Special Report

Pulp-dentin biology in restorative dentistry.Part 3: Pulpal inflammation and its sequelaeKarin J. Heyeraas, Cand Med Dent, Dr OdontV Odd B. Sveen, LDS, MS,Ivar A, Mjör. BSD, MSD, MD, Dr Odont

Physiologic and histopathologic experimental studies over the last 30 to 40 years haue demonstrated that

the healing of the dental pulp is comparable to that of connective tissue elsewhere, despite its location in

the low-compliance pulp chamber. The greatest difficulty from a clinical point of view is to assess cellular

changes and vitality of Ihe pulp. If il is vital, the potential for repair is present, especially in young individu-

als. fQuintessence Int 2001:32:611-625)

Toothache has plagued mankinid since prehistorictimes. It may originate in the pulp-dentin complex

or organ or it may be of periodontal origin. Pain asso-ciated with pulpal inflammation has been a majorissue in health care since the early days of medicine,and it represents the basis for the establishment ofdentistry as a separate branch of surgery.

The anatomic position of the dental pulp, enclosedwithin a rigid dentin chamber, has led to some mis-conceptions regarding the fate of pulpal inflammation.The principal signs and symptoms of infiammation areredness, heat, swelling, and pain. If these characteris-tics of inflammation are applied to the anatomic posi-tion of the pulp, the swelling of the tissue seems to bethe overwhelming sign, which led to the "seLf-strangu-lation theory" of pulpal necrosis. The basis for thistheory has been entirely hypothetical; it suggests thatthe pressure inside the pulp chamber would be so highthat it woidd cut off its own blood supply, resulting intotal necrosis of the pulpal tissue.

Clinical obset^ation alone should have proved thatthis sequence of events does not occur. Any clinicianwho has entered a necrotic pulp chamber will haveencountered vital tissue in the root canals in someteeth, evidence that total necrosis does not always

occur. In fact, the frequency of pulpal infiammationobserved in histopathologic studies of the pulp sug-gests that the reactions can usually resolve withoutnecrosis. Even experimentally induced localizedabscesses in monkey teeth may heal.' Because infiam-mation is a protective response of the body to injury, itis unlikely tbat the dental pulp would be left vulnera-ble and witbout mechanisms tc sustain its vitahly andprevent widespread damage to tissues. Such mecha-nisms are in place, and this review will examine thenature of these inflammatory responses.

Whenever the pulp is subjected to injury, theimmune system will trigger an infiammatory responseto limit tissue damage by eliminating and digestinginvading organisms and cellular debris. Paradoxically,these infiammatory responses can injure pulpal tissuein severe cases and lead to pulpal necrosis. However,the special conditions under which the infiammatoryreactions occur in this low-compliance tissue call forextraordinary mechanisms, especially in relation toreactions in specialized cells, blood flow, transcapil-lary transport of fluids, lymphatic drainage, and con-siderations related to pressure gradients. The interac-tion among these factors makes heahng the mostcommon outcome of pulpal inflammation.

•Protessor, Departmsnt of Physiology, University ot Bergen, Bergen,

Norway.

'Associate Professor, Eastman Department of Dentistry, University ot

Rochester, School of Medicine anû Denlistry, Rochester, New Vork.

^Professor Academy 100 Eminent Sotiolar, Department ot OperativeDentistry, University ot Florida, Coiiege of Dentistry, Gainesville, Florida;

NIOfui. Scandinavian Institute of Dental fJlaterials, Haslum, Nomvay.

Reprint requesls: Dr Ivar A. fuljör. University ot Florida, Coliege of

Dentistry, PO Box 100415, Gainesviiie, Florida 32610. E-mail: imjor©

dentai.ufl.edu

This is one of seven articles in a series ertiphasizing a biologic approach torestorative dentistry through an understanding oí the pulp-dentin complex.

HISTOPATHOLOGY OF PULPAL INFLAMMATION

Pulpitis is similar to inflammation in other connectivetissues anywhere in the body. It may vary in intensity,duration, and extent. Based on clinical symptoms andhistopathologic descriptions, acute and chronic pulpi-tis may be distinguished. Cells associated with inflam-matory reactions in connective tissue have been iden-tified as polymorphonuclear leukocytes, primarilyassociated with acute reactions, and a group of cellsoften referred to as mononuclear leukocytes, including

Quintessence International611

• Heyeraas et al

Fig 1 Ptiotcmicrograph ol a Slight puipaireaction.= Ttie most striking change Iromthe normal structure is the increased num-ber ot celis in the iocation cf the cell-treezone in the area corresponding to the "cau-ity tubules" (between dotted lines), ie. thosetubules exposed by the cavity preparation.The reaction is localized to the affectedtubules. ( He m atoxyl in-eos in-stained, dem-ineraiized section; ciiginal magniticationxio.)

Fig 2 Higher magnrlication ot the pulp-predentin area of the cavily tubuies shownin Fig 1. Tiie odontcbiastic layer (O] is visi-bie, bul il has more capiliaries than normaladjacent to it. The cells in the subcdonto-blastic region, wiiere the celi-tree zone isnormaliy tound, have a morphology corre-sponding tc that of fibroblasts and undiffer-entiated cells in the rest of the pulpal tissueA few inliammatory ceils are lound (Hema-toxylin-eosin-stained, demineralized sec-tion; original magnification X1OO.]

Fig 3 Pholomicrcgraph ot a moderate pui-pai reaction.Í- Many ceils have accumuiatedsubjacent to the affected tubules, but tiieyare weil deiimited to the cavity tubules{between dotted lines). (Hematoxylin-eosin-stained, demineralized section; origi-nal magnitication xtO.)

lymphocytes, plasma cells, and a series of macro-phages. Specialized immunocompetent, dendritic cellshave also been identified. Mast cells are not found inthe normal pulp, but they have been identified in theinflamed pulp. Thus, in normal tissue they may bepresent in a precursor form or they may enter via theblood circulation as part of the infiammatory exúdate.In addition, reacfions occur in the odontoblasts as aresult of pulpal inflammation, and these reactions willbe described in detail.

Reports on pulpal reactions and inflammatoryresponses are usually presented in a descriptive form.As such they are qualitative assessments, althoughattempts to define and illustrate different stages semi-quantitatively have been made by classifying the reac-tions as slight or mild, moderate, and severe. -

Slight reaction is described to differ from the nor-mal pulpal structure by an increased number of cellsin the so-called cell-free zone and in the adjacent pul-pal tissue (Figs 1 and 2). The majority of these cellshave morphologic characteristics of fibroblasts andundifferentiated cells, but a few inflammatory cells are

also involved. An increased number of capillaries arenoted, and a few extravasated red blood cells may befound. The response is localized to the affected denti-nai tubules.

Moderate reaction is predominantly characterizedby more cells in areas subjacent to the affected dentinthan are associated with a slight reaction (Figs 3 and4). Neutrophiiic and mononuclear leukocytes invadethe odontoblast-predentin area in proportions thatdepend on whether the reaction is predominantlyacute or chronic. Odontoblasts cannot be identified intheir normal pseudostratified appearance, but individ-ual odontoblasts may be discerned. Sometimes odon-tobiastic nuclei can be observed in tbe dentinaitubules.

Increased numbers of capillaries and vessels arefound in the infiltrated tissue and at its border. Thepulpal reacüon is iocalized (Fig 3). The width of thepredentin may or may not deviate from normal,depending on the duration of the reaction.

Severe reaction is described as an area with markedcellular infiltration, including abscess formation (Figs

612 Volume 32, Number 8, 2001

Heyeraas et ai •

Fig 4 Higfier magnitication of the pulp-predentin area ot the cavity tubules shownin Fig 3. No odontoblastic iayer can beidentified, but individual odontoblasts are intheif normal position. Some ocfonloblasficnuclei iiaue been displaced into the denti-nal tubules. The cells in the subcdontoblas-tic region are well delimited to the exposeddentinai tubules and they comprise neu-tropiniiic and mononuclear ieukocytes asweil as tibrobiasts and unditferentiatedcells Many capillaries are present.(Hematoxylin-eosin-stained, demineraiizedsection; originai magnifioation x too.)

Fig 5 Photomicrograph of ,"i fifvoro pulpi.reaction.' The cellular iniikiation is localizedto ihe cavity tubules (above the dotted line),is intense subjacent to the exposed tubules,arid corresponds to that of an abscess(Hemafcxylin-eosin-stained. demineraiizedsection; original magnilicaticn xtO.)

Fig 6 Higher magnification ol the pulp-pre-dentin area of the cavity tubuies shown inFig 5. No odontobiasts are recognizabieand no predentin oan be identified 7 daysatter the experimental procedure was initi-ated It IS prcbable that the predentin pres-ent at the start cf the treatment has mineral-ized and nc new predentin has termed.Some nuclei have been dispiaced into thedentinai tubuies. Poiymorphonuciear andmononuclear leuiiocytes predominate in theattested area, and evidence ot ohemotaxisis visible as cells are closely associated withthe dentinal tubules. (Hematoxylin-eosin-stained, demineraiized section; originalmagnification xtOO.)

5 and 6). Poiymorphonuciear and mononuclearleukocytes predominate in the affected area, and theresponse is well delimited. The odontohlastic layercannot he identified as a morphologic entity or asindividual cells shortly after the response is estah-lished. No predentin is formed, and within days theexisfing predentin apparently mineralizes and cannothe distinguished from the adjacent dentin. Odon-toblastic nuclei can be seen in the dentinal tuhulesprovided the changes do not represent a long-standingreacfion. Numerous biood vessels are found in the tis-sue surrounding the intense cellular infiltrafion.

The predentin reactions, tbe degeneration of theodontoblasts, and the cellular infiltration are all welldelimited to the affected dentinal tuhules (Fig 5)- Thehyperemia is difficult to ascertain hased on histo-pathologic examination, hut, because a nontraumaticpreparafion tecbnique was used for the specimen inFigs 4 and 5, the displacement of odontoblastic nuclctinto the affected dentinal tubules is considered to heevidence of a locahzed increased fissue fiuid pressurethat results from the hyperemia.

Ttie uitrastructure of puipai infiammation

The ultrastructural changes in the odontoblasts andthe inflammation in the adjacent fissue after grindingof rat molars have heen studied in detail.^ Results fromthis study will be used to outline the structuralchanges associated with pulpal infiammafion, includ-ing the reparative phase. The immediate responses (15to 60 minutes) included displacement of odontoblasticnuclei into the denfinai tuhules and disturbances inthe subodontoblastic tissue. The denfin exposed hy thegrinding was exposed to the oral environment for peri-ods ranging from 6 hours to 8 days postoperatively.

Six hours postoperatively, the inflammatory changeswere recognizable by the presence of engorged bloodvessels, leukocytes, and exúdate (Fig 7). Degenerativechanges in the remaining odontohtastic iayer were pre-dominant {Fig 8), including dilated, rough endoplasmicreticulum and swollen mitochondria. Deteriorafion inthe subodontohlastic cells was also visible {Fig 9). After12 and 24 hours, these changes were more advanced,and evidence of necrosis was found (Figs 10 and 11).

Quintessencelnternationaf 613

Heyer aas et al

Fig 7 (left) PhotomicrogrcJfih cf "' ' j : ' " ' ^ ^ "mineralized section 6 hour " ' " ' " î " ^a lat molar. The prepared (Sh) wasclose to tne pulp, and ,i • '"O" «' "^^pulp and predentin has • '• (pfobabiyas a histoiogic artifact) philic leuko-cytes have intiitrated the ..lUijiitoblastiC layer(O) and more blood vessels (BV) than nor-mal are found in the odontobiastic and sub-odontcblastic region. (Basic fuchsin andméthylène blue-stained, undemineraiizedsection; original magnification X500.)

Fig 8 (right) Eleotron micrograph of odon-tobiasts (O) corresponding to those in Fig 7,showing marked degenerative and necroticchanges both in the nucieus and in the cyto-piasm. Exúdate (EX) separates those cdcn-tobiasts that have not tjeen displaced. Thenuciei (N] display peripheral accumuiationof chromatin and cytoplasmic organellessuch as rough-surfaced endoplasmic reticu-ium (RER), and rnitochondria (M) areswolien. (Originai magnification x6,Ó00.)

Fig 9 Eiectrcn micrograph showingdegenerative changes in the subcdonto-biastic region 6 hours attei grinding ot ratdentin. The subodontobiastic ceii has aiarge vacuole (Va] filled with granuiar mate-rial. (Originai magnification xl6,000.1

Fig 10 Photomjcrograph showing severeinjury in a pulp horn 24 hours after grindingot a rat moiar. Necrotic tissue (Ne) sepa-rates the dentin from the pulp. Macro-phages (MP) at the border between necroticand vitai putpai tissue constitute a part otthe inflammatory infiitration. Note the pres-ence ot many engorged biood vessels (BV).(Basic fuchsin and methyiene blue-stained,undemineralized section; original magnifica-tion X500.)

Fig 11 Eiectron micrograph of ceils at theborder ol necrotic tissue simiiar to the situa-tion shown in Fig 10 Numerous ceiis iine upagainst the subodontobiastic necrotic tissue(Ne). Many ceiluiar processes (CP) are seenalong the border toward the necrotic tissue.(N) Nucieus of subodontobiastic cell.[Original magnification x 11,300.)

614 Volume 32, Number a,

Heyeraas et al •

Active phagocytosis was seen at the border betweenvital and necrotJc tissue (Fig 12), and distinct degenera-tive changes in the odontoblastic processes were noted.

After 48 hours, inflammatory changes were still evi-dent, but many cells assumed an elongated form withpolarization of the nucleus (Fig 13). These cells wereidentifled as new or secondary odontoblasts (Fig 14).Macrophages were active at the border of the vital tis-sue (Fig 15). Evidence of collagen synthesis by thenew odontoblasts was noted after 48 hours. After 3 to8 days, the healing phases predominated, includingcontinued coilagen synthesis (Fig 16), and mineraliza-tion of the intertubular matrix occurred (Fig 17). Thus,the inflammation that resulted from the inflictedtrauma resolved,

A similar sequence of events of healing is believedto occur following trauma from cavity and crownpreparations or from various clinical conditions andprocedures carried out in human teeth. However,iocalized necrosis is unlikely to occur as a result ofcavity or crown preparations alone. It is also knownthat the healing processes will take longer to manifestthemselves in human teeth. Furthermore, confoundingfactors, such as caries, age, and previous trauma, mayplay a role. Some of these factors include defensereactions, eg, obturation of dentinal tubules, whichreduces dentin permeability. Other reactions may leadto scar tissue in the pulp, resulting in pulpal tissuewith more collagen fibers than normal and reducedcellularity. Healing is less likely to take place in afiber-rich pulp with reduced number of cells than in acell-rich pulp with many undifferentiated cells and fewfibers. Therefore, it is always the clinician's obligationto minimize the trauma to the pulp, and in that wayreduce the injury, to provide the best possible oppor-tunities for future pulpal healing.

Quantitative methods

Because histopathologic descriptions are inherentlysubjective, attempts have been made to quantify pul-pal reactions by morphometric analysis.'"" This tech-nique allows detailed quantitative analyses of volumedensities, including an account of the different cellularcomponents of inflammatory cells, such as a distinc-tion between polymorphonuclear and mononuclearleukocytes. The cytoplasma-nucleus ratio of the odon-toblasts and the area of predentin subjacent to a denti-nal lesion may also be measured.

The use of quantitative methods also allows morepowerful statistics to be employed than can be used forresults based on qualitative descriptions. Statisticallysignificant differences that would have been easilymissed with descriptive histopathology can often bedetected between small measurements. In the evalua-

Fig 12 Electron micrograpti of a macrophage similar to those inFig 10. A necrotic, engulfed cell (EC) is being digested within alarge vacuoie (Va) in the cytoplasm of a macrophage. (N) Nucleusof macrophage. (Original magnification x16,300.)

tion of localized reactions subjacent to a cavity in thedentin, the semiquantitative descriptive method wasconsidered adequate,"* but detailed studies of the odon-toblast-predentin region associated witb formation oftertiary dentin allowed distinction between reactions toactive and arrested caries lesions. However, it remainsto be determined at what levei statistically significantdifferences are clinically significant.

Etiology of pulpal inflammation

Inflammatory reactions in the pulp may be caused bytrauma to tbe tooth, by toxic or allergenic agents inrestorative materials, and by bacterial products, manyof whicb may cause immunologie reactions. Followingthe identification of dendritic, immunocompetent cellsin the dental pulp.^'^ research has focused on antigen-specific inflammatory reactions in the pulp. Bacterialproducts may act as antigens, and as such the immunesystem may play an important roie in defense mecha-nisms in the pulp-dentin organ. It has long been rec-ognized that bacteria play a central role in inducingpulpai reactions, both in association with dental cariesand from their presence at the tooth-restoration inter-face. Histopathologic studies of the pulp have repeat-edly demonstrated that lymphocytes in large numbersarc associated with pulpal inflammation, and thesecells are known for their ability to recognize antigens.

The dendritic cells play an important role in captur-ing the antigens, migrating to the iymph nodes, andpresenting them to the lymphocytes. Other dendritic

Quintessence International 615

Heyeraas et ai

Fig 13 Ptiotomicrograpti of celis subjacentto a lesion v itti less injury to ttie puip thanthat shown in Fig 10. Note ttie predomi-nance of spindle-shaped oelis. probabiy dif-ferentiating into secondary odontobiast-i ikeceils, and the high vascuiarity in Ihe area(Basic fjohsin and méthylène blue-stained,undemineraiized section; original magnifioa-tion x500.)

Fig 14 Eiectron micrograph ol a celi nearan Injured area ot the puip differentiatinginto an odontobiast-iike ceii. Its cytopiasmhas an abundance of ribosomes (Ri) andrough-surfaceü endoplasmic relicuium(RER). Note Ihe iarge areas of granuiarmaterial (GM) and newly formed coiiagenfibers (CF). (N) Nucieus of odon loblast-i ikeceli. (Originai magnification x24,100.)

Fig 15 Eiectron micrograph of a macro-pnage an ihe boider of injured pulpal lis-sue. Numerous organeiles are lound in thecytopiasm, including mitochondria (M) andmany lysosome-like (Ly). electron-densebodies. (N] Nucieus of macrophage. (Ori-ginai magnification xtt.300.)

Fig 16 (left) Eiectron micrograpn of part cfa nawiy deveioped secondary odontobiast(O) with large areas of granular material(GM) within Itie ceil. A round body of granu-lar materiai is being disoharged from theceli surface into an area with an abundanceof fine fibriis (FF). (Originai magnificaticnX42.000.)

Fig 17 (right) Electron micrograph show-ing part of an odontoblast (0) with an abun-dance ot intraceliuiar granuiar material(GM,) and ribosomes (Ri). Granuiar mater-ial is also found outside Ihe oeii (GMj).Collagen libers (CF) have formed, corre-sponding to predentin, and they becameincorporated into mineralised tertiary dentin(TD). (Original magnification x20,t)00.)

616 Volume 32, Number 8, 2001

Heyeraas et al •

ConnectiveDerim Odón toD lasts tissue

Tlie deiidiitie celpresents peptide fragir conjugation mithi class IImolecules lor antigen-specT cells

The activated T cells leave Ihe Inode and start to scan penplieral lissuesuch as pulp The individuai is now ¡mmCI sensitized depending on the type otantigen. The dendritic ceii can nowpresent antigen directly to the pulp tcrmemory T cells and thus initiatean irtlammation.

Fig 18 interaction behween dendritic celis in the odontobiastic region and T iymphccytes in theinduction of a primafy immune response. The dendritic oelis are stained immunohistochemicaily withanti-HtA-DR antibodies. (From Jontell et al.i^ Modified with permission.)

cells are linked to the macrophages.i' Entry of anti-gens to the pulp is through the dentin. Therefore, thepermeability of the dentin is of utmost importance indetermining the amount of antigen entering the pulpand consequently the magnitude of the pulpal reac-tion. The dendritic cells may also interact with nervesand vessels in the pulp. Thus, neuroimmunologicresponses of the pulp may be the prime defense reac-tion in the pulp-dentin organ. ^

The sequence of events in the interaction betweendendritic cells and lymphocytes in the induction ofprimary immune responses of particular importance torestorative dentistry is illustrated in Fig 18. The den-dritic cells in the odontobiastic region of the pulp arestrategically positioned as a primary immunosurveil-lance system. Because dendritic cells are also oftenfound along blood vessels, they may also have impor-tant functions in regulating blood flow. Thus, the cel-lular elements in the immune system represent struc-tural entities that are essential for the physiology ofthe pulp and its defense mechanisms.

PATHOPHYSIOLOGY OF PULPAL INFLAMMATION

The blood flow, ie, the volume of blood passingthrough the vessels per unit of time, determines thespeed of diffusion between blood and interstitial tissuefluid. The higher the blood fiow, the faster the difhi-sion. Tbus, more oxygen and nutrients are delivered tothe pulp and more carbon dioxide and waste productsare removed by high blood flow. Therefore, anyinflammation-induced increase in blood flow isregarded to be a protective response, which enablesthe tissue to survive a noxious stimulus. A decrease inblood flow would cause delayed removal and accumu-lation of injurious agents, which in turn might lead tothe demise of the pulp.

Nerve impulses are the prevailing mechanism forregulation of pulpal blood fiow. In the dental pulp,both the autonomie efferent nerves of sympathetic ori-gin and the afferent sensory nerve fibers from thetrigeminal ganglion have important roles in blood flowregulafion.»-"

Quintessence International617

• Heyeraas et al

Fig 19 The dark fibers are sympathetic nerves immunoreactedfor neuropeptide Y in a rat dental puip. Most tibers follow Itie longaxis ot blood vessels (BV) through the root to the coronal pulp. Theneuropeptide Y fibers are thin and have a delicate beadedappearance. (O] Odontoblastic layer. (Original magnificationX400,)

Fig 20 The dark fibers in the odontoblastic area are sensorynerves immunoreacted for calcitonin gene-relaied peptide in a ratmoiar pulp. (CA) Dentin exposed to the oral enviionment for 5weeks; (D) dentin, (Original magnitication xt2.)

Activation of sympathetic nerve fihers causesmainly vasoconstriction hecause of activation of Q-adrenoreceptors^'-'^'ä and neuropeptide Y, which isreleased together with norepinephrine from sympa-thetic nerve endings' ^" {Fig 19). The carofid harore-ceptor reflex also activates the sympathetic fibers tothe dental pulp^ ; thus falls in systemic blood pressurecause sustained and prolonged reduction in the bloodcirculation in teeth.

Prolonged sympathetic vasoconstriction in the den-tal pulp has been shown in animal experiments tomarkedly decrease the excitability of intradental sen-sory nerves." During pulpal ischemia, the sensory ter-minals of the A-type nerve fihers lose their normal sen-sitivity." Therefore, during pulp testing, such asapplication of heat and cold and recording ofresponses to electrical pulp tests, it should be remem-bered that it is not the vitality but the excitability ofthe sensory nerves that is examined. The pulp may sur-vive fairly prolonged, severe reductions of blood flowwithout permanent damage." Thus, any clinical situa-tion that decreases pulpal blood flow will reduce theresponses to these pulp tests, although the vitality ofthe pulp may be unchanged.

While the autonomie sympathetic nerves are mainlyresponsible for vasoconstriction, a certain population ofafferent sensory nerve fihers seems to be the principalsource of vasodilafion during inflammation in the pulp.These peptidergic, small and medium neurons associ-ated with itnmyelinated C or myelinated A-delta fihersare excited hy a variety of noxious stimulL^o. '' Theirmain function is associated with ptalpal pain; however,these nerves contain vasodilating neuropeptides such as

neurokinin A, calcitonin gene-related peptide (CGRP),and suhstance P {SP), which are released from thenerve terminals in response to activation of intradentalsensory fibers^^'^* (Fig 20). Thus, stimuli laiown to causetooth pain may initiate vasodilation.

These neuropeptides cause a relatively long-lastingincrease in pulpal hlood flow and tissue pressure (Fig21), Infusion of specific antagonists to CGRP and SP ordisruption of the sensory innervation results in a signifi-cantly lower blood flow than is recorded in innervatedteeth.'^ These findings indicate that a resting vasodilatortone arising from a spontaneous, basal release of CGRPand SP exists in the dental pulp. Supporting evidencehas been provided hy intra-arteHal infusion of agoniststo sensory neuropeptides. These agonists cause a signif-icant increase in pulpal blood fiow and intersfitial fluidpressure.'* Thus, available evidence demonstrates theinvolvement of the sensory nerves in the inflammatoryprocess, and the term neurogenic infiammation is usedto collecfively refer to the pulpai responses initiated bynerves following injury of any sort.

Sfimulafion of adjacent tissues, inciuding adjacentteeth, gingiva, and the fip may also cause increasedblood flow in the unstimulated piilp, ''. '' and painfulstimtilation of teeth may result in increased hlood flowin adjacent sofi tissues. These findings indicate exten-sive branching of sensory axons, suggesting an axonreflex-mediated spread of the neurogenic infiamma-fion beyond the site of stimulafion. It is considered tobe of clinical reievance that cutting and probing ofexposed dentin not only may evoke vasodilation in thepulp but also may result in an infiammatoiy reactionin adjacent teeth and soft tisstie.

618 Volume 32, Number 8, 2001

Heyeraas et ai

Fig 21 Simuitaneous originai recordingsof biood tlow (laser Doppler flow [LDF]) inthe coronal pulp (upper curve), interstitialfluid pressure (IFP), systemic arterial pres-sure (Pj), and blood flow in the apical pulp(lower curve) before, during, and after elec-trical tooth stimulation (100 uA). (FromHeyeraas and Kvinnsland.^' Reprinted withpermission.)

Interstitial fluid pressure

The hydrostatic pressure in the interstitial fluid sur-rounding the cells and the other structural compo-nents of the pulp has been shown to be 5 to 20 mm Hgahove atmospheric pressure.''-^'' Because the pulp isenclosed in a rigid dentin chamber, even small changesin pulpal fluid volume will be noted in the interstitialfluid pressure. This low-compliance system will resultin an increased tissue fluid pressure following anincrease in fluid volume, and conversely a drop inpressure following a reduction in fluid volume. In gen-eral, inflammation is associated with a raised fluid vol-ume in the tissues arising from vasodilation andedema. Severe inflammation in the monkey pulp hasheen shown to result in localized interstitial tissuefluid pressures as high as 60 mm Hg."

Regardless of the tissue, the immediate inflamma-tory response is virtually identical and is characterizedby hyperemia and increased vascular permeability thatallows the escape of plasma proteins. These events willincrease both the blood volume and the interstitialfluid volume in the locally inflamed area and thereforeraise the pulpal pressure (Fig 22). However, theincrease in interstitial fluid pressure in the puip is usu-ally limited to the locally inflamed area and does notextend to the rest of the pulp. -J* This finding is con-sistent with the histopathologic observation that pul-

pal reactions are localized to the affected dentinaltubules and are well delimited from the rest of the pul-pal tissue (Fig 23}.

The mechanisms involved in limiting the inflamma-tory increase in pressure are dependent on severallocal feedback mechanisms. The increased interstitialfluid pressure will lower the transcapillary hydrostaticpressure difference and therefore oppose further filtra-tion. In addition, the locally increased pressure in theinflamed area will favor net absorption of interstitialfluid in adjacent capillaries in uninflamed tissue.Discontinuities in the endothelium and fenestrationsof pulpal capülaries^s ^lay facilitate these exchangemechanisms. Furthermore, increased interstitial fluidpressure will most likely increase the lymphaticdrainage. Provided that these feedback mechanismsare functioning, the tissue fluid pressure will be limitedto the affected area, corresponding to that shownhistopathologically (see Figs 1 to 6).

The main feedback mechanisms counteracting abuildup and spread of the tissue fluid pressure areillustrated in Fig 22. They involve a net absorption otfluid into capillaries in adjacent uninflamed tissue andan increased lymphatic drainage. ^ Thus the increasein blood and interstitial fluid volume arising from theinflammatory process is effectively counteracted, andthe pulpal volume is kept relatively constant. No gen-eralized increase in pulpal fluid pressure occurs, and

Quintessence International 619

Heyeraas et al

Increased vessei permeability

Increased capillarybtod pressure /-^¡^¡^^^

^ I, circleIncreased venous

blocd pressure

Increasedfluid voiume

Fig 22 Buildup of a so-calied VICIOUS cir-cle during puipal inflammation The edema-preventing mechanisms that i.eive to keepthe pulpal tluid volume relatively constantare indicated by open arrows. These mech-anisms oppose a rise in tissue pressureand break the vioious oircle. The idea ol thevicious circle was the basis for the tor mer iyheld selt-strangulaticn theory of puipalinflammation. (From Heyeraas and Berg-gren.^' Modified v ith permission.)

Fig 23 Rat rnoiar with circumscribed abscess (AB) in the pulpsubjacent to a oavity (CA) that had been left open to the oral envi-ronment for 5 weeks. Nerve sprouting is seen as dark fibers bor-dering the abscess. The section was immunoreacted lor caloitoningene-related peptide. (D) Dentin. [Ofigirtai magnification xi2.)

healing will usually prevail provided that the injuriousagents are removed. However, persistent severe irri-tants may lead to a spread of the tissue fluid pressure,causing total pulpal necrosis.

The magnitude and the duration of the increasedinterstitial fluid pressure will vary, depending on thedegree and severity of the inflammation. Both the cel-

lular infiltration and the tissue fluid pressure willchange in succession with the severity of the inflam-matory reaction. Although the basic nature of theinflammatory process is stereotyped, its intensity andextent will depend on the severity of the injury and thereactive capability of tbe bost

The relatively high pulpal tissue fluid pressureunder normal and pathologic conditions may beiinked to a neurogenic defense mecbanism that helpsto protect the pulp against the entry of harmful agentsthrough exposed dentinal tubules^'^^ (Fig 24). Anincreased interstitial fluid pressure will promote out-ward flow of fluid through dentinal tubuies. The out-ward flow may protect the pulp against entry of harm-ful agents. Because the pulp has an interstitial fluidpressure above atmospheric pressure and has a lowcompliance, the outward flow through the dentin wiflincrease quickly following injury. This mechanism mayalso explain the sensitivity noted as soon as theperipheral dentin is exposed during cavity preparation.The sudden outflow of dentinal fluid will initiatehydrodynamic movements in the contents of thetubules and tbus initiate pain, as explained by thehydrodynamic theory of dentinal pain.*"

Simultaneously, as the sensory nerves are exited,they may release vasodilatory neuropeptides in thepulp, via a so-called axon reflex, which will causevasodilation and consequently a further increase inpressure and, therefore, an increase in outward flow(Fig 24).

620 Voiume 32. Number 8. 2001

Heyeraas et al •

Cortex pain

Vessel

Fig 24 Diagram showing how stimuli in the dentin may excite nerves in the deniin-odcnioblastregion, causing pain. At the same time, these stimuii can brirtg about release of sensory neuropep-tides such as calcitonin gene-reiated peptide (CGRP) and substance P (SP) Irom nerve endings inthe pulp via an axon rellei. The neuropeptides induce vasodilation, which in turn increases the tissuepressure looaily in the low-compliance pulp. The increased tissue pressure will increase the outwardtlow of fluid trom the exposed dentinal tubules and thereby help protect the pulp against inward diffu-sion of harmful substances. (Frcm Mjor and Heyeraas. s Reprinted with permission.)

CORRELATION BETWEENPULPAL HISTOPATHOLOGY ANDINTERSTITIAL FLUID PRESSURE

The interstitial fluid pressure in the locally inflamedarea of pulpal tissue has been sbown to vary consider-ably."^-' This condition is not unexpected, because tbemagnitude of the interstitial fluid pressure in thelocally inflamed area simply reflects the increase influid voiume. Depending on the severity of the injury,the state (acute or chronic) of the pulpitis, and thehealing capacity of the pulp, both the blood volumeand the interstitial fluid volume will vary.

In fact, sticcessive recordings over time of intersti-tial tissue fluid pressure in monkey pulps haverevealed marked variations in tissue pressure." Afterdeep cavities had been prepared under nonsterile con-ditions, tbe pulp was exposed and prepared for tissuefluid pressure recordings. Initial pressure was recordedin all teetb. In some teeth, additional recordings weretaken from the sartie experimentally exposed site afterit had been temporarily closed for variable periods of

time. Tbis experimental procedure resulted in a rangeof different degrees of pulpal inflammation dependingon the time sinee the pulp was exposed and the num-ber of successive recordings.

After tbe teeth were removed following the lastrecording, they were subjected to histopathologic exam-ination. Some of the variations in tissue pressurerecorded in this experirtiental series were undoubtedlyof méthodologie origin. However, the experimentaldesign allowed correlation between the final tissuepressure recorded and the histopathology of the pulp.The clinical appearance of the exposed site was alsonoted as hemorrhagic, wet, dry, or exuding at each tis-sue pressure recording. Despite variations in the resultsand the limited material studied (25 teeth in total, 15 ofwhich had undergone repeated, intermittent recordingsup to 24 days), a correlation appeared to exist amongthe pulpal tissue pressure, the inflammatory responseassessed histopathologically, and the rise of fluid in thecavity as evaluated clinically at successive recordings, ^

An interstitial fluid pressure above 40 to 50 mm Hgwill most likely cause stasis, ischemia, and development

Quintessence internalionai 621

• Heyeraas et al

Dl

E

i

tial t

luid

pre

60

50

40

30

20

10-

0

m' ^ '

0 4

/ / ' , • • • \ \ " '

' " ' ' - • • \ ^ ^ ^ \

•••' V — - ^ - ^

9 t4 17 22Day

Fig 25 Three ouives showinr; yj'.,oessiveregistration ol interstitiai tissu fi'-'d pres-sure in ihe puips ot three moi.i'fy mcisorsover a 22-day period. The pror-sdure gen-eraily caused severe puipai recelions andnecrosis^' [see Figs 26 to 29). There werevariations in recordings in ttie same toothover time. These variations may be ofméthodologie origin, but they may alsoretlect diflerences in biologic responses.Maximum tissue pressure was usualiyrecorded after about 14 days.

of necrosis. What seems difficult to ascertain by suchcorreiations is how much of the necrosis is due to stasiscaused by a high tissue fluid pressure and how much isthe resuh of direct cellular damage caused by bacteriaand injurious agents. On the other hand, histopatho-logic studies aionc seem unsuitable for evaiuation of thegeneral circulatory state of the pulp during inflamma-tion, because diiated and congested vessels might justas well indicate stasis as a hyperemia, or the vesselsmay be overstretched during the preparation of the his-tologie sections.

Successive, representative tissue pressure record-ings in three monkey teeth over a 22-day period illus-trated the marked variation in the initial recordings,which in the experimental series averaged 16.0 ± 11.4mm Hg (range of 0 to 50 mm Hg), as well as in theindividual teeth over time" (Fig 25). The histopathol-ogy 45 minutes after an initial recording had beencompleted is presented in Fig 26. The blood vesselslocated centrally in the pulp appeared congested and ageneral increase in cellularity was apparent. An exam-ple of severe inflammatory reaction 21 days after thepulpal exposure, followed by four successive tissuepressure recordings, is shown in Fig 27. The tissue fluidwas chnically seen to be rising in the prepared cavityof this tooth prior to the final tissue pressure record-ing, which was 48 mm Hg. Complete necrosis of thepulp was noted in two teeth after 24 days. Other teethshowed coronal necrosis and severe inflammation inthe root pulp {Fig 28). In the tooth shown in Fig 28,the exposed site appeared dry and the tissue pressurewas recorded as 5 mm Hg.

Despite the méthodologie difficulties in tissue pres-sure recordings under the extreme experimental condi-tions described, the results are considered to illustratethe diversity in responses of the dental pulp with re-gard to correlation of physiologic parameters andhistopathologic assessment. Some of the variations inthe pulpal tissue pressure recordings could be due tothe occurrence of heahng processes even under theextreme experimental conditions.

Undoubtedly, healing of the pulp is the most com-mon outcome of pulpal inflammation under clinicalconditions. Even localized abscesses will heal by theformation of irregular tertiary dentin in affected areas,provided that the cavities are cleaned and restoredwith commonly used materials. The tertiary dentin isformed during healing of pulpitis either by survivingodontoblasts [reactionary dentin) or by newly differen-tiated odontoblasts (reparative dentin),'" Such healingwas strikingly demonstrated in a series of experimentsin which localized abscesses were induced by theplacement of soft, carious dentin in experimentallyprepared cavities in intact monkey teeth.» After thecarious dentin was removed and the cavities wererestored with a variety of different materials, theabscesses healed by the formation of tertiary dentin'(Fig 29). Healing even occurred without removal ofthe carious dentin in some teeth hut less predictablythan after routine operative treatment, tn some teethwhere the carious dentin was left in the cavity, necro-sis occurred, but usually after some tertiary dentin hadformed (Fig 30). Changes in the dentin that reduce itspermeability will facilitate this healing.

622 Volume 32, Number 8, 2001

Heyeraas et al •

Fig 26 Structure in a monkey pulp after ithad been experimentally exposed and sub-jected to tissue tiuid pressure recordingsfor 45 minutes. The recorded pressure was18 mm iHg. Note the congested vesselslocated centraliy in the pulp and a generalincrease m cellulanty of the pulp. (CA)Prepared cavity; (E) exposure site.(Hematoxylin-eosin-stained, demineraiizedsection; original magnitication x25,¡

Fig 27 Severe pulpal reaction 21 daysafter multiple tissue pressure reoordings.The tissue pressure reoorded at the perfo-rated site (E) immediately pnor to removalol the tcoth was 48 mm Hg. The inflamma-tory exúdate is extruded through fhe perfo-ration site (Hematoxylin-eosin-stained,demineraiized section; original magnifica-tion X25.)

Fig 28 Coronal necrosis (hJe) of a mcnkeypulp atter a 24-day observation period fol-iowing multiple recordings of tissue pres-sure. The root pulp adjacent to the necroticarea is severely infiamed, TUe pressurereccrded at the perfcraficn site immediatelyprior to removal of the tooth was 5 mm Hg.(Hematoxylin-eosin-stained, demineraiizedsection; originai magnification X25.)

Fig 29 (ieft) Extensive tertiary dentin (TD)formation in an area of a monkey puip thathad been subjected to an expérimentai pro-cedure that routineiy caused localizedabscess formation similar to that shown inFig 5. The cavity (CA) was reopened,cieared, and restored with zino oxide-eugenol cement and ailowed to heai lor 3months. Note the marked irreguiarity of thefirst formed tertiary dentin. (iHematoxylin-eosin-stained, demineraiized section; origi-nal magnification x45.)

Fig 30 (rigiit) Pulpal necrosis after cariousdentin was sealed in with amaigam and ieftin an experimentally prepared, deep cavityfor 82 days in a monkey tooth. Tertiarydentin (TD) had formed subjacent to thecavity prior to the necrosis. (Hematoxylin-ecsin-stained, demineraiized section; origi-nai rqagnitication X25.)

Ouintessenoeinternational 623

Heyeraas et al

CONCLUDING REMARKS

Inflammation in the pulp, as in other connective tissue,is a basic defense mechanism to limit or prevent tissuedamage. It took a long time for the dental profession toaccept that inflammation, which involves increasedblood flow and swelling of the connective tissue, couldbe compatible with maintenance of pulpal vitality. Justthe fact that the great majority of teeth remain vital evenafter restorative treatment of severe attacks of rapidlyprogressing caries should have indicated that mecha-nisms exist to control the severiti' of inflammation of thepulp without detrimental consequences.

Physiologic and histopathologic experimental stud-ies over the last 30 to 40 years have demonstrated thatthe healing of the dental pulp is comparable to that ofconnective tissue elsewhere, despite its location in thelow-compliance compartment of the pulp chamber.The greatest difficulty from a clinical point of view isto assess the cellular changes and vitality of the pulpbased on symptoms and results from various pulp testssuch as application of heat and cold and recording ofresponses to electrical pulp tests. The main decision tobe made is to determine if the pulp is necrotic or vital.If it is vital, the potential for repair is present, espe-cially in young individuals.

Regeneration of connective tissue has beenobserved in root canals that have been cleaned outafter pulpal necrosis.-' .-" These observations highlightthe potentials for development of new treatmentmodalities that encourage regeneration of tissue archi-tecture. Tissue engineering and biomimetic signalingwith growth factors offer exciting opportunities fordevelopment of novel approaches to clinical manage-ment of injured dental pulps, especially if theseapproaches can work cooperatively with the inflam-matory defense reactions already taking place.

ACKNOWLEDGMENTS

The authors would like to thank Dr A. J. Smith, Professor andChairman. Unit of Oral Biology, University of Birmingham,Birmingham. England, for reviewing (he manuscript.

A Guest Research Fetlowîhip from the Research Council ofNorway partly in .suppori of Ivar A. Mjiir's Faculty DevelopmentalLeave at NIOM, Scandinavian Institute of Dental Materiais, is grate-fully acknowledged.

REFERENCES

1. Mjör IA, Tronstad L. The healing of experimentally inducedpulpitis. Oral Surg 1974;38:115-121.

2. Jontell M, Gunraj MN, Bergenholtz G. Immunocompetentcells in the normal dental pulp. J Dent Res 1987;66:1149-1153.

12.

15.

16

20.

21.

23.

Langeland K. Tissue changes in the dental pul|. N expen-mcntal histologie study, Odontol Tidskr 1957;6"i : 146.Stanley HR. Design for a iiuman pulp study 1 Oral Surg1968;25:633-647Stanley HR. Design for a human pulp study. II. Oral Surg1968;25:756-764.Mjör ¡A, Tronstad L. Experimentally induced pulpitis. OralSurg 1972;34:]02-108.Browne RM, Plant CG, Tobias RS. Quantification of the his-tologie features of pulpal damage. Int Endod J 1980;13:104-111.Tobias RS, Plant CG, Browne RM. A comparative study ofthe irritating effects of silicate cements. Br Dent J 1981;150:119-124.Sveen OB. An Ultrastructurai Study of Pulpal Responses toInjury [thesis] Rochester, NY: University of Rochester,1972.Warfinge ]. Morphometrie analysis of teeth with inflamedpulp J Dent Re.s 1987 ;66:78-83.Bjorndai L, Darvann T, Thyistrup A. A quantitative lightmicroseopic study of the odontoblast and subodontobiasticreactions to active and arrested enamel caries without cavi-tatiun. Caries Res 1998;32:59-69.Jontell M. Bergenholtz G, Seheynius A, Ambrose W.Dendritic cells and macrophages expressing class II antigensin the normal rat ineisor pulp, j Dent Res 1988;67:1263-1266.Jontell M, Okiji T, Dahlgren U, Bergenholtz G. Immunedefense mechanisms of the dental pulp. Crit Rev Oral BiolMed 1998 ;9:179-200.Heyeraas KT, Nœss G. Nervous control of blood flow in thedental pulp in dogs. Acta Physiol Scand 1978;104:13-23.Uddman R, Grunditz T, Sundler R Neuropeptide Y:Occurrenee and distribution in dental pulps. Acta OdontolScand 1984;42:361-365.Byers MR, Swift ML, Wheeler EF. Reactions of sensorynerves to dental restorative procedures. Proc Finn Dent Soc1992;88(suppl l):73-82.Heyeraas KJT, Kvinnsland 1, Byers MR, Jacobsen EB. Nervefibres i mm uno reactive to protein gene produet 9.5, calei-tonin gene-related peptide, substance P and neuropeptide Yin the dental pulp, periodontal ligament, and gingiva in cats.Aeta Odontol Seand 1993 ;51:207-221.Edwall L, KJndlova M. The effeet of sympathetic nerve stim-ulation on the rate of disappearance of tracers from variousoral tissues. Acta Odontol Scand 1971 ;29:387-400.Kim S. Regulation of Blood Flow of the Dental Pulp:Macrocirculation and Microcirculations Studies [thesis],New York: Columbia University, 1981.Olgart L. Neurogenic components of pulp inflammation. In:Shimono M, Maeda T, Suda H, Takahashi K (eds).Pulp/Dentin Complex. Tokyo: Quintessence, 1996:169-175.Heyeraas K. The effect of variations in arterial blood pres-sure and baroreceptors reflexes on pulpai blood flow indogs. Arch Oral Biol 1975;20:345-349.Edwali L, Scott D Jr. Influence of changes in microcirctila-tion on the excitability of the sensory unit in the tooth of thecat. Acta Physiol Scand 1971;82:555-566.Olgart L, Gazelius B. Effects of adrenaline and felypressin(octapressin) on blood flow and sensory nerve activity in thetooth. Aeta Odontol Seand 1977;35:69-75.

624 Volume 32, Number 8, Ê001

• Heyeraas el al

24. Matthews B, Vongsavan N. Interactions between neural andhydrodynamic mechanisms in dentine and pulp. Arch OralBiol 1994;39(suppl):87S-95S.

25. Gazelius B, Edwall B, Olgart L, Lundberg JM, Hokfelt T,Fischer JA. Vasodilatory effects and coexistence of calcitoningene-related peptide (CGRP) and substance P in sensorynerves of cat dental pulp. Acta Physiol Scand 1987;130;33-40.

26. Heyeraas K, Jacobsen EB, Fristad I, Raab WH-M Vascularand immunoreactive nerve fiber reactions in the pulp afterstimulation and denervation. In: Shimono M, Maeda T,Suda H, Takahashi K (eds). Dentin/Pulp Complex. Tokyo:Quintessence, 1996:162-168.

27 Heyeraas KJ, Kvinnsland I. Tissue pressure and blood flowin pulpal inflammation. Proc Finn Dent Soc 1992;88(suppll):393-401.

28. Jacobsen EB, Heyeraas KJ. Pulp interstitial fluid pressureand blood flow after denervation and electrical tooth stimu-lation in the ferret. Arch Oral Biol 1997 ;42:407-415.

29. Berggreen E, Heyeraas KJ. The role of sensory neuropep-tides and nitric oxide on pulpal blood flow and tissue pres-sure in the ferret J Dent Res 1999i78:1535-1543.

30. Sasano T, Kuriwada S, Shoji N, Sanjo D, Izumi H, Karita K.Axon reflex vasodilatation in cat dental pulp elicited by nox-ious stimulation of the gingiva. J Dent Res 1994;73;1797-1802.

31. Heyeraas KJ, Berggren E. Interstitial fluid pressure in nor-mal and inflamed pulp. Crit Rev Oral Biol Med 1999;10:328-336.

32. van Hassel HJ. Physiology of the human dental pulp. OralSurg 1971132:126-134.

33. Stenvik A, Iversen J, Mjor lA. Tissue pressure and histologyof normal and inflamed tooth pulps in Macaque monkeys.Arch Oral Biol 1972;17:1501-15U.

34. Heyeraas KJ, Kvinnsland I. Micropuncture measurements ofinterstitial ñuid pressure in normal and inflamed dental pulpin cats. J Endod 1983;9;105-109.

55. Dahl E, Mjör IA. The fine structure of the vessels in thehuman dental pulp. Acta Odontol Scand 1973 ;31:223-230.

36. Heyeraas K. Blood flow and vascular pressure in the dentalpulp. Acta Odontol Scand 1980;38:135-144.

37 Matthews B. Sensory physiology: A reaction. Proc FinnDent Soc 1992;88(suppl l):529-532.

38. Vognsavan N, Matthews B. Changes in pulpal blood flowand in fluid flow through dentine produced by autonomieand sensory nerve stimulation in the cat. Proc Finn DentSoc 1992,88(suppl l):491-497

39. Mjör IA, Heyeraas KJ. Putp-denlin and periodontal anatomyand physiology. In: 0rstavik D, Pitt-Ford TR (eds). EssentialEndodontology. Oxford: Bladtwell Science, 1998:9-41.

40. Brännström M. A hydrodynamic mechanism in the trans-mission of pain-producing stimuli through the dentine. In:Anderson DJ (ed]. Sensory Mechanisms in Dentine. Oxford:Pergamon Press, 1963:73-79.

41. Smith AJ, Cassidy N, Perry H, Bègue-Kirn C, Ruch ]-V,Lesot H. Reactionary dentiiiogenesis. Int J Dev Biol1995:39:273-280.

42. Nygaard-0stby B. The role of the blood clot in endodontictherapy. Acta Odontol Scand 1961;19:323-353.

43. Hersted P, Nygaard-0stby B. Tissue formation in the rootcanal after total pulpectomy and partial root filling. OralSurg 1978;46:275-282.

Quintessence International625