crowns

Question 1

when are crowns indicated

Answer 1

root canal treated teeth
used as retainers for bridgework
improve or restore aesthetics
protect weakened tooth structure

Question 2

what are the principles of tooth preparation

Answer 2

preservation of tooth structure
retention and resistance
structural durability
marginal integrity
preservation of periodontium
aesthetic considerations

Question 3

discuss preservation of tooth structure

Answer 3

where possible preserve sound tooth structure to avoid weakening the tooth unnecessarily and damaging the pulp
must balance the criteria for retention and resistance with structural durability
under preparation results in poor aesthetics and over built crowns
over preparation results in pulp and tooth strength being compromised
anterior teeth are prepared for crowns in three planes to be respectful of proximity to the pulp

Question 4

discuss retention and resistance in relation to crown preparation

Answer 4

retention = prevents removal of the restoration along the path of insertion or the long axis of the tooth preparation

resistance = prevents dislodgement of the restoration by forces directed in apical or oblique direction, and prevents any movement of the restoration under occlusal forces

aided by 6 degree taper, length of the walls, retentive grooves, limiting the number of paths of insertion

Question 5

discuss structural durability

Answer 5

restoration must contain a bulk of material that is adequate to withstand the forces of occlusion

achieved through occlusal reduction, functional cusp bevel, axial reduction

Question 6

discuss marginal integrity

Answer 6

finish lines can be chamfer (rounded for metal) or shoulder (right angle for ceramic)

Question 7

discuss preservation of the periodontium

Answer 7

margins of the restorations should be:
smooth and fully exposed to a cleansing action
placed where the dentist can finish them and the patient can clean them
placed at gingival margin whenever possible (sub gingival margins may be required)
respect the biological width (from the base of the gingival pocket to the alveolar crest)

Question 8

discuss aesthetic considerations for crowns

Answer 8

smile lines
metal margins of metal ceramic crowns may being to show following gingival recession

Question 9

what are the stages in crown preparation

Answer 9

occlusal reduction
- maintaining some occlusal morphology using a diamond tapered fissure bur
- most common reason for lab techs to not be able to construct a crown is insufficient occlusal clearance

separation
- using long tapered diamond bur (6 degree taper)
buccal reduction prepared in two planes (follow the original anatomy of the tooth to avoid buccal pulp horn)

palatal or lingual reduction
- prepared using diamond chamfer bur in one plane for premolars and molars (follow palatal contour for upper canines and incisors)

shoulder or chamfer margin

check occlusal surface and clearance (use amalgam plugger to ensure it is 2mm)