title: Influence of different convergence angles and tooth preparation heights on the internal adaptation of Cerec crowns.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  This study evaluated the influence of different convergence angles and tooth preparation heights on the internal adaptation of Cerec crowns.
  
  
  METHODS
  Tooth preparations were made on typodont teeth with different combinations of convergence angles and occlusal-cervical heights: Group I = 20 degrees angle, 6 mm height; Group II = 20 degrees angle, 4 mm height; Group III = 12 degrees angle, 6 mm height; and Group IV = 12 degrees 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 degrees convergence angle demonstrated the best internal fit (cement space in Groups III and IV = 121 +/- 41 microm and 115 +/- 42 microm, respectively). The difference between the 2 convergence types was within the range of the scanning error (25 microm) 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 degrees and 20 degrees convergence angles also had no significant effect on internal fit (P>.0083). Cement space at distal walls (185 +/- 28 microm) was the thickest among all axial walls (P=.0001) and was twice as thick as that at the facial (90 +/- 14 microm) and palatal walls (92 +/- 15 microm).
  
  
  CONCLUSIONS
  Within the limitations of this study, there was little difference in the internal fit of Cerec crowns prepared with convergence angles of 12 degrees and 20 degrees. 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.
authors.affiliation: Department of Prosthodontics, School of Dentistry, National Taiwan University, Taipei, Taiwan.
authors.forename: Shu-Hui
authors.initials: SH
authors.lastname: Mou
authors.forename: Tsongi
authors.initials: T
authors.lastname: Chai
authors.forename: Juo-Song
authors.initials: JS
authors.lastname: Wang
authors.forename: Yuh-Yuan
authors.initials: YY
authors.lastname: Shiau
mesh_terms: D000465:Algorithms; D000704:Analysis of Variance; D002516:Ceramics / Q000737:chemistry*; D017076:Computer-Aided Design / Q000295:instrumentation; D003442:Crowns; D003738:Dental Cements / Q000737:chemistry; D003776:Dental Porcelain / Q000737:chemistry*; D017267:Dental Prosthesis Design; D006801:Humans; D007091:Image Processing, Computer-Assisted; D003765:Models, Dental; D008963:Molar; D016628:Replica Techniques; D013223:Statistics as Topic; D013499:Surface Properties; D019231:Tooth Preparation, Prosthodontic / Q000379:methods*
chemical_list: D003738:Dental Cements; D003776:Dental Porcelain; C074543:Cerec