Cutting instruments used in cavity preparation and their importance

Introduction

1. Hand-cutting instruments
2. Rotary cutting instruments
3. Sonic/ultrasonic instruments and auxiliary abrasives

1. Hand-cutting instruments

• Handle - held by the operator
• Shank - connects handle to blade and determines angulation
• Blade - active cutting part

A. Excavators

Types

• Has a spoon-shaped working end with a curved cutting edge.
• Available as single-ended or double-ended instruments.
• Used with a scraping or scooping motion.

• Removal of soft, infected dentine.
• Removal of residual caries near the pulp where rotary instruments may risk pulp exposure.
• Cleaning the cavity floor and walls.
• Removal of temporary restorative material.

• Provides good tactile sensation, helping the dentist distinguish soft infected dentine from firmer affected dentine.
• Allows conservative caries removal, especially in deep lesions.
• Reduces the risk of inadvertent pulpal exposure.

• Has a disc-shaped end and a claw-like cleoid end.
• Mainly used for carving amalgam restorations, but may assist in removing carious dentine in small areas.

B. Chisels

Types and uses

• Blade lies in the same plane as the handle.
• Used to plane enamel walls and remove unsupported enamel.

• Blade is angled relative to the handle.
• Useful for cutting and shaping proximal walls and line angles.

• Has a curved shank and is used in anterior teeth.
• Useful for refining cavity walls and removing enamel in areas of limited access.

• Produce smooth enamel margins.
• Remove unsupported enamel prisms that may fracture after restoration.
• Help create definite internal line angles and sound cavity walls.
• Particularly valuable when controlled hand cutting is needed rather than rapid rotary cutting.

C. Hatchets

Types

• Used to cut enamel and dentine.
• Useful for planing cavity walls, sharpening line angles, and removing unsupported enamel.

• A modified enamel hatchet with a curved blade.
• Available as mesial and distal pairs.

• Trimming and beveling gingival enamel margins of proximal boxes.
• Refining cavosurface margins in Class II preparations.
• Producing a smooth, well-defined gingival seat.

• Helps establish a strong marginal seal.
• Reduces marginal leakage and recurrent caries by eliminating defective enamel margins.
• Improves adaptation of amalgam or other restorative material at the gingival margin.

D. Hoe excavators

• Have a blade set at a right angle to the handle.
• Used with a pulling motion.

• Removal of soft carious dentine from cavity floors.
• Smoothing and planing pulpal or gingival floors.
• Removal of unsupported enamel in accessible areas.

• Effective for flattening and refining cavity floors.
• Permits controlled cutting in areas where rotary access is difficult.

E. Angle formers

• Have a small, angled blade.
• Commonly used to accentuate line angles and create retention features.

• Refining internal line angles.
• Preparing retention grooves in Class II amalgam preparations.
• Sharpening the axiopulpal line angle.

• Creates mechanical retention where required, especially for non-adhesive restorations.
• Produces a more definite cavity outline and internal form.

2. Rotary cutting instruments

A. Dental burs

• Shank - inserted into the handpiece
• Neck - connects shank and head
• Head - active cutting portion

Types according to material

i. Steel burs

• Made of hardened steel.
• Used mainly at slow speed.

• Removal of soft caries.
• Excavation of dentine.
• Finishing of temporary restorative materials.

• Become dull rapidly.
• Less effective on enamel than tungsten-carbide burs.
• May corrode if improperly maintained.

ii. Tungsten-carbide burs

• Have sharp flutes that cut tooth structure by a shaving action.
• Used in high-speed or low-speed handpieces.

• Cutting enamel and dentine.
• Establishing outline form and initial cavity depth.
• Removal of old amalgam and restorative material.
• Finishing cavity walls and margins.

• Efficient cutting reduces chairside time.
• Produces comparatively smooth cavity walls.
• Enables precise preparation when the appropriate bur shape is selected.

iii. Diamond instruments

• Consist of diamond particles bonded to a metal blank.
• Cut by abrasion rather than by a blade-like shaving action.

• Cutting hard enamel.
• Finishing enamel margins.
• Tooth preparation for adhesive restorations, veneers, and crowns.
• Refining margins, bevels, and surface contours.

• Highly effective for enamel reduction.
• Available in coarse, medium, fine, and superfine grits.
• Fine-grit diamonds are useful for producing smooth margins before adhesive restorations.

3. Shapes of rotary cutting instruments and their uses

A. Round bur

• Spherical head.

• Initial entry into pits and fissures.
• Removal of carious dentine.
• Creating retention pits in amalgam cavity preparations.
• Opening small cavities conservatively.

• Enables controlled entry into tooth structure.
• Particularly useful for selective caries excavation.

B. Pear-shaped bur

• Pear-shaped head with rounded end.

• Initial cavity preparation, particularly Class I and small Class II cavities.
• Produces slightly convergent walls and a rounded pulpal floor.
• Commonly used for conservative amalgam cavity preparations.

• Its shape helps create resistance and retention form in conventional amalgam preparations.
• Rounded internal angles reduce stress concentration in tooth structure and restoration.

C. Inverted cone bur

• Wider at the cutting end than at the neck.

• Creating undercuts and retention.
• Flattening pulpal and gingival floors.
• Preparing retention grooves.

• Useful for mechanical retention in amalgam restorations.
• Must be used cautiously because excessive undercut weakens tooth structure and may endanger the pulp.

D. Straight fissure bur

• Cylindrical with parallel sides.

• Cutting straight cavity walls.
• Extending preparations.
• Preparing slots and grooves.
• Sectioning teeth or restorations in some procedures.

• Produces uniform walls and controlled cavity extension.
• Useful where parallelism is required.

E. Tapered fissure bur

• Tapers toward the shank.

• Preparing slightly convergent cavity walls.
• Establishing retention form in conventional cavity preparation.
• Refining proximal boxes.

• Helps produce a cavity form that resists displacement of a non-adhesive restoration.

F. End-cutting bur

• Cutting blades are mainly at the end rather than on the sides.

• Deepening cavity floors without excessive lateral enlargement.
• Refining pulpal floors.
• Working close to a proximal surface or adjacent tooth.

• Allows controlled vertical cutting.
• Minimizes unnecessary removal of tooth structure from cavity walls.

G. Finishing burs

• Usually have more flutes than regular carbide burs.

• Refining cavity margins.
• Smoothing walls.
• Finishing composite and amalgam restorations.

• Smooth cavity surfaces improve adaptation of restorative material.
• Reduce marginal irregularities that can promote plaque retention and microleakage.

4. Handpieces used with cutting instruments

A. Low-speed handpiece

• Caries excavation with round steel or carbide burs.
• Finishing procedures.
• Polishing and removal of temporary restorations.

• Less aggressive cutting.
• Better tactile feedback in deep caries excavation.
• Useful near the pulp.

B. High-speed air turbine or electric handpiece

• Rapid cutting of enamel and dentine.
• Cavity outline formation.
• Removal of old restorations.
• Preparation of bevels and cavity margins.

• Greatly improves efficiency and patient comfort when used correctly.
• Water spray is necessary to limit heat generation, clear debris, and maintain cutting efficiency. Evidence from handpiece testing shows that spray pattern affects coolant delivery and cutting rate, especially during groove cutting and proximal preparation (JADA study).

5. Importance of cutting instruments in cavity preparation

1. Conservation of tooth structure

2. Efficient caries removal

3. Establishment of proper cavity outline form

4. Development of resistance form

5. Development of retention form

6. Smooth cavity walls and margins

7. Removal of unsupported enamel

8. Protection of pulp

9. Better visibility and access

10. Improved longevity of restoration

Precautions during use

• Use a sharp, undamaged bur. Dull burs produce heat and require excess pressure.
• Use adequate air-water spray with high-speed instruments.
• Apply intermittent, light pressure rather than continuous heavy force.
• Protect adjacent teeth with matrix bands or guards during proximal preparation.
• Avoid overextension and excessive undercuts, especially in adhesive restorative dentistry.
• Use spoon excavators carefully in deep lesions to avoid pulp exposure.
• Sterilize reusable instruments and discard damaged burs.

Conclusion