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principles of tooth preparation
Transcript of principles of tooth preparation
leaving a lip of proximal enamel soft tissues
careful retraction of lips and cheeks to prevent damage pulp
injury can be caused by
1. temperature - blunt / bald cutting burs generate alot of heat, excessive pressure, higher rotational speeds . use highspeed handpiece with water coolant
2. chemical - irritation by luting cements, solvents, restorative resins etc.
3. bacterial action - bacteria ingress due to microleakage, or residual infected tooth structure contours
adequate axial reduction to maintain plaque control over gingival margin
duplicate contours of original tooth under most circumstances.
adequate reduction at furcal involvement areas (adequate fluting to be prepared to allow just adequate thickness of material)
and interproximal regions margin placement
supragingival margins whenever possible.
1. easy to maintain plaque control around gingival margin
2. easy to prep teeth without associated tissue trauma
3. easy impression registration
4. restoration easy to fabricate and easy to evaluate during recall
However, subgingival margins can be justified if
1. teeth in question lies within the aesthetic zone and margin is to be hidden behind the labiogingival crest.
2. dental caries, cervical erosion, or previous subgingival restorations are present, and crown lengthening is not indicated
3. proximal contacts extend to gingival crest
4. additional resistence and retention required
5. root sensitivity cannot be treated by other conservative options
6. modification of axial contour is needed. margin adaptation
smooth margins!!!! for:
1. ease of prep without overextension or unsupported enamel
2. ease of recognition of margins on impression and die
3. distinct boundary to which wax pattern should be finished
4. sufficient bulk of meta (so wax can be handled without distortion)
5. conservation of tooth structure margin geometry
smooth margins!!!! tissue preservation prevent harmful pulpal effects
usage of partial coverage crown when possible
prep of teeth with minimal taper
prep occlusal surface such that it follows the anatomic contours to give uniform thickness in restoration
retain maximum thickness of residual tooth. consider ortho for malpositioned teeth resistence retention
consideration: preventing deformation of restoration geometry of tooth prep
essentially cylindrical : occlusal axial reduction should follow gingival margin geometry
small taper of preparation will prevent movement of cemented restoration
crowns with tall axial walls are more retentive
round sharp edges of prep to minimize stresses concentrated at these areas.
retention of complete crown > partial crown material being cemented
fitting surface adhered better to amalgam than CR roughness of fitting surface.
air-abrading fitting surface with 50micron alumina luting agent
adhesive cements most retentive film thickness of luting agent
conflicting evidence magnitude and direction of dislodging forces.
resistence is affected if prosthesis occlusion carefully designed
bruxer and smoker might have fairly large oblique forces geometry of tooth prep
function between axial wall taper, preparation diameter and height
molar teeth require more parallel prep tha premolars due to larger diameter: height physical properties of luting agent
compressive strength and modulus of elasticity
GIC and resin have higher compressive strength
temperature affects ZOE compressive strength
zinc phosphate has highest modulus of elasticity alloy selection
type III or IV alloys with lower gold content but higher strength and hardness adequate tooth reduction
minimum reduction of 1.5 mm over functional cusps for alloy
sufficient occlusal clearance magins
kept 1.5 mm away from occlusal contacts all ceramic metal-ceramic characteristics:
shoulder/ heavy chamfer margin
additional palatal reduction require
contraindicated on teeth with thin labio-lingual width facial reduction
minimum thickness for optimum appearance is 1.5mm
needed to create a sense of color depth and translucency
cervical and incisal thirds commonly encounter shade problems
in thin teeth, less than ideal appearance may have to be accepted
two planes, or insufficient reduction for in cervical or incisal area results. incisal reduction
incisal reduction of 2mm is recommended for good aesthetics proximal reduction
determined by the location of the metal-ceramic junction.
anterior teeth looks most natural without metal backing. labial margin placement
subgingival for patient with high lip line and when usage of metal collar margin is contemplated
metal can be hidden below the gingival crest. although if gingival biotype is thin, metal can show through and result in some discoloration.