CROWN AND BRIDGE: LECTURE 7 PREPARATION FOR FULL VENEER CROWNthis lecture contains practical information more than basic science, and it is mainly a view of steps that you will do in the clinic.it needs good concentration and linking between its parts. Unfortunately, the doctor did not give us the soft copy of the lecture to get the pictures from.but he explained the pictures of your book.we try the best to make the steps clear even without pictures. please go to the book to see the pictures........

In the last lecture we talked about the principles of tooth preparation. And those principles are general guides for you during working. Today we will apply those for full veneer crown. At first, why do we choose full crown to concentrate on? And why we prefer full crown over other retainer types? Because it is the most retentive retainer type, it has superior resistance feature, and it is ideal retainer for short abutment tooth and for long edentulous span. SOoOoO full veneer design must not used on every tooth. Instead it should be used on those teeth whose restoration demands maximum retention (i.e. commonly not in placement of single restoration but in bridges with short abutment or long span.) KEEP IN YOUR MIND: we aim in our work to replace the lost tooth structure AND to preserve what is remaining of the tooth. Therefore removal of tooth structure during preparation has to be in rational with the thickness of crown material that we are going to use and this also has to preserve as much as we can of tooth structure. And all this is to prevent future failure of our restoration. Nte: the steps we will discuss today are applied on preparation of abutment tooth to receive the retainer, and on preparation of the tooth to receive crown. As a rule: the preparation steps that we follow in our work are influenced by the material of the full veneer crown which can be: *full metal or *metal ceramic or *all ceramic.

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NOW starting with the full metal crown preparation: Because of cosmetic result, full metal crowns are contraindicated in the anterior teeth. it is used in the posterior teeth when all axial surfaces (not the center of the tooth) have been attacked by the caries, or decalcifications, or previously restored. BUT it is contraindicated in the case of uncontrolled caries. the preparation for full metal crown is less destructive than that required for either metalceramic or all-ceramic. what are the instruments(armamentarium) that we use ? 1-handpiece. 2-No.171L bur. 3-round end tapered diamond. 4-short needle diamond. 5-torpedo diamond. 6-torpedo bur. 7-red utility wax.(it is used to get an idea about the thickness that am reducing) As a beginning, we have to: 1do occlusal reduction by rounded end tapered diamond bur, creating 1.5 mm clearance on the functional cusps and 1 mm on the non functional cusps. It is preferred during this step to do orienting grooves(grooves of 1-1.5 mm depth) on the occlusal surface.and this is done as first step by: Round end tapered diamond is used to place the grooves on the ridges and the primary grooves of the occlusal surface.then the tooth structure remaining between the orientation grooves is removed to accomplish the occlusal reduction.see the pictures below

Rounded end diamond bur.

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This picture shows the orienting grooves(notice their position in

the primary grooves and the ridges of the occlusal surface.

Note :similar(vertical) grooves can be done on the smooth surfaces of anterior teeth during their preparation-keep this in your mindPrepared occlusal surface.

why we have to do occlusal reduction at first? *this gives us an idea about how much the remaining height of the tooth after preparation.then we can decide if this height is enough to be retentive or not.and this gives a clue about how much we need to remove from the axial surfaces. -recall: the more height of the prepared tooth, the more retentive restoration. *when we reduce the occlusal surface first, the vision of the distal surface will be better so easier preparation of the distal surface. 2-functional cusp bevel(by 1.5 mm): this is done on the functional cusps: we make wide bevel on the buccal inclines of mandibular buccal cusp and on lingual inclines of maxillary lingual cusps. And again by round-end tapered diamond. orienting grooves are also helpful in obtaining this reduction.

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the left picture shows functional cusp bevel preparation and

the result is in the right picture. Is functional cusp bevel essential step? Yes it is, otherwise thin casting or poor morphology in the restoration will result. Now when you finish occlusal preparation and functional cusp bevel: 3-we use no.171L bur to smooth what we have done(it is carbide bur,and the carbide texture leaves smoother surface after cutting) .and be sure that no sharp angles or ridges at the planes joint, otherwise: 4- remove them by tapered fissure bur. 5-Then you can check the occlusal clearance by red utility wax(2mm thick strip). 6- moving to the lingual and buccal surfacesbuccal and lingual reduction (.5-1mm): Here we use torpedo diamond (see the picture below,it looks like al 9aro5 as the Dr described)

Sides of the bur will produce the desired axial reduction while its tapered tip forms a chamfer finish line.

7-moving to the mesial and distal surfaces: we stop using torpedo diamond because there is big chance to harm the adjacent tooth.so we begin the proximal preparation with short needle diamond(see the picture below). pencil like bur is worked carefully. once enough room has been obtained, the torpedo diamonds is introduced again.

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So the chamfer finish line is made all around(buccally ,lingually,mesially,distally) 8-after that I use torpedo carbide finishing bur to smoothen the axial surfaces.this bur will make the chamfer finish line clearer and easier to pick up by my impression. (be aware that :*we have torpedo diamond-to do preparation- and torpedo bur-to smoothen*the occlusal preparation is smoothened by no.171L, while the axial preparation is smoothened by the torpedo bur.) 9-placement of seating groove: *by no. 171L bur. *on the buccal surface of mandibular preparation and on the lingual surface of maxillary preparation.(because these surfaces have the greatest bulk)

*it aims to: 1-prevent any rotational tendencies during cementation. 2-it will help guide the cast(restoration, crown) to place. 3-to prevent mesiodistal movement(in case of long span we make to seating grooves buccally and lingually to achieve this). Now.what are the rationales for previous steps? -Occlusal reduction structural durability because we need that much thickness for metal crown to stand occlusal forces. -Functional cusp bevel structural durability because much of occlusal load will be over this area. -Axial and proximal structure reduction for retention, resistance as will as structural durability -Finish line marginal integrity and periodontal preservation -Seating groove in case if we need to increase any retention and resistance.

So we have finished with metal crown.. metal-ceramic crown(also called porcelain-fused-to-metal crown):5

If we want to form crown for anterior teeth we have to do it from metal-ceramic or full ceramic. the reduction of tooth structure should be adequate to receive the metal as will as ceramic thickness with adequate contouring: *if you make reduced contouring(less preparation than adequate), you will end up with very thin area of porcelain so the metal will see through, and if your technician over contour that to hide the metal , it will be ugly and more prominent and it will affect the health of surrounding gingiva. *on the other hand if you make increased contouring(more preparation than adequate), you will make the pulp of the tooth in danger. metal ceramic crown has inner full metal layer and outer porcelain layer which reaches to the cingulum. such restoration combines the strength and accurate fit of metal crown with the cosmetic effect of a ceramic crown.it was found that metal ceramic restoration is 2.8 times as strong as all ceramic restoration. keep in your mind : since this restoration is combined of metal and ceramic, then you will find that their preparations are also combined. In fact, there is DEEP facial preparation to accommodate both layers of metal and ceramic. And there is SHALLOWER preparation on the lingual surface and on the lingual aspect of the proximal surfaces. among this topics, it is important for you to be familiar with index term: definition:it is a positive check on reduction produced by the preparation. when to be made? Before the preparation is begun. where it is made? Intra-orally if the existing tooth contour is correct. And extra-orally on the diagnostic cast if the tooth is badly broken. what the doctor explained is the intra orally method. how to make it? We prepare a putty of silicone and put it on the labial and lingual surfaces of the tooth to be prepared and on AT LEAST one adjacent tooth on each side.this will be mold of what we have before preparation. what is to do after the index set and being removed? Cut the index into a labial and lingual half. then cut the labial half into gingival and incisal half. then bring the lingual half and check

if it fits the lingual side of soft and hard tissue, and bring the gingival half of buccal side and check if it fits the corresponding area . what the benefit of these pieces? Notice the index is for original tooth before the preparation and this will give me an idea that I'm doing right or I need to cut more or I did cut for too much.6

So we check the labial reduction after doing preparation-by gingival half of the index. And we evaluate the incisal reduction by the lingual index. Lets go back to our preparation: Which burs and instruments we need? 1. laboratory knife with No 25 blade 2. silicon impression 3. hand piece 4. small needle diamond 5. long needle diamond 6. topedo diamond 7.Topdeo bur 8. H 158 -012 radial fissure bur 9. Rs-1 binarple chisel 10. flat end diamond bur.

Preparation steps: Note:the labial surface of the anterior teeth is composed of 2 planes: if the below draw represents the labial surface of the anterior teeth ,then the upper part is the gingival half and the lower part is the incisal half.

1- the initial step is to make orienting grooves on the labial(gingival) and incisal surfaces with a

flat end tapered diamond. So the labial grooves are 2 sets: *one set parallel with the gingival half of the labial surface. ** other set is parallel to the incisal half. Therefore, so we do 3 grooves in cervical half above the gum in which depth? Thickness for metal ceramic is 1.5 at least so to be in safe side we make the grooves depth 1.2 mm.

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in theory and doctor philosophy in crown preparation is when start to remove we should remove less at beginning because we will remove more after but dont remove more because we can't correct that after and be away from the gum so any time we could bring the finish line below but if we start from the beginning with gingival or subgingival level we can't correct it. Then I will do another two grooves in incisal half : * these two will be within those three ( interdigitate) the incisal grooves should be cut all the way through the incisal edge-all the tooth* thickness is cut-but this cut is only in the incisal end of these 2 grooves and the gingival (!!end of these 2 grooves are of 2mm in depth. (can you imagine @@In here we need 2 mm because we need enough thickness for metal and porcelain to build up the nice shape and translucency of enamel.2- incisal reduction (2mm):

it is done by flat-end tapered diamond (see the picture at side) this is done first-before smooth surfaces preparation-to allow easy Instruments access to the axial surfaces and the gingival finish line. when you do this step, be sure to make the reduced incisal edge parallel to the inclination of the unprepared incisal edge-original one-. 3. labial reduction ( incisal half ) until we demolish these two grooves, and this is done by the same flat end tapered diamond 4. labial reduction ( gingival half ) until the indentation of my grooves disappear so we have 1.2 mm reduction if we need more we go slightly more with my bur rubbing the whole surface to be more rounded. again this is done by flat-end tapered diamond.5- lingual reduction ( wheel diamond bur -rounded angle-see the picture below-) lingual surface

for anterior teeth little bit odd because we have concavity and convexity .

Starting with the concave incisal-part

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Reduce the concavity above the cingulum-more occlusally- to have occlusal clearance(minimum of 0.7mm,and you need 1mm of clearance in the areas of lingual surface that will be covered with BOTH metal and porcelain) because this area where lower teeth contact the upper so all protrusive movement in mandible will be carried in this area. To reduce that,it is difficult to use normal bur the best way is to have a wheel diamond bur ( two types, one with sharp angle and other with rounded angle-always choose the rounded angles-). moving to the convex part: be sure that the junction between the cingulum and the prepared concave part is not overreduced. We talked in retention and resistance that we need the two opposing walls to be parallel but in anterior teeth, we rely only in the parallism of labial surface with the upper part of the lingual surface.( from cingulum downward to the gingival area). the part that is covered by metal only:we reduce in our preparation 0.5mm . then I have to break the contact so we need needle bur to make initial reduction from-6 .proximal surface 7.go back again to the lingual surface, Use torpedo diamond for lingual cervical area ,and make chamfer finish line. We extend our chamfer line until we meet the shoulder finish line that we have prepared labially,

so: 1-anterior prepared tooth to receive metal-ceramic crown has shoulder finish line labially and chamfer finish line lingually. 2-the two finish lines meet proximally.so the proximal labial part will be prepared with shoulder line and the proximal lingual part will be prepared with chamfer line. 3-the junction where the two finish line meet is as a wing that will add to the retention and decrease the freedom of rotation. 8- axial finishing and smoothening by carbide torpedo bur. This is done for proximal and lingual surfaces. 9- labial and shoulder finishing : We use H158-012 radial fissure bur the labial surface, how this bur works:

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This bur is too similar to the fissure bur that we use in conservative clinic. it is made of carbide. During working its sides will smoothen the labial surface and round the angles and the end is forming radial shoulder finish line. So: to convert shoulder to radial shoulder I have radial shoulder finishing bur from carbide and I will go around my shoulder to remove the sharp angles that we create it by flat ended bur. 10. to check our work we use the index (labial index checking). 11. lingual index checking. there is a common mistake that some dentist do, sometime when you do your preparation, if you take more from tooth structure(i.e. more than the width of the bur) there is a chance that part of enamel become more prominent, why? Because the bur is now narrower than the finish line(due to excess preparation). This will break in the stone so when the technician make the crown either will be over or there will be a little gap under the crown so you have to make sure that you take this chisel and go around to remove all this prominent enamel and make them smooth. Now.what are the rationales for previous steps? -Radial shoulder line marginal integrity and periodontal preservation and structural durability. -chamfer line marginal integrity and periodontal preservation. -axial reduction retention and resistance and structural durability. -the meet lingual part with buccal one( I will have like little wing because the reduction will be less start narrow then become wide). This wing is for retention and resistance and the freedom of displacement become much less and path of insertion become more accurate. -At the tip of that wing we have like slip, we call it incisal notch this will help for structural durability because my restoration we be seated there (more stable). **clinical case** - common preparation for central, lateral and canine, why we do metal ceramic for canine while full ceramic for central and lateral?? Because the canine is exposed to more load especially during lateral excursion (recall: 2 types of lateral excursion canine guidance and group function).

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and lingual surface of canine, we make it from metal because as we notice the lower teeth are worn of the porcelain(which is very abrasive) so to decrease the wearing during lateral excursion we make the palatal surface of canine from metal.e pos Now moving to the Posterior preparation for metal-ceramic crown: used for posterior teeth that are in the appearance zone(maxillary premolars,maxillary first molars,mandibular first premolars, and may be mandibular second premolar there are many steps similar to those of anterior teeth. again.make index (similar steps, similar purposes for what we have discussed above) preparation steps: 1.occlusal reduction 1.5 mm ( again it is preferred to do orientation grooves with round end tapered diamond) 2. function cusp reduction 2mm (is also begun with depth orientation grooves) then I go to the buccal surface. 3. orientation grooves 1.2 mm ( because premolar is narrow at cervical area we put 2 grooves cervically and three occlusally, why occlusally we put three?? Because there is a cusp with mesial and distal slope so to remove uniform from both side we put one groove mesially and one distally and one on tip(cusp itself).so by this step we prepare the occlusal half then the gingival half. 4.initial proximal reduction. 5-lingual reduction (making chamfer finish line). 6. radial shoulder finish line facially. 7. in case (optional) if I want to make gingival bevel facially I will do withflame diamond bur. ( in maxillary premolar we dont do it because if we make it the bevel will be covered by metalaesthetically compromised- ). Again .what are the rationales for previous steps? -gingival bevel (optional)marginal integrity. - radial shoulder marginal integrity and periodontal condition , -wings more resistant and retention .

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-Chamfer line marginal integrity periodontal preservation. -Axial reduction retention and resistance. -Functional cusp bevel structural durability. -Occlusal reductionstructural durability.

all ceramic crown preparation :preparation should be made as long as possible to give maximum support for porcelain, so.short crown will be worried to make them from full porcelain -A shoulder with a uniform width at least ( 1 mm) all around. -All sharp angels should be rounded to not create stress in ceramic because it is brittle. -Should be avoided in : 1. patient with edge to edge relation very deep bite, opposing teeth occlude in cervical fifth of the lingual surface give chance .2 .fore chipping for porcelain and fracture 3.short teeth. Preparation steps: 1. orientation grooves(labial-gingival half- 1.2 mm, incisal 2 mm) -these grooves:three gingivally and two incisally. -these grooves are made with flat-end diamond. 2. incisal reduction (2 mm by flat-end diamond) 3. labial reduction incisally with flat-end diamond 4.Labial reduction gingivally with flat-end diamond 5. lingual reduction with small wheel diamond. In our text book: not saying to do initial proximal reduction with needle bur because we need to do shoulder all around ( myself-Dr- I dont like it because there is a chance for damaging adjacent tooth or over prepare so: 6. initial proximal reduction 7. lingual axial reduction 8. radial shoulder finishing all around Radial shoulder for marginal integrity , structural durability, periodontal preservation and resistance.

?Now what to use metal- ceramic or all ceramic It depends on occlusion. .But what ever is the case we will have full radial shoulder for all ceramic12

and for metal ceramic: radial shoulder labially, chamfer lingually

very important note:in case of metal- ceramic crown in upper anterior teeth. Particularly, on the lingual side, DO NOT make the metal porcelain junction where the lower anterior teeth occlude because this will be a week point ( the joint between ceramicand metal) so there is chance to start chipping . full ceramic crown could also used for posterior teeth. Finally, good knowledge with skillful hand will have good preparation .

DONE BY:

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