Gates-Glidden and Peeso drills: sizes, diameters and safe use

A Gates-Glidden drill (GG for short) is a slender, parallel-sided shaft that ends in a small oval cutting head. The shaft stays the same diameter along its length; only the small head cuts. The very tip of that head does not cut, which is why it is called a safety tip, and the instrument cuts on its side. In practice this means a GG does not bore into the canal; it shaves the wall as it comes out.

• The design has been in use for almost 150 years without a noteworthy change, and it is still made with the same geometry today.
• Its job is to enlarge the coronal part of the canal; preparing the whole canal is not what it is for.
• It is effective at removing the calcifications that are often found just below the canal orifice.
• The shaft does not bend, so it cannot follow a curve; its working area is the straight portion of the canal.
• It is made in stainless steel and in NiTi, in six sizes and several lengths.

Six sizes are made, and the cutting head grows with the number. The figure quoted for each size is the maximum diameter, measured at the widest point of the head; the non-cutting tip is narrower. The table gives the maximum diameters of the first four sizes, which the teaching sources state numerically. Sources give different values for sizes 5 and 6, so no figure is given here; what matters is that those two sizes are clearly wider than the rest and carry the highest risk in the furcation area.

As the size goes up, two things change together: the instrument gets wider and the dentin left behind gets thinner. Larger sizes have been reported to raise the risk of perforation in the furcation area, so they are not taken deep into the canal. Work with the large sizes stays close to the orifice, and depth is left to the smaller sizes.

Two sequences are described for how the sizes follow one another. In a step-down sequence the largest size goes first and the sizes get smaller; in a step-back sequence the smallest goes first and the sizes get larger. Both open the orifice effectively, and both work best when the canals leave the access cavity without a sharp angle.

There is no single correct number; the speed depends on what the instrument is doing at that moment. The three values below are given for different tasks and are not interchangeable.

The temperature of the outer root surface has been measured to rise by 3 to 5 °C during use; the rise depends on dentin thickness, working time, pressure, drill size and speed. That reported range is considered to be within what the periodontal ligament can tolerate.

In a multi-rooted tooth, the side of the root that faces the furcation is thinner than the outer side. In a curved canal an instrument naturally tends to rest against that thin side. Because a GG cuts on its side and does not bend, that tendency thins the wall quickly unless it is resisted, and the root is perforated through its lateral wall. That is a strip perforation.

• Used incorrectly, a GG can reduce root-wall thickness markedly.
• High speed, excessive pressure, an incorrect angle of insertion and aggressive drilling into the canal are the factors named together in reported strip-perforation cases.
• Larger sizes raise the risk of perforation in the furcation area; the risk grows with the size.
• A crown-down approach and anticurvature preparation are described to reduce this risk.

Anticurvature preparation aims to keep instruments away from the furcation area in curved canals, so that too much dentin is not removed from that critical zone and a strip perforation is avoided. In practice it means precurving the instrument so that it rests on the outer side of the curve and directing the cutting force toward the outer wall.

1. 01Straight-line access firstIf the access cavity does not give a straight line to the orifice, the drill enters the canal at an angle and finds the wrong wall on first contact. The drill is not picked up until access is complete.
2. 02Open the path with hand filesThe smallest GG is 0.50 mm. Whether the canal can take that diameter is learned by opening a path with small hand files first.
3. 03Work only in the straight portionThe drill can be taken safely up to the point where the curve begins. An unbending shaft forced into the curve pushes against the outer wall.
4. 04Decide the size sequence in advanceStep-down runs from large to small, step-back from small to large. Whichever is chosen, the depth at which each size stops is planned before starting.
5. 05Keep the large sizes coronalLarger sizes have been reported to raise the risk of perforation in the furcation area. Work with them stays near the orifice; depth is left to the smaller sizes in the sequence.
6. 06Brush away from the furcationA light lateral force is applied as the drill is withdrawn, and it is directed toward the outer contour of the tooth. Deliberately moving the coronal third of the canal away from the furcation preserves the remaining dentin.
7. 07Advance by repetition, not pressureThe instrument advances through several light in-and-out passes, not by forcing. When resistance is felt, a smaller size is used or the operator returns to hand files.
8. 08Keep irrigatingThe drill produces dentin chips; chips that are not flushed out block the canal and bind the instrument. The canal is irrigated at every change of size, and patency is checked with a small hand file.

At first glance a Peeso reamer looks like a Gates-Glidden: the same long shaft, the same working principle. The difference is the length of the cutting section. A Peeso's cutting part is markedly longer, and that length makes it good at preparing a straight post space and hazardous in a curved canal.

A Peeso may still appear in coronal flaring or in post-space preparation. The same caution applies in both: over-preparation and thinning of the radicular dentin walls. Dentin removed while preparing a post space does not come back, which is why the sources advise against excessive preparation and thinning of the root walls.

The purpose of coronal flaring has not changed; the tools have multiplied. Three groups of instruments now do the same job, and all serve the same goals: freeing the instrument from coronal interferences, giving straight-line access to the middle and apical thirds, removing reparative dentin at the orifice, reducing the angle of curvature while increasing its radius, and getting plenty of irrigant into the canal early.

• Flexible-shaft burs: the same design as a Gates-Glidden but with a shaft that bends, so they tolerate a curve a little better.
• Orifice openers and shapers: short, high-taper rotary NiTi instruments. In one example the cutting section is 12 mm long with an 11% taper; in another it is 14 mm long with a 19% taper at the tip falling to 3.5% at 9 mm. Neither has a cutting tip.
• Rotary systems that hand coronal flaring to the first file of the sequence: in a crown-down system the first file already opens the coronal part.

Being inexpensive, safe and effective keeps the Gates-Glidden useful in the student clinic, and it remains an effective instrument for removing the calcifications just below the orifice.