C & B

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

‫بس‬ ‫م الله الرحمن الرحيم‬ :In this lecture we will talk about Bridges configuration Biomechanical principles of tooth preparation -

Bridges configuration Last lecture we ended up with bridges configuration which is a .'separated chapter in the book 'shillingburg It will be a quick revision for bridge configuration because the .principles almost mentioned in the abutment evaluation lecture This chapter is divided into simple design bridges and complex design bridges, you are requested for simple design bridges(which means single ..tooth replacement bridges) and canine replacement bridges Dr. said: There is few details in the book, you have to come cross them, this details will be more clear when we get resin-bonded bridges lecture.. ((I added them next Usually, we replace the central incisor using resin-bonded bridges, conventionally they used to be 2 wings, one on each side of the adjacent .central and lateral incisors Lateral incisor is the same, 1st premolar also the same( upper and lower), but in the 2nd premolar we start talking about conventional bridges, those bridges that are luted using conventional cement which …are: GIC, RMGI When you go back to the book they are talking about three-quarter crown retainers, and in the treatment plan I mentioned that three-quarter crown retainers are not the best, the best is full veneer crowns, so I need you to escape this little piece of details from the book and not to use three-quarter crown because we don't recommend this crown in this .institute, so you go for full veneer crown to retain conventional bridges 1st molar again it's a conventional bridge where we use the lower 7 and .lower 5 as conventional retainers All the mentioned bridges are simple bridges by design because you .have single missing tooth In canine replacement bridges, it's again single tooth replacement bridges, but I mentioned last lecture that they aren't along the interabutment line, so we have a torque, unfavourable leverage, or unfavourable load distribution, then conventional bridge won't work, I ..mean 2 abutments won't work 1

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

In canine replacement bridges we use the lateral, central and 1st premolar as retainers, so we use double abutments in the “mesial side”, by that we consider the central incisor a secondary abutment, and secondary abutment should have more or less, comparable crown-root ratio, comparable periodontal ligament surface area, comparable retention and resistance features, then you can use double abutments on the mesial side, but you can't use distal secondary abutment, you can't use 4 and 5 and lateral incisor, because the lateral incisor is quite weak and if you use it alone mesially, this bridge is going to fail, but if we use lateral and central they will reinforce themselves, the .same is applicable in the lower canine Examples: to replace the 5 we make conventional bridge using the 4 and 6 as retainers with full veneer crowns. And to replace the lateral incisor .we use a resin-bonded bridge In the book there is details where you can't use resin-bonded bridges you need to come cross them, it will be more obvious and clear when I …discuss with you the resin-bonded bridges lecture :From the book Simple fixed partial dentures Missing: maxillary central incisor Abutments: central incisor and lateral .incisor .Retainers: resin-bonded retainer .Pontic: metal-ceramic . Abutment-pontic root ratio: 1.9

Missing: madibular central incisor

Abutments: central incisor and lateral .incisor .Retainers: resin-bonded retainers .Pontic: metal-ceramic . Abutment-pontic root ratio: 2.1 2

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

Missing: maxillary lateral incisor

.Abutments: central incisor and canine .Retainers: resin-bonded retainers .Pontic: metal-ceramic . Abutment-pontic root ratio: 2.6

Missing: mandibular lateral incisor .Abutments: central incisor and canine .Retainers: resin-bonded retainers .Pontic: metal-ceramic . Abutment-pontic root ratio: 2.5

Missing: maxillary 1st premolar .Abutments: canine and 2nd premolar Retainers: resin-bonded retainers, if .teeth are unblemished .Pontic: metal-ceramic . Abutment-pontic root ratio: 2.1

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

Missing: mandibular 1st premolar

.Abutments: canine and 2nd premolar Retainers: resin-bonded retainers, if teeth .are unmarred .Pontic: metal-ceramic . Abutment-pontic root ratio: 2.5 Missing: maxillary 2nd premolar

.Abutments: 1st premolar and 1st molar .Retainers: full veneer crowns .Pontic: metal-ceramic . Abutment-pontic root ratio: 3.1

Missing: mandibular 2nd premolar .Abutments: 1st premolar and 1st molar Retainers: metal-ceramic crown on .premolar and full veneer crown on molar .Pontic: metal-ceramic . Abutment-pontic root ratio: 3.1

Missing: maxillary 1st molar .Abutments: 2nd premolar and 2nd molar .Retainers: full veneer crowns .Pontic: metal-ceramic . Abutment-pontic root ratio: 1.5 4

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

Missing: mandibular 1st molar .Abutments: 2nd premolar and 2nd molar Retainers: metal-ceramic crown on premolar and full veneer crown on .molar .Pontic: all-metal hygienic . Abutment-pontic root ratio: 1.5

Canine replacement bridges Missing: maxillary canine Abutments: central incisor, lateral .incisor and 1st premolar .Retainers: metal-ceramic .Pontic: metal-ceramic . Abutment-pontic root ratio: 2.3

Missing: mandibular canine Abutments: central incisor, lateral incisor .and 1st premolar .Retainers: metal-ceramic .Pontic: metal-ceramic . Abutment-pontic root ratio: 1.9

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

Biomechanical principles of tooth preparation We will go for further details in fixed prosthodontics, the first lecture was definitions, second lecture was treatment planning either about extracoronal, intracoronal or replacement bridges, third lecture was about abutment evaluation mainly for bridges, now and in this specific time you should be able to formulate a treatment plan for your patient, if you do that then you are able to start fabrication of this prostheses, I assume if I asked any body of you about replacement or restoration of any tooth in terms of fixed prosthodontics you should be able to tell me what is your restoration, if you all reach this stage then the next step is how to .fabricate your prostheses (…Midterm exam will have a lot of questions about treatment plan) Today we are going talk about proper work, we decide the treatment ..plan and we go to the next step which is tooth preparation Tooth preparation will be only for crown and bridges, we won't talk about intracoronal restoration.. we will talk mainly about extracoronal restoration, some concepts might be valid for both but you need mainly to ..focus on extracoronal restoration

:Principles of preparation When you prepare a tooth for any reason actually, for fillings, crown or :bridges, there are 5 main principles you need to follow them .A. preservation of tooth structure .B. retention and resistance .C. structural durability .D. marginal integrity .E. preservation of periodontium A. Preservation of tooth structure Generally in dentistry you have to be conservative, it means you don't☻ need to cut teeth unnecessarily, at the same time you need to cut enough .tooth structure to make a durable restoration There is equation between durable, strong, and stable restoration,☻ there is a limit, a red line you don't have to cross it and there is extra or .unnecessary tooth removal which could sacrifice your tooth structure 6

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

retentive, strong and stable restoration is a golden aim, and removal☻ of enough tooth structure to achieve the goal is mandatory, but there is no .need to remove sound tooth structure which could endanger it's integrity Imagine that you need to make a metal-ceramic crown, and for ☼ ► metal-ceramic crown you need to reduce the occlusal surface about 1.52mm, this is the least amount of tooth structure to be reduced, if you decide to make metal-ceramic crown you don't need to cut 3mm because this is unnecessary cut and you are sacrifice your tooth structure, at the same time you can't cut only 1mm because less than 1.5-2mm you will have weak restoration, weak structure and weak material. Then you need to stop at a border line where this is the optimum reduction you need, and beyond this you sacrifice your tooth structure, also you sacrifice your .restoration You need to be conservative without compromising the principles of ☼ ..further issues If you sacrifice more tooth structure, as any other tissue, you will ☼ .(endanger its integrity (pulp, periodontium and structures Look at the table (the last page in the slides) for mandibular teeth, ☼ there is a double line underlining the lingual surface of the incisor, the lingual wall is 0.6mm enamel and 0.9mm dentin, imagine that you need 1.5mm for metal-ceramic crowns, what does that mean?? You are encroaching the pulp, then in such a scenario, your treatment planning should not include a crown which need such tooth destruction, you can use for example all-ceramic crowns that need 1mm, then it's more safe to use all-ceramic crown in lower central incisors rather than to use metal..ceramic crowns If you have to use metal-ceramic crowns you need to prepare your pulp first which means making endodontic treatment for the tooth. You have .to consider all of these things in your treatment plan Another thing, If you use all-ceramic crown and you need 1mm only ☼ reduction, you don't need to go for 1.5mm because you will encroach the pulp. So this is unnecessary cut, and if you go for 0.5mm which is less .than you need then the crown will be weak So you need to decide which type of crown, many reason behind it, one of them is the amount of tooth structure you need to remove, and how .much you will damage the integrity of the tooth

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

B. Resistance and retention Retention prevents the removal (displacement) of the restoration☻ .along the path of insertion or along the long axis of the tooth preparation Resistance prevents the dislodgment of the restoration by forces☻ .directed in any direction except the direction of the path of insertion The important pit in crown and bridges is the resistance, because you☻ need the retention for one direction of force ( the direction of path of ..insertion) and you need the resistance for multiple directions of forces In the oral cavity we have more non long axis directing forces, and if we put the crown, it will move buccally or lingually, mesialy or distally if it was bridge, but if it move upward, the only thing to do that, is to eat a sticky food or try to remove it, so for one direction of force we need the retention and for multiple directions of forces we need the resistance, then we conclude that the resistance is more important than the retention but you can't say I can have resistant crown but not retentive, .you need them both but the priority is for the resistance Both of them are interrelated topics and together they are called☻ stability, you need a stable restoration which means a retentive and ..resistant restoration we fix the restoration to the tooth structure by cement(in conventional☻ prostho) or screw(in implants), the strongest cement can't replace proper retentive and resistant feature, which means if you depend solely on cement your restoration will fail, you need something else to help the cement to prevent the dislodgement of the restoration, these things are the preparation features or what is called geometric configuration of your .preparation When we talk about retention and stability we don't talk about☻ cement, the cement is an adjunct to help you stabilize your restoration (it's not the primary retaining and stabilizing forces for the restoration), if you don't have enough geometric configuration in your preparation, your .restoration will be dislodged Geometric configuration must place cement under compression when☻ .possible :Direction of forces► When you consider the geometry of your preparation, you need to☼ consider the direction of forces, the best force or reaction of the cement .is the compressive force and the worst thing is the shear force :In the picture below notice A. tensile force B. shear force 8

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

C. oblique force( which could be divided into shear and compressive .(forces D. compressive force

Shear Force

Tensile Force

Compressive Force

Oblique Force

When you prepare a tooth you have to consider that, whenever☼ possible put your cement under compression and avoid shear, this is by …principles and we will see later how we apply it I spoke about retention and resistance and I mentioned that the geometric configuration of your preparation is very essential, this geometry is :affected by

Taper .1 Freedom of displacement .2 Length .3 Substitution of internal features .4 Path of insertion .5 Taper .1 .Taper is the relationship between 2 opposing surfaces☻ The opposing surfaces of tooth structure must converge/diverge☻ occlusally slightly with no undercuts to allow seating the final .restoration The opposing surfaces must be converge occlusally in case of extracoronal restoration (crowns), and diverge occlusally in case of intracoronally restorations…the reason for that is because you need no undercut to allow seating the final restoration, which means if you have intracoronal restoration for example amalgam, what do you do in amalgam?? We make undercut, we bring the inverted bur and walk all 9

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

around the walls, but we don't have problems with these undercuts .because we can condense plastic restorations In fixed prosthodontics it's vice versa, I need it without undercuts and flare upward, because when we bring the inlay or onlay, this shape will .allow us to seat them without problems So we need it diverge occlusally in intracoronal restoration and converge .occlusally in extracoronal restoration The relationship between the 2 opposing walls mesial and distal, ☻ buccal and lingual is called taper.. and the relationship between the long .axis of the tooth and one of these single walls is called inclination The more the parallel the opposing surfaces, the better the stability of ☻ .the restoration. More forgiving in big teeth Look at this premolar(in the next picture), there is an intracoronal☼► restoration, imagine the gingival floor is wider than the occlusal access, would this restoration fit in?? of coarse not, it's impossible to enter some thing wide, rigid and non-flexible into a narrow opening, then you need your walls to be divergent occlusally. Extracoronal restorations or crowns .are vice versa, we need to converge the walls occlusally The angle they meet with, which is 6°, this is taper or angle of☼ convergence. The angle between the long axis of the tooth and the preparation wall which is 3° is the inclination, and we create this by using inclined bur, so the inclination is related to the bur. Taper is related to the .angle which composed of 2 inclinations When you prepare your tooth, the ideal taper is to be parallel to get☼ the best stability with the least tooth destruction, and to avoid the loss of geometrical configuration, but with very parallel walls the restoration won't fit in easily, so we make a little taperness, which is about 6°, this is the ideal, but it's unfortunately unachievable and we can't create it in the patient mouth, by making slight taperness, it will guarantees a good geometrical configuration for best retention and resistance and it .guarantees full seating of the restoration If you look at the diagram(in the same picture) the Y-axis represent☼ the retention or the stability force and the X-axis represent the degree of taper, at 5° or 6° you need 100 grams to displace your restoration , the .more you go for 'more taperness' the less the retention The 20° of taper is the critical point, beyond it you will lose a lot of your retention and stability, above it you can achieve reasonable good .retention and stability features plus saving the structure of the tooth The best taperness or the achievable taper is 16°, you can achieve it in the clinic, give you a good retention and stability and allow the seating of 10

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

your restoration. If you achieve something above it this is fine but try to …achieve any thing under it, so 16° is your target but the ideal is 6°

TAPERNESS 3° 3°

6°

INCLINATION

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Freedom of displacement .2 Best retention and resistance is achieved with a single path of☻ .withdrawal The addition of auxiliary features (grooves and boxes.. ) will limit☻ .the paths of withdrawal to one .The greater the surface areas, the better the stability☻ If you look at diagram A in the lower picture, the restoration can be☼► placed and displaced along all of these arrows, if you start adding feature like a grooves and boxes it will limit the number of paths of insertion and removal, and we will end up with one path of insertion(diagram B). ☼ Imagine you have a crown and on the fitting surface of it we have something protruding outward, they won't get into the preparation unless .. you orient it well and seat it All auxiliary features (grooves, boxes, pin holes..) help you to☼ minimize the number of paths of insertion and removal, if you have less number of paths of withdrawal this mean automatically that you have less .chance of displacement This is how you create more retention and more resistance by☼ modification of your preparation rather than depending on the cement .itself

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

Last time we said that in bridges we need buccal and lingual grooves☼► .because the movement is mesiodistal Imagine you prepare these grooves on this premolar(in the lower☼ picture), the only force which will let the grooves to resist movement is buccal or lingual forces, so this is shear forces, which is the worst and to minimize its effect we need to adjust our preparation and geometry, one .of these adjustments is to create a grooves Notice that in the previous picture the grooves help us to minimize☼ the number of paths of withdrawal and in this picture they help us to .resist the shear forces When you design your geometry, you consider where are shear☼ forces and compressive forces then you adjust your preparation to let the .shear forces to be minimal and compressive forces to be maximal Look at diagram B, the grooves are flared, and you have three-☼ quarter crown, if you push the restoration with the direction of the arrow, let us say buccally, the restoration will flex outward with excessive forces, if you have definitive walls like in diagram A the force will be perpendicular to the tooth structure then the chance of displacement is less. In other words, as we are aiming parallel walls for extracoronal restoration we need parallel walls for our features (grooves, boxes, pin holes..) because you need to let the forces applied to the tooth structure to be perpendicular rather than in oblique direction that will lead to .displace and flex the restoration

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

Length .3 .the longer and the larger the preparation, the better the retention ☻ length/width ratio is very critical specially in posterior teeth (not ☻ less than 0.4) for resistance. which means after you prepare your tooth, the cervicoocclusal dimension should be at least 0.4 of the mesiodistal dimension. Example: if the width was 10mm the length must be 4mm, this is the minimum otherwise you sacrifice the retention and the .resistance In anterior teeth, they are narrow teeth but the minimum length is 3mm .(correct it in the slides), otherwise you can't restore your crown short wide preparation should be modified by auxiliary features like☻ .grooves and boxes which will improve the stability If you have lower molar with a length/width ratio of 0.3, this is insufficient to retain or stabilize a crown, if you need to improve it you have to create a grooves, which needs parallel walls to reduce the shear .forces Retention► The longer the preparation the better the retention, this is goes mainly☼ with surface area, so also the wider the preparation the better the .retension Retention increases inversely with surface area and when un-☼ .polished Retention is just expresses itself along the long path of insertion,☼ .then adding any surface area will enhance the retention Any increase in the surface area(by increase length or width) will☼ result in improvement of retention, but this is not applicable for .resistance

:Oblique forces and resistance► we said oblique forces are very common in the oral cavity, imagine ☼ you have oblique force on the right diagram in the lower picture, there is 13

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

something called arc of rotation which represented by p1, p2, p3, if you need to displace your restoration with the direction of the arrow, there is something called pivot point (center of rotation) that located on the finish line and where the rupture of the restoration occur, if you make a perpendicular line from this point crossing the preparation, then any area below this line won't resist displacement (non-resistance area), and any area above it will resist displacement (resistance area). And what is .your aim?? To increase the resistance area

Arc of rotation

Resistance area Non-resistance area Pivot point

:Preparation width and resistance► If you have long restoration, what it will have, higher resistance☼ .area or less?? There will be higher resistance area .If you have wider restoration, it will have less resistance area☼ The reason that this perpendicular line will go more distance and the more it goes it will go up as well, so the resistance area will be less in wide preparation. (see the comparison between wide and narrow .(preparation in the lower picture Remember retention enhanced by increasing the surface area,☼ whatever the surface area is: length, mesiodistal dimension or buccolingual dimension, but resistance increasing directly with length .and narrow preparation

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

:Oblique forces, auxiliary features and resistance► If we consider resistance is more important than retention then I need☼ to adjust any wide preparation, and this happens by grooves, boxes… and these auxiliary features won't only reduce the number of paths of insertion and resist the shear forces, also it will reduce the arc of .displacement In the lower picture, look at diagram C and compare it with diagram☼ A, in diagram A there is no modification and the resistance area is the dark area only, in diagram C we have 2 arc of displacement: r1 and r2, r2 is shorter, this mean that we have a good resistance area created by the groove, again if you have parallel walls in your groove you will enhance .your resistance

:Oblique forces, taper and resistance► We said previously that resistance and retention enhanced by☼ geometry, So your geometry is very important for the stability of your .restoration When we talk about taper, we mention that it's important to have as☼ parallel surfaces as possible, the reason behind it is that the more parallel the surfaces, the better is the stability because you have larger area of .resistance Look at diagram A in the lower picture, the surfaces are very parallel☼ and the whole area is an area of resistance, because if you have oblique force and the pivot at the finish line, the perpendicular line will go to the other finish line, because if you increase the taper a little bit, it will become higher a little bit and the dark area will become less, then the resistance will become less as in diagram B. If you go for further taperness this perpendicular line will go higher and the dark area will be .much less, which means the resistance will be less as in diagram C …So taper play a crucial role in resistance

15

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

:substitutions of internal features .4 .It's grooves, boxes and pin holes☻ .used when needed☻ parallelism is important for their opposing walls and easier to achieve☻ than preparation walls. So parallelism applies on external walls as well as .on the walls of auxiliary retentive features :Path of insertion .5 .we are aiming all the time for one path of insertion ☻ to reduce the number of paths of insertion, what we did?? We did ☻ .grooves, boxes.. so you adjust your geometry .it is usually along the long axis of the tooth ☻ the stability is affected, which is resistance and retention, so if you ☻ have multiple path of insertions; the stability will be poor, and if you .have one path of insertion; you will have a good stability .(aesthetic and biology will be affected (explained later ☻ .in bridges, all abutments should have common path of insertion ☻ You remember in treatment plan, if you plant for a bridge you need to create a single path of insertion for both preparation, and I mentioned sometimes you have to sacrifice unnecessary tooth structure to create this parallelism, if you have inclined 7 and you want to replace 6 , you need to create a retainer on the 5 and 7 and you need to cut unnecessarily from the mesial surface of the 7 to have common path of insertion, by this unnecessary cut you sacrifice in retention and resistance because you are sacrifing the taper. If you have parallel teeth from the start, then the mesiodistal taper and buccolingual taper can be created perfectly with 16° here and 16° there to have common path of insertion. If you have a tooth which is very tilted, so you need to cut much more from the tooth .structure

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

look at diagram A in the lower picture, this path of insertion is along☼► the long axis of the tooth, provided you have no tilted tooth, the more .safe definition it should be parallel to the adjacent contact area Diagram C, the molar is tilted mesially, if you create the path of insertion along the long axis of the tooth, when you try to insert your crown it will be blocked by the distal surface of the 5(the dark spot). in this scenario ??what is the solution we prepare a crown which is parallel with the adjacent contact areas as in diagram D, so your aim is to create a preparation which is parallel with the adjacent contact area and this is applicable all the way through, but parallel with the long axis is not applicable all the way through, because when the tooth is tilted, the path of insertion will be interfered by the .adjacent tooth

This is another example, the preparation in the anterior teeth,☼► usually, it's along the long axis of the tooth, but if you put it more lingually or more labially it will affect the excellence of aesthetic and .pulp tissue

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

:From the book In the above picture, the path of insertion should be parallel the long axis .(of the tooth (diagram A If the path is directed facially, the prominent facioincisal angle may (create aesthetic problems of over-contouring.(diagram B However, if the path is directed lingually, the facial surface will intersect the lingual surface, creating a shorter preparation. It also may encroach .the pulp

C. Structural durability when I talked about preservation of tooth structure, I mentioned ☻ that we have compromise, you need to cut enough tooth structure to fabricate strong restoration, and you should not cut unnecessarily because you will endanger the integrity of the tooth, and you should not cut less because you will endanger the restoration, this is called structural ..durability restoration must contain enough bulk of material and have enough ☻ .thickness to withstand the occlusal forces this bulk must be confined to the space created by tooth ☻ preparation, it means if I need for metal-ceramic crown 1.5mm occlusal reduction and I prepared only 0.5mm, so the technician will add another 1mm to have a good thickness of the material, but when you seat it in the patient mouth there will be 0.5mm which is for fitting and 1mm out of the contours, how it will affect?? It will interfere with the occlusion, with .the contour of the tooth because you are adding extra material :The techniques for that including .occlusal reduction .1 .functional cusp bevel .2 .axial reduction .3 .reinforcing struts .4 occlusal reduction .1 how much you need to cut of the occlusal surface, depends on the ☻ type of the material that you will use(1-1.5mm for gold, 1.5-2mm for metal-ceramic crown, 2mm for all-ceramic),these numbers are your minimum preparation, and try to avoid extra cut to avoid damaging the .tooth, also try to avoid undercut to avoid damaging the restoration should keep the same morphology, it means for example if you ☻ prepare a molar, after you finish, you must get a smaller molar, which means 2mm reduction is homogenous all the way through; in the fissures, 18

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

cusps, fossa.. so you are creating a mini molar, you can't reduce from the cusp 2mm and leave the fissures and cut only 1mm, so you have to be .homogenous on the occlusal surface .inclined surfaces may not need tooth preparation ☻ in the lower picture, if you look at diagram A (ideal cut), the☼► preparation on the molar is exactly the same shape but with 1-1.5 ..(reduction, so you create a small molar (mini shape of the original tooth diagram B (under cut), the cusps are cut but the fissures are remained without preparation, so when you create a crown, this area will be very .thin to prevent the interference with the opposing tooth So if we don't have sufficient tooth structure, the technician has 2 options either to create thin restoration and this is weak and it tends to fracture, or .to make it thick and it will interfere with the opposing occlusion diagram C (extra cut), if you try to create 2mm without following the morphology and in the area of the fissures you cut 2mm and cut the whole tooth at this level, so in certain areas you cut more than 2mm, for example in the area of the cusps you will cut 4mm, assuming the cusp .height is 2mm

.Inclined planes may not need preparation☼► If you look at diagram A, let us say that the ideal preparation is to☼ cut 2mm, the mesial side of the lower molar is inclined, there is already space between it and the opposing tooth, in this case you don't need to cut unnecessary tooth structure to create 2mm, say there is a space of 1mm so you can cut further 1mm and you end up with something like in diagram B, and when the technician build the crown there will be a .contact on the mesial side without problems

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

functional cusp bevel .2 The buccal cusp is the functional cusp in the lower archThe palatal cusp is the functional cusp in the upper arch.A wide bevel on the functional cusp to withstand heavy forces ☻ The functional cusp bevel is the area where we need to create further ☻ space because this area represent the centric stop where the opposing tooth meet and where the load occur (excessive load area), so you create a bevel of 45° on the cusps to create more space, we talk about 2.5mm .reduction in this area so the bevelling is about 0.5mm

:From the book In the next picture, look at the functional cusp bevel on the buccal☼► .cusp

Lack of functional cusp bevel can cause a thin area or perforation in ☼ .the casting, also may result in over-contouring 20

C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

axial reduction .3 we need to create a space, this space usually 1.2mm for metal- ☼► ceramic crowns , 1mm for all ceramic , for gold or all-metal crowns we talk about 0.7mm.. It’s more conservative to do metal-crown for posterior teeth rather than to do metal-ceramic crown. But you can't do metal .crowns anteriorly because it's not aesthetic

:From the book In the above picture, diagram A represent an ideal axial reduction. Inadequate axial reduction can cause thin walls and a weak restoration .((diagram B), or a bulbous, over-contoured restoration (diagram C (Reinforcing struts(trusses .4 We can join the grooves by struts which are another simple grooves, it's .a reinforcing design, just to enhance the structural durability .Notice the reinforcing struts in the next picture

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C&B lec#:4 Dr. Mousa Marashdeh

Done by: Ruba Sameer Date: 11-3-09

D. marginal integrity Your crown or restoration should meet the tooth at a margin which is ☻ .the finish line Finish line: is the line where the restoration meets the preparation, and☻ the restoration must be will adapted to the finish line to prevent any .(marginal defect (recurrent caries, periodontal problems The restoration must closely adapt to the cavosurface finish line of the ☻ .preparation the configuration of the finish line will affect the bulk of material, ☻ .adaptation, seating and aesthetics We will continue this topic in the next lecture enshaAllah E. Preservation of periodontium .We will talk about it in a separate lecture

The End : ‫بالنهاية حابة اوجه تحية ل‬ ‫ زينببب)بببدنا‬,‫ جمانببة شمسبباه‬,(..‫ جمانببة حسببن) يببا بابببا سببناني واوا‬,(..‫شهد قعببدان) غلبتببك معببي‬ ‫ امل‬,‫ مجد الصيدلي‬,‫ فاطمة الطائي‬,‫ فرح عبد المعيد‬,‫ نور حمدان‬,(..‫ سناء) هدافة الدفعة‬,(..‫كعكة‬ ,‫ ملببك‬,‫ سببكينة‬,‫ زينببات‬,‫ شببيرين‬,‫ رونببزى‬,‫ فببرح القببرم‬,‫ لينببا‬,‫ داليا‬,‫ مرام ونور الرحمون‬,‫العمري‬ ....‫ ميمنة‬,‫ اسيل الرفاعي‬,‫ ايمان الراس‬,‫ فكرية‬,‫ نجلء‬,‫ منى‬,‫ نادية‬, ‫اروى‬ ,‫ عمببار‬,‫ اميببن محاجنببة‬,‫ باسل‬,‫ امين مدلج‬,‫ اياس خليلية‬, (..‫عبدال عوضي)شكراً على المساعدة‬ ......‫ نعيم‬,‫ معاذ‬,‫ محمد شعبان‬,‫ محمد خزعلي‬,‫محمود‬

‫تمت وبحمد الله‬ …Best wishes for you all

Ruba Sameer Aburima

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