The effects of acid-etching on enamel from differentclinical variants of amelogenesis imperfecta:an SEM studyW. Kim Seow MDSc, DDSc, PhD, FRACDS A. Amaratunge RDS, PhD

AbstractPurpose: Successful bonding of resins to teeth affected

by amelogenesis imperfecta (AI) may be highly de-pendent on how the enamel responds to acid etching. Theaim of this study was to determine, using scanning elec-tron microscopy (SEM), the types of etching patternachieved with 37% phosphoric acid on dental enamel of5 clinical variants of AI, namely, pitted hypoplastic,smooth hypoplastic, X-linked (male), X-linked (female),and hypomineralized.

Methods: A normal premolar and primary molar3Ckomtwo healthy patients were used as controls. The enamel wasscanned before and after acid etching for 1 min. In thenormal, control teeth, the three classical etching patternswere produced: type 1, in which the prism cores are pref-erentially removed; type 2, in which the prism peripher-ies are removed, and type 3 in which the removal of enameldoes not relate to prism structure.

Results: In the normal primary molar, patterns of types2 and 3 were generallyproduced. In the AI teeth, the ef-fects of acid etching reflected the clinical variant ofAI. Allthree etch patterns were observed in the enamel surround-ing the pits in the pitted type of AI and in the bands ofnormal enamel in the female with X-linked AL as well asin the hypomineralized variant. In contrast, no typical etchpatterns could be detected in the enamel j~om the malepatient with X-linked variant, as well ashore the enamelaffected by the smooth hypoplastic variant.

Condusions: The lack of typical etchingpatterns in thesevariants may be the result of abnormal prism structure, orthe standard etching time and~or acid concentration maybe inappropriate for the abnormal enamel. The results ofthis study may have useful applications in the restoration ofteeth affected by AL (Pediatr Dent 20:1 37-42, 1998)

Aacid etching of enamel to create a surface suitble for bonding resins is now a cornerstoneechnique of clinical dentistry. Phosphoric acid

at a concentration of 37% is now the most widely usedacid in bonding techniques for esthetic restorations,

fissure sealants, and splinting of teeth. 1_4 Acid etchingof enamel results in many surface irregularities, whichgreatly increases the surface area for mechanical bond-ing, as well as its wetability, which enhances the flowof resin. ~4

The distinct morphological surface changes causedby acid etching have been clearly demonstrated bySEM.4 In general, the surface modifications may beclassified into three types of etching patterns.4 In thetype 1 pattern, a honeycomb appearance is producedby prism core material being preferentially removed,leaving the prism peripherally intact. In the type 2pattern, a cobblestone effect is produced by the pe-ripheral regions of the prisms being dissolved pref-erentially, leaving the prism cores intact. In the type3 pattern, surface loss occurs without exposing theunderlying enamel prisms. Although the relative sig-nificance of the etching patterns to bond strength isstill unclear, it is likely that all three types contrib-ute to clinical bonding on any one surface.1

While the etching patterns produced in normalenamel have been well described, little is known re-garding the changes produced in abnormal enamel,and in particular, for developmental abnormalitiessuch as amelogenesis imperfecta (AI). M is a heterog-enous group of inherited conditions affecting enamel,and shows prevalence rates ranging from 1:4000 to1:14,000.5-7 As the diagnosis and classification of Mbased on molecular criteria8-1° are yet to be determined,diagnosis of AI is usually based on clinical and inher-itance criteria.’~-J3 Witkop7 suggested four broad cat-egories of AI in his classification, namely, hypoplastic,hypocalcified, hypomaturation, and hypomaturation!hypoplastic with taurodontism, and included severalsubtypes dependant on inheritance patterns. In clini-cal practice, however, the lack of clear familial histo-ries in the majority of patients often prevents definitivediagnosis of affected individuals into a particular sub-type. Furthermore, extensive overlap of clinical pre-

Pediatric Dentistry - 20.’1, 1998 American Academy of Pediatric Dentistry 37

sentations of the hypocalcified and hypomaturationtypes often make it extremely difficult to distinguishclinically between these two types of Al. Hence, forpractical purposes, three broad clinical variants are gen-erally recognized, namely, 1) hypoplastic, in which theenamel is deficient in quantity, 2) hypomineralized/hypomaturation in which mineralization/maturationof enamel is altered, and 3) X-linked, in which femalesclassically display vertical striping of enamel, and malesshow an even loss of enamel.

Because a significant complication of Al is pooresthetics, an important aspect of clinical managementis improvement of the appearance using bonded res-ins.12'15 However, in many patients with Al, adhesiverestorations may show high failure rates that are asso-ciated with inadequate bonding between the restora-tions and enamel. This area of restorative dentistry forAl has not been well investigated. As the bond betweenenamel and restoration is highly dependent on theenamel surface changes after acid etching, the exami-nation of these surfaces may provide clues as to thepotential success of the acid etch technique on thedifferent types of enamel surfaces in Al variants. Theaim of this investigation was to use SEM to examinethe effects of acid-etching on teeth from patients withdifferent variants of Al compared with normal teeth.

MethodsNaturally exfoliated primary teeth, as well as pre-

molar teeth extracted for orthodontic reasons, were do-nated by patients. All the Al patients had beenpreviously diagnosed by Dr. Seow using clinical and ra-diographic criteria.12'" The teeth had been kept dry bythe patients until the time of study. One tooth from apatient of each of the following clinical variants of Alwas randomly selected for study: 1) pitted hypoplastic,2) rough hypoplastic, 3) X-linked hypoplastic (female),4) X-linked hypoplastic (male), and 5) hypomineralized.A total of five Al-affected teeth and two normal con-trol teeth were used for the study.

The teeth were cleaned in an ultrasonic bath con-taining 0.5% sodium hypochlorite solution for 30 min

to remove surface debris, and dried with compressedair. Half the facial surface of each tooth was maskedwith masking tape and the unmasked half treated with37% phosphoric acid for 1 min, thoroughly washedwith distilled water for another minute, and dried withcompressed air. The teeth were then vacuum coatedwith 50 mm of silver and prepared for SEM in aPhillips 505 scanning electron microscope (PhillipsElectronic Eindhoven, Netherlands).

ResultsControl Teeth

A premolar extracted for orthodontic reasons froma healthy female was used as a control tooth for theAl-affected premolar. A naturally exfoliated mandibu-lar primary first molar from a healthy male was usedas a comparison for the Al-affected primary teeth.

Normal maxillary premolar-untreatedLow-power SEM showed a generally uniform sur-

face with distinct perikymata showing as wave-likehorizontal bands. Higher magnification showed thesurface to be covered with many shallow depressionsand a few micropits.

Normal maxillary premolar-acid etchedAfter treatment with acid, the etched area gener-

ally showed a type 1 pattern with the prism corespreferentially removed (Fig 1). However, in small,isolated areas the etching pattern was similar to thatof type 2, i.e., prism peripheries were preferentiallyremoved. A type 3 etching pattern (general removalof tooth structure without exposing prism structure)was also observed in other isolated areas.

Normal primary mandibular molar-untreatedRounded enamel rod endings were faintly visible

on the surface of the normal primary molar, and gaveit a uniformly speckled appearance. Apart from a fewscratches, the untreated enamel surface was generallyunremarkable.

Normal primary mandibular molar-acid etchedAcid etching generally produced a type 2 pattern.

4 Fig 1. Facial surface of normal premolarafter treatment with phosphoric acid.Type 1 etch pattern in which the prismcores were preferentially removed wasgenerally observed. Mag. x3000

> Fig 2. Facial surface of normal primarymolar treated with phosphoric acid.

Type 2 etch pattern was predominantlypresent Mag. xlOOO

38 American Academy ofPediatric Dentistry Pediatric Dentistry -20:1, 1998

In some isolated areas, the removal of tooth mineralaround the prism borders appeared more extensive,leaving behind irregular clumps of fractured sheetsof enamel prisms.

Hypoplastic pined AlClinically, all permanent teeth of the patient

showed many round, pin head-sized pits, which wereconcentrated mainly on the occlusal half of the fa-cial and lingual surfaces.

Maxillary premolar-untreatedIn the untreated surface, the pits appeared round

or elongated oval under low-power SEM, arranged lin-early, and surrounded by apparently normal enamel(Fig 3a). At high magnification, the pits were clearlydemarcated as depressed areas containing rough, dys-plastic enamel and organic material. The enamel sur-face in the cervical parts of the tooth appeared to benormal, although containing a few pits.

Maxillary premolar-acid etchedAfter acid etching, the enamel surrounding the pits

generally showed mixture of either type 1 or type 2patterns (Fig 3b). In other isolated areas, the patternof etching was generally that of type 3.

Smooth, hypoplastic AlIn this patient, the entire primary dentition showed

thin enamel which appeared extremely abraded.

Primary mandibular first molar-untreated

The untreated tooth surface of the primary first

molar tooth showed a generally unremarkable andrelatively smooth surface (Fig 4a).

Primary mandibular first molar-acid etchedAfter acid etching, the enamel showed a generally

uniform fibrillar surface, without any of the classicalfeatures of etched enamel (Fig 4b).

X-linked Hypoplastic (Male)Clinically, the entire primary dentition of the pa-

tient showed thin enamel, of normal color, and ap-peared smooth on the surface.

Primary maxillary incisor-untreatedLow-power SEM of a primary maxillary incisor

from this patient showed extensive surface loss ofenamel from abrasion. Higher magnification showedthe intact parts of the enamel surface to be relativelysmooth (Fig 5a).

Primary maxillary incisor-acid etchedIn the areas where intact surface enamel was

present, acid etching showed patchy loss of surfacetooth structure without evidence of etching patterns(Fig 5b).

X-linked Hypoplastic (Female)

The primary dentition of this patient was heavilyrestored with amalgam and showed areas of extensivebreakdown. The enamel appeared rough and grooved.

Primary maxillary first molar-untreatedLow-power SEM showed alternating bands of

relatively smooth, normal-appearing enamel inter-

•4 Fig 3a. Untreated facial surface ofpremolar affected with pinedhypoplastic Al. The pits were mainlyconcentrated on the occlusal halves ofthe facial and lingual surfaces. Mag. x30

*• Fig 3d. Facial surface of premolaraffected with pined hypoplastic Al aftertreatment with phosphoric acid. In the

center is a large oval pit Filled arrowshows area of type 1 etch pattern. Open

arrow shows area of type 2 etch pattern. Inthe areas Immediately adjacent to the pit,a type 3 etch pattern was found. Mag. xSOO

4 Fig 4a. Untreated facial surface ofprimary molar affected with the smoothhypoplastic variant of Al. Apart from a fewnarrow linear depressions, the surfacewas generally unremarkable. Mag. xf 000

> Fig 4b. Facial surface of primary molaraffected with the smooth hypoplastic

variant of Al after treatment withphosphoric acid. The enamel shows a

uniform fibrillar surface without any of theclassical features of etching. Mag. xlOOO

Pediatric Dentistry -20:1, 1998 American Academy of Pediatric Dentistry 39

•4 Fig 5a. Untreated facial surface of a pri-mary incisor from a male affected with X-linked Al. The right side of the figuredepicts an area of relatively smooth, intactsurface, wheras the left side shows areasof abrasion with a granular appearanceand shallow grooves and pits. Mag. xSOO

^ Fig 5b. Primary incisor from a male affect-ed with X-linked Al after phosphoric acid

treatment On the right, there was patchy lossof surface enamel with no etch pattern. On the -'

left, which had partial loss of surface enamel,there was irregular removal of tooth

structure [type 3 etch pattern). Mag. xluOspersed with abnormal, granular, and rough enamel.Higher magnification of the abnormal areas showeda complex pattern of interconnecting rounded ridgesseparated by furrows (Fig 6a) containing roughgranular enamel and small pits (Fig 6b).

Primary maxillary first molar-acid etchedAcid etching of the surface revealed a variety of

etching appearances. In the bands of smooth normalenamel, a type 2 etching pattern predominated (Fig6c). In areas of abnormal granular enamel, there wasirregular removal of tooth structure without definiteetching patterns.

HypomineralizedAIA premolar was available from a female who had

been diagnosed as having hypomineralized variantAl. Clinically, both primary and permanent teeth ofthe patient appeared yellow-brown, and showed ar-eas of posteruptive fracture and abrasion.

Maxillary premolar-untreatedFig 7a shows the intact areas of the facial surface.

Higher magnification of this area showed the presenceof many irregular, shallow pits and fine cracks.

The mesial half of the facial surface showed a largearea of partial enamel loss. SEM of the fractured

enamel in this area revealed a rough surface contain-ing irregular patches of rough, granular enamel (Fig7b). In the vertical parts of the fractured enamel sur-face, irregular, longitudinal prism structure was ob-served (Fig 7c).

Maxillary premolar-acid etchedAcid etching of the enamel surface with phospho-

ric acid produced a random mixture of the three clas-sical etching patterns.

DiscussionAs the failure rates of adhesive restorations in Al

may be high, the question often arises as to whetherthis type of dental enamel may be successfully etched.The present study addresses this important clinicalissue in five clinical variants of Al, namely, pitted hy-poplastic, smooth hypoplastic, X-linked (female), X-linked (male), and hypomineralized, using extractedpremolar and exfoliated primary teeth.

The common features of normal enamel, as wellas the abnormal enamel in Al, have been describedin previous publications.15"23 Our results on the un-treated surfaces of the different variants of Al showedmany of these abnormalities. However, there havebeen no previous studies comparing the effects of acid

Fig 6a. Ontreated facial surface of aprimary first molar from a femaleaffected with the X-linked variant of Al.The surface showed a complex patternof interconnecting, rounded ridgesseparated by grooves. Mag. X110

Fig 6b. Untreated facial surface of aprimary first molar from a femaleaffected with the X-linked variant of Al.Higher magnification of a furrowbetween two bands of normal enamelshowed the presence of rough, gran-ular enamel and small pits. Mag. X800

Fig 6c. Facial surface of a primary firstmolar from a female affected with the X-linked variant of Al after treatment withphosphoric acid. A type 2 etch pattern wasgenerally observed in the bands of normalenamel depicted in Fig 6a. Mag. 1300

40 American Academy ofPediatric Dentistry Pediatric Dentistry - 20:1, 1998

etching on enamel from different Al variants.Our study shows that the three classical acid-etch-

ing patterns found in normal enamel may be producedin most of the clinical variants of Al, although each vari-ant tended to show a predominant etch pattern. In thecase of the pitted hypoplastic variant, the predominantetch pattern was that of type 1, in which the prism coreswere preferentially removed. In contrast, in the X-linked(female) variant, the main etching pattern was type 2,in which the peripheral boundaries of the prisms weredissolved. In the X-linked (male) variant, the etchedenamel generally showed a type 3 pattern, in which thepattern of prism dissolution was irregular and did notappear to be related to prism structure. In thehypomineralized type of Al, all three types of etchingpatterns were found to be distributed equally.

In contrast, the enamel surface of the smooth hy-poplastic variant did not change significantly afteretching with phosphoric acid. It is possible that re-moval of a thin surface layer of enamel had occurredwithout production of etch patterns because of ab-normalities in prism structure, such as the presenceof a prismless layer. Alternatively, it is also likely that,because of smaller or weaker prisms, the length oftime of the acid etch or the concentration of etchantmay not be optimal to produce the classical etch pat-terns. These hypotheses are based on findings of pre-vious studies which found abnormalities of prismstructure, as well as reduction in enamel thickness bymore than half compared with normal enamel in thesmooth hypoplastic variant of Al.'8

In the hypomineralized variant, the findings onthe untreated surface such as micropits and cracks aresimilar to those noted in a previous report.17 The factthat acid etching produced all the three classical etchpatterns suggested that the prism structure in thisvariant may be generally normal, a finding which had

been observed in a previous ultrastructural study onthis Al variant.19

Thus, based on the findings of this study, acid etch-ing for bonded restorations may be possible inhypomineralized Al despite the presence ofhypomineralization abnormalities and morphologicalchanges detected at the crystallite level.19 In this regard,pretreatment of enamel with 5% sodium hypochlo-rite had been shown to be effective in removing ex-cess protein from teeth affected by thehypomineralized Al, with enhancement of clinicalbonding.21 Of further interest is the recent finding ofWright et al.24 that decreased mineral content andincreased protein was found not only inhypomineralized Al, but also in the hypomaturationand hypoplastic variants. This suggests that pretreat-ment with sodium hypochlorite is likely to be of valuein enhancing the effect of acid etching in cases of Alin general. However, our use of only 0.5% sodiumhypochlorite in the ultrasonication bath was mainlyfor the removal of gross plaque and other organic de-bris present on the tooth surface. This low concentra-tion of the reagent would be unlikely to have affectedthe tooth surface to significantly alter the effects of theacid etching as reported by Venezie et al.21

Our study thus showed that the presence or ab-sence of useful etch patterns for bonding largely re-flects the preoperative clinical appearance/morphology of the Al-affected teeth. In this regard,it is of interest to note the differences between maleand female teeth in the X-linked variant. Because ofthe homozygous Al gene defect in the males, thetooth from the male patient showed uniformly ab-normal enamel. When treated with acid, classical etchpatterns did not appear, suggesting abnormal prismstructure on the entire surface. In contrast, in the fe-male patient, the untreated enamel presented in al-

Fig 7a. low-power SEN of facial surfaceof unteated premolar of patient affectedby hypomineralized Al. A few crackswere present most likely the result ofextraction. On the left side (arrow) areareas of shallow depressions containingrough, granular enamel. Mag. X100

Fig 7b. Higher magnification of areashowing surface loss of enamel in Fig.la. The surface showed rounded ridgesand depressions which appear tocontain shallow, irregular honeycomboutlines resembling a developingenamel morphology. Mag. X2000

Fig 7c. Facial surface of premolar ofpatient affected by hypomineralized Alafter treatment with phosphoric acid. Amixture of the three clasical etchpatterns were observed on the surface.Mag.X2000

Pediatric Dentistry - 20:1, 1998 American Academy of Pediatric Dentistry 41

ternating bands of normal and abnormal enamel asa result of the Lyonization phenomenon which maybe observed in X-linked conditions.21 After treatmentwith acid, only the bands with normal enamel showedthe typical etch patterns, suggesting that enamel withnormal prism structure was found only in the bandscontaining normal enamel.

From a clinical viewpoint, the presence of the typi-cal etch patterns in most variants of AI suggests thatbonding of composite resins may be feasible in mostpatients with AI. High failure rates of adhesive restora-tions on M-affected teeth compared with normal teethcould be due to factors other than bond failure, such asfractures within weak enamel or dentin supporting therestorations. Alternatively, total or partial loss of enamelmay have occurred prior to placement of restorations,so that reduced areas were available for bonding.

Conclusions

1. In the pitted hypoplastic and hypomineratizedvariants of AI, classical etch patterns as seen innormal, control enamel may be produced aftertreatment with phosphoric acid

2. In the smooth hypoplastic and male X-linkedvariants, classical etch patterns are generally notobserved, and in the female X-linked variant,these are seen only in the bands of normal enamelwhich alternate with the abnormal bands.

The authors thank staffofthe Electron Microscope Division, Centerfor Microscopy and Microanalysis, University of Queensland foruse of SEM facilities.

Dr. Seow is associate professor in pediatric dentistry, University ofQueensland Dental School, Brisbane, Australia. Dr. Amaratungeis senior dentist, Aboriginal and Islander Community Health Ser-vice, Woolloongabba, Australia.

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42 American Academy of Pediatric Dentistry Pediatric Dentistry - 20.’1, 1998