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Restoring Strength of Incisors with Veneers Ceramic Crowns
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Yong-Hee P. Chuna/Constanze Raffeltb/Hanjo Pfeifferc/Mozhgan Bizhangd/Gunnar Saule/ Uwe Blunckf/Jean-François Rouletg
Purpose: The aim was to determine the in vitro fracture resistance of incisors restored with veneers and full ceramic crowns compared to unrestored teeth. Materials and Methods: Seventy intact, extracted human maxillary central incisors were randomized and assigned to 7 groups (n = 10). The teeth in group 1 remained intact (control). The teeth in groups 2 to 6 were prepared and IPS Empress restorations were conditioned and bonded using an adhesive luting cement, Variolink II/Syntac (group 2: labial veneer with incisal overlap, group 3: 3/4 veneer with margin in enamel, group 4: 3/4 veneer with margin in dentin, group 5: crown with margin in enamel, group 6: crown with margin in dentin group 7: veneer on worn tooth. After finishing and polishing, specimens were stored in water and thermocycled for 2000 cycles between 5°C and 55°C. The maximal fracture load of the specimens (40-degree inclination) was determined using the universal testing machine (Zwick) at a constant crosshead speed (0.5 mm/min). The statistical analysis was performed using the Kruskal-Wallis test with Bonferroni correction (p < 0.05). Fracture surfaces were qualitatively analyzed by SEM. Results: All restored teeth with cervical preparation margins in enamel showed a fracture load not significantly different from the intact teeth (control). Restored teeth with cervical preparation margins in dentin showed a significantly lower fracture load. All restorations showed a fracture load far above 400 N, serving as functional reference for anterior teeth. The failures were predominantly cohesive. Conclusion: For the restoration of tooth strength, defining the finishing lines of veneers and crowns in enamel is recommended. Restorations with finishing lines in dentin resulted in significant loss of strength. Three-quarter veneers with finishing lines in enamel are functionally equal to crowns with the advantage of conserving tooth structure. Keywords: dental material, ceramic veneer, full ceramic crowns, adhesion, adhesive luting, fracture load. J Adhes Dent 2010; 12: 45-54. doi: 10.3290/j.jad.a17533
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hile the treatment objectives of today’s dentistry regarding the restoration of form and function are unchanged, the demands on quality and esthetics by patients have grown.24,44,74 Modern materials and adhe-
a
Postdoctoral Fellow, Biologic and Materials Sciences, University of Michigan, Ann Arbor, MI, USA.
b
Dentist, Private Practice, Heilbronn, Germany.
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Dentist, Medeco Dental Clinic, Düsseldorf, Germany.
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Associate Professor, Department of Operative and Preventive Dentistry and Endodontics, Heinrich-Heine University, Düsseldorf, Germany.
e
Dentist, Private Practice, Berlin, Germany.
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Associate Professor, Department of Operative and Preventive Dentistry, Charité, Berlin, Germany.
g
Head of Research and Development-Clinical, Ivoclar Vivadent, Schaan, Liechtenstein.
Correspondence: Yong-Hee P. Chun, DDS, MS, Dr. med. dent., Biologic and Materials Sciences, University of Michigan, 1210 Eisenhower Place, Ann Arbor, MI 48108, USA. Tel: +1-734-975-9326, Fax: +1-734-975-9329. e-mail:
[email protected]
Vol 12, No 1, 2010
Submitted for publication: 20.07.08; accepted for publication: 15.10.08.
sive techniques have innovated dentistry towards restorations which are truly invisible to the patient.35,36,59 With the improvement of ceramic materials and luting composites, ceramics are successfully bonded to teeth,5 conceptualizing towards minimally invasive dentistry.63,73 The basis for a tight bond is an intimate ultrastructural interlocking at the interfaces between tooth and ceramic.51 Bonding of ceramic restorations offers two major advantages. The quality of esthetics achieved with bonded ceramics is considered state of the art. Bonding of ceramics to the tooth recovers strength that is lost as an inherent part of tooth preparation for restorative procedures.39 Following a facial veneer preparation, the increase in flexure can amount to 91% as measured by 3-D finite element analysis.42 Bonded ceramic veneers are capable of restoring the fracture strength of teeth to values of intact teeth.1,35,38,65 Bonding has proven clinically acceptable survival of anterior veneers for up to 10 years in function ranging from 64% to 91%.9,50 The major complication of veneers are fractures in up to 3% of the placed veneers according to clinical studies.9,41 45
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Fig 1a Group 2: Labial veneer preparation Fig 1b Group 3: 3/4 veneer with finishing Fig 1c Group 4: 3/4 veneer with finishing with incisal overlap. line in dentin. line in enamel.
Fig 1d Group 5: Crown with finishing line in Fig 1e Group 6: Crown with finishing line in enamel. dentin.
Similar to veneers, anterior all-ceramic full crowns have a 5-year clinical success of 94.9% as determined by pooling data from 2048 crowns53 and 98.9% as determined in an 11-year retrospective study.13 The most common complications for crowns in general are loss of pulp vitality and fracture of the core. These events may be related to the expansive preparation design for crowns. Comparing the volume of tooth structure removed for an all-ceramic crown preparation to a veneer preparation, double the amount of tooth structure is removed for a crown than a veneer.10 As pulpal trauma is correlated to the extent of preparation, less pulpal complications are expected with veneer preparations than with crown preparations. In the interest of preserving pulp vitality and sound tooth structure, preparation designs for crowns are modified towards minimally invasive dentistry. Translated into the clinical situation, indications for anterior full crowns may be reconsidered in favor of extended veneers. 46
Fig 1f Group 7: Tooth with attrition (short clinical crown to be lengthened).
Veneers extended into the interproximal and palatal surfaces demonstrated 97.6% to 100% survival 5 years after cementation.20 Examples for clinical indications are open contact, change in tooth shape, decay, and pre-existing restorations. The retreatment of Class III composite restoration in anterior teeth often poses a challenge when deciding between a full crown and an extended veneer. In conjunction with retreatment, the cervical margin may be defined in dentin. However, evidence for extended veneers with cervical definition in dentin is not yet available. Therefore, the aim of this study was to evaluate the performance of bonded veneers extended interproximally and cervically upon in vitro fracture load in comparison to labial veneers, full crowns, and intact teeth. The null hypothesis is that regardless of the preparation design, bonded ceramic veneers and full crowns will achieve fracture loads comparable to intact incisors. The Journal of Adhesive Dentistry
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Preparation Teeth serving as controls for the veneer and crown preparations were left unprepared. For group 2 (Fig 1a), the buccal enamel reduction was standardized by using a depth cutter (# 834.314.021 LVS-1, Gebr. Brasseler; Lemgo, Germany) resulting in 0.5 mm depth.16 A 0.5-mm-deep chamfer finishing line was set 1.0 mm above the CEJ in the enamel using a 100-μm cylindrical bur with a rounded tip (# 881.314.014, Gebr. Brasseler) at 160,000 rpm. Half of the width of the interproximal area was left unprepared. For the incisal overlap, the incisal edge was reduced by 1 mm, and a 1.0-mm-deep and 0.5-mm-wide chamfer was prepared on the incisal/palatal side. The entire preparation was finished at 40,000 rpm with a 15-μm finishing diamond (# 881EF.314.014, Gebr. Brasseler) of the same shape and size as the preparation diamond. All preparations were performed with copious water irrigation. The incorporation of Class III composite restorations was simulated in 3/4 veneers by placing the finishing line in enamel (group 3) (Fig 1b). The same systematic approach was used as in group 2. The only difference was that the interproximal area was included in the preparation of about 2.0 mm into the oral direction. The group 4 veneers had the same dimensions as the group 3. With the exception of the cervical finishing line of the facial and interproximal aspects, 3/4 veneers were placed in dentin (Fig 1c). For the full crown in group 5, the crown finishing line ended in enamel (Fig 1d). According to recommendations for the preparation of full all-ceramic crowns,18 the incisal edge was shortened by 2.0 mm and the cervical finishing line was positioned 1.0 mm above the CEJ into enamel. The amount of circumferential reduction was standardized
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The extracted human teeth used for this study were contributed by different dentists. During the tooth measurement and selection, teeth were stored in a 20% alcohol solution for disinfection purposes. Subsequently, the storage medium was changed to water. Selection of study teeth was restricted to maxillary central incisors devoid of tooth fractures or cracks. The selection took place under a light microscope at 12X magnification (Wild Heerbruck; Heerbruck, Switzerland). Since the pool of extracted teeth was not able to supply a sufficient number of intact maxillary teeth of the same dimension, the selected teeth were divided into three categories by size. The dimensions of the selected teeth were measured with vernier calipers (Tesa Digit – Cal II; Renens, Switzerland). By adding the mesio-distal width, incisal-cervical length and buccal-oral depth, a value was generated that served to assign a tooth into one of the three categories: A: 24.8 to 25.8 mm, B: 25.9 to 26.9 mm, and C: 27.0 to 28.0 mm. In order to obtain a similar representation of tooth dimensions in each experimental group (groups 1 to 6), 3 teeth from category A and C and 4 teeth from category B were randomly chosen. For the group 7 (teeth with attrition), only teeth with a sum of 24.4 to 25.4 mm were used. The teeth were debrided and stored in water in between procedures to prevent dehydration.
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opyretigal No CChun t fo by grinding 1.0-mm-deep horizontal cuts using r Pa disk ubThe shaped diamond (# 824.314.037, Gebr. Brasseler). lica rounded chamfer was cut at 160,000 rpm using a 125-μm ti cylindrical diamond with a round tip (# 881.314.016, te Gebr. on ss e n c e Brasseler). Finishing was done using the corresponding 15-μm diamond bur (# 8881.314.016, Gebr. Brasseler) at 40,000 rpm. The palatal reduction above the tubercle was performed using a coarse Heidenreich diamond (# 899.314.031, Gebr. Brasseler) and finishing was done with the same type, but with 15-μm grit (# 8899.314.031, Gebr. Brasseler). With this preparation design, the majority of the prepared surface consisted of dentin. Teeth of group 6 were prepared as full crowns with finishing lines in dentin (Fig 1e). This preparation was done in analogy to group 5, with the difference that the cervical finishing line was placed at the CEJ in dentin, thus removing all enamel. In group 7, teeth with attrition were simulated, aiming for restoration of the crown to the original crown length (Fig 1f). They were prepared in a manner similar to group 2. The same systematic approach was used as in group 2. The only difference was that the incisal edge was shortened to create a palatal finishing line 1.0 mm above the tubercle in order to simulate a worn tooth. Veneer and Crown Manufacture Impressions from all prepared teeth were taken using a polyvinylsiloxane impression material (Silagum, AV putty batch # 991502-36 and -37, AV quick normal set batch # 992400-07 and -17, DMG; Hamburg, Germany). Models were made from stone cast (Esthetic-rock, Dentona Dental; Dortmund, Germany), and hardened with plaster hardener (Yeti; Engen, Germany). A spacer varnish (Duo-Spacer Silver, batch # CH 40235262, Yeti) was applied 1 mm above the finishing line. After insulation of the stone models, veneers were waxed in the dimensions of the original crown by measuring width, height, and depth. The thickness of the veneers was controlled with vernier calipers at several sites. In group 7, the incisal edge was built up for 4.0 mm. According to the manufacturer’s instructions, IPSEmpress veneers and crowns were fabricated using the lost wax technique (IPS Empress, color 01, batch # B19504, Ivoclar Vivadent; Schaan, Liechtenstein). After the fitting of the veneer on the stone model, the veneers and crowns were painted twice (IPS Empress Shades 110/120, IPS Empress Glazing and Staining Liquid, Ivoclar Vivadent) and baked before the glazing material (IPS Empress Glazing Material, Ivoclar Vivadent) was applied and baked. Veneers and crowns were then fitted on the original tooth. To determine the amount of removed tooth tissue, every restoration was weighed before the insertion with an analytical scale (Sartorius calibrated precision scale 1404004, Sartorius; Göttingen, Germany). The volume was calculated using the specific weight of IPS Empress (2.8 g/cm3). Luting Procedures Table 1 lists the materials used for the luting procedures conducted. Only ceramic restorations without fractures, cracks, or flaws as identified under light microscopy at 12X magnification were luted to the tooth. Before bonding 47
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Vita Ceramics Etch ( 0.05. Letters a, b: No statistically significant differences were found between groups of the same letter.
dental hard tissue removed increased significantly (Fig 5, Table 3). Positioning the finishing line in dentin in 3/4 veneers and in crowns resulted in significantly greater reduction of the dental structure compared to a finishing line in enamel. 50
DISCUSSION Fracture load studies typically suffer from high variability within a test group. This variability can result from many different sources: handling of specimen, quality of teeth, The Journal of Adhesive Dentistry
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tooth dimensions, and specimen preparation. Since the history and the origin of the teeth are unknown, it is difficult to evaluate the quality of the dental hard tissues. For our in vitro study, we decided to use extracted human teeth, since they best represent the clinical situation compared to teeth of other species. Freshly extracted teeth are usually not available in desired quantity, and thus require storage in a disinfecting solution. Adverse effects induced by the storage solution on the tooth structure are unwanted, since they could interfere with bonding. The teeth used in this study were collected by local dentists; their age, storage time, and storage medium before delivery were unknown. Teeth were stored in 20% ethanol at the beginning of the study. Dehydration by ethanol can cause volumetric shrinkage contributing to crack formation of the tooth structure. Storage in 75% ethanol (v/v ethanol/ water)33 or in methanol69 resulted in significantly lower shear bond strength. Teeth frozen immediately following extraction have shear bond strengths similar to freshly extracted teeth, suggesting that bonding is affected by processing of dentin matrix after extraction. In contrast, the bond strength for teeth stored in 70% ethanol remained the same over time and was comparable to teeth stored in water.19 Three- to 4-week storage in 70% ethanol after extraction did not influence microleakage in Class V restorations.23 Since we used a lower ethanol concentration to store the teeth, the effects on the tooth structure and bonding interface may be less severe. During the specimen selection, strict selection criteria were applied, and great effort was taken to eliminate all teeth with cracks potentially harboring fracture induction sites. The coronal dimensions of teeth passing macroscopic and microscopic inspection were measured. Since the tooth dimensions varied greatly, the teeth were grouped by size. Within each group of a given tooth size, randomization into the test groups was carried out, in order to accomplish a similar distribution of tooth size in each test group. The materials used in this study were selected because of the availability of long-term clinical and in vitro data. In anterior crowns, Empress ceramic has demonstrated clinical success with high survival and highly satisfactory service.13,64,70 For esthetic results and retention to the tooth, ceramic restorations are bonded to the tooth via resin cements.49 The bonding system Variolink II/Syntac achieves acceptable gap-free margins in enamel and dentin,14,71 and has proven to be clinically successful.17,20 Light curing of a luting composite is influenced mainly by the thickness and the color of the ceramic.3,31 When the luting composite is only light cured, ceramic layers thicker than 2 or 3 mm, as typically found in inlays, may be problematic, since the degree of cure may decrease.11 In our study, the ceramic thickness of the crowns was 1.0 to 2.0 mm and of the veneers 0.5 to 1.0 mm. The ceramic thickness was not standardized for the staining of the IPS Empress ceramic. The use of a dual-curing luting cement was indicated, offering polymerization of the luting resin in areas of thick restorations otherwise not penetrated by sufficient light intensity.
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Fig 5 Dental hard tissue removed during preparation. The bars indicates nonsignificant differences between groups at the level of p > 0.05.
Using the ultrasonic insertion technique,58 the viscosity of the composite is lowered,32 resulting in minimal thickness of the composite layer.52 Excess luting composite was carefully removed before light polymerization. The best bond strength is obtained when Heliobond is light cured before the insertion of the restorations.31 However, in the present study, the luting protocol was modified. Heliobond was not cured before seating the restorations to avoid a bonding “puddle” resulting in the inability to completely seat the restoration. A thicker composite layer could have confounded the fracture load.43 The glycerin gel used prevents the formation of an oxygen inhibition layer and therefore contributes to a good marginal quality. Veneers made with IPS Empress and luted with Variolink II/Syntac have demonstrated clinical success in a prospective trial.20 The veneer preparation was used according to the recommendation of Magne and Douglas, reducing the incisal edge and preparing a chamfer on the incisal/palatal side.36 Alternatively, the palato-incisal margin could have been prepared as a butt joint or a mini-chamfer.65 Extending into the proximal space behind the contact point in the palatal direction is clinically required if the goal is to “correct” the tooth position with veneers or if pre-existing composite restorations are present. 1 2 , 21, 6 0 The crown preparation forms used in this study are commonly recommended and applied to all-ceramic restorations.18,45 The oral cavity experiences thermal cycling during food intake and functional load.48. Restorations exposed to thermal cycling experience stress due to expansion and contraction of the material. Thermal stress increases the marginal leakage at composite/tooth interfaces46 and decreases the fracture resistance of restored teeth.8 In luted veneers, thermal stress causes compressive and tensile forces in the ceramic43 and induces cracks when the luting cement layer is thick compared to the ceramic layer.40 For thermal cycling, the parameters temperature ex51
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tremes, cycle number, and dwell time are selected arbitrarily, since they vary widely in the clinical situation. However, excessive conditions should be avoided in order to be able to discriminate differences in performance.15 Water storage and thermocycling were performed to simulate stress to the interfaces and to age the restorations in vitro.4,57,66 The conditions that we chose for the study are considered moderate with 2000 cycles between 5°C and 55°C and a dwell time of 30 s. However, it is questionable as to what extent thermal cycling is relevant to clinical failures. In the present study, increasing static load was applied to the restorations until fracture. Static load is commonly used for testing the strength of restored teeth,28,56 but in clinical reality, teeth are repeatedly loaded far below the maximum fracture load. The resulting fatigue decreases the maximum fracture strength of all-ceramic crowns.7,68 To simulate fatigue in vitro, a dynamic load should have been applied using a staircase procedure. For such an experimental design, a much larger number of specimens is required to gain statistical power. The results produced in the present study therefore point in the right direction and should be confirmed either with in vitro fatigue studies or clinical studies. Loading incisors at an angle of 40 degrees is physiological for the maxilla-mandible relation.61 However, this direction exerts mainly shear and compressive forces to the labial surface of the veneer and less tensile forces. In order to avoid uncontrollable stress concentrations at the incisal load insertion point, which would have certainly induced unphysiological cracks during the experiment, flattening under standardized conditions was attained, using a flat plunger. Our intention with this modification was to reduce variability in fracture load. We did not expect any negative effects caused by this gentle modification of the incisal edge. The average fracture load on intact teeth was 1160 N, confirming the results of Reeh and Ross.55 This is much higher than the maximum forces to which incisors are subjected under normal mastication, ranging from 108 to 250 N.25, 26 There is a consensus that initially, anterior restorations should withstand forces of 400 N as a minimum.30,72 All restorations in the present study were loadable far above this minimum value. The null hypothesis that bonded veneers of different preparation designs would restore teeth comparable to intact incisors could only partially be accepted. Only restorations with finishing lines in enamel fractured statistically at the same load as intact teeth. For restorations with finishing lines in dentin, the null hypothesis must be rejected. Traditional recommendations for veneer preparations limit veneers to finishing lines in enamel. One question of this study was to evaluate whether an extension of cervical finishing lines into dentin would decrease the strength of the tooth-restoration complex. This question was applied to veneers and crowns. Extending the removal of tooth structure would decrease the cross section of the remaining tooth significantly. As long as the finishing lines of veneers or crowns were in enamel, bonded restorations were able to restore the
pyrig No Co t fo tooth strength, regardless of the dimensions. r This Pu result supports the concept that lost enamel may be replaced bli by cat bonded ceramic. Using finite element analysis of bonded i 37 on veneers, initial strength can be restored tup essto 96%. Teeth with veneer preparations modified in incisal e n c ere-
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duction or palatal extension are conserved by bonded veneers.65 Even teeth with endodontic treatment in combination with bonded veneers are restored in their fracture strength.27 All veneer preparations had finishing lines defined in enamel. In contrast to our results, others have found that the reduction of the incisal edge and a palatal chamfer decreased the fracture load significantly. 6,22 Higher stability of veneers is achieved if prepared with a window preparation and an intact incisal edge.22,29,62 Finite element analysis revealed tensile stress concentrations at the palatal concavity.38,43 The stress increased with the depth of the margin in the concavity. The tensile forces remained low when the margins were butt-joined or chamfered and located in the convex portion of the palatal side.38,43 A palatal overlap preparation with chamfer experiences evenly distributed compression forces.75 For bonded all-ceramic restorations, a subgingival finishing line apical of the cementoenamel junction is not recommended, since control of oral fluids and removal of excess luting material are impossible.54 Nevertheless, in vitro experiments address questions under optimized conditions. In restorations with finishing lines placed in dentin, the fracture strength decreased significantly. It was not possible to correlate the amount of removed dental mineralized tissues with the obtained fracture load of the restorations, confirming the data of Edelhoff and Sorensen.10 This is also an indication that the adhesive technique used was able to provide a strong bond between the ceramic veneer and the enamel. Similarly, the bond between veneer and dentin is enhanced. In addition, as a measure of the quality of the bond, the area of exposed dentin could have been quantified and related to the fracture strength. In veneers with a palatal chamfer or window design, stress is highest in the incisal edge. As the load increases, the stress shifts to buccal and cervical areas.75 The chamfer design, in contrast to the window preparation, experienced reduced incisal stress on ceramic, cement, and tooth (bonding interface). The cement layer acted as a stress absorber.75 The chamfer with palatal overlap develops less stress than the window veneers. The palatal aspect is exposed to compressive forces. In our study, the forces required to fracture the veneers (average fracture load on intact teeth 1160 N) exceeded both the maximum forces to which incisors are subjected under normal mastication (between 108-250 N25,26) and the 400 N minimum advocated by consensus.30,72 Cohesive fractures and fractures at the tooth neck are therefore a strong indicator of the quality of bond between the restorations and the enamel. As in the study by Stokes and Hood,67 in the present study, intact teeth fractured exclusively at the tooth neck. In contrast, in the clinic the most common type of fracture is the vertical root fracture. Dentin thickness and tensile stress are inversely related to root fractures.34 Root anaThe Journal of Adhesive Dentistry
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Aiming for minimally invasive dentistry, 3/4 veneers with finishing lines in enamel are functionally equal to crowns with the advantage of conserving tooth structure. Restorations with finishing lines in dentin resulted in significant loss of strength.
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tomy with oval diameter, wide root canal shape, or root concavity are most susceptible to fractures. Cervical fractures can occur due to lack of flexibility of periodontal ligament and bone. The inelastic embedding resin serves as a bending point, triggering a horizontal cervical fracture. Teeth with veneer preparations that removed tooth structure sparingly (group 2) had a combination of fracture outcomes: cervical, cohesive, and adhesive. Bonded ceramic can functionally substitute for enamel as demonstrated by cohesive fractures in the ceramic.1,6 Conversely, in only 15% of all teeth among all groups, adhesive fractures were detected primarily in groups with finishing line in enamel. When the tooth structure was reduced in the vertical dimension (group 7, veneers on teeth with attrition), a higher proportion of cervical fractures were found (Table 2). The loss of vertical support possibly increased the stress on the cervical area, leading to fracture. When the cervical area is weakened by a circumferential preparation for crowns (groups 5 and 6), the number of cervical fractures increases. In crowns with a more apical location of the finishing line in dentin, the number of cervical fractures is even higher (group 6), possibly due to the apical tapering of the tooth. As the veneers are extended interproximally to 3/4 veneers (groups 3 and 4), the proportion of cohesive fractures increases compared to labial veneers (group 2). Due to the reduction of the interproximal tooth structure, the relative load on the remaining tooth structure increases the chances for cohesive failures in dental hard tissue. The low numbers of adhesive fractures even in restorations with the finishing line in dentin suggest that the position of the finishing line does not affect the interface of ceramic to tooth. Within the limitations of this in vitro, static load study, the null hypothesis stating that ceramic veneers and bonded full ceramic crowns are able to fully restore the strength of the tooth is partially accepted for finishing lines in enamel. With cervical finishing lines in dentin, the fracture load is significantly reduced but still far above the required minimum of 400 N for clinical function. The indication of veneers can be extended to teeth with reduced vertical height in order to lengthen the crown. In agreement with the interest in conservation of sound tooth structure, modifications in veneer preparation designs may be assets in clinical practice to save sound tooth structure. Preparation of a 3/4 veneer requires the sacrifice of only half the sound tooth structure compared to tissue loss in a full crown preparation. For a labial veneer, 25% of tooth structure is saved compared to a 3/4 veneer.
opyretigal No CChun t fo ACKNOWLEDGMENTS rP ub lica This study was kindly supported by Ivoclar Vivadent, Schaan, Liechtti enstein. The Empress veneers were fabricated in collaboration with on te the dental laboratory Locke, Berlin, Germany. We are grateful ss etonMs. ce D. Meyer and Ms. B. Haase for their valuable technical skills. The photographs and drawings were prepared with assistance of Mr. R. Hoey and Mr. Lorenz.
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Clinical relevance: For anterior restorations, veneers and 3/4 veneers should be considered as treatment options as long as the finishing line ends in enamel. Advantages are esthetics and sparing tooth structure from preparations.
The Journal of Adhesive Dentistry
