CLINICAL ARTICLES

A clearing technique for the study of root canal systems

Don Robertson, DDS; I. Joel Leeb, DDS; Mike McKee, DDS; and Erich Brewer, DDS

This art icle p r e sen t s a s imp le and i nexpens ive t e c h n i q u e for in vi t ro e x a m i n a t i o n of endodon t i c a l l y t r ea t ed or u n t r e a t e d roo t canal sys tems . T h e t e c h n i q u e consis ts of decalcif icat ion wi th nitr ic acid,

d e h y d r a t i o n wi th alcohol, and c lear ing wi th m e t h y l sal icylate, wh ich renders the tee th t ransparen t . T w o case s tudies , e x a m p l e s of the fine detail of roo t canal m o r p h o l o g y , are p resen ted . T h e y inc lude

u n i n s t r u m e n t e d , i n s t r u m e n t e d , a n d o b t u r a t e d tee th w h i c h d e m o n s t r a t e s o m e of the app l i ca t ions of the c lear ing t echn ique .

A variety of techniques have been proposed to permit visualization of the root canal system and the effect of biomechanical procedures on its morphology. Sectioning of the roots at right angles to the long axis is of value in determining canal contour and size? ~ But this technique does not permit a continuous view of the canal system. Longitudinal section- hag of the root compensates for the previous limitation by providing an intact canal surface? However, only a limited amount of the canal wall can be retained, and only the more gross morphologic details can be observed. Improved examination of the morphology of the canal wall is possible with use of the scanning electron microscope which permits magnifications in excess of 2,500 and allows for a greatly detailed study of the effect of biomechanical and irri- gation procedures? Again, the major disadvantage is the limited area available for study. An alternative technique has been the production of replicas of the root canal system. Resins, with or without vacuum

pressure, have been injected into intact teeth which then were dis- solved or clarified) '6 A modification of this technique using sodium hypochlorite and nitric acid to dis- solve the tooth has permitted de- tailed reproduction of the root canal system 7 and the physical appearance of endodontic obturation tech- niques. 8,9

The major disadvantage of all of these techniques is that the tooth is irreversibly altered as a result of sectioning, dissolution, or the injec- tion of a solid core material. A more ideal model would be one in which no physical change has been intro- duced to the tooth, thus permitting observation of the changes induced by root canal therapy. This paper describes a simple technique for making a tooth transparent.

M A T E R I A L S A N D M E T H O D S

A representative sample of ex- tracted human molars, premolars, and incisors that had been stored in water and Formalin, was selected for

clearing to demonstrate root canal morphology. The technique used is a modification of one described by Brain. 1~ The following protocol was followed:

The teeth were cleaned, and tissue and calculus were removed.

A round bur was used to produce a conservative cavity preparation in the pulp chamber.

The teeth were placed in a 5% sodium hypochlorite solution for 24 hours to dissolve organic debris from the root canal system and washed in running tap water for two hours.

The specimens were decalcified for three days in 5% nitric acid at room temperature. The nitric acid solution was changed ~daily and agitated by hand three times each day.

After completion of decalcifica- tion, the teeth were rinsed in running tap water for four hours.

The dehydration process consisted of a series of ethyl alcohol rinses starting with 80% solution overnight, followed by a 90% solution for an hour, and three 100% ethyl alcohol rinses for an hour each.

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The dehydrated teeth then were placed in methyl salicylate which made the teeth transparent after approximately two hours.

I n d i a ink was injected into the pulp chamber with a no. 27 gauge needle on a Luer-Lok plastic, dispos- able syringe. The ink then was drawn through the canal system by apply- ing negative pressure to the apical end of the tooth with use of the central suction system. The latter provided approximately 25 mm of mercury vacuum.

The excess ink then was removed from the surface of the root with gauze soaked in alcohol, and the tooth was returned to the methyl salicylate solution until needed.

To demonstrate a model for exam- ining endodontically treated teeth, 25 adult human teeth were chosen. Twelve teeth were filled by hand using a Kloroperka technique, ten teeth were filled by hand using later- al condensation, and one tooth was filled by hand using a warm gutta- percha technique. Two teeth treated in vivo using a lateral condensation technique were deemed clinical fail- ures and were extracted. All teeth were stored in 70% isobutyl alcohol until the aforementioned technique was used.

RESULTS

Examination of the transpareht teeth with the dissecting microscope allowed the examiner to study in detail the morphology of the root canal system or the endodontic fill without destroying the specimen.

The morphology of the root canal system is demonstrable in uninstru- mented molars. Visualization of canal orifices, constrictions, and con- tours as well as ink penetration of

Fill l--Maxillary first molar, uninstrumented, for examination of anatomy of root canal.

fins and accessory canals is accom- plished (Fig 1, 2).

Molars that have been instru- mented and obturated may be stud- ied for the completeness of obtura- tion into instrumented and uninstru- mented spaces, volume of gutta-per- cha present, indication of the thick- ness and amount of sealer and its location, the proximity of accessory cones to the master cone, and the irregularities of the surface of the filling material. Comparison of the effectiveness of obturation between lateral condensation and Kloroperka is possible (Fig 3, 4).

Technical errors not evident clini- cally may be shown in transparent teeth. Case 1 shows a perforation along the furcal aspect of the distal root. This was not detected in radio- graphs because of the overlap of the root, facial and lingual to the groove (Fig 5).

Case 2 shows a maxillary molar that had been extracted after a vari- ety of clinical procedures were per- formed. The technique allowed the visualization of the result of instru- mentation, obturation, and surgical procedures that further altered the tooth (Fig 6).

Fig 2--Mandibular third molar exemplifies fine detail provided by technique.

DISCUSSION

A variety of techniques to demin- eralize and clarify intact teeth have been reported. 11 1~ The most com- monly used clearing agent was xylene. 1'-1a Contrast between the canal system and dentin was ob- tained with either hematoxylin 1:~-~ or india ink. 11-1~'1'~

The major advantages of the method described in this paper are: the speed with which the entire pro- cess may be accomplished (approxi- mately five days are required as com- pared with the two or more weeks needed in other methods) ':~ and the simplicity of the technique. Little equipment and space are required. Less toxic and inexpensive chemicals than those used previously were needed. ~ .... Methyl salicylate wilt tolerate more water than other clear- ing agents and is not noxious? ~

The technique is simple, but there are some potential problems. These include the determination of the end point of decalcification. Although there are tests to determine the end point of decalcification, ~~ we fol- lowed an empirical method of decal- cifying the teeth 24 hours after the

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JOURNAL OF ENDODONTICS I VOL 6, NO 1, JANUARY 1980

Fig 3--Maxilla~y first molar fiUed using Kloroperka showing appearance of fine detail,

fins and accessory canals, and voids provided by technique.

enamel appeared to be dissolved. Another potential problem is that incomplete dehydration will leave the teeth with opaque areas. This is correctable by additional dehydra- tion in 100% ethyl alcohol. Incom- plete or uneven decalcification can be avoided by agitation of acid three times a day, having sufficient volume of acid, daily changes of acid solu- tion, and not stacking teeth during demineralization as demineraliza- tion occurs more rapidly at the top of the container than at the bottom. The decalcification process is slow but can be accelerated by heating the acid solution or by using a more concentrated acid solution. Either method can be used, but shrinkage may increase and digestion of the organic component of the tooth is probable. Another common problem is the development of opacity after air drying. However, this is readily

Fig 4--Maxillary first molar for visualization of sealer and accessory cones used in lateral condensation.

reversible by immersion in oil of wintergreen. Another solution for this may be embedding in acrylic resin. 13.~4

The current technique does not require a hardening agent, such as xylene. Therefore, biomechanical procedures could not be performed on the prepared models. However, it is not clear that the original hardness of the dentin is restored by xylene thus permitting in hand procedures to be viewed in the same light as the in vivo situation.

We have suggested a technique for studying the fine detail of the mor- phology of the root canal and the instrumentation and obturation of teeth in vitro. Two clinical endodon- tic failures were studied.

C a s e 1

Case I involved a mandibular left

first molar extracted two years after treatment because of an apparent return of symptoms. Shortly after the root canal treatment was completed, the patient's condition was diag- nosed as trigeminal neuralgia of the left mandibular division. Trea tment of this disorder resulted in a signifi- cant reduction in the pain. Approxi- mately a month before extraction, a burning sensation occurred in the region of this molar resulting in its removal. After the tooth was made transparent, examination disclosed the following: slight underfilling of the distal canal, a mesiolingual canal that was well filled using lateral condensation, and mesial-facial ca- nal that displayed both an inade- quate fill and a perforation. This canal contained a single gutta-per- cha cone that was encompassed by debris, sealer, and voids and ex- tended approximately 3 mm past the apex. The perforation was on the furcal aspect of the mesial root. Exit- ing from this defect was an at tempted gutta-percha filling that lay on the external surface of the root (Fig 5).

C a s e 2

Case 2 involved a maxillary right first molar extracted after two years of treatment because of intermittent pain. The tooth had been re-treated with use of lateral condensation, but the intermittent pain persisted. The next procedure was apical surgery which involved the placement of retrofill silver alloys in the buccal roots. The symptoms continued, and an intentional replant with retrofilts on all three roots was performed. Again, the pain persisted, so a resec- tion procedure was performed in which the mesiat root was removed.

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T h e pa in persis ted a n d the t oo th was

ex t rac ted .

T h i s t e c h n i q u e can p r o v i d e ins ight

in to those cases tha t fail because o f

b i o m e c h a n i c a l e r ror i n v o l v i n g inade-

q u a t e i n s t r u m e n t a t i o n or o b t u r a t i o n

(as in the too th in case 1). H o w e v e r ,

no t all ques t ions r e g a r d i n g c l in ica l

fa i lures are answered . T h e phys ica l

basis for fa i lure in case 2 r ema ins

unde tec t ed .

T h e t r a n s p a r e n t t ee th h a v e been

used on a l imi ted basis to deve lop a

be t t e r u n d e r s t a n d i n g a n d app rec i a -

t ion of root cana l a n a t o m y a n d endo-

d o n t i c p rocedures for c o n t i n u i n g

d e n t a l e d u c a t i o n a n d for the educa -

t ion of pat ients .

O t h e r app l i ca t i ons o f the tech-

n i q u e have been p r o p o s e d such as

e v a l u a t i o n of e n d o d o n t i c f i l l ing tech-

n iques , assessment o f e n d o d o n t i c fill-

ing mater ia l s , e v a l u a t i o n o f instru-

m e n t a t i o n t e c h n i q u e s , a n d a m e t h o d

of se l f -eva lua t ion o f m a n i p u l a t i v e

e n d o d o n t i c skills.

SUMMARY AND CONCLUSIONS

A s imple and i nexpens ive tech-

n i q u e for in v i t ro e x a m i n a t i o n of

e n d o d o n t i c a l l y t rea ted o r u n t r e a t e d

root cana l systems has been pre-

sented. T h e knowledge o b t a i n a b l e

us ing this mode l pe rm i t s an in-

c reased a p p r e c i a t i o n of the a n a t o m y

of the root cana l a n d the effects of

e n d o d o n t i c p rocedures on the tooth.

T h i s k n o w l e d g e cou ld resul t in h igh-

er q u a l i t y of and inc reased d e m a n d

for e n d o d o n t i c services. T h i s tech-

n i q u e has po t en t i a l for use in d e n t a l

e d u c a t i o n and in analysis o f c l in ica l

failures.

The authors thank Drs. Robert Shankle and Daniel Kolzet of the University of North Carolina School of Dentistry, department of

endodontics, for preparing teeth for examina- tion. Financial support for this project was provided by the Endodontics Fund of the Dental Foundation of North Carolina.

Dr. Robertson is an endodontic resident, University of North Carolina, and a commis- sioned officer, US Public Health Services, Indian Health Service. Dr. Leeb is associate professor and chairman, and Drs. McKee and Brewer are endodontic residents, University of North Carolina at Chapel Hill, NC. Requests for reprints should be directed to: Dr. I. Joel Leeb, associate professor and chairman, Department of Endodontics, School of Den- tistry 209H, University of North Carolina, Chapel Hill, NC 27514.

References 1. Kuttler, Y. Microscopic investigation of

root apexes. JADA 50:544-552, 1955. 2. Green, E.N. Microscopic investigation of

root canal diameters. JADA 57:636-644, 1958.

3. Weine, F.S., and others. Canal configu- ration in the mesiobuccal root of the maxillary first molar and its endodontic significance. Oral Surg 28:419-425, 1969.

4. Baher, N.A., and others. Scanning elec- tron microscopic study of the efficacy of vari- ous irrigating solutions. J Endod 1(4):127-135, 1975.

5. Cams, E.J., and Skidmore, A.E. Config- uration and deviations of root canals of max- illary first premolars. Oral Surg 36:88'0-886, 1973.

6. Fisher, D.E.; Ingersoll, N.; and Bucher, J.F. Anatomy of the pulpal canal: three dimensional visualization. J Endod 1(1):22- 25, 1975.

7. Davis, S.R.; Brayton, S.M.; and Gold- man, M. The morphology of the prepared root canal: A study utilizing injectable silicone. Oral Surg 34(4):642-648, 1972.

8. Brayton, S.M.; Davis, S.R.; and Gold- man, M. Gutta-percha root canal fillings. An in vitro analysis. Oral Surg 35(2):226-231, 1973.

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12. Pomaranz, H.H., and Fishelberg, G. The secondary mesiobuccal canal of maxillary molars. JADA, 88:119-124, 1974.

O F

Fig 5--Mandibular left first molar, endodon- tic clinical failure treated with lateral conden- sation. Shown are underfilld distal canal (U) and mesio-facial canal that is perforated (P), inadequately obturated (I), and over-

filled (OF).

Fig 6.--Maxillary right first molar, clinical failure treated with use of lateral condensa- tion, re-treated using two surgical procedures, and then extracted and cleared for examina- tion.

13. Vertucci, E.; Sealing, A.; and Gillis, R. Root canal morphology of the human maxil- lary second premolar. Oral Surg 38(3):456- 464, 1974.

14. Seeling, A., and Gillis, R. Preparation of cleared specimens for pulp cavity studies. J Dent Res 52:1154, 1973.

15. Hasselgren, G., and Tronstad, L. The use of transparent teeth in the teaching of preclinical endodontics. J Endod 1(8):278- 280, 1975.

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