D. Richardson. DOS Department oí Prostbodonlics

f. Tao, DDS Department oí Oral Biology

D.H. Pashley, DMD, PhD Department oí Oral Biology Permeability: Effects School of of Preparation Medical College oí Georgia

Sequential crown reductions of extracted human teeth were made to evaluate both regional differences in dentin permeability and the effects of reduction. Two different methods of tooth reduction were used. In group 1, tooth reduction was nonuniform but was done in a manner similar to conventional crown preparations. Maximum, total crown permeability was measured using a pressurized fluid filtration technique after removing the smear layer. Regional reductions in permeability were accomplished by creating smear layers on mesial, distal, buccal, lingual, and occlusal surfaces sequentially. Between each step, changes in dentin permeability were measured. In group 2, tooth reductions were uniform and regional permeability was reduced by localized application of potassium oxalate. Both methods demonstrated increased permeability of dentin as the prepared surfaces approached the chamber. Generally, the mesial surfaces were more permeable than the distal surfaces. The occlusal and lingual surfaces were the least permeable regions. Inrj Prostbodont 1991 ;4:219-225.

rown preparations are often necessary to potential for pulpal irritation, especially under pro- C restore teeth. However, preparation for com- visional restorations that may leak more than the plete-coverage restorations requires removal of final prosthesis, much of the enamel, thereby exposing a significant Dentin permeability is not homogeneous but var- fraction of dentin. Dentin contains approximately 2 ies widely in both occlusal dentin^ and in the dentin million tubules per cm-^,' and the surface area of of Class II cavities,^ Sturdevant et al' reported that crown preparations is approximately 1 cm^. the dentin ofaxial walls of Class II cavities was much Further, crown preparations reduce the thickness of more permeable than that on the occlusal surface, dentin from a normal value of 2.5 to 3.0 mm to even when the thickness was the same. However, between 1.0 and 1.5 mm, depending on the site little information is available on the permeability and type of preparation. Reductions in dentin thick- characteristics of complete crown preparations. ness increase dentin permeability^" and hence the The purpose of this experiment was to quantify the regional permeability characteristics of complete crown preparations. Correspondence to: Dr David H. Pashley, Oral Biology/Phys- iology, Medical College of Georgia. School of Dentistry. Materials and Methods Augusta, Georgia 30912-1129. Extracted, unerupted third molars from men and Editor's note: Dr Psshley holds the patent on potassium oxalate women between the ages of 19 and 28 years were as a dentin desensitizing treatment. He and the Medical Col- used in this study. The teeth were stored for less lege ol Georgia have licensed the lohn O. Sutler Company to market potassium oxalate under the trade name Protect, Den- than 1 month at 4°C in isotonic saline containing tin Desensitizing Solution. 0,2% sodium azide to inhibit bacterial growth. The

e4. Number 3, 1991 219 The Iniernarionai lournal of Pros Itio don lies Dentin Pecmeabililv: Effecls of Crown Preparation RiLhardsor et al

Crown Preparation N¡ Irom Pressure Tank

Pressurized Buffer MIcrosyrlnge Reservoir

Fig 1 Schematic diagram of the apparatus used to measure the permeability of dentin in crown preparations. Fluid movement across the dentin was measured by following the progress of a tiny air bubble in a micropipette.

roots were removed 2 mm below the cemento- After each regional creation of a smear layer, the enamel junction (CEJ) using an Isomet saw (Buehler reduction in dentin permeability was remeasured. Ltd, Lake Bluff, Illinois), The pulpal soft tissue was This was done in two stages. In the first stage, the removed with a cotton forcep, and the resulting crown was reduced through most of the enamel, crown was cemented to a 2 X 2 X 0,6-cm piece although a considerable amount of enamel of clear plastic (polv[methyl methacrylate]) using remained on the occlusal and lingual surfaces. This cyanoacrylate cement. The plastic sheet was pen- was designated as a shallow reduction, A second etrated by an 18-gauge stainless steel tube that per- reduction (designated as a moderate reduction) was mitted communication between the pulp chamber completed after making 0,5-mm-deep grooves in and the permeability measuring apparatus (Fig 1¡, the dentin (see group 2), The tooth structure Prior to crown preparation, the system was pres- between the grooves was then reduced to yield a surized to 10 psi to check for leakage. If the tiny air bubble moved in the micropipette (Fig 1), all junc- total reduction of approximately 2,0 mm. tions were checked for leakage and the tooth was In group 2, uniform grooves 0-5 mm deep were remounted if necessary. prepared vertically and horizontally into the enamel using laminate veneer diamond depth guides (cat- Crowns were reduced using two methods. In alog no, 834-021, Brassier USA, Savannah, Georgia), group 1, a conventional crown preparation was None ofthe depth cuts or tooth reduction extended made by an experienced prosthodontist using a beyond the CE), The tooth structure between the medium-grit diamond bur (flame type, catalog no, depth grooves was then removed using a medium 258,8M, Premier, Norristown, Pennsylvania), This diamond bur (catalog no, 258.8, 2 Striper, Premier) procedure followed typical clinical practice, and the buccal surface ofthe tooth was reduced more than operated in a high-speed handpiece with copious the lingual surface. The maximum permeability of air-water spray. By using a digital micrometer (no, the teeth was measured by treating the surface with MC 8206, General Hardware Mfg Co, New York, Û.5M EDTA ipH 7,4) for 2 minutes. Regional reduc- New York), it was found that this reduction removed tions in permeability were then accomplished by 0.7 ± 0,1 mm of tooth structure. The creation of creating smear layers on each region using a light depth guide cuts and subsequent tooth reH^ ction brushing action of a medium-grit diamond bur in a were repeated four times in this group cr-.,-,,jared high-speed handpiece. to two larger reductions in the first group, Th.^ sur- face area was measured after the first and sr-cond

The [fiternational loumal of Prosthodontics 220 Richardson et al Dentin Permeabiliiy: Eífecls ol Crown Preparation reductions in groijp î and the third and fourth distal, buccal, lingual, and occlusal) to the total per- reducfions (Tables 1 a and 1 b) in group 2, The crown meability was then evaluated by the careful appli- surface area was measured by covering it with cation of 'i'Vi) monopotassium-monohydrogen Reprosil (LD Caulk Div, Dentsply, Milford, Dela- oxalate (Protect, John 0. Butler Co, Chicago, Illinois) ware) and then carefully removing the material, slic- sequentially to each region to reocclude the tubules ing it until it would lay flat under a glass slide, where and reduce the permeability of the dentin,'" This it was photographed with an adjacent scale. The treatment reduced dentin permeability more than surface area of the material was calculated from the original smear layer. The area was outlined in photographic enlargements using a digitizing tablet pencil, and the oxalate solution was carefully (SummaSketch II, Summagraphics Corp, Seymour, applied to avoid any spread beyond the desired Connecticut). Impressions of ohjects of similar size area. The solution generally stayed within the con- yielded surface areas that were within ±4% of the fines of the pencil mark. The tooth was held under actual surface areas. The permeability of each crown a dissecting microscope in a position that permitted preparation was measured using the fluid under control of gravitation force. The excess oxalate solu- pressure system previously described."' New depth tion was removed with endodontic paper points to grooves were prepared and the procedure was avoid spreading by rinse water. As phosphate-buff- repeated to remove residual enamel and to reduce ered saline permeated through the nonoccluded dentin thickness. This procedure led to a uniform region of increased permeability, it was collected reduction in tooth structure compared to the sam- with endodontic paper points to avoid the spread ples in group 1, in which the heights of contour of any residual surface oxalate that might have been were completely removed. dissolved by the buffer. In this manner, the reduc- After the third (shallow) and fourth ¡moderate) tion in the permeability of each region could be reductions in group 2 (ie, removal of approximately evaluated. It was assumed that oxalate treatment 2 mm of tooth structure), the total permeahility of reduced the permeability of the treated area 100% the crown preparation was measured after removing rather than the actual value of 93%.''After treatment the smear layer using 0.5M EDTA (pH 7.4) for 2 of all five regions, the crown was immersed in the minijtes. The contribution of each region (mesial. oxalate solution for an additional 2 minutes to deter-

Table la Absolute Lp (hydraulic conductance) of Different Surfaces of Crown Preparation, in ;iL cnr^min-' cm H^O"', Mean ± SEM {intragroup comparisons) Shallow reduction Moderate reduction Group 1 Group 2 Group 1 Group 2

Mesial 0.0023 ±0.0001 0.0070 + 0.0022 0.0221 ±0.0074 0.0428 ±0.0164 Distal 0.0015±O.C001 0.0051+0.0025 0.0060 ±0.0004 0,0156 ±0.0093 Buccal O.G059±0.0018 0.0025 ±0 0008 0 0110±0.0032 0.0110±0,0073 Lingual 0.0019 ±0.0008 0.0012 ±0.0003 0 0022 ±0.0006 0.0129 = 0.0062 Occlusal 0.0020 ±0.0006 0.0007 ±0,0001 0.0056 ±0.0014 0.0180 ±0.0108

Groups connected by vertical lines m the same plane are not statisticalty different Irom one another at P -^ .05.

Table 1b Absolute LP (fiydraulic conductance) of Different Surfaces of Crown Preparation, in wL cm-^min-^ cm HjO"', Mean ± SEM (intergroup comparisons) Stiallow reduction Moderate reduction

Surface Group 1 Group 2 Group 1 Group 2

Mesial 0.0023 + 0.0001 0.0070± 0.0022 0.0221 ±0,0074 0.0428 + 0,0164 Distal 0.0015± 0.0001 0.0051 ±0.0025 0.0060 J: 0,0004 0.0156±0.0093 Buccal 0.0059 ±0.0018 0.0025 ±0,0008 0.0110± 0,0032 0.0110±0.0073 Linguai 0,0019 ±0.0008 0,0012±0,0003 0.0022 ±0.0006 0.0129 ±0.0062 Occlusal 0.0020 + 0.0006 0.0007 + 0,0001 0.0056 ±0.0014 0.0180 ±0.0108

Groups connected by horizontal lines are not statistically Oifterent irom each other al P = .05. Groups not canhecteO by the horizontal line are significantly datèrent at P< .05.

E4. Number 3, 1991 221 The International Journal ot Pros! fi odor lies ability: Effects of Cri mine if any areas had been missed. This had no of Table la lists the hydraulic conductances of each additional effect on permeability. of the five regions of the preparations after fhe tirs Finally, the crowns were sectioned longitudinally (shallow) and second (moderate) redn

Table 2a Relative Regional Dentin Permeability of Crown Preparation, Mean ± SEM (intragroup comparisons) Shallow reduction Moderate reduction

Surtace Group 1 Group 2 Group 1 Group 2

100 100 100 100 14,7 + 12,0(9) 34,4 + 4,6(9) 35,5-5.2(9; 40,8 + 5.0(7) 15.4+ 3.7(9) 16,0 + 6,1(9) 14,7 + 9.9(9; 7,7 + 3.9(7) 19.9+ 9,8(9) 21.7 + 4.8(9) 11,2 + 7,6(9; 13,2 ±5.0(7) 8,1 + 3,6(9) 10,7 + 4.0(9) 4,2±6.7(9' 10,3 ±3.5(7) Occlusal 16.'1± 4.8(9) 9,0 ±2.0(9) 9,5 ±2.5(9) 12,7±3,3(7)

l'allies connectsa by vertical Unes are not signitiçantly äittarant. Wumöers in parentt^eses indicate the numtier oi specimens studied.

Table 2b Relative Regional Dentin Permeability ol Crown Preparation, Mean ± SEM (intergroup comparisons) Shallow reduction Moderate reduction Surface Group 1 Group 2 Group 1 Group 2

Control 100 100 100 100 Mesial 14,7±12.0(9) 34 4 + 4 6(9) 35.5±5.2(9) 40.8 ±6.0(7) Distal 15.4± 3.7(9) 16.0 + 6.1(9) 14.7 ±9.9(9) 7.7 + 3,9(7) Buccal 19.9± 9.8(9) 21.7 ±4.6(9) 11.2 ±7.6(9) 13,2 ±5,0(7) Lingual 8.1 ± 3.6(9) 10 7±4.0(9) 4,2 + 6.7(9) 10,3 + 3.5(7) Occlusal 16,4± 4,8(9) 9.0 ±2,0(9) 9,5 ±2,5(9) 12,7 ±3,3(7)

Vatues connected by horizontal lines are not significantly dltterent. Numbers In parsnttieses indicate the numtier ot specimens studied.

Table 3 Remaining Dentin Thickness (RDT) After Moderate (fourth) Reduction in Group 2, Mean ± SEM,N = 7

Surface BDT (mm) Lp-

Mesial 1.80 + 0,18 0.0428 + 0.0164 distal 1,63 + 0.16 0.0156 + 0,0093 3uooal 1.59 + 016 0.0110 + 0,0073 Jngual 1.77 + 0,15 00129-0.0062 Occlusal 2,57 + 0,22 0.0180 ±0,0108

'Hyóraulic conductance, in /iL cm-'mirr' cm H,O-'.

The Internationa 222 Richardson et al Dentin l-ermeabiüly. Effects of Crown Preparation ity was highest over the buccal surface (Table la) surfaces. The occiusal dentin was the thickest (2.6 because the buccal surface was reduced more in mm), followed by the lingual surface (1.8 mm|. The the conventional crown preparatioti than in the uni- surface area of the crown preparations rapidly form reduction group 2. The buccal dentin perme- decreased from 1.44 ± 0.08 cm' (group 2) after the ability was twice as high as the next most permeable shallow reduction, to 1.04 ± 0.04 (group 1) and region, the mesial surface, in the group 1 specimens. 0.99 ± 0,08 cm^ (group 2) after the moderate reduc- After the third reduction in group 2, enamel tion, a significant (P< .01) 34% reduction (data not remained only on the lingual and occiusal surfaces, shown). which consequently had the lowest permeability. The mesial and distal surfaces were the most perme- Discussion able atter the third reduction (shallow reduction, Table 1 a). Their levels of permeability were not sta- Mjor," in his text on dentin reactions, places the tistically different from the buccal dentin permeabil- current work in proper perspective. He stated that ity in group 1 specimens. The mesial surface the lack of correlation between clinical symptoms permeability increased almost tenfold after the last of pain and the histopathoiogic appearance of pul- reduction (moderate reduction in Table 1 a) in group pal soft tissue in pulpal inflammatory conditions may 1 specimens, to become the most permeable largely be the result of differences in the permeabil- region, followed in order of decreasing permeability ity of the dentin, a variable that is not histologicallv by the buccal, distal, occiusal, and lingual surfaces. visible. He further suggested that pulpal reactions After the fourth reduction (moderate in Table la) in to restorative procedures and microleakage would group 2, the mesial surface exhibited the highest not occur if dentin were impermeable. The extreme permeability, followed by the occiusal, distal, lin- variability in pulpal responses to materials described gual, and buccal surfaces. The data listed in Table in the literature is probably the result of extreme 1a show statistical comparisons between regions, differences in dentin permeability. Although a few while the same data in Table 1b permit intergroup investigators are beginning to demonstrate the comparisons. Generally, the mesial dentin was more importance of dentin permeability in cytotoxicity permeable (P < .05) than any other region, with testing,'^'" most clinicians do not reali7e the central the exception of the moderate reduction in group role of dentin permeability in the development of 1 (Table 1 a). Within any region, there were generally pulpal pathosis. no statistically significant differences between the A lack of quantitative methods for measuring den- different groups (Table 1b). tin permeability has hampered progress in this In Table 2a, the permeability data are expressed research. However, a variety of methods are now as a percent of the maximum value for each spec- available for both in vivo and in vitro use.'= The imen. Qualitatively, the same permeability profiles current use of fluid under pressure to survey the are seen, although the standard errors of the mean permeability characteristics of complete crown are much smaller when the data are expressed as preparations was an extension of earlier work in a percent of a maximum value. Table 2a presents which the method was used to measure the perme- intragroup regional comparisons, while Table 2b ability and microleakage of Class I cavities.-•' The provides intergroup comparisons at each region. pressures employed (10 psi or 703 cm H,O) were The approximate decrease in dentin thickness far above physiologic pressures and were used as a between each reduction in group 2 was O.S to 0.7 matter of convenience to provide enough fluid mm. This led to large increases in dentin permeabil- movement to measure accurately in a few minutes. ity between the third and fourth reduction in group As pressure and flow are linearly related,^ physio- 2 and to occasional pulp exposures. Those speci- logic pressures would be expected to produce pro- mens with pulp exposures were excluded from portionately lower fluid flows across dentin. The measurement, accounting for the decrease in the pressures used in this study were insufticient to number of specimens measured following the third remove smear layers or force any intratubular mate- reduction. rial from the dentin. That is, these pressures did not When the thickness of dentin remaining between increase dentin permeability above that which the prepared dentin surface and the pulp chamber would have been found if measured at physiologic was measured after the fourth reduction in group pressures.'^ The results confirm the high permeabil- 2, the buccal dentin was found to be the thinnest, ity of proximal walls of preparations,' which was at 1.6 mm (Table 3). However, there was no sig- unrelated to dentin thickness (Table 3). This high nificant difference between the remaining dentin permeability on the mesioaxial surface remains thickness measured on the mesial, distal, or buccal unexplained, but may result from regional differ-

• 4, Number 3, t991 223 I of Proslhodorlics Dentir Pecmeabllity: Effects of Crown Preparjiri Richardson et al

enees in the number or diameter of dentinal tubules. to occur in response to thermal and osmotic stimuli. The dentin thickness in most human molars is about These may be responsible for patients' repoits of 2.6 mm on all but the occlusal surface, where it is sensitivity under provisional crowns and/'" 'allow- 3.8 to 4,0 mm in the regions,"' ing cementation of the final restoration if :lie res- The final remaining dentin thickness in group 2 toration does not cover the finish line or if the (Table 3) indicates that only 30% to 40% of the total cement is torn from the margin of the casting during dentin thickness was removed. That is, there was premature removal of excess cement. still 60% to 70% of the original dentin remaining. The results of this study clearly demonstrate that Clinically, remaining dentin surfaces more than 1.5 dentin permeability increases as dentin is prepared mm (Table 3) do not permit detection of the under- closer to the pulp. As dentin is thinned, pulpal lying pulp as either a shadow or as a pink region. response to tooth preparation becomes more In the conventional reduction of tooth structure severe,^"-' As the remaining dentin thickness during a crown preparation, the buccal and occlusal decreases, the total surface area of the crown avail- surfaces are normally reduced more than the lingual able for retention also decreases. Both results tend and proximal surfaces. For this reason, the perme- to compromise the desired result. ability of the buccal and occlusal surfaces was higher As deep dentin generally has a high permeability, in group 1 than in group 2, after the first reduction. it should be covered with materials that are bio- The first reduction in group 1 was roughly equiva- compatible with the pulp and will seal dentin well. lent to the third reduction in group 2. The second Several of the newer dentin bonding agents may be reduction in group 1 was equivalent to the fourth useful for sealing the dentin following crown prep- reduction in group 2. This was the reason the data aration, just prior to making impressions. It has been were stratified by approximate tooth reduction one of the author's (DHP) obsen/ations that a thin rather than sequence number of reductions. film of these agents decreases dentin permeability Although the absolute values of the regional and should protect the pulp should a provisional hydraulic conductances are very important, another restoration become loose. It would also prevent useful method of presenting the data is as a percent hydraulic fluid movement during impression making of the total permeability of the crown preparation and during final luting of restorations. Thin veneers at any given degree of reduction. Those data (Tables of residual enamel in conservative crown prepara- 2a and 2b) indicate that the most permeable region tions serve the same function and seal the dentin was the mesial surface, followed by the buccal or better than resins. Conservative crown preparations distal surfaces. The lingual or occlusal surfaces were that retain enamel should prevent the development usually the least permeable. of dentin sensitivity and pulpal irritation. Clinicians Table 3 lists the thickness of the remaining dentin who wish to identify areas of exposed dentin in after the last crown reduction in group 2, The crown preparations may use a commercial indicator remaining dentin thickness on the occlusal surface solution (Dentin Detector Solution, LD Caulk, Mil- was greatest {P < .05), followed by the lingual sur- ford, Delaware] or a 4% solution of D & C Red No, face. All other surfaces were significantly thinner (P 28, commonly called erythrosine dye and generally < .05), but they were not different among them- used as a plaque-disclosing solution. A 5-second selves even though buccal and mesial dentin were application of erythrosin dye, followed by a water more permeable. Shillingburg and Crace'^ and rinse, will stain exposed dentin light pink but will Stambaugh and Wittrock" reported that the thick- not stain freshly prepared enamel. ness of crowns was greatest on the occlusal surface, All of these results were obtained using extracted, followed by the lingual surface. The present study unerupted third molars. Caution should be exer- confirms those reports. cised in any attempts at extrapolating these data to The diffusion distances from the margins of pro- erupted teeth, to anterior teeth, or to the in vivo visional crowns to these regions of high permeability situation in general until more research of this type are relatively short (Fig 1). Even if zinc oxide- is done both in vitro and in vivo. eugenol cement can suppress bacterial colonization of these dentin surfaces and suppress inflamma- Conclusions tion,'"•" bacterial products from saliva may diffuse across the dentin and irritate pulpal soft tissues. Even 1. The permeability of complete crown prepara- if there is no pulpal inflammation, the smear layer tions was highest on the mesial sur^.ice, fol- may dissolve within a matter of days to weeks, leav- lowed by the buccal, distal, ociJu^al, and ing dentinal tubules open and permeable. Such lingual surfaces, open tubules may permit hydrodynamic fluid shifts 2, Creation of smear layers using a diamond bur

The Inteinationsl lournal of Proslhodoniics 224 Richardson el al abiliiy; Effects of Crown Preparation

and occlusion of dentina] tubules using potas- 9, Pashley DH, Depew DD: The effects of smear layer, sium oxalate were equally effective in reducing Copahle and oxaiate on microleakage, Oper Dent dentin permeability, 1986;11:95-IO2, 10. Pashley DH, Galioway SE: The effects ol oxalate treatment 3, Reductions in remaining dentin thickness led on ihe smear layer of ground surfaces of human dentin. to large increases in dentin permeability. Arch Oral S/o/1985^0731-737, n. Mjor iA; Dentin Reactions. Boca Raton, Ela, CRC Press, Acknowledgments 1983. 12, Hanks CT, Diehi ML, Craig RC, Makmen P-L, Pashley DH: The authors are gratel'ui to Shirley Johnston for her excellent secretarial support. This work was supported, in part, by Grant Characterization of the "in vitro puip chamber" using the DE06427 from the NIDR and by the Medicai College of Geor- cytutoxicity of phenol. I Ora/Pd)/joM989;1 8:97-107. gia Dentai Research Center. 13, Hanl

4, Number 3, 1991 225 The International Journal of Prosthodonlfcs