The major difference between the design of a scaler and a curette is in the shape of the blade. In cross section, the blade of a scaler is triangular, whereas a curette is semicircular. The rounded, convex back of the curette allows it to be placed within a periodontal pocket with minimal laceration or discomfort to the patient. Most curettes are smaller and finer than other scaling instruments, and end in a rounded toe - features which improve tactile feedback and also minimise gouging of the root surface.

There are basically two designs of curettes. Universal curettes are designed so that it is possible to adapt the one instrument to all tooth surfaces by making use of both cutting edges on each blade. Area-specific curettes, such as the Gracey curettes, are designed so that each blade adapts to a specific tooth surface or area. Only one cutting edge on each blade is used.

Universial Curette Design
1. Universal curettes are designed so that the working ends can be adapted to all tooth surfaces of all regions of the mouth with one double-ended instrument.

2. The blade of a universal curette is honed at 90 degrees to the lower shank. All universal curette shanks are designed so that when the handle is parallel to the root surface being instrumented, the lower shank is automatically tilted towards the tooth to provide proper working angulation.

3. Both cutting edges on each blade can be used by simply tilting the instrument one way or the other. 

4. The two cutting edges on a blade are straight and parallel to each other. There is no curvature of the blade except for the curve from the shank to the toe.

Gracey Curette Design
1. Gracey curettes are area specific instruments. There are seven mirror image pairs of curettes in a full set. 
 

2. The blade of a Gracey curette is offset at an angle to the lower shank, rather than being perpendicular to it. Most Gracey curettes are manufactured so that the face of the blade is offset at about 70 Dgrees to the lower shank. This allows for perfect working angulation when the lower shank of the curette is positioned parallel with the tooth surface. 

3. Only one cutting edge of the Gracey curette blade is used. Determining which side of the blade should be used and sharpened can be tricky, particularly if the instrument has been poorly sharpened in the past. By holding the instrument with the blade face up, and parallel to the floor, it is possible to view the curvature of the blade to the side. The outer edge of the blade that forms the larger outer curve is always the correct cutting edge.

4. The blade of the Gracey curette is not only curved from toe to shank, but it is also curved to the side. It is important to retain this curvature during instrument sharpening. Only the lower third or half of the Cracey blade contacts the root surface during instrumentation.

Choosing Instruments For General Dental Practice
If you are going to provide effective periodontal treatment for your patients, then it is worthwhile investing in a suitable range of quality instruments. In general, the Universal curettes are easier to use, particularly if you do not root plane very often. A useful combination of Universal curettes is: 
Columbia 2R/2L 
Columbia 4R/4L 
Columbia 13/14 
Younger-Good 7-8 

It is not necessary to use the full set of Gracey curettes every time you root plane. If you understand the principles of their use, it is possible to adapt instruments to other areas of the mouth other than those listed in the table above. Many dentists will chose to create smaller kits of instruments such as: 
Gracey 1-2 
Gracey 7-8 
Gracey 11-12 
Gracey 13-14

Hand and wrist fatigue can follow a session of root- planing. To minimise this, it is best to purchase your instruments with large hollow handles that are ribbed or scored for good finger grip. It is also important to sharpen your instruments regularly. The next round of Practice Information Sheets will cover the sharpening of Universal and Gracey Curettes.

A Final Tip
It is common practice for dentists to get their dental assistant to wipe the blade of the curette with gauze at frequent intervals throughout root-planing procedures. This activity is no longer recommended, as the risk of the blood-infected instrument piercing the nurse's glove and skin is too great a risk. Therefore, it is important to educate your dental assistant to use water and suction to remove debris from the instrument while you root-plane.

If you need further guidance on periodontal instrumentation, an excellent text is: Pattison G, Pattison AM. (1992) Periodontal instrumentation - a clinical manual. 2nd edition. Reston Publishing Company, Inc. Virginia, USA.

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