SCIENTIFIC ARTICLE The effect of dentin depth on the permeability and ultrastructure of primary molars V. Koutsi, DDS, MSR.G. Noonan, DMD,MS J.A. Horner, BS M.D. Simpson,BS W.G. Matthews, BS D.H. Pashley, DMD,PhD Abstract The purpose of this study was to measure the permeability of primary molar and permanentpremolar dentin at various thicknesses from the pulp and to correlate permeability with the tubule density and diameter of dentin using SEM.The data were examinedfor statistically significant differences using two-wayanalysis of variance, multiple comparisonScheffe, and regression analysis. The permeability of all teeth increased with decreasing dentin thickness. The removalof the smearlayer resulted in a significant increase in the permeability. However,the permeability of the dentin in premolarswas significantly higher than that in the primary molars. The density and diameter of the dentinal tubular in primarymolars were lower than the values reported for permanentteeth and mayaccount for the lower permeability of the primarymolars. (Pediatr Dent 16:29- 35, 1994) Introduction radius.) Since filtration varies with the fourth power of The intrinsic permeability of dentin is responsible the tubular radius, small changes in tubule radius or for permitting bacterial or chemical substances to dif- diameter have a large effect on fluid flow. The number fuse across dentin and irritate pulpal and periradicular and diameter of dentinal tubules have an obvious ef- tissues. If dentin was not permeable then the pulp fect on permeability. If the tubules of primary dentin would be spared a good deal of irritation. Normal are smaller and/or less numerous than in permanent dentin is more permeable than carious dentin1 or scle- dentin, then the permeability of primary dentin will be rotic dentin. 2 The permeability of dentin tends to de- lower than that of permanent dentin. Dentinal tubules creaseR, with age due to physiologic sclerosis of dentin. are less dense and narrower (1 ~tm) at the dentoenamel 3 Thus, the permeability of dentin is an important junction (DEJ), and become more dense and wider (3- biologic variable that can be measured and used to 4 ~tm) near the pulp. 7 This anatomical organization compare the barrier properties of dentin within teeth, indicates that superficial dentin is less permeable than between4 teeth, or between species. deep dentin, a notion that has been experimentally Most of the previous measurements of dentin per- confirmed. 5 The rate at which fluid filters through meability have been carried out on the coronaP-~ or dentin is also sensitive to the length of the tubules. As radicular 5 dentin of human permanent teeth. How- dentin thickness is decreased, filtration increases expo- ever, no studies have been performed on primary teeth. nentially for both coronal 6 and radicular 5 permanent Permeability is defined as the ability of a membraneto dentin. permit solutes or solvents to pass through it. 6 The A critical variable affecting permeability is the na- presence of numerous fluid-filled cylindrical dentinal ture of the dentin surface~that is, whether or not the tubules makes dentin a very poor barrier. Rather, den- surface is coated with a smear layer. The smear layer is tin can be regarded as a very porous membrane that a microscopic layer of cutting debris that is produced can be studied by quantitating the ease with which whenever dentin is cut by hand or rotary instruments. fluid can pass through it under defined conditions. Removal of the smear layer increases the hydraulic The most important variables that influence filtration conductanceof dentin by 30- to 40-fold.4’ 6, 8 Clinically, across dentin are the pressure difference across the removing the smear layer results in increased sensitiv- dentin (whether osmotic or hydrostatic), the length ity to osmotic, thermal, and tactile stimuli. 9 Smear the tubule (shorter tubules have less resistance than layer loss can occur gradually through dissolution longer tubules), and the radius of the tubule raised to caused by microleakage around restorations or sud- the fourth power.6 (Although diffusion varies with the denly following the acid conditioning step commonto tubule radius raised to the second power, fluid filtra- most marketed dentin bonding systems. Since the per- tion involves different mechanisms, one of which is the meability of primary dentin has never been reported, frictional resistance developed by concentric cylinders the purposes of this study were: to investigate the ef- of fluid slipping past one another, which cause convec- fect of dentin depth and the presence and absence of tive transport to vary with the fourth power of the the smear layer on the permeability of primary molars; Pediatric Dentistry: January/February1994 - Volume16, Number1 29 to compare the permeability of primary molars with lingual surface enamel of the primary molars and that of premolars; and to correlate permeability with premolars, which served as a reference mark to mea- density and diameter of primary molars. sure the buccolingual thickness of the crowns using a digital micrometer (Sylvae Ultra-Cal II, Fowler Co., Methods and materials Inc., Newton, MA). All reductions were done on the Tooth preparation buccal surface, thereby avoiding any influence that Fifteen extracted, human, posterior, noncarious pri- occlusal abrasion might have had on the production of mary teeth (from children aged 9 to 11 years) and sclerotic or reparative occlusal dentin. All buccal sur- premolars (from children aged 12 to 15 years) were faces were normal and intact. The enamel on the buccal placed immediately after extraction into 4°C phosphate surface of the teeth was removed using a diamond bur buffered saline (Dulbecco’s Phosphate Buffered Saline, in a high-speed handpiece under copious air/water Gibco Laboratories; Grand Island, NY) containing 0.2% spray. When all enamel was removed and the first sodium azide to inhibit microbial growth. All teeth dentin could be seen, the tooth thickness was mea- were removed for orthodontic reasons with informed sured and recorded again, which defined the DEJ. consent from parents. The teeth were used within The buccal dentin was ground further until it was three months of extraction. The roots of the teeth were visually estimated that the dentin surface area was removed approximately 1 mm apical to the sufficient for permeability measurements (ca. 2-3 mm cementoenamel junction using a low-speed diamond diameter). The buccolingual thickness was remeasured saw (Isomet, Buehler Ltd., Lake Bluff, IL). The pulp and the crowns were connected to the apparatus to tissue was removed with cotton forceps, avoiding con- measure the hydraulic conductance of the dentin cov- tact with the walls of the coronal pulp chamber. ered with smear layer. 1 Each permeability measure- The crowns were attached to 2x2x0o7-cm pieces of ment was repeated four times. Two drops of 0.5M Plexiglas ® (DuPont, Wilmington, DE) containing 15- EDTA(pH 7.4) were then placed on the dentin surface mmlengths of 18-gauge stainless steel tubing through for 2 min to remove the smear layer. The dentin surface their centers, using fast-setting cyanoacrylate adhesive was rinsed with distilled water for 5 sec and four addi- (Zapit, DVA,Yorba Linda, CA). This served to connect tional measurements were taken. Then, the DEJ was the tooth pulp chamber with the apparatus used to traced on the tooth using a sharp lead pencil, the crown measure dentin permeability at 10 psi 5 and it ensured was placed in a holding device, and a photograph of that the pulp chamber and dentin were always full of the dentin surface area was taken under standardized phosphate-buffered saline (Fig. 1). magnification. This procedure was followed by an additional grinding of approximately 0.4 mmof dentin Dentin reductions from the labial surface, remeasurement of thickness, A groove was prepared with a diamond bur in the dentin permeability measurements with smear layer rBe~lCuCc~li o n~ Lingual reparedCrown Segment ~Plexiglass N2 from Pressure Tank I 18GuageStainless Steel Tube ~luid FIO~V Air Bubble Distilled Pressurized Buffer Microsyrlnge Reservoir Fig 1. Schematicof the apparatusused to measuredentin permeability via fluid flow froma pressurizedreservoir through a micropipetteto the crown.The movement of an air bubble(insert) in the micropipettewas proportional to the permeability. 30 Pediatric Dentistry: January/February 1994 - Volume 16, Number 1 and after smear layer removal, and photographing of ally, this region was found to be the most permeable, the dentin surface. This routine was carried out four to confirming the results of Maroli et al. ~1 SEMexamina- five times until it was visually estimated that the pulp tion of the most permeable area permitted better corre- chamber was close to being exposed. The teeth were lation between permeability and morphology. Tubule removed from the Plexiglas and sectioned density was determined from SEMmicrographs taken buccolingually into two pieces with a diamond disc. at 480xo The calculation was facilitated by covering The minimumremaining dentin thickness between the each micrograph with tracing paper on which the tu- pulp chamber and the cut surface was measured to the bules could be marked off as they were counted. Based closest 0.01 mmusing a machinist’s micrometer. on the magnification, the area used for the calculation Total dentin thickness was determined from the ini- of tubule density was measured in mm2 and then the tial dentin thickness and minimumremaining dentin density2. was expressed in tubules/mm thickness. The dentin depth of each sequential step Tubule diameter was determined from SEMmicro- was then calculated and expressed as a per cent of the graphs taken at 6000x. A computer program was uti- total dentin thickness. Photographic enlargements (5x7- lized to measure the diameter in ~tm. For each micro- in. prints) were made, and the DEJ that had been marked graph, one tubule was selected for calibration.