Hyperalgesia

Three characteristics of hyperalgesia are (1) spontaneous pain, (2) a decreased pain threshold, and (3) an increased response to a painful stimulus.129 It is recognized that hyperalgesia can be produced by sustained inflammation, as in the case of sunburned skin. Clinical observation has shown that the sensitivity of dentin is often increased when the underlying pulp becomes acutely inflamed, and the tooth may be more difficult to anesthetize. This is due in part to the upregulation of tetrodotoxin-resistant (TTX-resistant) sodium channels in inflamed neural tissue.114,129 NGF seems to play an important role in hyperalgesia. NGF regulates chronic inflammatory hyperalgesia by controlling gene expression in sensory neurons,257 including genes involved in inflammatory hyperalgesia in the dental pulp.74 Although a precise explanation for hyperalgesia is lacking, apparently localized elevations in tissue pressure and inflammatory mediators that accompany acute inflammation play an important role (see Chapter 19 for additional detail).141,143,324,339,356 Clinically, we know that when the pulp chamber of a painful tooth with an abscessed pulp is opened, drainage of exudate soon produces a reduction in the level of pain. This suggests that mechanical stimuli may contribute substantially to pain during inflammatory hyperalgesia.

In addition, certain mediators of inflammation (e.g., bradykinin, 5-hydroxytryptamine [5-HT], prostaglandin E2) are capable of producing hyperalgesia.253,256 For example, 5-HT and CGRP are able to sensitize intradental fibers to hydrodynamic stimuli such as cold, air blasts, and osmotic stimulation.256 Unmyelinated fibers are activated by a number of inflammatory mediators. Bradykinin produces a dull, aching pain when placed in a deep cavity in a human tooth.187

Leukotriene B4 (LTB4) was shown to have a long-lasting sensitizing effect on intradental nerves, suggesting that it may potentiate nociceptor activity during pulpal inflammation.217 Both LTB4 and the complement component, C5a, stimulate neutrophils to secrete additional pain-producing cytokines.

Many silent nerve fibers are present in the normal pulp253,254 and are termed silent because they are not excited by ordinary external stimuli. Once they are sensitized through pulpal inflammation, they begin to respond to hydrodynamic stimuli.48,253,254,256 This phenomenon may provide an additional mechanism for dentin hypersensitivity. The molecular mechanisms of this activation are not known in detail but involve upregulation of numerous genes and their products.14,47,114

Painful Pulpitis

From the foregoing, it is apparent that pain associated with the stimulation of the A fibers does not necessarily signify that the pulp is inflamed or that tissue injury has occurred. Clinically, pain produced by A fibers in response to the hydrodynamic mechanism has a sharp or bright quality as contrasted with the dull, boring, or throbbing pain associated with C fibers. A fibers have a relatively low threshold of excitability to external stimuli,230,252 and painful pulpitis is more likely to be associated with nociceptive C fiber activity indicative of pulpal tissue injury.252-254256 The clinician should carefully examine symptomatic teeth to rule out the possibility of hypersensitive dentin, cracked or leaky fillings, or tooth fracture—each of which may initiate hydrodynamic forces—before establishing a diagnosis of reversible or irreversible pulpitis (see also Chapters 1 and 2).

Pain associated with an inflamed or degenerating pulp may be either provoked or spontaneous. The hyperalgesic pulp may demonstrate a lowered threshold of pain by responding to stimuli that usually do not evoke pain (allodynia), or the pain may be exaggerated and persist longer than normal (hyperalgesia).8 On the other hand, the tooth may commence to ache spontaneously in the absence of any external stimulus.129 Spontaneous, unprovoked pain generally indicates a pulp that is seriously damaged and generally will not respond to noninvasive therapy. For a more complete discussion of pain mechanisms of the dentin-pulp complex, the reader is referred to Chapter 19.

Several studies have elucidated the role of hydrostatic pressure changes in the activation of pulpal nerve fibers.249 In experiments involving cats and dogs, both positive and negative pressure changes were introduced into the pulp by means of a cannula inserted into the dentin. Using single-fiber recording techniques, a positive correlation was found between the degree of pressure change and the number of nerve impulses leaving the pulp (Figs. 12-42 and 12-43).230,250,362

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FIG. 12-42 Response of a single dog pulp nerve fiber to repeated hydrostatic pressure stimulation pulses. Lower solid wavy line of each recording indicates the stimulation pressure applied to the pulp. Upper line (kPa) is the femoral artery blood pressure curve recorded to indicate the relative changes in the pulse pressure and the heart cycle.

(Modified from Närhi M: Activation of dental pulp nerves of the cat and the dog with hydrostatic pressure. Proc Finn Dent Soc 74[suppl 5]:1, 1978.)

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FIG. 12-43 Relationship between the number of nerve impulses (N) and the pressure impulse (I) of a small group of pulpal cat nerve fibers with three suction stimuli and four pressure elevations (pressure impulse is labeled as both mm Hg × sec and kPa × sec).

(Modified from Närhi M: Activation of dental pulp nerves of the cat and the dog with hydrostatic pressure. Proc Finn Dent Soc 74[suppl 5]:1, 1978.)

Vascular Supply

Blood from the dental artery enters the tooth by way of arterioles having diameters of 100 µm or less. These vessels pass through the apical foramen or foramina with nerve bundles. Smaller vessels may enter the pulp by way of lateral or accessory canals. They are richly innervated by autonomic and sensory nerves, and the regulation of blood flow seems to be dominated by neuronal control1,23,181,267,340 (Fig. 12-44).

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FIG. 12-44 Substance P–positive nerve fibers in the wall of pulpal blood vessels.

(Courtesy Dr. K.J. Heyeraas.)

The arterioles course up through the central portion of the radicular pulp and give off branches that spread laterally toward the odontoblast layer, beneath which they ramify to form a capillary plexus188 (Fig. 12-45). As the arterioles pass into the coronal pulp, they fan out toward the dentin, diminish in size, and give rise to a capillary network in the subodontoblastic region329 (Fig. 12-46). This network provides the odontoblasts with a rich source of metabolites.

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FIG. 12-45 High-power scanning electron micrograph of vascular network in the radicular pulp of a dog molar showing the configuration of the subodontoblastic terminal capillary network (TCN). Venules (VL) and arterioles (AL) are indicated.

(Courtesy Dr. Y. Kishi, Kanagawa Dental College, Kanagawa, Japan.)

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FIG. 12-46 Subodontoblastic terminal capillary network (TCN), arterioles (AL), and venules (VL) of young canine pulp. Dentin would be to the far left and the central pulp to the right. Scale bar: 100 µm.

(From Takahashi K, Kishi Y, Kim S: A scanning electron microscopic study of the blood vessels of dog pulp using corrosion resin casts. J Endod 8:131–135, 1982.)

Capillary blood flow in the coronal portion of the pulp is nearly twice that in the root portion.176 Moreover, blood flow in the region of the pulp horns is greater than in all other areas of the pulp.232 In young teeth, capillaries commonly extend into the odontoblast layer, thus ensuring an adequate supply of nutrients for the metabolically active odontoblasts (Fig. 12-47). In the subodontoblastic capillaries, fenestrations are observed in the vessel wall.291 These fenestrations are thought to promote rapid transport of fluid and metabolites from the capillaries to the adjacent odontoblasts. The average capillary density is about 1400/mm2, which is greater than in most other tissues of the body.364

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FIG. 12-47 Blood vessels in the pulp horn fan out into the odontoblast layer.

(Courtesy Dr. S.R. Haug.)

Blood passes from the capillary plexus, first into postcapillary venules (see Fig. 12-46; Fig. 12-48) and then into larger venules.188 Venules in the pulp have unusually thin walls, and the muscular layer is discontinous,69 which may facilitate the movement of fluid in or out of the vessel. The collecting venules become progressively larger as they course to the central region of the pulp. The largest venules have a diameter that may reach a maximum of 200 µm, considerably larger than the arterioles of the pulp.

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FIG. 12-48 Postcapillary venule draining blood from subodontoblastic capillary plexus.

The resting pulpal blood flow is relatively high, averaging 0.15 to 0.60 ml/min/g tissue,228,337 and blood volume represents about 3% of pulpal wet weight,31 approximately the same as in mammary tumour tissue.377 As would be anticipated, pulpal blood flow is greater in the peripheral layer of the pulp (i.e., the subodontoblastic capillary plexus)188 where the oxygen consumption has been shown to be higher than in the central pulp.27

Changes in pulpal blood flow can be measured through dentin using laser Doppler flowmeters. Sensitivity to movement requires that they are stabilized in an occlusal stent or a modified rubber dam clamp.91,305 Because up to 80% of the Doppler signal originates from periodontal tissue, it is helpful to cover periodontal tissues with a black rubber dam.132 Laser Doppler flowmetry can be used to detect revascularization of traumatized teeth.76,86 Although measurement of pulpal blood flow would be an ideal tool for determining pulp vitality, the use of laser Dopplers and other techniques is limited due to sensitivity, specificity, reproducibility, and costs.

Regulation of Pulpal Blood Flow

Under normal physiologic conditions, pulpal vascular tone is controlled by neuronal, paracrine, and endocrine mechanisms that keep the blood vessels in a state of partial constriction. The pulpal blood flow is also influenced by vascular tone in neighboring tissues. Vasodilatation in these tissues has been shown to cause a drop in pulpal blood flow due to reduction in local arterial pressure of the teeth and thereby reduced pulpal perfusion pressure.338 The “stealing” of dental perfusion pressure makes the dental pulp vulnerable in clinical situations with inflammatory processes in the adjacent tissues, as in gingivitis and periodontitis.

Neuronal regulation of blood flow is extensive in the pulp. There is little or no vasoconstrictor tone of sympathetic origin in the dental pulp during resting conditions,158,340 but a neuronal vasodilator tone caused by release of sensory neuropeptides has been demonstrated (Fig. 12-49).21,23

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FIG. 12-49 Effect of antagonist infusion of h-CGRP(8-37) (calcitonin gene–related peptide inhibitor) and SR 140.33 (substance P inhibitor) on basal pulpal blood flow (PBF) and gingival blood flow (GBF).

(From Berggreen E, Heyeraas KJ: Effect of the sensory neuropeptide antagonists h-CGRP[8-37] and SR 140.33 on pulpal and gingival blood flow in ferrets. Arch Oral Biol 45:537–542, 2000.)

There are α-adrenergic receptors in the pulp,153 and stimulation of the cervical sympathetic trunk causes vasoconstriction and fall in pulpal blood flow that can be partially reversed by α-receptor blockade.181,340 NPY, colocalized with norepinephrine in pulpal sympathetic nerve fibers, contributes also to vasoconstriction in the pulp.79,178

Increase in pulpal blood flow is observed after electrical tooth stimulation and is caused by release of sensory neuropeptides followed by vasodilatation.23,144,169 CGRP released from sensory nerve fibers is mainly responsible for the observed vasodilatation.21,23

There is evidence for sympathetic modulation of sensory neuropeptide release in the dental pulp130; presynaptic adrenoceptors are found on the sensory nerve terminals and attenuate the release of vasodilators from the sensory nerves.36,171

Muscarinic receptors have been identified in the pulp,34 and the parasympathetic neurotransmitter acetylcholine (ACh) causes vasodilatation and increases blood flow in the tissue.385 The vasodilation evoked by acetylcholine has been demonstrated to be partly dependent on nitric oxide (NO) production.

VIP, which coexists with ACh in postganglionic neurons, is found in the dental pulp102,352 and has been demonstrated to cause vasodilatation and increase in pulpal blood flow in cats.267

On the other hand, Sasano and co-workers307 failed to demonstrate parasympathetic nerve-evoked vasodilatation in the cat dental pulp, leaving pulpal vascular responses to parasympathetic neurotransmitters with some uncertainty.

Local Control of Blood Flow

The microvascular bed in the dental pulp has the ability to regulate hemodynamics in response to local tissue demands. Endothelin-1 is located in the endothelium of pulpal vasculature,58 and close intraarterial infusions of endothelin-1 reduce pulpal blood flow.22,112,385 However, endothelin-1 does not seem to influence blood vessel vascular tone under basal, resting conditions.22

The endothelium in pulpal blood vessels modulates vascular tone by release of vasodilators such as prostacyclin and NO. A basal synthesis of NO provides a vasodilator tone on pulpal vessels.23,208 The shear forces that blood flow exert on endothelial cells seem to regulate the release of NO.73

Adenosine is released from ischemic and hypoxic tissue and is probably important in the metabolic regulation of blood flow in periods of low pulpal oxygen tension. When applied from the extraluminal side of the vessel wall, adenosine mediates vasodilatation in pulpal vessels.385

Humoral Control of Blood Flow

Evidence for humoral control of pulpal blood flow exists and takes place when vasoactive substances transported by the bloodstream reach the receptors in the pulp tissue. Angiotensin II is produced by activation of the renin/angiotensin system and exerts a vasoconstrictive basal tone on pulpal blood vessels.22 The angiotensin II receptors, AT1 and AT2, were recently identified in the rat pulp.322

Similarly to the effect of norepinephrine released from sympathetic nerve fibers in the pulp, epinephrine released from the adrenal medulla will cause vasoconstriction due to activation of α-adrenergic receptors in the pulp. Another catecholamine, dihydroxyphenylalanine (DOPA), also induces vasoconstriction in pulpal arterioles when applied intraarterially.385

Lymphatics

The lymphatic vasculature forms a vessel network in the interstitium which drains filtered fluid and proteins and returns it to the blood through the larger lymphatics.147,301 In addition, it serves an important role in the body’s immune defense, since dendritic cells enter the blind-ended lymphatic capillaries in the dental pulp and transport captured antigen to regional lymph nodes where they present the antigen to lymphocytes. The lymphatic fluid removes foreign invaders such as bacteria and their byproducts from the peripheral tissue and represents a transport system for infectious agents. During inflammation, new lymphatic vessels are formed to meet the demand of increased fluid transport and antigen presentation.5

Historically, the existence of lymphatics in the pulp has been a matter of debate because it is difficult to distinguish between blood and lymphatic vessels by ordinary microscopic techniques without specific lymphatic markers.

Recently, specific lymphatic markers have been applied (Fig. 12-50), and an extensive system of lymphatic vessels in the pulp is now recognized to exist (Fig. 12-51).20,283 These new data indicate that true lymphatics travel from the coronal pulp throughout the roots.28,262 Lymphatic capillaries are found in the pulp horn but not in the odontoblast layer.20 Moreover, bundles of lymphatics leave the pulp via the apical foramen and through lateral canals in the root. In the rodent incisor pulp, lymphatic capillaries are found only in the apical third. In the middle and coronal third of the periodontal ligament, lymphatic vessels seem to drain mainly into the alveolar bone, whereas in the apical third, vessels drain toward the apex, where they join into bundles of lymphatic vessels.

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FIG. 12-50 Lymphatic capillaries joining into larger lymphatic vessels in a rodent incisor. The lymphatic endothelium is visualized by whole mount immunohistochemistry utilizing an antibody against vascular endothelial growth factor receptor 3.

(From Berggreen E, Haug SR, Mkonyi LE, Bletsa A: Characterization of the dental lymphatic system and identification of cells immunopositive to specific lymphatic markers. Eur J Oral Sci 117(1):34–42, 2009.)

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FIG. 12-51 The lymphatic system in a mouse molar immunoreacted with antibody to vascular endothelial growth factor receptor 3. Note that the lymphatic vessels travel from the coronal pulp through the root toward apex.

(From Berggreen E, Haug SR, Mkonyi LE, Bletsa A: Characterization of the dental lymphatic system and identification of cells immunopositive to specific lymphatic markers. Eur J Oral Sci 117(1):34–42, 2009.)

Lymphatic capillaries are filled when pulpal interstitial pressure increases after bulk swelling of the interstitium and by strain on the extracellular matrix. When lymph is formed, deformation of interstitial tissue leads to propulsion of lymph by compression of the lymphatics,308 and inside the rigid pulp chamber, the only force that can cause tissue deformation and strain on the extracellular matrix is the arteriolar pulse pressure converted to a relatively high interstitial pulsatile pressure. It was demonstrated more than 70 years ago in the rabbit ear that the removal rate of an intravital stain injected subcutaneously was higher with than without arterial pressure pulsations in the tissue.273

As lymphatics from the pulp drain through the periodontal ligament, it is logical to assume that normal tooth movement during chewing will act as an additional extrinsic propulsion mechanism for the lymph.

Transcapillary Fluid Exchange

As in all other tissues in the body, the fluid transport between the pulpal blood vessels and the interstitial space is regulated by differences in colloid osmotic and hydrostatic pressures in the plasma and interstitium in addition to lymph flow (Fig. 12-52).

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FIG. 12-52 Interstitial structure and pressures that govern transcapillary fluid transport. Kf, Capillary filtration coefficient; σ, capillary reflection coefficient for plasma proteins.

(From Wiig H, Rubin K, Reed RK: New and active role of the interstitium in control of interstitial fluid pressure: potential therapeutic consequences. Acta Anaesthesiol Scand 47:111–121, 2003.)

During normal conditions, a steady state is achieved as the fluid filtered into the interstitial space equals the amount of fluid transported out of the same compartment. Using radioisotopes, the interstitial fluid volume in the pulp was measured and averaged 0.6 ± 0.03 ml/g wet weight,31 demonstrating that as much as 60% of the extracellular fluid in dental pulp is located outside the vascular system. Measurements of interstitial fluid pressure in the pulp with the micropuncture method have given values in the range from 6 to 10 mm Hg,23,140 but higher values measured with different methods have also been reported.41,357,361

Colloid osmotic pressure (COP) measurements in interstitial fluid isolated from rat incisors has shown a relatively high pulpal COP, reaching 83% of plasma COP.31 The high value may imply that the normal permeability of pulpal vessels to plasma proteins is relatively high and/or the drainage of plasma proteins is ineffective. The fact that lymphatic vessels are only found in the apical part of the rodent incisor may support the latter possibility. Filtered plasma proteins must be removed from the interstitial fluid to maintain homeostasis, and the only possibility for transport is by lymphatics, since proteins cannot be reabsorbed into blood vessels.

Circulation in the Inflamed Pulp

Inflammation in the pulp takes place in a low-compliance environment composed of rigid dentinal walls. Compliance is defined as the relationship between volume (V) and interstitial pressure (P) changes: C = Δ V/ Δ P. Consequently, in the low-compliant pulp, an increase in blood or interstitial volume will lead to a relatively large increase in the hydrostatic pressure in the pulp. The acute vascular reactions to an inflammatory stimulus are vasodilatation and increased vascular permeability, both of which will increase pulp interstitial fluid pressure141,143,340,356 and may tend to compress blood vessels and counteract a beneficial blood flow increase (Fig. 12-53).

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FIG. 12-53 Original simultaneous recordings of percent change in pulpal blood flow (ΔLDF%), interstitial fluid pressure (IFP), and systemic blood pressure (PA mm Hg) in cat during electrical tooth stimulation. Note that when IFP is first decreasing after an initial rise, the pulpal blood flow reaches its maximal level (arrows), demonstrating compression of vessels in the first phase.

(Courtesy Dr. K.J. Heyeraas.)

Classical studies have demonstrated that an increase in intrapulpal tissue pressure promoted absorption of tissue fluid back into the blood and lymphatic vessels, thereby reducing the pressure.141,143 This observation can explain why pulpal tissue pressure in inflamed pulps may persist in local regions for long observation periods,339 contradicting the old concept of a wide, generalized collapse of pulpal venules and cessation of blood flow (pulpal strangulation theory).

The delivery of dental restorative procedures may lead to substantial increases or decreases in pulpal blood flow, depending upon the precise procedure and time point sampled.180 Vasoactive mediators are locally released upon an inflammatory insult, and in the pulp, prostaglandin E2, bradykinin, SP, and histamine have all been demonstrated to increase pulpal blood flow after application.179,266 In contrast, serotonin (5-HT) is released primarily from the platelets, and given intraarterially, it has been shown to reduce pulpal blood flow.177,385

Acute inflammation in the dental pulp induce an immediate rise in blood flow and can reach a magnitude of up to nearly 200% of control flow followed by increased vascular permeability.142,143

A common outcome of pulpal inflammation is development of tissue necrosis. One study found circulatory dysfunction developed in the pulp after exposure to lipopolysaccharide (LPS) from gram-negative bacteria.31

In addition, the inflammatory cytokines IL-1 and TNF-α are elevated in the inflamed pulp. When the endothelium is exposed to endotoxin, it expresses cytokines, chemokines, and thromboxane A2. The latter has been demonstrated to be produced in the pulp exposed to LPS265 and induces vasoconstriction. The set of changes in endothelial function have been called endothelial perturbation and were first described in endothelial cells exposed to endotoxin or to cytokines such as IL-1, TNF-α, and IL-6.25,255 The activated endothelium also participates in procoagulant reactions that promote fibrin clot formation.325 A reduced pulpal perfusion due to endothelial perturbation might be the consequence of bacterial infection impairing pulpal defense mechanisms and promoting necrosis. Downregulation of vascular endothelial growth factor (VEGF) expression in stromal cells and reduced microvessel density have been observed in human dental pulps with irreversible pulpitis.12 VEGF is an essential proangiogenic factor, and the reduced microvessel density might also lead to reduced pulpal perfusion and contribute to development of pulpal necrosis.

The role of lymphatic vessels in dental inflammation is unknown, but Pimenta et al., found an increased number of lymphatic vessels in inflamed pulps from teeth with caries compared to noninflamed pulps, indicating that lymphangiogenesis takes place in the pulp.283

Vascular Permeability

Increased vascular permeability takes place as a result of acute inflammation, and vascular leakage has been demonstrated in the pulp after release of inflammatory mediators such as prostaglandin, histamine, bradykinin, and the sensory neuropeptide, SP.170,179,220

LPS and lipoteichoic acid (LTA) from gram-negative and gram-positive bacteria, respectively, cause upregulation of VEGF in activated pulpal cells.35,331 VEGF increases vascular permeability,88,312 and it is likely that it also causes leakage in pulpal vessels. It is a very potent agent, since its ability to enhance microvascular permeability is estimated to be 50,000 times higher than that of histamine.316 Cytokines such as IL-1 and TNF-α are released into the pulp interstitial fluid during inflammation31 and upregulate VEGF mRNA gene expression in pulpal fibroblasts.60

The resulting increased vascular permeability allows increased transport of proteins through the capillary vessel wall and results in increased COP in the tissue. In acute pulpitis induced by LPS, it has been shown that COP in the pulp can reach the level of plasma COP, meaning that a barrier between plasma and interstitium can be eleminated.31

Clinical Aspects

The influence of posture on pulpal blood flow has been observed in humans.60 Significantly greater pulpal blood flow was measured when subjects changed from an upright to a supine position. The supine position increases venous return from all tissues below the level of the heart, thereby increasing cardiac output and producing a transient increase in systemic blood pressure. The increase in blood pressure stimulates baroreceptors that reflexively decrease sympathetic vasoconstriction to all vascular beds and thereby increases peripheral blood flow.

Patients with pulpitis often report an inability to sleep at night because they are disturbed by throbbing tooth pain. In addition to the lack of distractions normally present during the day, the following mechanism may be operative in patients with inflamed pulps. When these patients lie down at the end of the day, their pulpal blood flow probably increases due to the cardiovascular postural responses described earlier. This may increase their already elevated pulpal tissue pressure,141,143,324,339,356 which is then sufficient to activate sensitized pulpal nociceptors and initiate spontaneous pulpal pain. Thus the “throbbing” sensation of a toothache is due to the pulsation in the pulp that follows heart contractions (systole), causing intermittent increases in pulpal tissue pressure.

Pulpal Repair

The inherent healing potential of the dental pulp is well recognized. As in all other connective tissues, repair of tissue injury commences with débridement by macrophages, followed by proliferation of fibroblasts, capillary buds, and the formation of collagen. Local circulation is of critical importance in wound healing and repair. An adequate supply of blood is essential to transport immune cells into the area of pulpal injury and to dilute and remove deleterious agents from the area. It is also important to provide fibroblasts with nutrients from which to synthesize collagen. Unlike most tissues, the pulp has essentially no collateral circulation; for this reason, it is theoretically more vulnerable than most other tissues. In the case of severe injury, healing would be impaired in teeth with a limited blood supply. It seems reasonable to assume that the highly cellular pulp of a young tooth, with a wide-open apical foramen and rich blood supply, has a much better healing potential than an older tooth with a narrow foramen and a restricted blood supply.

Dentin can be classified as primary, secondary, or tertiary, depending on when it was formed. Primary dentin is the regular tubular dentin formed before eruption, including mantle dentin. Secondary dentin is the regular circumferential dentin formed after tooth eruption, whose tubules remain continuous with that of primary dentin. Tertiary dentin is the irregular dentin that is formed in response to abnormal stimuli, such as excess tooth wear, cavity preparation, restorative materials, and caries.64,65 In the past, tertiary dentin has been called irregular dentin, irritation dentin, reparative dentin, and replacement dentin. Much of the confusion was caused by a lack of understanding of how tertiary dentin is formed.

If the original odontoblasts that made secondary dentin are responsible for focal tertiary dentin formation, that particular type of tertiary dentin is termed reactionary dentin.320 Generally, the rate of formation of dentin is increased, but the tubules remain continuous with the secondary dentin.323 However, if the provoking stimulus caused the destruction of the original odontoblasts, the new, less tubular, more irregular dentin formed by newly differentiated odontoblast-like cells is called reparative dentin. In such dentin the tubules are usually not continuous with those of secondary dentin. Initially, the newly formed cells tend to be cuboidal in shape, without the odontoblast process that is necessary to form dentinal tubules. They seem to form in response to the release of a host of growth factors that were bound to collagen during the formation of secondary dentin.90,294,320 The loss of the continuous layer of odontoblasts exposes unmineralized predentin that is thought to contain both soluble and insoluble forms of TGF-β, insulin-like growth factor (IGF)-1 and IGF-2, BMPs, VEGF, and other growth factors that attract and cause proliferation and differentiation of mesenchymal stem cells to form reparative dentin and new blood vessels. During caries progression, bacterial acids may solubilize these growth factors from mineralized dentin, liberating them to diffuse to the pulp, where they could stimulate reactionary dentin formation. This is also thought to be the mechanism of action of calcium hydroxide during apexification treatment. Despite its high pH, calcium hydroxide has a slight demineralizing effect on dentin and has been shown to cause the release of TGF-β.319 TGF-β and other growth factors stimulate and accelerate reparative dentinogenesis. Other researchers have attempted to apply growth factors to dentin to allow it to diffuse through the tubules to the pulp.299,300,321 Although this has been successful, the remaining dentin thickness must be so thin that this approach may not be practical from a therapeutic perspective. Others have inserted deoxyribonucleic acid (DNA)-sequenced BMP-7 into retroviruses to transfect ferret pulpal fibroblasts to stimulate increased BMP-7 production. Although this was successful in normal pulps,299 it was unsuccessful in inflamed pulps.298 Specific amelogenin gene splice products, A+4 and A−4, adsorbed onto agarose beads and applied to pulp exposures, induced complete closure and mineralization of the root canal in rat molars.116 The regulation of peritubular dentin formation is not well understood. Some have claimed that this is a passive process resulting in occlusion of the tubules over time, but it has also been claimed that this is a mechanism under odontoblast control. If odontoblasts could be stimulated to form excessive peritubular dentin by the application of an appropriate biologic signaling molecule to the floor of cavity preparations, then the tubules of the remaining dentin could be occluded, rendering such dentin impermeable and protecting the pulp from the inward diffusion of noxious substances that might leak around restorations.277 These are examples of how molecular biology may be used in future restorative dentistry.

The term most commonly applied to irregularly formed dentin is reparative dentin, presumably because it frequently forms in response to injury and appears to be a component of the reparative process. It must be recognized, however, that this type of dentin has also been observed in the pulps of normal, unerupted teeth without any obvious injury.259

It will be recalled that secondary dentin is deposited circumpulpally at a very slow rate throughout the life of the vital tooth.323 In contrast, when a carious lesion has invaded dentin, the pulp usually responds by depositing a layer of tertiary dentin over the dentinal tubules of the primary or secondary dentin that communicate with the carious lesion (Fig. 12-54). Similarly, when occlusal wear removes the overlying enamel and exposes the dentin to the oral environment, tertiary dentin is deposited on the pulpal surface of the exposed dentin. Thus the formation of tertiary dentin allows the pulp to retreat behind a barrier of mineralized tissue.345

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FIG. 12-54 Reparative dentin (RD) deposited in response to a carious lesion in the dentin.

(From Trowbridge HO: Pathogenesis of pulpitis resulting from dental caries. J Endod 7:52–60, 1981.)

Compared with primary or secondary dentin, tertiary dentin tends to be less tubular, and the tubules tend to be more irregular with larger lumina. In some cases, particularly when the original odontoblasts are destroyed, no tubules are formed. The cells that form reparative dentin are often cuboidal and not as columnar as the primary odontoblasts of the coronal pulp (Fig. 12-55). The quality of tertiary dentin (i.e., the extent to which it resembles primary or secondary dentin) is quite variable. If irritation to the pulp is relatively mild, as in the case of a superficial carious lesion, then the tertiary dentin formed may resemble primary dentin in terms of tubularity and degree of mineralization. On the other hand, dentin deposited in response to a deep carious lesion may be relatively atubular and poorly mineralized, with many areas of interglobular dentin. The degree of irregularity of this dentin is probably determined by numerous factors, such as the amount of inflammation present, the extent of cellular injury, and the state of differentiation of the replacement odontoblasts.

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FIG. 12-55 Layer of cells forming reparative dentin. Note the decreased tubularity of reparative dentin compared to the developmental dentin above it.

The poorest quality of reparative dentin is usually observed in association with marked pulpal inflammation.64,345 In fact, the dentin may be so poorly organized that areas of soft tissue are entrapped within the dentinal matrix. In histologic sections, these areas of soft-tissue entrapment impart a Swiss-cheese appearance to the dentin (Fig. 12-56). As the entrapped soft tissue degenerates, products of tissue degeneration are released that further contribute to the inflammatory stimuli assailing the pulp.345

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FIG. 12-56 Swiss-cheese type of reparative dentin. Note the numerous areas of soft-tissue inclusion and infiltration of inflammatory cells in the pulp.

It has been reported that trauma caused by cavity preparation that is too mild to result in the loss of primary odontoblasts does not lead to reparative dentin formation, even if the cavity preparation is relatively deep.73 This has been confirmed both in rat teeth242 and human teeth.240 However, chronic pulpal inflammation associated with deep caries produces reparative dentin. This reparative dentin is formed by new odontoblast-like cells. For many years, it has been recognized that destruction of primary odontoblasts is soon followed by increased mitotic activity within fibroblasts of the subjacent cell-rich zone. It has been shown that the progeny of these dividing cells differentiate into functioning odontoblasts.96 Investigators383 have studied dentin bridge formation in healthy teeth of dogs and found that pulpal fibroblasts appeared to undergo dedifferentiation and revert to undifferentiated mesenchymal stem cells (Fig. 12-57). The similarity of primary odontoblasts to replacement odontoblasts was established by D’Souza et al.75 They were able to show that cells forming reparative dentin synthesize type I (but not type III) collagen, and they are immunopositive for dentin sialoprotein.

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FIG. 12-57 Autoradiographs from dog molars illustrating uptake of 3H-thymidine by pulp cells preparing to undergo cell division after pulpotomy and pulp capping with calcium hydroxide. A, Two days after pulp capping. Fibroblasts, endothelial cells, and pericytes beneath the exposure site are labeled. B, By the fourth day, fibroblasts (F) and preodontoblasts adjacent to the predentin (PD) are labeled, which suggests that differentiation of preodontoblasts occurred within 2 days. C, Six days after pulp capping, new odontoblasts are labeled, and tubular dentin is being formed. (Titrated thymidine was injected 2 days after the pulp capping procedures in B and C.)

(From Yamamura T, Shimono M, Koike H, et al: Differentiation and induction of undifferentiated mesenchymal cells in tooth and periodontal tissue during wound healing and regeneration. Bull Tokyo Dent Coll 21:181, 1980.)

Destruction of primary odontoblasts can occur from cutting cavity preparations dry,77,192 from bacterial products such as endotoxins shed from deep carious lesions,19,370 or from mechanical exposure of pulps.241 Such pulpal wounds do not heal if the tissue is inflamed.64 Local fibroblast-like cells divide, and the new cells then redifferentiate in a new direction to become odontoblasts. Recalling the migratory potential of ectomesenchymal cells from which the pulpal fibroblasts are derived, it is not difficult to envision the differentiating odontoblasts moving from the subodontoblastic zone to the area of injury to constitute a new odontoblast layer. Activation of antigen-presenting dendritic cells by mild inflammatory processes may also promote osteoblast/odontoblast-like differentiation and expression of molecules implicated in mineralization. Recognition of bacteria by specific odontoblast and fibroblast membrane receptors triggers an inflammatory and immune response within the pulp tissue that would also modulate the repair process.118

Although many animal studies have shown dentin bridge formation in healthy pulps following pulp capping with adhesive resins,64 such procedures fail in normal human teeth.62 When small mechanical pulp exposures are inadvertently made in healthy teeth, the current recommendation is to place a small, calcium hydroxide–containing dressing on the wound. After setting, the surrounding dentin can be bonded using a no-rinse, self-etching primer adhesive.165 Like calcium hydroxide, mineral trioxide aggregate (MTA) has also been recognized to promote hard-tissue formation.3,7,245

The formation of atubular “fibrodentin” is another potential product of newly differentiated odontoblasts, provided that a capillary plexus develops beneath the fibrodentin.17 This is consistent with the observation made by other researchers64,96 that the newly formed dentin bridge is composed first of a thin layer of atubular dentin on which a relatively thick layer of tubular dentin is deposited. The fibrodentin was lined by cells resembling mesenchymal cells, whereas the tubular dentin was associated with cells closely resembling odontoblasts.

Other researchers323 studied reparative dentin formed in response to relatively traumatic experimental class V cavity preparations in human teeth. They found that seldom was reparative dentin formed until about the 30th postoperative day. The rate of dentin formation was 3.5 µm/day for the first 3 weeks after the onset of dentinogenesis, after which it decreased markedly. By postoperative day 132, dentin formation had nearly ceased. Assuming that most of the odontoblasts were destroyed during traumatic cavity preparation, as was likely in this experiment, the 30-day delay between cavity preparation and the onset of reparative dentin formation is thought to reflect the time required for the proliferation, migration, and differentiation of new replacement odontoblasts.

Does reparative dentin protect the pulp, or is it simply a form of scar tissue? To serve a protective function, it would have to provide a relatively impermeable barrier that would exclude irritants from the pulp and compensate for the loss of developmental dentin. The junction between developmental and reparative dentin has been studied using a dye diffusion technique, which demonstrated the presence of an atubular zone situated between secondary dentin and reparative dentin (Fig. 12-58).92 In addition to a dramatic reduction in the number of tubules, the walls of the tubules along the junction were often thickened and occluded with material similar to peritubular matrix.310 Taken together, these observations would indicate that the junctional zone between developmental and reparative dentin is an atubular zone of low permeability. Moreover, the accumulation of pulpal dendritic cells was reduced after reparative dentin formation, which may indicate the reduction of incoming bacterial antigens.303

image

FIG. 12-58 Diffusion of dye from the pulp into reparative dentin. Note atubular zone between reparative dentin (RD) and primary dentin on the left.

(From Fish EW: Experimental investigation of the enamel, dentin, and dental pulp, London, 1932, John Bale Sons & Danielson, Ltd.)

One group335 studied the effect of gold foil placement on human pulp and found that this was better tolerated in teeth in which reparative dentin had previously been deposited beneath the cavity than in teeth that were lacking such a deposit. It would thus appear that reparative dentin can protect the pulp,19 but it must be emphasized that this is not always the case. It is well known that reparative dentin can be deposited in a pulp that is irreversibly injured and that its presence does not necessarily signify a favorable prognosis (see Fig. 12-55). The quality of the dentin formed, and hence its ability to protect the pulp, to a large extent reflects the environment of the cells producing the matrix. The presence of a single tunnel defect64 through reparative dentin would circumvent the protective effect of atubular reparative dentin. Therefore any clinical attempt at pulp therapy must include sealing dentin with bonding agent.

Periodontally diseased teeth have smaller root canal diameters than teeth that are periodontally healthy.193 The root canals of such teeth are narrowed by the deposition of large quantities of reactionary dentin along the dentinal walls.311 The decrease in root canal diameter with increasing age, in the absence of periodontal disease, is more likely to be the result of secondary dentin formation.

One study showed that in a rat model, frequent scaling and root planing resulted in reparative dentin formation along the pulpal wall subjacent to the instrumented root surface.134 However, given that normal rat root dentin is only 100 µm thick, such procedures are probably more traumatic to the pulp in the rat model than in humans, where normal root dentin is more than 2000 µm thick.

Not uncommonly, the cellular elements of the pulp are largely replaced by fibrous connective tissue over a span of 5 decades. It appears that in some cases, the pulp responds to noxious stimuli by accumulating large fiber bundles of collagen, rather than by elaborating reparative dentin (Fig. 12-59). However, fibrosis and reparative dentin formation often go hand in hand, indicating that both are expressions of a reparative potential.

image

FIG. 12-59 Fibrosis of dental pulp showing replacement of pulp tissue by large collagen bundles (CB).

With the expanding knowledge of tooth regeneration and biologic mechanisms of functional dental tissue repair, current treatment strategies are beginning to give way to evolving fields such as tissue engineering and biomimetics. Pulpal stem cells in scaffolds have been shown to produce pulp-like tissues with tubular-like dentin,85 and in animal models, root perforations have been treated with scaffolds of collagen, pulpal stem cells, and dentin matrix protein 1, resulting in organized matrix similar to that of pulpal tissue.288

A case report indicates that it might be possible to revascularize the pulp in infected necrotic immature roots (Fig. 12-60; see also Chapter 16 ).16 A young patient presented with an immature second lower right premolar with radiographic and clinical signs of apical periodontitis with the presence of a sinus tract. The canal was disinfected without mechanical instrumentation, with copious irrigation and the use of a mixture of antibiotic agents. Later, a blood clot is created in the canal space and the access is filled with an MTA base. The treatment allows revascularization of the immature tooth and regain of a vital state of the pulp chamber, as well as normal root development below the restoration similar to the adjacent and contralateral teeth.

image

FIG. 12-60 Immature tooth with a necrotic infected canal with apical periodontitis. The canal is disinfected with copious irrigation with sodium hypochlorite and an antibiotic paste. Seven months after treatment, the patient is asymptomatic, and the apex shows healing of the apical periodontitis and some closure of the apex.

(From Banchs F, Trope M: Revascularization of immature permanent teeth with apical periodontitis: new treatment protocol? J Endod 30:196–200, 2004.)

In the future, the field of pulpal repair will probably develop rapidly, and new treatment strategies will appear.

Pulpal Calcifications

Calcification of pulp tissue is a very common occurrence. Although estimates of the incidence of this phenomenon vary widely, it is safe to say that one or more pulp calcifications are present in at least 50% of all teeth. In the coronal pulp, calcification usually takes the form of discrete, concentric pulp stones (Fig. 12-61), whereas in the radicular pulp, calcification tends to be diffuse (Fig. 12-62).349 There is no clear evidence whether pulp calcification is a pathologic process related to various forms of injury or a natural phenomenon. The clinical significance of pulp calcification is that it may hinder root canal treatment.

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FIG. 12-61 Pulp stone with a smooth surface and concentric laminations in the pulp of a newly erupted premolar extracted in the course of orthodontic treatment.

image

FIG. 12-62 Diffuse calcification near the apical foramen.

Pulp stones (denticles) range in size from small, microscopic particles often seen in association with the wall of arterioles to accretions that occupy almost the entire pulp chamber (Fig. 12-63). The mineral phase of pulp calcifications has been shown to consist of typical carbonated hydroxyapatite.349 Histologically, two types of stones are recognized: (1) those that are round or ovoid, with smooth surfaces and concentric laminations (see Fig. 12-61) and (2) those that assume no particular shape, lack laminations, and have rough surfaces (Fig. 12-64). Laminated stones appear to grow by the addition of collagen fibrils to their surface, whereas unlaminated stones develop by way of the mineralization of preformed collagen fiber bundles. In the latter type, the mineralization front seems to extend out along the coarse fibers, making the surface of the stones appear fuzzy (Fig. 12-65). Often these coarse fiber bundles appear to have undergone hyalinization, thus resembling old scar tissue.

image

FIG. 12-63 Pulp stones occupying much of the pulp chamber.

image

FIG. 12-64 Rough surface form of pulp stone. Note hyalinization of collagen fibers.

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FIG. 12-65 High-power view of a pulp stone from Fig. 12-57, showing the relationship of mineralization fronts to collagen fibers.

Pulp stones may also form around epithelial cells (i.e., remnants of Hertwig’s epithelial root sheath). Presumably the epithelial remnants induce adjacent mesenchymal stem cells to differentiate into odontoblasts. Characteristically these pulp stones are found near the root apex and contain dentinal tubules.

The cause of pulpal calcification is largely unknown. Calcification may occur around a nidus of degenerating cells, blood thrombi, or collagen fibers. Many authors believe that this represents a form of dystrophic calcification. In this type of calcification, calcium is deposited in tissues that are degenerating. Calcium phosphate crystals may be deposited within the cells themselves. Initially this takes place within the mitochondria because of the increased membrane permeability to calcium resulting from a failure to maintain active transport systems within the cell membranes. Thus degenerating cells serving as a nidus may initiate calcification of a tissue. In the absence of obvious tissue degeneration, the cause of pulpal calcification is enigmatic. It is often difficult to assign the term dystrophic calcification to pulp stones because they so often occur in apparently healthy pulps, suggesting that functional stress need not be present for calcification to occur. Calcification in the mature pulp is often assumed to be related to the aging process, but in a study involving 52 impacted canines from patients between 11 and 76 years of age, there was a constant incidence of concentric denticles for all age groups, indicating no relation to aging.259 Diffuse calcifications, on the other hand, increased in incidence to age 25 years; thereafter they remained constant in successive age groups.

At times, numerous concentric pulp stones with no apparent cause are seen in all the teeth of young individuals. In such cases, the appearance of pulp stones may be ascribed to individual biologic characteristics (e.g., tori, cutaneous nevi).259

Although soft-tissue collagen does not usually calcify, it is common to find calcification occurring in old hyalinized scar tissue in the skin. This may be due to the increase in the extent of cross linking between collagen molecules (because increased cross linkage is thought to enhance the tendency for collagen fibers to calcify). A relationship may exist between pathologic alterations in collagen molecules within the pulp and pulpal calcification.

Calcification replaces the cellular components of the pulp and may possibly hinder the blood supply, although concrete evidence for this strangulation theory is lacking. Idiopathic pulpal pain was classically attributed to the presence of pulp stones. Modern knowledge of mechanisms of nociceptor activation, coupled with the observation that pulp stones are so frequently observed in teeth lacking a history of pain, have largely discounted this hypothesis. Therefore, from a clinical perspective, it would be very unlikely that a patient’s unexplained pain symptoms are due to pulpal calcifications, no matter how dramatic they may appear on a radiograph.

Luxation of teeth as a result of trauma may result in calcific metamorphosis, a condition that can, in a matter of months or years, lead to partial or complete radiographic obliteration of the pulp chamber. The cause of radiographic obliteration is excessive deposition of mineralized tissue resembling cementum or, occasionally, bone on the dentin walls, also referred to as internal ankylosis (Fig. 12-66). Histologic examination invariably reveals the presence of some soft tissue, and cells resembling cementoblasts can be observed lining the mineralized tissue. Such calcific metamorphosis of the pulp has also been reported in replanted teeth of the rat.258

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FIG. 12-66 A, Calcific metamorphosis of pulp tissue after luxation of tooth as a result of trauma. Note presence of soft-tissue inclusion. B, High-power view showing cementoblasts (arrows) lining cementum (C), which has been deposited on the dentin walls.

Clinically, the crowns of teeth affected by calcific metamorphosis may show a yellowish hue compared with adjacent normal teeth. This condition usually occurs in teeth with incomplete root formation. Trauma results in disruption of blood vessels entering the tooth, thus producing pulpal infarction. The wide periapical foramen allows connective tissue from the periodontal ligament to proliferate and replace the infarcted tissue, bringing with it cementoprogenitor and osteoprogenitor cells capable of differentiating into either cementoblasts or osteoblasts or both.

When calcific metamorphosis is noted on a patient’s radiograph, it is sometimes suggested that the tooth be treated endodontically because the pulp is expected to be secondarily infected, and endodontic therapy should be performed while the pulp canal is still large enough to instrument. In a classic study of luxated teeth, Andreasen9 found only 7% of the pulps that underwent calcific metamorphosis exhibited secondary infection. Since the success rate for nonsurgical endodontic therapy, not only in general373 but also for obliterated teeth,68 is considered high, prophylactic intervention does not seem to be warranted.

Age Changes

Continued formation of secondary dentin throughout life gradually reduces the size of the pulp chamber and root canals, although the width of the cementodentinal junction appears to stay relatively the same.105,323 In addition, certain regressive changes in the pulp appear to be related to the aging process. There is a gradual decrease in the cellularity and a concomitant increase in the number and thickness of collagen fibers, particularly in the radicular pulp. The thick collagen fibers may serve as foci for pulpal calcification (see Fig. 12-64). The odontoblasts decrease in size and number, and they may disappear altogether in certain areas of the pulp, particularly on the pulpal floor over the bifurcation or trifurcation areas of multirooted teeth.

With age there is a progressive reduction in the number of nerves99 and blood vessels.24,26 Evidence also suggests that aging results in an increase in the resistance of pulp tissue to the action of proteolytic enzymes,387 hyaluronidase, and sialidase,26 suggesting an alteration of both collagen and proteoglycans in the pulps of older teeth. The main changes in dentin associated with aging are an increase in peritubular dentin, dentinal sclerosis, and the number of dead tracts.*323 Dentinal sclerosis produces a gradual decrease in dentinal permeability as the dentinal tubules become progressively reduced in diameter.328

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* The authors acknowledge the outstanding work of Drs. Henry Trowbridge, Syngcuk Kim, Hideaki Suda, David H. Pashley, and Fredrick R. Liewehr in previous editions of this text. The present chapter is built on their foundational work.

* References 44, 47, 73, 77, 192, and 241.

* A receptor cell is a non–nerve cell capable of exciting adjacent afferent nerve fibers. Synaptic junctions connect receptor cells to afferent nerves.

* To appreciate fully the dimensions of dentin tubules, understand that the diameter of the tubules (about 1 µm) is much smaller than that of red blood cells (about 7 µm). The thickness of coronal dentin is about 3 mm, so each tubule is 3000 µm long but only 1 µm in diameter. Thus each tubule is 3000 diameters long.

* A force of 44 cM (44 g) applied to an explorer having a tip 40 µm in diameter would produce a pressure of 2437 MPa on the dentin.55 This is far in excess of the compressive strength of dentin, listed as 245 MPa, as evidenced by the shallow grooves lined by smear layers created in dentin using this force.56

* The term dead tract refers to a group of dentinal tubules in which odontoblast processes are absent. Dead tracts are easily recognized in ground sections because the empty tubules refract transmitted light, and the tract appears black in contrast to the light color of normal dentin.