0099-2399/90/1609-0442/$02.00/0 JOURNAL OF ENDODONTICS Copyright 9 1990 by The American Association of Endodontists

Printed in U.S.A.

VOL. 16, NO. 9, SEPTEMBER1990

Dentin Permeability: Effects of Endodontic Procedures on Root Slabs H. M. Fogel, DDS, MS, and D. H. Pashley, DMD, PhD

The permeability of human radicular dentin was measured as a hydraulic conductance before and after treatment with K files and before and after subsequent treatment of the endodontic smear layer with NaOCI, 50% citric acid, or 3 % monopotassiummonohydrogen oxalate. Filing reduced dentin permeability 25 to 49%, respectively, depending upon whether outer or inner root dentin was filed. The permeability of these smear layers was unaffected by 5% NaOCI but increased many times after treatment with 50% citric acid for 2 min. Oxalate treatment lowered root dentin permeability to levels below that produced by creation of smear layers due to the production of a crystalline precipitate.

sealing benefits are lost. Others have tried to cross-link the smear layer to improve its bonding to the underlying dentin (5). Still others have tried to remove the smear layer and replace it with an uncontaminated chemical sealing layer (6, 7). Such a sealing layer may be less permeable than the original smear layer. Pashley and colleagues (7, 8) have shown that potassium oxalate greatly reduces coronal dentin permeability by forming crystals that block the tubule orifices. This is the proposed mechanism of action of oxalate as a desensitizing agent in dentin hypersensitivity, although it has only been tested on coronal dentin. It may not be necessary to remove the smear layer in order to effectively reduce dentin permeability with potassium oxalate (8); however, it may be desirable to acid etch the smear layer if it is contaminated with bacteria and bacterial products. The purpose of this study was to investigate the effects of endodontic instrumentation, irrigation, and potassium oxalate application on the permeability of root dentin. M A T E R I A L S AND M E T H O D S

Smear layers occupy a unique place in dentistry and are a subject of controversy in endodontics. When canals are enlarged, a smear layer is produced. The smear layer masks the underlying dentin matrix and decreases permeability by blocking the tubule openings (1). Dental materials that are capable of flowing into open tubules (e.g., resins, root canal sealers, cements) are prevented from doing so by the presence of the smear layer. The smear layer also interferes with the direct bonding of materials capable of adhering to the dentin matrix (e.g. glass ionomers, polycarboxylate cements). This results in a decrease in retention and increased marginal leakage (2). The smear layer may be beneficial in that it provides an obstruction to bacterial penetration of dentin and decreases dentin's permeability. However, Akpata and Blechman (3) found that a smear layer created by endodontic instrumentation was permeable to streptococci. In addition, the smear layer may slowly dissolve if exposed to subsequent leakage or plaque acids. This would leave a void between the dentin and the adjacent dental material which saliva could fill. Bacterial colonization of the unprotected dentin surface might then occur (4). The smear layer may also harbor bacteria or bacterial products and so provide a reservoir of potential irritants beneath a restoration or in a treated root canal (3). For these reasons, many investigators have looked at ways of removing the smear layer (2, 5). When the smear layer is removed its

Extracted, unerupted, human third molars from young patients (age 19 to 23 yr) were placed in isotonic saline containing 0.2% sodium azide (to inhibit microbial growth) and stored at room temperature. Only teeth with relatively broad, fiat root surfaces were selected. The crowns were removed using a high-speed diamond saw. Inner and outer root slabs were prepared as described previously (9). The outer root slabs consisted of cementum and superficial dentin. The inner root slabs consisted of dentin alone. Each specimen was dipped in 50% citric acid for 2 min to remove any smear layers created by the diamond saw. The specimens were placed in a plastic split chamber device and connected to the apparatus described by Pashley and Galloway (8). Briefly, a nitrogen cylinder provided the hydrostatic pressure gradient which moved isotonic saline containing 0.1% trypan blue dye from the reservoir, through a micropipette (Fig. 1), into the lower compartment of the split chamber and through the dentinal tubules to the open upper compartment. The movement of an air bubble in the micropipette was measured in m m per min and, knowing the volume of the micropipette, this was converted to ~zl per min. The dentin surface area studied was limited by identical rubber O rings on each side of the slab. A microsyringe was used to control the position of the air bubble in the system. Experiments were conducted at room temperature. Each measurement was repeated four times.

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Vol. 16, No. 9, September 1990

Root Dentin Permeability

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vol) monopotassium monohydrogen oxalate was used (Protect; John O. Butler Co., Chicago, IL) and the hydraulic conductance (L~) of the dentin remeasured. Selected samples were washed in water, air dried overnight, coated with gold, and examined in a JOEL scanning electron microscope. RESULTS

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FiG 1. Schematic demonstrating how the root slabs were prepared from teeth and mounted in a split chamber device to measure Lp.

Hydraulic conductance (Lp) was calculated using this formula: Jv

Lp = AAP where Jv = fluid flow in ul min-~; A = dentin surface area in cm2; AP = hydrostatic pressure gradient in cm H20; and Lp = hydraulic conductance in tA cm-2min-~cm n 2 0 -t. Fluid movement (Jv) was calculated from m m of bubble movement per min. Hydrostatic pressure was controlled by the regulator on the nitrogen tank. Due to variations in permeability between different teeth, results were expressed as percentage of m a x i m u m hydraulic conductance of the acidetched (50% citric acid, 2 min) root slabs prior to any manipulation. Thus, each slab acted as its own control. The hydraulic conductances calculated using the formula were entered into a microcomputer statistics program (Star Pack; The Basic Business Software, Co., Inc., Portland, OR). The means and standard deviations of the different groups were examined for statistically significant differences using a one-way analysis for variance and ranked using Duncan's multiple range test. Student's t test was used to determine whether significant differences existed between the mean L j s before and after surface manipulations. All tests for statistical differences were conducted with a = 0.05. Pilot trials had shown that in most cases, for radicular dentin slabs, the use of 25-#1 pipettes (65-ram long) and a hydrostatic pressure gradient o f 15 psi (1050 cm H20) resulted in rates of bubble movement that could be accurately measured over a 10-min period. The surface area studied was always 8.76 • 10-2 cm. This was the largest area possible due to the topography of the samples and represented the surface area within the O rings. Filing was done using #60 K files (Union Broach, New York, NY) on the inner surfaces of the root slabs. About three to four passes were made over each part o f the dentin which was kept moist with saline solution. The hydraulic conductance of each sample was measured after acid etching, after using K files with lateral force. Irrigants tested were saline, 5.25 % sodium hypochlorite, and 50% citric acid, in that order. These irrigants were rinsed over the surfaces to be tested for 2 rain. The effect of potassium oxalate application to inner root surfaces was also studied. A 2-min exposure to 3% (wt/

The scanning electron micrographic appearance of acidetched radicular dentin revealed the typical appearance of etched dentin (Fig. 2). The use of K files on that surface created a smear layer which obscured the orifices of the underlying dentinal tubules (Fig. 3). Although it appeared that the tubules were sealed by smear layer, fluid continued to penetrate through the dentin albeit at a slower rate. Treatment of acid-etched inner dentin with 3% monopotassiummonohydrogen oxalate completely covered the tubule orifices with a fine precipitate (Fig. 4). The use of endodontic files on the inner surface of the inner root slabs resulted in a mean reduction in Lp of 48.6% (Table I) to 51.4% of maximum values (p < 0.001). NaOC1 application for 2 min did not affect the Lp of the endodontic smear layer (Table 1) nor did it change the scanning electron microscopic (SEM) appearance of the smear layer (not shown). This file-created smear layer could be removed by acid etching and the Lp of the specimens rose to values higher than the prefiled levels (i.e. 118%). The post-etch L; was higher than the pre-etch values because the dentin had been made thinner by filing. The SEM appearance of the K-filed dentin after acid etching was no different from the original appearance of the acid-etched dentin (Fig. 2). There was no difference in the response to acid if the smear layer was pretreated with NaOC1 prior to acid etching (not shown). Application of potassium oxalate to this acid-etched surface caused a mean reduction in Lp of 88.7% (Table I) to values that were 11.3% of maximum. There was a significant difference between the mean reductions in Lp caused by file-created smear layers and by potassium oxalate (p < 0.01).

F~G 2. SEM of inner surface of inner half of root dentin etched with 50% citric acid for 2 min (original magnification x2000).

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Journal of Endodontics

Fogeland Pashley

TABLE 1. Changes in the hydraulic conductance of root dentin before and after various treatments (percentage of maximum permeability*: X -+ SEM) Inner Dentin Before K file 100 After K filet 51.4 +- 12.0 (5)~t After NaOCIw 46.4 -+ 6.3 (5) After etchll 118.3 -+ 29.0 (5) [ After oxalate82 11,3 -+ 6.6 (5) [

Outer Dentin 74.8 73.6 110.9 38.7

100 _+ 12.8 (5) [ + 9.3 (5) -+ 11.5 (5) __ 11.8 (5) I

I

9 The maximum Lp of outer dentin was 2 x 10 -s and that of inner dentin was 2 x 10 -3 /~l cm-=min -1 cm H~O-1, "i" After K filing and saline rinse_ :~ Values joined by vertical lines in the same plane were not statistically different at p < 0.05. w Five percent NaOCI was applied for 2 rain followed by water rinse. II Etched with 50% citric acid for 2 rain followed by water rinse. 82Applied 3% (wt/vol) potassium-hydrogen oxalate for 2 min followed by a water rinse.

FIG 3. SEM of inner surface of inner half of root dentin previously etched with citric acid and then filed with K file. Underlying tubules are completely obscured with smear layer. Diagonal groove is deep file mark (original magnification x5000).

FiG 4. SEM of inner surface of inner half of root dentin previously acid etched with citric acid followed by a 2-min treatment with 3% oxalate. No tubule orifices were visible (original magnification x 10,000).

The use of endodontic files on the inner surface of the outer root slabs resulted in a mean reduction in Lp of only

25.2% (Table 1) to values of 74.8% of maximum. This smear layer was also insensitive to NaOCI but acid labile, and the Lp returned to its original value or higher (i.e. 110.9%, Table 1) after acid-etching. DISCUSSION The permeability of root dentin has been reported to be low relative to coronal dentin (10). This was related to low dentinal tubule density and small tubule diameter. Inner dentin was found to be more permeable than outer dentin. The results of this experiment confirmed those previous conclusions. When smear layers are created on coronal dentin, they decrease dentin hydraulic conductance (Lp) 70 to 90% (10-

13). Generally, coronal dentin smear layers have been created using abrasive paper or burs. The reductions in the Lp of radicular dentin produced by K files was much lower (25 to 49%, Table 1) than the reported reductions in coronal dentin produced by burs (70 to 90%), probably because there is less heat generated in filing and less burnishing of the cutting debris. Inner root dentin smear layers reduced Lp more than outer root dentin smear layers (Table 1) although the difference was not statistically significant due to the high standard errors. Inner root dentin is probably softer than outer root dentin (14) and may be more easily "smeared." Endodontic preparation techniques remove the innermost layers of predentin and root dentin. The precise effects of such procedures on the permeability of the remaining root structure in vivo are difficult to predict. Reducing dentin thickness tends to make the remaining dentin more permeable. However, as dentin thickness is reduced from the pulpal surface outward, the actual number and diameter of the exposed dentinal tubules per unit surface area decreases due to the divergence of the tubules. The net effect is a slower rate of increase in permeability than would be predicted from the decreased thickness alone (15). Canal enlargement increases the canal wall surface area as well as decreasing the dentin thickness. Both diffusion (15) and filtration increases with available surface area. Since canal anatomy is complex, endodontic instruments do not reach all canal walls (16). Thus, the final shape of the prepared canal is also geometrically complex and this makes estimates of canal wall surface area rough at best. In a study of the efficacy of several endodontic irrigating solutions, Goldman et al. (17) found that sodium hypochlorite alone did not remove the smear layer. This is supported by the finding of the present investigation since sodium hypochlorite was found to have no effect on the hydraulic conductance of smear layer-covered radicular dentin slabs (Table 1). In a subsequent paper however, Goldman et at. (18) found that when sodium hypochlorite was used as a final flush in an instrumented canal after pretreatment with EDTA, the smear layer was more effectively removed than when EDTA was used alone. For this reason, they speculated that the smear layer may contain an organic component which was partially masked by mineral. It seems likely that this is true since endodontic instruments in contact with healthy and necrotic pulp tissue, bacteria, and odontoblast processes must smear some tissue on to the canal walls (19). Nevertheless,

Vol. 16, No. 9, September 1990

sodium hypochlorite treatment in the absence of pretreatment with EDTA does not seem to influence the permeability of the remaining dentin (Table l). Citric acid is also used as an endodontic irrigant. A 50% solution is reported to remove the smear layer and open the tubule orifices to allow the penetration of resin to seal the canal (19). A 10% solution alternated with NaOCI produced clean canal walls and patent tubules (20). In this study, 50% citric acid effectively removed the smear layers created by K files and returned the hydraulic conductance to control values or slightly higher depending on the amount of thickness reduction (Table 1). However, the files were not used in the usual manner since they were not within the confines of a root canal. Rather, they were used with lateral force on flat slabs of root dentin. Further work needs to be done on the effects of different file sizes and techniques on root dentin permeability using intact roots. Potassium oxalate was shown to significantly reduce the permeability of root dentin. However, it reduced Lp more when applied to inner root dentin than to outer dentin (reductions to 11.3 versus 38.7% of maximum, respectively, p < 0.05). The hydraulic conductance of coronal dentin has been reported to be reduced to 4% of maximum permeability by a 2-min application of 3% monopotassium oxalate (9). Thus, the reductions in permeability achieved after potassium oxalate application in root dentin are somewhat less than expected (7, 8). The monopotassium-monohydrogen oxalate (pH 2.5) used in our study mobilizes calcium from the dentin matrix to form calcium oxalate crystals. These crystals obstructed the tubule orifices (Fig. 4), and caused a profound reduction in the hydraulic conductance of dentin to levels below that produced by smear layer creation (Table l). All previous studies on the effects of oxalate treatment on dentin permeability were done with coronal dentin (7-9). Potassium oxalate may also be of use in endodontics. The smear layer created by endodontic instrumentation may harbor bacteria or bacterial products. It may also reduce the ability ofirrigants and medicaments to penetrate into dentinal tubules to dissolve tissue and kill bacteria. For these reasons many investigators have looked at ways of removing the smear layer (5). However, the benefits of reduced dentin permeability are then lost. The creation of a potassium oxalate layer after chemomechanical canal preparation has been completed may be an advantage. Since potassium oxalate reduces dentin permeability, any bacteria remaining in the tubules would be entombed between the sealed layer on the pulpal side and the relatively impermeable (10) dentin/cementum layer found peripherally. The diffusion of toxic substances from necrotic tissue in such tubules would also be greatly reduced. However, potassium oxalate would not seal large canal ramifications that could potentially harbor greater amounts of irritants than could the dentinal tubules. The actual clinical effect of potassium oxalate used in this way is unknown.

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CONCLUSIONS The use of endodontic files created smear layers which produced modest reductions in the permeability of inner and outer root dentin. Sodium hypochlorite or saline application did not affect the hydraulic conductance o f such smear layers. These smear layers could be removed by acid-etching to restore the Lp to unoccluded values. Application of potassium oxalate to radicular dentin resulted in dramatic reductions in hydraulic conductance. This research was supported, in part, by Grant DE06427 from the National Institutes of Dental Research and by the Medical College of Georgia Dental Research Center. The authors are grateful to Shirley Johnston for secretarial and editorial assistance. Dr. Fogel is a member of the Department of Endodontics, Faculty of Dentistry, University of Manitoba, Winnipeg, Manitoba. Dr. Pashley is a member of the Department of Oral Biology, School of Dentistry, Medical College of Georgia, Augusta, GA.

References 1. Dippel HW, Borggreven JMPM, Hoppenbrouwers PMM. Morphology and permeability of the dentinal smear layer. J Prosthet Dent 1984;52:657-62. 2. Goldman M, Devitre R, Pier M. Effect of the dentin smeared layer on tensile strength of cemented posts. J Prostet Dent 1984;52:485-8. 3. Akpata ES, Blechman H. Bacterial invasion of pulpal dentin wall in vitro. J Dent Res 1982;61:435-8. 4. Vojinovic O, Nyborg H, B(bnnstrbm M. Acid treatment of cavities under resin fillings: bacterial growth in dentinal tubules and pulpal reactions. J Dent Res 1973;52:1189-93. 5. Pashley DH, Smear layer: physiological considerations. Oper Dent 1984;3(suppl): 13-29. 6. Bowen RL, Cobb EN. A method for bonding to dentin and enamel. J Am Dent Assoc 1983;107:734-6. 7. Greenhill JD, Pashley DH. The effects of desensitizing agents on the hydraulic conductance of human dentin in vitro. J Dent Res 1981 ;60:686-98. 8. Pashley D, Galloway S. The effects of oxalate treatment on the smear layer of ground surfaces of human dentine. Arch Oral Bio11985;30:731-7. 9. Pashley DH, Leibach JG, Homer JA. The effects of burnishing NaF/ kaolin/glycerin paste on dentin permeability. J Pedodonto11987;58:19-23. 10. Fogel H, Marshall F, Pashley D. Effects of distance from the pulp and thickness on the hydraulic conductance of human radicular dentin. J Dent Res 1988;67:1381-5. 11. Pashley DH, Livingston MJ, Greenhill JD. Regional resistances to fluid flow in human dentine in vitro. Arch Oral Biol 1978;23:807-11. 12. Boyer DB, Svare CW. The effect of rotary instrumentation on the permeability of dentin. J Dent Res 1981 ;60:966-71. 13. Tao L, Pashley DH. The relationship between dentin bond strengths and dentin permeability. Dent Mater 1989;5:133-9. 14. Pashley DH, Okape A, Parham T. The relationship between dentin microhardness and tubule density. Endod Dent Traumato11985;1:176-9. 15. Outhwaite WC, Livingston MJ, Pashley DH. Effects of changes in surface area, thickness, temperature and post-extraction time on human dentine permeability. Arch Oral Bio11976;21:599-603. 16. Walton RE. Histologic evaluation of different methods of enlarging the pulp canal space. J Endodon 1976;2:304-11. 17. Goldman LB, Goldman M, Kronman JH, Lin PS. The efficacy of several irrigating solutions for endodontics: a scanning electron microscopic study. Oral Surg 1981 ;52:197-204. 18. Goldman M, Goldman LB, Cavaleri R, Bogis J, Un PS. The efficacy of several endodontic irrigating solutions: a scanning electron microscopic study: part 2. J Endodon 1982;8:487-92. 19. Tidmarsh BG. Acid-cleansed and resin-sealed root canals. J Endodon 1978;4:117-21. 20. Wayman BE, Kopp WM, Pinero GJ, Lazzari E. Citric and lactic acids as root canal irrigants in vitro. J Endodon 1979;5(9):258-65.

Dentin permeability: effects of endodontic procedures on root slabs.

The permeability of human radicular dentin was measured as a hydraulic conductance before and after treatment with K files and before and after subseq...
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