Daniel Habor

A voice-coil actuated ultrasound micro-scanner for intraoral high resolution impression taking

Silicone based impression-taking of prepared teeth followed by plaster casting is well-established but potentially less reliable, error-prone and inefficient for newly emerging techniques such as computer aided design and manufacturing (CAD/CAM) of dental prosthetics. Intra-oral optical scanners have been introduced to increase efficiency of CAM but no breakthrough occurred so far. Oral liquids such as saliva, blood and sulcular fluid are still one of the main problems since the preparation area must be completely dry. Moreover, sub-gingival preparations need to be uncovered invasively prior to scanning and a reflecting powder coating is required in some cases. High frequency ultrasound (HFUS) has been recently introduced as an alternative to optical scanning. Ultrasound is less sensitive against oral fluids and in principal able to penetrate gingiva in a patient-friendly and cost-effective way. Although HFUS systems have been introduced for ophthalmology, dermatology or small animal imaging, none of them suits the challenging requirements and high accuracy demands for intra-oral micro-scanning of prepared teeth. For this reason, we conceived a new ultrasonic micro-scanning device based on a voice-coil actuated spherically focused HFUS transducer for intra-oral use. The system, which is designed for both highly dynamic accurate positioning and micrometer-resolution, is supplied with a sensor providing position feedback for motion control as well as the ultrasound trigger engine. In this contribution, we describe the set-up and evaluate the lateral displacement of the micro-scanners end-effector with respect to the oscillation rate using laser triangulation. The results are in good agreement to the requirements of an intra-oral ultrasound based micro-scanner.

Concept and evaluation of a synergistic controlled robotic instrument for trepanation in neurosurgery

A robotic instrument which synergistically cooperates with the surgeon for opening the skull in neurosurgery is proposed. To reduce frequent complications of this intervention, tear of the dura mater and bad reintegration of the skull bone a soft tissue preserving saw is combined with automatic depth regulation on the basis of a priori acquired medical imaging data (CT/MRI). By fusing the individual capabilities of the surgeon and a robotic device, it is possible to design an instrument which is significantly smaller than a fully autonomous system. The acceptance is enhanced by the integration of the surgeon into the process with direct control over the procedure. During the intervention, the instrument is manually guided by the surgeon on a freely defined trajectory. To be able to control this instrument, a method for real-time depth regulation, medical imaging data pre-processing and reduction as well as appropriate interfaces for the surgeon have been developed. In an experimental setup with phantom skull caps the system has been evaluated and has shown promising results, with a mean error of 0.62mm. Future work will include a detailed analysis of the persisting errors, integration of different sensors to control the instrument and preclinical trials.

An ultrasonic micro-scanner for thickness assessment of the vestibular jawbone: In-vitro accuracy evaluation

Progressive peri-implant bone loss may lead to implant failure. Conventionally, the vestibular jawbone thickness (VJT) is monitored via cone beam computed tomography. Ionizing radiation and artifacts due to metallic implants, as well as superstructures, are major drawbacks of x-ray-based imaging techniques. As a non-invasive and patient-friendly alternative, high frequency ultrasonic (HFUS) micro-scanning can be used to assess the jawbone surface. In this study, we present a HFUS scanner and algorithm for assessing the jawbone thickness which is based on a priori information of the superstructure surface and which does not require ultrasound penetration of the jawbone. Four implants were inserted into bovine ribs. Prior to mounting the polymer superstructures, the position and orientation of the implant relative to the superstructure surface was determined based on optical 3-D scans. Subsequently, porcine gingiva samples were attached to the ribs. The specimens were fixed in a water basin filled with isotonic saline solution. Ultrasound data was acquired with a HFUS micro-scanner (center frequency: > 50 MHz, relative bandwidth: > 70%, aperture 4 mm, focus 8 mm) designed for intra-oral use supporting highly dynamic accurate positioning in micrometer-resolution. Within this study, the ultrasound trigger spacing was set to 39 μm. Bone and superstructure surfaces were segmented out of the ultrasound data and converted to polygon meshes. These meshes were matched to an a priori acquired 3-D model of the superstructure. Finally, the vestibular bone thickness was calculated from the matched 3-D model. For evaluation of the proposed ultrasonic technique all specimens were cut into slices (thickness approx. 1 mm), examined using a stereo-microscope and compared to the calculated bone thickness. The overall error of ultrasound based bone thickness determination was 38±99μm (max: 260μm, min: -240μm) which is in good agreement to clinical requirements and which outperforms conventional x-ray based cone beam computed tomography.

Distortion reduction for a dental HFUS microscanning device

Silicone impression-taking of teeth is an established but inefficient technique for computer-aided manufacturing of dental prosthetics. Hence, intra-oral scanners based on optical technologies have been developed to optimize the process of impressioning and to enable a complete chairside workflow. However, during the scan, the presence of blood or saliva can cause critical model defects and the need to invasively expose subgingival areas remains essential for scanning. We introduced high frequency ultrasound (HFUS) as a new method for scanning dental structures. Ultrasound is less sensitive to oral fluids and is inherently able to penetrate gingiva non-invasively. HFUS-based intra-oral microscanning (USM) of teeth requires both a high spatial resolution and a mechanical precision to finally achieve an optimal impression detail. We designed a microscanner based on a direct drive mechanism with 2 degrees of freedom. In a preliminary study we tested the systems 2D-distortion by scanning a ball grid array as a reference body (RB) in degassed and tempered water. The ball grids accuracy was measured by a commercial optical reference scanner (OS). The Euclidean distance (ED) between ball centers of the RB and the USM were calculated. Afterwards, we implemented a 2D-distortion reduction method (DRM). The scaling factors (coefficients of the compensation matrix) were calculated by a linear regression analysis on the EDs. Finally, the systems precision was evaluated by scanning the occlusal surface of a molar tooth with the USM and applying the DRM. The acquired model was aligned and compared to a scan obtained with the OS by using a best-fit algorithm. Without using the DRM we measured a mean deviation of 14.8 microns (positive) and 21.2 microns (negative) (SD 22.7 microns). After the application of the DRM we achieved a mean deviation of 7.6 microns (positive) and 19.9 microns (negative) (SD 16.9 microns). The scan quality of the USM was improved by the developed DRM and thus reached the anticipated accuracy range for intra-oral impressioning.

Model based assessment of vestibular jawbone thickness using high frequency 3D ultrasound micro-scanning

Endosseous implants are well-established in modern dentistry. However, without appropriate therapeutic intervention, progressive peri-implant bone loss may lead to failing implants. Conventionally, the particularly relevant vestibular jawbone thickness is monitored using radiographic 3D imaging methods. Ionizing radiation, as well as imaging artifacts caused by metallic implants and superstructures are major drawbacks of these imaging modalities. In this study, a high frequency ultrasound (HFUS) based approach to assess the vestibular jawbone thickness is being introduced. It should be emphasized that the presented method does not require ultrasound penetration of the jawbone. An in-vitro study using two porcine specimens with inserted endosseous implants has been carried out to assess the accuracy of our approach. The implant of the first specimen was equipped with a gingiva former while a polymer superstructure was mounted onto the implant of the second specimen. Ultrasound data has been acquired using a 4 degree of freedom (DOF) high frequency (<50MHz) laboratory ultrasound scanner. The ultrasound raw data has been converted to polygon meshes including the surfaces of bone, gingiva, gingiva former (first specimen) and superstructure (second specimen). The meshes are matched with a-priori acquired 3D models of the implant, the superstructure and the gingiva former using a best-fit algorithm. Finally, the vestibular peri-implant bone thickness has been assessed in the resulting 3D models. The accuracy of this approach has been evaluated by comparing the ultrasound based thickness measurement with a reference measurement acquired with an optical extra-oral 3D scanner prior to covering the specimens with gingiva. As a final result, the bone thicknesses of the two specimens were measured yielding an error of −46±89μm (first specimen) and 70±93μm (second specimen).

Analyse der Regelungsstrategien eines Osteotomie-Instruments auf Basis multimodaler Informationen / Analysis of the Control Strategies of an Instrument for Osteotomy on the Basis of Multimodal Information

Zusammenfassung Um den Chirurgen bei der Eröffnung des Schädels in der Neurochirurgie oder des Brustbeins in der Herz- und Thoraxchirurgie zu unterstützen, wurde ein semiautomatisches, handgeführtes Sägeinstrument entwickelt. Die Schnitttiefe wird auf Basis von drei unterschiedlichen Messverfahren (Computertomographie, Ultraschall, Licht) automatisch geregelt. Nach erfolgreichen Machbarkeitsstudien zu den einzelnen Ansätzen wird in diesem Beitrag darauf aufbauend ein detaillierter Vergleich der Verfahren mit einer Untersuchung der strukturellen Vor- und Nachteile sowie Optimierungspotentiale durchgeführt. Summary To support the surgeon during opening of the skull in neurosurgery or of the sternum in cardiothoracic surgery, a semiautomatic, hand guided saw was developed. The cutting depth is automatically controlled on the basis of three different modalities (computed tomography, ultrasound, light). In addition to the general successful proof of general feasibility, the present paper provides the results of a detailed comparison of the different systems in combination with an analysis of the structural advantages and disadvantages and their individual optimization potential.