principles of preparation design

lecture given 9/3/2026

what are the biologic goals of preparation design?

conservation of tooth structure

harmonious occlusion

proper contour

what are the esthetic goals of preparation design?

subgingival margins

restorations

material used

design

contours

what are the mechanic goals of preparation design?

resistance form

retention form

principles of tooth preparation

preservation of tooth structure

preservation of the periodontium

retention and resistance

structural durability

marginal integrity

preservation of tooth structure

must preserve remaining tooth structure

intact surfaces of tooth structure that can be maintained while producing a strong, retentive restoration should be saved

what are the structure/guidelines for preservation of tooth structure?

preparation of teeth with the minimum practical convergence angle between axial walls

orientation of the diamond parallels the long axis of the tooth being prepared

avoid damage to adjacent tooth

occlusal surface reduction: follow anatomic planes

supra gingival margins

least traumatic of the soft tissues

usually situated on enamel

easily finished

impressions are easily made

most accessible for cleansing

restorations can be easily evaluated

sub gingival margins

clinical crown is short

esthetic appearance

old restorations with subgingival margin

caries, abrasion, erosion

what are the disadvantages of sub gingival margins?

root sensitivity

soft tissue injury

plaque retention

possible violation of biological width

biologic width

the dimension of space that the healthy gingival tissues occupy above the alveolar bone

evaluated with radiographs, probing, and bone sounding

what did wilson and majnard caution against when it comes to biologic width?

some distance of unprepared tooth structure should remain between the finished line and junctional epithelium and this distance should ideally be 0.5mm

between supragingival and subgingival margins, there is no difference in the health of gingiva, the change of the sulcus depth, gingival contour, and plaque accumulation when…

the crown margins are well fitted and well contoured

retention form

the quality inherent in the dental prosthesis acting to resist the forces of dislodgement along the path of placement

what factors affect retention form?

magnitude of disloding forces- sticky food, floss under connectors/temp crowns

path of insertion

geometry of tooth preparation- taper, freedom of displacement, surface area, stress concentration

material being cemented

type of luting agent and film thickness of luting agent

path of insertion

the specific direction in which a prosthesis is placed on the abutment teeth or dental implants

the mesiodistal inclination of the path must be parallel to the contact areas of adjacent teeth

if path is inclined mesially or distally, the restoration will be held up at the proximal contact areas and be locked out

taper

the axial walls of the preparation must taper slightly to permit the restoration to seat

recommended 2 to 6 degree minimum angle

total occlusal convergence (TOC)

the angle of convergence between 2 opposing prepared axial surfaces

what is the average taper of a crown prep by a dentist?

14.3 to 22.1

posterior teeth TOC (>/<) anterior teeth TOC

>

mandibular teeth TOC (>/<) maxillary teeth

>

mandibular molars are prepared with the (least/greatest) TOC

greatest

faciolingual surfaces have (greater/lesser) TOC than mesiodistal

greater

FPD TOC (>/<) single crowns

>

freedom of displacement

number of paths along which restorations can be removed from the tooth preparation

long, parallel axis walls and grooves has high retentive values

a short, over tapered preparation has low retention values

if only one path exists, there is maximum retention

surface area

the greater the surface area, the more area available for the restoration to be cemented/bonded

the total surface area of the preparatio is influenced by the size of the tooth, extent of coverage (full or partial), and internal features such as grooves and boxes

a more tapered prep means…

decreased retention, and less resistance

a single path of insertion means…

increased retention

more surface area means…

increased retention

short occlusocervical dimension (length) means…

decreased retention

bonded restoration vs cemented restoration means…

increased retention

resistance form

the features of a tooth preparation that enhances the stability of a restoration and resist dislodgement along an axis other than the path of placement

what factors affect resistance form?

magnitude and direction of forces (mastication and parafunctional habits, interferences, anterior guidance)

leverage

length and width of preparation

taper

type of preparation

luting agent

leverage

occurs when the line of action of a force passes outside the supporting tooth structure

forces outside the margin are caused by- wide occlusal table, crowns on tipped teeth, cantilevers

preparation length

shortening the preparation will reduce the resistance

when the preparation is short, the lifting force required is small

small restoration on a small preparation will less likely fail than a large restoration on the same length preparation

what is the proposed minimal OC dimension for premolar and anterior teeth according to parker, maxwell, woosley, and matich?

3mm for teeth that are prepared within the recommended TOC range of 10 to 20 degrees

what is the proposed minimal OC dimension for molar teeth according to parker, maxwell, woosley, and matich?

4mm is proposed as the minimal OC dimension

usually prepared with greater convergence than anterior teeth, have a greater diameter than other teeth, and are located where occlusal forces are greater

where is the taper resisting area on an over tapered prep?

near the occlusal surface

whenever possible, teeth should be prepared so that the _________ corners are preserved thereby sustaining variation in the circumference morphology that enhances resistance form

facioproximal and linguoproximal

teeth without natural circumference morpholgy after tooth preparation (round teeth) or teeth that lack adequate reistance form should be modified with…

the creation of grooves/boxes

axial grooves/boxes should be used rountinely when…

mandibular molars are prepared for fixed partial denture

the parker, malone III, and trier study that examined 294 preparations found that

___% of incisors had resistance form

___% of the caninines had resistance form

___% of the premolars had resistance form

and only ___% of molars had resistance form

96, 92, 81, 46

the parker, malone III, and trier study that examined 294 preparations found that

of the molars with grooves, ___% had resistance form

___% of preparations with buccal or lingual grooves had resistance form to mesial and distal dislodgement

___% of preparations with mesial or distal grooves had resistance form to buccal and lingual dislodgement

62%

100%

100%

the parker, malone III, and trier study that examined 294 preparations found that

___% of crowns that fell off lacked resistance

___% of crowns in commercial labs lacked resistance

95

25

over tapered prep means…

decreased resistance

short occlusocervical dimension means…

decreased resistance

small tooth with large restoration means…

decreased resistance

bonded restoration vs cemented restoration means…

increased resistance

structural durability

the ability of a restoration to withstand destruction due to external forces

what factors affect structural durability?

adequate tooth reduction- occlusal and axial

material selection

margin design

what is the occlusal and axial reduction for a gold crown?

1mm, 0.5 - 1.0mm

what is the occlusal and axial reduction for a PFM crown?

1.5mm, 1.2 - 1.5mm

what is the occlusal and axial reduction for a all ceramic crown?

2mm, 1.2 - 1.5mm

adequate tooth reduction should also have…

round line and point angles, avoid deep grooves in the center of the occlusal surfaces to prevent stress concentration and to distribute the forces over a larger surface area

chamfer

a finish line design for tooth preparation in which the gingival aspect meets the external axial surface at an obtuse angle

it is cut using a round end diamond

mainly used with PFM and zirconia crown

heavy (deep) chamfer

has a 90 degree cavo surface angle, with a large radius

radial shoulder- same with lesser radius

rounded tapered diamond

used with ceramics

shoulder

a finish line design for a tooth preparation in which the gingival floor meets the external axial surfacess at approx a right angle

it is formed using a flat ended diamond

mainly used fror all ceramics

the sharp 90 internal line angle, concentrates stress in the tooth

shoulder with bevel

a shoulder finish line with the cavosurface angle modified with a bevel

formed using a torpedo bur

can be used for PFMs where a shoulder was prepared but because of caries or presence of restoration, a bevel is added

bevel

a slanting edge, the process of slanting the finish line and curve of a tooth preparation

allows the cast metal margin to be bent or burnished against the prepared tooth structure

minimizes the marginal discrepancy

removes unsupported enamel

according to pascoe jpros dent, crowns cemented to shoulder finish line fit (better/worse) than the crowns that are fitted to shoulder with bevel

better

there is no evidence that the bevel will imrpove the marginal seal

knife edge

a clearly defined junction of prepared and unprepared tooth structure that lacks a concavity at the gingival termination

conservative of tooth structure

it is difficult to wax castings

do not use!

feather edge

the demarcation between prepared and unprepared tooth structure created by minimal tooth preparation without a defined visible line of reference for the cavosurface finish line such as a shoulder or chamfer finish line

conservative of tooth structure

it is difficult to wax castings

not enough bulk for restorative

do not use!

what are characteristics of an ideal margin?

smooth

linear margin

provide enough material strength

on enamel

easy access- for dentist and pt

easy for impression

there’s a lot going on here