L1: Partial ceramic restorations part I Flashcards
1
Q
Main objectives:
A
- respect tooth structure!!!
- Control your hunger for using your high speed!!!
Always remember
- We can remove huge amount of tooth structure in seconds compared to caries
2
Q
Reduce or maybe interrupt restorative cycle:
HOW?
A
- Postpone the first restoration as long as possible
- Use less aggressive excavation and caries removal methods to maintain pulp vitality
- Reduce the amount of tooth substance loss by using minimally invasive preparation and restorative techniques
- Improve the restoration seal**, **bonding**, and overall **quality for longer restoration survival
- Use a more conservative approach toward restoration replacement and maintenance by postponing, repairing, or refurbishing rather than always replacing completely
3
Q
Applied Biomechanics:
Knowledge of the mechanical properties and microstructural features of dental enamel is important:
A
- to understanding stress dissipation in the tooth
- for developing biomimetic restorative materials for the execution of clinical dental preparations
Hierarchical microstructures endow enamel unique anisotropic mechanical properties, which ensure its life-time survival in the mouth as a load bearing organ
4
Q
Enamel vs Dentin
A
Enamel:
- Densely mineralized brittle yet hard outer shell of the tooth that envelopes/engulfs the softer dentin core
- Carbonate-rich hydroxyapatite crystals are arranged in enamel rods
- The convex enamel gives strength (protects the dentin)
Dentin:
- Collagen-rich apatite reinforced bio- composite
- Resilient yet tougher than enamel
- Similar at the nanostructural level to the bone
- Unique structural architecture consisting of dentinal tubules surrounded by peritubular dentin cylinders of randomly orientated apatite crystallites, embedded in an intertubular dentin matrix
- The concave resilient dentin gives support (prevents enamel fracture)
The tooth behaves as a natural bio-composite material
5
Q
A
- To map the in plane strain distribution under compression
- There is roughly 200μm thick zone in dentin beneath the DEJ which is softer and less mineralized than the bulk of the dentin.
- In essence this layer acts as a buffer or cushion, and minimizes thermal stress.
- When the enamel tip was ground to one-half its original thickness the strain in the dentin beneath the cusp tip is higher than in the rest of the dentin adjacent to the DEJ
- Stress patterns generated by a vertical load
- Obliquely vertical on enamel
- Horizontal in the dentin
- Increased width of the fringes and where fringes are closer together is an indication of increased stress
6
Q
Cross bracing:
A
- In construction, cross bracing is a system utilized to reinforce building structures in which diagonal supports intersect.
- Cross bracing is usually seen with two diagonal supports placed in an X-shaped manner.
- Under lateral force (such as wind or seismic activity) one brace will be under tension while the other is being compressed. In steel construction, steel cables may be used due to their great
7
Q
Decussate:
A
- crossed or intersected in the form of an X;
- (verb) cross or intersect so as to form a cross;
8
Q
Gnarled enamel:
A
- They initially follow a curving path through one third of the enamel next to the DEJ.
- After that, the rods usually follow a more direct path through the remaining two thirds of the enamel to the enamel surface.
- Groups of enamel rods may entwine (twist/wrap around) with adjacent groups rods and follow a curving irregular path toward the tooth surface.
These constitute gnarled enamel
9
Q
Hunter- Schreger bands:
A
- Changes in the direction of enamel rads, which minimize the potential for fracture in the axial direction. produce an optical appearance called Hunter-Schreger bands.
- HSBS are most concentrated in regions exposed to the greatest functional demand, such as the occlusal surfaces of posterior teeth for chewing and the surfaces of maxillary and mandibular canines for guiding mandibular movement
10
Q
Describe.
A
Diagrammatic Reconstruction of a cusp
- The upper cut surface shows a concentric (simplified) Hunter-Schreger band system.
- Directions of prisms within the bands are indicated by arrows.
- The vertical cut surface shows these same bands marked in heavy lines.
- The prisms on each side of a cup-shaped segment of a Hunter-Schreger band system are oppositely directed with respect to the viewer.
- A single prism within one of the cones (indicated by the fine lines) changes its direction around the central axis of the tooth from clock wise to counterclockwise and back again as it passes from Hunter-Schreger band to Hunter Schreger band
11
Q
Vertical lamellae:
A
hypomineralised structures extending from enamel towards DEJ, contain proteins proteoclycans and lipids, stress relief, open and close during masticatory cycle
12
Q
Enamel tufts:
A
hypomineralised structures, similar role to lamellae, self heal by filling with protein rich fluids
13
Q
To unite or not to unite cavities? To extent occlusally:
A
- Premolar Teeth
- Prepared with occlusal, mesio- occlusal & MOD with different buccolingual isthmus widths (one fourth, one third and one half)
- Average depth 2.5mm
14
Q
Mondelli et al (1980):
A
All occlusal cavity preparations decrease the strength of teeth in propportion to the width of the preparation.
- The Class I occlusal preparation reduces the strength of the tooth less than the occlusal portion of the Class II preparation with equal width
- The width of the isthmus was a statistically significant factor. A desirable width of the isthmus is one-fourth the inter-cuspal width
In vitro study
- The force required to fracture teeth with occlusal and MOD cavities was determined and compared with that required to fracture sound teeth Depth 0.5 into Dentin
15
Q
Larson (1981):
Effect of Prepared Cavities on the Strength of Teeth
A
- In all instances, teeth with cavity preparations were significantly weaker
- The width of the occlusal portion of the cavity affects the strength of the crown.
- The extension of a preparation to involve proximal boxes does not significantly reduce the strength of a tooth
16
Q
A
- The occlusal cavity outline for the MOD preparations was extended approximately 1/3 the distance between the cusp tips and the pulpal floor level was 0.5 mm into dentin
- Clinical Relevance:
- MO/DO slot amalgam restorations will minimize extent of cuspal fracture compared to conventional MOD amalgam restorations in maxillary premolars
17
Q
Width vs Depth:
A
Outward bending increased dramatically with exaggerated cavity depth and width
18
Q
FRACTURES AND RESTORATIVE MATERIAL
Amaglam Vs Composite:
A
- Conclusion: Regarding restorative materials, teeth restored with resin or amalgam fractured equally. It is normally believed that teeth with amalgam restorations are more associ ated with fractures than those restored with composites
- Conclusion: The prevalence of cusp fractures in amalgam-restored teeth and resin-based composite restored teeth is not significantly different. Teeth with more than one surface restored with either resin- based composite or amalgam and teeth in older subjects were more likely to suffer a cusp fracture.
- Clinical implications: Teeth restored with amalgam and with resin-based composite exhibited equally low cusp fracture prevalence. When choosing between amalgam and resin-based composite in consideration of the likelihood of a future cusp fracture, either restorative material is acceptable
22
Q
Fracture prevention study:
A
- North Carolina GDP’s indicated that 44% of the crowns they placed were for the principle reason of fracture prevention
- When different groups of dentists examined the same patients, there was little agreement about which teeth should be crowned due to risk of fracture
24
Q
A
- Initial situation small caries lesions must be treated on the first molar(occlusal) and on both premolars (proximal)
- The finished cavity preparations. The reinforcing structures of the tooth, the oblique ridge and transverse ridges, were maintained,
- The various small cavities on the occlusal surface of the first molar were not connected.
- Maintaining these reinforcing structures contributes to the fracture resistance of the teeth
