Since 1766, when extensive tooth structure was lost, crowns were a valid treatment option. The earliest crowns were made from seasoned white hickory wood which would be inserted into the root canal, with the moisture causing it to swell up for retention. In 1904, an all ceramic crown fabrication was created. Today, popular crown options include gold crowns, porcelain fused to metal (PFM) and all ceramic crowns (ACC). Research into how to prepare a tooth to achieve aesthetic and sustainable crowns began as early as the 1950s. Later, in 2001, Goodacre published an article describing the nine critical aspects of tooth preparation based on scientific principles for crowns.
1) Total occlusal convergence
Total occlusal convergence (TOC) describes the angle formed when the two opposing prepared axial surfaces converge. The ideal TOC angle is between two to six degrees to ensure retention against vertical displacement. Clinically, TOC between 10 and 20 degrees are more achievable and thus advised. To have good TOC, Goodacre recommends binocular vision and the use of a mirror for indirection vision. Given the limitations in vision, mandibular teeth are prepared with larger TOC than maxillary teeth, and posterior teeth with greater TOC than anteriors.
2) Occluso-cerival/ inciso-cervical dimension
The height of the crown preparation is important in resistance against horizontal forces. It is also referred to as the occluso-cervical / inciso-cervical dimension For anteriors and premolars, 3mm is the minimum height . For molars, they will need 4mm of OC. Teeth which do not meet this OC dimension will need additional resistance features such as grooves and boxes.
3) Ratio of Occlusocervical/ Incisocervical Dimension to Faciolingual Dimension
The ratio of OC: FL is to be at least 0.4 for all teeth. In other words, the wider the tooth is in FL direction, the higher OC should be to preserve that natural tooth structure.
4) Circumferential Morphology
After reducing the tooth's anatomy, all the teeth will take on different geometry. Mandibular molars look rectangular, maxillary molars look rhomboidal, premolars and anterior teeth look oval. It is important to preserve these corners of a tooth preparation. Teeth which lack these corners lack resistance to dislodgment and will require auxiliary features such as grooves and boxes. To provide resistance to the masticatory cycles and parafunctional habits, these grooves are most often added on the proximal surfaces of teeth.
5: Finish Line Location
To preserve periodontal health, the finish line location is to be supra-gingival, at least 0.4mm occlusal to the base of the gingival crevice.
Subgingival finish lines may be used if it is:
required to achieve OC dimension
encounter tooth defects such as dental caries, fractures, or erosion/ abrasion
to produce a ferrule on endodontically treated teeth
to improve aesthetics of discolour teeth.
Subgingival finish lines must not extend to epithelial attachments.
6: Finish Line Form and Depth
Recommended finish line depths for all ceramic crowns have ranged from 0.5 to 1.0mm. With the use of a semi translucent all ceramic material, it is not advantageous to have finish line depths greater than 1mm. Both chamfer and shoulder can be used with all-ceramic crowns bonded to the prepared teeth
For all porcelain fused metal crowns, finish line depths of 1.0mm or more is recommended. Chamfer finish lines of 0.3mm is deep enough for all metal crowns.
7: Axial and incisal / occlusal reduction depth
For PFM crowns, axial reduction is to be at least 0.5mm and occlusal reduction to be at least 1.0mm. Occlusal reduction of 2.0mm is achievable even in young teeth.
For all ceramic crowns, axial reduction for all ceramic crowns does not need to exceed 1.0mmm when semi-translucent. Incisal / occlusal surfaces can be reduced 2mm.
8: Line angle form
Sharp line angles create stress in the restoration so they should be rounded to preserve tooth integrity. This can also optimise fit and reduce air bubbles when pouring impressions and casting modules.
9: Surface Texture
Smooth surfaces are advised. However, surface roughness can improve crown retention with zinc phosphate luting agents, but not been true for polycarboxylate, glass ionomer, and resin luting agents are used.
APA Reference
Goodacre, C. J., Campagni, W. V., & Aquilino, S. A. (2001). Tooth preparations for complete crowns: an art form based on scientific principles. The Journal of prosthetic dentistry, 85(4), 363–376. https://doi.org/10.1067/mpr.2001.114685
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