- Email: [email protected]
Influence of different convergence angles and tooth preparation heights on the internal adaptation of Cerec crowns
Influence of different convergence angles and tooth preparation heights on the internal adaptation of Cerec crowns Shu-Hui Mou, DDS,a Tsongi Chai, DDS, DScD,b Juo-Song Wang, DDS, MS,c and Yuh-Yuan Shiau, DDS, MSd School of Dentistry, National Taiwan University, Taipei, Taiwan Statement of problem. Because of an imagining principle called active triangulation in the Cerec system, a shadow is cast distal to the illuminated objects. This distal shadow may be enlarged when the occlusal-cervical height of the prepared tooth is increased. Depth data of the shadow are unreliable, so the internal fit of Cerec crowns has been questioned. Purpose. This study evaluated the influence of different convergence angles and tooth preparation heights on the internal adaptation of Cerec crowns. Material and methods. Tooth preparations were made on typodont teeth with different combinations of convergence angles and occlusal-cervical heights: Group I = 20° angle, 6 mm height; Group II = 20° angle, 4 mm height; Group III = 12° angle, 6 mm height; and Group IV = 12° angle, 4 mm height. Ten Cerec crowns were fabricated for each type of tooth preparation. Measurements of the internal fit were performed with the cement space replica technique and an image analysis system. Three-way analysis of variance was used to analyze the differences in cement space with different tooth preparations and the number of times that milling tools were used to prepare the Cerec crowns (P<.05). Multiple comparisons were made to evaluate differences between groups (P<.0083). Results. Cerec crowns with a 12° convergence angle demonstrated the best internal fit (cement space in Groups III and IV = 121 ± 41 µm and 115 ± 42 µm, respectively). The difference between the 2 convergence types was within the range of the scanning error (25 µm) produced by the Cerec camera. The number of times that milling tools were used had no significant effect on internal fit (P=.78). Tooth preparation height equal to or shorter than 6 mm occlusal-cervically with both 12° and 20° convergence angles also had no significant effect on internal fit (P>.0083). Cement space at distal walls (185 ± 28 µm) was the thickest among all axial walls (P=.0001) and was twice as thick as that at the facial (90 ± 14 µm) and palatal walls (92 ± 15 µm). Conclusion. Within the limitations of this study, there was little difference in the internal fit of Cerec crowns prepared with convergence angles of 12° and 20°. Distal shadows influenced the thickness of the cement spaces, particularly at the distal walls. However, tooth preparations with an occlusal-cervical height not greater than 6 mm did not exaggerate the effect of the distal shadows. (J Prosthet Dent 2002;87:248-55.)
CLINICAL IMPLICATIONS This in vitro study suggests that to obtain acceptable internal fit of a Cerec crown, the convergence angle and occlusal-cervical height of prepared teeth should not exceed 20° and 6 mm, respectively. The average internal gap of Cerec crowns recorded in this study ranged from 100 to 200 µm, with the widest gap located at the distal surface. This thick or uneven cement space may reduce the strength of the adhesive cement and thus deserves further investigation.
T
he Cerec CAD/CAM system was designed to produce ceramic inlays, onlays, veneers, and crowns at chairside. The principal advantage of this system is its
This study was supported by the National Taiwan University Hospital (Grant No. NTUH.S90-1500-70). aGraduate student, Department of Prosthodontics. bAssistant Professor, Department of Prosthodontics. cAssociate Professor and Chairman, Department of Prosthodontics. dProfessor, Department of Prosthodontics. 248 THE JOURNAL OF PROSTHETIC DENTISTRY
ability to provide long-lasting, tooth-colored restorations in one appointment without provisional restorations.1 However, the marginal fit and adaptation of these restorations have been criticized.2-7 A cementation width of 50 to 100 µm traditionally has been considered acceptable.2 Adhesive cementation with a 200 to 300 µm space has been mentioned without scientific evidence.2,8 However, based on a study on the bond strength of resinous cement,9 a 50to 100-µm cement space appears to be more satisfacVOLUME 87 NUMBER 3
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tory. Leinfelder et al10 suggested that the interfacial gap for resinous cement should not exceed 100 µm since poorly fitting restorations are supported mainly by the luting cement and their longevity might be jeopardized.11,12 Furthermore, thick adhesive cement under a ceramic restoration may reduce the support from the tooth, thus increasing the risk of ceramic fracture.9 Conversely, extremely thin adhesive cement layers may also negatively impact the longevity of cemented restorations.9,12,13 The contraction stress generated during polymerization is more prominent in a thin resinous cement layer. If the setting reaches a critical stress, premature debonding of the adhesive joint may be induced.12,13 The active triangulation principle is applied in the Cerec system.14 One optical path is converted from a receptor to a pattern projector. Due to the physical separation of the 2 channels, a certain amount of parallax is formed. The parallax is constant, and the resulting range-dependent shift of the pattern projected onto the object can be converted accurately into depth values.14 The Cerec camera projects a beam at a specific angle with regard to the recording axis and creates a shadow distally to the illuminated object; this is called the distal shadow phenomenon15 (Fig. 1). The shadow appears at the distal surface only. Theoretically, the effect of this shadow on tooth preparation may increase when the occlusal-cervical height of the prepared tooth is increased. Improvements in the Cerec 2 and 3 systems over Cerec 1 overcame some problems associated with resolution and milling precision and thus increased the accuracy of the marginal fit of Cerec restorations.2,16,17 However, the active triangulation principle remains a feature of both Cerec 2 and Cerec 3 (W. H. Mörmann, written communication, April 2000).17 The internal gap for a Cerec 2 anterior crown has been reported as 141 ± 21 µm.18 Thus, internal fit remains a weak point of Cerec restorations because the internal configurations of these restorations are based on images scanned from the Cerec camera, and the distal shadow problem seems unavoidable. Resistance and retention forms of crown restorations are affected by the convergence angle of prepared teeth.19 It has been demonstrated that clinicians tend to prepare teeth with convergence angles of 10° to 25°.20-23 One study found that when adhesive resinous cement was used, there was no difference in retention between teeth prepared with 0° and 10° convergence angles or between teeth prepared with 15° and 20° angles.24 With all-ceramic crowns, a 10° convergence angle has been recommended to obtain retention and retain ceramic strength with minimum tooth reduction, but convergence angles of up to 20° have been considered acceptable.19,25 The manufacturer of the Cerec system recommends a 12° MARCH 2002
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Fig. 1. Distal shadow phenomenon caused by Cerec camera imaging principle called active triangulation. (A), Beam of light projected through aperture on tooth. (B), Imaging of tooth through second aperture. (C), Distal shadow.
convergence angle, but with no consideration of the influence of tooth height. The applicability of this recommended angle has yet to be evaluated on teeth with different preparation heights. The purpose of this in vitro study was to investigate the effects of tooth preparation tapering and height on the distal shadow phenomenon and the internal adaptation of Cerec crowns.
MATERIAL AND METHODS Four maxillary right first molar typodont teeth (FDI tooth #16; KaVo Elektrotechnisches Werk GmbH, Lleutkirch im Allgäu, Germany) (Fig. 2) were prepared with a parallelometer. Teeth were assigned to 4 groups according to the convergence angle and occlusal-cervical height of the prepared tooth measured on the facial and palatal walls: Group I = 20° angle, 6 mm height; Group II = 20° angle, 4 mm height; Group III = 12° angle, 6 mm height; and Group IV = 12° angle, 4 mm height. The tooth height 249
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Fig. 2. Schematic drawings of 4 types of tooth preparation designs. Group I = 20° convergence angle, 6 mm height. Group II = 20° convergence angle, 4 mm height. Group III = 12° convergence angle, 6 mm height. Group IV = 12° convergence angle, 4 mm height.
at the center of the mesial and distal wall was 0.5 mm lower than the facial and palatal walls. The butt shoulder was 1.0 mm in width, and the finishing line was placed 0.5 mm above the gingival tissue. All preparations were completed by one operator.
Imaging and Cerec crown fabrication Cerec 2 crown software (version 1.21; Siemens AG, Bensheim, Germany) and Vita Mark II ceramic blocks (size 1-14; Vita Zahnfabrik H. Rauter GmbH and Co KG, Bad Säckingen, Germany) were used. Cerec powder (Siemens AG) was applied evenly on the prepared tooth and adjacent teeth before scanning was initiated.26 An optical impression of each tooth was made with a Cerec intraoral scanner (Siemens AG). To minimize the image distortion, the intraoral scanner was calibrated initially and then positioned with a customized fixation device while the typodont model with the prepared tooth was held on a survey table (Fig. 3).27 The distance and angulation between the scanner and the tooth were then fixed. The Correlation I program (Siemens AG) was selected for the fabrication of Cerec crowns. An image of each prepared typodont tooth (Groups I to IV) was made first, followed by an image of an unprepared standard typodont tooth. The image of the unprepared tooth served as a reference for crown fabrication. The 2 images were overlapped with the Correlation program to create a Cerec crown (Fig. 4). Ten crowns were produced for each group. 250
Fig. 3. Customized fixation device (A) held intraoral scanner (B) used to capture image of typodont tooth model (C), which was secured on surveying table (D).
Cylindrical burs 1.6 mm in diameter and milling wheels (Siemens AG) were used in the milling processes. Each milling tool and water coolant with lubricant (Siemens AG) were replaced after 5 milling processes. The dimensional setting of the expected cement space was 0 µm.
Measurement of internal fit The crown fitting surfaces and typodont teeth were cleaned with 70% alcohol and then dried with air to remove the Cerec powder and dust. The cement space replica technique28-32 (Fig. 5) was used to measure the internal fit of the Cerec crowns. Fit tester (Tokuso Fit Tester; Tokuyama Corp, Tokyo, Japan) was applied to the inner surface of each crown, which was placed on the prepared tooth. Before evaluation, a reference line was drawn on the crown and extended to the typodont tooth. This line served as an identification for correct crown placement. To simulate finger pressure applied during crown placement, a load of 3 kg was applied on the occlusal surface of the crown for 5 minutes with a customized loading jig.33 VOLUME 87 NUMBER 3
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b
Fig. 4. a, Prepared typodont tooth. b, Image of prepared tooth and design lines on Cerec monitor. M, Mesial; D, distal; B, facial; P, palatal.
The Cerec crown and fit tester were removed from the typodont tooth, and the crown was boxed at its margin with a 3.5-mm wax dam to form a stage. The bottom of the stage was flat and parallel to the crown margin. Additional silicone impression material (Aquasil Monophase; Dentsply, Konstanz, Germany) was mixed and injected onto the intaglio surface of the crown, which was coated with fit tester. Injection was continued until the wax dam was filled. Fit tester, which served as the cement space replica, is an addition silicone material, so it bonded firmly with the silicone impression material. After the material set, the crown was carefully removed from the replica. The replica specimen consisted of the cement space replica and tooth replica. The replica specimen was placed in a copper band (12 mm in diameter and length; E. Hanhnenkratt GmbH, Stein, Germany) with marks to indicate 4 axial wall locations. The copper band was filled with the silicone impression material to form a specimen cylinder. The specimen cylinders were cross-sectioned with no. 11 scalpels (Medicon Instrumente, Medicon EG, Tuttlingen, Germany). The cylinders were first cut from the stage by pushing 4.0 mm of the cylinder out of the copper band. The subsequent cuts of the cylinders were in 1.5 mm section intervals. A new scalpel was used for each section. Specimens with a nonparallel stage or an uneven cut surface were discarded. Three slice surfaces located at the cervical, middle, and occlusal thirds were selected for measurement (Fig. 6, A). An image processing and analysis system (Matro Inspector 2.2; Leica Cambridge Ltd, Cambridge, England) with a measuring accuracy of 1 µm was used to record the film thickness. Distinct color contrast of the specimens was established as the critical criterion for reliable measurement. The cement space replica was white (fit tester), which provided good contrast to MARCH 2002
Fig. 5. Flowchart of cement space replica technique.
the violet background of the silicone impression material. Measurements were accomplished by calculating the area of the cement space replica of a selected axial wall (A) and then the distance between the 2 ends (D) (Fig. 6, B). The average film thickness (T) of each axial wall was calculated by dividing the area by the distance: Cement space = T (µm) = A (µm2)/D (µm) Four measurements (4 axial walls) were recorded for each section in all 4 groups for a total of 480 measurements (4 x 3 x 10 x 4). All sectioning, observation, 251
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Fig. 6. a, Cross-section of cement space replica. b, Image of cement space replica under image processing and analysis system. White area is cement space replica of fit tester (A). Violet areas are tooth replica of Aquasil Monophase (B) and investing material of Aquasil Monophase (C). Table I. ANOVA on cement space in relation to groups, axial walls, and milling tool use Source
df
Groups Axial walls Milling tool use (5 times)
Type I SS
3 3 4
F value
10645.13 713788.96 859.35
7.26 486.75 0.44
Pr >F
.0001 .0001 .7801
Table II. Cement space of Cerec crowns at axial walls of 4 types of tooth preparations (µm) Group
I II III IV Average
Mesial
115 120 133 112 119
± ± ± ± ±
30 23 24 26 26
Distal
190 200 175 179 185
± ± ± ± ±
31 29 26 22 28
Facial
91 97 89 85 90
Palatal
Average
± 17 100 ± 210124 ± 46 ± 90 95 ± 90127 ± 46 ± 18 88 ± 16 121 ± 46 ± 11 85 ±1 90115 ± 42 ± 14 92 ± 15 122 ± 45
Group I = 20° angle, 6 mm height; Group II = 20° angle, 4 mm height; Group III = 12° angle, 6 mm height; Group IV = 12° angle, 4 mm height.
and measurement procedures were performed by one examiner.
Statistical analysis Group means and standard deviations were calculated for film thickness for each group. Three-way analysis of variance (ANOVA) was used to analyze the cement space of Cerec crowns in relation to the 4 tooth preparation designs, 4 axial walls, and 5 uses of the milling tools. Bonferroni’s multiple comparison procedure was used as a correction for multiplicity. Each two-group comparison was conducted at the α*=.05/6=.0083 level of significance. Average scores across walls and groups were used for orthogonal-contrast multiple comparisons of the differences between 252
Table III. Multiple comparisons of cement space of Cerec crowns among 4 groups Groups
df
I vs. II I vs. III I vs. IV II vs. III II vs. IV III vs. IV
1 1 1 1 1 1
Contrast sum of squares
843.75 498.81 5005.06 2640.06 9958.81 2343.75
Mean square
843.75 498.81 5005.06 2640.06 9958.81 2343.75
F value
P value*
1.78 1.05 10.54 5.56 20.96 4.93
.1833 .3061 .0013 .0188 .0001 .0268
*Bonferroni multiple comparison procedure was used as a correction for multiplicity. Each two-group comparison was conducted at the α*=.05/6=.0083 level of significance.
preparation designs, axial walls, and number of times that milling tools were used.
RESULTS Three-way ANOVA showed that variation in the internal fit of Cerec crowns was related to tooth preparation designs and the 4 axial tooth walls (P<.05) (Table I). The cement space recorded on each of the 4 axial walls for each of the 4 groups is listed in Table II. Average values were as follows: 124 ± 46 µm (Group I), 127 ± 46 µm (Group II), 121 ± 41 µm (Group III), and 115 ± 42 µm (Group IV). The average cement space was thicker in teeth with a 20° convergence angle than in teeth with a 12° angle. There was no significant difference in the cement space when different preparation heights were used with both 12°and 20° convergence angles (P>.0083). The thinnest cement space was recorded for Group IV and the thickest for Group II. The average cement space was thickest at the distal wall (185 ± 28 µm), followed by the mesial wall (119 ± 26 µm). Cement VOLUME 87 NUMBER 3
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Fig. 7. Cement spaces of Cerec crowns at four axial walls in four groups. M, Mesial; D, distal; B, facial; P, palatal.
spaces were thin at facial and palatal walls (90 ± 14 µm and 92 ± 15 µm, respectively) (Table II). Bonferroni’s multiple comparison procedure was used as a correction for multiplicity, and a test with Type I error of .05 was expected. There were no significant differences in cement spaces for Groups I and II (P=.1833), Groups I and III (P=.3061), Groups II and III (P=.0188), or Groups III and IV (P=.0268). Significant differences between Groups I and IV (P=.0013) and between Groups II and IV (P=.001) were recorded (Table III). Differences in the cement space between mesial and distal walls, mesial and facial walls, mesial and palatal walls, distal and facial walls, and distal and palatal walls were all significant (P=.001). There was no difference in the cement space between facial and palatal walls (P=.6107) (Table IV). In general, cement spaces in Groups III and IV were thinner than those in Groups I and II. Distal cement space was approximately 20 µm thinner in Groups III and IV than in Groups I and II (Table II, Fig. 7). The number of times that milling tools were used did not have significant influence on cement space for any groups (P=.7801) (Table I, Fig. 8).
DISCUSSION Cerec CAD/CAM restorations were introduced more than 15 years ago. Several researchers have criticized the marginal fit of Cerec restorations.2-7 However, improvements in the Cerec machine have made the margins more acceptable through precise operating procedures.15 Internal fit is now the main concern since internal geometry of the crown is obtained by a 3-dimensional scanning of the prepared MARCH 2002
Table IV. Multiple comparisons of cement spaces at 4 axial walls of Cerec crowns Contrast
M vs. D M vs. B M vs. P D vs. B D vs. P B vs. P
df
1 1 1 1 1 1
Contrast sum of squares
Mean square
F value
P value*
260041.66 51450.81 46537.35 542830.81 526594.01 123.26
260041.66 51450.81 46537.35 542830.81 526594.01 123.26
547.35 108.30 97.96 1142.59 1108.41 0.26
.0001 .0001 .0001 .0001 .0001 .6107
M, Mesial; D, distal; B, facial; P, palatal. *Bonferroni multiple comparison procedure was used as a correction for multiplicity. Each two-group comparison was conducted at the α* = 0.05/6 = 0.0083 level of significance.
tooth, and the scanning accuracy may not always be reliable.2,26 Bindl et al18 reported that the mean internal gap of Cerec 2 anterior crowns was 141 ± 21 µm. In the present study, the average internal gap of Cerec 2 posterior crowns was 122 ± 45 µm. The improvement may be attributed to the dimensional setting of the cement space, which was set at 0 µm in the present study and at 30 µm in Bindl et al.18 The improvement also may be related to the customized fixation device used in the present study. Since the exposure time of the Cerec camera is up to 0.2 seconds, it requires a steady hand to avoid image distortion.26 The custom camera fixation device used in this study provided a more steady scan than is possible with a hand-held technique, which is used in most clinical situations. Differences in average cement space among the 4 253
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Fig. 8. Cement space related to number of times milling tools were used in four groups.
groups were less than 12 µm; such differences may not be detectable clinically. Although it has been suggested that the distal shadow may be enlarged if the occlusal-cervical height of a prepared tooth is increased, the internal fit of Cerec crowns with preparation heights of 4 and 6 mm was similar in this study. This result may be attributable to the fact that all the heights in this study were within the measuring capability (10 mm) of the Cerec 2 camera. The internal fit of prepared teeth higher than 6 mm was not investigated because this height is rarely encountered after a 2 mm occlusal reduction of the tooth for an all-ceramic crown. The new Cerec 3 system, which has improved milling accuracy, was developed for the fabrication of Cerec crowns.17 The Cerec 3 camera makes use of the active triangulation principle with double apertures to increase the primary depth measuring range to 20 mm. Nevertheless, the distal shadow problem still remains (W.H. Mörmann, written communication, April 2000) and may still influence the internal fit of Cerec restorations. There are controversial opinions about the optimal adhesive cement space to achieve acceptable bond strength. Interfacial gaps of 200 to 300 µm can be considered acceptable but have been mentioned without scientific evidence.2 Taking into account the physical and clinical properties of resin-based luting agents, an interfacial gap of 50 to 100 µm seems to result in optimal resin cement performance.8 The average internal gap of Cerec crowns in the present study was approximately 100 to 200 µm, with the widest gap 254
located at the distal surface. The probability of bond failure due to uneven cement space remains unknown and deserves further investigation. In restorations with a wide internal gap, fine-hybrid composite luting materials with a modulus of elasticity close to that of dentin are recommended to provide high fracture resistance to the bonded restorations.8 In this study, the number of times that milling tools were used did not have a significant influence on the internal fit of Cerec crowns. This may be due to the automatic calibration system of the Cerec 2 milling machine, which can adjust the grinding procedures when the burs are not as sharp or when dust has accumulated.16 In general, a better internal fit was obtained with a tooth convergence angle of 12° than with an angle of 20°. The distal cement space in teeth with a 12° convergence angle was on average 20 µm thinner than that in teeth with a 20° angle. However, the uniformity of Cerec powder applied on each axial wall was controlled only by visual means. In a previous study, the reported error during powder application was 20 to 40 µm.27 Even with this possible error, the distal cement space was still the thickest among all axial walls. However, since the resolution of the Cerec 2 camera is 25 µm,2 any internal gap difference of less than 25 µm could represent nothing more than the scanning error. Based on the findings of this study, it is not possible to state that 12° convergence angle results in better internal fit of a Cerec crown than a 20° angle. It seems reasonable to believe that there is not much difference between these 2 convergence angles. VOLUME 87 NUMBER 3
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CONCLUSIONS Within the limitations of this study, there was little difference in the internal fit of Cerec crowns with 12° and 20° convergence angles and 4- to 6-mm axial walls. Tooth preparation height equal to or less than 6 mm occlusal-cervically provided adequate fit for Cerec crowns. The distal shadow created by the Cerec scanning system may have played a dominant role in cement thickness at the distal wall, but tooth preparation heights of 6 mm or less did not exaggerate the effect of this shadow. We acknowledge W. H. Mörmann, J. Pfeiffer, A. Schwotzer, and the Sirona Company for providing technical information. Thanks are extended to Erin Chen for assistance with the statistical analysis.
REFERENCES 1. Calamia JR. Advances in computer-aided design and computer-aided manufacture technology. In: Golub-Evans J, editor. Current opinion cosmetic dentistry. 2nd ed. Philadephia: Current Science; 1994. p. 67-73. 2. Hickel R, Dasch W, Mehl A, Kremers L. CAD/CAM—fillings of the future? Int Dent J 1997;47:247-58. 3. Siervo S, Pampalone A, Siervo P, Siervo R. Where is the gap? Machinable ceramic systems and conventional laboratory restorations at a glance. Quintessence Int 1994;25:773-9. 4. Hass M, Arnetzi G, Pertl C, Polansky R, Smetan M. Cerec and dental technicians. CAD/CIM in aesthetic dentistry—Cerec 10-year anniversary symposium. Berlin, Germany: Quintessence Publishing Co; 1996. p. 293-8. 5. Magne P, Dietschi D, Holz J. An in vitro evaluation of the internal seals of CEREC overlays. Quintessence Int 1991;22:425-41. 6. Schmalz G, Reich E, Federlin M. Gap dimension and marginal quality of Cerec inlays in vitro. In: Mörmann WH. International symposium on computer restorations. Proceedings. Chicago: Quintessence Publishing Co; 1991. p. 441-52. 7. Mörmann WH, Krejci I. Clinical and SEM evaluation of Cerec inlays after 5 years in situ. In: Mörmann WH. International symposium on computer restorations. Proceedings. Chicago: Quintessence Publishing Co; 1991. p. 25-32. 8. Mörmann WH, Bindl A, Luthy H, Rathke A. Effects of preparation and luting system on all-ceramic computer-generated crowns. Int J Prosthodont 1998;11:333-9. 9. Molin MK, Karlsson SL, Kristiansen MS. Influence of film thickness on joint bend strength of a ceramic/resin composite joint. Dent Mater 1996;12:245-9. 10. Leinfelder KF, Isenberg BP, Essig ME. A new method for generating ceramic restorations: a CAD-CAM system. J Am Dent Assoc 1989;118:703-7. 11. Sjögren G. Marginal and internal fit of four different types of ceramic inlays after luting. An in vitro study. Acta Odontol Scand 1995;53:24-8. 12. Feilzer AJ, De Gee AJ, Davidson CL. Increased wall-to-wall curing contraction in thin bonded resin layers. J Dent Res 1989;68:48-50. 13. Rees JS, Jacobsen PH. Stresses generated by luting resins during cementation of composite and ceramic inlays. J Oral Rehabil 1992;19:115-22. 14. Mörmann WH, Brandestini M. The fundamental inventive principles of Cerec CAD/CAM and other CAD/CAM methods. CAD/CIM in aesthetic dentistry—Cerec 10-year anniversary symposium. Berlin, Germany: Quintessence Publishing Co; 1996. p. 81-110.
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15. Mörmann WH, Bindl A, Richter B, Apholt W, Toth RT. CEREC computer aided design/computer integrated manufacturing. Vol. 2. Zurich, Switzerland: Foundation for the Advancement of Computer-Assisted Dentistry Publishers; 1999. p. 21. 16. Mörmann WH, Schug J. Grinding precision and accuracy of fit of CEREC 2 CAD-CIM inlays. J Am Dent Assoc 1997;128:47-53. 17. Mörmann W. The right step to Cerec 3. Int J Comput Dent 2000;3:3-4. 18. Bindl A, Windisch S, Mörmann WH. Full-ceramic CAD/CIM anterior crowns and copings. Int J Comput Dent 1999;2:97-111. 19. Dodge WW, Weed RM, Baez RJ, Buchanan RN. The effect of convergence angle on retention and resistance form. Quintessence Int 1985;16:191-4. 20. Eames WB, O’Neal SJ, Monteiro J, Miller C, Roan JD, Cohen KS. Techniques to improve the seating of castings. J Am Dent Assoc 1978;96:432-7. 21. Ohm E, Silness J. The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 1978;5:371-5. 22. Mack PJ. A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J Oral Rehabil 1980;7:255-65. 23. Nordlander J, Weir D, Stoffer W, Ochi S. The taper of clinical preparations for fixed prosthodontics. J Prosthet Dent 1988;60:148-51. 24. Sarafianou A, Kafandaris NM. Effect of convergence angle on retention of resin-bonded retainers cemented with resinous cements. J Prosthet Dent 1997;77:475-81. 25. Doyle MG, Goodacre CJ, Munoz CA, Andres CJ. The effect of tooth preparation design on the breaking strength of Dicor crowns: 3. Int J Prosthodont 1990;3:327-40. 26. Hembree JH Jr. Comparisons of fit of CAD-CAM restorations using three imaging surfaces. Quintessence Int 1995;26:145-7. 27. Wiedhahn K. The optical Cerec impression—electronic model production. Int J Comput Dent 1998;1:41-54. 28. May KB, Russel MM, Razzoog ME, Lang BR. Precision of fit: the Procera AllCeram crown. J Prosthet Dent 1998;80:394-404. 29. Boening KW, Wolf BH, Schmidt AE, Kastner K, Walter MH. Clinical fit of Procera AllCeram crowns. J Prosthet Dent 2000;84:419-24. 30. Kelly JR, Davis SH, Campbell SD. Nondestructive, three-dimensional internal fit mapping of fixed prostheses. J Prosthet Dent 1989;61:368-73. 31. McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11. 32. Harris IR, Wickens JL. A comparison of the fit of spark-eroded titanium copings and cast gold alloy copings. Int J Prosthodont 1994;7:348-55. 33. Wang CJ, Millstein PL, Nanthanson D. Effects of cement, cement space, marginal design, seating aid materials, and seating force on crown cementation. J Prosthet Dent 1992;67:786-90. Reprint requests to: DR YUH-YUAN SHIAU DEPARTMENT OF PROSTHODONTICS SCHOOL OF DENTISTRY, NATIONAL TAIWAN UNIVERSITY NO. 1 CHANG-TE ST TAIPEI, TAIWAN FAX: (886)2-2389-3211 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 + 0. 10/1/122011
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CLINICAL IMPLICATIONS This in vitro study suggests that to obtain acceptable internal fit of a Cerec crown, the convergence angle and occlusal-cervical height of prepared teeth should not exceed 20° and 6 mm, respectively. The average internal gap of Cerec crowns recorded in this study ranged from 100 to 200 µm, with the widest gap located at the distal surface. This thick or uneven cement space may reduce the strength of the adhesive cement and thus deserves further investigation.
T
he Cerec CAD/CAM system was designed to produce ceramic inlays, onlays, veneers, and crowns at chairside. The principal advantage of this system is its
This study was supported by the National Taiwan University Hospital (Grant No. NTUH.S90-1500-70). aGraduate student, Department of Prosthodontics. bAssistant Professor, Department of Prosthodontics. cAssociate Professor and Chairman, Department of Prosthodontics. dProfessor, Department of Prosthodontics. 248 THE JOURNAL OF PROSTHETIC DENTISTRY
ability to provide long-lasting, tooth-colored restorations in one appointment without provisional restorations.1 However, the marginal fit and adaptation of these restorations have been criticized.2-7 A cementation width of 50 to 100 µm traditionally has been considered acceptable.2 Adhesive cementation with a 200 to 300 µm space has been mentioned without scientific evidence.2,8 However, based on a study on the bond strength of resinous cement,9 a 50to 100-µm cement space appears to be more satisfacVOLUME 87 NUMBER 3
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tory. Leinfelder et al10 suggested that the interfacial gap for resinous cement should not exceed 100 µm since poorly fitting restorations are supported mainly by the luting cement and their longevity might be jeopardized.11,12 Furthermore, thick adhesive cement under a ceramic restoration may reduce the support from the tooth, thus increasing the risk of ceramic fracture.9 Conversely, extremely thin adhesive cement layers may also negatively impact the longevity of cemented restorations.9,12,13 The contraction stress generated during polymerization is more prominent in a thin resinous cement layer. If the setting reaches a critical stress, premature debonding of the adhesive joint may be induced.12,13 The active triangulation principle is applied in the Cerec system.14 One optical path is converted from a receptor to a pattern projector. Due to the physical separation of the 2 channels, a certain amount of parallax is formed. The parallax is constant, and the resulting range-dependent shift of the pattern projected onto the object can be converted accurately into depth values.14 The Cerec camera projects a beam at a specific angle with regard to the recording axis and creates a shadow distally to the illuminated object; this is called the distal shadow phenomenon15 (Fig. 1). The shadow appears at the distal surface only. Theoretically, the effect of this shadow on tooth preparation may increase when the occlusal-cervical height of the prepared tooth is increased. Improvements in the Cerec 2 and 3 systems over Cerec 1 overcame some problems associated with resolution and milling precision and thus increased the accuracy of the marginal fit of Cerec restorations.2,16,17 However, the active triangulation principle remains a feature of both Cerec 2 and Cerec 3 (W. H. Mörmann, written communication, April 2000).17 The internal gap for a Cerec 2 anterior crown has been reported as 141 ± 21 µm.18 Thus, internal fit remains a weak point of Cerec restorations because the internal configurations of these restorations are based on images scanned from the Cerec camera, and the distal shadow problem seems unavoidable. Resistance and retention forms of crown restorations are affected by the convergence angle of prepared teeth.19 It has been demonstrated that clinicians tend to prepare teeth with convergence angles of 10° to 25°.20-23 One study found that when adhesive resinous cement was used, there was no difference in retention between teeth prepared with 0° and 10° convergence angles or between teeth prepared with 15° and 20° angles.24 With all-ceramic crowns, a 10° convergence angle has been recommended to obtain retention and retain ceramic strength with minimum tooth reduction, but convergence angles of up to 20° have been considered acceptable.19,25 The manufacturer of the Cerec system recommends a 12° MARCH 2002
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Fig. 1. Distal shadow phenomenon caused by Cerec camera imaging principle called active triangulation. (A), Beam of light projected through aperture on tooth. (B), Imaging of tooth through second aperture. (C), Distal shadow.
convergence angle, but with no consideration of the influence of tooth height. The applicability of this recommended angle has yet to be evaluated on teeth with different preparation heights. The purpose of this in vitro study was to investigate the effects of tooth preparation tapering and height on the distal shadow phenomenon and the internal adaptation of Cerec crowns.
MATERIAL AND METHODS Four maxillary right first molar typodont teeth (FDI tooth #16; KaVo Elektrotechnisches Werk GmbH, Lleutkirch im Allgäu, Germany) (Fig. 2) were prepared with a parallelometer. Teeth were assigned to 4 groups according to the convergence angle and occlusal-cervical height of the prepared tooth measured on the facial and palatal walls: Group I = 20° angle, 6 mm height; Group II = 20° angle, 4 mm height; Group III = 12° angle, 6 mm height; and Group IV = 12° angle, 4 mm height. The tooth height 249
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Fig. 2. Schematic drawings of 4 types of tooth preparation designs. Group I = 20° convergence angle, 6 mm height. Group II = 20° convergence angle, 4 mm height. Group III = 12° convergence angle, 6 mm height. Group IV = 12° convergence angle, 4 mm height.
at the center of the mesial and distal wall was 0.5 mm lower than the facial and palatal walls. The butt shoulder was 1.0 mm in width, and the finishing line was placed 0.5 mm above the gingival tissue. All preparations were completed by one operator.
Imaging and Cerec crown fabrication Cerec 2 crown software (version 1.21; Siemens AG, Bensheim, Germany) and Vita Mark II ceramic blocks (size 1-14; Vita Zahnfabrik H. Rauter GmbH and Co KG, Bad Säckingen, Germany) were used. Cerec powder (Siemens AG) was applied evenly on the prepared tooth and adjacent teeth before scanning was initiated.26 An optical impression of each tooth was made with a Cerec intraoral scanner (Siemens AG). To minimize the image distortion, the intraoral scanner was calibrated initially and then positioned with a customized fixation device while the typodont model with the prepared tooth was held on a survey table (Fig. 3).27 The distance and angulation between the scanner and the tooth were then fixed. The Correlation I program (Siemens AG) was selected for the fabrication of Cerec crowns. An image of each prepared typodont tooth (Groups I to IV) was made first, followed by an image of an unprepared standard typodont tooth. The image of the unprepared tooth served as a reference for crown fabrication. The 2 images were overlapped with the Correlation program to create a Cerec crown (Fig. 4). Ten crowns were produced for each group. 250
Fig. 3. Customized fixation device (A) held intraoral scanner (B) used to capture image of typodont tooth model (C), which was secured on surveying table (D).
Cylindrical burs 1.6 mm in diameter and milling wheels (Siemens AG) were used in the milling processes. Each milling tool and water coolant with lubricant (Siemens AG) were replaced after 5 milling processes. The dimensional setting of the expected cement space was 0 µm.
Measurement of internal fit The crown fitting surfaces and typodont teeth were cleaned with 70% alcohol and then dried with air to remove the Cerec powder and dust. The cement space replica technique28-32 (Fig. 5) was used to measure the internal fit of the Cerec crowns. Fit tester (Tokuso Fit Tester; Tokuyama Corp, Tokyo, Japan) was applied to the inner surface of each crown, which was placed on the prepared tooth. Before evaluation, a reference line was drawn on the crown and extended to the typodont tooth. This line served as an identification for correct crown placement. To simulate finger pressure applied during crown placement, a load of 3 kg was applied on the occlusal surface of the crown for 5 minutes with a customized loading jig.33 VOLUME 87 NUMBER 3
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b
Fig. 4. a, Prepared typodont tooth. b, Image of prepared tooth and design lines on Cerec monitor. M, Mesial; D, distal; B, facial; P, palatal.
The Cerec crown and fit tester were removed from the typodont tooth, and the crown was boxed at its margin with a 3.5-mm wax dam to form a stage. The bottom of the stage was flat and parallel to the crown margin. Additional silicone impression material (Aquasil Monophase; Dentsply, Konstanz, Germany) was mixed and injected onto the intaglio surface of the crown, which was coated with fit tester. Injection was continued until the wax dam was filled. Fit tester, which served as the cement space replica, is an addition silicone material, so it bonded firmly with the silicone impression material. After the material set, the crown was carefully removed from the replica. The replica specimen consisted of the cement space replica and tooth replica. The replica specimen was placed in a copper band (12 mm in diameter and length; E. Hanhnenkratt GmbH, Stein, Germany) with marks to indicate 4 axial wall locations. The copper band was filled with the silicone impression material to form a specimen cylinder. The specimen cylinders were cross-sectioned with no. 11 scalpels (Medicon Instrumente, Medicon EG, Tuttlingen, Germany). The cylinders were first cut from the stage by pushing 4.0 mm of the cylinder out of the copper band. The subsequent cuts of the cylinders were in 1.5 mm section intervals. A new scalpel was used for each section. Specimens with a nonparallel stage or an uneven cut surface were discarded. Three slice surfaces located at the cervical, middle, and occlusal thirds were selected for measurement (Fig. 6, A). An image processing and analysis system (Matro Inspector 2.2; Leica Cambridge Ltd, Cambridge, England) with a measuring accuracy of 1 µm was used to record the film thickness. Distinct color contrast of the specimens was established as the critical criterion for reliable measurement. The cement space replica was white (fit tester), which provided good contrast to MARCH 2002
Fig. 5. Flowchart of cement space replica technique.
the violet background of the silicone impression material. Measurements were accomplished by calculating the area of the cement space replica of a selected axial wall (A) and then the distance between the 2 ends (D) (Fig. 6, B). The average film thickness (T) of each axial wall was calculated by dividing the area by the distance: Cement space = T (µm) = A (µm2)/D (µm) Four measurements (4 axial walls) were recorded for each section in all 4 groups for a total of 480 measurements (4 x 3 x 10 x 4). All sectioning, observation, 251
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Fig. 6. a, Cross-section of cement space replica. b, Image of cement space replica under image processing and analysis system. White area is cement space replica of fit tester (A). Violet areas are tooth replica of Aquasil Monophase (B) and investing material of Aquasil Monophase (C). Table I. ANOVA on cement space in relation to groups, axial walls, and milling tool use Source
df
Groups Axial walls Milling tool use (5 times)
Type I SS
3 3 4
F value
10645.13 713788.96 859.35
7.26 486.75 0.44
Pr >F
.0001 .0001 .7801
Table II. Cement space of Cerec crowns at axial walls of 4 types of tooth preparations (µm) Group
I II III IV Average
Mesial
115 120 133 112 119
± ± ± ± ±
30 23 24 26 26
Distal
190 200 175 179 185
± ± ± ± ±
31 29 26 22 28
Facial
91 97 89 85 90
Palatal
Average
± 17 100 ± 210124 ± 46 ± 90 95 ± 90127 ± 46 ± 18 88 ± 16 121 ± 46 ± 11 85 ±1 90115 ± 42 ± 14 92 ± 15 122 ± 45
Group I = 20° angle, 6 mm height; Group II = 20° angle, 4 mm height; Group III = 12° angle, 6 mm height; Group IV = 12° angle, 4 mm height.
and measurement procedures were performed by one examiner.
Statistical analysis Group means and standard deviations were calculated for film thickness for each group. Three-way analysis of variance (ANOVA) was used to analyze the cement space of Cerec crowns in relation to the 4 tooth preparation designs, 4 axial walls, and 5 uses of the milling tools. Bonferroni’s multiple comparison procedure was used as a correction for multiplicity. Each two-group comparison was conducted at the α*=.05/6=.0083 level of significance. Average scores across walls and groups were used for orthogonal-contrast multiple comparisons of the differences between 252
Table III. Multiple comparisons of cement space of Cerec crowns among 4 groups Groups
df
I vs. II I vs. III I vs. IV II vs. III II vs. IV III vs. IV
1 1 1 1 1 1
Contrast sum of squares
843.75 498.81 5005.06 2640.06 9958.81 2343.75
Mean square
843.75 498.81 5005.06 2640.06 9958.81 2343.75
F value
P value*
1.78 1.05 10.54 5.56 20.96 4.93
.1833 .3061 .0013 .0188 .0001 .0268
*Bonferroni multiple comparison procedure was used as a correction for multiplicity. Each two-group comparison was conducted at the α*=.05/6=.0083 level of significance.
preparation designs, axial walls, and number of times that milling tools were used.
RESULTS Three-way ANOVA showed that variation in the internal fit of Cerec crowns was related to tooth preparation designs and the 4 axial tooth walls (P<.05) (Table I). The cement space recorded on each of the 4 axial walls for each of the 4 groups is listed in Table II. Average values were as follows: 124 ± 46 µm (Group I), 127 ± 46 µm (Group II), 121 ± 41 µm (Group III), and 115 ± 42 µm (Group IV). The average cement space was thicker in teeth with a 20° convergence angle than in teeth with a 12° angle. There was no significant difference in the cement space when different preparation heights were used with both 12°and 20° convergence angles (P>.0083). The thinnest cement space was recorded for Group IV and the thickest for Group II. The average cement space was thickest at the distal wall (185 ± 28 µm), followed by the mesial wall (119 ± 26 µm). Cement VOLUME 87 NUMBER 3
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Fig. 7. Cement spaces of Cerec crowns at four axial walls in four groups. M, Mesial; D, distal; B, facial; P, palatal.
spaces were thin at facial and palatal walls (90 ± 14 µm and 92 ± 15 µm, respectively) (Table II). Bonferroni’s multiple comparison procedure was used as a correction for multiplicity, and a test with Type I error of .05 was expected. There were no significant differences in cement spaces for Groups I and II (P=.1833), Groups I and III (P=.3061), Groups II and III (P=.0188), or Groups III and IV (P=.0268). Significant differences between Groups I and IV (P=.0013) and between Groups II and IV (P=.001) were recorded (Table III). Differences in the cement space between mesial and distal walls, mesial and facial walls, mesial and palatal walls, distal and facial walls, and distal and palatal walls were all significant (P=.001). There was no difference in the cement space between facial and palatal walls (P=.6107) (Table IV). In general, cement spaces in Groups III and IV were thinner than those in Groups I and II. Distal cement space was approximately 20 µm thinner in Groups III and IV than in Groups I and II (Table II, Fig. 7). The number of times that milling tools were used did not have significant influence on cement space for any groups (P=.7801) (Table I, Fig. 8).
DISCUSSION Cerec CAD/CAM restorations were introduced more than 15 years ago. Several researchers have criticized the marginal fit of Cerec restorations.2-7 However, improvements in the Cerec machine have made the margins more acceptable through precise operating procedures.15 Internal fit is now the main concern since internal geometry of the crown is obtained by a 3-dimensional scanning of the prepared MARCH 2002
Table IV. Multiple comparisons of cement spaces at 4 axial walls of Cerec crowns Contrast
M vs. D M vs. B M vs. P D vs. B D vs. P B vs. P
df
1 1 1 1 1 1
Contrast sum of squares
Mean square
F value
P value*
260041.66 51450.81 46537.35 542830.81 526594.01 123.26
260041.66 51450.81 46537.35 542830.81 526594.01 123.26
547.35 108.30 97.96 1142.59 1108.41 0.26
.0001 .0001 .0001 .0001 .0001 .6107
M, Mesial; D, distal; B, facial; P, palatal. *Bonferroni multiple comparison procedure was used as a correction for multiplicity. Each two-group comparison was conducted at the α* = 0.05/6 = 0.0083 level of significance.
tooth, and the scanning accuracy may not always be reliable.2,26 Bindl et al18 reported that the mean internal gap of Cerec 2 anterior crowns was 141 ± 21 µm. In the present study, the average internal gap of Cerec 2 posterior crowns was 122 ± 45 µm. The improvement may be attributed to the dimensional setting of the cement space, which was set at 0 µm in the present study and at 30 µm in Bindl et al.18 The improvement also may be related to the customized fixation device used in the present study. Since the exposure time of the Cerec camera is up to 0.2 seconds, it requires a steady hand to avoid image distortion.26 The custom camera fixation device used in this study provided a more steady scan than is possible with a hand-held technique, which is used in most clinical situations. Differences in average cement space among the 4 253
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Fig. 8. Cement space related to number of times milling tools were used in four groups.
groups were less than 12 µm; such differences may not be detectable clinically. Although it has been suggested that the distal shadow may be enlarged if the occlusal-cervical height of a prepared tooth is increased, the internal fit of Cerec crowns with preparation heights of 4 and 6 mm was similar in this study. This result may be attributable to the fact that all the heights in this study were within the measuring capability (10 mm) of the Cerec 2 camera. The internal fit of prepared teeth higher than 6 mm was not investigated because this height is rarely encountered after a 2 mm occlusal reduction of the tooth for an all-ceramic crown. The new Cerec 3 system, which has improved milling accuracy, was developed for the fabrication of Cerec crowns.17 The Cerec 3 camera makes use of the active triangulation principle with double apertures to increase the primary depth measuring range to 20 mm. Nevertheless, the distal shadow problem still remains (W.H. Mörmann, written communication, April 2000) and may still influence the internal fit of Cerec restorations. There are controversial opinions about the optimal adhesive cement space to achieve acceptable bond strength. Interfacial gaps of 200 to 300 µm can be considered acceptable but have been mentioned without scientific evidence.2 Taking into account the physical and clinical properties of resin-based luting agents, an interfacial gap of 50 to 100 µm seems to result in optimal resin cement performance.8 The average internal gap of Cerec crowns in the present study was approximately 100 to 200 µm, with the widest gap 254
located at the distal surface. The probability of bond failure due to uneven cement space remains unknown and deserves further investigation. In restorations with a wide internal gap, fine-hybrid composite luting materials with a modulus of elasticity close to that of dentin are recommended to provide high fracture resistance to the bonded restorations.8 In this study, the number of times that milling tools were used did not have a significant influence on the internal fit of Cerec crowns. This may be due to the automatic calibration system of the Cerec 2 milling machine, which can adjust the grinding procedures when the burs are not as sharp or when dust has accumulated.16 In general, a better internal fit was obtained with a tooth convergence angle of 12° than with an angle of 20°. The distal cement space in teeth with a 12° convergence angle was on average 20 µm thinner than that in teeth with a 20° angle. However, the uniformity of Cerec powder applied on each axial wall was controlled only by visual means. In a previous study, the reported error during powder application was 20 to 40 µm.27 Even with this possible error, the distal cement space was still the thickest among all axial walls. However, since the resolution of the Cerec 2 camera is 25 µm,2 any internal gap difference of less than 25 µm could represent nothing more than the scanning error. Based on the findings of this study, it is not possible to state that 12° convergence angle results in better internal fit of a Cerec crown than a 20° angle. It seems reasonable to believe that there is not much difference between these 2 convergence angles. VOLUME 87 NUMBER 3
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CONCLUSIONS Within the limitations of this study, there was little difference in the internal fit of Cerec crowns with 12° and 20° convergence angles and 4- to 6-mm axial walls. Tooth preparation height equal to or less than 6 mm occlusal-cervically provided adequate fit for Cerec crowns. The distal shadow created by the Cerec scanning system may have played a dominant role in cement thickness at the distal wall, but tooth preparation heights of 6 mm or less did not exaggerate the effect of this shadow. We acknowledge W. H. Mörmann, J. Pfeiffer, A. Schwotzer, and the Sirona Company for providing technical information. Thanks are extended to Erin Chen for assistance with the statistical analysis.
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15. Mörmann WH, Bindl A, Richter B, Apholt W, Toth RT. CEREC computer aided design/computer integrated manufacturing. Vol. 2. Zurich, Switzerland: Foundation for the Advancement of Computer-Assisted Dentistry Publishers; 1999. p. 21. 16. Mörmann WH, Schug J. Grinding precision and accuracy of fit of CEREC 2 CAD-CIM inlays. J Am Dent Assoc 1997;128:47-53. 17. Mörmann W. The right step to Cerec 3. Int J Comput Dent 2000;3:3-4. 18. Bindl A, Windisch S, Mörmann WH. Full-ceramic CAD/CIM anterior crowns and copings. Int J Comput Dent 1999;2:97-111. 19. Dodge WW, Weed RM, Baez RJ, Buchanan RN. The effect of convergence angle on retention and resistance form. Quintessence Int 1985;16:191-4. 20. Eames WB, O’Neal SJ, Monteiro J, Miller C, Roan JD, Cohen KS. Techniques to improve the seating of castings. J Am Dent Assoc 1978;96:432-7. 21. Ohm E, Silness J. The convergence angle in teeth prepared for artificial crowns. J Oral Rehabil 1978;5:371-5. 22. Mack PJ. A theoretical and clinical investigation into the taper achieved on crown and inlay preparations. J Oral Rehabil 1980;7:255-65. 23. Nordlander J, Weir D, Stoffer W, Ochi S. The taper of clinical preparations for fixed prosthodontics. J Prosthet Dent 1988;60:148-51. 24. Sarafianou A, Kafandaris NM. Effect of convergence angle on retention of resin-bonded retainers cemented with resinous cements. J Prosthet Dent 1997;77:475-81. 25. Doyle MG, Goodacre CJ, Munoz CA, Andres CJ. The effect of tooth preparation design on the breaking strength of Dicor crowns: 3. Int J Prosthodont 1990;3:327-40. 26. Hembree JH Jr. Comparisons of fit of CAD-CAM restorations using three imaging surfaces. Quintessence Int 1995;26:145-7. 27. Wiedhahn K. The optical Cerec impression—electronic model production. Int J Comput Dent 1998;1:41-54. 28. May KB, Russel MM, Razzoog ME, Lang BR. Precision of fit: the Procera AllCeram crown. J Prosthet Dent 1998;80:394-404. 29. Boening KW, Wolf BH, Schmidt AE, Kastner K, Walter MH. Clinical fit of Procera AllCeram crowns. J Prosthet Dent 2000;84:419-24. 30. Kelly JR, Davis SH, Campbell SD. Nondestructive, three-dimensional internal fit mapping of fixed prostheses. J Prosthet Dent 1989;61:368-73. 31. McLean JW, von Fraunhofer JA. The estimation of cement film thickness by an in vivo technique. Br Dent J 1971;131:107-11. 32. Harris IR, Wickens JL. A comparison of the fit of spark-eroded titanium copings and cast gold alloy copings. Int J Prosthodont 1994;7:348-55. 33. Wang CJ, Millstein PL, Nanthanson D. Effects of cement, cement space, marginal design, seating aid materials, and seating force on crown cementation. J Prosthet Dent 1992;67:786-90. Reprint requests to: DR YUH-YUAN SHIAU DEPARTMENT OF PROSTHODONTICS SCHOOL OF DENTISTRY, NATIONAL TAIWAN UNIVERSITY NO. 1 CHANG-TE ST TAIPEI, TAIWAN FAX: (886)2-2389-3211 E-MAIL: [email protected] Copyright © 2002 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2002/$35.00 + 0. 10/1/122011
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