Since the late 1950s, the use of ultrasonic technology has become a staple in endodontic practices around the world for hygiene and periodontal procedures. More recently, endodontists have introduced us to the clinical advantages of their use in root canal therapy, and we are just beginning to see their effectiveness for endo-restorative and microrestorative work.
Piezoelectric ultrasonic equipment
Distinguishing Features and Improved Technology
The first piezoelectric ultrasonic was introduced to the market by Satelec in the late 1970s. Since that time, there has been an evolution in the technology, operation, ergonomics, and the design of the machines. The most important feature of any ultrasonic unit is how it maneuvers the tip and adjusts to many different tip designs (each with a unique intended function). A good ultrasonic machine also must adapt to several unpredictable clinical conditions and tooth anatomy. It must be remembered that "the mechanism of ultrasonics is very different than other instruments,"1 such as a high-speed handpiece. As Dr. David Clark (Tacoma, Wash) has noted in this journal, it is somewhat "counterintuitive"; increased manual pressure or cranking up the power does not necessarily add cutting or vibrating efficiency. In many cases, it reduces efficiency. There are 2 variables of ultrasonic tip vibration: the frequency (number of times the tip moves back and forth every second) and power or intensity (the amplitude or arc of the tip movement, which is usually operator controlled). When the power is turned up, the frequency does not change; the "power" simply increases the back-and-forth arc (amplitude) of the tip. One of the latest advances in piezo "drive" technology is the Newtron (Satelec) line of ultraonics (Figures 2 and 3).
The module, which controls each unit, features "cruise control" consisting of 3 quite advanced functions:
1. Newtron type ultrasonics operate in a frequency range of 28 kHz to 36 kHz. Ideally, each tip and has its own ideal frequency. The Newtron module constantly monitors the weight and dimension of each tip and adjusts it accordingly (in real time) to that tip's optimal frequency.
2. With most ultrasonic units, the electronics push the tip to the farthest extent of its amplitude; the return is simply recoil action. The Newtron module controls both the push and the pull of the tip, resulting in even more precise tip control. The more tip control, the less likely the operator will break the ultrasonic tips. In the case of scaling procedures, better control results in more patient comfort.
3. As already stated, ultrasonic tips are most effective when they are passively applied to tooth structure and allowed to "dance on the surface," as opposed to being pushed. The Newtron module will adjust the power intelligently, depending on the load that it encounters in use; this is similar to torque control in electric endo motors.
WATER, AIR, VISION, AND LIGHT
Many design elements are incorporated into a number of high-quality piezoelectric models. Optional fiber-optic or LED handpieces (Figure 4) (ie, optional on the Varios by NSK and the Satelec Newtron models) certainly enhance visibility, but may be an unnecessary expense if the operator is using a high-powered LED headlight (Figure 5) or a microscope. In any event, intense light, good magnification through loupes or a microscope, and reliable ultrasonic technology are all a must if you are doing molar endodontics. Water has always been inherent to ultrasonic use; being necessary for magnetorestrictive units to cool the handpiece, the tooth surface, and the tip. Piezoelectric units still require the use of water in periodontal and hygiene procedures, but for most endodontic applications water is discretionary. For example, in operations such as access refinement and chasing calcified canals, much dentinal dust is created. This, when mixed with water, tends to become a slurry that might bog down the ultrasonic instrument, and will certainly obscure the clinician's vision.
However, there are advantages to intermittent water use. For example, when using an ultrasonic tip to remove a metal post by vibrating it, the heat generated can cause thermal injury to teeth and their supporting structures. In reporting on this possibility, Gluskin, et al2 recommended as part of a good protocol for post removal using ultrasonics, "Use devices that allow water to reach the working end of the ultrasonic tip to provide the maximum cooling effect." Furthermore, the occasional use of water in endodontic access refinement yields a clean surface and can expose what many clinicians refer to as a "dentinal roadmap"; for instance: differentiating the pulp chamber floor from secondary dentin, possibly providing clues to the location of a hidden canal, and properly orienting the operator in order to begin achieving a good Glidepath, and getting the cleaning and shaping process off to a good start. As it would appear that intermittent water use is advantageous, it would follow that ultrasonic tips that have water ports are preferred over tips without (Figure 6). The availability of a stream of air during ultrasonic use is also necessary. It also will act as a coolant, and a steady stream of precisely directed air can help keep clear visibility, blowing away dentinal and other dust and debris. The use of a Stropko irrigator (Stropko) (Figures 7 to 10), adapted to your air-water syringe, allows your assistant to direct an optimal fine stream of air to the operative field.
Finding and Breaking Into Hidden Canals and Access Enhancement
"Tooth retention has increased significantly in older adults, and dentists are now challenged by the need to preserve critical teeth."3 Clinicians are confronted with calcified canals, pulp stones, and hidden canals such as in maxillary molars (MB2).4,5 According to peer-reviewed literature, second mesial buccal canals are present in most maxillary molars. Dr. John Stropko examined 1,732 conventionally treated maxillary molars over a period of several years in a very impressive clinical study. He noted, "As the operator became more experienced, scheduled sufficient clinical time, routinely employed the dental operating microscope, and used specific instruments adapted for microendodontics, MB2 canals were located in 93.0% of first molars and 60.4% in second molars."6 There are 3 key factors mentioned here: a certain level of familiarity in treating MB2s, allowing adequate time, proper magnification, and using the correct instruments (Figures 11a and 11b). For both general practitioners and endodontists, the ultrasonic is a valuable tool to treat calcified and difficult to find canals, as long as it is complemented by the proper tip, and sufficient magnification and light.7,8 It is less aggressive than a high-speed handpiece, and with high magnification the operator can always see where the tip is and where it is heading. By contrast, the high-speed handpiece affords little if any visibility in access procedures, so the dentist must constantly stop, look, and start again to avoid procedural mishaps. As Dr. L. Stephen Buchanan9 says, "Any clinician who performs molar endodontics without ultrasonics is working too hard, is experiencing more anxiety than is necessary, and is most likely not finding MB2 canals in more than 40% of their maxillary molar cases."
Dental manufacturers today have brought us a variety of tips and cutting surfaces such as smooth stainless steel, zirconium nitride, and diamond coated tips. Recent studies have suggested that diamond coated surfaces offer some of the best cutting efficiency;10 however, diamond particles can fall out and wear down rapidly, making ultrasonic tips dull and much less efficient all too soon. A newer entry into the marketplace is the BL series of tips (B&L Biotech/Clinical Research Dental) (Figure 12a). Instead of a diamond coating, BL tips have integrated sharp micro-projections (small raised bumps) on the tip surface, which engage the tooth surface precisely and efficiently. The active abrasive surface has considerably more longevity than other coated tips, and they are less expensive due to the lack of an additional coating process.
In the natural course of the life of a dental practice, specialty or general, cases will present that will require retreatment of a previously restored, endodontically treated tooth. This will often entail disassembly of the restoration, perhaps including removal of a metal post, a silver point, gutta-percha, obturation carriers, etc. There is also the possibility of instrument separation during root canal therapy, which usually requires immediate intervention. The piezoelectric ultrasonic is also an invaluable tool for these procedures. In such situations, good case selection is very critical; frequently the endodontist has the proficiency to handle these procedures, having been well educated on the justification and protocols for retreatment, possessing valuable experience in retreating many cases over many years, and having all the instruments and tools essential to deal with the most difficult, unusual, and sometimes unforeseen conditions under which most retreatment is done.16
Metal post removal is a laborious and time-consuming chore that can be made easier with the use of the proper ultrasonic tip. A tip, which will engage and fit over the post or contact directly and vibrate the post, can break up the crystalline structure of the post cement, thus loosening it enough to be removed. Sometimes the operator must take this a step further, following up with a long and more slender tip to trough around the post, steadily loosening and eventually extricating it. In these cases, water should be used periodically to keep the tooth cool to avoid the earlier stated "thermal injury." The same approach can be used to remove other obstructions such as silver points, obturation carriers, pins, and separated nikel-titanium or stainless steel files. Choice of the proper tip is paramount. For removal of obstructions in the canal, here are some guidelines to keep in mind:
• There is no such thing as a one-size-fits-all tip. Try to use the largest diameter and shortest tip possible (similar to a using a 21-mm file in a shorter canal), depending on where the top of the obstruction to be removed is located. In some cases, more than one tip size will be needed to remove a stubborn post or file. Patience is essential.
• Always use a low power setting (10% to 20% power is usually more than enough) when using a tip further down into the canal. It is a constricted area to work in, making the use of minimal tip amplitude better.
• If you are attempting to remove a threaded post or an endodontic file, sometimes using a counterclockwise motion might aid removal as most files have a clockwise cut flute.
• Don't be a hero. If you cannot see it, you probably cannot retrieve it.
In addition to the above, it cannot be overstated that excellent visibility, good magnification, and proper illumination are all vital for success in these ultrasonic uses. Experience and good case selection are also vitally important.
Other Miscellaneous Uses in Endodontics
Ultrasonics are also presently used in removing gutta-percha, condensing gutta-percha, vibrating perforation repair material, such as mineral trioxide aggregate, into location, and root end surgical preparation. Currently, there are an increasing number of restorative procedures, which are ideal for ultrasonic applications such as margin finishing, interproximal, and small lingual restorations, and more advances in oral surgical applications are in development.