BDS4 principles of Crown prep Flashcards
when to provide extra coronal restorations?
- protect weakened tooth structure
- improve or restore aesthetics
- retainer for bridge
- design of RPD
- Restore tooth function
what are principles of tooth prep?
1- Preservation of tooth structure
2- Retention and resistance
3- Structural durability
4- Marginal integrity
5- Preservation of the periodontium
6- Aesthetic considerations
what is involved with preservation of tooth structure?
- where possible preserve sound tooth structure
- balance against needs for retention, resistance and structural durability
what does under prep result in?
Poor aesthetics
Over built crown with periodontal and occlusal consequences
what does over prep result in?
Pulp and tooth strength being compromised
what is retention for prep? how is it improved?
- Prevents removal of the restoration along the path of insertion or the long axis of the tooth preparation
- limit number of paths of insertion
what is resistance for prep?
Prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces
what is involved with retention and resistance of prep stage?
- taper - 6 degrees
- length walls
- extra means retention (grooves, slots)
- path of insertion
effect of longer walls?
interefere with tipping displacement
what must be decided before prep?
path of insertion
- imaginary line - where rest will be placed and removed from prep
- all features of prep must coincide with line
what is structural durability stage and how is it acheived?
- rest must contain bulk of material that i enough to withstand occlusal forces
- achieved through
*occlusal reduction
*functional cusp bevel
*axial reduction
what are finish line configurations for marginal integrity stage of prep?
(a) Knife edge
(b)Bevel
(c) Chamfer
(d)Shoulder
(e) Bevelled shoulder
what is this?
chamfer finish line
What is this?
shoulder finish line
how is the preservation of periodontium stage done?
margins of rest must be
- smooth and exposed to clean
- dentist can finish them and patient can clean
- placed at gingival margin
*potential subginigval may be required
what is this? what is size?
biological width - 2mm from CT to sulcular epithelium
what should you consider regarding aesthetic consideration stage?
smile lines
what are reductions for crown prep?
- axial reduction
- occlusal reduction
*functional cusp
*non-functional cusp
-finish line
what is reductions for metal crown
(e.g gold)
axial - 0.5mm
occlusal
*functional cusp - 1.5
*non-functional cusp - 0.5
finish line - chamfer 0.5mm
what is reductions for ceramic crown
(e.g traditional porcelain)
axial - 1mm
occlusal
*functional cusp - 1.5
*non-functional cusp - 1
finish line - shoulder (1mm)
what is reductions for MCC?
axial - 1.3mm
occlusal
*functional cusp - 1.8
*non-functional cusp - 1.3
finish line -
A) chamfer 0.5mm where only metal required
B) shoulder 1.3mm (metal 0.4mm) (porcelain 0.9mm)
what is reductions for all ceramic crowns?
(e.g porcelain bonded to aluminia or zirconia)
axial - 1.5mm
occlusal
*functional cusp - 2
*non-functional cusp - 1.5
finish line - chamfer (1-1.5mm)
what are stages in prep?
- Occlusal reduction
- Separation (long tapered diamond bur)
- Buccal reduction (prepare in 2 planes)
- Palatal or lingual reduction (diamond chamfer)
- Shoulder and chamfer finish
- Check occlusal surface and clearance
degree of taper for separation?
5-10 degrees
how do you check occlusion and what is most common reason lab can’t construct crown?
- check in ICP and excursive movement
- insufficent occlusal clearance. -lab
