Crown Down Prep with the STS System

Note: The STS has been replaced by the Fine-Cut Endo Handpiece but everything except as noted applies.

Santa Cruz, CA May 12, 1996
Latest Update Saturday, January 17, 1998

Watch for an announcement soon.

The STS sysem which we have been using for the last three or four years will soon be replaced with the new Fine Cut Endo Handpiece

After nearly two years experience with the most rapid cutting endo handpiece ever, the technique has evolved and it's time for an update.

One has to understand that the STS system is more than a quantitative increase in endo preparation speed.

It is similar to the change that we saw with the introduction of air turbines in operative.

Not only did it not require strength to be a dentist, but it was a lot more precise and safer.

Since we can now cut dentin very fast we can make changes in the very approach we take to endo.

The Crown-Down-Spiraling-Step-Back technique is the result.

The hardest part of learning to use the STS sytem is to NOT move the file up and down in the canal.

The STS does that for you.

You just move the file circumferentially.

The word circumferentially comes from the word circumference.

This means that you bear against the side walls of a canal and let the up and down motion of the STS do the work for you.

If you are going to fill with cement with the Passive Filling Technique it matters very little what the cross sectional shape of the canal is.

You've seen ads various endo preps in plastic blocks asking "Which of these canals would you rather fill?"

Quite frankly, to me, it doesn't matter.

They are all equally easy with cement.

You only have trouble when you have to pack partially molten gutta percha to the apex.

Remember, cement filling is an extrusion or casting process.

The cement will flow to fill the nooks and crannies.

You don't have to pack it in.

Your pluggers do not have to fit.

An elliptical canal is the same as a round one as far as flow is concerned.

If a canal is ovoid it can be left ovoid.

If it's round, that's OK too.

Filling a canal is more like taking an impression than packing an amalgam.


The cement will adapt.

Start your prep my making your access cavity with a round bur.

Break thru the occlusal and lift up, de-roofing the pulp chamber with the upper hemisphere of the bur.

Flush the pulp chamber well and identify the orifices of the canals.

Next use a #30 or #25 25mm Fine-Cut file in the STS introducing just the tip into the orifice.

The tip of a #30 is 0.30mm in diameter.

If you cannot get the tip of a 30 into the orifice, you may have to do some hand probing first.

The reason we use a 30 is because it is unlikely that you will go out the apex with a 30.

Move the tip around the rim of the orifice.

This will flare the orifice and get rid of that ledge of secondary dentin that is usually there.

Very frequently you'll find that the canal will suddenly open up and you can now fit a #30 in easily.

As you widen the orifice drop into the canal further and further.

Eventually the file will get stuck.

One of two things has happened.

As you try to force a #30 around a bend it will get stuck.

At this point, stop.

You have gone as far as you need to.

Usually you will be 5-10mm from the apex.

You have not measured yet, so now take a #15, drop it in and take your measurement x-ray.

You will find that by doing a Crown-Down-Orifice-Opening procedure, that you can get your measurement file in much easier.

An additional advantage is that you clean the dirtiest portion of the canal first, without pushing the debris into the bone.

Also without realizing it you slightly straighten the canal by filing the outside of the curve at the orifice.

After you have reached the apex with a #15 hand file, you know that a #15 STS file will also fit.

Now using the STS again, circumferentially file the area from about 2 mm from the apex up to the crown.

This time starting at the 2mm point and spiralling outward.

Do not dwell at the apex.

Being moving upward immediately.

After five or six passes at it you can now carefully move the file tip toward the apex starting at deeper and deeper penetration until you have filed the apex.

At this point you have cleaned the canal from the crown down to the middle and from the apex up to the middle.

So now what's left to do.


It all depends what you intend to fill with.

If you are going to pressure fill, then the prep is DONE.

If you are going to passive fill with cement then you still have to prepare a size 30 seat for the needle at the 1 mm point.

If you intend to fill with gp you have to do what you have to do to use gp.

It is indeed possible to prep and entire canal with two files.

A #30 crown-down and a #15 apex-up.

This is in addition to whatever files you need to get to the apex in the first place.

It does require some care to avoid getting vertical grooves and ending up with an amoeba-shaped canal.

This can be avoided in ribbon shaped canals by using a sweeping motion while circumferentially filing, changing the angle of attack.

Total time elapsed in preparing.

Less than 2-3 minutes.

Remember when crown preps took an hour.

I'll bet not too many dentists take longer than five minutes now.

It'll soon be the same way in endo.

If the apical portion of the canal is curved, you will have to precurve the #15 to fit the canal.

You dare not cut this fast with a straight file in a curved canal.

Even if you do not break the file, you may strip the canal.

My suggestion is that you pre-curve the file, then try it in my hand before putting it in the handpiece.

It must drop to place freely.

If you have to push it in against resistance, you are flexing it.

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