Complete cast crown preparation


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simplified from contemporary fixed prostho

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Complete cast crown preparation

  1. 1. THE COMPLETE CAST CROWN PREPARATION *summarized from Contemporary Fixed Prosthodontics - refer page 271 (summary) indications contraindication 1. extensive destruction from caries/trauma 1. less than maximum retention necessary 2. endodontically treated teeth 2. high esthetics need (anterior teeth) 3. existing restoration 4. necessity for max retention & strength 5. to provide contours to receive a removable appliance 6. other recontouring of axial surfaces (minor corrections of malinclinations) advantages 1. strong 2. high retention & resistance form 3. usually easy to obtain 3. intact buccal/lingual wall adequate resistance form exists 4. option to modify form & occlusion 4. if treatment objectives - especially for supraerupted can be met with more teeth conservative restoration 5. less easily to deform 6. its cylinder-like configuration encircles tooth and is reinforced by a corrugated occlusal surface disadvantages 1. removal of large amount of tooth struc 2. adverse effects on pulp & periodontium 3. vitality test cannot be done 4. display of metal 5. restorations may be restricted to max molars/mand molars/premolars 6. patients object to display metal 7. correction of occlusal plane PREPARATIONS : 1) Guiding grooves for occlusal reduction 1. use tapered carbide/narrow,tapered diamond 2. place depth holes 1mm deep in central, distal, mesial fossae 3. connect the holes 4. place guiding grooves in ; - buccal & lingual developmental grooves - each triangular ridge 5. place func cusp bevel in the area of contact with opposing tooth - to protect it by adequate thickness of metal - depth slightly less than 1.5mm (allow for smoothing) 6. ensure that occlusal reduction follows anatomic configuration - to minimize loss of tooth struc 7. must be placed with accuracy - should concentrate on position, depth, angulation of each groove 8. position : - in low point (central & dev grooves) & high point (cusp tips & triangular ridges) of each cusp 9. depth : - central groove & non-func cusp : 0.8mm - func-cusp : 1.3mm 10. corect angulation of grooves is important - to ensure that occlusal reduction is situated beneath occlusal surface of restoration 11. on non-func cusp, groove should // the intended cuspal inclination 12. on func cusp, groove should be angled slightly flatter - to ensure additional reduction of func cup 13. use periodontal probe to measure extent of reduction 2) occlusal reduction 1. remove the tooth structure that remains between grooves 2. use carbide/narrow, round-end, tapered diamond
  2. 2. 3. first, reduce half of occlusal surface - the other half can be maintained as reference 4. then reduce the other half 5. func cusps : 1.5mm non-func cusps : 1mm 6. how to verify the reduction/clearance ; - patient should close into several layers dark-colored utility wax in max intercuspation - remove the wax from mouth & evaluate it for thin spots - then measured with wax caliper - place the wax again in patient's mouth and ask patient to move his mandible into potrusion & excursive positions - remove again from mouth - measure the thickness of the utility wax - alternative way : use occlusal reduction gauge 7. refer page 263 (fig 8-10) 3) alignment grooves for axial reduction 1. place 3 alignment grooves in each buccal & lingual wall - use narrow/ round end, tapered diamond - 1 is placed at center, 1 in mesial & distal transitional line angle 2. these grooves determine the p.o.p of restoration 3. shank of the diamond must // to p.o.p of restoration - this will produce a convergence between axial walls of alignment grooves that is identical to the taper of the diamond (6' taper) 4. dont let the diamond cut into the tooth beyond the point where its tip is buried in tooth struc up to midpoint - to prevent creation of enamel lip 5. gingivally, there will be the resulting depth from the grooves alignment - should be no more than 1 ½ the width of tip of diamond 6. occlusocervically, placement of the tip of intrument determines the location of the margin 7. use periodontal probe to asses the parallelism of alignment grooves with one another or with proposed p.o.p of a 2ndry retainer 4) axial reduction 1. remove remaining tooth structure between alignment grooves 2. place chamfer margin 3. use narrow/ round tipped diamond 4. reduce half of the axial surface first - the other half as a reference 5. then reduce the other half 6. pay special attention to interproximal areas - to prevent unintentional damage to adjacent teeth - give time for cutting instrument to create its own space - enamel lip is maintained between the diamond & adjacent tooth - place metal matrix band to protect the adjacent teeth - most difficult areas to reduce : those with significant buccolingual dimension & root proximity 7. cut into proximal area from both sides until few mm of interprox island remain - use thinner, tapered diamond (needle diamond) - if damage, polish with white stones, silicone points, prophylaxis paste 8. place cervical chamfer concurrently with axial reduction - width : 0.5mm – allow adequate bulk of metal at the margin - must be smooth & continuos mesiodistally 9. remove unsupported enamel - liable to fracture - create open margin - failure of the restoration
  3. 3. 5) finishing 1. functions - to aid phases of fabrication of restoration - to facilitate impression making, waxing investing, casting - to enhance retention & resistance form 2. use fine-grit diamond/carbide bur 3. wider diamond is recommended - it smooths out any unwanted ripples - eliminate any unsupported enamel at margin 4. should be done smoothly with reduced speed handpiece 5. margin must be glassy smooth 6. round all the line angles 7. place non func cusp bevel if necessary 8. use air cooling to improve visibility 9. use water spray (when use air cooling) to prevent tooth from dehydrating and wash away debris 10. place additional retentive features (grooves/boxes) - use tapered carbide bur - slow handpiece 6) evaluation 1. detect the common errors that can reduce the retention - overtapered - undercuts 2. how to correct errors by using less conservative approach - upright the overtapered axial walls to increase retention - or use grooves/boxes/pinholes 3. tip of diamond should rest on the chamfer while making the axial reduction 4. assess occlusal & proximal clearances - should be adjusted if inadequate 5. correct any probs before making interim restoration & impression 6. refer page 268 (fig 8-25) Special Considerations Functional cusp bevel 1. 1.5mm occlusal clearance 2. bevel must be angled flatter than external surface 3. bevel is placed 45' to long axis Non-functional cusp bevel 1. min 0.6mm of clearance is needed for adequate strength 2. maxillary molars often require additional reduction bevel - to prevent overcontoured restoration 3. unnecessary for mandibular molars - as they are lingually inclined - profile relatively straight Chamfer Width 1. common error – increasing faciolingual width - lead to periodontal disease 2. min 0.5mm is adequate - to develop optimum axial contour 3. on premolars, make slightly narrower chamfer - to conserve tooth struc & retention form pae.