64

Since Buonocore1 first described the use of phos-phoric acid for enamel etching, various alternativeacids and chelating agents have been studied.2-7

None have proved to have significant advantages, andmost have shown poorer etch pattern production, lowerbond strength, or even excessive enamel dissolution. Dif-ferent strengths of phosphoric and other acids have alsobeen studied. Again, phosphoric acid has proved to be themost effective, at approximately 40% wt/wt.8,9 Nitricacid is also commercially available as an etching product,but an investigation into its effectiveness at etchingenamel for orthodontic bonding has not been undertaken.

Clinical application times for acid have decreasedover the past few years, following evidence that shortertimes are equally effective.10 The 60-second etch hasbeen recommended for direct bonding of molar tubes inadults,11 but shorter times are now suggested for bondingbrackets to premolars, canines, and incisors.12,13 Recentwork has shown that only a small percentage of the

enamel surface area is etched ideally with a 30-secondapplication of 37% phosphoric acid.14

This study was to determine the optimal acid appli-cation time for the orthodontic bonding area (OBA) ofthe buccal enamel of mandibular premolars with the useof phosphoric 37% wt/wt and nitric acid 2.5% wt/wt.

MATERIAL AND METHODS

Sixty mandibular premolars were extracted from ado-lescent white boys and girls between the ages of 10 and 18years before orthodontic treatment. The patients were life-time residents of nonfluoridated regions of northwestEngland and southwest Scotland. The teeth were clini-cally sound. The teeth were stored in aqueous 0.5% Chlo-ramine-T, as recommended by the International StandardsOrganization (ISO 1944), before the study. The etchingand preparation for scanning electron microscope (SEM)observation was carried out within 6 months of extraction.

The extracted teeth were rinsed with distilled water.Their buccal surfaces were cleaned with a pumice andwater slurry with the use of a rotary brush in a dentalhandpiece; they were rinsed again with water from a 3-in-1 syringe for 30 seconds and dried with compressedair for 30 seconds. The teeth were divided into 6 groupsof 10 and etched with either 37% phosphoric acid(Right-On Etchant; TP Orthodontics, Leeds, UK) or2.5% nitric acid (Mirage Bond; Chameleon Interna-tional, Kansas City, Kan) for 15, 30, or 60 seconds.

aSpecialist Registrar (FTTA) in Orthodontics, Liverpool University DentalSchool and Warrington Hospital, United Kingdom.bLecturer in Orthodontics, Newcastle Dental Hospital and School, United Kingdom.Reprint requests to: Alastair Gardner BDS, Specialist Registrar (FTTA), Ortho-dontic Department, Liverpool University Dental School, Pembroke Place, Liv-erpool LP 5PS, United Kingdom; e-mail, alastair_gardner@hotmail.com.Submitted, March 2000; revised and accepted, November 2000.Copyright © 2001 by the American Association of Orthodontists.0889-5406/2001/$35.00 + 0 8/1/114643doi:10.1067/mod.2001.114643

ORIGINAL ARTICLE

Variations in acid-etch patterns with differentacids and etch timesAlastair Gardner, BDS, MFGDP, FDS, MSc, MOrth,a and Ross Hobson BDS, MDS, FDS, MDO, PhDb

Liverpool, United Kingdom

Research has shown that mandibular premolars have the highest rate of orthodontic bracket bond failure. Bondstrength is thought to depend on the quality and quantity of enamel etch produced by the acid. Phosphoric acidis the most commonly used acid for etching before bonding. Nitric acid is also commercially available for etchingenamel; however, it has not been studied specifically for etching in orthodontic bonding. This ex vivo studycompared the enamel etch patterns achieved on the orthodontic bonding area of extracted mandibularpremolars treated with 37% wt/wt phosphoric acid and 2.5% wt/wt nitric acid applied for 15, 30, or 60 seconds.The etch patterns were viewed with a scanning electron microscope and assessed with a 5-point etch scale.Statistical analysis showed that increasing the duration of acid application (irrespective of acid type)significantly increased the amount of better quality etch. Phosphoric acid at 37% concentration was moreeffective at producing a good-quality etch than was nitric acid at 2.5% for all 3 application times. The quantityof good-quality etch produced by phosphoric acid at 37% was time specific, with 15 seconds being significantlyless effective than 30 or 60 seconds. However, 60 seconds was not significantly better than 30. Comparing theupper half of the orthodontic bonding area with the lower half showed no statistically significant differences foreither acid type or length of application (P > .005).These findings support the use of 37% wt/wt phosphoric acidand indicate an optimum application time of 30 seconds. (Am J Orthod Dentofacial Orthop 2001;120:64-7)

American Journal of Orthodontics and Dentofacial Orthopedics Gardner and Hobson 65Volume 120, Number 1

After etching, the teeth were washed for 30 secondsand dried for 30 seconds with a 3-in-1 syringe. Theteeth were then placed on SEM stubs and desiccated ina warm-air oven at 37°C for 24 hours. Finally, the teethwere gold coated to a depth of 15 nm and examined inthe SEM microscope at 10 kV and ×1820 magnifica-tion; the image was displayed on a 14-in monitor.

The area examined was the region of the buccal sur-face used by orthodontists to place preadjusted edge-wise brackets. Because preadjusted brackets aredesigned to be placed on the midpoint of the clinicalcrown, both in the horizontal and vertical planes, andthe average bracket size is approximately 4 mm2, it istherefore possible to accurately define the OBA as anarea of 4 mm2 centered on the midpoint of the buccalaspect of the clinical crown (Fig 1).14

Histometric point sampling is the method of count-ing a number of points on or within a sample to esti-mate the area (or volume) occupied by a particular fea-ture within the specimen. It is based on geometricprinciples first described by Delesse in 1848. Mattickand Hobson14 have found that point sampling 1,920intersections over the OBA gives 95% confidence inthe data. On the monitor, a hairline grid consisting ofhorizontal and vertical lines 38 mm apart was overlaid,producing a total of 30 line intersections per screen.Measurement of 30 line intersections on 64 screens,distributed evenly over the OBA, gave 1,920 pointscounted. The quality of etch at each point of line inter-section was assessed and scored according to the etchscale described by Galil and Wright.15 The data weretranscribed into ASCII text files and analyzed withMinitab for Windows 12.0 (Minitab, State College, Pa).

A general linear model analysis of variance and aTukey post-hoc test were used to examine for differencesbetween the 2 acids and the 3 etch durations and to lookfor differences between the upper and lower halves of theOBA. Statistical significance was set at 95% confidence.

The validity of the data obtained was assessed byrepeating the observation on a random sample of 6 buc-cal enamel surfaces at the end of the main observationperiod and, on the same sample, 3 months later. Therewas no significant difference between the first and sec-ond observations (P < .05).

RESULTS

The mean data table (Table I) shows the percentagearea occupied by each type of etch produced by the 2acids for each time of etch.

For 37% phosphoric acid, there was a reduction inthe area occupied by the type 5 etch pattern, from 55%at 15 seconds to a minimum of 15% at 30 seconds; how-ever, it increased to 23% at 60 seconds. The area occu-pied by the type 3 etch pattern increased with the appli-cation time from 1% at 15 seconds to 19% at 30 secondsto 20% at 60 seconds. The area occupied by the types 1,2, and 4 etch patterns increased from 15 to 30 seconds ofapplication but then dropped slightly at 60 seconds.

For 2.5% nitric acid, the area occupied by the type 5etch pattern decreased from 81% at 15 seconds to 76% at30 seconds to a minimum value of 28% at 60 seconds.The reverse was true for the area occupied by etch patterntype 3, which increased from 0% at 15 seconds to 3% at30 seconds and 17% at 60 seconds. The type 4 etch pat-tern increased from 19% at 15 seconds to 21% at 30 sec-onds and 54% at 60 seconds. Nitric acid produced no type2 etch patterns and extremely small amounts of type 1.

The 1-way analysis of variance for time of applica-tion versus etch type observed indicates that thechanges in the amount of etch type 1 (P > .05) and type2 (P > .05) were not significant. However, the changein the amount of etch type 4 was significant (P < .05),

Fig 1. Diagram of orthodontic bonding area.

Table I. Mean percentage area occupied by each etch typeproduced by 37% wt/wt phosphoric acid and 2.5%wt/wt nitric acid at 15, 30, and 60 seconds of application

Type Type Type Type Type Acid Time 1 2 3 4 5

Phosphoric 15 0% 0% 1% 44% 55%Phosphoric 30 9% 3% 19% 54% 15%Phosphoric 60 4% 2% 20% 51% 23%Nitric 15 0% 0% 0% 19% 81%Nitric 30 0% 0% 3% 21% 76%Nitric 60 1% 0% 17% 54% 28%

Type 1, ideal etch, honeycomb pattern; type 2, ideal etch, reverse honey-comb pattern; type 3, mixture of types 1 and 2; type 4, surface roughen-ing, no pattern apparent; type 5, no apparent etch.

66 Gardner and Hobson American Journal of Orthodontics and Dentofacial OrthopedicsJuly 2001

and the amounts of type 3 and type 5 were highly sig-nificant (P < .001).

The percentages of etch types 1, 2, and 3 were allgreater with phosphoric acid compared with nitric acidat all 3 application times. The opposite was true foretch types 4 and 5, where there were greater amountswhen nitric acid was used.

The 1-way analysis of variance for acid type versusthe etch pattern produced indicated that there was nosignificance between phosphoric acid or nitric acid atproducing type 2 (P > .05) or type 3 (P > .05) etch pat-terns. Phosphoric acid produced greater amounts oftype 1 etch than did nitric acid (P < .05). Conversely,nitric acid produced greater amounts of type 4 and type5 etch patterns ( P < .001).

The percentages of the etch types produced on theupper half of the OBA were compared with the per-centages of the etch types on the lower half. One-wayanalysis of variance found no significant difference (P> .05) for each of the 5 etch types.

DISCUSSION

Past studies into enamel etching have used a varietyof materials and methods, which has made interstudycomparison difficult. This investigation controlled thestudy sample by using mandibular premolars from 10-to 18-year-old white adolescents with a lifelong historyof residence in nonfluoridated areas.

The study found that phosphoric acid was statisti-cally significantly better than nitric acid at producingetch type 1 (P < .05) and etch type 4 (P < .01). Thelevel of significance for etch type 2 was P = .058, justbeyond significance. 2.5% nitric acid produced signifi-cantly greater amounts of etch type 5 (P < .001).

Nitric acid is clearly less efficient at 2.5% concen-tration than is phosphoric acid at 37% in producing anideal etch (types 1 and 2), and it significantly increasesthe amount of enamel showing no apparent etch (type5). This applied to all 3 acid application times. The evi-dence that surface roughness improves bond strength isby no means completely understood; however, failureto produce surface roughness cannot be any help inimproving bond strength. Overall, phosphoric acid37% wt/wt is better than nitric acid 2.5% wt/wt at pro-ducing a higher percentage of ideal etch on the buccalenamel surface of mandibular premolars.

This study found small quantities of type 1 etch onmost teeth etched with phosphoric acid, with a maxi-mum of 9.5% at 30 seconds. This agrees with Mattickand Hobson14 who reported that less than 5% of theOBA was ideally etched. The amount of type 2 etchwas seen at a maximum of 3% with phosphoric acidapplied for 30 seconds and was almost absent from the

surfaces etched by nitric acid. It seems likely that this isdue to the surface enamel rather than the etching tech-nique. Studies that have reported large amounts of etchpatterns that correspond to Galil and Wright’s type 1and type 2 etch patterns have ground their enamel sur-face before acid etching.15-20 Alternatively, becauseprevious studies have not used a systematic method ofevaluating the quantity of acid etch, they may havedescribed the best etch pattern observed, although itwas not necessarily present in large quantities.

The results of varying the acid application times areless obvious than those for acid type. There was no sta-tistical significance for either etch type 1 or type 2 (P >.05) between nitric acid and phosphoric acid at any ofthe 3 application times. The amount of etch types 3 and4 produced increased with the duration of acid applica-tion (type 3, P < .001; type 4, P < .05). Type 5 signifi-cantly decreased as the application time increased (P <.001). This means that longer application times reducedthe amount of area with no etching (type 5) andincreased amounts of types 3 and 4 but did not signifi-cantly increase the quantity of ideal etch (types 1 and 2).

Whereas longer applications of nitric acid contin-ued to increase the amounts of type 3 and type 4 etchpatterns, phosphoric acid had already produced a peaketch quality at 30 seconds of application. This wasshown as an increase in etch type 1, compared with 15and 60 seconds of application. A 30-second applicationof phosphoric acid also reduced type 5 etch to a lowerlevel than 15- and 60-second applications. This was inagreement with research by Oshawa21 who proposedthat an ideal strength-to-time ratio of acid could beachieved. Silverstone22 theorized that application timesand acid strengths above this ideal ratio would actuallyreduce the etch achieved and perhaps have an adverseeffect on the bond achieved; this was supported by ourstudy. Nitric acid 2.5% showed an improvement in etchwith time. It may be that a higher concentration ofnitric acid would show a similar trend to that of phos-phoric acid, and this is an area for further investigation.

This study supports the application of 37% phos-phoric acid for 30 seconds. Mandibular premolars areconsidered to etch less well than are more anteriorteeth,14 and it may be that etch times of less than 30seconds might be equally efficient at etching incisorsand canines to their maximum potential.

There were no statistical differences between the etchquality produced on the cervical area of the OBA com-pared with that on the occlusal area, for either acid typeor application time (P > .05). It has been shown that cer-vical enamel etches poorly because of the increasedaprismatic enamel in this region 14,23-25 However, thesestudies compared enamel areas from distinctly separate

American Journal of Orthodontics and Dentofacial Orthopedics Gardner and Hobson 67Volume 120, Number 1

regions on the crown of the tooth, rather than comparingthe lower half of the OBA with the upper half, whichmight not show such differences in etch patterns becausethey were not sufficiently cervically placed.

This study found that 37% wt/wt phosphoric acidremains the gold standard in enamel etching, and 30seconds duration is the ideal time for mandibular pre-molars. However, even this etch regime produces onlya small percentage of ideal etch (type 1 and type 2etches). Bond strength and etching studies equate idealetch with higher bond strength, and it would be usefulif future research could determine the amount of type 1and type 2 etch required to provide clinically usefulbond strengths. It would then be possible to directefforts into methods of obtaining the particular type ofetch pattern that provides an improved bond.

What is evident from this study was that individualteeth react differently to phosphoric acid, and, whereassome etch poorly even with 60 seconds of acid appli-cation, the majority reach a peak in etch quality by 30seconds. The reasons behind this are still to be eluci-dated. Studies in this area would be useful, especially ifteeth that were going to etch poorly could be identifiedbefore bonding.

For orthodontists, the use of phosphoric acid at 37%wt/wt concentration with a 30-second application timeshould continue as the gold standard for enamel etching.

CONCLUSIONS

1. Phosphoric acid at 37% wt/wt is more effective atproducing enamel etch than is nitric acid at 2.5%wt/wt when applied to the orthodontic bonding areaof mandibular premolars for 15, 30, or 60 seconds.

2. The optimal length of time for applying phos-phoric acid 37% wt/wt to the orthodontic bondingarea of mandibular premolars is 30 seconds.

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