RESEARCH AND EDUCATION Reporting numeric values of complete crowns. Part 2: Retention and resistance theories Janine Tiu, BDentTech, PGDipDentTech, a Basil Al-Amleh, BDS, DClinDent, MDipTech, PhD, b J. Neil Waddell, HDE, PGDipCDTech, MDipTech, PhD, c and Warwick J. Duncan, ED, MDS, PhD d Clinicians use tooth prepara- tion principles to maximize the retention and resistance of complete crowns. This in turn is thought to influence the longevity and survivability of the restoration. Manufacturers have provided recommenda- tions produce an ideal prepa- ration that maximizes the retention and resistance form of the prepared tooth. Ac- cording to the “Glossary of Prosthodontic Terms,” 1 reten- tion is the ability to resist removal along the path of insertion, and resistance is the ability to prevent dislodge- ment by oblique or horizontal forces. In practice, both reten- tion and resistance are closely related and are described as phenomena that cannot be separated. Several factors are under the control of the operator during tooth preparation and known to affect retention and resistance. These include the total occlusal convergence (TOC) angle, total surface area, surface roughness, preparation height and width, and auxiliary features such as boxes or grooves. 2,3 Parameters of ceramic lithium disilicate complete crown preparations by general dentists such as TOC, preparation height, and margin width were collected and reported in part 1 of this study. The TOC angles produced were always greater than the recommended angles for both anterior and posterior teeth, and preparation widths were always less than the recommended minimal width of 1 mm. In part 2 of a Doctoral student, Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. b Senior lecturer and Prosthodontist, Department of Oral Rehabilitation, Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. c Associate Professor, Department of Oral Rehabilitation, Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. d Professor, Department of Oral Sciences, Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand. ABSTRACT Statement of problem. Determining the retention and resistance of a tooth preparation for a complete crown has only existed in theory, and these theories have never been measured on tooth preparations performed in vivo. Purpose. The purpose of this study was to measure the theoretical retention and resistance of clinically produced complete crown preparations by using an objective measuring method. Material and methods. Stone dies from 236 complete crown preparations were collected from dental laboratories. The dies were scanned and analyzed with the coordinate geometry method. Cross-sectional images were captured, and the surface area was measured with a cone frustum and right truncated pyramid formula. Two different theories of resistance form, the “on” or “off” theory (limiting taper) and the linear model (resistance length), were calculated for premolar and molar preparations. Results. The mean surface areas ranged from 33.97 mm 2 to 105.44 mm 2 for the cone frustum formula and 41.75 mm 2 to 117.50 mm 2 for the right truncated pyramid formula. The facial side of maxillary premolars exhibited the highest percentage of resistance form with the limiting taper, at 58%, and the mesial side of the mandibular molars exhibited the lowest percentage of resistance form, at 6%. Conclusions. The objective method used in this study provides a way for retention and resistance theories to be tested and for further clinical implications to be investigated. (J Prosthet Dent 2015;-:---) THE JOURNAL OF PROSTHETIC DENTISTRY 1