principle of tooth preparation

indications for an indirect restoration (7)

• missing cusp(s)

• gross caries causing an unsupported cusp

• protection of posterior ecodontically treated teeth

• caries associated w a pre-existing indirect restoration causing weakened cusps

• worn teeth w moderate to severe dentin exposure

• cracked teeth (to encircle tooth)

• complete crowns are indicated on teeth w 5 affected surfaces 

contraindications for indrect restorations (4)

• pt w uncontrolled high caries risk

• to remove sound tooth structure that could be preserved w a direct restoration or be remineralized

• pt w TMD symptoms (pain) should first have such concerns addressed prior to tooth preparation

• teeth w a poor or guarded prognosis (e.g. loss of a tooth is anticipated within ~5 yrs due to periodontal disease)

what are the 3 things to consider in crown preparation

• biologic

• mechanical

• esthetic

biologic considerations in crown preparations (5)

• conservation of tooth structure

• avoidances of over-contouring/over-prepping

• supragingival margin

• harmonious occlusion

• protection against tooth fx

what structures should you be aware of preventing damage to during a prep 

• adjacent teeth 

• soft tissues 

• pulp 

what is the usual cause of injury to soft and hard tissues during preps

• temperature 

• chemical action 

• bacterial action 

what can cause heat by friction during a crown prep

• pressure 

• speed 

• bur 

  • type 

  • shape and size 

  • condition- make sure not worn down 

purpose of water spray

• helps avoid overheating tooth 

• prevents tooth desiccation 

• removes debris, avoiding bur clogging, and avoids a reduction in cutting efficiency  

which groups can we guess used no water 

Group II and IV → can notice an inc in temperature 

the remaining dentin thickness is inversely proportional to the _______ _______ according to Sheltzer 

the pulpal response 

how do the number of dentinal tubules change as you move from the DEJ, closer to the pulp, and why is this important 

number significantly increases; the more you prep → the more dentinal tubules get exposed → the more likely your pt is to experience sensitivity 

what is the finish line referring to 

a boundary surface of the actual tooth preperation

what is the margin referring to

the outer edge of an indirect restoration e.i. crown, inlay, onlay, etc

whenever possible, the finish line/crown margin should be _________ (subgingival/supragingival)

supragingival

advantages of placing your margin supragingival (8)

• improved bonding w no cervicular seepage 

• preservation of cervical tooth structure maintains structural integrity of the abutment 

• a dental dam could be used if indicated 

• they can be easily finished w/o associated soft tissue trauma 

• more likely to be kept plaque free 

• impressions are more easily made, w less potential for soft tissue damage 

• restoration can be easily evaluated at the time of placement and at recall apts 

• elevation of the restorative margin eliminates potential risks of chronic periodontal complications contributed by restorative dentistry 

what are the indications for subgingival margin placement (6)

• dental caries, cervical erosion, or restorations extend subgingivally and crown lengthening procedure is contraindicated 

• proximal contact area extends apically to the level of the gingival crest 

• additional retention, resistance, or both are need 

• the margin of an esthetic restoration is to be hidden behind the labiogingival crest 

• root sensitivity cannot be controlled by more conservative procedures 

• axial contour modification is indicated 

what is the biological width

dimension of the soft tissue, that is attached to the portion of the tooth coronal to the crest of the alveolar bone

placing restorative margins within the biollgic width frequently leads to…

gingival inflammation, clinical attachment loss, and bone loss 

a minimum of ____ mm of space is needed between the retorativemargins and the alveolar bone 

3 mm 

what is an inlay

an intracoronal cast restoration, does NOT include cusp coverage in restoration 

how can fx occur in an inlay 

inlays can act as a wedge during cementation or funx, but if cusps are weakened → fx will occur 

what is an onlay

a cuspal-coverage restoration

an onlay provides better ________ (retention/protection) but often lacks _______ (protection/retention)

provides better protection but often lack retention 

what indirect restoration provides the best protection against tooth fx AND has the best retention

complete crown

a complete crown can be associated w _______ ________ and _____ ________

associated w periodontal disease and poor esthetics

what are the mechanical considerations of tooth preps

• providing retention form 

• providing resistance form 

• preventing deformation of the restoration (structural durability) 

what is the path of insertion 

is an imaginary line along which the restoration will be places onto or removed from the preparation 

should the path of insertion be determined before or after the tooth is cut

before → this path should NOT encroach upon th epulp or adjacent teeth

what is the ideal path of insertion for a full or partial veneer crown

parallel w the long axis of the tooth 

the path of insertion must be parallel to the __________ or it will be prevented from seating

parallel to the adjacent proximal contacts

in what case would the path of insertion NOT be parallel to the long axis of the tooth 

if the tooth was tilted (fig B), then the path of insertion paralleling the long axis of the tooth may be blocked by the proximal contours of the adjacent (fig C) → so path of insertion is made perpendicular to the occlusal plane (fig D)

what is the typical incline of a mandibular molar

9 to 14 degrees lingually → path of insertion should coincide w this how

what is the common clinical error that results in additional unecessary removal of tooth structure (cross-hatched area) 

preparing such a tooth w a path of placement that is perpendicular to the occlusal plane of the mandibular arch 

how can ortho aid in crown preparation

ortho repositioning can help the crown prep be more conservative

definition of retention 

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

what is the essential element of retention

the two opposing vertical surfaces of the same preparation such as the buccal and lingual // mesial and distal walls 

definition of resistance

prevents dislodgment of the restoration by forces directed in apical or oblique direction and prevents any movement

retention and resistance are ______, so they are often inseparable qualities 

they are interrelated 

what are the factors influencing retention for a given fixed restoration in order (5) 

1. magnitude of dislodging force 

2. geometry or tooth preparation 

3. roughness of the fitting surface of the restoration 

4. material being cemented 

5. film thickness and properties of the luting agent 

forces that tend to remove a cemented restoration along is path of placement are very _____ (big/small) compared to ones that seat or tilt 

v small 

what are the 3 properties of retention-geometry of the prepared tooth 

• diameter of the preparation 

• length of the preparation 

• taper of the preparation 

what will inc retention:

• a slightly tapered wall 

• a slightly parallel wall 

slightly parallel wall 

which would inc resistance: 

• a slightly tapered wall 

• a slightly parallel wall 

a slightly parallel wall 

definition of taper

the convergence of two oppsoite external walls of a crown preparation as viewed in a given plane 

what is the angle of convergence/ total occlusal convergence (TOC)

the extension of the two opposite-facing external walls in a given plane forming an angle 

what effect does even a slight taper have on retention 

significantly reduces retention 

the recommended convergence between opposing walls is ____ degrees, which has been shown to optimize retention for _____________ cement 

6 degrees; zinc phosphate cement 

what is a clincially acceptable taper range for a complete crown 

range from up to 5-20 degrees 

why can the TOC not be as parallel as possible 

• to allow escape of excess luting agent during seat of the crown 

• slight undercuts are often present in preparations that are too cylindrical and prevent the restoration from seating 

definition of undercut on a complete crown preparation

any irregularity in the wall of a prepared tooth that prevents the withdrawal or seating of a wax pattern of crown

how can undercuts be present

• can be present whenever two axial walls face opposite directions

• divergence is inadvertently created between opppsite-facing external axial wall, or wall segments in a cervico-occlusal direction

how do you evaluate preparation taper to ensure all negative taper/undercuts are eliminated 

view w one eye from a distance of approximately 30 cm or 12 inch

why do we only use one eye during evaluation of preparation taper 

an undercut as great as 8 degrees can be overlooked if both eyes are used 

what will inc retention:

• dec in height

• inc in height

taller preps help retain crowns better, but beyond a certain height, the extra retention gain is less significant

what will inc resistance:

• dec height 

• inc height 

inc height 

what will inc retention:

• dec in diameter

• inc in diameter

inc in diameter gives a linear inc in retention

what will inc resistance:

• dec in diameter 

• inc in diameter 

dec in diameter 

__________ ___________ is an important factor in both retention and resistance

occlusogingival length

what is the minimal incosocervical dimension of incisors and premolars when prepared w 10-20 degrees of TOC 

3 mm

what is the minimal occlusocervical dimension of molars when prepared w 10-20 degrees of TOC 

4 mm 

what helps in adequate clearance and does not require excessive tooth reduction

an anatomically prepared occlusal surface

what can a flat occlusal surface preparation result in (2)

• insufficient clearance 

• or an excessive amount of reduction 

what factors are affected (2) of the occlusal surface if the preparation does NOT follow the contour of the unprepared tooth 

• height dec 

• surface area dec (friction) 

metal castings are most effectively prepared by…

airborne particle abrading the fitting surface w 50 microns of alumina

______ _______ of the fitting surface of restorations can improve retention w certain luting agents 

acid etching 

overall, roughening the tooth preparation has what influence on retention and is it recommended 

hardly influences retention, NOT recommended 

how is evidence supporting: retention is affected by both the type of castign alloy and any core buildup material that is present on the axial walls of the crown prep 

• clinical significance is not confirmed 

• conflicting results w different in vitro studies 

how is evidence supporting: the effect of inc thickness of the cement film on retention of a restoration 

conflicting evidence 

what 6 factors influence retentive properties of a restoration 

• taper

• surface area

• type of preparation 

• surface texture 

• film thickness 

• luting agent 

rank these factors from greater retentive properties to lesser:

• 6 degree taper

• parallel

• excessive taper

parallel > 6 degree > excessive taper

rank these factors from greater retentive properties to lesser:

• small surface area

• large surface area

large > small surface area

rank these factors from greater retentive properties to lesser:

• smooth surface texture of intaglio of restoration

• rough surface texture of intaglio of restoration

rough > smooth

retentive effect of film thickness 

effect uncertain 

rank these factors from greater retentive properties to lesser:

• glass ionomer luting agent

• polycarboxylate/zinc oxide-eugenol luting agent

• adhesive resin luting agent

• zinc phosphate luting agent

adhesive resin > GI > polycarboxylate/zinc oxide-eugenol > zinc phosphate

adequate resistance depends on what 3 things

• magnitude and direction of the dislodging forces 

• geometry of the tooth preparation (primary and secondary retentive features) 

• physical properties of the luting agents 

resistance is a funx of the relationship between what factors

• axial wall taper

• preparation diameter 

• preparation height 

what factors can interfere w rotational radius of a tooth preparation

• tooth diameter

• occlusal-gingival length

• amount of taper

• retentive features (bow, groove)

how can lateral forces displace a restoration, specifically talking about the rotational radius

causes the rotation around the gingival margin, effectively tipping the crown off its preparation

what area of a complete crown is placed under compression when a lateral force is applied

the resistance area (RA)

______ (short/tall) tooth preparations w ________ (small/large) diameter were found to have very little resistance form

short tooth preparations w large diameter

which teeth require a more parallel preparation than other teeth to achoeve adequate resistance form 

molar teeth require more than premolar or anterior teeth 

why does retention inc when the diameter is larger 

bc this inc surface area 

why does resistance inc when the diameter is smaller 

bc the shorter rotational radius for the arc of displacement 

what can be placed in healthy tooth structure that are effective in enhancing the resistance form of crown preparations 

proximal grooves or boxes 

why does proximal grooves or boxed improve resistance 

these interfere w rotational movement (tipping) of the crown and thereby subject additional areas of the luting agent to compression 

resistance to deformation is affected by physical properties of…

the luting agent, such as compressive strength and modulus of elasticity 

what are the 6 factors that influence resistance

• dislodging forces 

• taper 

• diameter 

• height 

• type of preparation 

• luting agent 

rank these dislodging forces from most to least resistant:

• habits 

• anterior guidance 

• eccentric interferences 

habits > eccentric interferances > anterior guidance 

rank these from most to least resistant:

• minimum taper

• excessive taper

• 6 degrees

minimum > 6 degrees > excessive

rank these from most to least resistant:

• small diameter premolar

• large diameter molar

premolar > molar

rank these from most to least resistant:

• average height

• large height 

• short height 

long > average > short 

rank these from most to least resistant:

• onlay 

• partial coverage 

• complete coverage

complete coverage > partial > onlay

rank these luting agents from most to least resistant:

• glass ionomer 

• adhesive resin 

• polycarboxylate 

• zinc oxide 

• zinc phosphate 

adhesive resin > GI > zinc phosphate > polycarboxylate > zinc oxide 

how can you prevent deformation of a restoration

adequate tooth reduction

what are the 3 componenets of adequate tooth reduction

• occlusal reduction 

• functional cusp bevel 

• axial wall reduction 

what can be made before tooth preparation to facilitate evaluationof tooth reduction uniformity 

putty index 

the finish line design requires a design that can provide enough …. to resist deformation 

must provide enough bulk of the material 

what are the 2 finish line designs we practice

• chamfer (what we doing now) 

• shoulder 

advantages of chamfer margin design 

distinct margin, adequate bulk, easier to control 

disadvantages of chamfer margin design 

care needed to avoid unsupported lip of enamel