Research in laser hard tissue applications at UQ

The major theme of the plenary lectures on the first day of the inaugural scientific congress of the Australian Association for Laser Dentistry (AALD) will be laser hard tissue applications, showcasing research work from Australia and New Zealand over recent years. The author will describe work undertaken at the University of Queensland, and then Graeme Milicich will present the finding of his research into hard tissue ablation using pulsed middle infrared lasers.

The congress will be held at the Gold Coast on July 24-26, 2009. See the website www.aald.asn.au for details of the meeting's scientific programme.

Hard tissue laser research began at UQ in September 1988, just over two decades ago, when we began to look for wavelengths which would effectively ablate enamel, as a replacement for a high speed turbine. That launch date was almost exactly 10 years after I had seen a laser for the very first time - a low power (1 milliwatt) visible red helium-neon gas laser used for holographic imaging at James Cook University, during a senior high school visit to that university in 1978. The power of that first laser is 5 million times less that that of the largest laser we have in our facility at the present time, but their difference in size is only about 20 fold!

When our research began in 1988, we had a strong link with Laserdyne Technologies, a leading Australian "high tech" company which designs and manufactures advanced optical technologies including lasers for military as well as medical purposes, producing units from 50 milliwatts to 1500 watts for a range of applications (Figure 1). We worked with Laserdyne staff over a number of years on cavity preparation, CAD-sintering and other laser technologies. By 1991, there were a handful of dentists in Australia including myself using lasers in clinical practice. Stephen Marks and Ambrose Chan in Sydney were notables in that regard, and Ambrose later went on to undertake a very interesting research study which resonates back to today since it laid the groundwork for our later work on laser-induced analgesia which has now become a standard application in cavity preparation laser systems.

In 1991, the knowledge base for clinical applications of lasers for hard tissue procedures in dentistry was small, and no such applications were recognized by the US Food and Drug Administration. Our publications for many years were required to carry an explicit statement regarding such lack of approval, as a disclaimer for the benefit of North American readers. The first hard tissue applications were approved by the FDA in May 1997, which meant that no such disclaimers were required after that watershed date. By then, laser research outside North America had progressed considerably, and fears laser-generated heat would damage the dental pulp had been dispelled by the work of many groups which tracked thermal stresses using miniature thermocouples and other methods. Today, there is a strong evidence base of over 200 published papers, and most laser systems come preprogrammed with the settings for procedures that were determined in those earlier years.

From determining the dose response effects for enamel ablation, we then systematically worked through a range of hard tissue applications, running into occasional blind alleys along the way, but also making some unexpected discoveries that are now incorporated into the everyday practice of laser dentistry across the world. There is still much that can be done, even though the most recent textbook of laser dentistry at some 592 pages would lead one to think that all the answers are known, which is certainly not the case.

Over the past 20 years, we have worked with wavelengths from the short ultraviolet through to the far infrared, finding the optimal wavelengths to undertake certain tasks, going back to first principles and the basic sciences of chemistry and physics. Many times we have used existing lasers in new ways not anticipated by their designers, and at other times have designed and built our own laser equipment. One of the more interesting pieces in my collection of lasers is the "Photon Force 5", a system which we designed that has one fiber optic connector but which can deliver 4 different wavelengths of light or white light into that one fiber - and in a small box with only one moving part (the selector knob).

The research work has led our group to gain experience with a range of analytical methods and technologies which have been useful for our other (non-laser) research, including electron microscopy, fluorescence spectroscopy, Raman spectroscopy and X-Ray photoelectron spectroscopy, to name but a handful. We have been fortunate having access to outstanding facilities at the UQ Centre for Microscopy and Microanalysis, the Brisbane Surface Analysis Facility and at the Department of Chemistry at QUT.

A major theme of our recent work has been improved laser-based methods for biomechanical preparation of root canals. These studies will be a major part of the AALD plenary lecture. Trials have been undertaken with a range of middle infrared lasers (holmium:YAG, erbium:YAG and erbium, chromium:YSGG), which showed the ability of laser pulses to shape the radicular dentine of the walls of the root canal, and at the same time remove smear layer and debris. The presence of the correct amount and flow of water was found to be critical for achieving controlled ablation and smooth sides of the preparation.

We developed a range of techniques to modify optical fibers, so that lateral emissions of laser energy were increased and forward emissions were reduced. This pathway led to not one but several different configurations of side firing fibers and safe-ended side firing fibers. A unique honeycomb surface was created which had unique light emission and light collection properties. We also developed a rapid way of shaping conical ends onto glass fibers. This changes their optical performance and also allows them to penetrate better than conventional fibers into small and highly curved root canals.

A major recent development has been the use of lasers to generate pressure waves in aqueous media in the root canal. To measure the speed of these waves, the movement of microscopic bubbles has been tracked inside longitudinally split roots attached to glass slides, creating in effect an "ant farm" in which the events in the canal can be observed under a microscope and simultaneously filmed using a super high speed video camera. The actual speeds determined for these pressure waves were as fast as 120 km/hr. The waves cause a scouring action which gives outstanding cleaning of debris from root canals. In one series of studies, thick smear layers were created intentionally by preparing canals with rotary NiTi (ProTaper) instrumentation using water rather than EDTAC as the irrigant for the rotary files. Even in this extreme situation, the laser-generated pressure waves removed the smear layer in a matter of seconds.

To quantify this cleaning action, a digital method was developed so that the quality of smear layer removal could be assessed objectively from high power scanning electron microscope images. Using this same analytical method it was possible to show for the first time that laser-generated pressure waves activated solutions of EDTAC and improved their ability to remove smear layers. This could be documented even in the apical third of the canal.

To determine if there were new risks associated with this technique, we measured microscopic displacement of fluid from the apical foramen, and compared this to a range of conventional irrigation methods. Thermal changes on the outer root surface during laser treatment was also measured, and found to be so low as not to pose any concerns for the health of the periodontal ligament.

The ability of laser fluorescence to detect bacteria within the root canal environment has also been examined in a series of studies. Initially, root canals from unerupted teeth were deliberately infected in the laboratory and the levels of fluorescence from bacteria assessed over time. Threshold readings for sound healthy dentine were established. Examination of extracted teeth with known periapical pathology confirmed high fluorescence readings in infected canals, and examination under the scanning electron microscope of these same teeth confirmed the presence of bacterial biofilms. Treatment of infected canals reduced the fluorescence signal back to the baseline of healthy dentine.

Taken together, the results of the separate components of the endodontic research theme point to the viability of stand alone systems for detection of residual infection in the root canal. Such devices could assist in determining a biological endpoint for treatment. Detection systems could also be combined with a medium power visible red laser or red LED for photoactivated disinfection, so that such deposits, once detected could then be inactivated.

Laser fluorescence could also be combined with a pulsed high power laser capable of generating pressure waves and activating irrigant solutions to provide a "seek and destroy" autopilot system. This is an area where further research is being directed. If successful, less removal of dentine may be possible, meaning that root filled teeth would be less compromised physically than with current methods. A device of this type might enable endodontics to be completed in a single short visit, with the benefit of a known biological endpoint.

As well as the upcoming Australian Association for Laser Dentistry meeting, we will also be presenting UQ laser dentistry research at the FDI World Congress in September 2009.

An academic pursuit is never the work of one investigator, and over the past 20 years the UQ laser hard tissue applications research program has involved many students and colleagues, whose input and hard work I now wish to recognize formally:

Undergraduate UQ students: Tony Mendoza, Shari Prove, Stephen Perham, Kim Forrest-Winchester, Dan Abood, Mark Sandford, Tammer Amyra, Jihyo Kim, John Clark, Jeffrey Kho, Andrew McQuillan, Emma Wainwright, Danielle Do-Vuong and Justin Ooi.

Undergraduate overseas students: Elanor Roberts, Steven Armitage, Alasdair Cattanach, Vanessa Hoppe, Wim van Uum, Filis Keles, Gaby Groenveld and Kristina Ankerst.

Masters students: Graeme Stringer, Jackson Liu, Stephen Smith, Tim Silbert, Michael Lee, Karen Wong, Ward Rutley, Ola El-Bataina, Sarliza Sanusi, Jeff Hislop, Andrew Sainsbury and Quan Ho.

Doctoral and postdoctoral students: Sima Shahabi, Zahra Mazouri, Saso Ivanovski, Jelena Vlacic, Roy George, Suhayla Mubarak, Nghiem Doan, Srdjan Diklich, Raghad Hmud, Fardad Shakibaie, Laurent Lamard and Maryam Kuzekanani.

Clinical support staff: Kathy Liew, Rachel Peltola, Melanie Savins, Sarah Davies, Casey Black, Angela Lathouras, Renate Provost, Neil Bruce and Neil Taylor.

Collaborating staff at UQ: Peter Brockhurst, Ian Meyers, Kim Seow, Aart Taverne, Greg Seymour, Gerard Milburn, Peter L'Estrange, Philip Bird, Neil Bruce, Doug Harbrow, Halina Rubinzstein-Dunlop, Norman Heckenberg, Paul Meredith and Bill Kahler.

Other colleagues collaborating in the UQ research program: David Cox, Alan Forrest-Winchester, Peter Clark Ryan, Michael Walsh, Tim White, Helen Clifford, Andrew Brostek, Peter Verheyen, Armin Aminzadeh, Eric Reynolds Matt Moncreiff and Robert Gall.

I also wish to acknowledge the granting bodies which have funded our work (NH&MRC, ARC, ADRF, APRF, APS and ASE), and the laser manufacturers who have worked collaboratively with us over many years to advance the science and clinical practice of laser dentistry.

Table 1. UQ laser hard tissue research contributions over 2 decades

Cavity preparation

• Trials of laser wavelengths and systems
• Ablation rates of permanent teeth
• Ablation rates of deciduous teeth
• SEM comparative studies of laser vs turbine drills
• Patient clinical trials of anxiety levels
• Tips for real time fluorescence detection
• Dentine thresholds for fluorescence guidance
• Laser effects on restorative materials
• Selective removal of restorative materials and bonding agents

Etching of enamel and dentine

• Identification of optimal parameters
• SEM characterization of the surface
• Chemical characterization of the surface
• Laboratory bond strength tests
• Thermocycling studies
• Microleakage studies
• Clinical trials for sealant therapy
• Clinical trials for restorations

Hard/soft tissue combinations

• Clinical trial of laser pulp capping
• Desensitization by surface dentine tubule sealing
• Desensitizing using low level biostimulation
• Mechanisms for laser-induced analgesia
• Optimal parameters for laser-induced analgesia

Laser-activated fluoride

• Wavelength dependence
• Laboratory trials of caries prevention
• Clinical trial of caries prevention and arrest
• Laboratory trials of erosion prevention
• Use of lasers vs LEDs
• Chemical characterization of the surface
• Mechanisms for photonic conversion of apatite

Root surface debridement

• Dentine thresholds for fluorescence guidance
• Testing of fluorescence vs. other optical approaches
• Novel optical analytical methods for calculus recognition
• Alternative optical methods of calculus detection

Photoactivated disinfection for dental caries

• Trials of laser wavelengths
• Development of PAD laser and LED systems
• Use of different optical delivery systems
• Effect on cariogenic bacteria
• Penetration into dentine
• Effect on cervical and root surface caries lesions

Photoactivated disinfection in endodontics

• Trials of laser wavelengths
• Development of PAD laser and LED systems
• Use of different optical delivery systems
• Development of new fiber optical technologies
• Testing of resistant bacteria
• Testing of biolfilms

Surface sintering

• Fusion of synthetic apatites
• Fusion of enamel
• Fusion of restoratives
• CAD-sintering workstation prototypes

Bleaching

• Improved gels for photothermal bleaching
• Improved methods for photodynamic bleaching
• Thermal stress assessments in the laboratory
• Clinical trials of bleaching methods

Endodontic biomechanical preparation

• Effects of wavelengths on dentine ablation
• Ablation properties of radicular dentine
• Dehydration of canals
• Shockwave generation using lasers
• Novel optical fibers for laser endodontics
• Root thermal stresses during laser therapy
• Removal of smear layer
• Activation of aqueous irrigants
• Detection of bacteria
• Negotiation of curved canals
• Autopilot systems for laser endodontics