CMMC preparartion

8/3/2019 CMMC preparartion

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Indirect RestorationsMetal-Ceramic (PFM) Crown Prep. Upper 1st Premolar

Metal-ceramic crowns are also known as "Porcelain Fused to Metal" (PFM)crowns. Where porcelain is needed, extra reduction must be undertaken tomake room for both metal and ceramic.

In places where porcelain is not needed, for example the palatal margin, lessreduction is done.

Slide show: Metal-Ceramic Crown Prep. Upper 4.


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ASSESSMENT.Study the unprepared tooth; visualize the preparation shape you want to achieve.Decide which areas will be restored with alloy and which areas will require more room for alloy andporcelain only.Plan where you will place your finishing lines, and what type of margin will be where.


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ASSESSMENT.Study the unprepared tooth; visualize the preparation shape you want to

achieve.Decide which areas will be restored with alloy and which areas will requiremore room for alloy and porcelain only.Plan where you will place your finishing lines, and what type of margin willbe where.


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OCCUSAL DEPTH SLOTS (GROOVES) BUCCAL CUSPUsing 501 round-ended tapered diamond* cut a depth slot into the central

ridge of the buccal cusp. As this cusp will be covered in alloy and porcelain,reduction should be 2.0mm. Cut 2 more occlusal slots mesially and distallyto the first.*The 501 bur acts as your depth gauge, its dimensions are: Tip 1.1mm;Maximum diameter 1.6mm; length 7.0mm ; Taper 4.5 (9 convergence)


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OCCLUSAL DEPTH SLOTS PALATAL CUSPCut 3 slots in the palatal cusp, at depth 1.5mm if it is to be covered in alloy

only or 2.0mm if porcelain coverage is required.


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JOIN DEPTH SLOTS BUCCAL CUSP.Using 501 join depth slots on the buccal cusp to create a smooth, uniform,

planar reduction that follows the general occlusal morphology.Note that a thin band of marginal ridge can be left unprepared to preventbur contact with the adjacent tooth and aid in depth orientation.


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JOIN DEPTH SLOTS PALATAL CUSP.Repeat for palatal cusp.


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FUNCTIONAL CUSP BEVEL.Use 501 to prepare a smooth, uniform functional cusp bevel on the palatal

cusp, at an angle that approximates the inclination of the opposing cusps(about 45).Reduction is 1.5mm for alloy; 2.0mm for gold, and can be begun with depthgrooves.


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BUCCAL DEPTH SLOTS OCCLUSAL HALF.Buccal axial reduction is completed in 2 (or more) planes because of the

curvature usually present in premolars.Use 501 bur to cut 1.5mm depth slots in the occlusal half of the buccalsurface, fading out at the maximum bulbosity.


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BUCCAL DEPTH SLOTS CERVICAL HALF.Using 501 bur cut 1.0mm deep marginal depth slots parallel to the cervical

half of the buccal surface. Finish depth slots supragingivally.


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BUCCAL DEPTH SLOTS CERVICAL HALF.Alternative view


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PLANAR BUCCAL REDUCTION.Attempting the buccal axial reduction in one plane will:

- Cut too much tooth away in the middle of the buccal surface.- Create an irretrievable over-taper of the buccal surface.- Risk pulpal exposure.- Leave occlusal 1/3 of axial wall under-prepared, resulting in a finishedcrown of poor contour.


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BUCCAL REDUCTION OCCLUSAL HALF (View 1)Join the buccal depth slots in the occlusal half of the preparation.


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BUCCAL REDUCTION OCCLUSAL HALF (View 3)Join the buccal depth slots in the occlusal half of the preparation.


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BUCCAL REDUCTION / SHOULDER CREATION.Using 501 join the marginal depth slots and create a shoulder that:

- Is 1.0mm in depth.- Is supra-gingival, even if the finished shoulder is to be sub-gingival- Has a rounded internal line angle.- Follows the contour of the gingivae buccallyKeep the long axis of the bur parallel to the buccal surface, in the same wayas the depth slots.


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EXTEND SHOULDER INTERPROXIMALLYExtend the shoulder interproximally until it is out of sight buccally when

looking along the line of the arch. This creates the so-called wings whichconserve tooth tissue and mark the point at which porcelain coverage willterminate.NB: These are not slots/grooves.


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CLEAR CONTACTSUse 582 needle point diamond to cut through the marginal ridge inside the

contact point. Great care is needed to prevent damage to the adjacentteeth.Cut through the tooth so that a strip of proximal enamel protects theadjacent tooth from bur contact; do not attempt to take the bur betweenthe teeth.The aim of this part of the preparation is simply to separate the teeth.


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PLANE PROXIMAL SURFACES.Once the contacts are clear, use the needle point diamond to plane the

proximal surfaces until there is sufficient access for your torpedo diamond,without contacting the adjacent teeth.Clinical note: your technician is going to have to saw the model betweenyour prep and the adjacent tooth. If you dont leave enough room, yourmargin and/or the proximal surface of the adjacent tooth may be lost.


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15.PALATAL CHAMFER.Using your 877 Torpedo diamond prepare the palatal axial surface to create

the correct convergence relative to the cervical half of the buccal axialpreparation.


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INTERPROXIMAL CHAMFER.Use the torpedo diamond to join the palatal chamfer and the buccal

shoulder.- Keep the margin supra gingival.- Monitor the bur angulation to create the correct taper between proximalaxial walls.


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BLEND PALATAL AND INTERPROXIMAL CHAMFERSRound any line angle between palatal and proximal axial preparations to

ensure alloy will have the correct contour and a natural emergence profile.Do not over-round your preparation.


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SHOULDER FINISHING (View 1)The position for the buccal margin should be planned in advance.

Keep the margin supragingival.Preparation with 501 diamond will produce a rounded internal line anglethat will reduce stress concentration when fabricating, cementing and usingthe crown.The 501 will also create a rounded lip towards the outside of the shoulder,this must now be carefully flattened with a flat-tipped bur eg: 847 tapereddiamond (Tip diameter 1.01mm) and/or a sharp margin trimmer.


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SHOULDER FINISHING (View 2)... using the chisel


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CHECK AXIAL TAPERDry and carefully view your preparation from all angles and check the

following:No undercuts. 10-20 convergence angle.The whole margin and all internal angles are visible, when viewed from oneposition above.Smooth any sharp line angle where the buccal axial planes meet, to revealany part of the shoulder hidden when viewed from above.


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SMOOTH PREPARATIONUsing composite finishing burs, fine discs or rubber cups + pumice, gently

smooth line and point angles.


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FINISH.


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FINISH.

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CMMC preparartion

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