DENTAL–PULP COMPLEX
As long as dentin is covered peripherally by enamel on coronal surfaces and cementum on radicular surfaces, the dental pulp will generally remain healthy for life, unless the apical blood supply is disrupted by excessive orthodontic forces or severe impact trauma. Most pathological pulp conditions begin with the removal of one or both of these protective barriers via caries, fractures, or abrasion. The result is the communication of the pulp soft tissue with the oral cavity via dentinal tubules. A classic work done by Pashley et al.,20using radioactive131I in dog teeth, clearly demonstrated that once dentin tubules are opened, molecules can perme- ate to the pulp easily (Figure 5). This dentin perme-
ability has a huge impact on the pathogenesis of the pulp in many clinical situations. This will be discussed later.
It is apparent that substances easily permeate den- tin, permitting thermal, osmotic, and chemical insults to act on the pulpal constituents. The initial stages involve stimulation or irritation of odontoblasts and may proceed to inflammation and often to tissue destruction. First, one must examine the dentin struc- ture.
DENTIN STRUCTURE
Dentin is a calcified connective tissue consisting of approximately 70% inorganic material and 10% water. Organic matrix accounts for 20% of dentin, of which about 91% is collagen. Most of the collagen is type I, but there is a minor component of type V. Noncollagenous matrix components include phos- phoproteins, proteoglycans, g-carboxyglutamate- n-containing proteins, acidic glycoproteins, growth factors, and lipids.
Dentin is penetrated by millions of tubules; their density varies from 40,000 to 70,000 tubules per square millimeter21,22 (Figure 6A). Tubules are from 1 mm in diameter at the dentinoenamel junction to 3 mm at their pulpal surface. These variations in the size of the tubules, in the dentin surface area, and in the surface area covered by the tubules are important in the pathogenesis of pulpal inflammation. For instance, permeation of toxic substances to the pulp created by caries or restorative procedures from the dentinoenamel junction is less likely compared to permeation from the middle of the dentin due to the degree of surface area covered by the tubules. From a purely anatomical standpoint, possible pulpal damage increases exponentially the deeper toxins reach into the dentin.
Dental tubules contain a fluid that has a composi- tion similar to extracellular fluid.23If the fluid becomes contaminated with carious bacterial endotoxins and exotoxins, it becomes a reservoir of injurious agents that can permeate through dentin into the pulp to initiate inflammation.24It is useful to understand the important variables that control dentin permeability.
DENTIN PERMEABILITY
Dentinal tubules in the coronal dentin converge from the dentinoenamel junction to the pulp chamber.25 This tends to concentrate permeating substances into a smaller area at their terminus in the pulp. The surface area occupied by tubules at different levels indicates the effect of tubule density and diameter. One can Figure 5 Pashley’s experimental setup for the study of dentin perme-
ability. 131I, as the marker, was diffused through the buccal chamber attached to the naked dentin of the tooth. Blood was drawn from the venous system and checked for131I radioactivity. The appearance of131I
in the blood demonstrates the degree of permeability of the dentin. Reproduced with permission from Pashley DH et al.20
calculate from Garberoglio and Bra¨nnstro¨m’s22obser- vations that the area of dentin occupied by tubules is only 1% at the dentinoenamel junction and increases to 45% at the pulp chamber (Figure 6B).
The clinical implications of this are enormous. As dentin becomes exposed to increasing depths by
restorative procedures, attrition, or disease, the remaining dentin becomes increasingly perme- able.26,27Thus, dentin removal renders the pulp more susceptible to chemical or bacterial irritation. This functional consequence of tubule area is also respon- sible for the decrease in dentin microhardness closer to the pulp28,29; as tubule density increases, the amount of calcified matrix between the tubules decreases. This relative softness of the dentin lining the pulp chamber facilitates canal enlargement during endodontic treatment.30
Overall dentin permeability is directly proportional to the total surface area of exposed dentin. Obviously, a leaking restoration over a full crown preparation provides more surface for diffusion of bacterial pro- ducts than would a small occlusal restoration.31 Restorations requiring extensive and deep removal of dentin (i.e., preparation for a full crown) would open more and larger tubules and increase the rate of injur- ious substances diffusing from the surface to the pulp—thus the importance of ‘‘remaining dentin thickness.’’32,33 The permeability of the root is 10 to 20 times less than that of a similar thickness of coronal dentin.34This may account for the lack of pulpal reac- tions to periodontal therapy that removes cementum and exposes root dentin to the oral cavity.
Recent evidence indicates that dentin permeability is not constant after cavity preparation. In dogs, den- tin permeability fell over 75% in the first 6 hours following cavity preparation.35 Although there were no histological correlates of the decreased permeabil- ity, dogs depleted of their plasma fibrinogen did not decrease their dentin permeability following cavity preparation.36The authors speculated that the irrita- tion to pulpal blood vessels caused by cavity prepara- tion increased the leakage of plasma proteins from pulpal vessels into the dentinal tubules, where they absorb to the dentin, decreasing permeability. Future study of this phenomenon is required to determine if it occurs in humans.
The character of the dentin surface can also modify dentin permeability. Two extremes are possible: tubules that are completely open, as seen in freshly fractured37or acid-etched dentin,38 and tubules that are closed either anatomically39 or with microcrystalline debris.40 This debris creates the ‘‘smear layer’’ that forms on dentin surfaces whenever they are cut with either hand or rotary instruments.41The smear layer slows bacterial penetra- tion,42,43but permits a wide range of molecules to readily permeate dentin. Small molecules permeate much faster than large molecules. Smear layers are often slowly dis- solved over months to years as oral fluids percolate around microleakage channels between restorative
A
B
Figure 6 A, This picture shows the heterogenous nature of the dentin tubular diameters along the dentin length and dentin surface. The tubular diameter increases (up to 4 mm) and the dentin surface area occupied by the tubules increases logarithmically approaching the pulp. Courtesy of Dr. H. Trowbridge. B, Scanning electron micrograph (SEM) of a cross section of tubules. Courtesy of Dr. D. Pashley.
materials and the tooth.44 The removal of the ‘‘smear layer’’ by acid etching or chelation increases dentin per- meability45because the microcrystalline debris no longer restricts diffusion of irritants and also permits bacteria to penetrate into dentin46There is considerable debate as to whether smear layers created in the root canal during biomechanical preparation should be removed.47 Its removal may increase the quality of the seal between endodontic filling materials and root dentin. It may also increase the bond strength of resin posts.48
PULP REACTION TO PERMEATING SUBSTANCES
What happens when permeating substances reach the pulp chamber? Although bacteria may not actually pass through dentin, their by-products49,50have been shown to cause severe pulp reaction.49,51 The broad spectrum of pulp reaction, from no inflammation to abscess formation, may be related to the concentra- tion of these injurious substances in the pulp. Although exposed dentin may permit substances to permeate, their concentrations may not reach levels high enough to trigger the cascade of events associ- ated with inflammation. This would indicate that the
interstitial fluid concentration of these substances can be maintained at low concentrations. As long as the rate of pulpal blood flow is normal, the microcircula- tion is very efficient at removing substances diffusing across dentin to the pulp chamber. This delicate bal- ance between the dentin and the pulp is illustrated in Figure 7. There is enough blood flowing through the pulp each minute to completely replace between 40 and 100% of the blood volume of the pulp.52 Since blood is confined to the vasculature, which comprises only about 7% of the total pulpal volume,52,53 the blood volume of the pulp is replaced 5 to 14 times each minute.
If pulpal blood flow is reduced,54–67there will be a resultant rise in the interstitial fluid concentration of substances that permeated across dentin. The increased concentration of injurious agents may degranulate mast cells,68–70 release histamine70 or SP,71–75produce bradykinin,76or activate plasma pro- teins.77,78All of these effects would initiate inflamma- tion. The endogenous mediators of inflammation produce arteriolar vasodilation, elevated capillary hydrostatic pressure, increased leakage of plasma pro- teins into the pulp interstitium,79 and increased pulp tissue pressure.80–82 These events, by causing the Figure 7 Diagram showing the delicate balance between the dentin and the pulp. On the left side of the balancing beam are factors governing the dentin and on the right side are factors that govern the pulp.
collapse of local venules, lead to a further reduction in pulpal blood flow.83 With an even higher interstitial concentration of irritants, a vicious cycle84 is created that may terminate in the pulp (Figure 8).
DENTIN SENSITIVITY
Clinicians recognize that dentin is exquisitely sensitive to certain stimuli.85,86It is unlikely that this sensitivity results from direct stimulation of nerves in dentin (Figure 9). As previously stated, nerves cannot be shown in peripheral dentin.87,88 Another speculation is that the odontoblastic process may serve as excitable ‘‘nerve endings’’ that would, in turn, excite nerve fibers shown to exist in deeper dentin, closer to the plup.87–89 The experiments of Anderson et al.90 and Bra¨nn- stro¨m91 suggest that neither odontoblastic processes nor excitable nerves within dentin are responsible for dentin’s sensitivity. This led Bra¨nnstro¨m et al. to propose the ‘‘hydrodynamic theory’’ of dentin
sensitivity, which sets forth that fluid move- ment through dentinal tubules, moving in either direc- tion, stimulates sensory nerves in dentin or the pulp.91,92Further support for the hydrodynamic the- ory came from electron microscopic examination of animal93–95 and human dentin93,96,97 demonstrating that odontoblastic processes seldom extend more than one-third the distance of the dentinal tubules. A work by LaFleche et al.98 suggested that the process may retract from the periphery during extraction or processing. Obviously, more investigation will be required before any definitive statement can be made regarding the distribution of the process. The tubules are filled with dentinal fluid that is similar in com- position to interstitial fluid.99The hydrodynamic the- ory satisfies numerous experimental observations. Although it cannot yet be regarded as fact, it has provided and will continue to provide a very useful perspective for the design of future experiments.100–102 Figure 8 Vicious cycle of pulpal inflammation that begins with irritation (top), leads to a localized response, and may progress to a lesion of increasing severity and eventual irreversible pulpitis.
DENTAL PULP
The dental pulp consists of a loose connective tissue. A single layer of dentin-producing cells, the odontoblasts, lines the peripheral part of the pulp separating the loose connective tissue of the pulp from the predentin. The odontoblasts represent the link between dentin and the pulp. The dental pulp connective tissue is somewhat special in regard to its encasement within rigid dentin walls (see Figure 1). The pulp receives its blood supply from arterioles and lacks a collateral blood supply. The cellular constituents of a human pulp specimen examined under the microscope are presented in Figure 10, and these are explained in subsequent sections. Also shown in Figure 11 are dif- ferent layers in the pulp and its constituents.
INTERSTITIAL FLUID
The rigid encasement allows the tissue-limited possibi- lities to expand and keeps the extracellular fluid volume, that is, blood and interstitial fluid, relatively constant. The extracellular fluid volume in the dental pulp is normally relatively high of about 63%.103Due to the low compliance a small increase in the pulpal volume caused by an increase in blood or interstitial
fluid will raise the hydrostatic pressure inside the tooth. This is shown to happen normally by any increase in blood flow and thus blood volume.104However, as long as there are no noxious stimuli that increase the vessel permeability, any change in tissue pressure caused by blood volume changes will be transitory, because inter- stitial fluid is absorbed back to the blood vessels, and no harm will happen to the pulp (Figure 12).
The hydrostatic pressure in this fluid is the inter- stitial fluid pressure, or the so-called tissue pressure. The interstitial fluid is similar to blood plasma, except for a lower concentration of plasma proteins (albu- min and globulin). In health, plasma proteins do not permeate through the capillary wall and the concen- tration in the interstitial fluid is normally low. How- ever, in a recent study in rat incisors, surprisingly high protein concentration in the pulp interstitial fluid during physiological conditions was reported.103 The main function of the interstitial fluid is as a transport medium for nutrients and waste products between cells and capillary blood. Accordingly, the interstitial fluid acts as a middleman between cells and blood, or as an extension of the plasma. Every cell must have nutrition and the means to rid itself of waste pro- ducts, and these metabolic requirements are taken Figure 9 Schematic diagram of essentials of three theories of dentin sensitivity. A, Classical theory proposed that stimuli applied to dentin caused direct simulation of nerves in dentin. B, Modified theory proposed that stimuli applied to the odontoblastic process would be transmitted along the odontoblast and passed to the sensory nerves via some sort of synapse. C, Hydrodynamic theory proposed that fluid movement within tubules transmits peripheral stimuli to highly sensitive pulpal nerves. C more accurately represents the actual length of the odontoblastic process relative to the tubules. Nerves are seldom found more than one-third the distance from the pulp to the surface. Modified with permission Torneck.
care of by the blood and the interstitial fluid (see Figure 12). Blood is brought to the tissues by the smallest blood vessels, the capillaries. Substances are transported between the blood and the interstitial fluid mainly by diffusion through the capillary wall
(see Figure 12). These capillaries are so widely dis- tributed that no cells are more than 50 to 100 mm from the blood vessels. Due to the short distance, the exchange of substances between blood and interstitial fluid may take place rapidly by simple diffusion. Figure 11 Illustration of different layers in the dental pulp (left) and its constituents (right).
Figure 10 Top left, medium-power photomicrograph of a human pulp specimen showing dentin (D), predentin (P), odontoblast layer (O), cell-free zone (CF), cell-rich zone (CR), and central pulp (CP). Top right, Region similar to the area bracketed in A. CF zone contains large numbers of small nerves and capillaries not visible at this magnification. Underlying CR does not have high concentration of cells but contains more cells than the central pulp. Courtesy of Drs. Dennis Weber and Michael Gaynor. Bottom left, Diagram of peripheral pulp and its principal elements. Middle right, Scanning electron micrograph (SEM) of the dentin–pulp junction. Note corkscrew fibers between odontoblasts (arrow). Reproduced with permission from Jean A, Kerebel JB, Kerebel LM. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1986;61:592. Bottom right, SEM of the pulpal surface of the odontoblast layer. Thread-like structures are probably terminal raveling of nerves. Courtesy of Drs. R. White and M. Goldman.
CELLULAR STRUCTURES
The odontoblasts are derived from ectomesenchymal cells and are responsible for the formation of dentin. These cells line the periphery of the pulp and consist of a cell body and a cytoplasmic/odontoblastic process. The shape of the odontoblast varies according to the functional state of the cell. The cells are tall and colum- nar in shape about 50 to 60 mm long in the coronal occlusal portion while they are cuboidal in the floor of the pulp chamber and the inner surface of the pulp. The odontoblast layer is separated from the minera- lized dentin by a 10- to 40-mm-thick layer of unminer- alized matrix, the predentin. Odontoblasts are involved in the production of predentin–dentin matrix, an extracellular framework that becomes mineralized. The matrix includes a complex mixture of proteogly- cans, glycoproteins, sialoproteins, phosphoproteins, and a variety of other molecules.
FIBROBLAST
Fibroblasts originate from the mesenchymal tissue and are the predominant cells of the connective tissue. Fibroblasts are elongated with little cytoplasm and nucleus-containing condensed chromatin. They are responsible for the formation and maintenance of the fibrous components and ground substances of the
connective tissue. Fibroblasts can synthesize and secrete a wide variety of extracellular molecules that include fibrous elements of extracellular matrix (ECM) such as collagen, elastin, proteoglycans, glycoproteins, cyto- kines, growth factors, and proteinases. They are also involved in the remodeling of connective tissue through the degradation of collagen and other ECM molecules and their replacement by newly synthesized molecules. Fibroblasts release proteolytic enzymes such as the matrix metalloproteinase (MMP) family. MMPs are secreted as inactive precursors and cleave to become active. Extracellular degradation often occurs in inflam- matory lesions. Inhibitors to MMPs, called tissue inhibitors of metalloproteinases, are also secreted by fibroblasts to regulate extracellular degradation. Intra- cellular degradation is for the normal physiological turn- over and remodeling of collagenous connective tissue.
ODONTOBLASTS
Odontoblast, the principal cell of the dentin-forming layer, is the first cell type encountered as the pulp is approached from the dentin (Figure 13). These cells Figure 12 The interstitial fluid is the fluid that surrounds all cells in the
body.
Figure 13 Pseudostratified appearance of odontoblasts. Dark horizontal line (arrow) delineates the cell body from the odontoblastic processes and predentin and was once termed ‘‘pulpodentinal membrane.’’ Ultra- structural examination has shown this to be a terminal cell web that forms a support and attachment area between adjoining cells. Repro- duced with permission from Walton R, Leonard L, Sharawy M, Gang- arosa L. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1979;48:545.
arise from peripheral mesenchymal cells of the dental papilla during tooth development and differentiate by acquiring the characteristic morphology of glyco- protein synthesis and secretion105(Figure 14). Glyco- protein forms the predentin matrix that is rendered mineralizable by the odontoblast, a unique cell pro- ducing a unique tissue, dentin. Synthesizing and secretory activities render the odontoblast highly polarized, with synthesis occurring in the cell body and secretion from the odontoblastic process. The cell body contains organelles that represent different stages of secretion of collagen, glycoproteins, and calcium salts.106 Matrix secretion precedes minerali- zation, with these two events separated in time and space by the predentin.
As happens in bones, the initial mineral seeding of predentin at the dentinoenamel junction is by the formation of ‘‘matrix vesicles.’’107,108 Classic studies by Weinstock et al.,109–111using an autoradiographic technique, have demonstrated the functional sequence of matrix production and secretion. This material has
recently been reviewed by Holland.112 In histological sections viewed under a light microscope, odonto- blasts appear to vary from tall, pseudostratified columnar cells in the coronal pulp to a single row of cuboidal cells in the radicular pulp to a flattened, almost squamous shape near the apex.113,114 These