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In vitro study investigating the mass of tooth structure removed following endodontic and restorative procedures
In vitro study investigating the mass of tooth structure removed following endodontic and restorative procedures Sela K.F. Hussain, BDS, MSc,a Ailbhe McDonald, BDentSc, PhD, MSc,b and David R. Moles, BDS, PhD, MScc Eastman Dental Hospital and UCL Eastman Dental Institute, London, UK Statement of problem. There is limited scientific evidence which quantifies the amount of tissue removed during endodontic and restorative procedures. Purpose. The purpose of this study was to measure and compare the mass of tissue structure removed from incisor and canine teeth following successive preparations. Material and methods. Twenty-two intact, disease- and restoration-free teeth (n=11/group) were collected from consenting patients undergoing dental extractions at Eastman Dental Hospital. The teeth were stored in 4% formaldehyde saline and successively prepared for an access opening (AC), endodontic instrumentation (EI), porcelain laminate veneer (PC), metal-ceramic (MC) crown, and post-and-core (PC) preparations. The baseline mass for each tooth was measured and recorded, in grams, at baseline and after each preparation, on a digital analytical balance. A standard protocol was applied to ensure accurate mass measurements. Repeated measures analysis of variance (ANOVA) was used to make comparisons between the incisor and canine groups (α=.05) for actual mass of tooth in milligrams, percentage of tooth mass remaining compared to baseline, and percentage decrease in mass compared to the preceding procedure. Results. The estimated marginal percentage mass lost was significantly greater (P<.001) in the incisor group compared to the canine group, as an overall trend, with the incisors losing proportionally more mass for each procedure (P<.001). Conclusions. Mean percentage of removed tooth tissue increased successively from EI, AC, PC, and PV preparation, with greatest change from the previous procedure occurring for MC crown preparation. (J Prosthet Dent 2007;98:260-269)
Clinical Implications
In a sequence of preparations, metal-ceramic crown preparations removed a greater mass of tooth structure than other preparations in endodontically treated teeth. Overall, considerable tooth structure is removed after completion of these procedures.
Teeth receiving endodontic therapy typically lose both coronal and radicular tooth structure. Thus, restorations for endodontically treated teeth
are designed to return teeth to acceptable form, function, and esthetics. Restorations should also provide an environment that protects and/or
allows healing of the periapical tissue. It is intended, therefore, that restored, endodontically treated teeth should also prevent both tooth fracture1 and
This study was a poster presentation at the annual 83rd IADR meeting, Baltimore, Md, March 2005. Specialist Registrar, Restorative Dentistry. Consultant, Restorative Dentistry. c Senior Clinical Lecturer, Health Services Research. a
b
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October 2007 endodontic failure through coronal leakage, by means of an inadequate restoration.2,3 It is accepted that the greater the amount of residual dentin, the more predictable the longevity and strength of a tooth and its restoration.4 Preservation of tooth structure to provide strength and fracture resistance is important, therefore, when restoring endodontically treated teeth.4-6 Sedgley and Messer’s5 in vitro study concluded that the apparent greater susceptibility to fracture of endodontically treated teeth was due to the cumulative loss of tooth structure during restorative and endodontic procedures. Hansen and Asmussen6 suggested that coronal flare with rotary canal preparation burs and subsequent canal instrumentation may also result in further weakening of tooth structure. This opinion concurs with the in vitro findings of Trabert et al,4 who reported that the fracture resistance of endodontically treated teeth diminishes with a decrease in dentin. Similarly, Mattison7 stated that the remaining strength of a tooth is directly related to the remaining bulk of dentin. Endodontically treated teeth have been considered to have altered physical characteristics when compared with their vital counterparts.8 This difference was initially thought to be due to dentin dehydration causing an increased brittleness in endodontically treated teeth.8 Helfer et al8 examined teeth from dogs after pulp extirpation to determine the moisture content of vital and pulpless teeth. The authors established that nonvital teeth contained 9% less moisture than their vital counterparts. However, this was shown not to be statistically significant. Papa et al9 examined the moisture content of matched vital and nonvital root-treated human teeth. The authors concluded that there was no significant difference in moisture content between extracted vital and nonvital teeth. Thus, it appears that the physical properties of roottreated dentin are not dissimilar from
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those of vital dentin, but the clinical effects of endodontic procedures on these properties remain unclear. Magne and Douglas10 concluded from an in vitro study of extracted mandibular incisors, that endodontic procedures were responsible for a 38% reduction in flexural strength of crowns. Similarly, Lang et al11 showed significant reductions in rigidity following access cavity and post preparations. The authors concluded that both substance loss and alteration of canal morphology played an important role in tooth rigidity. Ho et al12 demonstrated in an in vitro study that conservative access preparations restored with composite resin in endodontically treated mandibular incisors showed no differences in fracture resistance and modes of failure when compared to intact teeth. Thus, it may be concluded that endodontically treated anterior teeth (incisors and canines) with conservative access preparations may be restored without the need for further tooth destruction to accommodate a cast restoration. Edelhoff and Sorensen13 attempted to quantify and compare the amount of tooth structure removed from acrylic resin typodont teeth with preparations for porcelain laminate veneers, all-ceramic crowns, and conventional metal-ceramic crowns. The authors reported that there were significant differences in the amount of tooth structure removed between different preparations. Seow et al14 suggested that the amount of sound tooth structure that remains following endodontic therapy is an important factor to consider when planning the subsequent definitive restoration. The authors also indicated that it is important to preserve as much tooth structure as possible to prevent clinical fracture of a tooth, and the use of minimal preparation techniques should be adopted wherever possible. Therefore, the recommendations for restoring endodontically treated teeth are commonly based on clinical experience, as there is little evidence-
based information upon which a clinician can rely in the decision-making process.15 Thus, the aims and objectives of this in vitro study were to measure the mass of tooth structure removed in making an endodontic access cavity (AC), endodontic instrumentation (EI), porcelain laminate veneer preparation (PV), conventional metal-ceramic (MC) crown preparation, and a cast post-and-core (PC) preparation. This study also aimed to compare the mass of tooth structure removed from maxillary and mandibular central and lateral incisors (incisor group) with maxillary and mandibular canine teeth (canine group). It was considered important to measure the difference in the amount of tooth tissue removed for differing endodontic and restorative procedures for 2 different tooth groups. A certain minimal amount of tooth tissue needs to be removed for a restoration such as a porcelain laminate veneer or metalceramic crown to ensure strength and rigidity in the definitive restoration. However, there is a proportional difference in mass between maxillary and mandibular central and lateral incisors and maxillary and mandibular canine teeth, which may be clinically significant with regards to the remaining tooth tissue and subsequent longevity of a tooth. The null hypothesis was that there was no difference in the mean percentage of tooth mass removed between the 2 groups at each stage of preparation.
MATERIAL AND METHODS Prior to commencement of this study, ethical approval was sought by application to the Joint Research and Ethics Committee, Eastman Dental Institute, and the Research and Development Directorate, University College London NHS Hospital Trust. Patients were given an explanation of the research project by either the author or the dentist performing the extraction and provided with an informational leaflet before providing consent. Twenty-two intact, restora-
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Table I. Preparation design and armamentarium used for each procedure Procedure 1. Access opening
Preparation Design
Burs Used
From incisal edge, midway between
L767.9C
mesio-distal surfaces for each tooth.
(Two Striper green stripe bur; Claudius Ash, Potters Bar, Hertfordshire, UK)
2. Endodontic
Coronal preparation: step-down technique.
instrumentation
Gates Glidden No. 1 & 2 (Gates Glidden; Pulpdent Corp, Watertown, Mass). K-Flexofile size 8 (K-Flexofile; Dentsply Ltd, Weybridge, UK), to establish patency
Apical preparation: step-back technique with watch-winding motion.
K-Flexofile sizes 10, 15, 20, 25, master apical file 30, K-Flexofile sizes 35, 40, 45, 50
3. Porcelain-
Chamfer finish line1 mm coronal to CEJ.
L767.9C
laminate veneer
0.5-mm labial reduction extending interproximally.
LSP767.9VF (Two Striper, yellow stripe; Claudius Ash)
2-mm incisal reduction with 1-mm incisal overlap, with chamfer finish line. 4. Metal-ceramic
Buccal shoulder margin and lingual chamfer margin
L767.9C
crown
1 mm coronal to CEJ.
285.5VF (Two Striper, yellow stripe); Claudius Ash)
1.5-mm labial reduction extending interproximally through contact point. 0.8-mm reduction palatally/lingually,
LSP767.9VF
2-mm incisal reduction was maintained. 5. Cast post and core
3-mm coronal height retained above and followed
L722.10C (Twostriper, Premier; Claudius Ash)
contour of CEJ. Incisor group: 10-mm post length and yellow
Yellow 1-mm ParaPost (ParaPost XT system;
parallel-sided ParaPost.
Coltene/Whaledent Inc, Cuyahoga Falls, Ohio)
Canine group: 10-mm post length and black
Black 1.5-mm ParaPost (ParaPost XT system;
parallel-sided ParaPost.
Coltene/Whaledent Inc)
CEJ = cementoenamel junction
tion-free, disease-free, single-rooted teeth were collected from consenting patients undergoing dental extractions as part of orthodontic, orthognathic, or periodontal treatment at the Eastman Dental Hospital. They were divided into 2 groups (n=11), the incisor group (maxillary and mandibular central and lateral incisors) and the canine group (maxillary and mandibular canines), and stored in 4% formaldehyde-saline (35% (w/w) formaldehyde and 10% (w/w) methanol; JM Loveridge Ltd, Southampton,
Hampshire, UK) throughout the experiment. Initial preparation of the teeth involved the removal of any superficial staining, calculus, and adherent soft tissue using an ultrasonic scaler (Piezon Master 400; EMS SA, Nyon, Switzerland) and subsequent polishing with a rotary brush (White bristle; Stoddard Mfg Co, Letchworth Garden City, Hertfordshire, UK) and pumicewater mixture (Pumice powder, coarse grit; Bracon Ltd, Etchingham, East Sussex, UK). The teeth were examined
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under a microscope at x2.5 magnification (GX Microscope model C2D; GT Vision Ltd, Haverhill, Suffolk, UK) to ensure they were free from caries, restorations, crazing, and fractures. The buccolingual and mesiodistal dimensions were measured at the cementoenamel junction using a thickness gauge (Thickness gauge, series 547; Mitutoyo, Kawasaki, Japan) accurate to 0.001 mm, to ensure that teeth in each group were matched in size. The baseline mass for each tooth
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October 2007 was measured and recorded at the start of the study. All teeth were blotted for 10 minutes on absorbent paper towels (UnoDent green hand towels; Unodent, MarktSchwaben, Germany) prior to weighing on a digital analytical balance (Analytic balance, Model SI 124; BDH, London, UK), accurate to 0.0001 grams. After the completion of each stage of tooth preparation outlined in Table I, the teeth were rehydrated for 24 hours in the storage medium. They were then placed on an absorbent paper towel, with 2 size 40 paper points (Roeko Paper Points, white; Coltene/Whaledent Inc, Cuyahoga Falls, Ohio) within the root canal system and dehydrated for 10 minutes. The mass was then calculated using the digital analytical balance. This technique was proven accurate in a pilot study, evaluating the minimum time necessary to produce consistent mass measurements. Standardized preparations were completed for all teeth as shown in Table I, following a sequence of tooth preparation procedures as follows: access cavity preparation (AC), endodontic instrumentation (EI), porcelain laminate veneer preparation (PV), complete metal-ceramic (MC) crown preparation, and cast post-and-core (PC) preparation. Eleven teeth per group were prepared by a single operator. Table I shows the preparation designs, including the armamentarium used for each procedure. Vinyl polysiloxane putty (Lab-Putty; Coltene/Whaledent Inc) was used as a holding jig prior to tooth preparation. Two additional polymerized silicone (Doric Flo-Light; Davies Schottlander and Davis Ltd, Letchworth, UK) reduction guides were fabricated for each tooth, which were sectioned in a mesial-distal and buccal-lingual plane and used as reference guides to standardize tooth reduction (Fig. 1). This was achieved by measuring the distance between the tooth surface and the fitting surface of the reduction guide. This was supported by the in vitro study conducted by Brunton et al,16 who recommended the use of silicone indices
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1 Silicone holding jig for canine specimen 1 and reduction guide sectioned in mesial-distal plane prior to preparation. to produce appropriate reduction of both the labial and palatal surfaces of teeth when preparing them for porcelain laminate veneers. The authors reported that an operator’s ability to distinguish comparative depths of preparation was accurate to within 0.1 mm when using a silicone reduction guide. The access cavity was made slightly lingual to the incisal edge to allow for straight line access to the root canal, while preserving the labial enamel. A bur diameter of 1.5 mm and a length of 3 mm was used (L767.9C Two Striper green stripe bur) and held parallel to the long axis of the tooth. The bur was replaced after every fourth preparation. In this way, the access cavity preparation was standardized. Following access cavity preparation (procedure 1), each single canal was identified using a size 08 K-Flexofile, and pulpal remnants were removed using a Hedstrom size 10 and 15 file. Measurement of the working length to the apex was performed both visually and using a periapical radiograph. Canal patency was obtained and maintained throughout the instrumentation of the root canal sys-
tem for each test tooth. Canals were instrumented by hand, using a standardized watch-winding technique (45-degree rotational movement clockwise and counterclockwise, with gentle apical pressure). The coronal and apical portions of the tooth were prepared as shown in Table I, as are the preparation designs for the remaining 3 procedures. The data were transcribed to a statistical software program (SPSS 12.0.1; SPSS Inc, Chicago, Ill) to enable statistical analysis. Repeated measures analysis of variance (ANOVA) was used to make comparisons between the 2 different tooth types (incisor and canine group) and across each restorative procedure type (α=.05). Analyses were made separately for 3 outcome measures: actual mass of tooth in milligrams, percentage of tooth mass remaining compared to baseline, and percentage of decrease in mass compared to the preceding procedure. Residual and predicted values were generated and used for statistical model evaluation purposes. Model evaluation confirmed the validity of the analytical approach.
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Volume 98 Issue 4 RESULTS An example of each procedure (tooth preparation) is represented in the illustrations for Incisor 1 from the incisor group (Figs. 2 through 6). The results in Table II present the estimated marginal mean mass of the canine group and incisor group in milligrams, along with the associated 95% confidence intervals (95% CI) at baseline and at each procedural stage, as derived from the repeated measures ANOVA. The absolute mass of the incisor group was less than the canine group at baseline and following each procedure (P<.001). Also, by definition, the mass decreased within each tooth group as progressively more tooth structure was cumulatively removed with each subsequent procedure (P<.001). Table III demonstrates the estimated marginal mean values for the percentage of the original mass remaining in both tooth groups following each procedure. This standardization allows direct comparison between the 2 tooth groups. The mass lost in percentage terms is significantly greater in the incisor group than the canine group, both as an overall trend and following each sequential procedure (P<.001). Table IV presents the estimated marginal mean mass lost at each cumulative procedure, as a percentage of the mass at the preceding procedural stage for both tooth groups. The overall trend is for the incisor group to lose proportionally more mass (tooth material) at each stage as compared to the canine group (P<.003). The notable exception to this trend is at the final transition from the metalceramic (MC) crown preparation to the post-and-core (PC) preparation, where the canine group lost proportionately more material than the incisor group (P<.003 for interaction from the repeated measures ANOVA). The proportion of material sacrificed at each stage was not constant. For the incisor group, 3.9% (95% CI, 3.4% to 4.4%) of tooth structure was lost,
A
B
2 Incisor group specimen at baseline. A, Labial view. B, Proximal view.
A
B
3 Incisor group specimen showing amount of remaining tooth structure following tooth preparation for access opening. A, Lingual view. B, Incisal view.
A
B
4 Incisor group specimen showing amount of remaining tooth structure following tooth preparation for porcelain veneer preparation. A, Labial view. B, Proximal view.
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A
B
5 Incisor group specimen showing amount of remaining tooth structure following tooth preparation for metal-ceramic crown preparation. A, Labial view. B, Proximal view.
A
B
6 Incisor group specimen showing the amount of remaining tooth structure following tooth preparation for post and core preparation. A, Proximal view. B, Incisal view.
Table II. Estimated marginal mean mass (mg) at each cumulative procedural stage 95% Confidence Interval Group
Procedure
Mean (SD)
Lower
Upper
Incisor
Baseline measurement
535.9 (98.8)
417.9
6534.0
Access opening
515.1 (95.2)
400.4
629.8
Endodontic instrumentation
504.3 (94.6)
391.7
616.9
Porcelain veneer preparation
432.8 (81.7)
330.4
535.1
MC crown preparation
329.4 (65.9)
241.5
417.3
Post preparation
313.2 (61.9)
235.1
391.2
Baseline measurement
1129.3 (246.4)
1011.3
1247.4
Access opening
1103.8 (239.7)
989.1
1218.5
Endodontic instrumentation
1087.5 (234.8)
974.9
1200.1
Porcelain veneer preparation
973.0 (215.2)
870.6
1075.3
MC crown preparation
795.3 (186.4)
707.4
883.2
Post preparation
716.3 (164.2)
638.2
794.3
Canine
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Table III. Estimated marginal mean percentage of baseline mass remaining at each cumulative procedural stage 95% Confidence Interval Group
Procedure
Mean %
Lower
Upper
Incisor
Baseline measurement
100.0
100.0
100.0
Access opening
96.1
95.6
96.6
Endodontic instrumentation
94.0
93.6
94.5
Porcelain veneer preparation
80.7
79.8
81.6
MC crown preparation
61.3
59.4
63.2
Post preparation
58.3
56.2
60.4
Baseline measurement
100.0
100.0
100.0
Access opening
97.8
97.3
98.2
Endodontic instrumentation
96.3
95.9
96.8
Porcelain veneer preparation
86.1
85.2
87.0
MC crown preparation
70.4
68.4
72.3
Post preparation
63.5
61.4
65.6
Canine
and 2.2% (95% CI, 1.8% to 2.7%) was lost for the canine group when making an access opening preparation. Further tissue loss occurred in the order of 2.1% (95% CI, 1.8% to 2.4%) for the incisor group, and 1.4% (95% CI, 1.1% to 1.8%) for the canine group, when converting the access opening preparation to a complete endodontic preparation. The relative magnitude of tooth tissue loss for the porcelain laminate veneer increased to 14.2% (95% CI, 13.3% to 15.1%) for the incisor group and 10.6% (95% CI, 9.7% to 11.5%) for the canine group. The relative magnitude of tooth tissue loss for the MC crown preparation was 24.0% (95% CI, 22.2% to 25.9%) for the incisor group and 18.3% (95% CI, 16.4% to 20.2%) for the canine group.
The final procedure in the sequence, the cast post-and-core (PC) preparation, was the only one in which the canine group sacrificed proportionately more tooth tissue than the incisor group, with a further 4.9% (95% CI, 3.5% to 6.3%) tissue loss in the incisor group compared to 9.8% (95% CI, 8.4% to 11.1%) tissue loss in the canine group.
DISCUSSION The null hypothesis was rejected, as there was a difference in the mean percentage of tooth tissue removed between the 2 groups at each stage of preparation. This study showed the mass decreased within each tooth group as progressively more tooth
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structure was cumulatively removed with each successive preparation. This was in agreement with the findings by Magne et al,10 who reported an increase in tooth structure removal following each subsequent procedure (porcelain veneer, composite resin restoration, and endodontic treatment). This reduction in tooth structure resulted in an increase in mandibular incisor crown flexibility. The authors concluded that endodontic procedures were responsible for most of the loss in crown stiffness. However, if the estimated marginal mean percentage of tooth structure removed for each procedure from this study is examined, the total mean cumulative percentage of tooth structure removed for performing both endodontic pro-
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October 2007
Table IV. Estimated marginal mean percentage decrease in mass compared to preceding stage 95% Confidence Interval Group
Procedure
Mean %
Lower
Upper
Incisor
Access opening
3.9
3.4
4.4
Endodontic instrumentation
2.1
1.8
2.4
Porcelain veneer preparation
14.2
13.3
15.1
MC crown preparation
24.0
22.2
25.9
Post preparation
4.9
3.5
6.3
Access opening
2.2
1.8
2.7
Endodontic instrumentation
1.4
1.1
1.8
Porcelain veneer preparation
10.6
9.7
11.5
MC crown preparation
18.3
16.4
20.2
Post preparation
9.8
8.4
11.1
Canine
cedures successively (access cavity and endodontic instrumentation) was 6.0% (95% CI, 5.2% to 6.8%) for the incisor group and 3.6% (95% CI, 2.9% to 4.5%) for the canine group from the baseline mass (Table IV), respectively. These values are relatively small when compared to the mean percentage of tooth structure removed for the restorative preparations, such as the porcelain veneer and MC crown preparation. There was a significant difference (P<.001) for the estimated marginal mean total of tooth structure (mass) removed between the incisor and canine group as calculated by the repeated measures analysis (ANOVA) for both endodontic procedures AC and EI (Table III). Therefore, proportionately larger amounts of tooth tissue were removed from the smaller teeth (incisor group) when performing these procedures, although this may be offset by the preparation of a larger post diameter in the canine group. The reported increase in crown
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flexure due to endodontic procedures by Magne et al10 may be related to the site of hard tissue removal from a more critical area (near the cingulum of the mandibular incisor teeth) and less related to the amount removed. Similarly, Lang et al11 evaluated the effects on tooth rigidity of maxillary central incisors and different endodontic procedures: access preparations, manual instrumentation, and tapered and parallel-sided post preparations. Significant reductions in rigidity occurred after access cavity preparations and post preparations. The authors concluded that both substance loss and alteration of the canal morphology had an important role in tooth rigidity. The porcelain laminate veneer preparation removed a mean of 14.2% (95% CI, 13.3% to 15.1%) of tooth tissue for the incisor group and 10.6% (95% CI, 9.7% to 11.5%) from the canine group (Table IV). This is comparable to results reported by Edelhoff et al13 for anterior typodont
teeth. Edelhoff et al13 found that a mean of 16.7% of tooth structure was removed for a similar design of veneer preparation. However, in the present study, teeth underwent endodontic treatment prior to the porcelain laminate veneer preparation, compared to only the porcelain laminate veneer preparation in Edelhoff et al’s study. A limitation of this study was the standardized reduction for the laminate veneer preparation for both groups, limiting the reduction to the specific requirements of the material. This may not reflect a true clinical scenario, as esthetic or occlusal reasons for removing greater or lesser amounts of tooth tissue would normally be considered. The results of this study demonstrated that the porcelain laminate veneer preparation is a relatively conservative option for restoring anterior teeth, particularly for small teeth such as maxillary and mandibular central and lateral incisors. This is in agreement with findings from Magne et
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Volume 98 Issue 4 al10 and Ho et al,12 who advocate the use of porcelain laminate veneers for endodontically treated anterior teeth. Magne et al10 concluded from an in vitro study of mandibular incisors that pulpless veneered incisors performed similar to natural teeth. Similarly, in an in vitro study, Ho et al12 confirmed that mandibular endodontically treated incisors, using conservative access and porcelain veneer restorations, were able to withstand the same oblique loading as intact mandibular incisors. Thus, it seems that the loss of tooth structure associated with extensive restorative procedures in endodontically treated teeth may be important to their function, while the effect of pulpal removal on the remaining tooth may have a lesser role.4,9 The MC crown preparation was shown in the present study to remove the greatest amount of tooth structure, 24.0% (95% CI, 22.2% to 25.9%) for the incisor group and 18.3% (95% CI, 16.4%to 20.2%) for the canine group, under the current experimental design where sequential preparations were performed on the same tooth (Table IV). The mean percentage of tooth structure removed from baseline was 38.7% (95% CI, 36.8% to 40.8%) for the incisor group and 29.6% (95% CI, 27.7% to 31.6%) for the canine group (Table III). These values were again significant (P<.001), showing that more tooth structure was removed from the incisor group than the canine group, and may reflect the anatomical shape of these teeth. However, the results obtained from the present investigation following the MC crown preparation are smaller than the values reported by Edelhoff et al,13 who stated a mean removal of 71.9% for an MC crown preparation similar to that performed in this study. The large difference between the 2 studies in the mean amount of tooth structure removed may be explained by the fact that Edelhoff et al13 used typodont teeth. This suggests that typodont teeth may
not be a suitable substitute for natural teeth, which differ greatly in their anatomical shape, the presence of a pulp chamber, intertubular dentin, and dentinal tubules. Additionally, the tactile feedback of human teeth differs from typodont teeth, which may have caused a difference in the preparation procedure. One advantage of using typodont teeth would be the standardization in the size of teeth. However, by conducting this experiment on similarly sized, singlerooted natural teeth with a single canal, as well as by a single operator, the authors attempted to minimize both the amount of morphological and operator variability encountered, and, subsequently, ensured that the results obtained were as accurate as possible with regards to changes in mass. This may have resulted in conservative preparations, which could be investigated further by conducting a similar study with differing operators. Seow et al14 investigated the amount of tooth structure remaining following preparations for various all-ceramic restorations and an MC crown preparation that may be used to restore an endodontically treated maxillary second premolar. The authors used illustrations of endodontically treated maxillary second premolars in buccopalatal, mesiodistal, and occlusal sections and superimposed the outline of the various ideal intraand extracoronal preparations. They found that approximately 1.0 mm of tooth structure remained buccally and 1.6-1.8 mm palatally, following a reduction of 1.2 mm for the buccal shoulder and a 0.8-mm palatal chamfer and axial reduction for the MC crown preparation. Thus, the authors concluded that the completed crown would be largely supported by a foundation restoration, and the fracture resistance of the tooth may be compromised given the absence of remaining tooth tissue. The anatomical variations that exist between the same type of tooth, as well as alterations in preparation design due to occlusal and esthetic requirements, were not
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considered. This study examined the mass of tissue removed for both the coronal and root dentin, even though it is the coronal dentin which provides structural integrity for the prepared tooth. In this manner, it was dissimilar to the study by Edelhoff et al13 in which only the coronal dentin was weighed, following teeth preparation. Thus, it would have been interesting to calculate the mass removed from the coronal dentin only by removing the root below the cemento-enamel junction, and investigating the impact on the percentage of coronal mass removed. An unexpected finding was that the post-and-core preparation removed a further 4.9% (95% CI, 3.5% to 6.3%) and 9.8% (95% CI, 8.4% to 11.1%) of tooth tissue for the incisor and canine groups, respectively (Table IV). This equated to a mean difference from baseline of 41.7% (95% CI, 39.6% to 43.8%) for the incisor group and 36.5% (95% CI, 34.4% to 38.6%) for the canine group (Table III). Performing a post-and-core preparation with 3 mm of coronal tissue for both incisor and canine teeth may not be overly destructive of tooth structure. Post sizes used were clinically appropriate, but conservative, ensuring removal of a minimal amount of tooth tissue. Additionally, these teeth had already been prepared for MC crowns and may not have needed extensive amounts of further reduction. Lang et al11 reported a significant loss of tooth rigidity as a result of post preparation. Modification of the anatomical root canal shape to accommodate tapered and parallel-sided posts produced a large reduction in rigidity. There is general agreement in the literature that conservation of tooth structure is one of the critical aspects for increasing the longevity of a tooth and its subsequent restoration. Despite the limitations of this study, the authors are not aware of other research that has attempted to quantify the amount of tooth structure removed with both endodontic and restorative procedures in natural teeth. Further
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October 2007 investigations are needed to confirm the relative contribution of the loss of tooth structure for each design preparation to the structural rigidity of the tooth.
CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. The greatest relative amount of tooth structure was removed for the metal-ceramic (MC) crown preparation when sequential preparation was performed. 2. The estimated marginal mean mass removed following access cavity preparation, endodontic instrumentation, and laminate veneer or metalceramic crown preparations, as a percentage, was found to be significant (P<.001) when comparing the incisor and canine groups. The incisor group lost a proportionately greater amount of tooth structure (mass). 3. The mean percentage of tooth tissue removed following the cast post-and-core preparation was the only sequential procedure that resulted in the canine group losing propor-
tionately more tooth tissue than the incisor group.
REFERENCES 1. Hansen EK. In vivo cusp fracture of endodontically treated premolars restored with MOD amalgam or MOD resin fillings. Dent Mater 1988;4:169-73. 2. Swartz DB, Skidmore AE, Griffin JA. Twenty years of endodontic success and failure. J Endod 1983;9:198-202. 3. Ray HA, Trope M. Periapical status of endodontically treated teeth in relation to the technical quality of the root filling and the coronal restoration. Int Endod J 1995;28:12-8. 4. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture – a comparison of endodontic and restorative treatments. J Endod 1978;4:341-5. 5. Sedgley CM, Messer HH. Are endodontically treated teeth more brittle? J Endod 1992;18:332-5. 6. Hansen EK, Asmussen E. Efficiency of dentin-bonding agents in relation to application technique. Acta Odontol Scand 1989;42:117-20. 7. Mattison GD. Photoelastic stress analysis of cast-gold endodontic posts. J Prosthet Dent 1982;48:407-11. 8. Helfer AR, Melnick S, Schilder H. Determination of the moisture content of vital and pulpless teeth. Oral Surg Oral Med Oral Pathol 1972;34:661-70. 9. Papa J, Cain C, Messer HH. Moisture content of vital vs endodontically treated teeth. Endod Dent Traumatol 1994;10:91-3. 10.Magne P, Douglas WH. Cumulative effects of successive restorative procedures on an-
terior crown flexure: intact versus veneered incisors. Quintessence Int 2000;31:5-18. 11.Lang H, Korkmaz Y, Schneider K, Raab WH. Impact of endodontic treatments on the rigidity of the root. J Dent Res 2006;85:364-8. 12.Ho HH, Chu FC, Stokes AN. Fracture behavior of human mandibular incisors following endodontic treatment and porcelain veneer restoration. Int J Prosthodont 2001;14:260-4. 13.Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87:503-9. 14.Seow LL, Toh CG, Wilson NH. Remaining tooth structure associated with various preparation designs for endodontically treated maxillary second premolar. Eur J Prosthodont Restor Dent 2005;13:57-64. 15.McDonald A, Setchell D. Developing a tooth restorability index. Dent Update 2005;32:343-4, 346-8. 16.Brunton PA, Aminian A, Wilson NH. Tooth preparation techniques for porcelain laminate veneers. Br Dent J 2000;189:260-2. Corresponding author: Dr Sela K. F. Hussain Restorative Dentistry, Second Floor Eastman Dental Hospital and UCL EastmanDental Institute 256 Grays Inn Rd London WC1X 8LD UNITED KINGDOM Fax: 44 20 7915 1028 E-mail: [email protected] Copyright © 2007 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Comparison of the retentive characteristics of cobalt-chromium and commercially pure titanium clasps using a novel method Tse ET, Cheng LY, Luk HW, Chu FC, Chai J, Chow TW. Int J Prosthodont 2006;19:371–2. This study aimed to compare the retentive forces of cast cobalt-chromium (Co-Cr) and commercially pure titanium (cpTi) clasps. A clasp assembly comprising a pair of symmetrical clasps was made to fit the opposite halves of a hardened stainless-steel sphere. This twin clasp was designed to counterbalance the tipping forces when the clasp assembly was drawn from the sphere. A total of 120 clasp assemblies were fabricated in cast Co-Cr and cpTi and placed at undercut depths of 0.25 mm, 0.50 mm, and 0.75 mm (n = 20 for each). For Co-Cr clasps, the retentive forces at these undercuts depths were 2.34 ± 0.23 N, 4.65 ± 0.35 N, and 7.56 ± 0.50 N, respectively. The corresponding retentive forces for cpTi clasps were 1.24 ± 0.13 N, 2.34 ± 0.23 N, and 3.70 ± 0.27 N. The retentive force of cpTi clasps was approximately half that of Co-Cr clasps for the same undercut depth. Reprinted with permission of Quintessence Publishing.
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Clinical Implications
In a sequence of preparations, metal-ceramic crown preparations removed a greater mass of tooth structure than other preparations in endodontically treated teeth. Overall, considerable tooth structure is removed after completion of these procedures.
Teeth receiving endodontic therapy typically lose both coronal and radicular tooth structure. Thus, restorations for endodontically treated teeth
are designed to return teeth to acceptable form, function, and esthetics. Restorations should also provide an environment that protects and/or
allows healing of the periapical tissue. It is intended, therefore, that restored, endodontically treated teeth should also prevent both tooth fracture1 and
This study was a poster presentation at the annual 83rd IADR meeting, Baltimore, Md, March 2005. Specialist Registrar, Restorative Dentistry. Consultant, Restorative Dentistry. c Senior Clinical Lecturer, Health Services Research. a
b
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October 2007 endodontic failure through coronal leakage, by means of an inadequate restoration.2,3 It is accepted that the greater the amount of residual dentin, the more predictable the longevity and strength of a tooth and its restoration.4 Preservation of tooth structure to provide strength and fracture resistance is important, therefore, when restoring endodontically treated teeth.4-6 Sedgley and Messer’s5 in vitro study concluded that the apparent greater susceptibility to fracture of endodontically treated teeth was due to the cumulative loss of tooth structure during restorative and endodontic procedures. Hansen and Asmussen6 suggested that coronal flare with rotary canal preparation burs and subsequent canal instrumentation may also result in further weakening of tooth structure. This opinion concurs with the in vitro findings of Trabert et al,4 who reported that the fracture resistance of endodontically treated teeth diminishes with a decrease in dentin. Similarly, Mattison7 stated that the remaining strength of a tooth is directly related to the remaining bulk of dentin. Endodontically treated teeth have been considered to have altered physical characteristics when compared with their vital counterparts.8 This difference was initially thought to be due to dentin dehydration causing an increased brittleness in endodontically treated teeth.8 Helfer et al8 examined teeth from dogs after pulp extirpation to determine the moisture content of vital and pulpless teeth. The authors established that nonvital teeth contained 9% less moisture than their vital counterparts. However, this was shown not to be statistically significant. Papa et al9 examined the moisture content of matched vital and nonvital root-treated human teeth. The authors concluded that there was no significant difference in moisture content between extracted vital and nonvital teeth. Thus, it appears that the physical properties of roottreated dentin are not dissimilar from
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those of vital dentin, but the clinical effects of endodontic procedures on these properties remain unclear. Magne and Douglas10 concluded from an in vitro study of extracted mandibular incisors, that endodontic procedures were responsible for a 38% reduction in flexural strength of crowns. Similarly, Lang et al11 showed significant reductions in rigidity following access cavity and post preparations. The authors concluded that both substance loss and alteration of canal morphology played an important role in tooth rigidity. Ho et al12 demonstrated in an in vitro study that conservative access preparations restored with composite resin in endodontically treated mandibular incisors showed no differences in fracture resistance and modes of failure when compared to intact teeth. Thus, it may be concluded that endodontically treated anterior teeth (incisors and canines) with conservative access preparations may be restored without the need for further tooth destruction to accommodate a cast restoration. Edelhoff and Sorensen13 attempted to quantify and compare the amount of tooth structure removed from acrylic resin typodont teeth with preparations for porcelain laminate veneers, all-ceramic crowns, and conventional metal-ceramic crowns. The authors reported that there were significant differences in the amount of tooth structure removed between different preparations. Seow et al14 suggested that the amount of sound tooth structure that remains following endodontic therapy is an important factor to consider when planning the subsequent definitive restoration. The authors also indicated that it is important to preserve as much tooth structure as possible to prevent clinical fracture of a tooth, and the use of minimal preparation techniques should be adopted wherever possible. Therefore, the recommendations for restoring endodontically treated teeth are commonly based on clinical experience, as there is little evidence-
based information upon which a clinician can rely in the decision-making process.15 Thus, the aims and objectives of this in vitro study were to measure the mass of tooth structure removed in making an endodontic access cavity (AC), endodontic instrumentation (EI), porcelain laminate veneer preparation (PV), conventional metal-ceramic (MC) crown preparation, and a cast post-and-core (PC) preparation. This study also aimed to compare the mass of tooth structure removed from maxillary and mandibular central and lateral incisors (incisor group) with maxillary and mandibular canine teeth (canine group). It was considered important to measure the difference in the amount of tooth tissue removed for differing endodontic and restorative procedures for 2 different tooth groups. A certain minimal amount of tooth tissue needs to be removed for a restoration such as a porcelain laminate veneer or metalceramic crown to ensure strength and rigidity in the definitive restoration. However, there is a proportional difference in mass between maxillary and mandibular central and lateral incisors and maxillary and mandibular canine teeth, which may be clinically significant with regards to the remaining tooth tissue and subsequent longevity of a tooth. The null hypothesis was that there was no difference in the mean percentage of tooth mass removed between the 2 groups at each stage of preparation.
MATERIAL AND METHODS Prior to commencement of this study, ethical approval was sought by application to the Joint Research and Ethics Committee, Eastman Dental Institute, and the Research and Development Directorate, University College London NHS Hospital Trust. Patients were given an explanation of the research project by either the author or the dentist performing the extraction and provided with an informational leaflet before providing consent. Twenty-two intact, restora-
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Table I. Preparation design and armamentarium used for each procedure Procedure 1. Access opening
Preparation Design
Burs Used
From incisal edge, midway between
L767.9C
mesio-distal surfaces for each tooth.
(Two Striper green stripe bur; Claudius Ash, Potters Bar, Hertfordshire, UK)
2. Endodontic
Coronal preparation: step-down technique.
instrumentation
Gates Glidden No. 1 & 2 (Gates Glidden; Pulpdent Corp, Watertown, Mass). K-Flexofile size 8 (K-Flexofile; Dentsply Ltd, Weybridge, UK), to establish patency
Apical preparation: step-back technique with watch-winding motion.
K-Flexofile sizes 10, 15, 20, 25, master apical file 30, K-Flexofile sizes 35, 40, 45, 50
3. Porcelain-
Chamfer finish line1 mm coronal to CEJ.
L767.9C
laminate veneer
0.5-mm labial reduction extending interproximally.
LSP767.9VF (Two Striper, yellow stripe; Claudius Ash)
2-mm incisal reduction with 1-mm incisal overlap, with chamfer finish line. 4. Metal-ceramic
Buccal shoulder margin and lingual chamfer margin
L767.9C
crown
1 mm coronal to CEJ.
285.5VF (Two Striper, yellow stripe); Claudius Ash)
1.5-mm labial reduction extending interproximally through contact point. 0.8-mm reduction palatally/lingually,
LSP767.9VF
2-mm incisal reduction was maintained. 5. Cast post and core
3-mm coronal height retained above and followed
L722.10C (Twostriper, Premier; Claudius Ash)
contour of CEJ. Incisor group: 10-mm post length and yellow
Yellow 1-mm ParaPost (ParaPost XT system;
parallel-sided ParaPost.
Coltene/Whaledent Inc, Cuyahoga Falls, Ohio)
Canine group: 10-mm post length and black
Black 1.5-mm ParaPost (ParaPost XT system;
parallel-sided ParaPost.
Coltene/Whaledent Inc)
CEJ = cementoenamel junction
tion-free, disease-free, single-rooted teeth were collected from consenting patients undergoing dental extractions as part of orthodontic, orthognathic, or periodontal treatment at the Eastman Dental Hospital. They were divided into 2 groups (n=11), the incisor group (maxillary and mandibular central and lateral incisors) and the canine group (maxillary and mandibular canines), and stored in 4% formaldehyde-saline (35% (w/w) formaldehyde and 10% (w/w) methanol; JM Loveridge Ltd, Southampton,
Hampshire, UK) throughout the experiment. Initial preparation of the teeth involved the removal of any superficial staining, calculus, and adherent soft tissue using an ultrasonic scaler (Piezon Master 400; EMS SA, Nyon, Switzerland) and subsequent polishing with a rotary brush (White bristle; Stoddard Mfg Co, Letchworth Garden City, Hertfordshire, UK) and pumicewater mixture (Pumice powder, coarse grit; Bracon Ltd, Etchingham, East Sussex, UK). The teeth were examined
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under a microscope at x2.5 magnification (GX Microscope model C2D; GT Vision Ltd, Haverhill, Suffolk, UK) to ensure they were free from caries, restorations, crazing, and fractures. The buccolingual and mesiodistal dimensions were measured at the cementoenamel junction using a thickness gauge (Thickness gauge, series 547; Mitutoyo, Kawasaki, Japan) accurate to 0.001 mm, to ensure that teeth in each group were matched in size. The baseline mass for each tooth
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October 2007 was measured and recorded at the start of the study. All teeth were blotted for 10 minutes on absorbent paper towels (UnoDent green hand towels; Unodent, MarktSchwaben, Germany) prior to weighing on a digital analytical balance (Analytic balance, Model SI 124; BDH, London, UK), accurate to 0.0001 grams. After the completion of each stage of tooth preparation outlined in Table I, the teeth were rehydrated for 24 hours in the storage medium. They were then placed on an absorbent paper towel, with 2 size 40 paper points (Roeko Paper Points, white; Coltene/Whaledent Inc, Cuyahoga Falls, Ohio) within the root canal system and dehydrated for 10 minutes. The mass was then calculated using the digital analytical balance. This technique was proven accurate in a pilot study, evaluating the minimum time necessary to produce consistent mass measurements. Standardized preparations were completed for all teeth as shown in Table I, following a sequence of tooth preparation procedures as follows: access cavity preparation (AC), endodontic instrumentation (EI), porcelain laminate veneer preparation (PV), complete metal-ceramic (MC) crown preparation, and cast post-and-core (PC) preparation. Eleven teeth per group were prepared by a single operator. Table I shows the preparation designs, including the armamentarium used for each procedure. Vinyl polysiloxane putty (Lab-Putty; Coltene/Whaledent Inc) was used as a holding jig prior to tooth preparation. Two additional polymerized silicone (Doric Flo-Light; Davies Schottlander and Davis Ltd, Letchworth, UK) reduction guides were fabricated for each tooth, which were sectioned in a mesial-distal and buccal-lingual plane and used as reference guides to standardize tooth reduction (Fig. 1). This was achieved by measuring the distance between the tooth surface and the fitting surface of the reduction guide. This was supported by the in vitro study conducted by Brunton et al,16 who recommended the use of silicone indices
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1 Silicone holding jig for canine specimen 1 and reduction guide sectioned in mesial-distal plane prior to preparation. to produce appropriate reduction of both the labial and palatal surfaces of teeth when preparing them for porcelain laminate veneers. The authors reported that an operator’s ability to distinguish comparative depths of preparation was accurate to within 0.1 mm when using a silicone reduction guide. The access cavity was made slightly lingual to the incisal edge to allow for straight line access to the root canal, while preserving the labial enamel. A bur diameter of 1.5 mm and a length of 3 mm was used (L767.9C Two Striper green stripe bur) and held parallel to the long axis of the tooth. The bur was replaced after every fourth preparation. In this way, the access cavity preparation was standardized. Following access cavity preparation (procedure 1), each single canal was identified using a size 08 K-Flexofile, and pulpal remnants were removed using a Hedstrom size 10 and 15 file. Measurement of the working length to the apex was performed both visually and using a periapical radiograph. Canal patency was obtained and maintained throughout the instrumentation of the root canal sys-
tem for each test tooth. Canals were instrumented by hand, using a standardized watch-winding technique (45-degree rotational movement clockwise and counterclockwise, with gentle apical pressure). The coronal and apical portions of the tooth were prepared as shown in Table I, as are the preparation designs for the remaining 3 procedures. The data were transcribed to a statistical software program (SPSS 12.0.1; SPSS Inc, Chicago, Ill) to enable statistical analysis. Repeated measures analysis of variance (ANOVA) was used to make comparisons between the 2 different tooth types (incisor and canine group) and across each restorative procedure type (α=.05). Analyses were made separately for 3 outcome measures: actual mass of tooth in milligrams, percentage of tooth mass remaining compared to baseline, and percentage of decrease in mass compared to the preceding procedure. Residual and predicted values were generated and used for statistical model evaluation purposes. Model evaluation confirmed the validity of the analytical approach.
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Volume 98 Issue 4 RESULTS An example of each procedure (tooth preparation) is represented in the illustrations for Incisor 1 from the incisor group (Figs. 2 through 6). The results in Table II present the estimated marginal mean mass of the canine group and incisor group in milligrams, along with the associated 95% confidence intervals (95% CI) at baseline and at each procedural stage, as derived from the repeated measures ANOVA. The absolute mass of the incisor group was less than the canine group at baseline and following each procedure (P<.001). Also, by definition, the mass decreased within each tooth group as progressively more tooth structure was cumulatively removed with each subsequent procedure (P<.001). Table III demonstrates the estimated marginal mean values for the percentage of the original mass remaining in both tooth groups following each procedure. This standardization allows direct comparison between the 2 tooth groups. The mass lost in percentage terms is significantly greater in the incisor group than the canine group, both as an overall trend and following each sequential procedure (P<.001). Table IV presents the estimated marginal mean mass lost at each cumulative procedure, as a percentage of the mass at the preceding procedural stage for both tooth groups. The overall trend is for the incisor group to lose proportionally more mass (tooth material) at each stage as compared to the canine group (P<.003). The notable exception to this trend is at the final transition from the metalceramic (MC) crown preparation to the post-and-core (PC) preparation, where the canine group lost proportionately more material than the incisor group (P<.003 for interaction from the repeated measures ANOVA). The proportion of material sacrificed at each stage was not constant. For the incisor group, 3.9% (95% CI, 3.4% to 4.4%) of tooth structure was lost,
A
B
2 Incisor group specimen at baseline. A, Labial view. B, Proximal view.
A
B
3 Incisor group specimen showing amount of remaining tooth structure following tooth preparation for access opening. A, Lingual view. B, Incisal view.
A
B
4 Incisor group specimen showing amount of remaining tooth structure following tooth preparation for porcelain veneer preparation. A, Labial view. B, Proximal view.
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October 2007
A
B
5 Incisor group specimen showing amount of remaining tooth structure following tooth preparation for metal-ceramic crown preparation. A, Labial view. B, Proximal view.
A
B
6 Incisor group specimen showing the amount of remaining tooth structure following tooth preparation for post and core preparation. A, Proximal view. B, Incisal view.
Table II. Estimated marginal mean mass (mg) at each cumulative procedural stage 95% Confidence Interval Group
Procedure
Mean (SD)
Lower
Upper
Incisor
Baseline measurement
535.9 (98.8)
417.9
6534.0
Access opening
515.1 (95.2)
400.4
629.8
Endodontic instrumentation
504.3 (94.6)
391.7
616.9
Porcelain veneer preparation
432.8 (81.7)
330.4
535.1
MC crown preparation
329.4 (65.9)
241.5
417.3
Post preparation
313.2 (61.9)
235.1
391.2
Baseline measurement
1129.3 (246.4)
1011.3
1247.4
Access opening
1103.8 (239.7)
989.1
1218.5
Endodontic instrumentation
1087.5 (234.8)
974.9
1200.1
Porcelain veneer preparation
973.0 (215.2)
870.6
1075.3
MC crown preparation
795.3 (186.4)
707.4
883.2
Post preparation
716.3 (164.2)
638.2
794.3
Canine
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Table III. Estimated marginal mean percentage of baseline mass remaining at each cumulative procedural stage 95% Confidence Interval Group
Procedure
Mean %
Lower
Upper
Incisor
Baseline measurement
100.0
100.0
100.0
Access opening
96.1
95.6
96.6
Endodontic instrumentation
94.0
93.6
94.5
Porcelain veneer preparation
80.7
79.8
81.6
MC crown preparation
61.3
59.4
63.2
Post preparation
58.3
56.2
60.4
Baseline measurement
100.0
100.0
100.0
Access opening
97.8
97.3
98.2
Endodontic instrumentation
96.3
95.9
96.8
Porcelain veneer preparation
86.1
85.2
87.0
MC crown preparation
70.4
68.4
72.3
Post preparation
63.5
61.4
65.6
Canine
and 2.2% (95% CI, 1.8% to 2.7%) was lost for the canine group when making an access opening preparation. Further tissue loss occurred in the order of 2.1% (95% CI, 1.8% to 2.4%) for the incisor group, and 1.4% (95% CI, 1.1% to 1.8%) for the canine group, when converting the access opening preparation to a complete endodontic preparation. The relative magnitude of tooth tissue loss for the porcelain laminate veneer increased to 14.2% (95% CI, 13.3% to 15.1%) for the incisor group and 10.6% (95% CI, 9.7% to 11.5%) for the canine group. The relative magnitude of tooth tissue loss for the MC crown preparation was 24.0% (95% CI, 22.2% to 25.9%) for the incisor group and 18.3% (95% CI, 16.4% to 20.2%) for the canine group.
The final procedure in the sequence, the cast post-and-core (PC) preparation, was the only one in which the canine group sacrificed proportionately more tooth tissue than the incisor group, with a further 4.9% (95% CI, 3.5% to 6.3%) tissue loss in the incisor group compared to 9.8% (95% CI, 8.4% to 11.1%) tissue loss in the canine group.
DISCUSSION The null hypothesis was rejected, as there was a difference in the mean percentage of tooth tissue removed between the 2 groups at each stage of preparation. This study showed the mass decreased within each tooth group as progressively more tooth
The Journal of Prosthetic Dentistry
structure was cumulatively removed with each successive preparation. This was in agreement with the findings by Magne et al,10 who reported an increase in tooth structure removal following each subsequent procedure (porcelain veneer, composite resin restoration, and endodontic treatment). This reduction in tooth structure resulted in an increase in mandibular incisor crown flexibility. The authors concluded that endodontic procedures were responsible for most of the loss in crown stiffness. However, if the estimated marginal mean percentage of tooth structure removed for each procedure from this study is examined, the total mean cumulative percentage of tooth structure removed for performing both endodontic pro-
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October 2007
Table IV. Estimated marginal mean percentage decrease in mass compared to preceding stage 95% Confidence Interval Group
Procedure
Mean %
Lower
Upper
Incisor
Access opening
3.9
3.4
4.4
Endodontic instrumentation
2.1
1.8
2.4
Porcelain veneer preparation
14.2
13.3
15.1
MC crown preparation
24.0
22.2
25.9
Post preparation
4.9
3.5
6.3
Access opening
2.2
1.8
2.7
Endodontic instrumentation
1.4
1.1
1.8
Porcelain veneer preparation
10.6
9.7
11.5
MC crown preparation
18.3
16.4
20.2
Post preparation
9.8
8.4
11.1
Canine
cedures successively (access cavity and endodontic instrumentation) was 6.0% (95% CI, 5.2% to 6.8%) for the incisor group and 3.6% (95% CI, 2.9% to 4.5%) for the canine group from the baseline mass (Table IV), respectively. These values are relatively small when compared to the mean percentage of tooth structure removed for the restorative preparations, such as the porcelain veneer and MC crown preparation. There was a significant difference (P<.001) for the estimated marginal mean total of tooth structure (mass) removed between the incisor and canine group as calculated by the repeated measures analysis (ANOVA) for both endodontic procedures AC and EI (Table III). Therefore, proportionately larger amounts of tooth tissue were removed from the smaller teeth (incisor group) when performing these procedures, although this may be offset by the preparation of a larger post diameter in the canine group. The reported increase in crown
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flexure due to endodontic procedures by Magne et al10 may be related to the site of hard tissue removal from a more critical area (near the cingulum of the mandibular incisor teeth) and less related to the amount removed. Similarly, Lang et al11 evaluated the effects on tooth rigidity of maxillary central incisors and different endodontic procedures: access preparations, manual instrumentation, and tapered and parallel-sided post preparations. Significant reductions in rigidity occurred after access cavity preparations and post preparations. The authors concluded that both substance loss and alteration of the canal morphology had an important role in tooth rigidity. The porcelain laminate veneer preparation removed a mean of 14.2% (95% CI, 13.3% to 15.1%) of tooth tissue for the incisor group and 10.6% (95% CI, 9.7% to 11.5%) from the canine group (Table IV). This is comparable to results reported by Edelhoff et al13 for anterior typodont
teeth. Edelhoff et al13 found that a mean of 16.7% of tooth structure was removed for a similar design of veneer preparation. However, in the present study, teeth underwent endodontic treatment prior to the porcelain laminate veneer preparation, compared to only the porcelain laminate veneer preparation in Edelhoff et al’s study. A limitation of this study was the standardized reduction for the laminate veneer preparation for both groups, limiting the reduction to the specific requirements of the material. This may not reflect a true clinical scenario, as esthetic or occlusal reasons for removing greater or lesser amounts of tooth tissue would normally be considered. The results of this study demonstrated that the porcelain laminate veneer preparation is a relatively conservative option for restoring anterior teeth, particularly for small teeth such as maxillary and mandibular central and lateral incisors. This is in agreement with findings from Magne et
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Volume 98 Issue 4 al10 and Ho et al,12 who advocate the use of porcelain laminate veneers for endodontically treated anterior teeth. Magne et al10 concluded from an in vitro study of mandibular incisors that pulpless veneered incisors performed similar to natural teeth. Similarly, in an in vitro study, Ho et al12 confirmed that mandibular endodontically treated incisors, using conservative access and porcelain veneer restorations, were able to withstand the same oblique loading as intact mandibular incisors. Thus, it seems that the loss of tooth structure associated with extensive restorative procedures in endodontically treated teeth may be important to their function, while the effect of pulpal removal on the remaining tooth may have a lesser role.4,9 The MC crown preparation was shown in the present study to remove the greatest amount of tooth structure, 24.0% (95% CI, 22.2% to 25.9%) for the incisor group and 18.3% (95% CI, 16.4%to 20.2%) for the canine group, under the current experimental design where sequential preparations were performed on the same tooth (Table IV). The mean percentage of tooth structure removed from baseline was 38.7% (95% CI, 36.8% to 40.8%) for the incisor group and 29.6% (95% CI, 27.7% to 31.6%) for the canine group (Table III). These values were again significant (P<.001), showing that more tooth structure was removed from the incisor group than the canine group, and may reflect the anatomical shape of these teeth. However, the results obtained from the present investigation following the MC crown preparation are smaller than the values reported by Edelhoff et al,13 who stated a mean removal of 71.9% for an MC crown preparation similar to that performed in this study. The large difference between the 2 studies in the mean amount of tooth structure removed may be explained by the fact that Edelhoff et al13 used typodont teeth. This suggests that typodont teeth may
not be a suitable substitute for natural teeth, which differ greatly in their anatomical shape, the presence of a pulp chamber, intertubular dentin, and dentinal tubules. Additionally, the tactile feedback of human teeth differs from typodont teeth, which may have caused a difference in the preparation procedure. One advantage of using typodont teeth would be the standardization in the size of teeth. However, by conducting this experiment on similarly sized, singlerooted natural teeth with a single canal, as well as by a single operator, the authors attempted to minimize both the amount of morphological and operator variability encountered, and, subsequently, ensured that the results obtained were as accurate as possible with regards to changes in mass. This may have resulted in conservative preparations, which could be investigated further by conducting a similar study with differing operators. Seow et al14 investigated the amount of tooth structure remaining following preparations for various all-ceramic restorations and an MC crown preparation that may be used to restore an endodontically treated maxillary second premolar. The authors used illustrations of endodontically treated maxillary second premolars in buccopalatal, mesiodistal, and occlusal sections and superimposed the outline of the various ideal intraand extracoronal preparations. They found that approximately 1.0 mm of tooth structure remained buccally and 1.6-1.8 mm palatally, following a reduction of 1.2 mm for the buccal shoulder and a 0.8-mm palatal chamfer and axial reduction for the MC crown preparation. Thus, the authors concluded that the completed crown would be largely supported by a foundation restoration, and the fracture resistance of the tooth may be compromised given the absence of remaining tooth tissue. The anatomical variations that exist between the same type of tooth, as well as alterations in preparation design due to occlusal and esthetic requirements, were not
The Journal of Prosthetic Dentistry
considered. This study examined the mass of tissue removed for both the coronal and root dentin, even though it is the coronal dentin which provides structural integrity for the prepared tooth. In this manner, it was dissimilar to the study by Edelhoff et al13 in which only the coronal dentin was weighed, following teeth preparation. Thus, it would have been interesting to calculate the mass removed from the coronal dentin only by removing the root below the cemento-enamel junction, and investigating the impact on the percentage of coronal mass removed. An unexpected finding was that the post-and-core preparation removed a further 4.9% (95% CI, 3.5% to 6.3%) and 9.8% (95% CI, 8.4% to 11.1%) of tooth tissue for the incisor and canine groups, respectively (Table IV). This equated to a mean difference from baseline of 41.7% (95% CI, 39.6% to 43.8%) for the incisor group and 36.5% (95% CI, 34.4% to 38.6%) for the canine group (Table III). Performing a post-and-core preparation with 3 mm of coronal tissue for both incisor and canine teeth may not be overly destructive of tooth structure. Post sizes used were clinically appropriate, but conservative, ensuring removal of a minimal amount of tooth tissue. Additionally, these teeth had already been prepared for MC crowns and may not have needed extensive amounts of further reduction. Lang et al11 reported a significant loss of tooth rigidity as a result of post preparation. Modification of the anatomical root canal shape to accommodate tapered and parallel-sided posts produced a large reduction in rigidity. There is general agreement in the literature that conservation of tooth structure is one of the critical aspects for increasing the longevity of a tooth and its subsequent restoration. Despite the limitations of this study, the authors are not aware of other research that has attempted to quantify the amount of tooth structure removed with both endodontic and restorative procedures in natural teeth. Further
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October 2007 investigations are needed to confirm the relative contribution of the loss of tooth structure for each design preparation to the structural rigidity of the tooth.
CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. The greatest relative amount of tooth structure was removed for the metal-ceramic (MC) crown preparation when sequential preparation was performed. 2. The estimated marginal mean mass removed following access cavity preparation, endodontic instrumentation, and laminate veneer or metalceramic crown preparations, as a percentage, was found to be significant (P<.001) when comparing the incisor and canine groups. The incisor group lost a proportionately greater amount of tooth structure (mass). 3. The mean percentage of tooth tissue removed following the cast post-and-core preparation was the only sequential procedure that resulted in the canine group losing propor-
tionately more tooth tissue than the incisor group.
REFERENCES 1. Hansen EK. In vivo cusp fracture of endodontically treated premolars restored with MOD amalgam or MOD resin fillings. Dent Mater 1988;4:169-73. 2. Swartz DB, Skidmore AE, Griffin JA. Twenty years of endodontic success and failure. J Endod 1983;9:198-202. 3. Ray HA, Trope M. Periapical status of endodontically treated teeth in relation to the technical quality of the root filling and the coronal restoration. Int Endod J 1995;28:12-8. 4. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture – a comparison of endodontic and restorative treatments. J Endod 1978;4:341-5. 5. Sedgley CM, Messer HH. Are endodontically treated teeth more brittle? J Endod 1992;18:332-5. 6. Hansen EK, Asmussen E. Efficiency of dentin-bonding agents in relation to application technique. Acta Odontol Scand 1989;42:117-20. 7. Mattison GD. Photoelastic stress analysis of cast-gold endodontic posts. J Prosthet Dent 1982;48:407-11. 8. Helfer AR, Melnick S, Schilder H. Determination of the moisture content of vital and pulpless teeth. Oral Surg Oral Med Oral Pathol 1972;34:661-70. 9. Papa J, Cain C, Messer HH. Moisture content of vital vs endodontically treated teeth. Endod Dent Traumatol 1994;10:91-3. 10.Magne P, Douglas WH. Cumulative effects of successive restorative procedures on an-
terior crown flexure: intact versus veneered incisors. Quintessence Int 2000;31:5-18. 11.Lang H, Korkmaz Y, Schneider K, Raab WH. Impact of endodontic treatments on the rigidity of the root. J Dent Res 2006;85:364-8. 12.Ho HH, Chu FC, Stokes AN. Fracture behavior of human mandibular incisors following endodontic treatment and porcelain veneer restoration. Int J Prosthodont 2001;14:260-4. 13.Edelhoff D, Sorensen JA. Tooth structure removal associated with various preparation designs for anterior teeth. J Prosthet Dent 2002;87:503-9. 14.Seow LL, Toh CG, Wilson NH. Remaining tooth structure associated with various preparation designs for endodontically treated maxillary second premolar. Eur J Prosthodont Restor Dent 2005;13:57-64. 15.McDonald A, Setchell D. Developing a tooth restorability index. Dent Update 2005;32:343-4, 346-8. 16.Brunton PA, Aminian A, Wilson NH. Tooth preparation techniques for porcelain laminate veneers. Br Dent J 2000;189:260-2. Corresponding author: Dr Sela K. F. Hussain Restorative Dentistry, Second Floor Eastman Dental Hospital and UCL EastmanDental Institute 256 Grays Inn Rd London WC1X 8LD UNITED KINGDOM Fax: 44 20 7915 1028 E-mail: [email protected] Copyright © 2007 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Comparison of the retentive characteristics of cobalt-chromium and commercially pure titanium clasps using a novel method Tse ET, Cheng LY, Luk HW, Chu FC, Chai J, Chow TW. Int J Prosthodont 2006;19:371–2. This study aimed to compare the retentive forces of cast cobalt-chromium (Co-Cr) and commercially pure titanium (cpTi) clasps. A clasp assembly comprising a pair of symmetrical clasps was made to fit the opposite halves of a hardened stainless-steel sphere. This twin clasp was designed to counterbalance the tipping forces when the clasp assembly was drawn from the sphere. A total of 120 clasp assemblies were fabricated in cast Co-Cr and cpTi and placed at undercut depths of 0.25 mm, 0.50 mm, and 0.75 mm (n = 20 for each). For Co-Cr clasps, the retentive forces at these undercuts depths were 2.34 ± 0.23 N, 4.65 ± 0.35 N, and 7.56 ± 0.50 N, respectively. The corresponding retentive forces for cpTi clasps were 1.24 ± 0.13 N, 2.34 ± 0.23 N, and 3.70 ± 0.27 N. The retentive force of cpTi clasps was approximately half that of Co-Cr clasps for the same undercut depth. Reprinted with permission of Quintessence Publishing.
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