Lasers in Surgery and Medicine 46:573–581 (2014)
An Automatic Robotic System for Three-Dimensional Tooth Crown Preparation Using a Picosecond Laser Lei Wang, MSc,1 Dangxiao Wang, PhD,1 Yuru Zhang, PhD,1 Lei Ma, MSc,1 Yuchun Sun, MD,2 and Peijun Lv, MD2 State Key Lab of Virtual Reality Technology and Systems, Beihang University, Beijing, China 2 School and Hospital of Stomatology, Peking University, Beijing, China
1
Background: Laser techniques have been introduced into dentistry to overcome the drawbacks of traditional treatment methods. The existing methods in dental clinical operations for tooth crown preparation have several drawbacks which affect the long-term success of the dental treatment. Objective: To develop an improved robotic system to manipulate the laser beam to achieve safe and accurate three-dimensional (3D) tooth ablation, and thus to realize automatic tooth crown preparation in clinical operations. Method: We present an automatic laser ablation system for tooth crown preparation in dental restorative operations. The system, combining robotics and laser technology, is developed to control the laser focus in threedimensional motion aiming for high speed and accuracy crown preparation. The system consists of an end-effector, a real-time monitor and a tooth fixture. A layer-by-layer ablation method is developed to control the laser focus during the crown preparation. Experiments are carried out with picosecond laser on wax resin and teeth. Results: The accuracy of the system is satisfying, achieving the average linear errors of 0.06 mm for wax resin and 0.05 mm for dentin. The angle errors are 4.338 for wax resin and 0.58 for dentin. The depth errors for wax resin and dentin are both within 0.1 mm. The ablation time is 1.5 hours for wax resin and 3.5 hours for dentin. Conclusions: The ablation experimental results show that the movement range and the resolution of the robotic system can meet the requirements of typical dental operations for tooth crown preparation. Also, the errors of tooth shape and preparation angle are able to satisfy the requirements of clinical crown preparation. Although the experimental results illustrate the potential of using picosecond lasers for 3D tooth crown preparation, many research issues still need to be studied before the system can be applied to clinical operations. Lasers Surg. Med. 46:573–581, 2014. ß 2014 Wiley Periodicals, Inc. Key words: tooth crown preparation; picosecond laser; robotic system; layer-by-layer ablation INTRODUCTION Tooth crown preparation in dental restorative operations refers to the process of removing hard tissue of a decayed tooth and forming an expected tooth shape. There ß 2014 Wiley Periodicals, Inc.
are two typical methods for the hard tissue removal in present clinical operations: grind using turbine-driven drills and ablation using Er:YAG or Er,Cr:YSGG lasers. The former may generate mechanical and thermal stress, resulting in micro cracks of several tens of microns in enamel [1,2]. These cracks are starting points for new carious attacks and have to be avoided for long-term success of the dental treatment. Besides, the vibrations and the noise produced by drills will make patients uncomfortable. Laser ablation of hard tissue for tooth crown preparation is considered to be safer and more comfortable compared with traditional drills or burs since it produces less pain and reduces noise and vibrations [3]. Researches of applying ultra-short pulsed lasers (USPL) in restorative dentistry have been increasing in the past decades [4]. Bello-Silva et al. explored different wavelengths, pulse durations and irradiation parameters of USPL in dentin and enamel removal, and showed that USPL is suitable for cavity preparation [5]. Lizarelli et al. [6] carried out ablation experiments using USPL on a tooth surface. They demonstrated the possibility of selective control of refractive index change and application in the preferential removal of portions of dental hard tissues. Kraft et al. used USPL for calculus removal on a root cement surface [7]. Daskalova et al. demonstrated that by selecting suitable parameters one can obtain efficient dentin surface preparation without evidence of thermal damage [8]. Niemz [9] demonstrated the advantages and limitations of using USPL in dentistry. Rode et al. [4] demonstrated
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and have disclosed the following: No authors hold stock, or receive royalties from any companies. Contract grant sponsor: National Key Technology R&D Program of China; Contract grant number: 2012BAI07B04; Contract grant sponsor: National Natural Science Foundation of China; Contract grant numbers: 61170187, 61190125. Correspondence to: Dangxiao Wang, PhD, The State Key Lab of Virtual Reality Technology and Systems, Beihang University, Beijing 100191,China. E-mail: [email protected] Correspondence to: Peijun Lv, MD, Peking University School and Hospital of Stomatology, Beijing, China. E-mail: [email protected] Accepted 11 June 2014 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/lsm.22274
574
WANG ET AL.
ablation of dental enamel using a subpicosecond pulsed laser. Kru¨ger et al. [10] pioneered the study on using femtosecond lasers in dental surgery. Previous research on USPL for tooth crown preparation has been carried out to manipulate the laser beam manually. Compared with manual manipulation, robotic manipulation has potential benefits in 3D motion planning for achieving higher accuracy, avoiding hand trembling and reducing operation time. Meanwhile, robotic manipulation can help patients to relieve fear and discomfort associated with conservative dental treatment [11]. Furthermore, a robotic device may provide high accuracy performance by carefully designed sensors and actuation systems in various surgical operations [12]. Our long-term goal is to develop a robotic system for tooth crown
preparation that can achieve high efficiency and accuracy. The work reported here is the first step toward the goal. METHODS Design Requirements of the Robotic End-Effector Architecture of the automatic crown preparation system. The automatic crown preparation system includes the following components: a miniature robotic end-effector, a tooth fixture, a laser generator, a laser transmission arm, a laser scanner and a computer console (Fig. 1a). The original shape of the target tooth (Fig. 1b) is acquired by a laser scanner (3Shape D700, Denmark). The expected crown preparation shape is designed using the STL Module of Mimics software (Mimics 10.0,
Fig. 1. a: Conceptual illustration of the automatic crown preparation system; (b) shape of a target tooth before and after preparation: the blue part indicates the original shape of the target tooth before preparation, and the brown part is the expected shape of the target tooth after crown preparation; (c) a Gaussian laser beam focused by a protruding optical lens; (d) the optical components and transmission path of the laser beam.
AN AUTOMATIC ROBOTIC SYSTEM FOR TOOTH PREPARATION
Materialize, Inc., Leuven, Belgium) based on the scanning data, which has already taken into account the retention and resistance form. Trajectory of the robotic end-effector is planned according to the data from the target tooth and the expected crown shape. With the scanned data of the target tooth, adjacent teeth and soft tissue, a fixture can be designed (Imageware 13, EDS Corp., CA) and printed in resin material through a rapid prototype machine (Evisiontec Corp., Berlin, Germany). The fixture is fixed steadily on the dentition with dental silicon material, and is used to define the location of the end-effector relative to the target tooth. In this paper, we focus on the design and experiments of the robotic end-effector, which will be installed on the transmission arm, and then connected and fixed on the tooth fixture. All the other components of the system (including laser scanner, laser transmission arm and laser generator etc.) are available and provided by our collaborators. Performance requirements of the robotic endeffector. The goal of crown preparation is to remove decayed tissues from a target tooth and prepare the tooth into an expected shape that a crown can be placed. Manual operation for crown preparation is difficult to maintain high accuracy because of the narrow space (open distance of the mouth in vertical direction is 25–50 mm) within patients’ oral cavity, unexpected movement of patient’s head and tongue, visual judgment error of the dentist and location error of the dentist’s hand. The robotic system should overcome those manual drawbacks and take into account the clinic needs comprehensively. In the design of the robotic end-effector, we consider the following requirements: (1) High accuracy: The accuracy of the tooth shape and the smoothness of the tooth surface are two main performance metrics for evaluating the quality of the preparation procedure. The shape accuracy of the prepared tooth preparation includes: the linear error is less than 0.2 mm, and the angle error is required to be l,000 millimeters per second; ablation time: 1 hours Diameter of the tip of the end-effector: 25 mm; weight: 500 g Real-time monitoring of the target tooth Accurate and reliable connections between end-effector and tooth fixture
576
WANG ET AL.
Within the focal depth, the focused laser has the ability to remove stiff materials such as enamel and dentin. The parameters of the laser need to be explored to avoid overheating and to prevent carbonization of tooth materials. Design solution for 3D motion control of the laser focus. In order to generate 3D ablation trajectory for tooth crown preparation, the 3D motion of the laser focus is decomposed into a 2D planar motion and a linear motion along the tooth’s depth (z-axis). The 2D planar motion can be realized by changing the direction of the laser beam through the pendular movement of the two vibrating mirrors. And the linear motion can be realized by the translation of the protruding optical lens. Pendular motors are chosen to drive the vibrating mirrors. To ensure the accuracy of 2D scanning, closedloop control is adopted with high-resolution position sensors. A linear motor is adopted to drive the protruding optical lens moving along a linear guide. Furthermore, closed-loop control is adopted with a high resolution grating ruler to get real-time feedback of the lens’ position. Design solution for real-time monitoring. Realtime monitoring is vital to safe 3D USPL automatic ablation as the real-time information of the target tooth is needed for the dentists to adjust the operation, and the operation should be stopped in case of emergency. CCD is adopted as the optical tracking sensor in our system. There are mainly two solutions for using CCD sensors: intraoral and extraoral imaging [18–22]. The intraoral space is quite narrow, and the tooth fixture will cover the target tooth during the whole ablation process, making it difficult for dentists to check the real-time ablation situation directly. So extraoral real-time monitoring is selected. The information of the target tooth is fed back to the CCD sensor which is mounted out of the mouth.
TABLE 2. Given Parameters of the System Parameter D D1 L L1
Meaning Diameter of the target tooth [0–11.7 mm] Diameter of the incoming laser beam (3.5 mm) The range of dental drilling along the z-axis [0–6 mm] The distance from the first molar to the mouth entrance cavity [0–60 mm]
Prototypes Selection of motors and optical components. Parameters shown in Table 2 are provided by the dentists, which provide a foundation for selecting the optimal components. The diameter of the protruding optical lens is decided to be 25.4 mm, and the diameter of the dichroscope is 28 mm and that of the reflecting mirror I and II both to be 18 mm. The focal length is 175 mm. These parameters also depend on the specifications of available protruding optical lens in the market. Specifications for all the selected components are shown in Table 3. Prototype. A compact prototype is built based on the selected components (Fig. 2a,b). The overall size of the physical prototype (Fig. 2c) is 108 mm 56 mm 43 mm (except the CCD camera), and the diameter of the parts that works inside the mouth is 20 mm. Besides, a fixture for the first molar is designed and made as shown in Figure 2d,e. In the next step, we will design fixtures for other teeth. Ablation Method The layer by layer ablation method. A layer-bylayer ablation method is proposed to generate a 3D path
TABLE 3. Specifications of the Selected Components Components Pendular motors
Voice coil motor AVM 12-6.4
Specifications
Components
Size and weight
F 22 mm 38 mm, 50 g
Optical aperture (mm) Maximum scan angle Marking speed (millimeter per second) Repeatability (mRad) Size and weight
7
Performance
12.5 4000
Distance-coded reference MercuryTM 2000
Specifications Size
Performance CCD (Microvision MV-VE 120 SM)
Dimension (mm) Max resolution
An Automatic Robotic System for Three-Dimensional Tooth Crown Preparation Using a Picosecond Laser Lei Wang, MSc,1 Dangxiao Wang, PhD,1 Yuru Zhang, PhD,1 Lei Ma, MSc,1 Yuchun Sun, MD,2 and Peijun Lv, MD2 State Key Lab of Virtual Reality Technology and Systems, Beihang University, Beijing, China 2 School and Hospital of Stomatology, Peking University, Beijing, China
1
Background: Laser techniques have been introduced into dentistry to overcome the drawbacks of traditional treatment methods. The existing methods in dental clinical operations for tooth crown preparation have several drawbacks which affect the long-term success of the dental treatment. Objective: To develop an improved robotic system to manipulate the laser beam to achieve safe and accurate three-dimensional (3D) tooth ablation, and thus to realize automatic tooth crown preparation in clinical operations. Method: We present an automatic laser ablation system for tooth crown preparation in dental restorative operations. The system, combining robotics and laser technology, is developed to control the laser focus in threedimensional motion aiming for high speed and accuracy crown preparation. The system consists of an end-effector, a real-time monitor and a tooth fixture. A layer-by-layer ablation method is developed to control the laser focus during the crown preparation. Experiments are carried out with picosecond laser on wax resin and teeth. Results: The accuracy of the system is satisfying, achieving the average linear errors of 0.06 mm for wax resin and 0.05 mm for dentin. The angle errors are 4.338 for wax resin and 0.58 for dentin. The depth errors for wax resin and dentin are both within 0.1 mm. The ablation time is 1.5 hours for wax resin and 3.5 hours for dentin. Conclusions: The ablation experimental results show that the movement range and the resolution of the robotic system can meet the requirements of typical dental operations for tooth crown preparation. Also, the errors of tooth shape and preparation angle are able to satisfy the requirements of clinical crown preparation. Although the experimental results illustrate the potential of using picosecond lasers for 3D tooth crown preparation, many research issues still need to be studied before the system can be applied to clinical operations. Lasers Surg. Med. 46:573–581, 2014. ß 2014 Wiley Periodicals, Inc. Key words: tooth crown preparation; picosecond laser; robotic system; layer-by-layer ablation INTRODUCTION Tooth crown preparation in dental restorative operations refers to the process of removing hard tissue of a decayed tooth and forming an expected tooth shape. There ß 2014 Wiley Periodicals, Inc.
are two typical methods for the hard tissue removal in present clinical operations: grind using turbine-driven drills and ablation using Er:YAG or Er,Cr:YSGG lasers. The former may generate mechanical and thermal stress, resulting in micro cracks of several tens of microns in enamel [1,2]. These cracks are starting points for new carious attacks and have to be avoided for long-term success of the dental treatment. Besides, the vibrations and the noise produced by drills will make patients uncomfortable. Laser ablation of hard tissue for tooth crown preparation is considered to be safer and more comfortable compared with traditional drills or burs since it produces less pain and reduces noise and vibrations [3]. Researches of applying ultra-short pulsed lasers (USPL) in restorative dentistry have been increasing in the past decades [4]. Bello-Silva et al. explored different wavelengths, pulse durations and irradiation parameters of USPL in dentin and enamel removal, and showed that USPL is suitable for cavity preparation [5]. Lizarelli et al. [6] carried out ablation experiments using USPL on a tooth surface. They demonstrated the possibility of selective control of refractive index change and application in the preferential removal of portions of dental hard tissues. Kraft et al. used USPL for calculus removal on a root cement surface [7]. Daskalova et al. demonstrated that by selecting suitable parameters one can obtain efficient dentin surface preparation without evidence of thermal damage [8]. Niemz [9] demonstrated the advantages and limitations of using USPL in dentistry. Rode et al. [4] demonstrated
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and have disclosed the following: No authors hold stock, or receive royalties from any companies. Contract grant sponsor: National Key Technology R&D Program of China; Contract grant number: 2012BAI07B04; Contract grant sponsor: National Natural Science Foundation of China; Contract grant numbers: 61170187, 61190125. Correspondence to: Dangxiao Wang, PhD, The State Key Lab of Virtual Reality Technology and Systems, Beihang University, Beijing 100191,China. E-mail: [email protected] Correspondence to: Peijun Lv, MD, Peking University School and Hospital of Stomatology, Beijing, China. E-mail: [email protected] Accepted 11 June 2014 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/lsm.22274
574
WANG ET AL.
ablation of dental enamel using a subpicosecond pulsed laser. Kru¨ger et al. [10] pioneered the study on using femtosecond lasers in dental surgery. Previous research on USPL for tooth crown preparation has been carried out to manipulate the laser beam manually. Compared with manual manipulation, robotic manipulation has potential benefits in 3D motion planning for achieving higher accuracy, avoiding hand trembling and reducing operation time. Meanwhile, robotic manipulation can help patients to relieve fear and discomfort associated with conservative dental treatment [11]. Furthermore, a robotic device may provide high accuracy performance by carefully designed sensors and actuation systems in various surgical operations [12]. Our long-term goal is to develop a robotic system for tooth crown
preparation that can achieve high efficiency and accuracy. The work reported here is the first step toward the goal. METHODS Design Requirements of the Robotic End-Effector Architecture of the automatic crown preparation system. The automatic crown preparation system includes the following components: a miniature robotic end-effector, a tooth fixture, a laser generator, a laser transmission arm, a laser scanner and a computer console (Fig. 1a). The original shape of the target tooth (Fig. 1b) is acquired by a laser scanner (3Shape D700, Denmark). The expected crown preparation shape is designed using the STL Module of Mimics software (Mimics 10.0,
Fig. 1. a: Conceptual illustration of the automatic crown preparation system; (b) shape of a target tooth before and after preparation: the blue part indicates the original shape of the target tooth before preparation, and the brown part is the expected shape of the target tooth after crown preparation; (c) a Gaussian laser beam focused by a protruding optical lens; (d) the optical components and transmission path of the laser beam.
AN AUTOMATIC ROBOTIC SYSTEM FOR TOOTH PREPARATION
Materialize, Inc., Leuven, Belgium) based on the scanning data, which has already taken into account the retention and resistance form. Trajectory of the robotic end-effector is planned according to the data from the target tooth and the expected crown shape. With the scanned data of the target tooth, adjacent teeth and soft tissue, a fixture can be designed (Imageware 13, EDS Corp., CA) and printed in resin material through a rapid prototype machine (Evisiontec Corp., Berlin, Germany). The fixture is fixed steadily on the dentition with dental silicon material, and is used to define the location of the end-effector relative to the target tooth. In this paper, we focus on the design and experiments of the robotic end-effector, which will be installed on the transmission arm, and then connected and fixed on the tooth fixture. All the other components of the system (including laser scanner, laser transmission arm and laser generator etc.) are available and provided by our collaborators. Performance requirements of the robotic endeffector. The goal of crown preparation is to remove decayed tissues from a target tooth and prepare the tooth into an expected shape that a crown can be placed. Manual operation for crown preparation is difficult to maintain high accuracy because of the narrow space (open distance of the mouth in vertical direction is 25–50 mm) within patients’ oral cavity, unexpected movement of patient’s head and tongue, visual judgment error of the dentist and location error of the dentist’s hand. The robotic system should overcome those manual drawbacks and take into account the clinic needs comprehensively. In the design of the robotic end-effector, we consider the following requirements: (1) High accuracy: The accuracy of the tooth shape and the smoothness of the tooth surface are two main performance metrics for evaluating the quality of the preparation procedure. The shape accuracy of the prepared tooth preparation includes: the linear error is less than 0.2 mm, and the angle error is required to be l,000 millimeters per second; ablation time: 1 hours Diameter of the tip of the end-effector: 25 mm; weight: 500 g Real-time monitoring of the target tooth Accurate and reliable connections between end-effector and tooth fixture
576
WANG ET AL.
Within the focal depth, the focused laser has the ability to remove stiff materials such as enamel and dentin. The parameters of the laser need to be explored to avoid overheating and to prevent carbonization of tooth materials. Design solution for 3D motion control of the laser focus. In order to generate 3D ablation trajectory for tooth crown preparation, the 3D motion of the laser focus is decomposed into a 2D planar motion and a linear motion along the tooth’s depth (z-axis). The 2D planar motion can be realized by changing the direction of the laser beam through the pendular movement of the two vibrating mirrors. And the linear motion can be realized by the translation of the protruding optical lens. Pendular motors are chosen to drive the vibrating mirrors. To ensure the accuracy of 2D scanning, closedloop control is adopted with high-resolution position sensors. A linear motor is adopted to drive the protruding optical lens moving along a linear guide. Furthermore, closed-loop control is adopted with a high resolution grating ruler to get real-time feedback of the lens’ position. Design solution for real-time monitoring. Realtime monitoring is vital to safe 3D USPL automatic ablation as the real-time information of the target tooth is needed for the dentists to adjust the operation, and the operation should be stopped in case of emergency. CCD is adopted as the optical tracking sensor in our system. There are mainly two solutions for using CCD sensors: intraoral and extraoral imaging [18–22]. The intraoral space is quite narrow, and the tooth fixture will cover the target tooth during the whole ablation process, making it difficult for dentists to check the real-time ablation situation directly. So extraoral real-time monitoring is selected. The information of the target tooth is fed back to the CCD sensor which is mounted out of the mouth.
TABLE 2. Given Parameters of the System Parameter D D1 L L1
Meaning Diameter of the target tooth [0–11.7 mm] Diameter of the incoming laser beam (3.5 mm) The range of dental drilling along the z-axis [0–6 mm] The distance from the first molar to the mouth entrance cavity [0–60 mm]
Prototypes Selection of motors and optical components. Parameters shown in Table 2 are provided by the dentists, which provide a foundation for selecting the optimal components. The diameter of the protruding optical lens is decided to be 25.4 mm, and the diameter of the dichroscope is 28 mm and that of the reflecting mirror I and II both to be 18 mm. The focal length is 175 mm. These parameters also depend on the specifications of available protruding optical lens in the market. Specifications for all the selected components are shown in Table 3. Prototype. A compact prototype is built based on the selected components (Fig. 2a,b). The overall size of the physical prototype (Fig. 2c) is 108 mm 56 mm 43 mm (except the CCD camera), and the diameter of the parts that works inside the mouth is 20 mm. Besides, a fixture for the first molar is designed and made as shown in Figure 2d,e. In the next step, we will design fixtures for other teeth. Ablation Method The layer by layer ablation method. A layer-bylayer ablation method is proposed to generate a 3D path
TABLE 3. Specifications of the Selected Components Components Pendular motors
Voice coil motor AVM 12-6.4
Specifications
Components
Size and weight
F 22 mm 38 mm, 50 g
Optical aperture (mm) Maximum scan angle Marking speed (millimeter per second) Repeatability (mRad) Size and weight
7
Performance
12.5 4000
Distance-coded reference MercuryTM 2000
Specifications Size
Performance CCD (Microvision MV-VE 120 SM)
Dimension (mm) Max resolution
