🌹‏إِنَّ رَبِّي لَسَمِيعُ الدُّعَاءِ🌹

#CrownـBridge #Designing_crown_preparations The principles of crown preparation design 🔶 The following factors need to be considered: 🔹 Materials 🔹Function 🔹Appearance 🔹Adjacent teeth 🔹Periodontal tissues 🔹Pulp ✅ Related to materials: 🌀Metal crowns: 🐚normally no metal surface should be less than 0.5mm thick and occlusal surfaces should be more 🐚 When distorting forces are anticipated, the design can be modified either by reducing the tooth more, and producing a thicker metal layer, or by introducing grooves or boxes into the preparation to stiffen the metal by producing ridges on the fit surfaces. 🌀 Ceramic crowns : 🐚 it is necessary to provide a sufficient thickness of more translucent porcelain on the labial/buccal surface of the crown to simulate the appearance of a natural tooth. 🐚 The edge strength of porcelain is low, and therefore the compromise at the cavosurface angle between brittle enamel and brittle porcelain is a 90°butt joint 🌀 Metal–ceramic materials: 🐚 A thickness of 2mm is ideal, but in many situations, for example lower incisor crowns, this is impossible because of the smallness of the tooth. 🔴 Margin configurations for crown preparations: ☀️a 👉 A section of a molar tooth. ☀️b👉 A knife-edge margin with a cavosurface angle approaching 180°. ☀️ c 👉 A chamfer margin with a cavosurface angle of 130°–160°. This is the most commonly used margin for metal finishing lines. It can be deepened for metal–ceramic margins. ☀️d👉 A finishing line with minimal tooth reduction but with a sharp step, prepared with a square-ended instrument producing a cavosurface angle of approximately 90°. ☀️ e 👉 A full shoulder with a 90°cavosurface angle which is used for ceramic crowns. When used for a metal–ceramic crown, the metal is either brought to the margin or finished short, leaving a porcelain margin. ☀️f 👉A full shoulder with bevelled margin for a metal–ceramic crown when the margin cannot be seen, for example at the buccal margin of lower molar teeth. A metal margin may be easier to finish and be less plaque retentive than porcelain. It is also easier to make in the laboratory. 🔴Related to function: 🔶 Occlusion - the cusps that function against each other in this way should be reduced more than other parts of the preparation. - With tilted teeth the occlusal reduction is often not uniform. 🔴 Related to appearance: 🔶 Labial, buccal, incisal and proximal reduction: 🔺 Adequate reduction of the tooth surface must be carried out on those surfaces where the appearance of the crown will be important. 🔶 Crown margins: The position of the crown margin in relation to the gingival margin affects the appearance. 🔺 Subgingival margins may have a better appearance initially but will sometimes produce a degree of gingival inflammation that, apart from possibly leading to more serious periodontal disease 🔴 Related to adjacent teeth: 🔶 Clearance to avoid damage to adjacent teeth 💥 the tooth surface must be reduced sufficiently to allow the full thickness of the bur to pass across the contact area within the contour of the tooth being prepared. 🔶 Path of insertion: When teeth are unevenly aligned, The solution is either to reshape the adjacent teeth or to design the preparation at an angle that permits the insertion of the crown. 🔴 Related to periodontal tissues: ♣️ The shape of the crown margin (the cavosurface angle) should be designed so that the crown surface can conveniently be made in line with the tooth surface. ♣️ Insufficient reduction at the margin can result in an overbuilt crown, which in turn produces a plaque retention area at the margin. 🔴 Related to the pulp: 🔶 When a vital pulp is to be retained within the crown preparation, a minimal thickness of dentine must be preserved to protect it. 🔶 The thickness of this layer will depend upon: 🔹 the age of the patient 🔹the condition of the dentine (i.e. the amount of peritubular and secondary dentine) 🔹and the type of preparation. 🔶 As with all tooth preparation https://t.me/densitysecond99

#crown_bridge The fracture repaired with composite following grit- blasting with alumina oxide particles and silane priming.

#crown_bridge An intra-oral grit-blasting machine.

#crown_bridge 5-Fractured porcelain at the tip of a bridge retainer that was 5 years old and otherwise satisfactory.

#crown_bridge 3-Polishing the composite. 4-The finished result. This would have been better if an opaquer had been used over the metal to stop the ‘shine through’.

#crown_bridge 2-After polishing with pumice and water, a silane coupling agent is painted over the surface, followed by a resin bonding agent and light-cured composite.

#crown_bridge 1-The porcelain of the lateral incisor facing has chipped. The bridge is more than 10 years old.

#CrownـBridge #Porcelain_fracture 🔴At one time pieces of porcelain fracturing off metal–ceramic restorations, 💢 or the loss of the entire facing due to failure of the metal–ceramic bond, were relatively common. 😏With modern materials and techniques this is much less common; It can often be repaired with composite, 🙁the repair is less satisfactory than the original porcelain 😊Even if the repair only lasts a few years👉👉 before discolouring or wearing, it is a cost-effective way to extend the life of a restoration. 😃To prevent this type of damage to metal–ceramic bridges, 💠the framework must be properly designed with an adequate thickness of metal to avoid distortion 👌👌👌👌 particularly with long- span bridges. 😳If there is any risk of the pontic area flexing, 📢the porcelain should be carried on to the lingual side of pontics to stiffen them further. ✅A ceramic crown or bridge that is fractured must be replaced. 🔰Sometimes the cause is a blow, and then the choice of material can be regarded as fortunate: 👉had a metal–ceramic material been used it is more likely that the root of the tooth would have fractured. 👉🤕 If the fracture is due to trauma, and particularly if the crown or bridge had served successfully for some time⤵️⤵️⤵️ ✅ it should be replaced by means of another all-ceramic restoration. 😬😖if the failure occurs during normal function, shortly after the crown or bridge is fitted👇👇👇 ❌the implication is that the conditions are not suitable for a ceramic restoration, ✅and the replacement should be metal–ceramic.

#CrownBridge 🔴Causes of failure and some solutions ⚫️Mechanical failure of crowns or bridge components: 🔻Typical mechanical failures are: ✳️• Porcelain fracture✳️ ✳️• Failure of connectors: solder and laser welded joints. ✳️• Distortion. ✳️• Occlusal wear and perforation. ✳️• Lost acrylic facings.

#crownBridge ✍Solutions for loss of retention If there is no extensive damage to the prepara- tion, it may be possible to re-cement the crown or bridge, provided that the cause can be identi- fied and eliminated. ✅It may be that a bridge was dislodged by a blow or that some problem during cementation was the cause. ✅However, if the underlying reason is that the preparations are not adequately retentive, they can sometimes be made more retentive and the crown or bridge (or at least the unsatisfactory retainer) remade. ✅A more adhesive cement could be tried such as a chemi- cally active cement as used for minimum-prepara- tion bridges. ✅ It is always wise to re-check the occlusion in case this contributed to the failure. ✅Alternatively it may be necessary to include additional abutment teeth in a bridge to increase the overall retention or to change the design in some other way. 🔴Reference Planning and MakingCrowns and Bridges Fourth Edition Bernard GN Smith Leslie C Howe

#crownBridge ✅There is some evidence that minimum-prep- aration bridges are retained for longer periods when they have been re-cemented. ✅It is difficult to imagine why this should be, other than perhaps the operator taking greater care the second time around. ✅If a minimum-preparation bridge debonds soon after placement, this is often due to poor cementation technique and if re-cementation is done with more care, the bridge is likely to last for longer. ✅Bridges that survive for many years and then debond may well not last for as long a second time. ✅There is now good evidence that fixed–fixed minimum-preparation bridges fail through loss of retention more readily than cantilever (with one abutment tooth) and fixed–movable designs. ✅This is why these designs have been advocated earlier in this book. ✅ It is very unusual for a minor retainer for a fixed–movable minimum-prepar- ation bridge to lose its retention because there are no significant forces to dislodge it.

#crownBridge ♻️Minimum-preparation bridges ✅Loss of retention of one part of a fixed–fixed minimum-preparation bridge also occurs but, although caries does sometimes develop rapidly, because the surface of the tooth is enamel rather than dentine, the development of caries is usually slower than with a convential bridge. ✅ If one retainer does become loose, it is a matter of urgency to remove at least that retainer, and usually the whole bridge. ✅If a fixed–fixed minimum-preparation bridge becomes loose at one end but seems firmly attached at the other, one option is to cut off the loose retainer, leaving the bridge as a cantilever.

#crownBridge ✅There is some evidence that minimum-prep- aration bridges are retained for longer periods when they have been re-cemented. ✅It is difficult to imagine why this should be, other than perhaps the operator taking greater care the second time around. ✅If a minimum-preparation bridge debonds soon after placement, this is often due to poor cementation technique and if re-cementation is done with more care, the bridge is likely to last for longer. ✅Bridges that survive for many years and then debond may well not last for as long a second time. ✅There is now good evidence that fixed–fixed minimum-preparation bridges fail through loss of retention more readily than cantilever (with one abutment tooth) and fixed–movable designs. ✅This is why these designs have been advocated earlier in this book. ✅ It is very unusual for a minor retainer for a fixed–movable minimum-prepar- ation bridge to lose its retention because there are no significant forces to dislodge it.

#crown_bridge Carious abutment teeth (the upper right canine and upper left first premolar) revealed by removing a bridge that was still firmly attached to the sound central incisors.

#crown_bridge The upper central incisor had a post-retained crown but no diaphragm covering the root face. The tooth has split longitudinally and the crown has fallen off. It must now be extracted.

#crownBridge ♻️Conventional fixed–fixed bridges and splinted retainers ✅When only one retainer of a conventional bridge becomes loose, this can be disastrous. ✅Without a cement seal, plaque forms in the space between the retainer and the abutment tooth and caries develops rapidly across the whole of the dentine surface of the preparation (Figure 14.2). ✅Sometimes the patient is aware of movement developing in the bridge or experiences a bad taste from debris being pumped in and out of the space with intermittent pressure on the bridge. ✅A good diagnostic test for a loose retainer is to examine the bridge carefully without drying the teeth, pressing the bridge up and down and looking for small bubbles in the saliva at the margins of the retainers.

#crownBridge 🚫Causes of failure and some solutions✅ ❌Loss of retention ✅With the exception of post crowns, where failure is usually due to inadequate post design or construction (Figure 14.1), ✅loss of retention is not a common cause of failure of individual crowns. 👉However, because of the leverage forces on fixed–fixed bridges, one of the more common ways in which they fail is by one of the retainers becoming loose but the other remaining attached to the abutment tooth.

#CrownBridge 🔮Components of bridge🔮.? 1-Abutment Is the natural tooth or root which support the bridge on which the retainer is cemented.... 2-Retainer: lt is that part of bridge 'which is cemented to the Abutment tooth 3-pontic It is that part of bridge 'which replace the lost natural tooth. 4-connector: That part of a bridge which connect the pontic to the retainer 'lt could be rigid or non rigid A-Span :is the space between natural teeth'that is to be filled by the bridge. B-Saddle :is the area of the edentulous ridge'over which the pontic will lie.. C-Pier : is an abutment tooth standing between and supporting tow pontics. D-Unit :when applied to bridgework 'means either a retainer or pontic

#CrownBridge Marginal deficiencies: 🔴 Positive ledge (overhang) A positive ledge is an excess of crown material protruding beyond the margin of the preparation. 🔴what can i do❓ 🍃Considering that this is a fairly easy fault to recognise and correct before the crown or bridge is fitted, it is surprising how frequently overhangs are encountered. 🍃However, it is often possible to correct them without otherwise disturbing the restoration. 🍃A positive ledge should be adjusted until the probe passes smoothly from tooth to crown without a catch. 🍃A negative ledge is a bigger problem and often means that the crown has to be remade.

#CrownBridge The impression material divided into two types lmpression materials which used in crown and bridges......🔮..? 🔮Classification.....🔮 1-Non Elastic:🔮 A-plaster of paris. B-lmpression compound. C-zinc oxide Eugenol. D-some waxes. 2-Elastic:🔮 1-Hydrocolloids:🔮 A-Reversible(Ager-Ager). B-Irreversible(Alginate). 2-Elastomers(Rubber Base):🔮 A-polysulfide. B-condensation silicone. C-Additional silicone. D-polyether.