Instrumentation - the Move from Push Pull to Rotary Motion - What happened and why
The past few years have seen Endodontics move away from the traditional push-pull file motion and step back preparation technique to rotary motion. It is ironic that while these techniques are considered "new", rotary motion (in the form of the "Envelope of Motion") has been a recognized part of the Boston University Endo technique since its inception in the early 1960s. Schilder was the first to publish guidelines (Dent. Clin. N. Am. Endodontics 1974) for what is now considered the standard in endodontic canal preparation. This typical tapering funnel shape as produced by those taught the technique is called "The Look". Better shaping in the midroot and canal body facilitates cleaning through deeper use of irrigants. Once filled, the canal flows with the root anatomy. The small apical diameter allows for apical control of filling materials. It is this typical canal shape (rather than the often referred to "Boston puff") that is the defining characteristic of the "Boston University" technique. It was Schilder who first realized the most profound concept of canal instrumentation: the prepared canal shape is primarily influenced by the MOTION of the instrument in the canal, NOT the type of instrument used. Research later continued to prove that round canals could be obtained only by use of instruments in rotary motion. Once this was understood, the push pull file motion fell into disfavor because of its inability to produce a funnel shape and problems with ledging in curved canals.
Limited by instruments of the day, Schilder compensated for the "Standardized" untapered instrument design by creating the "Envelope of Motion". Placing a gentle consistent curve on the fluted portion of the reamer induced the creation of this Envelope. Through the use of a rotary/withdrawal motion, the curved reamer creates the effect of a giant flexible Gates-Glidden bur. The advantage of this motion was that unlike other instrumentation techniques (where the tip of the instrument influenced the canal design), this Envelope worked mainly on the body of the canal. Because the tip of instrument was not forcibly but passively inserted into the canal, tip design was irrelevant. This body enlargement allowed for unrestricted apical access of less flexible files and deeper penetration of irrigants. The cleaner, wider canals were easier to fill with the use of the warm gutta percha technique. Proponents of cold Lateral Condensation techniques became increasingly defensive when shown the multiple lateral and accessory canals that were produced by this method of cleaning, shaping when combined with warm gutta percha. For that reason, Warm Vertical Gutta percha techniques continue to gain popularity at the expense of Cold Lateral condensation.
Roan's "Balanced Force" method has also gained acceptance since its introduction in 1986. Again, instruments are used in a rotary rather than push-pull motion. Because of the way the files are used, the aggressive tip design of the standard K file required modification. These files with modified tips are known as the Flex-R.
The Crown-Down technique has also become popular because of previous problems with apical control of files. By first enlarging the orifice and body of the canal with rotary motion instruments, access to the smaller apical portions is more easily achieved. While some may hail this as a revolution in endodontic instrumentation, it merely represents an acknowledgement of the principles that Schilder outlined in 1974, albeit with a slightly different method. Advanced practitioners of the BU technique have long understood the advantages of first reaming "short" in order to open the canal orifices to gain access to working length. Although some may label it "Crown-Down", experienced clinicians will recognize that this technique is merely an extension of the principles of proper access preparation to the canal space (i.e. / removal of triangles 1 and 2).
More recently, Ni-Ti metallurgy and instruments of differing taper have finally caught up with these concepts. It is now possible to purchase motorized versions of these very flexible Ni-Ti instruments with differing tapers that can produce these shapes consistently.
Whether performed with a pre-curved reamer (Envelope of Motion), a file of different taper in a engine driven handpiece or with crown down technique, several conclusions are inevitable:
(1) The step-back preparation is dead.
(2) Continuously tapering funnel shaped preparations can only reliably be achieved by instruments used in rotary motion
(3) The principle of canal body enlargement followed by minimal apical manipulation and finishing has finally been generally accepted as the standard.
(4) Initial criticisms of the Warm Gutta Percha canal shape (previously described as overenlarged) have given way to the acknowledgement that adequate body enlargement is necessary for both efficient cleansing and proper obturation with Warm GP techniques.
These concepts, currently being aggressively marketed by Buchanan, Ruddle and others, have their origins in The Boston University technique. As instrument flexibility and design continue to improve, modifications in the actual physical technique may occur, but the design parameters and principles as outlined some 30 years ago remain the gold standard of Endodontics.