The investigation of microleakage in root canal therapy An
9. :?I. Ja.cab.so,t, B.A., Il.D.A’.,* nxd J. A, ,COZLFrcmdwfer, A.I.M., F.R.I.C.,“” Imdm, Englnnd INSTITUTE
OF LONDO?: An t~lec~trochemicnl technique employing zero-resistance ammetry is described for evaluating the quantity, rate, and areas permeated in the canal system of a rootfilled tooth by 1 per cent potassium chloride. Single-rooted extracted teeth mtre rootfilled by a vertical condensation guSutta-percha technique. The teeth were coated from the eementoenamel junction to 3 mm. from the apex with a stopping-off medium, so that the apical foramen and the accessory canals in the area would play a role, lmt not the lateral canals in the gingival area. The teeth were placed in a potassium chloride solution. When tlw potassium chloride permeated the apical seal and reached a mild steel rod placed tllrough the occlusxl access opening extending 2 mm. into t.he coronal end aspect of the canal, corrosion of the steel was established and was measured for quantity and rate. This trvAniquo suggests the possibility of anothc~r pathway for tissue fluid to effect coronal dentinal discoloration. The twhnique also suggwts :I quantitatiw mwns of mwsuring apical sealing procedures.
clinical situation often encountered is an endodontically treated tooth which may be satisfactory in most respects but may still require further rcstorative treatment for esthetic reasons. The latter may arise because of dentinal discoloration. This article reports an investigation of a technique which provides eridenre for consideration of an additional cause for dentinal discoloration. There have been extensive investigations of the leakage pattern connected with the various apical sealing techniques, and commonly dye-penetration and radioisotope methods are usecl in these studies.l-l1 Swh investigations have pro*Department **Department
of Conservative Dentistry. of Dental Materials. 817
and van Praunhofer
vided a broad measure of agreement, but in general quantitative results cannot be obtained; particularly the time scale of leakage is difficult to determine. This preliminary communication reports an electrochemical technique which permits the rapid quantitative evaluation of geriapical leakage in a coronal direction. METHOD
The specimens used in this study were ext.racted mandibular and maxillary anterior teeth and single-rooted premolars. They were collected without regard to the patient’s age and were stored in distilled water at 20 + lo C. The root canals of the teeth were prepared with hand-held reamers and files to a size that would allow an acceptable gutta-pcrcha vertical condensation technique to be performed. This method was selected because,in our opinion, it would give the best apical obturation. Rickert’s sealant was used in conjunction with the obturating technique. The teeth were then coated with two layers of inert impermeable stopping-off medium * from the apex of the tooth to the gingival enamel, but leaving the apical 3 mm. uncoated. This procedure was adopted to permit functioning of both the apical foramen and any accessory canals in this area (Fig. 1). A mild steel rod was then placed through the ocelusal accessopening of the tooth HOthat it traversed the pulp chamber and entered the top 2 mm. of the root canal and was held in position by plasticine. The canal was dried with paper points and compressed air to ensure that it was dry before placement of the mild steel rod. A copper lead connected the steel rod to one input of a potentiostat operating in the zero-resistance ammeter modc.12rI:5Stainless steel tape functioned as the other electrode in the galvanic cell established between the stainless steel cathode and the mild steel rod placed in the mouth, with 1 per cent potassium chloride serving as the electrolyte. A galvanic corrosion current in this system will flow only when there has been leakage into the root canal and a continuous electrolytic path has thus beromc established. The time elapsed between immersion and current flow accurately denotes the potassium chloride penetration rate, while the magnitude of the current will indicate the degree of penetration. The principle of the technique is that the mild steel rod, in contact with potassium chloride and coupled to stainless steel, will function as the anode and corrode. The stainless steel functions as the cathode in the galvanic corrosion cell and does not. corrode. The magnitude of the current detected by the zero-resistance ammeter is an accurate indication of the rate of corrosion which will be determined by the aggressivenessof the solution, the area of metal corroding, the nature of the metal itself, and various other electrochemical factors.” Three teeth that had been root fi!led in the samemanner as the test teeth were coated (including the apical foramen and accessory canal zone to the cementoenamel junction with the stopping-off medium) to act as controls. They were placed in solution with a mild steel rod in the same position as before and were *Lacomitc
& Co., Birmingham,
of test specimen.
coupled to the measuring apparatus at the end of 21 days, and the galvanic current flow was measured. Four test teeth were intermittently monitored for 11 days, and the observed current patterns were recorded. Measurements were also made in all teeth at the end of 21 days. RESULTS
The control teeth did not exhibit galvanic current flow at the end of 21 days, although the four test teeth monitored for 11 days showed a mean current of 0.2 microamperes. A current first appeared on the seventh day for one tooth, on the eighth day for two teeth, and on the ninth day for the remaining test teeth, so that, on the average, leakage as evidenced by corrosion occurs within 8 days. A constant current was observed up to 11 days and immersion was continued for a further 10 days. The mean current for the test teeth after immersion for 21 days was 2.0 _+0.3 microamperes. Thus, a tenfold increase in the current density occurred between 11 and 21 days’ immersion. The control teeth showed no current flow for the 21-
Fig. Z. SEM nodular corrosion
photograph of mild steel rod after insertion products indicated by arrows. (Magnification,
in test x100.)
day test period, suggesting that potassium chloride permeation occurred only through the apical foramcn or the accessory canal in the apical zone. However, it must 1~ pointed out that the significance of the magnitude of the current (other than the fact that. it was detected) is not apparent at this time and it will be necessary to test many more teeth in order to make a comparison bctwccn the various apical sealing procedures. The current was calculated according to Ohm’s law (E = IR., E being the potential diffcrcnce established across a standard resistance of 10,000 ohms inserted in the zero-resistance ammeter circuit, I being the current in microamperes, and R the 10,000 ohms standard resistance). The important finding at this time is the production of a current resulting from corrosion of the steel rod. In Fig. 2 a scanning electron micrograph of the end of the steel rod from the cervical aspect of the canal shows that it is obviously co\-wed by corrosion products. In Fig. 3 corrosion products are also visible on the canal walls. This corrosion would appear to have been a result of 1 per cent potassium chloride solution permeating the apical seal and migrating up the canal to come into contact with the mild steel rod. DISCUSSION
The underlying principle of this technique is that a metal, such as mild steel, corrodes when in contact with a corrosive, such as potassium chloride solution. If a galvanic cell is created by coupling the mild steel to a more electropositive
Pig. 3. SEM 1 xnd 2 indicate x700.1
photogrspt~ of corrosion products deposited deposits of corrosion products approximating
in root caml
on the cervical canal wall; arrows dentinal tubules. (Magmfication,
metal, such as stainless steel, the corrosion rate of the steel is accelerated and the current flowing between the two metals can be measured by external means, namely, zero-resistance ammetry. The latter technique is used because it allows the galvanic corrosion current to be measured without interference with the corrosion reaction. In t.hc method reported here, the mild steel is “protected” from exposure to the c*hloricle solution (and hence is protected from corrosion) by enclosing it in an cndodontically treated tooth. Corrosion will occur only when the chloride solution contacts the mild steel, that is, when leakage has occurred and a continuous electrolytic pathway has become established. 14t this point a current will flow between the mild steel and the stainless steel. Thus, the method permits the accurate detection of the onset of leakage into the coronal portion of the tooth. Furthermore, the magnitude of the current can be utilized to indicate the volume of leakage since the corrosion current will increase as more of the steel surface is covered by the diffusing liquid. Clinical evidence indicates that teeth which have received root canal therap! often become esthetically unsatisfactory because of discoloration. This discoloration has been ascribed to a variety of sources, including pulpal tissue remnants, root canal and restorative filling materials, and blood-degradation products.14-1’ The discoloration of coronal dentin, however, has been noted even when meticulous attention has been paid to the root canal preparative and sealing procedures.
This would suggest that discoloration of the coronal enamel of root-filled teeth can arise from other sources. Previous studies on the prcpara.tion’s and obturation’“, “” of root vanal systems have indicated that a perfect obturation of the canal, particularly a lateral canal, is not possible. On this basis, therefore, it is conceivable that periapical ingress of tissue fluids due to leakage and capillary flow up the root canal could lead to discoloration of the tooth tissue. The studies reported here clearly demonstrate that such leakage does occur in vitro and its onset is rclat.ively rapid, bcroming est,ablishcdwithin approximatcl) 1 week. Thus, the prcsencc of tissue fluids at the leaseof the pulpal chamber might bc anticipated within a short time following the root treatment of a tooth in the clinical situation. Clearly, this fluid would tend to diffuse into dentinal tubules of the crown because of the anatomy of the tubules in the coronal area of the canal,?‘~*” as well as flow into the pulp chamber. Thus, a transport system for tissue fluids, elements from the root-filling material, and corrosion products from posts or other metallic crown supports could become establishecl. If diffusion of these liquids into the dentin occurs, then in the absenec of full carowncoverage the long-term staining of tlcntin will bccomc apparent. This study indicates that leakage within the root canal and its attendant dentin discoloration might be expcatcd to occur at some time following root canal therapy. The clcgrce and extent of staining will, of course, be dependent on a number of factors, including the effectiveness of canal obturation, the materials used, and the anatomy and phJ-siology of the individual tooth and its environment. The results of this study would suggest that methods for the sealing of the cervical and coronal dentin from the ranal system must be investigated in greater depth than in the past. CONCLUSION
This study demonstrates t.hat leakage into the upper part of the eanal occurs in vitro in root-filled teeth placed in potassium chloride solution. In a clinical situation this leakage might lead to dentin discoloration because of the areas of the canal system into which the solution permeated and caused corrosion. Clearly, effective root canal therapy requires complete obturation of the canal to prevent leakage. However, clinical attempts at accomplishing this obturat.ion may not suffice for csthctic successand, therefore, measuresto seal the dentin in the coronal aspect of the eana.1as well as the pulp chamber must be adopted. Future work will bc devoted to the comparison of root-filling techniques by zero-resistance ammetry in order to suggest to the clinician the best procedure for a given clinical situation. Investigations of sealing procedures of cervical and coronal dentin will bc, carried out, as this would appear to be one of the methods of preventing dentinal discoloration by an apical foramen or accessory canal pathway. REFERENCES
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J. I. : Isotope
ORAL SURG. 8:
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