A Postoperative Evaluation Guideline for Computer-Assisted Reconstruction of the Mandible

In a recently published systematic review, we analyzed studies that assessed accuracy of computer-assisted reconstructions of the mandible. Because of a lack of uniformity in planning and evaluation methodologies, the ability to compare postoperative outcomes was limited. Therefore, we proposed a practical, feasible and reproducible evaluation guideline.

With this guideline, we aim to create uniformity between future studies regarding postoperative accuracy evaluation. We feel there is abundant room for further progress in determining tolerable outcome ranges regarding acceptable mandibular angle deviations and regarding deviations of the virtually planned dental implants in relation to future prosthetics. The procedure will be demonstrated by Lars Leeuwrik, a dentistry student and member of our oral reconstruction research group.

Begin by opening the image-based 3D medical software. Click file and new from disk. Then select the folder that contains the DICOM images of the postoperative scan.

Choose the study from the list and click convert. A window will pop up for the assessment of the orientation of the skull. Change the orientation by left clicking the orientation characters.

Then click OK to validate. To create an ROI, click on the threshold tool and define a threshold containing all voxels of the mandibular bone with a certain interval of gray value. Manually tweak the Hounsfield range.

Then click the green button to validate the segmentation. To edit the ROI, choose the isolate tool. Then click the mandible in the 3D view point.

Select create result in new ROI and click the green button to validate the isolation which will cause all non-connected structures to disappear. When the ROI is finished, click the add part button on the workflow toolbar. Then click the solid part tool, select the mandible postop, and select off in the smoothing options.

Click the green button to validate. Finally, in the export menu, select the mandible postop part, choose the output directory, and use the standard scale one. Click the green check mark to validate.

To determine the Frankfurt plane, midsagittal plane, and nasion for uniform orientation of the preoperative STL model of the skull, open the evaluation software and drag the preoperative STL file into the popped up screen. Click construct, plane, and three-point plane, and create a virtual point by using Control Left Click on both the internal acoustic foramina and the left infraorbital margin. Then click create and close.

Then select line and two-point line from the construct menu and create a virtual point by Control Left Clicking on the nasion and the basion. Finally, select point and Control Left Click the nasion. Click operations, main alignment, and plane line point, and designate the parameter plane one to the nominal parameter plane Z, line one to line Y, and 0.1 to global coordinate system.

Select the STL file of the preoperative mandible. Then click operations, CAD, and actual mesh to CAD. Select the new CAD data in the popup menu and rename the file.

Drag the postoperative STL model into the software, rename the file and select the STL file under actual elements in the left explorer menu under meshes. Click operations and define actual master. Then click operations, alignment, single element transformation and three-point alignment.

In the popup menu, combine the three nominal points on the mandible preop with three similar actual points on the mandible postop by using Control Left Click. Then select apply and close. Deselect the mandible preop and select the mandible postop in the left explorer menu.

Use the select/deselect on surface tool to select a surface on the remnant mandible on both the lateral and medial sides. Click operations, alignment, main alignment, and local best fit. Select all CAD groups as the targeted element and set a maximum distance of 10 millimeters.

Next, click the select/deselect on surface tool and select a surface on only the lateral side within the surface selected in the previous step. Click inspection, CAD comparison, and surface comparison on actual. In the popup menu, set a maximum distance of 10 millimeters and click OK.Turn off the toggle visibility of the mandible postop.

Use the select patch tool, left click on the surface, then click the magnifying glass on the toolbar. When the round toolbar pops up, click check and deviation label arithmetic mean to show the mean in millimeters. Deselect the mandible preop and select the mandible postop.

Then select the surface according to manuscript directions. Click operations, alignment, main alignment, and local best fit. Make sure that only both the condyles are selected for the superimposition algorithm of the software.

Select all CAD groups as the targeted element and take a maximum distance of 10 millimeters. Select the mandible preop, then click construct, point, and surface point to determine virtual points on the condyle superior, condyle posterior, vertical corner, and horizontal corner. Next, determine the virtual points on the mandible postop by clicking on construct, point, and projection point.

To create a line between two virtual points, click construct, line, and two-point line. Select two points under construction elements. Then click create and close.

To create an angle between two lines, click construct, angle, two directions angle. Select line one and line two. Then click create and close.

Use this process to determine the right and left sagittal mandibular angles. Create a midsagittal plane by clicking construct, plane, plane and viewing directions. Select two points on the Z axis.

Use this plane to create the coronal and axial angles. Similar to the previous step, create a midsagittal plane for the mandible preop. Determine the same mandibular angles on the mandible preop.

Connect the mandible preop angles to the mandible postop angles by selecting a mandible preop angle in the explorer menu. Then click the magnifying glass, measuring principle, link to actual element. Select the corresponding mandible postop angle and click OK.Working with the mandible preop file, select point from the construct menu and create a virtual point by Control Left Clicking in the middle and top of the cover screws of the dental implants.

Then select point from the construct menu and Control Left Click in the middle and top of the cover screws of the dental implants in the mandible postop file. Right click on the first dental implant in the mandible preop, then click measuring principle and link to actual element. Select the same dental implant in the mandible postop and repeat this for all dental implants.

Click check and select dXYZ to show the distance in millimeters for the dental implants. The reconstruction of a James Brown class III mandibular defect was evaluated with the presented protocol. The coronal, axial, and sagittal mandibular angle deviations and six dental implant XYZ distances were calculated.

This guideline is applicable to all different types of mandibular defect classes according to the classification of James Brown and to all different types of bony flaps and fixation methods. Currently, our group is conducting a global multicenter study to validate this guideline in a large cohort. We expect to present the first results in the summer of 2020.