Koutarou Maki

Comparison of cone beam computed tomography imaging with physical measures

OBJECTIVES The goal of this study was to determine the accuracy of measuring linear distances between landmarks commonly used in orthodontic analysis on a human skull using two cone beam CT (CBCT) systems. METHODS Measurements of length were taken using volumetric data from two CBCT systems and were compared with physical measures using a calliper applied to one human adult skull. Landmarks were identified with chromium steel balls embedded at 32 cranial and 33 mandibular landmarks and the linear measures were taken with a digital calliper. The skull was then scanned with two different CBCT systems: the NewTom QR DVT 9000 (Aperio Inc, Sarasota, FL) and the Hitachi MercuRay (Hitachi Medico Technology, Tokyo, Japan). CT data including the landmark point data were threshold segmented using CyberMeds CB Works software (CB Works 1.0, CyberMed Inc., Seoul, Korea). The resulting segmentations were exported from CB Works as VRML (WRL) files to Amira software (Amira 3.1, Mercury Computer Systems GmbH, Berlin, Germany). RESULTS The error was small compared with the gold standard of the physical calliper measures for both the NewTom (0.07+/-0.41 mm) and CB MercuRay (0.00+/-0.22 mm) generated data. Absolute error to the gold standard was slightly positive, indicating minor compression relative to the calliper measurement. The error was slightly smaller in the CB MercuRay than in the NewTom, probably related to a broader greyscale range for describing beam attenuation in 12-bit vs 8-bit data. CONCLUSIONS The volumetric data rendered with both CBCT systems provided highly accurate data compared with the gold standard of physical measures directly from the skulls, with less than 1% relative error.

Changes in Cortical Bone Mineralization in the Developing Mandible: A Three‐Dimensional Quantitative Computed Tomography Study

Quantitative computed tomography (QCT) was completed in 34 subjects between the ages of 9 and 33 years with symmetrical mandibles in order to investigate the three‐dimensional cortical bone mineral density (BMD) distribution in the mandible. The number and distribution of the pixels were determined at three levels: (1) representing the entire mandibular bone; (2) the cortical bone at 60% above the baseline defined as the segmentation level (around 1050 mg/cm3) and representative of only cortical bone; and (3) the highest mineralized cortical bone (>1250 mg/cm3). The geometrical distribution of the highest mineralized areas was evaluated by three‐dimensional reconstruction of the images. The total number of pixels for the entire mandible increased significantly at each time point represented at four increasing ages groups (9–11 years of age, 12–14 years of age, 15–17 years of age, and >18 years of age). The male and female subjects had a similar total number of pixels for the entire mandible before the age of 11, but the male subjects showed a significantly larger total number of mandibular pixels after that age. Comparison of the number of pixels for pure cortical bone (60% segmentation level) and the highest mineralized cortical bone indicated a significant increase with maturation with the greatest change occurring between the 13‐year and 16‐year age groups. However, the ratio of cortical bone/total bone increased at a more rapid rate in the male subjects and reached a plateau by the 16‐year age group, showing distinct differences in mineralization of the mandible between the sexes.

Functional Adaptation of Mandibular Bone

This study examined the human mandible from the biomechanical point of view. This chapter covers two subjects: mechanical events that occur in the human mandible during biting, and the basic behavior of the functional adaptation of bone. To estimate mechanical response in the mandible, we proposed an individual modeling method based on X-ray computed tomography (CT) data of the individual that consists of four parts. First, we extracted contour images of the mandibular shape from the X-ray CT data. Second, we made a surface model covered with polygons. Third, we provided an approximate model modified from a standard type of model. Finally, we obtained an individual finite-element model by transforming the approximate model so that it fitted the shape of the surface model. Using the model, we analyzed the stress distribution of the mandible during biting. The stress distributes in the whole area of the mandible, although there are some regions that are highly stressed. The stress distribution was compared with the bone density distribution, and a strong correlation was found. This correlation tells us that the human mandibular bone also has functional adaptation. Based on the analytical results, we discussed the mechanical rationality of the human mandible and the basic behavior of functional adaptation. To examine mechanical rationality, we determined bone robustness by calculating the ratio of stress value to bone strength for every element. The result shows that the human mandible takes a rational structure because the ratio is almost uniform throughout the mandible. To examine the basic behavior of functional adaptation, we proposed a model of functional adaptation and showed that the proposed model self-organizes a proper mechanical structure. We also showed that mechanisms of mandibular deformity can be explained successfully by the proposed model.

Dental Patient Robot

Presently, the simple head model (hereinafter referred to as phantom for practical training) with dentition models is used for dental therapy training. We suggested the patient robot for dental therapy training (hereinafter referred to as patient robot) as one of the practical applications of the humanoid robot technology, and we actually developed patient robots. One of them is the general model provided with 14 degrees of freedom (DOF) in addition to a tongue and lips that may interfere with treatment, allowing reflection of any change in simple expression. Also, active motions of the neck or hand allow various impediments so as to interfere with the actual potential treatment. The robot allowed trainees to do dental therapy training closer to the actual practice involving avoidance of these risks

Dental Patient Robot as a Mechanical Human Simulator

The aim of this research is the development of a patient robot for use in actual clinical training. Electro pneumatic regulators and electromagnetic valves incorporated in the robot is operated by manipulating air cylinders. It is possible to conduct training assuming several patients enabling trainees to learn a flexible response under a wide range of circumstances. A simple interface was used for ease of operation. Further, a built-in sensor inside the oral cavity responds to the trainees actions leading to a vomiting reflex and pain during drilling teeth. Attaching the pain sensor to the body of test subjects, will also be useful for training social service workers during nursing care examinations.

Application of cone-beam X-ray CT in dento-maxillofacial region

3-dimensional imaging system using a 2-dimensional detector and cone-beam x-ray was developed for dental field. This method reconstructs an isotropic 3-dimensional image from a set of projection images acquired during one rotation scan (9.6 Sec scan time). This system uses high-speed high-precision CCD television camera and Image Intensifier (4.0, 7.0 and 9.0 inches) as a detector and has new techniques to improve image quality and reduce x-ray dose. This system can reconstruct 5l2x5l2x5l2-voxel (130 milion-voxel) image with 0.20–0.35 mm/voxel resolution. Conventional X-ray images such as dental X-ray, panoramic, and cephalogram were reconstructed from volumetric data by image processing software. The application of this new method in dental field was examined with volunteers. From the results, the detailed structures of dento-maxillofacial region (from foramen mentale to TMJ) could be visualized in the 3D images. Bucco-labial extension of periapical lesion, related position of mandibular canal and dental implants, and position of impacted teeth were easy to detect more than conventional x-ray image. Accurate 3-dimensional image of dental arch was also reproduced from serial axial image. This system is widely applicable in dental field and can be used to derive clinically useful information about 3-dimensional maxillofacial structure, tooth position and bone quality.

Three-dimensional Display System of Individual Mandibular Movement

It is expected to develop an intelligible diagnostic system of temporomandibular disorders (TMD) for both medical doctors and patients. This study proposes a display system that visualizes motion of the human mandible. The system integrates two engineering methods. One is an optical motion capture technique for measuring the mandibular movements. The other is an individual modeling method based on the X-ray CT data. It is important to know exact mandibular movements for the proper diagnosis. This paper discusses experimental verification of the total performance of the system using a device of hinge movement. The verification clearly shows that precision of the model has a great effect on accuracy of the movements. The total performance of the system is achieved within an accuracy of 0.2mm at the hinge of the device. The system provides not only three-dimensional visual information of the mandibular movements as animations but also quantitative information of position, velocity and acceleration at an arbitral point of the model. The system will be useful for informed consent in medical treatments of TMD.

Patient Specific Finite Element Modeling of a Human Skull

This paper presents automated finite element modeling method and application to a biomechanical study. The modeling method produces a finite element model based on the multi-sliced image data adaptively controlling the element size according to complexity of local bony shape. The method realizes a compact and precise finite element model with a desired total number of nodal points. This paper challenges to apply this method to a human skull because of its intricate structure. To accomplish the application of the human skull, we analyze characteristics of bony shape for a mandible and a skull. Using the analytical results, we demonstrate that the proposed modeling method successfully generates a precise finite element model of the skull with fine structures.

Biomechanical Study of the Human Mandible on Mechanical Response of Its Shape and Structure

This paper describes a biomechanical study of the human mandible based on individual X-ray computed tomography (CT) data. The purpose of this study is to estimate the mechanical response in the human mandible during biting and to discuss the mechanical rationality of the mandible. It is indispensable for any biomechanical study to establish reliable mathematical models. We constructed two types of three-dimensional finite-element models based on individual X-ray CT data. One is symmetrical in shape, and the other one is nonsymmetrical. For stress analysis, we estimated the masticatory forces and directions from the CT images, and also estimated reaction forces acting on the teeth using a pressure sheet. The computational results showed that stress is distributed in the entire area of these mandibles, but stress around the condyle is less than in other regions. Bone density distributions of the mandibles were measured by the same X-ray CT. These results showed a strong relationship between stress and bone density distribution for both mandibles.

Masticatory muscles and mandibular bone growth

It is well known that bone density is well controlled by muscular systems and its biomechanical environment in living individuals. Investigating the growth changes in mandibular bone mineralization leads to further understanding of masticatory functions. On the other hand, medical imaging system such as computed tomography (CT) have provided a method to not only assess the infrastructure, but to quantify the bone. In our previous studies, we introduced a new calibration phantom developed from a Ca compound for applying quantitative computed tomography to the human craniomandibular skeleton (Maki et al., 1997). With this technique, the growth changes in three-dimensional distribution of the highest mineralized cortical bone of the mandible and relationship between muscle function were evaluated in this study.