Pulp-dentin biology in restorative dentistry.Part 2: initiai reactions to preparation of teethfor restorative procedures
Ivar A. Mjor, BSD, MSD, MD, Dr Odont'
Pulpal compiicaficns involving infiammation, degradation, and necrosis are fhe result of a series of frau-matic injuries. Tfie resforafive denfisf musf minimize ffie trauma fo denfin and pulp inflicfed during ciinicalprocedures, including fhaf infiicfed during fooffi preparation. Part li of tfiis series discusses ffie structuraiand physiologic changes in fhe pulp-denfin compiex fhaf result from crown and oavity preparation and theciinicai implication of fhese changes. (Quintessence Int 2001 ;32:537-551¡
Tbe eftects of restorative procedures on dentin andpulp represent a combined response to the prepa-
ration and to the restoration. Long-term effects ofpreparation events alone are difficult to assess,because the preparation will bave to receive a provi-sional or permanent restorative material or be leftexposed to the oral environment. Beeause no restora-tive material exists that is truly inert in a biologicsense, and because preparations left open to the oralenvironment will accumulate debris and bacteria, tbeonly way to evaluate structural cbanges in humandentin and pulp of a cavity or crown preparation is toextract tbe teetb immediately after tbe procedure iscompleted. Nondestructive, physiologic tecbniquesmay also be used in experimental studies on animals.
In 1955, a special experimental design wasattempted to evaluate the long-term effect of cavitypreparation.' Cavity preparations with a separate cen-tral deep cavity were prepared in premolars in chil-dren. The deep part was covered by a gold plate sealedin with zinc pbospbate cement. Tbe teeth wereextracted after observation periods ranging from 15minutes to 4 weeks. Histopatbologic examination ofthe pulp showed that tbe reactions subjacent to tbedeep cavity were the same as in teeth witb cavitiesfilled witb zinc oxide-eugenoi cement or gutta-percha.
'Professor, Academy 100 Eminenf Scholar, Department of Operative Den-tistry, LInwersity cf Florida, College of Dentistry, Gainesville, Florida;NIOM, Scandinaviar Institute of Dental Materials, Haslum, Norway.
This is one of seven articles in a series emptiasizing a biclogic approacti torestorative dentistry tfirough an understanding of the pjlp-denbri complex.
The eariy reactions included displacement of odonto-blastic nuclei into the dentinal tubules. After 4 weeks,reparative dentin had formed subjacent to the cavit\'.
Tbese results are considered to be a combinedeffect of two difterent procedures: restoration of a cav-ity with zinc phosphate cement and preparation of anopen cavity.' Apphcation of current knowledge indi-cates that the initial observations may have resultedfrom tbe higb interstitial fluid pressure in tbe pulp.Provided tbat the dentinal tubules are patent, cavity orcrown preparations cut into tbe high-pressure area ofthe vital pulp can permit dentinal fluid to leak out.This leakage may be tbe most important effect of cav-ity and crown preparation, and it will be dealt with insome detail in this article.
Any pulpal and dentinal cbanges that result fromtbe preparation can affect the evaluation of reactionsto the entire restorative procedure, including possibletoxic and allergic reactions to tbe restorative materialor to bacteria and salivary components. Much atten-tion has focused on bacteria present on tbe preparedsurface and those at tbe tootb-restoration interface.What actions to take against tbese bacteria is a con-troversial topic, but it is generally accepted tbat bacte-riostatic sealing of restorations is important in restora-tive dentistry to prevent bacterial leakage. However,extensive clinical experience bas sbown that contami-nation of dentin during cavity and crown preparationshas no major effect on tbe outcome of tbe treatment,and it may stimulate defense mecbanisms in tbe pulp-dentin organ. Long-term maintenance of a healthypulp is a result of atraumatic preparation and the useof biologically acceptable restorative materials thatcan seal the tootb-restoration interface to prevent orminimize bacterial leakage.
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Teetb tbat are to receive restorations bave or bavehad primary caries lesions, are worn, or bave fracturedbecause of trauma; many restorations are placedbecause of failure of a previous restoration. All tbeseconditions result in changes in the dentin and pulp.Intact teetb to be used for fixed partial denture abut-ments may also be involved. Thus, tbe condition of tbeteetb prior to cavity and crown preparation can bebighly variable.
In a researcb setting, the evaiuation of pulpal anddentinal responses to cavity or crown preparationmust be based on tbose responses tbat occur afterpreparation of intact teeth from young individtials,preferahly newly erupted teeth. Tbis selection of teetbis necessary because the norrnal structure of theseteeth is well estabiisbed, and any deviation in strtic-ture observed after tbe preparation can tben be attrib-uted to the specific procedure. Wben a preparationtecbnique that will not induce cbanges according totbe metbod of evaluation employed bas been estab-lished, tbis tecbnique can be used to prepare cavitiesand crowns for subsequent testing of reactions torestorative procedures, including agents applied to thedentin, sucb as cleansers, disinfectants, acids, bondingagents, and restorative materials.
Tbe initial reactions tbat will be described and dis-cussed in this article will be limited to proceduresimmediately preceding tbe restorative pbase of tbetreatment. Most studies on reactions to tbe cutting ofdental bard tissues bave been carried out witb rotaryinstruments using different types of burs, but air abra-sion and iasers bave also been used. Sucb techniquesbave not received \vide use in dental practice, but theywill be briefly reviewed in this article as alternativepreparation metbods.
Histologie cbanges associated with cavity andcrown preparation can be evaluated witb microscopictechniques. Pbysiologic changes can be assessed witbtechniques sucb as histocbemical demonstration ofneurogenic components, blood flow measurements, orrecording of interstitial tissue fluid pressure.
FORMATION OFTHE SMEAR LAYER
The normal structure of dentin comprises mineralizedintertubular and peritubtilar dentin and dentinaltubules (Figs 1 and 2) containing odontobiasticprocesses, or their remnants, and tissue fluid, oftenreferred to as dentinal fluid. If the surface of cutenamel and dentin is examined after preparation withband instruments or burs, no structural details sucb ascut dentinal tttbules {Fig 3) or enamel prisms will bevisible, even at high magnification. All such details areobscured by a covering layer of cutting debris from the
mineralized tissues.^ Tbis grinding debris consists ofground components of enamel and intertubular andperitubular matrix, including any content of tbe dend-nal tubules, mixed with water, dentinal fluid, and oftensaliva. Tbis layer is less than 2 jim thick and is termedtbe smear lay er.^-^
Because tbe dentinal substrate differs as a result ofage-related changes, caries, dentinal sclerosis, andrestorative procedures, tbe smear layer can vary in com-position.-' If the prepared dentinal surface bas opentubules, small plugs of debris may extend into any opendentinal tubule. The smear layer reduces tbe fluid flowfrom tbe dentin and decreases dentinal permeability. ''
Tlie composition of tbe smear layer varies, depend-ing not only on tbe substrate but also on tbe type ofbur used. If a higb-speed dental engine is used, thesmear layer will be tightly burnished to tbe preparedsurface. Tbe smear layer cannot be completelyremoved by a water spray (Fig 4) or by scrubbing, butit will dissolve during acid-etcbing procedures. Acidetcbing demineralizes tbe smear layer and tbe per-itubular and intertubuiar dentin of tbe prepared sur-face. It leaves the tubules wide open (Fig 5).
The smear layer may also be removed by the appli-cation of pumice to the prepared surface. Tbis proce-dure removes tbe smear layer and leaves the smearplugs in the openings of the tubules in place^ (Figs 6and 7). A similar selective removal of the smear layer,leaving the smear plugs intact, can be achieved by theuse of etbylenediamine tetra acetic acid [EDTA) forcavity cleansing.'
Tbe formation of the smear layer is a pbysicalprocess and not a biologic reaction per se. However, itdoes bave clinical implications that must be dealt within a biologic contexL
Tbe smear layer is not a stable structure, and itmust be removed in order to obtain optimal cbemicaland mecbanical bonding between restorative materialsand tooth structures. This demineralization will allowresin to infiltrate the tubules and tbeir branches, aswell as the collagen mesh of tbe intertubuiar matrixand the collagen in tbe walls of the tubules exposed bytbe acid {Fig 8).
Tbe presence of a smear layer can be beneñcial byphysically reducing the flow of fluid through dentinand thus decreasing its permeability. This reducedflow of dentinal fluid may have a protective effect onthe puipai tissue. The smear layer may also impede theentry of bacteria into the cut dentinal tubules."^ Analternative to removal of tbe smear layer for bondingto mineralized dentai tissues is to incorporate it as anintegral part of tbe adhesive system. Materials for suchbonding tecbniques bave been mariieted.
Routine restorative procedures that do not includeacid etching are performed witb the smear layer in
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Fig 1 í,---:-^ -.: =:ectron nucrûgrapn oí fractured coronal humandentm snowing a longitudinal view ol ene dentinai tubuie with dis-tinct peritubular dentin (PT) lining the tubule. Note the openings inthe wail of the tubuie tor numerous branches Irom the odontoblas-tic process. (ID) Intertubular dentin (Original magnifioationX7,400,)
Fig 2 Scanning eiect'O' • i_"_ ;-=!L" I.I -_!;•"•"! reraiizea njmandentin showing remnants ol the odonloblaslic process (OP) in adendnal tubule. The peritubuiar dentin is lost during demineraiiza-tion. (ID) Intertubular dentin. (Original magnifioation x t .250.)
Fig 3 Sr.íii.íiing eiectron micrograph oídenîin subjected to high-speed cutting.Some loose debris is seen, but nodentinai tubules can be discerned.(Originai magnifioation X3.000.)
Fig 4 Scanning eiectron micrograph o!dentin subjeoted io higfn-speed cuttingand subsequentiy washed with a waterspray. No dentinai tubuies can be dis-cerned, but less debfis is present thanin Fig 3. (Original magniücafionx3,000.)
Fig 5 Scanning eiectron micrograph otilie ground dentin surface after it hasbeen treated with 35% phosphoric aoidfor 60 seconds and washed with a waterspray. Note the open dentinai tubules.The perilubuiar dentin has been dem-ineraiized. (Original magniticationx3,000.) (From HOrsted-Bindslev P, fvljöriA (eds). Modern Concepts in OperativeDentistry. Copenhagen: Munksgaard.1988. Reprinted with permission )
FTg 6 (lefl) Scanning electron micro-graph of ground dentin atter the surfacehas been polished with pumice in a rub-ber cup. Smear plugs (SP) are present inthe openings of the tubules. One dentinaitubule (DTI does not have a smear plug:It was probably lost during preparation ofthe specimen. (Original magnification>:3,000.) (From Hörsted-Bindslev P, fwijörIA (eds). Modern Concepts in OperativeDentistry. Copenhagen: fvlunksgaard,1988. Reprinted with permission.)
Fig 7 (right) Scanning electron micro-graph of undemineralized dentin, sub-jected to cavity preparation, showing alongitudinal view of a dentinai tubule (DT)with a smear plug (SP). (Courtesy of DrM. Ferrari: original magnification x7.5iX).)
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Fig 8 Scanning electron micrograph of prepared and acid-elched dentin Note the inlerwoven libers on the cavity (CAl flooiand in the wall of the denlinal tubule (DTl (Courtesy ol Dr MFerran: original magnification x7,5QD.l
place. Because a sterile technique is rtot used, it islikely that bacteria can be found in the smear layer,even if a rubber datn is ernployed. This situation hasraised questions about the need to sterilize cavitypreparations prior to restoration." '- Any antibacterialtreatment applied to the prepared surface may alterthe conditions for adhesion and may lead to pulpalreactions. Many liners, bases, and luting cements haveantibacterial properties; it is also likely that the acid-etching procedure used in conjunction with adhesiverestorative techniques exerts an antibacterial effect.These various agents are likely to be supplemented byantibacterial defense mechanisms in the pulp, becauseit has been claimed that vital dentin resists infection.' "
REACTIONS TOCAVITY AND CROWN PREPARATION
Structurai changes
It has long been established that preparation tech-niques are available that cause nu or few histologiechanges in teeth after evaluation of demineraiized sec-tions stained with hematoxylin and eosin''-'' (Fig 9).Adequate cooling of a bur cutting at high speed isessential to prevent histologie changes in the dentinand injury to the underlying odontoblastie region ofthe pulp- Temperature increases can cause severeinjury to the pulp, and coolants should always be used
during cavity and crown preparation. Care must beexercised to ensure that the water spray effectivelycoois the bur at the cutting surface. "Shadowing"effects by the tooth may prevent the water spray fromreaching the hur (Fig 10). It is difficult to avoid histo-logie ehanges to the underlying pulp if a crown prepa-ration is performed at high speed, even if an adequatewater cooiing system is employed.' Intermittent cuttingusing light handpiece pressure can minimize tempera-ture increases during cavity and crown preparation.
If histologie sections showing so-called harmlesscavity preparations are scrutinized, a separation of thedentin and pulp is often found, localized to the tubulesexposed by the eavity preparation (Figs 11 and 12).This separation is likely to be a histologie artifact, butbecause it is often limited to the tubules exposed bythe cavity preparation, it is probable that some injuri-ous changes that predispose to the separation haveoccurred in the dentin or in the pulp-predentin region.
The injury inflicted on dentin and pulp when coolingof the bur is inadequate during cavity and crown prepa-ration of dentin can lead to displacement of odontoblas-tic nuclei into dentinal tubules {Figs 13 and 14). Similarhistologie changes can occur if the dentin is dried exces-sively after the preparation is eompleted. Marked disor-ganization in the organelles of the odontoblasts and intbe adjaeent cells can also be observed (Fig 15). Theseresponses must be regarded as gross reactions to injury.Overheating or burning of the dentin during erown andcavity preparation are tbe most common reasons fordisplacement of cells into the dentinal tubules and fordisruption of the eontents of the tubules.
Burning of the dentin was a frequent oecurrence inthe early days of restorative dentistry. Ahhough lowspeed was used, considerable pressure on the bur pro-duced frictional heat. Present-day high-speed equip-ment is designed to supply adequate cooling. However,care must be exercised to ensure that the water jetactually reaches the cutting edge of the bur at all timesand that no part of the preparation prevents the waterjet from reaching the cutting part of the bur,' as seenin Fig 10. If dentin is overheated or burned duringcavity or crown preparation, a color change will bevisible in the margin of the preparation, as seen on his-tologie sections when certain stains are used'^ (Fig 16).
Less dramatic changes than displacement of odon-toblastic nuclei can be demonstrated by staining sec-tions of prepared teeth to show the presence of glyco-saminoglycans. These dyes stain componetits that arelocated intratubularly in a specific segment of thedentinal tubule in newly erupted teeth, and they may,therefore, be used as a marker for changes occurringin the content of the tubules (Fig 17). The applicationof these special stains to demineraiized sections ofnewly erupted teeth can reveal distinct histologie
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Rg 9 Photomicro g rap h of a demineralized section ot a newlyempted premolar that was extraoted immediately atter cavity (CA)preparation with a bur supplied with an effective water coolingsystem, (tíotteü line) Extent of the tubules opened by tfie cavitypreparation. Note the intact odontoblastio (O] layer and distinctcell-free zone adjacent to it. (Hernatoxyhn-eosin stain; originalmagnification x220.)
Fig 10 Head ol a contra-angled handpiece with a bur in placeand with the water spray turned on. The water jet does not reachthe working part of the bur because the tooth intervenes. (Cour-tesy of Dr K. Langeland.)
Fig 11 Photomicri,g[apr, or a C5-ity (CA) prepared prior toe>araction oi the tooth, (dotted tine) Extent of the tubules cpehedby the oavity preparation. An intact odontoblaslic (O) layer is[weseht subjacent to the cavity, but the cell-free zone is not dis-tinct. The separation (S] between the predentin and the odonto-blastic layer is probably a histoiogic artifact: however, because itis limited to the tubules opened by the cavity preparation, it doessignify that some changes had resulted from the preparation.(Hematoxylin-eosin stain: original magnification x90.)
Fig 12 Higher magnification of the separation (S) between thepredentin (PD¡ and the cdontoblastic (O] layer as a result ol cavitypreparation similar to Ihat shown in Fig 11. (dotted iine) Extent ofthe dentinal tubules (DT] opened by the cavity preparation.(Hematoxylin-eosin stain; demineralized section, original magnifi-cation x350.]
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Fig 13 Pholomicrograph of a üemineral-ized ^ec'.ion Slowing odontobiastic nuclei{ON] displaced into the dentin as a result ofcavity préparation with ¡nadequafe coolingof the bur. The odontoblasfic layer is dis-rupted and vacuoiized. (Hematoxylin-eosinstain; original magnification x60.]
Fig 14 Pfiotomicrograpfi of a demineral-ized section showing odontoblastic nuciei(ON) dispiaced into tubuies of the pre-dentin (PD) subjacent to a cavity prepara-tion (Hematoxylin-eosin stain: originalmagnification x90D.)
Fig 15 Electron micrograph of jncJaiized dentin siiowing damaged and disoi.ganized cytoplasmic components fromodonloblasfs displaced into the prederlin(PD) foliowing (he grinding of a tat molar.(ívl¡ mitochondrium; (REB) rough endoplas-matio reticulum. (Courtesy of Dr O.B.Sueen; original magnification x15,000.)
cbanges in dentin in which cavities bave been pre-pared with a high-speed dental engine supplied withabundant water spray. This altered staining of thedentin occurs without displacement of odontoblasticnuclei into the tubules.
The teeth frequently used in such investigations areintact premoiars that are to be extracted from cbildrenaged 10 to 14 years for orthodontic reasons. The spe-cific change noted in these sections is based on tbepresence of intratubular reactive components tbathave shifted position following preparation of thedentin. The position of these reactive componentsafter preparation indicates outward movement or dis-placement of the tubular contents (Figs 17 to 19), Suchmovements occur even if a so-called nontraumaticpreparation technique is used.
The outward movement of the contents of thetubules is probably a result of tbe exposure of thedentin for the first time in an otherwise unaffectedtootb. Tbe preparation opens up into a high-pressurearea because tbe normal interstitial tissue fluid pres-sure of tbe pulp is in tbe range of 5 to 20 mm Hg.' ' *"It appears that this displacement of the contents of thetubules cannot be prevented wben preparations aremade in newly erupted, intact teeth,^' The movementof the contents of the tubules is dependent on tbetubules' being open. No studies similar to thosereferred to on newly erupted teeth have been per-
formed on intact teeth of adults or older individuals,in whom the tubules may be partly or completeiyobturated by grow'th of tbe peritubular dentin.
Cavity and crown preparations reaching dentin innewly erupted, intact teeth will expose tubules that arenormally open. Protrusion of the contents beyond thetubules is sometimes observed in histoiogic sections(Figs 18 and 19). The reason that protrusion is notobserved from all or most of the tubules may be thatthe reactive part of the contents has been completelyextruded through tbe dentin. Furtbermore, a smeailayer may obturate the opening of tbe tubules ancblock or reduce extrusion of the contents (see Figs 2and 4). The fact that the contents occasionally protrude beyond tbe cut tubules indicates tbat an activiforce has been applied, because capillary forces alomwould fill, but not overfill, the tubules. Tbese change;cannot be prevented, and they occur even where nidisplacement of odontoblastic nuclei takes place (Fig:20 and 21). Exudation of fluid from dentinal tubuleafter cavity preparation has also been shown througlreplica (impression) techniqttes.^^
The clinical signiflcance of outward movement cthe tubular contents bas not been established. It habeen suggested that the localized, reactive, and stainable contents of the tubules a short distance into thdentin from tbe dentin-predentin border are associatewitb secondary formation of peritubular dentin i
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Fig 16 (left) Demjneralized section show-ing altered staining (red) ol ttie cavity (CA)margin ttiat resulted wtien inadequate oool-ing ot ttie bur led to burning ol the dentin.(Original magnitication x25.)
Fig 17 (right) Photomicrograph ot a de-mineralized section ol an occlusal cavity(CA) preparation on a newly erupted, intactpremolar that was extracted immediatelyafter preparation. The normal staining pat-tern ot unattected dentin is shown on thelett. Note \Ue altered staining ot the dentinsubjacent to the oavity. (Z) Zone wiitiintratubular reaorive components in unaf-fected dentin. (Méthylène blue stain, origi-nal magnification x35.)
Fig 18 (left) Photomicrograph of a de-mineralized section showing tubular con-tení protruding into rhe cavity (CA] prepa-ration. The tooth was extracted immediatelyatter the cavity preparation. (Alcian blueand periodic acid-Schitt stain; originalmagnification x900.]
Fig 19 (right) Dentin at the cauity (CA)margin sfiowing dentinal tubules (DT) withintratubular staining, which is normally¡ound only in more puipally located dentin(corresponding to Z in Figs 17 and 21 iNote the protrusion ot tubular contents intcthe cavity preparation. (Original magnitica-tion x900.)
newly erupted teeth." Preparation and restoration ofteeth may, tberefore, have an effect on tbe secondarydevelopment and growth of peritubular dentin.
Tbe reactive components within tbe dentinaltubules include cytoplasmic constituents, Tbese con-stituents and any components present within tbe peri-odontoblastic space, including tissue fluid, apparentlybecome displaced. The disturbance and redistributionof tbese cellular constituents (Fig 15) will result indegeneration of the odontobiastic processes'"-* (Figs
22 and 23). If tbe preparation is restored, no reestab-lisbment of normal staining reactions is observed-' (Fig24). However, reestablishment of the normal stainingpattern has been shown to occur if coronal dentin isexposed to the orai environment in shallow, self-cleansing facets for at least 7 days ' {Fig 25).
The ciinical signiflcance of any change following thedisplacement of odontobiastic nuclei and/or the con-tents of the tubules has not been fully estabiisbed, butit is likely to have an effect on tbe physiology of the
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Fig 20 (lell) Photomicrograph ot a de-mineraiized section ot a cavity (CA) prepa-ration, prepared with an abundant waterccDiing of the bur The odontobiastic (0)iayer subjacent to tne cavity remainedintact and no dispiacement of odontobiastnuclei inio the dentinai tubules hasoccurred (dotted iine) Extent of the cavifypreparation (Hemato>7lin-eosin stain: origi-nai magnification x220.)
Fig 21 (right) Photomicrograph ct (heseclion adjacent to the one shown m Fig20, Note the difference in staining ot thedenlin subjacent to the cavity (CA) prepa-ration and that ol unaffected dentin to theleft of Ihe dotted line, which delimits (tieextent of the cavity preparation. (Z) Zonewith intratubular, stainabie components aspart of the normal staining pattern ofdentin. (Toiuidine blue stain; originai magni-lication X220.)
Fig 22 Electron micrograph ot an undemineraiized sectionshewing a dentinai tubuie (DT) with the remains ot a disintegratedodontobiastic process. (Ve) Vesicles coated with dark granuies.(Courtesy of Dr O.B. Sveen: originai magnification x 13,700.)
Fig 23 Eiectron micrograph ot an undemineralized sectionshowing a dentinai tubule (DT) with ceiluiar fragments undergoingnecrosis. The iarge, light, circular areas within the tubuies may beiipid dropiets. (Courtesy of Dr O.B. Sveen: originai magnificationX11,200.)
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Fig 24 P^otomlc^og^apn of a demineraiized section showing^tered staining of dentm subiaoent to oavities ttial nad beenrestored *ith calcium hydroxide and amalgam ¡CA-I] and zincoxide-eugenol oement (CA-2) 4 days prior to extraction of metooth. The zone (Z] with stainable inlratubular components ismissing sub|acent to the oavities. (dotted iines) Extent of thetubules opened by the cauity preparation. (Alcian blue stain: angi-ng magnification xd5 ¡
Fig 25 -'hoiümicrograph of a demineraiized section. The coronaldeniin had been exposed to the orai environment loilowing thegrinding ol a facet (FA) on the newly erupted premolar 35 daysbefore the tooth was extracted. The normai staining ot denlin inthe zone (Z] with siainable intratubular components is found bothsubjacent to the facet and in the unaffected dentin (Toluidine blueslain; originai magnification x35.)
affected dentin. The intratubular changes may be thestart of a "dead tract" reaction.-* The formation of adead tract may be dependent on the disturbance of thecontents of the tubules and subsequent formation oftertiary dentin subjacent to the affected dentinaltubules. However, a certain degree of trauma to tbeodontoblasts and their processes must take placebefore a dead tract will develop, including the forma-tion of an atubular "hyaline zone" between the physio-logic secondary dentin and the tertiary dentin. Thishyaline zone corresponds to the interfacial dentin.^'and the tertiary dentin compares to reparative dentin. *
Physioiogic changes
Immediate vascular responses have been demon-strated to result from the grinding of dentin. Olgart-^^"summarized the findings from a series of experimentson cats over a 25-year period that included commonclinical procedures using neurophysiologic and hemo-dynamic techniques. These procedures comprisedgrinding of dentin and percussion of the teeth. Briefgrinding (1 second, 3 times) of feline canines with adiamond bur flushed with saline at 6,000 rpm caused
an instantaneous increase in biood flow (Fig 26). Theblood flow in denervated contralateral teeth exhibitedsignificantly smaller and much delayed responses.Grinding halfway into dentin caused a 530'0 increasein blood flow lasting for about 10 minutes. Furthergrinding into deeper layers of dentin caused onlyminor differences in the magnitude of the response.The effect of anesthesia on the blood flow was illus-trated in a series of experiments using ultrasonic stitit-ulation-^ (Fig 21). The findings from these experimentswere interpreted to support the concept that extensivebranching of pulpal nerves is associated with func-tional connections to periodontal tissues.
In another experimental series, using teeth withintact enamel, a setup imitating the percussion of teethinstantly produced a 30% increase in blood flow last-ing about 2 minutes. The increase in blood flow fol-lowing percussion was smaller tban tbat after tbeexposure of dentin by grinding, perhaps because of thelack of exudation of dentinal fluid from the intactteetb. Tbese investigations were not supplemented bybistologic studies, which would have allowed correla-tion between the blood flow recordings and the degreeof movement of the contents of the tubules, including
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Fig 26 Pulpal blood How, expressed as a percentage of laserDoppler flowmetry (LDF) following grinding (3 times, 1 seccndeach] with a round diamond bur at 6,000 rpm. Grinding in theInner hall of the dentin (btack diamond) causes a rapid increasein pulpal blcod flew in a normal, innervated feline canine (uppercurve) In the denervated contralateral canine, the correspondingresponse is delayed and smaller (lower curve). (From Olgart^^ andthe Finnish Dental Society Appolonia. Reprinted with permission.)
SO
•3 60
œ
£ 40
3 20
T Ultrasonic stimulaer:) Before local ar
1^ ^ Atter local ane
8 •
1
6 1Low High
onestheticsthetic
1
11
High
Fig 27 The effect ot injection of local anesthetic solution on theblood flow following ultrasonic stimulation of a feline canine.Periapical injection with a local anesthetic solution, leaving thevasodilator response to electrical tooth stimulation intact (A), abcl-ished the vasodilator response to low-amplitude ultrasonic stimu-lation but not that induced by high-arnplitude stimulation.Additional subperiosteal apical injections blocked the intrapulpalnerves (B), and this procédure also reduced the response to high-amplitude ultrasonic stimulation. Numbers in bars indicate tiienumber of experiments performed. (LDF) Laser Doppler llow-metry. • = P < .0005. (From Olgart^ä a^d the Finnish DentalSociety Appolonia. Repnnted with permission.)
displaccmcnf of odonfoblasfic nuclei info the tubules.Such displaccmcnf is more likely to occur followingexposure of dentin than in teeth with intact enameLbecause fluid flows through enamel much less thanthrough exposed dentin.''"
Crown preparations made with a high-speed burwithout water spray have been sbown to decreaseblood flow in the pulp of dog canines.^' The magnitudeof the decrease in blood flow was dependent on theremaining dentin thickness. If the preparation reachedthe inner third of dentin, whicb was estimated to ieaveabout 1 mm of remaining dentin, the blood flow wasreduced by 90 /0 añer 1 hour. Preparation to tbe samedepth bad negligible effect on the pulpal blood flowwben abundant water spray was used to cool the bur.Dry preparation balfway into dentin resulted in a sig-niflcant increase in blood flow through shunt vessels,especially those in the apical part of the teeth.
Impressions of the prepared surfaces of rat teeth,which involved tbe use of a copper band witb warmwax, caused severe fluctuations in blood flow; minimalchanges were noticed if rubber-based impressionmaterials were used. A further reduction in blood flowresulted from epinephrine in anesthetic solution. Thus,both the combination of dry preparation with highspeed and the use of anesthetics with a vasoconstric-tor are considered to be particularly hartnful to the
pulp. These investigations were not supplemented byhistoiogic studies. Thus, no correlation with struijturalchanges can be made. However, bistologic cbanges intbe pulp and dentin following complete crown prepa-ration have been sbown to occur, and they are consid-ered difficult to avoid.*
The vasoconstriction noted after cavities are pre-pared deep in dentin without a coolant may be due toinhibition of sytnpatbetic nerve stimulation. ''' '"' It hasalso been sbown tbat stimulation of cervical sympa-tbetic nerves results in a significant decrease in fluidflow through dentin. Such changes in fluid flow mayincrease the rate of diffusion of agents from the denti-nal surface to the pulp. - Conversely, an increasedflow from the tubules, eg, because of inflammation inthe pulp, may prevent or reduce the diffusion of bacte-ria and toxic agents into the pulp.'^
It is evident that the dentinal fluid plays a centralrole in dentin physiology, ineluding those changes thatresult frotn restorative procedures. The peripheral flowof this fluid following cavity preparation allowsplasma proteins to enter the tubules. Clotting of theseproteins will reduce the functional diameter of thetubuies and reduce tbe permeability of the dentin."'*It was believed tbat fibrinogen caused the obstructionof the tubules, but the presence of fibrinogen has beendifficuh to demonstrate.
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Tbe presence of serum albumin in the tubules hasbeen established. -"* Cavity preparation into the innerthird of the dentin in intact premolars from childrencauses an albumin flux. This exudation of albumin wasmarkedly reduced after 2 days of exposure to the oralenvironment and after another 12 days following theplacement of a zinc oxide-eugenol cement to seal thecavity.-" The exposed dentin was covered by a tbin coatof Teflon to prevent the cement from blocking thetubules in these experiments. Histologie studies of thepulp showed good preservation of the pulpal tissuewithout cellular infiltration. Following bactériologiechallenge, marked cellular infiltration was found subja-cent to the tubules exposed by the ca\'ity preparation,including positive immunochemical reactivity formacrophages. Although the experimental proceduresdid not render the dentin impermeable, the reduction inperméabilité' may be clinically important in preventingbacteria and toxic agents from diffusing into the pulp.
It is apparent that preparation, impression, and per-cussion of teeth may result in significant vascularchanges in the dental pulp. These changes are tran-sient and usually resolve without clinical complica-tions. Whenever the blood flow is impeded in thepulp, locally or systemically, the reaction may be thestart of an adverse process. If tbe vascularity increases,tbe reaction may be looked on as a defense mecha-nism to initiate preparedness for subsequent insults.
The odontoblastic layer comprises closely packedcells that often appear pseudostratified in coronaldentin. A number of junctional complexes link theodontoblasts together. This layer of cells exhibits sev-eral characteristics similar to those of an epithelium. ^ït may act as a barrier and provide a protective efl'ectby preventing macromolecules from passing from thepulp into the dentin. Tbis barrier is often disruptedduring crown and cavity preparation and tbe physio-logic reactivity in the region will change. It is likelythat the junctional complexes become reestablished,but the nature and significance of this repair processare unclear.
Mechanisms governing displacementof odontoblasts and tubular contents
Displacement of odontoblastic nuclei into the dentinaitubules is a phenomenon that has long been recog-nized. It occurs regularly subjacent to marks on rootsfrom the beaks of extraction forceps-" but may alsooccur for a variety of other reasons, including cavityand crown preparation. Considerable attention waspaid to tbis pbenomenon when high-speed dentalengines were introduced in the late 1950s. The phe-nomenon has also been described as "aspiration" ofodontoblastic nuclei into dentinal tubules,"'^' but tbe
term displacement of odontoblasts'^ has come intocommon use, because it is not suggestive of any spe-cific mechanism for its occurrence. Other celis presentsubjacent to tbe tubules exposed by cavity preparationcan also be displaeed into tbe tubuies under extremeconditions.'^
Provided that an adequate water spray is used dur-ing tbe cavity preparation, displacement of nuclei wiflnot occur {see Figs 9 and 20). Intermittent, low-speedcutting of dentin witb light pressure and without waterspray is routinely performed, especially duringremoval of the final carious dentin in deep cavities. Ifperformed with care, this "excavation" with a largeround bur used witbout water spray is consideredacceptable treatment, but cavity depth is an importantmodifying factor.-*"-** Reduction in remaining dentinthickness makes the pulp more vulnerable to injuryfrom cavity and crown preparation trauma.
Electron microscopic studies of the pulp-predentininterface after cutting of dentin^^ bave revealeddisplaced cellular contents and odontoblastic nucleiin some of tbe tubules. A number cf morpbologicchanges bave also been noted, including intraceliulardisorganization and rupture of tbe nuclear membrane(see Figs 15, 22, and 23). Tbis process takes placequickly and causes disruption of the odontoblasticlayer. Lysis of the cellular elements takes place overtime, and an inflammatory reaction occurs in the adja-cent pulpal tissue. After about 20 days in human teeth,the displaced cells will have disintegrated, and nucleicannot be discerned in the dentin.'=
Tbe mechanisms involved in displacement of odon-toblastic cell bodies into tbe tubules and in displace-ment of the contents of the tubules are not fully under-stood. The displacement of nuclei is considered to be amore exireme reaction than that limited to movementof tubular contents. Many theories have been put for-ward to explain the displacement of nuclei into thetubules and the disorganization of the contents withinthe dentinal tubules. Because displacement of odonto-blastic nuclei is regularly found corresponding to theforceps marks on the root following tooth extraction,mechanical distoriion of tbe dentin is a likely explana-tion for the pbenomenon in that context.
Tbe distortion of teeth during extraction may betransmitted through the body of the pulp. If extractionforces are transmitted via the pulp, fluid flow is morelikely through exposed tubules than through tubulescovered by enamel and cementum. No such distortionoccurs when animals are killed by an overdose ofanesthesia and tissue blocks witb the teeth in place aredissected after histoiogic fixation, but displacement ofodontoblasts can sfiU occur. -' Furthermore, a compa-rable distortion is unlikely to occur during the prepa-ration of teeth and foliowing excessive drying of the
Quintessence International547
prepared dentin. Thus, tnechanical distortion of pulpaltissue alone cannot explain the phenomenon, but itmay be a contributing factor.
Other theories for displacement of tubular cotiientsinclude evaporation of fluid from the prepared surface,especially from heat generated during the preparation,marked differences in osmotic gradients,''^'*' andchemotaxis from toxic agents on the dentinal surface.Evaporation of the contents of tubules occurs follow-ing preparation without adequate cooling of the bur oras a result of excessive drying of the prepared surfaces.Capillary forces will then replace lost dentinai fluidwith the interstitial fluid in the pulp. A buildup ofintrapulpal pressure due to inflammation has also beensuggested,-" but bistopatbologic evidence of inflamma-tion is not an immediate response to crown or cavitypreparation. On the other hand, the increase in bloodflow as an immediate reaction to the grinding ofdentin-' is likely to increase the tissue fluid pressurelocally.
More than one mechanism may be in operation toexplain displacement of tbe contents of dentinaitubules, depending on the clinical situation. It is likelythat the normal high interstitial fluid pressure in tbepuip plays a role in displacing the tubular contents, atleast in teetb wbere tbe dentin is exposed and thetubules are open and not obturated by mineralizeddeposits. Because extraction forceps denude the rootdentin, the pressure gradient may also be a factor inthis connection.
Irrespective of the mechanism invoived, it is diffi-cult to explain part of the movement of tubuiar con-tents induced by cavity preparation in vitro, both priorto and after fixation of the teeth for histologie prepara-tion.""^'^^ These findings suggest that physical andpossibly chemical forces play a major role in tbe dis-placement of tbe contents of the dentinal tubules.
The outcome of displacement of cytoplasm, nuclei,and otber cellular components is a disintegration anddegeneration oí the contents of the tubules "* (see Figs22 and 23). Waste products will cause some degree ofinflammatory reaction in tbe pulp; based on bistologicand clinicai experience, the inflammatory response willusually be followed by heaiing. However, this type ofadditional trauma is unnecessary and should be pre-vented. In some teeth, cavity or crovm preparation maybe tbe additional trauma tbat results in pulpal compli-cations in an already compromised pulp or hypersensi-tivity and discomfort for the patient after treatment.
A number of factors affect the pulp-dentin repairprocesses following restorative procedures, includingremaining dentinal thickness, age of the patient, factorsrelated to cavity dimensions, and possihly tbe release ofgrowtb factors.''" Tbese issues will be discussed in detaiiin a later article on reactions to restorative procedures.
ALTERNATIVE PREPARATION METHODS
Rotary instruments witb stainless steel, tungsten car-bide, and diamond burs of different shapes and sizesare routinely employed to prepare cavities andcrowns. Alternative cutting methods include air abra-sion and lasers.
Air abrasion equipment tbat uses abrasive dust hasbeen developed for cutting tooth structure, but for sev-eral reasons tbe clinical application of tbis techniquenever became popular. It did not allow tactile senseduring tbe cutting procedure, and the abrasive dustobscured the field of operation. The dust may also beinhaled and therefore represents a potential healthproblem for tbe patient and tbe operator.
During tbe last few years, air abrasion equipmenthas been promoted for cleaning pits and fissures priorto tbe application of sealants. The need for such clean-ing has not been demonstrated. On the other hand,cleaning of pits and fissures can aid in the diagnosisof occtusal caries. However, the inherent problem withdust management remains; because tbe cleaning proce-dure only calls for removai of debris, the equipmentshould be used intermittenfly and for short periods. Theiacii of tactile sense, therefore, is of minor importance.
Laser equipment is based on tbe use of beams ofhigh light intensity, laser being an acrotiym for lightamplification by stimulated emission of radiation.Ligbt photons of characteristic wavelengths are pro-duced, amplifled, and flltered to make tbe laser beam.Carbon dioxide and neodymium:ytterium-aluminium-garnet lasers are most commonly used.
The main problem witb laser cutting of hard dentaltissues is tbe generation of beat. Increases in pulpaltemperature of more tban about 5°C may lead to dam-age. However, laser equipment is not used for cavilypreparation but has a number of potential applicationsin dental practice, including coalescence of pits andfissures to eliminate retention sites for bacteria, desen-sitization of exposed root surfaces, rougbening of bardtissue suriaces to promote bonding as an alternative orsupplement to acid etching, vaporization of carioustissue, and endodontically for vaporization of organictissue in the root canal." Limited research on pulpalreactions to laser cutting of dentin calls for caution inthe use of this technology in restorative dentistry. Tbeability of such tecbniques to coalesce deep, narrow fis-sures is also questionable.^*
FORMATION OFTHE HYBRID LAYER
Acid etcbing of enamel and dentin exposed by prep-aration, referred to as "tbe total-etch technique,"^^ hggbecome routine treatment in conjunction with adhesive
548 Volume32, Number 7. 2001
Mjöf
techniques. Acid etching deminerahzes hard tissuesand exposes the organic matrix. The scanty organicmatrix of enameJ is lost during the demineraJizationand subsequent washing. The components of dentin areselectively demineralized. The most significant expo-sure of collagen occurs after acid treatment of the inter-tuhular matrix of the dentin (see Fig 8).
The highly mineralized perituhular dentin deminer-ahzes quicker than does the intertubnlar matrix. Thisdemineralization widens the tubules, making themfunnel shaped toward the surface. It exposes collagenon the wall of the tubules and also uncovers tbe open-ings of a large number of lateral brancbes tbat may beimportant for penetration of resin to acbieve optimalbonding to dentin. *
The quality of the demineralized dentin is impor-tant for adhesion of resin, and the demineralizationshould not denature the collagen.=' Phosphoric acidand citric acid are the most commonly used acidetchants. The addition of 3% ferric chloride to 10%citric acid markedly enhances the adhesion to dentinby preventing the denaturation of collagen. The colla-gen exposed by acid etching forms an interwovenmesh of fibers that the resin will infiltrate {see Fig 8).This collagen mesh infiltrated by resin is referred to asthe hybrid layer.^*^ It is about 5 to 10 ym thick. Afterpolymerization, the resin-impregnated collagen,together with the resin in the dentinai tubules andtheir branches, constitutes tbe adhesion between tbedentin and the resin.
The formation of the hybrid layer is basically achemical process involving dissolution of primarilymineral salts and noncollagenous matrix componentsfollowed by diffusion of resin into tbe remaining colla-gen matrix. Because it bas clinical implications, it isimportant that such treatment be considered within abiologic context. The chemical treatment of dentinmay also release growth factors that may be importantfor subsequent reparative processes.'^
Mucb attention has been focused on the degree ofwetness of the hybrid layer at the time of applicationof the resin.*''-*^ ¡f ,he hybrid layer becomes too dry,the collagen mesb will collapse and penetration ofresin will be impaired. To obtain optimal bondingbetween the resin and the hybrid layer, the surfacemust have an adequate moisture content to preventcollapse of the collagen mesh. The ideal degree of wet-ness may vary from one resin-based product toanother. The wetness will certainly difter on the vari-ous parts of the prepared surface because of the differ-ent densities and the structure of dentinai tubules indifferent locations on this surface." Thus, the instruc-tions for use of resin-based materials must take theseissues into consideration, and they must be followedclosely to obtain the best possible clinical result.
Furtber details related to tbe bybrid layer will be out-hned in a discussion of adbesive restorative techniqueslater in this series of articles.
CONCLUDING REMARKS
Structural and pbysiologic cbanges resulting fromcrown and cavity preparation in vivo bave been out-lined in many experimental studies over the last 50years. A number of biologic reactions bave beenshown to occur. Some reactions are of a physical orcbemical nature, but tbey also bave biologic and clini-cal implications. Altbougb some of tbe reactions areclearly understood, for others the clinical implicationsare largely unknown, eg, the displacement of cellnuclei into dentinai tubules and tbe disruption of tubu-lar contents following crown and cavity preparation.Restorative dentistry is possible even if these reactionsare disregarded; however, if restorative dentistry is toevolve as a biologic science, tbey must receive atten-tion, clinically as well as in continued research efforts.Pulpal complications involving inflammation, degrada-tion, and necrosis are the result of a series of traumaticinjuries. It is, therefore, the responsibility of therestorative dentist to minimize the trauma to dentinand pulp inflicted during all clinical procedures,including that occurring during the preparation phase.
ACKNOWLEDGMENTS
A Guest Research Fellowship from the Research Council of Nor^'ay.parlly in support of the author's Facuky Developmental Leave atNIOM. Scandinavian Institute of Denial Materials, is gratefullyacknowledged.
The author would also like to thank Dr A. I. Smith, Professor andChainnan, and Dr P. E. Murray, Unit of Oral Biology. School ofDentistry, University of Birmingham. Birmingham, England, forreviewing Ihe manuscript.
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Quintessence International
New Frontiers inAdhesive Dentistry
HÏBRIDtZATION OFDENTAL HARD TISSUES
HYBRIDIZATION OF DENTAL HARD TISSUESNobuo Nakahayashi and David H. Pashley
The hybridization of dentin—a process that cre-ates a molectilar-level mixture of adhesive poly-tners and dental hard tissues—gives clinicians aversattle new materiai, useful in a wide array ofadvanced dental treatments. As the first in-depthexploration of the sub]ect, this book covers thedevelopment, present understanding, and futureresearch areas of thts multifunctional dental mate-rial. A thorough review of the current literaturerounds otjt the text.
Valuable for students, researchers, atid clini-cians seeking a greater understanding of resinhybridization of tooth struaure.
CONTENTS
! Evolution of Dentin-Resin Bonding2 Properries of Derttin3 Acid Conditioning and Hybridization of
Substrates4 Characterization of rhe Hybrid Layer5 The Quality of the Hybridized Dentin6 Ciinical Applications of Hybrid Layer
