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 mu m. 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 +/- 99 mu m (max: 260 mu m, min: 240 mu m) which is in good agreement to clinical requirements and which outperforms conventional x-ray based cone beam computed tomography.