3D Design and Printing of Highly Accurate Surgical Implant Guides
Factors, such as high investment costs, may make it feasible that this work could be performed by dental laboratories
Advances in dentistry have paved the way for improved implant planning techniques. At the forefront is the advancement of cone-beam computed tomography (CBCT), 3D planning/design software, and rapid prototyping using stereolithography and 3D printing. Studies have demonstrated the use of these new technologies to accurately establish implant spacing, depth, and angulation.1-3 Due to the steep learning curve that users typically face with the use of 3D software, the high investment costs associated with licensing CAD software, and the purchasing and maintenance of printing machines, it is conceivable that a portion, if not all, of the implant planning workload could be carried by the dental laboratory community. In a previous article, a procedure for overcoming CBCT scan scatter in the implant planning process was discussed. At the end of the process, two files were created: an implant file and a cast file. Each was oriented in 3D space to the patient scan. In this article, the discussion will center on the procedure for importing the files into 3D CAD software where guide cylinders and a surgical guide can be designed and then exported to CAM software to be fabricated by a stereolithography apparatus (SLA).
The first step in guide fabrication is creating guide cylinders based on the positions of the implants. The guide cylinders are designed in CAD software such as SolidWorks 2014 (SolidWorks, solidworks.com). Open SolidWorks and use the "New" wizard to navigate to and open the implant.stl file. The result will be four implants loaded on the desktop (Figure 1). Orient the XYZ triad so that the Z axis is pointing up or superiorly as it relates to the patient. This positions the implants at the orientation and inclination that would be observed in the patient’s mouth. Orient any one of the implants so that the apical surface is facing the monitor screen. Create a plane on the implant (Figure 2). On the plane labeled "Plane 1," sketch a circle and base the center of the sketch on the center axis of the implant. Create the dimension of the circle sketch to have a diameter appropriate to the clinician’s implant drill system. This dimension represents the inner diameter of the guide cylinder (Figure 3). Create a second circle sketch on "Plane 1," and make the diameter 5.5 mm, which represents the outside diameter of the guide cylinder (Figure 4). Select the 5.5-mm-diameter circle, ensure that the direction is oriented correctly, and extrude the circle sketch to an appropriate depth as prescribed by the clinician (Figure 5). Select the small inner-diameter circle, and use it to extrude a cut through the cylinder. The cylinder will now have a hole through its center with the appropriate diameter (Figure 6). Repeat the process for the other implants, then save the file in "STL (*.stl)" format to an appropriate location and name it "Cylinders" (Figure 7).
Surgical Guide Design
The second step is the design of the surgical guide. The guide is designed in software such as Geomagic® Freeform® Plus (Geomagic, geomagic.com) paired with a force feedback haptic device. In Freeform, navigate to the directory containing the cast and cylinder files, highlight both, and click "Import." The workspace will be populated with the guide cylinders created earlier in SolidWorks and the scanned cast (Figure 8). At this stage, the undercuts on the virtual cast must be blocked out. Create a plane perpendicular to the ideal path of insertion of the prosthesis, move the plane below the cast and extrude the blockout to the plane (Figure 9). Duplicate the "Blocked" cast, convert the cast to a buck state, and rename the duplicate cast "Guide." (Note: An object in the buck state cannot be altered in the software, which is essential for maintaining model integrity and ultimately results in an accurately fitting surgical guide.) To define the borders of the guide, move the plane up to within a few millimeters of the shortest cusp tip on the model. Use the plane to create the curves that represent the intended borders of the guide. Alter and delete the curves as necessary to ensure a well-designed guide (Figure 10). Emboss the area within the curve to the desired thickness to create the structure of the guide. Use the "Smooth" and "Hot Wax" tools to fill in and smooth any defects on the guide (Figure 11). Duplicate the cylinders, and make them visible but not active. Incorporate the cylinders into the guide design, ensuring that the cylinders are adequately supported in the guide. Hide the cylinders, and delete the buck portion of the model. A rough version of the final appliance is left. Smooth the peripheries, and use the Boolean subtraction algorithm to subtract the cylinders from the guide. Remove unnecessary floating material left by the Boolean and smooth operations (Figure 12). Combine the cylinders into the guide, and smooth the interface between the guide and the cylinders as needed. Subtract the "Blocked" cast from the "Guide" object in order to cut the tissue side of the cylinders to the correct height, and remove any material that bled onto the tissue surface during the smoothing operations. If any further smoothing or modifications are deemed necessary at this point, ensure that the last step is always to subtract the "Blocked" cast from the "Guide" (Figure 13).
Surgical Guide Fabrication
The last design step is to prepare and stage the guide STL file for building on the 3D prototyping machine. For this case, the guide is prepared and staged on a 3D Systems Viper si2 SLA (3D Systems, 3dsystems.com). Reduce the size and number of triangles in the file, then convert the guide into a mesh object. Navigate to an appropriate directory to export the file to, name the file "Guide_Mesh.stl," and click "Save." Save the entire Freeform project, and then close the session.
To stage the guide, open an instance of 3D Lightyear, load an empty-build platform, navigate to the directory containing the guide file, and double click the "Guide_Mesh.stl" file. Use the functions built into the staging software to verify the integrity of the file, orient and load the guide on the platform, create supports for the guide, and package the guide for the SLA machine (Figure 14). Transfer the file to the computer that controls the SLA, open the BuildstationTM software (3D Systems) on the SLA controlling computer, load the guide build file, and start the build (Figure 15). When the build is complete, post processing involves removing the guide from the build platform, removing the support structures, soaking and cleansing in an alcohol bath, performing final curing in a light-curing machine, and sanding where the support structures are attached to the guide. The guide will sit on the cast without the need for seating procedures and does not require finishing or polishing. Stainless steel guide tubes are placed in the cylinder channels to give the clinician an appropriate diameter pilot hole. With the guide tubes removed, the hole is the appropriate diameter to allow for a second pilot hole (Figure 16).
Companies offer the service of implant planning and guide fabrication using proprietary software. In this article, the process for designing and building a surgical guide using off-the-shelf software such as SolidWorks and Freeform was discussed. The CAD software and the technique described in this article enable any designer to consistently produce a highly accurate surgical guide to aid the clinician during implant surgery.
Part 1 of this article was published in the October issue. Please go to insidedentaltech.com/IDT766.
The clinician for this case was James Piper, DDS. Special thanks to Charles DeFreest, DDS; Ashley Reyes, DDS; and Alain David Carballeyra.
The opinions expressed in this article are solely those of the author and do not represent an endorsement by or the views of the United States Air Force, the Department of Defense, or the United States Government.
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