1. The document outlines the steps for preparing a tooth for an onlay restoration, including reducing the cusps, creating an occlusal step, and forming proximal boxes. 2. Key preparation steps include using a bur to reduce tooth cusps, create a 0.5mm occlusal step, and form 0.8mm wide proximal boxes removing 2/3 dentin and 1/3 enamel. 3. Additional steps include placing bevels, flares, and potentially retention grooves to complete the tooth preparation for an onlay restoration.

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2. INITIAL PREPARATION 1. OCCLUSAL REDUCTION FIRST STEP IS THE REDUCTION OF THE CUSPS WHICH IMPROVES THE ACCESS AND VISIBILITY FOR SUBSEQUENT STEPS IN TOOTH PREPARATION. EFFECIENCY OF THE CUTTING INSTRUMENT AND THE AIR-WATER COOLING SPRAY. IT IS EASIER TO ASSESS THE HEIGHT OF THE REMAINING CLINICAL CROWN OF THE TOOTH.

3. USING THE NO.271 CARBIDE BUR HELD PARALLEL TO THE LONG AXIS OF THE CROWN, PREPARE 2 mm DEEP PULPAL FLOOR ALONG THE CENTRAL GROOVE. THE OCCLUSAL PREPARATION IS EXTENDED FACIALLY AND LINGUALLY JUST BEYOND THE CARIES TO SOUND TOOTH STRUCTURE, TO VERIFY NEED FOR CUSP CAPPING.

4. WITH THE SIDE OF THE NO.271 CARBIDE BUR, PREPARE UNIFORM 1.5 mm DEEP DEPTH CUTS ON THE REMAINING OCCLUSAL SURFACE. THEY ARE USUALLY PLACED ON THE CREST OF THE TRIANGULAR RIDGES AND IN THE FACIAL AND LINGUAL GROOVE REGIONS. THE DEPTH CUTS SERVES AS GUIDES FOR THE AMOUNT OF REDUCTION.

5. CUSP REDUCTION IS COMPLETED WITH THE SIDE OF THE NO.271 BUR, AND THE REDUCTION SHOULD REFLECT THE GENERAL TOPOGRAPHY OF THE ORIGINAL OCCLUSAL SURFACE. SHOULD NOT ATTEMPT TO REDUCE THE MESIAL AND DISTAL MARGINAL RIDGES AT THIS TIME( TO AVOID HITTING ADJACENT TOOTH). THE GINGIVAL-TO-OCCLUSAL DIVERGENCE OF THESE PREPARATION WALLS SHOULD RANGE FROM 2-5 degrees DEPENDING ON THEIR HEIGHTS.

6. 2. OCCLUSAL STEP AFTER CUSP REDUCTION, THERE SHOULD BE A 0.5 mm DEEP OCCLUSAL STEP IN THE CENTRAL GROOVE REGION BETWEEN THE REDUCED CUSPAL INLCINES AND THE PULPAL FLOOR. OCCLUSAL STEP IS EXTENDED FACIALLY AND LINGUALLY BEYOND THE CARIOUS AREAS AND THE WALLS SHOULD GO AROUND THE CUSPS IN GRACEFUL CURVES THEN EXTENDED MESIALLY AND DISTALLY TO EXPOSE THE PROXIMAL DEJ IN ANTICIPATION OF PROXIMAL BOXING.

7. 3. PROXIMAL BOX CONTINUING WITH NO.271 CARBIDE BUR A PROXIMAL DITCH IS PREPARED. THE MESIODISTAL WIDTH OF THE DITCH SHOULD BE 0.8 mm AND PREPARED APPROX. TWO-THIRDS AT THE EXPENSE OF DENTIN AND ONE-THIRD OF ENAMEL. IDEAL EXTENSION GINGIVALLY OF A MINIMAL CAVITATED LESION ELIMINATES CARIES ON THE GINGIVAL FLOOR AND PROVIDES 0.5 mm CLEARANCE OF THE UNBEVELED GINGIVAL MARGIN WITH THE ADJACENT TOOTH.

9. FINAL PREPARATION REMOVAL OF THE INFECTED CARIOUS DENTIN AND DEFECTIVE RESTORATIVE MATERIALS ON THE PULPAL AND AXIAL WALLS. A CEMENT BASE CAN BE GIVEN IF IT IS INDICATED.

10. PREPARATION OF BEVELS AND FLARES. A DIAMOND INSTRUMENT IS USED TO PLACE COUNTER BEVELS ON THE REDUCED CUSPS, TO APPLY GINGIVAL BEVELS AND TO CREATE SECONDARY FLARES ON THE FACIAL AND LINGUAL WALLS OF THE PROXIMAL BOXES.

11. FISSURE THAT EXTENDS SLIGHTLY LINGUAL TO NORMAL POSITION OF COUNTER BEVEL MAY BE INCLUDED BY SLIGHTLY DEEPENING COUNTER BEVEL IN FISSURED AREA.

12. AFTER BEVELILNG AND FLARING SHARP JUNCTIONS BETWEEN THE COUNTER BEVELS AND THE SECONDARY FLARES ARE ROUNDED SLIGHTLY. LIGHTLY BEVEL AXIOPULPAL LINE ANGLE ALSO.

13. IF NECESSARY, SHALLOW( 3 mm DEEP) RETENTION GROOVES MAY BE CUT IN THE FACIOAXIAL AND THE LINGOAXIAL LINE ANGLES WITH NO.169 L CARBIDE BUR.

14. COMPLETED MESIO OCCLUSO DISTAL ONLAY PREPARATION