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Cavity Configurations for Indirect Partial-Coverage Adhesive-Cemented Restorations Guido Fichera, DDS1 Walter Devoto, DDS2 Dino Re, MD, DDS3

I

n d i rect esthetic adhesive restorations in composite resin and ceramics belong to the wider category of partial-coverage crown restorations. As such, they require pre p a r ation designs that leave a certain amount of intact clinical crown and generally have supragingival margins.1 To guarantee the fracture resistance of a p a rtial-coverage crown restoration over time, it is necessary to determine which part of the clinical crown that has, by itself or in combination with the buildup, appropriate structural characteristics and sufficient biomechanical strength. This decision influences the clinical outcome of the restoration. Respecting biologic principles and using a conservative approach are the building blocks for a successful outcome. For indirect tooth-colored adhesive-cemented restorations, the highest incidence of failure is f r a c t u re of the restoration material and intact tooth, together with secondary caries.2 To avoid fracture, cavity preparation should be considered as both a diagnostic and operative phase.3

Private practice, Monza, Italy.

1

Private practice, Sestri Levante (GE), Italy.

2

Researcher, Department of Prosthodontics, University of Milan, Italy.

3

Correspondence to: Dr Walter Devoto, Via E. Fico 106/8, 16039 Sestri Levante (GE), Italy. E-mail: [email protected]

FACTORS AFFECTING STRUCTURAL STRENGTH N u m e rous studies on the biomechanical and structural analysis of a tooth’s intact healthy stru ctures are available in the literature, especially from the era prior to the use of adhesive resins. From these re p o rts, it appears that the presence of the marginal ridge is fundamental4; if it is lacking, the occlusocervical and mesiodistal depth, and the size of the proximal boxes must be taken into consideration,5–8 as well as the intercuspal width (and thus, proximity to the tips of the cusps) and the depth 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 absence or presence of the pulp chamber roof4 (ie, vital or endodontically treated tooth), and the thickness and depth of interaxial dentin. 7 – 1 0 Restoration is further complicated as these factors must be related to the functional role of the tooth in question (eg, position in the arch, biotype, occlusal trauma, parafunctional habits, static and dynamic occlusion, condition of antagonistic teeth). Clinical re s e a rch shows that the adhesive bond between dentin and resin composite will, over time, decrease in strength,11,12 and that the extent of this decrease is in proportion to the mechanical, thermal, hydrolytic, and enzymolytic stresses to which the bond is subjected. It is also highly

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B ic ic mr

D

mr

id

mr

M

D

ic

ic

mr

M

id

ic

ic

pcr

L Fig 1 Occlusal view of structural model of the tooth. id = interaxial dentin, ic = intact cusp, mr = marginal ridge.

p robable that weakening of the adhesive bond over time is responsible for the mechanical failure of direct and indirect tooth-colored adhesivecemented restorations for which the restorationcavity interface is situated close to the tip of the cusp, or the intact tooth wall is insufficiently thick. These failures cannot easily be explained except by hypothesizing a failure over time of the adhesive bond at the interface, where it is subjected to high physical and chemical stress. It is the authors’ opinion that for posterior teeth it is important to place the margins of indire c t tooth-colored adhesive-cemented restorations in occlusal-axial areas subjected to lower mechanical s t ress. An analysis of the literature reveals four s t ru c t u res that determine the strength of intact tooth: the interaxial dentin, the pulp chamber roof, the marginal ridge, and the intact cusp. For their diagnostic and operative implications, these structures may be classified topographically as central or peripheral. The spatial relationship between the various structures is better clarified through a structural model of the tooth (Figs 1 and 2).

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Fig 2 Buccolingual view of the structural model of the tooth. id = interaxial dentin, ic = intact cusp, mr = marginal ridge, pcr = pulp chamber roof.

Central structures The central stru c t u res consist of the interaxial dentin and the pulp chamber roof. The interaxial dentin is the central core of the tooth. It may be seen as the occlusocervical continuation of the pulp chamber roof and, thus, occupies the area c o rresponding to the projection of the pulp chamber to the occlusal surface.13 The interaxial dentin connects the axial walls, in particular the buccal and lingual walls, and is the most important stru cture. When it is intact, the presence of other comp romised stru c t u res will not significantly undermine the overall fracture resistance of the intact tooth.14–17 Studies by Mondelli et al 14 and Larson et al 15 have shown that the loss of the marginal ridge (peripheral stru c t u re) does not produce significant structural weakening when the occlusal isthmus (ie, interaxial dentin) remains intact. On the contrary, exclusive preparation of the interaxial dentin is associated with significant stru c t u r a l weakening. The structural significance of the

Cavity Configurations for Indirect Partial-Coverage Restorations

marginal ridge is affected by any compromise of the interaxial dentin.16,17 The pulp chamber roof, contrary to common belief, is less important than the marginal ridge. Reeh et al 4 demonstrated that loss of the pulp chamber roof when both marginal ridges are kept intact (ie, endodontic treatment re q u i r i n g removal of some of the interaxial dentin and the pulp chamber roof) produces a less significant structural weakening than maintenance of the pulp chamber roof when one or two marg i n a l ridges are missing (ie, vital tooth with Class 2 cavity, occlusomesial, occlusodistal, and mesialocclusodistal). These considerations are important in making clinical choices based on the evaluation of healthy intact tooth structures and on scientific evidence, and not simply on empirical or preconceived notions. The hierarchy of tooth structures is thus: (1) the interaxial dentin, ( 2 ) the marginal ridge, (3) the roof of pulp chamber, and (4) the enamel-dentin complex of the intact cusp.

Peripheral structures The peripheral stru c t u res are the marginal ridge and the enamel-dentin complex of each intact cusp. The marginal ridge is the peripheral stru ct u re of the proximal wall, whereas the enameldentin complex of the cusp is the peripheral stru cture of the axial-buccal or palatolingual wall. The marginal ridge, its underlying enamel-dentin complex, and the interaxial dentin meet at the junction of the buccal wall with the palatolingual wall. The thickness of enamel-dentin complex at the base of each cusp does not participate in this structural junction but is rather the last support of the cusp itself. C o rrect evaluation of the marginal ridge must follow certain criteria. The loss of a marg i n a l ridge signifies the presence of a proximal box. If the interaxial dentin has been compromised and the presence of an occlusal isthmus is anticipated, the proximal box must be evaluated in

terms of the presence or absence of the pulp chamber roof and the thickness of the enameldentin complex at the adjacent intact cusp (it must be greater than1.5 to 2 mm in a vital t o o t h 18 and 2.5 to 3 mm in an endodontically treated tooth 13) and depth at the base.19 The nat u re of the structural interdependence determines whether or not it is necessary to cover the adjacent cusps. Articles by Linn et al20 and Panitvisai et al21 on the relationship between the marginal ridge and intact cusps in an endodontically treated tooth demonstrate a structural and functional dependence of the intact cusp on the adjacent marginal ridge. Likewise, the articles c o n f i rm the independent biomechanical behaviour among cusps as suggested by Sakaguchi et al,22 and which has been clinically confirmed by numerous studies.5–12 The loss of one marginal ridge in an endodontically treated molar, where the other marginal ridge is intact and adjacent cusps are well-supported, should be planned as a restoration with partial cusp coverage; the cusps adjacent to the lost marginal ridge are covered, while the cusps adjacent to the intact marginal ridge are maintained. The enamel-dentin complex of the intact cusp re p resents the most significant clinical factor in deciding whether to maintain or cover the cusp. Hood6 demonstrated that from the mechanical standpoint the enamel-dentin complex of the intact cusp adjacent to a proximal box behaves like a cantilever; the thickness and depth at the base of the intact cusp are the most important parameters since they vary with the cube of the d e f o rmation and, in the final analysis, are responsible for the strength of the cusp. This is why, with equal thickness and in the absence of a m a rginal ridge, intact cusps of endodontically treated teeth flex more than those of vital teeth. Keeping an intact cusp in a vital tooth is determined by an enamel-dentin thickness gre a t e r than 1.5 to 2 mm,20 whereas in an endodontically treated tooth the thickness must exceed 2.5 to 3 m m .1 3

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B ic

ic

D

mr

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id

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ic

L

TRANSITION AREAS AND POSSIBLE CAVITY CONFIGURATIONS After analyzing the peripheral and central stru ctures (ie, the marginal ridge, the intact cusp, and the interaxial dentin), it is possible, topographic a l l y, to outline the separation areas between each tooth structure. These areas are valuable in diagnosing cavity configurations, since they represent a line of transition between the restoration and intracoronal and extracoronal cavity to be outlined in partial-coverage crown preparation. They also act as spatial re f e rences and help accomplish a rapid buildup that is stereoscopically c o rrect. Taking, for example, a maxillary first molar, three areas of transition can be outlined between the marginal ridge and cusp, between cusp and cusp, and between the interaxial dentin and the peripheral marginal ridge–cusp unit (Fig 3). Given the anatomy of posterior teeth, two areas of transition are associated with each marginal ridge: a buccal transition, at the beginning of the adjacent buccal cusp; and a lingual transition, at the beginning of the adjacent lingual cusp. A number of cavity designs are derived from the possible combinations based on the absence or presence of the marginal ridge and on the maintenance or restoration of the two adjacent cusps

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Fig 3 Structural model of the tooth, illustrating the transition area (red) between marginal ridge and cusp, cusp and cusp, and interaxial dentin and peripheral marginal ridge–cusp unit. id = interaxial dentin, ic = intact cusp, mr = marginal ridge.

(buccal and lingual) (Fig 4). Configurations 1 to 4 (see Figs 4a to 4d) are characterized by the pre sence of the marginal ridge, while configurations 5 to 8 (see Figs 4e to 4h) are characterized by the a bsence of the marginal ridge, and, therefore, by the presence of a proximal box. These eight configurations cover all clinical possibilities for cavity design and cusp coverage in relation to the marginal ridge and account for half of the possibilities for a premolar (eight configurations associated with the presence of the opposed marginal ridge and eight configurations associated with its absence). By considering mirror images of the eight possible configurations for a marginal ridge and adjacent 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 combinations shows that 64 types of cavity preparation are possible for a partial-coverage crown in a tooth with four cusps. Thanks to the concept of principal transition areas, it is simple to standardize cavity design. Ascertaining the presence or absence of the marginal ridge is sufficient to determ i n e whether the adjacent cusp should be maintained or covered. Following this decision, the geometry of the cavity configuration can be outlined with certainty.

Cavity Configurations for Indirect Partial-Coverage Restorations

Fig 4 Basic cavity configuration depending on presence or absence of marginal ridge and maintenance restoration of adjacent cusps. bc = buccal cusp, lc = lingual cusp, mr = marginal ridge, pid = prepared interaxial dentin, cc = cusp cover, pb = proximal box.

mr

lc

cc

lc mr

pid

mr

pid

4a

4b

4d

lc

cc pb

pid

pb

pid

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cc pb

pid

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pid cc

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bc

bc

pid cc

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bc

bc

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4h

Fig 4a Cavity configuration 1: occlusal inlay.

Fig 4e Cavity configuration 5: occlusodistal inlay.

Fig 4b Cavity configuration 2: onlay with lingual cusp cover.

Fig 4f Cavity configuration 6: occlusodistal onlay with lingual cusp cover.

Fig 4c Cavity configuration 3: onlay with buccal cusp cover.

Fig 4g Cavity configuration 7: occlusodistal onlay with buccal cusp cover.

Fig 4d Cavity configuration 4: onlay with lingual and buccal cusp cover.

Fig 4h Cavity configuration 8: occlusodistal onlay with lingual and buccal cusp cover.

L 1-1

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1-6

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1-2

1-4

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1-5

1-8

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B Fig 5 Mirror-image associations of mesial configuration 1 with the 8 distal configurations.

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B Fig 6 Mirror-image associations of mesial configuration 2 with the 8 distal configurations.

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L 3.5 mm

cusp

2 mm

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M B 1.2 mm

L

1.2 mm

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3.5 mm mesiodistal levels

Fig 7 Horizontal cross section of intact tooth structure at a specific occlusocervical level to identify enamel-dentin thickness on the mesiodistal line.

pu

cu

pu

1.5-2 mm

de

Fig 8 Frontal cross section of intact tooth structure at a specific mesiodistal level to identify enamel-dentin thickness on the occlusocervical line.

TYPES OF CUSP COVER As stressed in a previous publication,3 cusp coverage in indirect esthetic adhesive restorations may be accomplished by either shoeing or capping. The diagnostic model for cusp cover outlines parameters of how to decide whether or not a cusp cover is necessary and what type of cover is appropriate. To use the model requires analyzing the cusps in three planes: transversal, frontal, and sagittal. The thickness of the enamel-dentin complex on the cusp dictates the need for cusp coverage. As p reviously indicated, the limit for enamel-dentin thickness in a vital tooth must be around 1.5 or 2 mm; if it is any less, the wall would be exclusively s u p p o rted by enamel and reinforcement provided by the buildup would not be reliable. An endodontically treated tooth requires greater enameldentin thickness; when the cusp has lost the adjacent marginal ridge, even if it is supported by an enamel-dentin thickness greater than 1.5 or 2 mm, it has lost all structural links (interaxial dentin, pulp chamber roof, and marginal ridge) to the opposite marginal wall and, thus, behaves as a cantilever. In this case, the cusp height becomes an essential factor, due to the loss of the pulp chamber roof. Covering the cusp is strongly recommended, unless the intact cusp has a remarkable thickness greater than 2.5 to 3 mm.

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cusp

marginal ridge

occlusocervical levels

me

Fig 9 Diagnostic model for type of cusp cover facilitates analysis of occlusocervical and mesiodistal enamel-dentin thickness of the intact cusp. pu = peripheral unit, cu = central unit, de = distal extremity, me = mesial extremity, red grids = transition area.

By definition, the thickness of the cusp wall is a spatial attribute on the transversal and fro n t a l planes. For an overall evaluation, the enameldentin thickness should be considered at various o c c l u s o c e rvical and mesiodistal levels. A transversal section at a specific occlusocervical level provides information on the thickness of the enameldentin complex along the mesiodistal line (Fig 7). A frontal cross section at a specific mesiodistal level provides information about the enameldentin thickness along the occlusocervical line (Fig 8). By combining the information concern i n g the enamel-dentin thickness at varying occlus o c e rvical and mesiodistal levels, it is possible to determine whether a cusp cover is required and the most appropriate type. Transversal and frontal examination is effectively supplemented by the diagnostic model for type of cusp cover, shown in lateral view (sagittal plane) with the start of the marginal ridge and the adjacent cusp. A grid of horizontal and vert i c a l lines can be traced on the cusp, to represent the o c c l u s o c e rvical levels (spaced by approximately 2 mm) and the mesiodistal levels (where the lines coincide with the mesial and distal extremities of the transition area, between cusp and marg i n a l ridge, and between cusp and cusp) (Fig 9). These lines produce a grid of structural units, which are valuable for ascertaining the enamel-dentin thick-

Cavity Configurations for Indirect Partial-Coverage Restorations

marginal ridge

cusp

cusp 1.5-2 mm

Figs 10a and 10b Structural deficiencies in the occlusal 2 mm of intact cusps require cusp coverage by shoeing. The model is shown on the left and the frontal cross section on the right. pb = proximal box, red grids = transition area.

B pb

10a

ness of the cusp. A Johannson thickness gauge for metals may be used as a diagnostic aid. The structural units (nine in all) may be subdivided into central3 and peripheral units (three on each side, ie, six per cusp). The peripheral stru ctural units coincide with the mesial and distal halves of the transition area, and, as such, will be the site of maintenance or restoration (ie, the passage of the cavity perimeter) depending on the clinical situation. By examining the various parts of this model, it is possible to obtain all the necessary diagnostic i n f o rmation related to cuspal coverage. Before analyzing the model, it is important to observe that a mesial and a distal extremity can be distinguished in the transition areas. For example, during the preparation phase the cutting line may be traced through the transition area between cusp and marginal ridge, or between cusp and cusp, anywhere from the mesial to the distal extremity, ie, within a range of approximately 1.5 to 2 mm. Conceptually, wherever the cutting line runs within this range of 1.5 to 2 mm, the design of the general cavity shape (types 1 to 8) does not change (see Fig 4). If the transition line is located at the mesial or distal extremity of the area, it will affect the type of cusp cover that is required. Identifying transition areas and their extremities limits the preparation line and avoids unnecessary

buildup

10b

preparation. To simplify analysis of the model, it is a p p ropriate to consider first the structural deficiency of the enamel-dentin thickness in the 2-mm occlusal area of the cusp. This deficiency indicates the need for shoeing (Figs 10a and 10b). When the perimeter of the intracoronal cavity is in proximity to or coincides with the tip of the cusp, shoeing is indicated. This is true for all situations in which the occlusal isthmus is intact and conventional capping is not considered a conservative enough approach. The lateral view shows the cusp cover with its mesiodistal extension, whereas the frontal cross section shows its conservative quality and the ease of perf o rming this type of cusp cover. Depending on the combination of mesial and distal extremities of the transition area, the shoeing will have four possible configurations; the most conservative approach does not involve the adjacent ridge or cusp, and the most extensive approach involves both. There are schematic distinctions between minimum-extension shoeing (re q u i red exclusively for a deficiency of a central structure), intermediate-extension shoeing (for a deficiency of a central stru c t u re plus one peripheral stru c t u re), and maximum-extension shoeing (for a deficiency of a central structure and both peripheral stru c t u res) (Figs 11a to 11f). Should the s t ructural deficiency in enamel-dentin thickness involve the middle third as well as the occlusal band,

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marginal ridge

cusp

cusp

11b

11a marginal ridge

cusp

cusp

11d

11c

marginal ridge

cusp

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11f

Figs 11a to 11f Models (left) and occlusal views (right) of cusp cover by shoeing with minimum (a and b), intermediate (c and d), and maximum (e and f) mesiodistal extension. red grids = transition area.

capping will be required (Figs 12a and 12b). Four combinations of capping are also possible, one classic total capping and three partial capping of minimum to intermediate extension (Figs 13a to 13f). A structural deficiency in the cervical third involves the same combinations as outlined above, the only clinical diff e rence being that the cervical margin will lie closer to the gingival margin, and, consequently, the axial wall will be longer (Figs 14 and 15). Out of the possible 64 cavity designs, configurations with the greatest space for cusp coverage are

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those in which an intact cusp is combined with a s t ructural deficiency involving a missing adjacent m a rginal ridge and an adjacent cusp re q u i r i n g complete restoration (eg, 2-6,2-8,3-7,3-8 [see Figs 5 and 6]). Such situations allow full exploitation of the operative space, due to the absence of transition areas adjacent to the cusp. Despite this, it is possible to re s o rt to maximum-extension cover in the presence of adjacent marginal ridge and cusp. By exploiting the transition areas bordering the intact cusp and using a cavity perimeter that passes through the more distant mesial and distal extremi-

Cavity Configurations for Indirect Partial-Coverage Restorations

marginal ridge

cusp

cusp 1.5-2 mm

Figs 12a and 12b Structural deficiency in the middle and occlusal thirds of the intact cusp requires cusp coverage by capping the middle third. The model is shown on the left and the frontal cross section on the right. pb = proximal box, red grids = transition area.

pb

12a

marginal ridge

cusp

13b cusp

cusp

13d

13c marginal ridge

13e

12b

cusp

13a marginal ridge

B buildup

cusp

cusp

13f

Figs 13a to 13f Models (left) and occlusal views (right) of cusp cover by capping the middle third with minimum (a and b), intermediate (c and d), and maximum (e and f) mesiodistal extension. red grids = transition area.

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marginal ridge

cusp

cusp 1.5-2 mm

Figs 14a and 14b Structural deficiency of the entire intact cusp requires cusp cover by capping the cervical third. The model is shown on the left and the frontal cross s e ction on the right. red grids = transition area.

pb

14a

marginal ridge

cusp

15b cusp

cusp

15d

15c

marginal ridge

14b

cusp

15a

marginal ridge

B buildup

cusp

cusp

pb

15e

15f

Figs 15a to 15f Models (left) and occlusal views (right) of cusp cover by capping the cervical third with minimum (a and b), intermediate (c and d), and maximum (e and f) mesiodistal extension. pb = proximal box, red grids = transition area.

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Cavity Configurations for Indirect Partial-Coverage Restorations

marginal ridge

16a

cusp

cusp

Figs 16a to 16b Structural deficiency of the central and lateral structures of the entire distal intact cusp and the occlusal third of the mesial intact cusp requires cusp cover by total capping and by shoeing with maximum extension. The model is shown on the left and the occlusal view on the right. red grids = transition area.

16b

Fig 17 Diagnostic algorithm with flowchart. Fig 18 Initial radiograph showing distal secondary caries at the second premolar and direct capping of mesial pulpal horn at the first molar, which is affected by pulpitis and occlusal caries.

18

Fig 19 Radiograph showing endodontic therapy.

ties, the minimum operative space needed to accomplish a maximum-extension cusp cover can be obtained. Furt h e rm o re, understanding of the mesial and distal extremities of a transition are a makes it possible to consider different designs for cusp coverage, provided that one or both transition areas adjacent to the undermined intact cusp are not involved. Partial capping and minimum- and int e rmediate-extension shoeing, combine the biomechanical benefits of cusp covers with the advantages of maximum conservation of tooth structure. Various combinations of enamel-dentin thickness on the intact cusp have been illustrated to

19

simplify explanation of the model and clarify the differences between various cusp covers. Clinical practice produces a wide range of possible combinations (Figs 16a and 16b), but by following a number of guidelines it is possible to make the best compromise between maximum conservation of tooth stru c t u re and optimal biomechanics of the tooth restoration (Fig 17). Furtherm o re, the grid and its central and peripheral subdivisions allow a realistic evaluation of buildup possibilities for re i n f o rcing the intact enamel-dentin complex (Figs 18 to 33).

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Fig 20 Examination of the first molar, with intact healthy tooth structure at and onlay treatment covering the mesiobuccal and mesiolingual cusps.

Fig 21 Placement of modified glass-ionomer cement at canal entrances.

Fig 22 Etching.

Fig 23 Application of primer and bonding agent.

Fig 24 Bright white chrome composite facilitates potential endodontic re-treatment and enhances value of buildup.

Fig 25 Application of fluid composite buildup.

Fig 26 Application of restoration composite buildup.

Fig 27 Preparation of onlay cavity with cusp cover by shoeing of mesiolingual cusp and by capping middle third of mesiobuccal cusp.

Fig 28 Sectional matrix, wedge, and retractor ring for Class II occlusomesial cavity at the second premolar.

Fig 29 Construction of mesial wall of the second premolar.

Fig 30 Direct resin composite restoration of the second premolar. Field for adhesive cementing of onlay to the first molar is placed under rubber dam.

Fig 31 Adhesive cementing of composite onlay to the first molar.

Fig 32 Direct resin composite restoration of the second molar.

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Fig 33 Removal of dam, followed by finishing and polishing of the restorations.

Cavity Configurations for Indirect Partial-Coverage Restorations

CONCLUSION By looking to biomechanical studies and a stru ctural model of the tooth, it is possible to standardize the diagnosis of cavity configuration and the application of clinical and operative guidelines. Determining the strength of the intact healthy tooth stru c t u res re p resents the starting point for all analysis. In examining the adequacy and deficiency of intact tooth stru c t u res, a clinician can det e rmine all biomechanically valid cavity configurations. Using a model of the enamel-dentin complex of the intact cusp, an analysis can provide indications for restoring any intact cusp, as well as the choice of type of cusp cover. This approach encourages cavity configurations and types of cusp cover that offer structural strength and maximum c o n s e rvation of the natural tooth.

REFERENCES 1. Carossa S, Pera P. Corone Parziali in Oro e in Ceramica. Milan: Masson, 1997:3–8. 2. Manhart J, Hickel R. Longevity of Restorations. In: Roulet JF, Wilson N, Fuzzi M (eds). Advances in Operative Dentistry, Vol 2: Challenges of the future. Quintessence 2001: 237–304. 3. Fichera G. Restauri indiretti in composito nei settori posteriori. Principi di preparazione cavitaria. Il Dentista Moderno, 2002;5:73–93. 4. Reeh ES, Messer HH, Douglas WH: Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endod 1989;15:512–516. 5. Blaser PK, Lund MR, Cochran MA, Potter RH. Effects of designs of Class 2 preparations on resistance of teeth to fracture. Oper Dent 1983;8:6–10. 6. Hood JAA. Methods to improve fracture resistance of teeth. In: Vanherle G, Smith DC (eds). International Symposium on Posterior Composite Resin Dental Restorative Materials. Amsterdam: Szulc, 1985:443–450.

7. Khera SC, Goel VK, Chen RCS, Gurusami SA. Parameters of MOD cavity preparations: A 3-D FEM Study, Part II. Oper Dent 1991;16:42–54. 8. Goel VK, Khera SC, Gurusami SA, Chen RC. Effect of cavity depth on stresses in a restored tooth. J Prostht Dent 1992;2:174–183. 9. Rees JS: The role of cuspal flexure in the development of abfraction lesions: A finite element study. Eur J Oral Sci 1998;6:1028–1032. 10. Lin CL, Chang CH, Wang CH, Ko CC, Lee HE. Numerical investigation of the factors affecting interfacial stresses in an MOD restored tooth by auto-meshed finite element method. J Oral Rehabil 2001;6:517–525. 11. Takahashi A, Inoue S, Kawamoto C, et al. In vivo longterm durability of the bond to dentin using two adhesive systems. J Adhes Dent 2002;2:151–159. 12. Hashimoto M, Ohno H, Kaga M, Endo K, Sano H, Oguchi H. In vivo degradation of resin-dentin bonds in humans over 1 to 3 years. J Dent Res 2000;6:1385–1391. 13. Becciani R, Castellucci A. La biomeccanica del dente trattato endodonticamente. Implicazioni cliniche. Dent Cadmos 2002;1:15–32. 14. Mondelli J, Steagall L, Ishikiriama A, de Lima Navarro MF, Soares FB. Fracture strength of human teeth with cavity preparations. J Prosthet Dent 1980;43:419–422. 15. Larson TD, Douglas WH, Geistfeld RE. Effect of prepared cavities on the strength of teeth. Oper Dent 1981;6:2–5. 16. el-Mowafy OM. Fracture strength and fracture patterns of maxillary premolars with approximal slot cavities. Oper Dent 1993;18:160–166. 17. Caron GA, Murchison DF, Cohen RB, Broome JC. Resistance to fracture of teeth with various preparations for amalgam. J Dent 1996;6:407–410. 18. Touati B, Miara P, Nathanson D. Estetica Dentale e Restauri in Ceramica. Milan: Masson, 2000:259–291. 19. Anderlini G. Moderni Orientamenti per la Restaurazione Dentale, Vol II. Edizioni Bologna: Martina, 1995:594–604. 20. Linn J, Messer HH. Effect of restorative procedures on the strength of endodontically treated molars. J Endod 1994;20:479–485. 21. Panitvisai P, Messer HH. Cuspal deflection in molars in relation to endodontic and restorative procedures. J Endod 1995;21:57–61. 22. Sakaguchi RL, Brust EW, Cross M, DeLong R, Douglas WH. Independent movement of cusps during occlusal loading. Dent Mater 1991;7:186–190.

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