)4LP 13.03 (0.35) 273.27 (15.07)

5DC 11.13 (0.52) 375.93 (34.36)_ 6BB 13.18 (0.70) 268.07 (27.64)

Table 3 shows the homogeneous subset identified by thepost hoc Scheffe test. The mean average for the three highcaries risk patients is VHN 269.07,273.27 and 285.40 andthey do not differ significantly. On the other hand, the hard-ness values in the adult patient (patient 5) for the low riskgroup was specially high (VHN 375.93). It was significantlydifferent than the hardness values for younger patients ofthe same group: VHN 324.67 and 327.20 respectively.

Table 3. Homogeneous subset for VickersHardness Values

PatientSubset for alpha = .05

1 2 36 269.074 273.273 285.40

.2 324.671 327.20

5 375.20

Sigo .762 1.00 1.00

Scheffe test

The SEM study of the structure of specimens surfaceshowed differences in the demineralising pattems. The teethfrom low risk patients presented preferential etchingeof theprism boundary while the specimens from high risk showedpreferential etching ofthe prism coreo Thefigures 4 and 5show the pattem for low and high riskrespectively,

The EDS analysis was done in patient 1 and 4, the chemi-cal composition shows differences mainly inthe sodium andmagnesium content. The figure 6 present the values forthese elements from enamel surface to amelo dentinaljuntion(ADJ).

Fig. 4. Prisms pattern in low caries risk patient.

Fig. 5. SEM image of prisms pattem in high caries risk patients.

l.2r-------------------,

0,8

-+- la Pac. ....•....la Pac 1

o,.

• (1,99

0,38.,(1.21

•• ----------*,0,21 • 0,21.-(1,19 (J 16 - 0,18-O,IJ9--------- '

0,2

(J+-----~r_----~-----~2

Fig. 6. Mean distribution of sodium and magnesium from enamelouter surface (1) to ADJ (3) in samples from patients 1 y 4

40 Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001.

4. Discussion

The hardness values found for sound enamel VHN 268 -375, are in agreementwith earlier published data [1--'"6]. Be-cause the special features of the hardness scale, careful useof measurements., is necessary. So in this way realible hard-ness values were obtained and differences in hardness valuereflect differences in the properties being studied.

In the younger patients, the comparison was made insound, no filled teeth for high and low caries risk categories.The age of these patients were similar whilethe differencesin their oral condition must be attributed to faster caries-pro-cess in the high risk group. The results ofthis study showedconsistently higher hardness values for low risk patients,classified on base of caries experience. The hardness valuesfor these patients were significantly higher than those forhigh caries risk group (Table2).Hardness is a property of enamel that can influence .caries.susceptibility because.the enamel.is exposed.to differentfactors.

Attin [9] used bovine enamel and found a significantcorrelation, within all experimental groups, between initialhardness and abrasion degree. He concluded that suscepti-bility of eroded enamel to toothbrushing abrasion may de-pend on the hardness ofthe enamel ofthe particular speci-men, because abrasion increases with decreasing hardnessvalue.

The main feature found in the high caries risk individualsis the faster process of caries lesions, and that many factorsare involved in this more susceptible condition. Anumber ofresults on salivary, microbiological and behavioral factorshave been obtained but less information on enamel.proper-ties regarding high caries risk condition was found.

The different reactions ofthe core prism and prism bound-ary to acid treatrnent, found in this study, favor the view thatdissimilar physical and/or chemical properties can beenpresent that affect the hardness value.

The differences in chemical composition may be alsoimportant factors. It is important that the distribution of so-dium and magnesium were similar for both patients, but thevalues for these elements, were different for low and highcaries risk patients.Among the patients selected on the basis of caries experi-ence the more susceptible individuals showed consistentlysignificantly lower hardness values (Table 2).The selection .ofhomogeneous subset (Table 3) shows the relationshipofhardness enamel with caries experience. The differencesfound in this study show an association of'lowenamel hard-ness with high susceptibility to caries disease. .The differences in hardness ofenamel can be associated tothe mineral content in enamel [8,12,13]. Kodaka [7] foundthat small amounts of organic substances at sites of similarmineral content can have a strong influence on themicrohardness values. Then, the presence of organic sub-

stances is another possible factor.More studies are necessary to know exact1y the factors

that determine the differences in enamel hardness.

Acknowledgements.We acknowledge the technica1 he1p from R. Trejo, J.

Cañetas, R.Hemández, P.Mexia. C. Flores, M. Aguilar, C.Zorrilla, S. Tehuacanero, L.Rendón. We thank the economi-cal support from CONACYT (Project 31695-N) andDGAPAUNAM (project IN-l 03700).

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