 Hello everyone welcome back to another session in dentistry ammo So we have the most important part of FPD there is a fixed partial denture the principles of tool preparations very very important question So it's been asked every two year as a essay question or you might get a Single principle as a short note. So you just cannot skip this chapter for your exam preparation So tooth preparation has basically five principle. They are preservation of tooth structure the retention and resistance form The structural durability the marginal integrity and preservation of the pedodontia Or we can classify a requirement of a tooth preparation into these three broad categories that is biological mechanical and aesthetic in Biological we need to think about the pulp vitality maintenance and The adjacent teeth and the soft tissues and we need to conserve the tooth structure in mechanical we have the resistance and retention form in aesthetic it includes a minimal display of metal and and The adequate thickness of porcelain and proper shade matching so when all these three factors combined to Produce the best tooth preparation. So we have the five principles of tooth preparation. So we'll start with the first one that is Preservation of the tooth structure So as per definition the tooth preparation is a process of removal of deceased and Or healthy enamel dentine anisement to shape a tooth to receive a restoration Okay, so the first principle preservation of tooth structure Where we need to Preserve the tooth structure that is a remaining tooth structure with some guidelines So the guidelines will change when the tooth we prepare is partial veneer or complete Restoration for the guideline changes according to the restoration we are planning or the crown we are planning and also it Changes with respect to the finish line that is supra gengeval and subgengeval Subgengeval has more chances of Disturbing the paradigm So the preparation of teeth with the minimal practical Convergence angle should be made that is between the axial walls. So it should be having minimal convergence of axial walls Then regarding the occlusion reduction we should make the anatomic planes with respect to the Reduction protocol. We just cannot reduce the surface as such We need to follow the occlusion plane then only there will be good preservation of tooth structure We should not destroy the tooth structure unnecessarily. You should just follow the curse and all the tooth Anatomy In order to receive the restoration. So that is our next point occlusion reduction We should follow the anatomic plane the first one was minimal axial wall convergence and it should change Based on the type of restoration and based on the type of Gengeval finish line And if suppose the axial surface is not in a proper angulation We need to Reposition it authentically the axial wall should be parallel to each other. So if necessary We need to go for authentic repositioning Okay, so that was about the preservation of tooth structure principle Next we have the retention and resistance form that is a second principle So retention and resistance They differs in the tooth anatomy based on the tooth anatomy and The forces being applied on the restoration So retention which prevents a removal of the restoration along the path of insertion or long axis of the tooth preparation as you see here It is along the Long axis of the tooth or the path of insertion whereas a resistance This picture It is a prevention of Dislourishment of the restoration by forces directed in an apical or oblique direction not the On the long axis it is on the apical or oblique direction and prevents any movement of the restoration under occlusion forces. So retention is basically on the forces which act on the long axis of tooth or Path of insertion Okay, whereas a resistance which the forces when act on apical or Oblique direction that is a basic difference between retention and resistance form So retention form we have many things to be considered. They are The dislodging forces and the geometry of the tooth preparation. So we are talking about the Retention form. Okay retention So the third one is the roughness of the fitting surface of the restoration Then the material being cemented then the looting agent being used. So these factors Affect the retention form of the restoration. So first one is a dislodging force The dislodging force. So forces that tend to remove a cemented restoration or a crown along its path of withdrawal Okay, so if we have a tooth and a restoration, so this is along the path of withdrawal So FPD which is subjected to dislodging forces. So those forces it can be a sticky food or application of flows we apply Flows under the connector for cleaning the cervical area So those things can dislodge the restoration. So they are the dislodging forces Second one is a geometry of tooth preparation. Okay, so in geometry we have many factors so they are the taper or Total Occlusional Convergence They're coming all under geometry of tooth preparation. Then Taper or TOC then the substitution of internal features then the path of insertion the freedom of Displacement then the length and surface area Then the stress Concentration and type of preparation all comes under the second factor in the retention form. Okay, I don't get confused So we're talking about the five principles of tooth preparation. So first one we finished that was the preserving the tooth structure the second one was retention form and resistance form in the second principle So this is a second principle So in that we are studying now is a retention form. It has got five features So among that the second feature geometry of tooth preparation has again many factors So that is taper and all these So we'll look into that one by one The first one is taper or total occlusional Convergence, okay taper. It's very important. That is nothing but the inclination That is a relationship of one wall of preparation to the long axis of the preparation. So how the Anculation of the axial wall with respect to the long axis can see the picture here. So usually it is two to three degree is Permissible that is a three degree. So when opposing surface both three degree combined to form a six degree Taper, okay, so six degree taper is the most Accepted one. So if we have the external walls if a preparation is receiving Arrestration or a retainer where the external walls should be converging whereas the Restoration or a preparation which is receiving a inlay or something like that the internal walls So the internal axial walls should diverge. Okay, so this is converging that is External wall should be converging whereas the internal walls should be Diverging, okay, you can see the picture. This is convergence. This is the divergence So as per theory the parallel axial walls provide maximum retention. Okay, so how do we make? Now visualize the taper of the preparation. So we need to Properly visualize it Proper visualization will prevent formation of any undercuts And as I said the ideal taper is six degree. So as per this Graph can see the retention form on the y-axis and the degree of taper on the x-axis You can see as the degree of taper increases the retention decreases So you can see the retention is highest at between the five and ten degree that is around six degree We get the highest retention And it decreases as we increase the taper. So you get the idea if we increase the taper will lose the retention, okay? So as I said Two to six degrees permissible Now we have the third one or the second one. Sorry second one the substitution of internal features Substitution of internal features in geometry of tooth preparation if the opposing walls are not available, okay? So basically we should provide opposing axial walls with minimal taper But if what if we don't have proper axial wall because of destroyed Wall because of severe attrition or if you are providing a partial veneer restoration So in such cases we need to Give the internal features for retention such as grooves box and pinholes, okay? So like this how we need to provide additional retention features Next is a path of insertion So path of insertion is nothing but an imaginary line along which the restoration will be placed onto and remote from the preparation this is the path of Insertion so path of all FPD about months must be parallel each other, okay? So it should be parallel each other do not get locked inside So we need to Have a visual survey to ensure the preparation is not having undercuts or over tapered just like this picture We need to have a visual survey So while doing visual survey the center of the occlusal surface of the preparation is viewed from 30 centimeter distance or 12 inch distance with just one eye, okay? Not a binocular vision just one night because binocular vision if we look at the preparation with two eyes This undercut preparation can appear to have an acceptable taper So never use both eyes to have the visual survey on the prepared tooth only use one eye And keep a distance of 12 inch or 30 centimeter So in patients mouth we can use a mouth mirror which can be held at an angle approximately half inch above the preparation, okay? so We can also keep a firm finger rest, okay? so firm finger rest can be established for the visual survey So this firm finger rest is nothing but the mirror which is manoeuvered until one preparation is entered So we place the mirror If we have to preparation We just keep the mirror where the one prepared tooth is at the center of the mirror Okay, then the mirror moved by pivoting on the finger rest without change In angulation till the second preparation is centered That is how we can make sure that it has having no undercuts or no Overtapering there is a firm finger rest the mirror Manoeuvering Manoeuvered until one preparation is entered then the mirror moved by Pivoting of the finger rest without changing the angulation. We should not change the angulation we keep the mirror in one position so that the prepared tooth is at its center then just change the Pivoting just pivot move by pivoting the finger rest without change its angulation Till the second preparation is entered just move it without changing the angulation and see the second preparation so that both the Prepared tooth is having or we can make sure that having the common Path of insertion so that is a process you can see the picture here So the path of insertion is considered in two-dimension that is a mesiodisctally and faciolinguli Mesiodisctal inclination should be parallel to the contact areas of adjacent teeth Okay, so it should be parallel to the contact area of adjacent teeth. Otherwise, it will be locked out We will not get the path of insertion So that was the mesiodisctally inclination whereas a faciolinguli inclination Which affects basically the aesthetics of metal ceramic and partial veneer crown. Okay, so for full veneer crown we need to have parallel Should be on the long axis parallel to the long axis of the tooth that is a facial and lingual orientation should be Parallel to long axis of the tooth whereas the facially inclined path of insertion Should be for the partial veneer crown or it should keep a facial Facial prominent facial occlusion line angle or Ovo contouring or opaque show-through for the partial veneer crown. Okay Whereas for full veneer crown we need to have the parallel long axis That is a facial angle orientation For posterior 3-4 count it should be parallel to the long axis of the tooth and anterior 3-4 count It should be parallel to the incisal one half of the labial surface Now we have the freedom of displacement freedom of displacement the number of paths Along which a restoration can be removed from the tooth preparation. That is a freedom of displacement only one path Will provide maximum retention? Okay, so that is a freedom of displacement now only one part should be there Now we have the length and surface area So the longer preparation there will be more surface area and it provides more retention and the length Length must be great enough to interfere with the arc of casting Pivoting about a point on margin on opposite side of restoration. Okay, so it should be Great enough to interfere with the arc of the casting pivoting Okay, about a point on the margin on opposite side of restoration And if there is short preparation the inclination will be on a critical site For smaller tooth it will be having a short Rotation radius so we need to provide grooves in the axial wall so it will reduce the rotation radius So if we don't have any Grooves it will be a bigger one so in order to reduce the rotation radius we provide grooves in the axial wall So that was a length and surface area now we have the stress concentration So stress concentration though it will be around the junction of axial and occlusal surface So we need to round the internal line angles to reduce a stress concentration And lastly the type of preparation which is coming under geometry of true preparation So complete crown there will be the type of preparation that is more preparation needed for the complete crown Compared to the partial crowns So we can provide more grooves boxes which will give better retention, okay so that was about the geometry of tooth preparation now we have the Roughness of the fitting surface of restoration. So roughening or grooving the restoration will improve the Retention, okay Which can be done by air abrasion that is using 50 micrometer alumina which will improve the Surface energy or the surface area which will increase the retention Whereas a roughening of the tooth preparation is not basically Recommended so roughening or grooving the restoration is recommended not the Tooth preparation, okay only the roughness of the fitting surface of the restoration will improve the retention not the prepared tooth Now the material being cemented so retention affected both by the casting alloy and the core buildup material So the more reactive the alloy is the more adhesion There will be with the certain looting agents such as type 1 and type 2 gold alloys for intra-coronal restoration and Type 3 and type 4 gold alloys for crown and FPD. So the type 1 and 2 gold for intra-coronal Whereas type 3 and 4 gold alloy for crowns and FPD, but as a nickel chromium for long span FPD There is a material being cemented Whereas a looting agent being used I just see cements are most retentive. There is a film thickness of looting agent There is a significantly an effect on the retention. So I just see resin can Put it like this I just see resin Then comes the GIC then comes Zinc phosphate Which is almost equal to polycarboxylate? Polycarboxylate and lastly the zinc oxide It is based on the retention. I just see resin GIC Zinc phosphate or polycarboxylate and lastly the zinc oxide Usional that is about the looting agent being used Whereas in resistance form we have Three factors basically one is the dislodging forces then looting agent being used and geometry of the tooth preparation So this Is after our retention form So the first one is dislodging forces. So mastication and para functional activity They are the substantial Horizontal or oblique forces. So resistance. Hope you remember the concept Retention is through the path of insertion or long access of the tooth, but the resistance Is at apical or oblique direction, okay? So the dislodging forces such as mastication and para functional activity But it should be on the horizontal or oblique directions So these lateral forces displays a restoration by causing rotation around the ginger well Margin next is a looting agent being used. So The resistance to deformation affected by compressive strength and modulus of elasticity so I do see resin then Just like our retention I do see resin then the glass and on a cement Then the zinc phosphate Then zinc polycarboxylate Polycarboxylate Finally the zinc oxide So this is the Resistance gradient Now we have the geometry of tooth preparation. It has got few features So the factors are type of preparation freedom of displacement in the occlusio Servical or incisor dimensions and the ratio of the Occlusio servical and facial angle dimension and finally the subconferential form of the prepared tooth So it all comes under the geometry of the tooth preparation So regarding the resistance form and type of preparation if it is a partial coverage restoration It was having less resistance than the complete crown because there is no buckle resistance area and partial coverage So we can give grooves or boxes to increase the resistance because it is greatest if walls are perpendicular in the direction of the force So that was about the Preparation type of preparation regarding the partial and complete if it is partial we can provide boxes with grooves now we The freedom of displacement So in freedom of displacement the lingual wall should be perpendicular to the reaction of force and In the proximal side the buckle and lingual wall must meet the pulpal wall at 90 degree There should not be any oblique ankle now. We have the occlusio incisor or incisor Servical dimension So the minimal occlusio servical dimension and t-rate is 3 mm premolars. It is 3 mm molars. It is 4 mm and The ratio with respect to the occlusio servical to facial lingual dimension So this is not actually facial lingual. This is incisor servical So ratio occlusio servical to facial lingual it should be 0.4 or higher for all teeth So if the total occlusio convosions is less than 6 degree it will be 0.1 Then less than 12 it is 0.2 less than 18.3 and less than 24 it is 0.4 Now we have the Circumferential form of preparatory so it should possess a circumferential irregularity that is a maxillary molars It should be in rhomboidal shape Mandupler molars, it should be rectangular Premolars and antires it should be oval. We should preserve corners of tooth preparation. There should not be Any axial grooves boxes should be provided in corners. Okay Because the chewing and para-functional habits can cause dislodging forces largely on the facial lingual aspect So grooves and boxes should be on the proximal surface So that was about the resistance form. Now we have a structural durability In structural durability Arrestration must contain a minimum bulk of material which is adequate to withstand the force of occlusion So bulk should be confined to the space created by the tooth preparation So to provide this adequate bulk we need to have proper occlusional reduction then there should be proper axial reduction and Also, we need to have proper functional cusp Bevel All are very important So in occlusion reduction for full metal restoration. We need to have 1.5 mm on the functional cusp Functional cusp and just one mm on non-functional cusp That is for Full metal restoration. Okay full metal crown for metal ceramic That is metal fused to ceramic in those cases. It is 1.5 to 2 mm for functional and Just 1 to 1.5 mm for non-functional Because it has a coping inside so we need to have more preparation Whereas for all ceramic crown, okay For all ceramic crown The reduction is 2 mm irrespective of the cusp that is both for functional and non-functional We need to have 2 mm reduction. That was about occlusional reduction Now regarding the functional cusp bevel So white bevel on the lingual inclines we should provide white bevel on lingual inclines on maxillary lingual cusp Okay for maxillary lingual cusp. We need to provide white bevel. That is a lingual incline of maxillary lingual cusp and the buckle inclines of Mantibular buckle cusp. Okay, so we need to provide white bevel. That is the lingual incline on the maxillary lingual cusp and the buckle incline of the mantibular buckle cusp should provide white bevel and There should be adequate bulk of metal in areas of heavy occlusional contact So if there is lack of functional cusp bevel, what happens is there will be fracture It happens in cases such as over inclination or over contouring or thin area in casting all will result in lack of proper functional cusp bevel and Ultimately the fracture of the restoration Now the axial reduction so we discussed about the Occlusional reduction and functional cusp bevel and finally the axial reduction. Okay, which all comes under structural durability so axial reduction the thin walls of casting which can Result in subject to distortion and over contouring which can also disastrous effect on the period on show So there will be adequate removal of the axial wall. Okay, so if there is no adequate removal it can result into distortion of the Retainer or the restoration or it can affect the period on show that is over contouring or under contouring That was the third principle structural durability. Now we have the fourth principle marginal integrity Marginal integrity is a closely adapted margins so these finish lines of the preparation which can Determine the aesthetics and the effect of period on shim So it also Depends on the survival of the restoration in the oral environment. So the configuration of finish line which Desides a shape and bulk of the metal at the margins it affects our marginal marginal effects a marginal adaptation and Degree of seating So all is different depends on the marginal integrity and we need to learn about various finish lines. So I'll be Taking finish lines as a separate session So that can be another execution or short essay the types of finish line So I'll be taking that in a separate video So this was the fourth principle marginal integrity So the many finish lines chamfer heavy chamfer shoulder Then the sloped shoulder radial shoulder shoulder with a bevel knife-witched and all those things and The last principle was fifth principle the preservation of period on shim So this was a fifth principle of to preparation. So It has a few factors The main thing is margin placement. So margin placement Will affect the period on shim So direct effect it has on the Success of the restoration so margin should be as smooth as possible and it should be placed in areas that can be finished Well by the dentist and kept clean by the patient so both factors should be considered that is It can be well finished by the dentist as well as It can be kept clean by the patient and it should be placed in enamel wherever possible And it should be supra gengeval whenever possible. Okay, it should be on Enamel and supra gengeval in order to preserve the period on shim if it is going deeper and deeper To have a better aesthetics. We might have problems in the period on shim so supra gengeval margins if we can have supra gengeval and Subgengeval margins So in supra gengeval margins there are less potential for the soft tissue damage and it can be easily prepared and finished More easily can be cleaned impressions can be more easily made Restorations easily evaluated at recall a point. So subgengeval margins Is Becoming a problematic if there is carries if there is so regular ocean or restoration which is extending subgengevaly And if proximal contact area extend to the gengeval crest we might Give the subgengeval margins Additional attention is needed for subgengeval margins and The margin of a metal ceramic crown is to be hidden behind the labio gengeval crest for the aesthetics Root sensitivity might be there Because of the subgengeval preparation A finish line should not be closer than 2 millimeter to the alveolar crest Okay placement in this area can pro and can result in gengeval inflammation pocket formation or loss of alveolar crest height. So That was all about the Principles That is a fifth principle that is preservation of around shim and Along with that, how do we prevent the damage? During tooth preparation. So we can prevent using various strategies One is prevention of adjacent teeth by using a metal matrix band leave a slight Lip or fin of proximal enamel there and Protection of soft tissue by careful retraction of lips and cheeks care to protect tongue when lingual surface of the mandible amolus are prepared and Pulp we need to think about the temperature Being created and the chemical action of the Simmons being applied and the chances of micro leakage So if the rise in temperature Without coolant you can see here 1 millimeter depth the rise in temperature without calling days point 54 degree 2 mm depth is 1 degree and 3 mm depth is 1.84. Okay with coolant It is comparatively low is point four zero point eight two and one point one three respectively So that was all about various principles of tooth preparation. So we discussed about five main principles And the second principle that is retention and resistance was a bit long. It has got many additional Factors so the five principles where The preservation of tooth structure retention and resistance from structural durability marginal integrity and preservation of the predominant ship so in that the retention and resistance form has Many other features such as dislodging force geometry of tooth preparation roughness of the fitting surface of the restoration material being cemented and looting agent being cemented in that The second one geometry of tooth preparation include lots of factors such as taper path of insertion freedom of displacement length and surface area stress concentration type of preparation and all that So this is very very very important for the exam is very commonly asked essay question principles of tooth preparation or each principle Can be asked as a short essay or short note. So never ever miss this question for your exam. So next I'll be coming with Finish lines which comes under actually marginal integrity. So we need to discuss in detail So I'll come up with that topic in my next video. So hope you understood make sure that you learn this For your final exam. Thank you