Tooth preparations for optimal success
Designing tooth preparations for optimal success.
Goodacre 2004, Dent Clin N Am
1-Total occlusal convergence (TOC) :
The angle formed between two opposing prepared axial surface
- 10-20 degrees posterior > anterior teeth
- Mandibular > maxillary teeth
- Fixed dental prosthesis > single crown
- If >20 degrees consider auxiliary features (grooves, boxes)
2- Occlusocervical/incisocervical dimension
- 3 mm for incisors and premolars prepared within the recommended 10 to 20.
- 4 mm for molars prepared with 10 to 20 degrees total occlusal convergence.
3- Ratio of occlusocervical/incisocervical dimension to faciolingual dimensios
- 0.4 or higher for all teeth
- If not achieved on (molars), use auxiliary features
- Tooth form
- Maxillary molars have a rhomboidal form
- Mandibular molars have a rectangular form
- Premolars and anteriors have an oval form
4- Circumferential form of the prepared tooth (anatomical reduction)
- Preserve facio-proximal and linguo-proximal corners
- Circumferential irregularities enhance resistance form
Axial grooves/ boxes should be used routinely when mandibular molars are prepared for fixed partial dentures, and they should be located on the proximal surfaces. Highly recommended to read Dr.Proussaefs article on this part.
5- Reduction Uniformity
- Makes it easier to create an esthetic restoration
- Promote development of normal contours
- Achieved by using depth grooves when preparing teeth
6- Finish line location
- Supragingival whenever esthetics permit
When subgingival finish lines are required, they should not be extended to the epithelial attachment.
7- Finish line form and reduction
- All-metal crowns : 0.3 mm deep chamfer finish line
- Metal-ceramic crowns: Finish line based on ease of formation, personal preferences
- All-ceramic crown: shoulder for cemented crowns; shoulder or chamfer for bonded
- Finish line + facial reduction depths >1mm not required when using glass ceramic
- More reduction required (>1 mm) for discolored tooth
- Finish line depths range from 0.5 – 1 mm Incisal/occlusal reduction of 2mm
8- Axial and occlusal reductions:
- All-metal crowns should be at least 0.5 mm deep and 1.0 mm deep, respectively.
- Metal-ceramic crowns: facial/axial reductions in excess of 1 mm can compromise the remaining tooth structure external to the pulp, whereas 2.0 mm of occlusal reduction is commonly achievable even on a young tooth.
- All-ceramic crowns: it is not necessary to exceed 1 mm of axial reduction with semitransluccnt systems and higher value, lower chroma shades.
- 2 mm incisal/occlusal reduction is recommended for allceramic crowns.
9- Line angle form ‘Rounded line angles’
- All ceramic crown:
- Decrease stress and increase longevity
- Metal ceramic crown , all-metal crown:
- Facilitate pouring impressions
- Investing wax patterns without trapping air bubbles
- Facilitate removal of casting nodules
10- Surface texture
- Smooth tooth preparation enhance the fit of restorations.
- Surface roughness generally increases retention with zinc phosphate cement, but its effect with adhesive cements (polycarboxylate, glass ionomer, resin).
- A reasonably smooth tooth preparation is therefore recommended.
