PEDIATRIC DENTISTRY/Copyright ©1985 byThe American Academy of Pediatric Dentistry

Volume 7 Number 3

Pulpal response to bleaching of teeth in dogs

N. Sue Seale, DDS, MSD Carolyn F.G. Wilson,

Abstract

Three different application times of hydrogen peroxideand heat were used to evaluate removal of tetracyclinestains from vital teeth of dogs and to evaluate histologicchanges in the pulp tissue. Fifteen-, 30-, and 45-mintreatments were applied at 2-week intervals for 4consecutive treatments. The animals were sacrificed at 13,62, and 92 days following the final bleaching treatment.The pulp tissue was examined histologically for treatmentresponses. Pulpal changes ranging from flattening anddisruption of the odontoblastic layer to total obliterationof the odontoblastic layer and internal resorption werefound in 17 of 18 treated teeth. The severity of theresponse appeared to be related to the length of treatment.The pulp tissues showed evidence of repair for all 3treatment times in 5 of 6 teeth at the 92-day observationperiod.

Tetracycline, when ingested during the tooth-

forming years, long has been known to cause un-sightly staining of the dentition. 1-6 Despite increasedawareness on the part of physicians and patients andpublished warnings by drug manufacturers, toothdiscoloration associated with tetracycline therapycontinues to be a problem. Successful color removalby bleaching the teeth has been reported.7-1~ Bleach-ing techniques vary, but one of the most widely re-ported procedures involves the use of externallyapplied 35% hydrogen peroxide and heat for from 20to 45 min.~-1° There are conflicting reports concerningpulpal response to bleaching with hydrogen peroxideand heat. 12-14 Cohen applied 35% hydrogen peroxideand 54°C heat for 15-min per surface for 3 treatmentsfor a total treatment time of 45 min.12 He found nohistologic changes in pulp tissue of human premolarswhen compared to control teeth. Robertson and Melfiapplied 35% hydrogen peroxide and 46-51°C heat for5-min intervals, in 2 appointments spaced 5 days apart

DDS, MSD

to the buccal surface of human premolars.13 Histo-logic evaluation disclosed mild inflammatory re-sponses limited to the superficial pulpal tissues of theexperimental teeth which were significant when com-pared with the control teeth. Seale et al., evaluatedthe 2 vital bleaching components, heat and hydrogenperoxide, for their histologic effects on the pulp tissueof dog teeth. 14 They applied 35% hydrogen peroxideand heat of 62°C separately and in combination for30 min per surface for 4 consecutive weekly treat-ments. They found that 35% hydrogen peroxide aloneproduced severe but apparently reversible destruc-tive changes in pulpal tissue. The addition of 62°Cheat did not appear to intensify the damage. Heatalone had no deleterious pulpal effect. None of theinvestigations involved tetracycline-stained teeth ormeasured color removal. The current study was un-dertaken as a part of a joint investigation to deter-mine the most effective bleaching procedure with theleast pulpal damage. The study also measured colorremoval as it related to application time of the bleach-ing agents.

Methods and Materials

Six healthy, approximately 14-month-old litter-matemongrel dogs with newly erupted permanent denti-tions were used as experimental animals. Five of thedogs were given feed mixed with tetracycline at adose of 22 mg/kg for 90 days beginning at 6 weeks ofage. The sixth animal received no drug and servedas the control. The 5 experimental animals receivingtetracycline developed bright yellow-stained denti-tion (the control animal had white teeth). Four fullyerupted, intact, noncarious canine teeth from eachdog were the experimental teeth, whereas all othersserved as unbleached stained controls. The animalswere anesthetized with 33 mg/kg pentobarbital IV priorto the bleaching procedures.

PEDIATRIC DENTISTRY: September 1985/Vol. 7 No. 3 209

A rubber dam was used to isolate all 4 canine teethsimultaneously in each animal in order to protect thegingival tissue from hydrogen peroxide. The teethwere treated in the following manner:

Tooth 1 -- Cotton pledgets, 1.5 mm thick, saturatedwith 35% hydrogen peroxide, were applied to thelabial aspect of the tooth from incisal tip to gingivalborder. The cotton pledget remained in place for 15min and was heated with a constant temperature hand-held device with a shaped metal tip. ~ This tip was 17x 8 mm and covered most of the cotton-covered toothsurface. It was heated to 62°C (144°F) and was appliedgently to the entire cotton surface for 1 sec and thenremoved for 8 sec in a continuous routine for the 15-rain treatment. The temperature of the metal tip wasdetermined using a thermistor probe which had beencalibrated to a temperature range of 140-147°F. Eachtooth receiving heat treatment was in contact withthe heated tip a total of 100 sec during the 15-mintreatment period, while the cotton pledget was incontact the total time. The pledget was kept moist-ened with hydrogen peroxide by periodically touch-ing it with another supersaturated cotton pledget. Atotal of 6 teeth, or 1 tooth from each animal, receivedthis treatment, 2 observed for 13 days, 2 observed for62 days, and 2 observed for 92 days.

Tooth 2 -- Cotton pledgets were applied and heatedin the same manner as described for -Tooth 1 andremained in place 130 min. A total of 6 teeth, or onetooth from each animal, received this treatment, 2observed for 13 days, 2 observed for 62 days, and 2observed for 92 days.

Tooth 3 -- Cotton pledgets were applied and heatedin the same manner as described for Tooth 1 andremained in place for 45 min. A total of 6 teeth, orone tooth from each animal, received this treatment,2 observed for 13, :2 observed for 62, and 2 observedfor 92 days.

Tooth 4 -- This tooth received no treatment otherthan isolation by the rubber dam for 45 min. A totalof 6 teeth received this treatment and served as thecontrol teeth, 2 each for the 13-, 62-, and 92-day ob-servation periods. The total number of teeth involvedwas 24 (18 experimental, 6 control).

The 6 animals were treated 2 weeks apart for 4consecutive treatments. The animals were anesthe-tized with pentobarbital and sacrificed with potas-sium chloride for removal of the 4 experimental teethat 3 observation periods following the last hydrogenperoxide treatment.: 13 days, 62 days, and 92 days.Two animals were sacrificed at each observation pe-riod. Each animal received bilateral perfusion of the

Union Broach Bleaching Instrument -- Union Broach Co: Long

Island, NY.

carotid arteries with 200 ml normal saline followedby 10% buffered formalin, until the jugular cathetersran clear.

The teeth were extracted following perfusion of thehead. The root tips were severed to allow for betterinfiltration of fixative. The teeth were fixed in 10%buffered formalin for I month. Following fixation, theteeth were demineralized is and embedded in Para-plast Plus.® Each tooth was sectioned at a thicknessof 5 ~x and every tenth section was collected. Twentysamples from each tooth were collected and stainedwith hematoxylin-eosin and examined. The pulp wasexamined and evaluated for its response in the cor-onal, middle, and apical thirds. Results were re-corded for each area according to the following criteria:(1) amount and quality of reparative dentin, (2) pres-ence and intensity of calciotraumatic lines in dentin,(3) integrity of odontoblastic layer and predentin, (4)morphology of odontoblastic cells, (5) evidence of in-ternal resorption, (6) presence and type of inflam-matory cells, (7) degree of inflammatory response,and (8) evidence of hemorrhage and edema. Differ-ences in type and severity of response between treat-ment times and between observation periods werenoted and compared.

Results

Two teeth for each treatment procedure, or a totalof 8 teeth, were available at each observation period.The distribution of teeth according to observation timeis shown in Table 1.

13 Days Following Final TreatmentThe 2 teeth receiving the 15-min application of hy-

drogen peroxide and heat showed different reactionsat this early time period. One tooth demonstrated amarked response in the incisal tip with a narrow zoneof amorphous reparative dentin, absence of preden-tin, flattened and discontinuous odontoblasts, andeosinophilic staining with vacuoles throughout thepulp tissue (Fig 1). The odontoblasts returned to nor-mal, with regular tubular dentin and predentin in themiddle and apical pulp. The other tooth showed no

TABLE 1. Distribution of Experimental Teeth According to Ob-servation Times (N = 24).

Group 13 Days 62 Days 92 Days15-min treatment 2 2 230-rain treatment 2 2 245-rain treatment 2 2 2No treatment control 2 2 2Total teeth in eachobservation time 8 8 8

210 PULPAL RESPONSE TO BLEACHING IN DOGS: Seale and Wilson

FIG 1. Thirteen days following the final 15-min application theincisal tip demonstrated a narrow zone of amorphous repar-ative dentin, absence of predentin, flattened and discontin-uous odontoblasts, and eosinophilic staining with vacuoles(90x).

FIG 3. Thirteen days following the final 30-min treatment, 1tooth demonstrated amorphous reparative dentin with trappedcell bodies, flattened odontoblasts, and a dense inflammatoryinfiltrate in the incisal third. (A) The other tooth in the 30-mintreatment group demonstrated a less severe reaction in theincisal tip with amorphous reparative dentin, no predentin,and flattened, discontinuous odontoblasts. (B) (225x).

*&** - T-. —?-.. " -=- - C • •̂ fir.

* •» » » <r •*-,. ->- •&.*•*•• 'zt V>. *.r

FIG 2. Control tooth for the 13-day observation period (225x).

response and was indistinguishable from the controltooth (Fig 2).

The teeth in the 30-min treatment sample showedresponses of 2 different degrees of severity in theincisal pulp tissue, but were comparable in the mid-dle and apical thirds. The tooth demonstrating themore severe reaction revealed a dense infiltrate ofpolymorphonuclear leukocytes, atubular reparativedentin with trapped cell bodies and flattened, dis-continuous odontoblasts. The other tooth demon-strated a less severe reaction with atubular reparativedentin, absence of predentin, and flattened odonto-blastic cells which were absent in places (Fig 3).

The teeth treated for 45 min exhibited results sim-ilar in severity to those in the 30-min group, includingthe 2 different degrees of severity in the incisal pulp

FIG 4. Thirteen days following the final 45-min treatment 1tooth demonstrated a severe inflammatory reaction withamorphous reparative dentin and flattened odontoblasts. (A)The other 45-min treated tooth demonstrated a less severereaction with hemorrhage, inflammatory cells, amorphous re-parative dentin, and flattened, discontinuous odontoblasts.(B) (225x).

tissue. The tooth demonstrating the more severe re-sponse revealed a dense inflammatory infiltrate, atu-bular reparative dentin and flattened odontoblasts.The tooth demonstrating the less severe reactionshowed some inflammatory cell infiltrate, amorphousreparative dentin, and flattened and discontinuousodontoblasts (Fig 4).

62 Days Following Final TreatmentThe two teeth receiving the 15-min application of

hydrogen peroxide and heat showed pulpal re-

PEDIATRIC DENTISTRY: September 1985/Vol. 7 No. 3 211

sponses at the incisal tip of atubular reparative den-tin, shortened odontoblasts, and a mild chronicinflammatory infiltrate (Fig 5). Odontoblasts and den tinin the apical region assumed a normal appearance.

The two 30-min treated teeth showed pulpal re-sponse of the incisal tips involving thick amorphousreparative dentin with cell bodies trapped and short-ened and occasionally missing odontoblasts (Fig 6).There was a mild chronic infiltrate which lessened asthe apex was approached. The odontoblasts returnedto normal with calciotraumatic lines appearing in thetubular reparative dentin and disappearing in the ap-ical area.

The two 45-min treated teeth varied in the severity

FIG 5. At the 62-day observation period, the teeth in the 15-min treatment group demonstrated atubular reparative den-tin, shortened odontoblasts, and a mild chronic inflammatoryinfiltrate in the incisal tip (225x).

of their response with 1 showing more active destruc-tive changes and the other showing healing of ap-parent earlier destructive changes. The incisal tip of1 tooth revealed no reparative dentin or odontoblasts.There were large numbers of giant cells involved inactive and extensive internal resorption (Fig 7). Mov-ing down from the incisal tip the pulp tissue ap-peared as granulation tissue with a moderate mixedinflammatory infiltrate and hemorrhage. There wasa gradual reappearance of flattened odontoblasts whichwere involved in repair of previous areas of internalresorption. Apically, the odontoblasts became morenormal with predentin, tubular reparative dentin, andcalciotraumatic lines, but the hemorrhage and inflam-matory cells persisted. The second tooth appeared tobe in a later stage of response as there were numer-ous areas of repaired internal resorption with cellularinclusions and a continuous flattened odontoblasticlayer. The pulp was infiltrated with chronic inflam-matory cells. In the middle of the pulp and apically,the structures all returned to a more normal appear-ance with 4 calciotraumatic lines clearly visible.

92 Days Following Final TreatmentThe teeth receiving the 15-min treatment showed

a large amount of amorphous reparative dentin in theincisal tip (Fig 8). The odontoblasts in this region wereshortened in height and abnormal in shape but con-tinuous. The pulp tissue stained eosinophilically. Theodontoblasts along the middle third assumed a morenormal shape with reappearance of predentin andtubular reparative dentin. Odontoblasts of the apicalthird were of normal height and shape. The pulptissue appeared similar to that of the control teeth.

The 30-min treated teeth showed shortened abnor-

Fic 6. At 62 days following the final 30-min treatment therewere shortened and discontinuous odontoblasts, thick amor-phous reparative dentin with cellular inclusions, and mildchronic inflammatory infiltrate in the incisal tip (225x).

FIG 7. At 62 days following the final 45-min treatment therewas complete absence of odontoblasts, reparative dentin, andnumerous giant cells involved in internal resorption in theincisal third (90x).

212 PULPAL RESPONSE TO BLEACHING IN DOGS: Seale and Wilson

Fie 8. Ninety-two days following the final 15-min treatment,the pulp in the incisal third revealed eosinophilic staining,shortened odontoblasts, and thick amorphous reparativedentin (90x).

FIG 9. Ninety-two days following the final 45-min treatment 1tooth revealed large amounts of amorphous reparative den-tin, cellular inclusions, and numerous areas of repaired in-ternal resorption in the incisal third (90x).

mal odontoblasts, wide zones of atubular reparativedentin with cell bodies trapped and areas of repairedinternal resorption in the incisal tip. The pulp tissuecontained large numbers of swollen phagocytic cellsfilled with gray and brown cytoplasmic inclusions.The middle of the tooth contained wide zones of tu-bular reparative dentin, tall columnar odontoblastsand a more normal-appearing body of pulp.

The 45-min treatment group showed one nonvitalpulp. The remaining tooth showed large amounts ofreparative dentin and numerous areas of repaired in-ternal resorption in the incisal tip (Figs 9 & 10). Theodontoblasts were shortened in height and predentinwas present. Chronic inflammatory cells were pres-

FIG 10. Higher magnification showing repaired internal re-sorption (225x).

ent throughout the entire pulp, but there were nosigns of hemorrhage or edema. All structures ap-peared normal in the apical third.

Discussion

There appears to be a wide variety of responsesrepresented in this investigation. The differences inseverity of pulpal reaction across treatment times atthe same observation period appears to be related tothe length of heat and peroxide application time withthe longer application times showing progressivelymore severe pulp pathology.

The results of this investigation would appear tosubstantiate an earlier report14 of severe but appar-ently reversible destructive changes in the pulps ofdog teeth in contact with 35% hydrogen peroxide aloneand in combination with 62°C heat for four 30-minapplications. They found similar pulpal responses ofinternal resorption and inflammation and speculatedthat the porous nature of the enamel and dentin al-lowed the agent to penetrate the tooth surface suffi-ciently to affect the odontoblasts and pulp tissue.

These current findings of increasingly severe pul-pal responses as bleaching agent application timesincrease would be consistent with enamel and den-tinal penetration by the agents. The longer the so-lution is in contact with the enamel surface, the deeperthe penetration and the greater the quantity of so-lution to penetrate. Cohen,12 and Robertson andMelfi13 reported mild or no pulpal changes associatedwith vital bleaching techniques. However, they werenot examining times of application which have beenreported to be clinically successful for color removal.A second reason for the differences in pulpal re-sponse in their studies compared with the presentinvestigation may be related to the particular teeth

PEDIATRIC DENTISTRY: September 1985/Vol. 7 No. 3 213

involved. Both Cohen and Robertson and Melfi usedhuman premolar teeth. The average thickness ofenamel and dentin, was not given; therefore, a directcomparison canno: be made. However, E1-Hadary etal.’6 reported an average thickness of 3.8 mm forenamel plus dentin in human premolars. The averagethickness of the enamel and dentin in the dog teethin the present study was 1.7 mm, thus making a dif-ference in enamel and dentin thickness of 2.1 mm. Ifthe teeth in the Cohen and Robertson and Melfi stud-ies were similar in thickness to those reported by E1-Hadary, then the thinner structure of the dog teethmay have allowed the bleaching solution to penetrateto the pulp more readily. The bleaching procedureunder study is per:formed most often on human max-illary central and lateral incisors. The average thick-ness of enamel and dentin in human incisors is moresimilar to that of dog canine teeth than that found inhuman premolars. If the thickness of enamel anddentin is a critical factor in permitting hydrogen per-oxide to affect the pulp adversely, this study moreaccurately demonstrates the probable effect of bleach-ing on the pulp of human maxillary anterior teeththan does the study which examined this phenome-non in human premolars.

All but one of the teeth in the present study showedresolution and healing of earlier changes, which wouldappear to indicate that the damage caused by all 3treatment times may be of a reversible nature. Therewas 1 nonvital tooth in the 92-day observation periodfor the 45-min treatment which represented 50% ofthe teeth in that group. This finding would most cer-tainly contraindicate the 45-rain treatment as it wasdone in this study. However, the associated clinicalstudy of color removal indicated that only two 45-rain treatments are necessary for successful color re-moval while this study reports pulpal changes asso-ciated with 4 treatment sessions.

The short observation times and the small numbersof teeth examined preclude direct clinical applicationof the results of this study. However, the consistentfindings of pulpal responses in 17 of 18 teeth receiv-ing hydrogen peroxide and heat would indicate theneed for further studies involving longer observationperiods and including greater numbers of teeth.

Conclusions

1. Heat of 62°C combined with 35% hydrogen per-oxide applied to the external surface of tetracycline-

stained dog teeth for 4 sessions each of 15, 30, or 45min, produced observable pulpal changes in 17 of 18teeth treated.

2. The severity of the pulpal response appeared tobe associated with the length of treatment time (thelongest time causing the most severe changes).

3. The pulp tissues showed evidence of repair forall 3 treatment times in 5 of 6 treated teeth in the 92-day sample.

Dr. Seale is an associate professor and Dr. Wilson is an assistantprofessor, pedodontics, Baylor College of Dentistry, 3302 GastonAve., Dallas, TX 75246. Reprint requests should be sent to Dr.Seale.

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214 PULPAL RI~SPONSE TO BLEACHING IN DOGS: Seale and Wilson