A system for safe and precise laser bone removal

Sebastian Stopp
Department of Micro Technology and Medical Device Technology
Technical University Munich, Germany

Emanuel von Kienlin
International Treatment Manager KaVo Dental GmbH

Herbert Deppe
Department of Oral and Maxillofacial Surgery
Technical University Munich, Germany

Tim C Lueth
Department of Micro Technology and Medical Device Technology
Technical University Munich, Germany

Usually, mechanically rotating instruments such as drills or mills are used for bone removal. In dental implantation, drills are used to produce cavities for implants. In Germany, 500,000 implants were set in 2005 using this technology. But the use of mechanically rotating instruments has several disadvantages. The main problem is the thermal damage generated by conventional drill systems made of steel or titanium. The generated heat can irreversibly damage the surrounding bone. To avoid frictional heat and vibrations, laser systems are an alternative for bone removal.(1,2) One application area for laser systems is dental implantation, where bone removal with lasers can be applied for cavity preparation.(3) Furthermore, the laser can be used for bone removal in osteotomy. In recent years, assistance systems for precise intraoperative realisation of preoperative planning have been developed. These assistance systems visualise the intraoperative position and orientation of surgical instruments relative to preoperative planning data.(4) Thus, the safety and the precision of an intervention can be increased.

[[HHE07_fig1_T21]]

Lasers for bone removal
The use of lasers is complicated by some disadvantages. During the surgery, the surgeon has only visual depth control. Optical depth control is not possible for narrow and deep cavities. In addition,  mistakes can be made through poor selection of the laser parameters and incorrect handling. Because the laser beam is focused, the energy on the bone surface changes with changing distance between laser focus and bone surface. With increasing distances between laser focus and bone surface, the energy for bone removal decreases and the risk of thermal damage increases.

Navigation for hand-moved lasers
To overcome the known deficits of the state of the art, the new approach is to use a navigated and power-controlled laser.(5,6) Pre­operative planning is done on the basis of CT data. During the surgery the position of the laser handpiece is visualised relative to the patient and to the preoperative planning. If the laser handpiece is positioned outside the planned area or the distance between laser focus and bone surface becomes too large, the laser is automatically deactivated by the system.

To calculate the applied energy on the bone surface and the resulting ablation and heat exposure, mathematical models are used.(7) To visualise the laser focus and measure the position and orientation of the laser handpiece relative to the patient, a navigation system is used. Thereby, the position and orientation of the laser handpiece are measured by an optical measurement system. To measure the position of the laser handpiece and the patient, optical trackers are used. The optical trackers are attached to the laser handpiece with a dental splint to the patient jaw. The position and orientation of the laser axis and the focal point are calculated relative to the optical measurement system using a specially designed calibration body. The distance between focus point and bone surface is calculated based on CT data and data from the optical measurement system. From the known laser parameters and the laser beam profile, the energy density can be calculated for the bone surface.

Prior to the treatment, plans have to be made. Plans are based on preoperative CT data. Therefore the MiMed-Dental-Navigation-Software was extended. During planning, the surgeon defines the size and position of cavities in the bone.

[[HHE07_fig2_T22]]

Visualization of measured and calculated information
During surgery, the measured data are visualised graphically. The navigation parameters are visualised in five different views. The visualisation is shown in Figure 3. In the upper left and upper middle view the position and orientation of the implants can be adjusted by the surgeon. The yellow line in the upper left view symbolises the nerve. Three implants are planned. One implant is coloured red. The red colour signalizes that the safety distance to the nerve is too small. The upper right view shows a top view of the planned cavity with a diameter of 5.00 mm. The cavity border is shown as a red circle. The blue point shows the laser on the bone surface and the green shows the orientation of the laser axis. The surgeon has to move the laser circularly over the surface to create a cavity. The lower left three dimensional view shows the reconstructed patient jaw and the position of the laser handpiece. The blue cylinders symbolize the planned cavity. The yellow part of the cylinders shows the actual cavity depth. The last view (lower right) shows the calculated energy proportion between dangerous heat energy and useful energy for bone removal.

[[HHE07_fig3_T23]]

In the cavity target view the planned cavity is visualised two dimensional from top. The laser beam position and orientation is visualised with two circles. One circle shows the position of the laser beam on the bone surface, the other one the angle between laser axis and planned cavity axis. This kind of visualization is well known in the navigated dental implantology. The energy and distance view is used to give information about the calculated proportion between ablation and heat energy. Additionally the actual distance between laser focus and bone surface is shown. Therefore, the lowering of the laser handpiece according to the reached depth is simplified for the surgeon.

Results
First experiments were performed on bovine bone. The positions of cavities were planned in the planning software. Following this, the navigated controlled laser was used to create cavities with a defined diameter and depth. A thread was cut into the cavities in a traditional manner and dental implants were inserted. The results show the possibility of laser cavity creation for dental implants.

Conclusion
The presented method is suitable to visualise the laser manufacturing process. The achieved accuracy of less then 1 mm in experimental setups is sufficient for dental implantology applications. The presented method was only verified for cylindrical cavities with a defined diameter and length until now. The method has to be analysed in further experimental studies for different cavity geometries. The presented method for visualising the reached depth of the material removal is verified with a laser handpiece with free focus position. Among laser hand pieces with free focus position, laser handpiece with contact tips or scanner system can be used for laser treatment. Therefore the presented method can be used as well because these cases can be considered as a special case of a free focused laser.

If a laser system with a scanner system is used, the scanner has to be adjusted for the diameter of the cavity. The user has only to lower the laser according to the navigation visualisation.

References

1. Sasaki KM, Aoki A, Ichinose S, Yoshino T, Yamada S, Ishikawa I. Scanning electron microscopy and Fourier transformed infrared spectroscopy analysis of bone removal using Er:YAG and CO2 lasers. J Periodontol 2002;73(6):643-52.
2. Kesler G, Romanos G, Koren R. Use of Er:YAG laser to improve osseointegration of titanium alloy implants – a comparison of bone healing. Int J Oral Maxillofac Implants 2006;21(3):375-9.
3. Schwarz F, Olivier W, Herten M, Sager M, Chaker A, Becker J. Influence of implant bed preparation using an Er:YAG laser on the osseointegration of titanium implants:a histomorphometrical study in dogs. J Oral Rehab In press 2007.
4. Lüth T, Bier J. Neue Technologien in der Mund-, Kiefer- und Gesichtschirurgie. In: Horch HH, editor. Mund, Kiefer- und Gesichtschirurgie. 4 ed.  Elsevier; 2007. p. 37-56.
5. Stopp S, Svejdar D, Deppe H, Lueth T. A new method for optimized laser treatment by laser focus navigation and distance visualization. Conf Proc IEEE Eng Med Biol Soc 2007;1:1738-41.
6. Stopp S, Deppe H, Lueth T. A new concept for navigated laser surgery. Lasers Med Sci 2007 Jul 28.
7. Bauer H. Laser als Strahlquelle. In: Bauer H, editor. Lasertechnik, Grundlagen und Anwendungen. Vogel Verlag Und Druck; 1991. p. 53-104.