The Time Has Come: Passive Ultrasonic Irrigation Techniques to be Replaced by Laser Photo-acoustics in Root Canal Treatment.

by Fernando J Meza, DMD

We have all been there before, at some point in our career when we realize we are missing the right tool for the job. Whether it’s working on a house project, cooking a meal or, as dentists, operating on teeth. Lasers have been around for many years in medicine and dentistry however, we are now finally realizing they are optimal in many clinical applications. In this article, I want to demonstrate how the Waterlase iPlus All-Tissue Laser (Er,Cr:YSGG), by Biolase is the right tool for root canal debridement and disinfection in our lifelong pursuit of the treatment of apical periodontitis.

It is always tempting to directly apply results from studies to real life. However, it may take years to develop adequate models that help us bridge the causal connections we are trying to establish. Materials and methods are everything in scientific studies. Different conclusions may emerge depending on how experiments are constructed. As they say: “the best test is the test, that tests”. What this means is: what if the methods applied do not accurately reveal what we are looking for? This was apparent decades ago when only culture-dependent methods to detect pathogenic organisms inside infected root canals were used. More recently, newer molecular approaches involving PCR sequencing reveal hundreds of more pathogenic bacteria that we never knew were present inside the infected root canal space.1 As we turn to lasers now, we will discuss how several studies conducted on enhanced canal debridement and disinfection utilizing ultrasonics are not keeping up with lasers and recent goals in root canal treatment.


Let us discuss how lasers can make our lives easier in terms of debridement and disinfection of the root canal system in treating apical periodontitis. Ultrasonic and endosonic devices have been tried in the past using different levels of oscillating frequencies applied to tip or files. Many studies show that ultrasonics are capable of acoustic streaming and cavitation effects as the tip is allowed to oscillate to its greatest potential in a water tank. However, when the same tip is placed in a confined root canal space, where oscillations are diminished and energy is dissipated against the canal walls, a different picture emerges.2-6

Although passive ultrasonic irrigation techniques have proven useful in debridement and disinfection of the root canal system, there are several limitations. We will discuss these limitations but first, let us describe the two main types of passive ultrasonic irrigation techniques: continuous and intermittent. The continuous version is when you are continually supplying the canal with either water from the active ultrasonic device or your irrigating solution of choice, which usually turns out to be sodium hypochlorite. The intermittent version is when you flood the pulp chamber or canals with water or irrigating solution, pause, then activate the oscillating ultrasonic tip inside the chamber or canal, and then repeat the cycle.

One example of continuous passive ultrasonic irrigation that incorporates the ultrasonic tip and delivery of solution in one complete mechanism is the ProUltra PiezoFlow system (by Dentsply Maillefer). By attaching a syringe to the specially designed tip, sodium hypochlorite solution can be directly injected into the canal at the orifice or root level that is 1 mm from binding to the canal wall while at the same time, the tip is made to oscillate. The tip size is equivalent to a #50 file and therefore an apical preparation of a size #40 0.04 taper is necessary.7,8 If using the intermittent technique, syringe irrigation is used to replenish the irrigating solution in between cycles of the oscillations. Studies have conflicting results between the effectiveness of different types of passive ultrasonic irrigation techniques.9

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Ten months after..
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One serious limitation to most of the in vitro and in vivo studies conducted with passive ultrasonic techniques is that the apical preparation size must be quite large to allow the full displacement amplitude of the oscillating ultrasonic tips.1-5 Apical sizes ranging between #50 0.02 taper, #40 0.04 taper, or #30 0.06 taper are often cited as necessary to achieve the space for the tip to oscillate and cause the shear forces necessary to enhance debridement.2-8 These sizes are needed for both types of passive ultrasonic irrigation techniques including intermittent and continuous versions with or without the use of the PiezoFlow system.7,8 One has to wonder whether it is always possible to instrument large to these sizes without causing ledges or transportation of the canal walls.

Another limitation to consider is the study design whereby closed or open systems are evaluated in apices and isthmuses since these opposing systems greatly affect the results of cleanliness.7,8

In addition to the canal preparation size, ultrasonic tip size, and efficiency of energy transfer, the direction of the oscillating tip within the canal space has an effect.10 Applying the correct direction of oscillation displacement may prove difficult to orient and control during use on a patient. While energy settings are easily controlled on the laser unit, ultrasonic settings may sometimes be too high for use inside the root canal space or just not as effective in terms of amplitude displacement.6 When laser fiber tips are used, orientation does not matter and contact with the canal walls does not dissipate the photo-acoustic streaming and cavitation effect.

Duration of treatment time with ultrasonic devices is also a factor. Passive ultrasonic studies will often mention a treatment time of 1 to 3 minutes using ultrasonic tips.7-9 As a comparison, laser studies often will mention 20-30 seconds of treatment time.11-13

Another grave limitation of passive ultrasonic irrigation is whether cavitation is actually observed within the root canal space. Acoustic streaming which encompasses fluid velocity and turbulence is the main effect achieved from ultrasonic tips. Cavitation, which is the formation of bubbles within the fluid in the canal that rapidly expand and implode causing jets and shear forces, is not as readily observed in ultrasonics.2-6 When a laser tip is activated in water or an irrigating solution within the root canal space or a small glass cylinder, a powerful cavitation effect occurs.14-16 This phenomenon is termed photoacoustics whereby the laser tip can efficiently convert light energy into powerful mechanical pressure waves capable of bubble formation, expansion, and implosion. It is precisely when the bubbles collapse that jets are formed that translate into shear forces. Within a confined root canal space, these formed cavitation bubbles can produce a pressure gradient that results in a fluid pump.14 It is this photo-acoustic effect producing both acoustic streaming and cavitation within a confined space that facilitates root canal debridement and disinfection when lasers are used.

In cases where root canal anatomy should be conservatively maintained and maximum apical sizes of #20 0.07 taper preparation or smaller are ideal, the acoustic streaming or cavitation effect is quickly dampened as the ultrasonic file contacts the root canal wall.17 Instead, a final apical preparation size of #40 0.04 taper or a #30 0.06 taper is advocated for proper use of passive ultrasonic irrigation techniques.2-8 This is not the case with laser fibers, particularly when using the Waterlase Er,Cr:YSGG 200 micrometer radial firing tip. Although the tip may be small enough to reach working length in some cases, current protocols with the laser show that considerable acoustic streaming and cavitation at the apical region is still achieved while the fiber tip remains either at the mid root, orifice, or pulp chamber.

An important distinction between ultrasonics and lasers that I am expressing in this article is that root canal preparation size may not be relevant since the photoacoustic effect is not dissipated when the laser fiber tip encounters the canal wall. The obvious question to ask is: “Can the original anatomy of the root canal space be respected or, at least, can a conservative enlargement of it be sufficient to address our goals of treating apical periodontitis?”

I present to you a case using the Waterlase iPlus with the RFT2 (200 micrometer, radial firing tip) by Biolase. Pre-operative and 10-month follow-up periapical images of a tooth #30 (#46 international), necrotic pulp with symptomatic apical periodontitis, completed in one visit, are shown. Powerful acoustic streaming and cavitation effects at the apices of this tooth which allowed for optimal canal debridement and disinfection, in my opinion, would not be possible without the use of the laser.

As a practicing endodontist, we need to have all the right tools for our patients. We can’t decide to use the microscope on one case and not on another or decide not to purchase a CBCT because it is not needed in every case. The same can be said about canal debridement and disinfection as it relates to apical size. If I have a large canal, I will reach for the ultrasonic unit, if not I will reach for the laser. The value of the laser in endodontics is that it can be used effectively in all cases with large or small apical sizes in less treatment time. Other benefits with lasers include soft tissue management with less bleeding in gingivectomies and apical surgery. The ultrasonic unit is a wonderful tool, and it too has its place for conservative removal of dentin to find canals, and for gutta percha and post removal. In terms of root canal debridement and disinfection, however, it’s time to retire the ultrasonic unit and the passive ultrasonic irrigation techniques and allow the incredible power of photoacoustics as mediated by the Waterlase All-Tissue Laser to take a front row seat.

Reprinted with permission, JLAD, Issue 5, 2022


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About the Author

Fernando Meza is a 2002 graduate of the University of Connecticut School of Dental Medicine. He is a recipient of the AADR/Dentsply Sirona sponsored Student Clinician Award. Dr. Meza received his Certificate in Endodontics from Temple University School of Dentistry in 2004. During his residency, he conducted research on the Biolase Waterlase All-Tissue Laser to explore the disinfecting capabilities of the laser on bacterially infected root canal systems. His research led to a publication in the Journal of the American Dental Association in 2007. He has been in private practice in endodontics in Northern Virginia since 2004. He has also volunteered as Assistant Clinical Professor of Endodontics at the University of Maryland School of Dentistry as part of the Dean’s Faculty from 2005 – 2017.