New approach to establish an object reference frame for dental arch in computer-aided surgical simulation

Automatic detection of symmetry plane for computer-aided surgical simulation in craniomaxillofacial surgery

Symmetry plane calculation is used in fracture reduction or reconstruction in the midface. Estimating a reliable symmetry plane without advanced anatomic knowledge is the most critical challenge. In this work, we developed a new automated method to find the mid-plane in CT images of an intact skull and a skull with a unilateral midface fracture. By use of a 3D point-cloud of a skull, we demonstrate that the proposed algorithm could find a mid-plane that meets clinical criteria. There is no need for advanced anatomical knowledge through the use of this algorithm. The algorithm used principal component analysis to find the initial plane. Then the rotation matrix, derived from an iterative closest point (ICP) registration method, is used to update the normal vector of the plane and find the optimum symmetry plane. A mathematical index, Hausdorff distance (HD), is used to evaluate the similarity of one mid-plane side in comparison to the contralateral side. HD decreased by 66% in the intact skull and 65% in a fractured skull and converged in just six iterations. High convergence speed, low computational load, and high accuracy suggest the use of the algorithm in the planning procedure. This easy-to-use algorithm with its advantages, as mentioned above, could be used as an operator in craniomaxillofacial software.

An Automatic Approach to Reestablish Final Dental Occlusion for 1-Piece Maxillary Orthognathic Surgery

Accurately establishing a desired final dental occlusion of the upper and lower teeth is a critical step in orthognathic surgical planning. Traditionally, the final occlusion is established by hand-articulating the stone dental models. However, this process is inappropriate to digitally plan the orthognathic surgery using computer-aided surgical simulation. To date, there is no effective method of digitally establishing final occlusion. We propose a 3-stage approach to digitally and automatically establish a desired final dental occlusion for 1-piece maxillary orthognathic surgery, including: 1) to automatically extract points of interest and four key teeth landmarks from the occlusal surfaces; 2) to align the upper and lower teeth to a clinically desired Midline-Canine-Molar relationship by minimization of sum of distances between them; and 3) to finely align the upper and lower teeth to a maximum contact with the constraints of collision and clinical criteria. The proposed method was evaluated qualitatively and quantitatively and proved to be effective and accurate.

Trueness and precision of scanning abutment impressions and stone models according to dental CAD/CAM evaluation standards

PURPOSE

The purpose of the present study was to compare scanning trueness and precision between an abutment impression and a stone model according to dental computer-aided design/computer-aided manufacturing (CAD/CAM) evaluation standards.

MATERIALS AND METHODS

To evaluate trueness, the abutment impression and stone model were scanned to obtain the first 3-dimensional (3-D) stereolithography (STL) file. Next, the abutment impression or stone model was removed from the scanner and re-fixed on the table; scanning was then repeated so that 11 files were obtained for each scan type. To evaluate precision, the abutment impression or stone model was scanned to obtain the first 3-D STL file. Without moving it, scanning was performed 10 more times, so that 11 files were obtained for each scan type. By superimposing the first scanned STL file onto the other STL files one by one, 10 color-difference maps and reports were obtained; i.e., 10 experimental scans per type. The independent t-test was used to compare root mean square (RMS) data between the groups (α=.05).

RESULTS

The RMS±SD values of scanning trueness of the abutment impression and stone model were 22.4±4.4 and 17.4±3.5 µm, respectively (P<.012). The RMS±SD values of scanning precision of the abutment impression and stone model were 16.4±2.9 and 14.6±1.6 µm, respectively (P=.108).

CONCLUSION

There was a significant difference in scanning trueness between the abutment impression and stone model, as evaluated according to dental CAD/CAM standards. However, all scans showed high trueness and precision.

Design, development and clinical validation of computer-aided surgical simulation system for streamlined orthognathic surgical planning

PURPOSE

There are many proven problems associated with traditional surgical planning methods for orthognathic surgery. To address these problems, we developed a computer-aided surgical simulation (CASS) system, the AnatomicAligner, to plan orthognathic surgery following our streamlined clinical protocol.

METHODS

The system includes six modules: image segmentation and three-dimensional (3D) reconstruction, registration and reorientation of models to neutral head posture, 3D cephalometric analysis, virtual osteotomy, surgical simulation, and surgical splint generation. The accuracy of the system was validated in a stepwise fashion: first to evaluate the accuracy of AnatomicAligner using 30 sets of patient data, then to evaluate the fitting of splints generated by AnatomicAligner using 10 sets of patient data. The industrial gold standard system, Mimics, was used as the reference.

RESULT

When comparing the results of segmentation, virtual osteotomy and transformation achieved with AnatomicAligner to the ones achieved with Mimics, the absolute deviation between the two systems was clinically insignificant. The average surface deviation between the two models after 3D model reconstruction in AnatomicAligner and Mimics was 0.3 mm with a standard deviation (SD) of 0.03 mm. All the average surface deviations between the two models after virtual osteotomy and transformations were smaller than 0.01 mm with a SD of 0.01 mm. In addition, the fitting of splints generated by AnatomicAligner was at least as good as the ones generated by Mimics.

CONCLUSION

We successfully developed a CASS system, the AnatomicAligner, for planning orthognathic surgery following the streamlined planning protocol. The system has been proven accurate. AnatomicAligner will soon be available freely to the boarder clinical and research communities.