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Cavity configurations for in direct partial coverage adhesive-cemented restorations ARTICLE in QUINTESSENCE OF DENTAL TECHNOLOGY · JUNE 2006

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Cavity configurations for in direct partial coverage adhesive-cemented restorations ARTICLE in QUINTESSENCE OF DENTAL TECHNOLOGY · JUNE 2006
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  See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/256589637 Cavity configurations for in direct partialcoverage adhesive-cemented restorations  ARTICLE   in  QUINTESSENCE OF DENTAL TECHNOLOGY · JUNE 2006 READS 174 3 AUTHORS , INCLUDING:Dino ReUniversity of Milan 84   PUBLICATIONS   134   CITATIONS   SEE PROFILE Available from: Dino ReRetrieved on: 10 November 2015  Cavity Configurations for Indirect Partial-CoverageAdhesive-Cemented Restorations Guido Fichera, DDS 1 Walter Devoto, DDS 2 Dino Re, MD, DDS 3 QDT 2006  55 ndirect esthetic adhesive restorations incomposite resin and ceramics belong tothe wider category of partial-coveragecrown restorations. As such, they require prepara-tion designs that leave a certain amount of intactclinical crown and generally have supragingivalmargins. 1 To guarantee the fracture resistance of apartial-coverage crown restoration over time, it isnecessary to determine which part of the clinicalcrown that has, by itself or in combination with thebuildup, appropriate structural characteristics andsufficient biomechanical strength. This decision in-fluences the clinical outcome of the restoration.Respecting biologic principles and using a conser-vative approach are the building blocks for a suc-cessful outcome.For indirect tooth-colored adhesive-cementedrestorations, the highest incidence of failure isfracture of the restoration material and intacttooth, together with secondary caries. 2 To avoidfracture, cavity preparation should be consideredas both a diagnostic and operative phase. 3 FACTORS AFFECTING STRUCTURALSTRENGTH Numerous studies on the biomechanical andstructural analysis of a tooth’s intact healthy struc-tures are available in the literature, especially fromthe era prior to the use of adhesive resins. Fromthese reports, it appears that the presence of themarginal ridge is fundamental 4 ; if it is lacking, theocclusocervical and mesiodistal depth, and thesize of the proximal boxes must be taken into con-sideration, 5–8 as well as the intercuspal width (andthus, proximity to the tips of the cusps) and thedepth of the occlusal isthmus, 5–9 the thickness of the enamel-dentin layer at the base of each cusp,the depth of the base of the intact cusp, 6 the ab-sence or presence of the pulp chamber roof  4 (ie,vital or endodontically treated tooth), and thethickness and depth of interaxial dentin. 7–10 Restoration is further complicated as these factorsmust be related to the functional role of the toothin question (eg, position in the arch, biotype, oc-clusal trauma, parafunctional habits, static and dy-namic occlusion, condition of antagonistic teeth).Clinical research shows that the adhesive bondbetween dentin and resin composite will, over time, decrease in strength, 11,12 and that the extentof this decrease is in proportion to the mechani-cal, thermal, hydrolytic, and enzymolytic stressesto which the bond is subjected. It is also highly 1 Private practice, Monza, Italy. 2 Private practice, Sestri Levante (GE), Italy. 3 Researcher, Department of Prosthodontics, University of Milan, Italy. Correspondence to:  Dr Walter Devoto, Via E. Fico 106/8,16039 Sestri Levante (GE), Italy. E-mail: dewal@tele2.it I   FICHERA ET AL QDT 2006 56 probable that weakening of the adhesive bondover time is responsible for the mechanical failureof direct and indirect tooth-colored adhesive-cemented restorations for which the restoration-cavity interface is situated close to the tip of thecusp, or the intact tooth wall is insufficiently thick.These failures cannot easily be explained exceptby hypothesizing a failure over time of the adhe-sive bond at the interface, where it is subjected tohigh physical and chemical stress.It is the authors’ opinion that for posterior teethit is important to place the margins of indirecttooth-colored adhesive-cemented restorations inocclusal-axial areas subjected to lower mechanicalstress. An analysis of the literature reveals four structures that determine the strength of intacttooth: the interaxial dentin, the pulp chamber roof,the marginal ridge, and the intact cusp. For their diagnostic and operative implications, these struc-tures may be classified topographically as centralor peripheral. The spatial relationship between thevarious structures is better clarified through a struc-tural model of the tooth (Figs 1 and 2). Central structures The central structures consist of the interaxialdentin and the pulp chamber roof. The interaxialdentin is the central core of the tooth. It may beseen as the occlusocervical continuation of thepulp chamber roof and, thus, occupies the areacorresponding to the projection of the pulp cham-ber to the occlusal surface. 13 The interaxial dentinconnects the axial walls, in particular the buccaland lingual walls, and is the most important struc-ture. When it is intact, the presence of other com-promised structures will not significantly under-mine the overall fracture resistance of the intacttooth. 14–17 Studies by Mondelli et al 14 and Larson et al 15 have shown that the loss of the marginal ridge(peripheral structure) does not produce signifi-cant structural weakening when the occlusal isth-mus (ie, interaxial dentin) remains intact. On thecontrary, exclusive preparation of the interaxialdentin is associated with significant structuralweakening. The structural significance of the Fig 1  Occlusal view of structural model of thetooth. id = interaxial dentin, ic = intact cusp,mr = marginal ridge. Fig 2  Buccolingual view of the structural modelof the tooth. id = interaxial dentin, ic = intactcusp, mr = marginal ridge, pcr = pulp chamber roof.B icicic icidicidicmr mr pcr mr mr  LMD MD  Cavity Configurations for Indirect Partial-Coverage Restorations 57 QDT 2006 marginal ridge is affected by any compromise of the interaxial dentin. 16,17 The pulp chamber roof, contrary to commonbelief, is less important than the marginal ridge.Reeh et al 4 demonstrated that loss of the pulpchamber roof when both marginal ridges arekept intact (ie, endodontic treatment requiringremoval of some of the interaxial dentin and thepulp chamber roof) produces a less significantstructural weakening than maintenance of thepulp chamber roof when one or two marginalridges are missing (ie, vital tooth with Class 2cavity, occlusomesial, occlusodistal, and mesial-occlusodistal).These considerations are important in makingclinical choices based on the evaluation of healthyintact tooth structures and on scientific evidence,and not simply on empirical or preconceived no-tions. The hierarchy of tooth structures is thus:  (1)  the interaxial dentin,  (2)   the marginal ridge,  (3)   theroof of pulp chamber, and  (4)   the enamel-dentincomplex of the intact cusp. Peripheral structures The peripheral structures are the marginal ridgeand the enamel-dentin complex of each intactcusp. The marginal ridge is the peripheral struc-ture of the proximal wall, whereas the enamel-dentin complex of the cusp is the peripheral struc-ture of the axial-buccal or palatolingual wall. Themarginal ridge, its underlying enamel-dentin com-plex, and the interaxial dentin meet at the junc-tion of the buccal wall with the palatolingual wall.The thickness of enamel-dentin complex at thebase of each cusp does not participate in thisstructural junction but is rather the last support of the cusp itself.Correct evaluation of the marginal ridge mustfollow certain criteria. The loss of a marginalridge signifies the presence of a proximal box. If the interaxial dentin has been compromised andthe presence of an occlusal isthmus is antici-pated, the proximal box must be evaluated interms of the presence or absence of the pulpchamber roof and the thickness of the enamel-dentin complex at the adjacent intact cusp (itmust be greater than1.5 to 2 mm in a vitaltooth 18 and 2.5 to 3 mm in an endodonticallytreated tooth 13 ) and depth at the base. 19 The na-ture of the structural interdependence deter-mines whether or not it is necessary to cover theadjacent cusps. Articles by Linn et al 20 and Pan-itvisai et al 21 on the relationship between themarginal ridge and intact cusps in an endodonti-cally treated tooth demonstrate a structural andfunctional dependence of the intact cusp on theadjacent marginal ridge. Likewise, the articlesconfirm the independent biomechanical be-haviour among cusps as suggested by Sakaguchiet al, 22 and which has been clinically confirmedby numerous studies. 5–12 The loss of one marginalridge in an endodontically treated molar, wherethe other marginal ridge is intact and adjacentcusps are well-supported, should be planned asa restoration with partial cusp coverage; thecusps adjacent to the lost marginal ridge arecovered, while the cusps adjacent to the intactmarginal ridge are maintained.The enamel-dentin complex of the intact cusprepresents the most significant clinical factor indeciding whether to maintain or cover the cusp.Hood 6 demonstrated that from the mechanicalstandpoint the enamel-dentin complex of the in-tact cusp adjacent to a proximal box behaveslike a cantilever; the thickness and depth at thebase of the intact cusp are the most importantparameters since they vary with the cube of thedeformation and, in the final analysis, are re-sponsible for the strength of the cusp. This iswhy, with equal thickness and in the absence of amarginal ridge, intact cusps of endodonticallytreated teeth flex more than those of vital teeth.Keeping an intact cusp in a vital tooth is deter-mined by an enamel-dentin thickness greater than 1.5 to 2 mm, 20 whereas in an endodonticallytreated tooth the thickness must exceed 2.5 to 3mm. 13  QDT 2006 58 FICHERA ET AL TRANSITION AREAS AND POSSIBLECAVITY CONFIGURATIONS After analyzing the peripheral and central struc-tures (ie, the marginal ridge, the intact cusp, andthe interaxial dentin), it is possible, topographi-cally, to outline the separation areas betweeneach tooth structure. These areas are valuable indiagnosing cavity configurations, since they repre-sent a line of transition between the restorationand intracoronal and extracoronal cavity to beoutlined in partial-coverage crown preparation.They also act as spatial references and help ac-complish a rapid buildup that is stereoscopicallycorrect.Taking, for example, a maxillary first molar,three areas of transition can be outlined betweenthe marginal ridge and cusp, between cusp andcusp, and between the interaxial dentin and theperipheral marginal ridge–cusp unit (Fig 3). Giventhe anatomy of posterior teeth, two areas of tran-sition are associated with each marginal ridge: abuccal transition, at the beginning of the adjacentbuccal cusp; and a lingual transition, at the begin-ning of the adjacent lingual cusp.A number of cavity designs are derived from thepossible combinations based on the absence or presence of the marginal ridge and on the mainte-nance or restoration of the two adjacent cusps(buccal and lingual) (Fig 4). Configurations 1 to 4(see Figs 4a to 4d) are characterized by the  pres-ence  of the marginal ridge, while configurations 5to 8 (see Figs 4e to 4h) are characterized by the  ab-sence  of the marginal ridge, and, therefore, by thepresence of a proximal box. These eight configura-tions cover all clinical possibilities for cavity designand cusp coverage in relation to the marginal ridgeand account for half of the possibilities for a pre-molar (eight configurations associated with thepresence of the opposed marginal ridge and eightconfigurations associated with its absence).By considering mirror images of the eight possi-ble configurations for a marginal ridge and adja-cent cusps, it is possible to obtain every type of cavity preparation for a partial-coverage crown—inlays, onlays, and overlays—in any combination(Figs 5 and 6). A simple calculation of the combi-nations shows that 64 types of cavity preparationare possible for a partial-coverage crown in a toothwith four cusps. Thanks to the concept of principaltransition areas, it is simple to standardize cavitydesign. Ascertaining the presence or absence of the marginal ridge is sufficient to determinewhether the adjacent cusp should be maintainedor covered. Following this decision, the geometryof the cavity configuration can be outlined withcertainty. Fig 3  Structural model of the tooth, illustratingthe transition area  (red)   between marginal ridgeand cusp, cusp and cusp, and interaxial dentinand peripheral marginal ridge–cusp unit. id =interaxial dentin, ic = intact cusp, mr = marginalridge.B icicicidicmr mr  LMD
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