Barry Musikant

1. Superelastic metals such as nickel-titanium flex far more than stainless steel instruments yet in clinical use have a much higher incidence of fracture.
2. The incidence of nickel-titanium fracture is related to defects from:
• work hardening of the alloy before machining
• surface defects produced during machining
• propagation of existing surface defects by cyclic fatigue, making the metal more brittle as the defects become larger and  more numerous

Score One for Empirical Reasoning

THE EMPIRICAL reasoning behind the manual SafeSider Instrumentation Technique was to develop a system that would create a canal space equivalent to the best shape produced by engine-driven rotary NiTi instruments while also replacing the vulnerable, unpredictable NiTi alloy with tough, predictable stainless steel wherever possible.
    To further enhance the ease, simplicity, and predictability of the SafeSider Technique, we designed a sequence of relieved flat-sided cutting-edge instruments that promote rapid negotiation to the apex.  Stainless steel instruments compose the first 80 percent of the system.  Only the last two instruments in the sequence are composed of nickel-titanium, and they, too, incorporate the SafeSider relieved flat.  By shaping  90 percent of the canal space prior to the use of NiTi instruments, the remaining 10 percent puts little stress on the subsequent NiTi hand instruments.  The forces applied to the NiTi instruments are further reduced by a significant straightening of the canal prior to their use.  Because they are never rotated beyond resistance, the amount of stress the instruments are subject to is limited.  Finally, the NiTi instruments are first test-bent in the hand approximately 90 degrees.  If they do not break in the hand, they will not break in the tooth if used in the prescribed manner.  If they do break in the hand, breakage is the ultimate confirmation that discarding the instrument at this point was the proper thing to do.  The bending test is highly significant in light of Kuhn’s paper, which states that crack propagation increases, rendering the instruments more and more brittle.
    The SafeSider Instrumentation System replaces and compensates for an engine-driven rotary NiTi system, delivering comparable shaping in less time and at far less cost.  The entire SafeSider System consists of ten instruments plus the No. 2 Peeso reamer used in a completely safe fashion.  The first four instruments enlarge the canal to the apex to a No. 20 stainless steel reamer, which is usually the starting point for all NiTi rotary systems.  After this only six more instruments and under four minutes are usually needed to produce a canal shape that accepts a medium or greater gutta-percha point.
    A tremendous amount of marketing money has been spent convincing dentists to use the rotary NiTi systems. Despite all the expensive secondary innovations to reduce the incidence of fracture plus the support of large segments of the endodontic community repeating the mantra that rotary NiTi is the new paradigm, there are more caveats today in the use of these systems than for any other systems.  The Kuhn paper clearly defines the basic weaknesses of rotary NiTi endodontics.  With full appreciation of the superior shaping results that can be attained, the SafeSider Instrumentation System delivers those results without yielding to rotary NiTi’s disadvantages.

Stress and the Thermal Gradient

IN ANOTHER PAPER, titled “The effect of thermal change on various dowel-and-core restorative materials,” published in the July 2001 issue of the Journal of Prosthetic Dentistry, Yang et al. demonstrated that the stress level developed in the restoration and the surrounding dentin was closely related to the degree of thermal gradient.  The nonmetallic dowel and cores generated greater thermal stresses than metal dowel and cores.   The combination of a resin core with a carbon-fiber dowel generated the highest stress in the cement layer core and metal-ceramic crown.  Thermal stresses generated from the thermal gradients of the nonmetallic dowel-and-core materials generated additional stresses in the cement and its interface.  With increasing thermal expansion, stresses in the restorations and coronal portion of the dentin increased more markedly than did stress levels in the supporting bone.
    As I often point out in my lectures, to me the worst restorative combination is a carbon fiber post with a full composite core surrounded by a ceramic crown ending in a butt joint onto dentin.  The Yang paper states that the cement layer between the core and the crown undergoes the highest degree of degradation, opening up the margins.  Open margins lead to secondary decay and internalized micromovement  in a vicious  cycle that shortens the life of the restoration  The esthetic goals that prompt the use of  a full ceramic restoration—which, in turn, requires a nonmetallic post—may be more functionally met by the use of a Captek-like restoration with a feathered metal margin that allows the use of a fully functional metal post without compromising esthetics.
    The Flexi-Post and Flexi-Flange design  have a metal dowel and a supporting core that is generally about 75 percent metal after the core has been prepared.  The small amount of additional composite should limit the amount of heat absorbed by the core material.  In addition, if Ti-Core with titanium is used, the metal component of the post and core is even greater, mitigating the thermal gradient of the composite portion of the core.
    As founders of Essential Dental Systems, we obviously support the products we developed and market.  However, it is nice to see that as the years go on the latest data from independent researchers support concepts that we have incorporated and, in turn, critically review concepts and designs that we have rejected.  These products were first developed for our own use and, if they worked as we planned, we then made them available to the profession.  It is a simple concept that still seems  to work.

An Interesting Insight

THE OTHER DAY, the suction in my rooms went down and I had to use another room.  The slow speed handpiece was different from the one I am used to, and I found using the No. 2 Peeso in this room more difficult than in my own.  Here are the particulars: In both my rooms I use a Titan slow speed unit.  It is connected to a rheostat that allows low rotations with good torque. A slight depression of the foot rheostat starts the Peeso slowly rotating exactly as it should.  However, in the other room, a Viper handpiece from Kinetics did not operate as smoothly.  Depressing the rheostat slightly did not start the handpiece slowly.  The rheostat had to be depressed more to make the handpiece start, and then it went faster than I wanted.  Once it had started, I could back off on the speed, but this movement did not give me the same control I have with the Titan unit.  In addition, despite the greater speed I did not have as much torque.  Ideally, I want slow speed and high torque to give me optimum control of where to remove circumferential dentin.  Bottom line: it did not give me the same control that I have with the Titan handpiece.  Certainly, you can use the Viper and get used to it.  Perhaps, the Viper handpiece can be better adjusted.  However, I believe in making life as easy as possible, and using the Titan makes my life easier. If this little insight strikes a chord with you, check out the Titan.  It can’t hurt, and it might help.
    P.S. I used Amy’s room.  Since then she has switched to the Titan slow speed handpiece and also finds life a lot easier.