Designing tooth preparations for optimal success

Dent Clin N Am 48 (2004) 359–385

Designing tooth preparationsfor optimal success

Charles J. GoodacreDepartment of Restorative Dentistry, School of Dentistry,

Loma Linda University, Loma Linda, CA 92350, USA

The form of prepared teeth and the amount of tooth structure removedare important contributors to the mechanical, biologic, and esthetic successof the overlying crown or fixed partial denture. Therefore, it is important todevelop clinical guidelines that can be used to optimize success in fixedprosthodontics.

Guidelines for tooth preparation

Total occlusal convergence

Total occlusal convergence (TOC) is the angle formed between twoopposing prepared axial surfaces (Fig. 1). An appropriate clinical goal forTOC is 10� to 20�. Historically, it has been recommended that the TOC beminimal, ranging between 2� and 6� [1–5]. There has been support for theseminimal convergence angles from research focused on pulling a crown offa prepared tooth in a direction parallel with the long axis of the preparedtooth [6,7]. However, resistance to lateral forces and not long-axis retentionis likely to be the determining factor in a crown’s resistance to dislodgment[8–10].

When retention and resistance forms were tested by cementing crowns onmetal dies, resistance testing was found to be more sensitive to changes inthe TOC than retention testing [9]. For this reason, recent testing hasfocused on resistance through the use of simulated lateral forces.

The tipping resistance of crowns has been tested for teeth with TOCangles of 10�, 16�, and 22� [9]. The teeth were similar in size to preparedmolars, and the results indicated that 22� of TOC provided inadequateresistance. The authors concluded that 10� of TOC was not easy to produce

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360 C.J. Goodacre / Dent Clin N Am 48 (2004) 359–385

clinically, and they therefore identified 16� as the best convergence angleamong those tested. A TOC between 10� and 22� has been recommended ina recent textbook [11].

When proposing guidelines for TOC, it is important to determine theangles formed by students and practitioners. Dental students, generalpractice residents, general practitioners, and prosthodontists do not usuallyproduce minimal TOC angles as have historically been recommended.Instead, the angles range between 12� and 17� (Fig. 2) [12–21]. This

Fig. 2. Posterior teeth such as this mandibular molar are usually prepared with greater TOC

than anterior teeth. This tooth has about 15� of TOC.

Fig. 1. The maxillary central incisor has been prepared with minimal convergence of the mesial

and distal surfaces.

361C.J. Goodacre / Dent Clin N Am 48 (2004) 359–385

information and the resistance testing data previously cited lend credence tothe proposal that 10� to 20� is an appropriate clinical goal.

It is important for practitioners and students to measure the convergenceangles they typically produce on various teeth and determine if the proposedgoal is routinely being achieved. Fig. 3 was developed to aid this evaluationprocess. Holding a die of the prepared tooth so the axial walls of the die aresuperimposed over the lines present on the drawing permits a closeapproximation of the TOC.

There are a number of factors that affect the amount of TOC andtherefore make it more difficult to achieve the goal of 10� to 20�. Thepreparation of posterior teeth (see Fig. 2) as opposed to anterior teeth (seeFig. 1) often leads to greater TOC [16,17,21], as does the preparation ofmandibular teeth compared with maxillary teeth [13,17,21]. Mandibularmolars have been identified as the teeth prepared with the greatest TOC[18,21]. One study reported greater TOC when the facial-lingual surfaces [16]were prepared, whereas another study identified greater convergence of themesial-distal surfaces [21], possibly indicating that variations occur betweenpractitioners. Fixed partial denture abutments are generally prepared withgreater convergence than teeth prepared for single crowns [19]. The use ofone eye when viewing teeth (monocular vision) produces greater convergencethan the use of both eyes (binocular vision) [19].

Fig. 3. This diagram can be used to measure the TOC of a die by aligning the axial walls of the

die so they become superimposed over the converging lines on the diagram that mostly closely

match the axial convergence of the die.


During clinical tooth preparation, an occlusal view is frequently used toassess TOC. However, this view is of limited value because small differencesin the TOC are difficult to ascertain from an occlusal view (Fig. 4). Facial orlingual views are more valuable when evaluating occlusal convergence.Therefore, it is recommended that TOC be assessed by evaluating the imageof the preparation(s) in a mirror.

In the presence of the factors that increase TOC beyond the recommended10� to 20� range, it is recommended that auxiliary tooth preparation features,such as grooves or boxes, be added to enhance the resistance of restorationsto dislodgment.

Occlusocervical/incisocervical dimension

It is proposed that anterior teeth and premolars have a minimalocclusocervical (OC) dimension of 3 mm and that molars have a minimaldimension of 4 mm. Critical convergence angles have been mathematicallycalculated and used to identify angles beyond which a crown wouldtheoretically not possess adequate resistance to dislodgment [22,23].Although the validity of such angles has been challenged [8,10], anassessment of the resistance form of dies from clinically failed restorationssupported a relationship between convergence angles and clinical failure [24].

At 1 and 2 mm of OC dimension, the critical convergence anglecalculations indicate a molar crown’s resistance to dislodgment wasinadequate unless the tooth was prepared with less than 6� and 12� ofTOC, respectively. Because it is unlikely that molars can be preparedroutinely at such minimal convergence angles, these dimensions areinadequate for molars. A molar can possess adequate resistance when thetooth possesses 3 mm of OC dimension and 17� or less of TOC. However,achieving 17� also is unlikely to routinely occur on molars, making 3 mm aninadequate OC dimension for molar tooth preparations. As a result, 4 mmis proposed as the minimal OC dimension for molars. It is important tomeasure prepared teeth with a periodontal probe to determine if the minimaldimensions have been reached (Fig. 5).

The resistance of crowns made for dies the size of prepared incisors andpremolars has been tested, and it was concluded that 3 mm providesadequate resistance [25], supporting the recommended OC dimension forpremolars and anterior teeth.

The tipping resistance of molar-sized crowns has also been measured [26].Three millimeters of OC dimension provided adequate resistance but only at10� TOC. Three millimeters was inadequate at 20� degrees of TOC, an anglefrequently found on molars. These data support the 4 mm minimalrecommended OC dimension for prepared molars. Therefore, 3 mm isproposed as the minimal OC dimension for premolars and anterior teeth.Because molars frequently are prepared with greater convergence thananterior teeth and premolars are larger and are subjected to greater occlusal


Fig. 4. (A) It is difficult to assess the degree of convergence from an occlusal view. (B) From

a lingual view, it is much easier to make a quantitative assessment of the TOC. The molar has

20� to convergence.


forces, 4 mm is proposed as the minimal OC dimension for prepared molars.Teeth lacking these minimal dimensions should be modified to enhance theirresistance form through the formation of proximal grooves/boxes.

Ratio of occlusocervical/incisocervical dimension to faciolingual dimension

The ratio of the OC dimension to the faciolingual (FL) dimension shouldbe 0.4 or higher for all teeth. Ninety-six percent of incisor crowns, 92% ofcanine crowns, and 81% of premolar crowns possess adequate resistancedespite variations in their preparation form and dimensions. However, only46% of molars possess appropriate resistance [27]. An important factor in

Fig. 5. (A) The maxillary premolar has the minimal recommended OC dimension of 3 mm for

anterior teeth and premolars. (B) The OC dimension of the mesial surface is being measured

with a periodontal probe. The wall has only 2 mm of OC dimension.


producing this adequate resistance is the favorable ratio of the OCdimension compared with the diameter of prepared incisors, canines, andpremolars. The favorable ratio is due to the typical anatomic dimensions ofthese teeth when they are prepared (Fig. 6A).

Because chewing and parafunctional habits place FL forces on singlecrowns and fixed partial dentures, it is recommended that the FL dimensionof the prepared tooth be used in making ratio calculations. Prepared molarshave a larger FL dimension than premolars and anterior teeth, and theyusually have a shorter OC dimension, thereby producing a lower ratio andpoorer resistance (Fig. 6B). Also, the TOC usually is greater on molars[16,17,21], and this accentuates the unfavorable ratio.

Calculations [23] indicate that adequate resistance can be achieved whenthe OC/FL ratio is 0.1, but the convergence angle must be less than 6�. Aratio of 0.2 requires less than 12� of TOC. A ratio of 0.3 requires less than18� of TOC. These convergence angles are difficult to produce on molars.The same calculations indicate that a ratio of 0.4 provides adequateresistance when the TOC is 24� or less [23], an angle that can be clinicallyproduced on molars.

The theoretical calculations have been tested by making crowns to fit dieswith five different convergence angles, and it was determined thatinadequate resistance is present when a molar-size die (10 mm in diameter)possessed a 0.35 ratio [28].

Circumferential form of the prepared tooth

Teeth should be prepared so they possess circumferential irregularitywhenever possible. When teeth are anatomically reduced, they possesscharacteristic geometric forms. For instance, when prepared maxillarymolars are viewed occlusally, they have a rhomboidal form (Fig. 7).Mandibular molars have a rectangular form (see Fig. 6B), and mostpremolars (see Fig. 7) and anterior teeth have an oval form. These shapesproduce circumferential irregularity. The value of these irregularities hasbeen evaluated by comparing the resistance areas of conical and pyramidaltooth preparation. The pyramidal preparations provided increased resistance[29]. Therefore, it is important to preserve the ‘‘corners’’ of a toothpreparation whenever possible.

When prepared teeth have no corners due to their round morphologicform or existing condition, they should be modified by forming axialgrooves or boxes that provide resistance to dislodging forces. Becausemolars are frequently prepared with greater convergence than other teethand because they usually have a smaller OC dimension and less favorableOC/FL dimension ratio, they often benefit from axial grooves or boxes thatenhance resistance form. Additionally, mandibular molars are the teethprepared with the greatest convergence angles [18–21], and it is suggested


that axial grooves/boxes be routinely used when mandibular molars areprepared for fixed partial dentures (Fig. 8A).

Because chewing and parafunctional habits place dislodging forces onsingle crowns and fixed partial dentures that are largely FL in direction,auxiliary resistance form features should be located in the tooth where theyprovide optimal resistance to these forces. Proximal grooves providedcomplete resistance to FL crown dislodgment, whereas facial or lingualgrooves provide only partial resistance (Fig. 8B) [26]. Therefore, auxiliaryresistance form features such as grooves and boxes should be located on theproximal surfaces of fixed partial denture abutments.

Fig. 6. (A) When anterior teeth are prepared for complete coverage crowns, they usually

possess a favorable ratio between the incisocervical crown dimension and the mesiodistal/

faciolingual dimension. (B) Molars have less favorable ratios between the OC dimension and

the FL dimension than anterior teeth. When mandibular molars are prepared, they have

a rectangular form with rounded corners that enhance resistance form.


Reduction uniformity

Teeth should be uniformly reduced, thereby enhancing the potential fornormal crown form and an improved esthetic result. Although there is noscientific evidence to support the benefits of uniform reduction of the facialand proximal surfaces, clinicians and technicians have long recognized thebenefits. Uniform reduction makes it easier for a dental laboratorytechnician to create esthetic restorations where the color and translucencycan approximate that of a natural tooth. The reduction uniformity alsopromotes the development of normal contours in conjunction with thedesired color and translucency. Reduction uniformity is best achieved byplacing depth grooves into the surface to be reduced (Fig. 9) and thenreducing the tooth in accordance with the grooves.

Finish line location

Finish lines should be positioned supragingivally whenever the estheticand resistance form requirements permit such a location. The periodontalbenefits of supragingival finish lines (Fig. 10) have been well documented[30–38]. However, subgingival finish lines often are used for appropriatereasons that include the need to achieve adequate OC dimension forretention and resistance form; to extend beyond caries, fractures, anderosion/abrasion; or to encompass a variety of tooth structure defects.Subgingival finish lines are also used to produce a cervical crown ferrule onendodontically treated teeth and to improve the esthetic result achieved ondiscolored teeth and with certain types of restorations. Althoughperiodontal health can be retained when subgingival margins are used,

Fig. 7. The prepared maxillary premolar has an oval crown form, and the prepared molar

resembles a rhomboid.


other factors must be present. The restorations must be properly contouredand exhibit good marginal fit [39–41]. Additionally, there must be carefulexecution of the hard and soft tissue procedures performed in conjunctionwith tooth preparation [39–41].

When a subgingival finish line is required, multiple studies indicate thatextension to the level of the epithelial attachment should be avoided. Pocketdeepening does not occur when the margin is at least 0.4 mm occlusal to thedepth of the gingival crevice [42], whereas more severe gingivitis occurswhen subgingival margins approximate the depth of the crevice [37]. Whenteeth were prepared so provisional crown margins were located farther

Fig. 8. (A) The mandibular molar has been modified by placing a groove into the mesial

surface. (B) The prepared maxillary molar has been modified with a facial groove. Facial

grooves are less effective at enhancing resistance to FL dislodging forces than grooves placed

into proximal surfaces.


apically than recommended, about a millimeter of gingival recession wasnoted within 2 weeks and a little over 1 mm of recession was recorded within8 weeks [43]. Histologic evaluation indicated the recession mechanism wasactivated during the first 7 days [43]. When crown margins were extended tothe bone crest, 1 mm of crestal bone loss was observed [44].

Finish line form

It is recommended that chamfer finish lines be used for all-metal crowns.For metal ceramic crowns, chamfer, shoulder, or beveled finish lines can be

Fig. 9. (A) Depth grooves have been placed into the facial surfaces of both maxillary central

incisors to aid in the production of uniform facial reduction. (B) Completed metal ceramic tooth

preparations. The facial depth grooves were helpful in the creation of tooth preparations that

possessed adequate and uniform reduction.


used. With all-ceramic crowns, shoulder finish lines should be used withnonbonded crowns, whereas a chamfer or shoulder can be used when thecrowns are to be bonded to the prepared tooth.

All-metal crowns

Although no scientific studies are available that support the superiority ofchamfer finish lines for all-metal crowns, they have frequently been usedbecause they are easy to form and sufficiently distinct to be visible on thetooth and die. They also provide space for adequate metal thickness com-bined with normal axial contours (Fig. 11).

Metal-ceramic crowns

Chamfer (Fig. 12A), beveled chamfer, shoulder with a rounded axiogin-gival line angle (Fig. 12B), shoulder with a sharp axiogingival line angle(Fig. 12C), and beveled shoulder (Fig. 12D) finish lines have been used formetal ceramic tooth preparation. Although initial research identified greatermarginal metal distortion when porcelain was fused to castings made forchamfer finish lines [45,46], subsequent studies failed to measure significantdifferences in marginal fit [47–49]. Additional studies that evaluated the effectof cementation also failed to identify significant fit differences as related tothe type of finish line [50,51]. Therefore, the selection of finish line formshould not be based upon expectations of enhanced marginal fit but ratheron factors such as formation ease, personal preference, esthetic requirements,and the type of crown being fabricated.

Fig. 10. Supragingival finish lines were placed on the mandibular canines because the cervical

aspects of these teeth were not visible during smiling and because sufficient retentive length had

been achieved.


All-ceramic crowns

Chamfer finish lines produced lower strengths with nonbonded crownsin laboratory tests [52,53]. However, the negative effect was not replicatedwhen the crowns were bonded (internally etched crowns cemented toetched prepared teeth with resin) to the teeth [54,55]. It therefore seemsappropriate to recommend shoulder finish lines (Fig. 13) for all-ceramiccrowns that are not to be bonded to underlying tooth structure, whereaschamfer or shoulder finish lines can be used when the crowns are to bebonded.

Fig. 11. A chamfer finish line is being formed on the mandibular first molar using a tapered

round-end diamond instrument. The rotary instrument was extended into the tooth by

a distance equal to one half its tip diameter, and then the instrument was carefully and

methodically moved around the circumference of the tooth.


Reduction depths

All-metal crowns should have chamfer depths of at least 0.3 mm, axialsurface reductions of at least 0.5 to 0.8 mm, and occlusal reduction depths of1 to 1.5 mm. For metal ceramic crowns, depths of 1.0 mm or more havebeen proposed and are esthetically desirable for the finish line and facialsurface reductions. However, it does not seem that depths of that magnitudeare regularly achieved clinically, and the presence of younger teeth andgreater TOC angles make it more unlikely that teeth can be reduced to thosedepths. Occlusally, reduction depths of 2 mm are achievable even on youngteeth that are being prepared for metal-ceramic crowns. Finish line andfacial reductions depths of 1.0 mm are recommended for all-ceramic crowns.An incisal/occlusal reduction of 2 mm for all-ceramic crowns permits thedevelopment of appropriate color, translucency, and morphology.

Fig. 12. (A) Chamfer finish lines have been established on the maxillary anterior teeth for metal

ceramic crowns. (Courtesy of Dr. Wayne Campagni, Loma Linda, CA). (B) A shoulder finish

line (finish line meets the unprepared tooth at about 90�) has been prepared with a rounded

axiogingival (internal) line angle. (C) A shoulder finish line has been prepared with a sharp

axiogingival line angle. (D) The metal ceramic crown preparations have been prepared with

beveled shoulder finish lines. (Courtesy of Dr. Tony Daher, LaVerne, CA.)


Fig. 12 (continued )


All-metal crowns

For all-metal crowns, finish line depths of 0.3 to 0.5 mm have beenrecommended [3,5]. This depth allows restorations to closely approximatenormal tooth dimensions, whereas finish lines\0.3 mm in depth are morelikely to produce overcontoured restorations [56]. Therefore, 0.3-mm-deepchamfer finish lines are well suited for all-metal crowns (see Fig. 11).

For all-metal crowns and fixed partial dentures, there are no data thatidentify the ideal axial reduction depths. Therefore, the experience ofclinicians and laboratory technicians forms the basis for recommending 0.5to 0.8 mm of reduction be developed near the occlusal aspect of the facial andlingual surfaces. This depth of reduction provides adequate space for thedevelopment of normal axial contours and material thickness for strength.

Fig. 13. A shoulder finish line has been developed for an all-ceramic crown.


Because proximal reduction usually exceeds facial/lingual reduction, specificnumeric depth suggestions are not necessary. However, proximal reductionshould include the formation of an adequately distinct finish line and provideaccess for impression making.

One millimeter of occlusal reduction provides space for the fabrication ofall-metal crowns, but reduction depths of 1.5 mm provide the space wherebywell-defined occlusal grooves and convex ridges can be developed. Also,should modest amounts of occlusal adjustment be required clinically, thecrowns can retain adequate ridge and groove form rather than become flator concave with a lack of occlusal grooves.

Metal-ceramic crowns

Finish lines for metal ceramic crowns should be 1.0 to 1.5 mm deep[2–5,11,57], and the facial surface be reduced between 1.0 and 1.7 mm[2–5,11,57]. These recommendations are supported by research that de-termined 1.0 mm or more of translucent porcelain is required to reproduceshade guide specimens [58–60]. However, actual measurements of preparedteeth [56,61] indicate reduction depths are often[1 mm (Fig. 14). The reasonfor failure to achieve the recommended depths may be related to the amountof tooth structure available for reduction. The combined dentin and enamelthickness ranged from 1.7 to 3.1 mm in one study [62], from 2.2 to 2.5 mm inanother study [63], and from 2.0 to 3.0 mm in a third study [64]. Thesethicknesses indicate that some teeth can safely be reduced in accordance withthe recommended depth range of 1.0 to 1.7 mm (Fig. 15), whereas other teethwould have thin areas of remaining dentin.

Younger teeth have tooth structure thicknesses of\2 mm and thereforecannot be reduced to meet the greater depths that have been proposed [26].Additionally, the effect of TOC must be considered because it can havea profound effect on the amount of remaining tooth structure for a certainreduction depth. The thickness of the remaining tooth structure has beenmeasured on adolescent premolars prepared with two finish line depths (0.8and 1.2 mm deep) and four TOC angles (5�, 10�, 15�, and 20�) [65]. Thecombination of a 1.2-mm-deep finish line and 20� of TOC produced a 0.3-mmthickness of remaining dentin. Therefore, the presence of younger teeth orgreater TOC angles make it unlikely that teeth can be reduced to the greaterproposed depths.

When esthetic materials are to be placed over incisal/occlusal surfaces,reduction depths of 2.0 to 2.5 mm have been recommended for metal ceramicrestorations to provide space for the development of appropriate color,anatomic form, and occlusion [2–5,11]. There is[4 mm of combined enamel-dentin thickness present on young teeth [5,62]. Teeth in older patients havegreater combined enamel and dentin thicknesses that range from 5 to 7 mm[5,62–64]. It seems that reduction depths of 2 mm or more are achievablerecommendations (Fig. 16).


Fig. 14. The metal ceramic tooth preparation has a facial cervical reduction depth of\1 mm.

Fig. 15. The two metal ceramic tooth preparations have facial reduction and finish line depths

that exceed 1 mm.


All-ceramic crowns

All-ceramic finish line depth recommendations range from 0.5 to 1.0 mm(Fig. 17) [2–5,11,57]. From a facial reduction standpoint, there is littleimprovement in shade matching when the thickness of all-ceramic crowns isincreased beyond 1 mm with semi-translucent, all-ceramic systems (eg,Empress and InCeram Spinell) and high-value, low-chroma shades (eg, A1)[60]. However, thicknesses in excess of 1 mm are beneficial when using moreopaceous all-ceramic systems or when using lower value, more chromaticshades such as C2 and A3 [60]. In addition, the inherent color of the preparedtooth can influence the color of the overlying all-ceramic crown, requiringgreater ceramic thickness when the dentin is discolored.

Malament and Socransky [66] investigated the effect of ceramic thicknesson the strength of all-ceramic crowns but were not able to correlaterestoration failure with thickness when the crowns were bonded to theprepared tooth with resin cement. They found no significant differences in theprobability of survival after 11.7 years (3430 cumulative monitoring years)between bonded crowns that were\1 mm thick and those that were >1 mmthick. The midaxial thickness of crowns in this study averaged approximately1.5 mm [66]. Therefore, if the crown is to be bonded with resin cement, thereduction should be based on the ceramic thickness required to achieveproper color and contour.

Line angle form

Line angles should be rounded on all-ceramic tooth preparations toincrease crown strength. For other types of crowns, rounding is advocated

Fig. 16. The incisal edges of the two central incisors have been reduced by a dimension that

exceeds 2 mm.


because it facilitates laboratory fabrication and restoration fit. Sharp lineangles (Fig. 18A, B), located where prepared tooth surfaces meet each other,create stress concentration [67–69], and rounding these line angles increasesthe strength of all-ceramic crowns. With all-metal and metal-ceramic crowns,line angles are rounded to facilitate laboratory procedures and restoration fitrather than to enhance restoration strength (Fig. 18C). Round line anglesmake it easier to pour impressions without trapping air bubbles that producevoids in the die, and the rounding also facilitates investing wax patternswithout air inclusions. Trapped air bubbles lead to nodules in castings thatcan impede complete seating of a restoration if they are not detected andremoved. Also, if casting nodules occur, they are easier to detect and removewhen the lines angles are rounded during tooth preparation.

Fig. 17. (A) The facial shoulder finish line on the maxillary lateral incisor is 0.5 mm in depth.

(B) The maxillary central incisor finish line depth is 1.0 mm.


Fig. 18. (A) The maxillary left central incisor has a sharp mesiofacial line angle that should be

rounded. Although the sharpness of the mesioincisal and distoincisal corners of the maxillary

right lateral incisor has been reduced, there are still sharp angles that could be rounded to

reduce stress concentration. (B) There are multiple proximoincisal line angles that could be

further rounded to decrease the stress concentration in the all-ceramic crowns. (C) There are

sharp distofacial and distal cusp tips on the mandibular molar that has been prepared for an all-

metal crown. Rounding these sharp cusps facilitates laboratory fabrication by making it less

likely that air bubbles will be trapped during pouring and die and investing the wax pattern.


Surface texture

Tooth preparations should be reasonably smooth to enhance restorationfit. Some surface roughness improves retention when using zinc phosphatecement but does not seem to have as substantial of an effect when adhesivecements are used (polycarboxylate, glass ionomer, and resin).

Tooth preparation smoothness has been found to improve the marginal fitof restorations in two studies [70,71], whereas another study [72] found nodifference in the marginal seating of complete crowns when the axial surfaceswere prepared with coarse diamond instruments (120 lm grit size) and whenthey were prepared with fine diamond (50 lm grit size) instruments.

The effect of smoothness on retention seems to be related to the type ofdefinitive cement. With zinc phosphate cement, seven of nine studies indicatethat preparations with some roughness (see Fig. 11) provide improvedretention compared with smoother preparations. However, when testing theeffect of adhesive-type cements (polycarboxylate, glass ionomer, and resin),there is no clear benefit to surface roughness. Three studies indicate thatroughness increased retention [72–74], whereas three other studies found thatroughness did not improve retention [71,75,76]. Therefore, when using zincphosphate cement, one can expect an increase in retention through surfacetexture, whereas the benefits of surface roughness have not been definitivelyestablished when using polycarboxylate, glass ionomer, and resin cements.

Summary

The following guidelines are proposed when preparing teeth for completecoverage crowns and fixed partial dentures:

1. The TOC (angle of convergence between opposing prepared axialsurfaces) should range between 10� and 20�. However, posterior teethare frequently prepared with greater convergence angles as are fixedpartial denture abutments. When the TOC angles exceed the recom-mended levels, the tooth preparation should be modified to includeauxiliary features such as grooves or boxes.

2. Three millimeters should be the minimal occlusocervical/incisocervical(OC/IC) dimension of incisors and premolars when they are preparedwithin the recommended TOC range of 10� to 20�. The minimal OCdimension of molars should be 4 mm when prepared with 10� to 20�

TOC. When the OC dimension is less than the recommended dimension,the tooth preparation should be modified to include auxiliary featuressuch as grooves or boxes.

3. The ratio of the OC/IC dimension to the FL dimension should be 0.4 orhigher for all teeth. When this ratio is not present, as on large diametermolars, the tooth preparation should be modified to include auxiliaryfeatures such as grooves or boxes.


4. Teeth should be prepared in a manner that preserves the facioproximaland linguoproximal corners whenever possible because circumferentialirregularities enhance resistance form. When prepared teeth lack‘‘corners’’ and are round after tooth preparation, they should bemodified to include auxiliary features such as grooves or boxes.

5. When auxiliary features are placed into teeth, the preferred locations arethe proximal surfaces. The facial and lingual surfaces are secondarylocations to be used when the addition of proximal features leaves thetooth in a state of questionable resistance form. Proximal grooves/boxesshould routinely be used when mandibular molars are prepared for fixedpartial dentures because mandibular molars often are prepared with thegreatest TOC and frequently have limited OC dimension and large FLdimensions that lead to unfavorable ratios.

6. For the purpose of optimizing periodontal health, finish lines should belocated supragingivally when the condition of the tooth and estheticrequirements permit such a location. When subgingival finish lines arerequired, they should not be extended to the epithelial attachment.

7. For all-metal crowns, 0.3-mm-deep chamfer finish lines should be used.The axial and occlusal reduction depths for all-metal crowns should beat least 0.5 mm and 1.0 mm, respectively.

8. For metal ceramic crowns, finish line selection should be based onformation ease, personal preference, esthetic requirements, and the typeof crown being fabricated rather than on expectations of enhancedmarginal fit with one type of finish line compared with the others. Manyteeth, because of available tooth structure thickness external to the pulp,cannot be reduced facially to depths that exceed 1 mm. Two millimetersof occlusal/incisal reduction can be achieved even on young teeth becausesufficient tooth structure thickness is present incisal/occlusal to the pulp.

9. Shoulder finish lines are recommended for all-ceramic crowns when theyare not bonded to the underlying tooth. However, shoulder and chamferfinish lines can be used with all-ceramic crowns that are bonded to theprepared tooth using a resin cement and acid etching. Finish line andfacial reduction depths [1 mm are not required when using a semi-translucent type of all-ceramic crown but are beneficial with moreopaceous porcelain systems are used or when the tooth structure isdiscolored. Incisal/occlusal reduction depths of 2 mm are achievable dueto the available thickness of tooth structure.

10. Rounded line angles on tooth preparations for all-ceramic crownsdecrease the stress placed on the crowns and thereby increase crownlongevity. With crowns that contain metal (all-metal and metal-ceramiccrowns), line angles are rounded to facilitate pouring impressions andinvesting wax patterns without trapping air bubbles and to facilitate theremoval of casting nodules.

11. Tooth preparation smoothness seems to enhance restoration fit, but itseffect on retention appears to be related to the type of cement used.


Surface roughness generally increased retention with zinc phosphatecement, but no definitive relationship has been established when crownsare cemented with adhesive cements (eg, polycarboxylate, glass ionomer,resin). It therefore seems appropriate to recommend that teeth beprepared with a reasonable degree of surface smoothness.

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Designing tooth preparations for optimal success