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Notes: Abstract: Objective: The aim of this study was to establish a correlation between bone mineral density measured preoperatively with dental computed tomography (CT), and insertion torque of screw-shaped dental implants.Material and methods: In eight human mandibles obtained postmortem, bone mineral density (BMD) was measured with dental quantitative CT (DQCT) and correlated with insertion torque values at 45 implant sites during insertion of screw-shaped dental implants (Brånemark System MKIII, Nobel Biocare, AB, Göteborg, Sweden).Results: A significant correlation (r=0.86, P〈0.001) between BMD and torque values was observed, indicating that local BMD at a specific implant position is related to the supportive capacity of the jawbone. BMD exhibited no correlation with bone height or position.Conclusion: The noninvasive assessment of BMD using a DQCT scan employing a low-dose protocol may be used to estimate expected primary stability depending on BMD, implant type and preparation procedure. These data may therefore help the surgeon to select the optimum implant position, implant type and operation technique.

Notes: Abstract: In this study a computer aided navigation technique for accurate positioning of oral implants was assessed. An optical tracking system with specially designed tools for monitoring the position of surgical instruments relative to the patient was used to register 5 partially or completely edentulous jaw models. Besides the accuracy of the tracking system, the precision of localizing a specific position on 3-dimensional preoperative imagery is governed by the registration algorithm which conveys the co-ordinate system of the preoperative computed tomography (CT) scan to the actual patient position. Two different point-to-point registration algorithms were compared for their suitability for this application. The accuracy was determined separately for the localization error of the position measurement hardware (fiducial localization error – FLE) and the error as reported by the registration algorithm (fiducial registration error – FRE). The overall error of the navigation procedure was determined as the localization error of additional landmarks (steel spheres, 0.5 mm diameter) after registration (target registration error – TRE). Images of the jaw models were obtained using a high resolution CT scan (1.5 mm slice thickness, 1 mm table feed, incremental scanning, 120 kV, 150 mAs, 512 · 512 matrix, FOV 120 mm). The accuracy of the position measurement probes was 0.69±0.15 mm (FLE). Using 3 implanted fiducial markers, FRE was 0.71±0.12 mm on average and 1.00±0.13 mm maximum. TRE was found to be 1.23±0.28 mm average and 1.87±0.47 mm maximum. Increasing the number of fiducial markers to a total of 5 did not significantly improve precision. Furthermore it was found that a registration algorithm based on solving an eigenvalue problem is the superior approach for point-to-point matching in terms of mathematical stability. The experimental results indicate that positioning accuracy of oral implants may benefit from computer aided intraoperative navigation. The accuracy achieved compares well to the resolution of the CT scan used. Further development of point-to-point/point-to-surface registration methods and tracking hardware has the potential to improve the precision of the method even further. Our system has potential to reduce the intraoperative risk of causing damage to critical anatomic structures, to minimize the efforts in prosthetic modelling, and to simplify the task of transferring preoperative planning data precisely to the operating room in general.