Designing CAD/CAM Surgical Guides for Maxillary Reconstruction Using an In-house Approach

Summary

Methods for designing a computer-aided design/computer-aided manufacturing (CAD/CAM) surgical guide are shown. Cutting planes are separated, united, and thickened to easily visualize the necessary bone transfer. These designs can be three-dimensional printed and checked for accuracy.

Abstract

Computer-aided design/computer-assisted manufacturing (CAD/CAM) is now being evaluated as a preparative technique for maxillofacial surgery. Because this technique is expensive and available in only limited areas of the world, we developed a novel CAD/CAM surgical guide using an in-house approach. By using the CAD software, the maxillary resection area and cutting planes and the fibular cutting planes and angles are determined. Once the resection area is decided, the necessary faces are extracted using a Boolean modifier. These superficial faces are united to fit the surface of the bones and thickened to stabilize the solids. Not only the cutting guides for the fibula and maxilla but also the location arrangement of the transferred bone segments is defined by thickening the superficial faces. The CAD design is recorded as .stl files and three-dimensionally (3-D) printed as actual surgical guides. To check the accuracy of the guides, model surgery using 3-D-printed facial and fibular models is performed. These methods may be used to assist surgeons where commercial guides are not available.

Introduction

The use of CAD/CAM techniques has recently increased in dental and denture work. Following this evolution of CAD/CAM, osteocutaneous flap transfers using CAD/CAM are now used in the field of mandibular reconstruction after an oncologic wide resection of malignant tumors^1,2,3. Several companies in Western countries have begun to supply and sell a CAD/CAM cutting guide for the mandible region. A CAD/CAM reconstruction of the mandible is considered to have an advantage in terms of accuracy^{4,5,6,7,8,9,10,11}. However, a disadvantage is that this technique is available in limited areas worldwide and it is very expensive¹². Thus, CAD/CAM reconstruction for maxillary lesions has not yet become popular. The number of the cases of maxillary reconstruction is lower than that for the mandible, and commercial guides are not common.

Because commercial maxillary CAD/CAM guides are not sold in Japan, we have developed CAD/CAM surgical guides using an in-house approach. The clinical effectiveness of the CAD/CAM guides has already been reported^{13,14,15,16,17,18,19}, but there is no report of how to design them. The purpose of the present report is to show the CAD/CAM design method using a low-cost in-house approach.

Protocol

This study was approved by the authors' institutional review board, and written consent forms were completed by all patients.

1. Preparation of Materials

1. Use a personal computer, computed tomographic (CT) data of facial bone and fibula, conversion software such as InVesalius²⁰, and three-dimensional (3-D) CAD software (e.g., Blender²¹).
   NOTE: A maximum thickness of 1 mm slices of CT data is recommended for an accurate design. For the actual surgical simulation, use the patient's CT data. For research, use free human 3-D data²².
2. Use a 3-D printer²³, screws, metal plates, and a small saw, to check not only the designs but also real objects and results.
   NOTE: The present study is experimental. Metal plates, screws, and a small saw can be used for the model surgery. Instead of metal plates, plastic-fixation plates can be also printed by the 3-D printer, together with the surgical guides.
3. Transfer the imaging data of both facial bone and fibula into 3-D data (.stl format) using InVesalius²⁰.
   NOTE: CT data is essentially recorded in the form of two-dimensional (2-D) pictures. Thus, before using the 3-D data, it is necessary to convert the data into 3-D data. Free software is sufficient for this purpose. This report does not explain how to transfer the data into a 3-D file; instructional videos and guides are available elsewhere.
4. Import each .stl file into Blender²¹.
   NOTE: CAD software usually accepts a .stl-style 3-D format. At first, maxillary and fibular .stl files should be opened in the specific CAD software by importing them.

2. Design

1. Deciding on the area of bone removal and solidifying a bone defect
   1. Decide on the area to be excised.
      NOTE: In this experimental simulation surgery, any part of the maxilla can be set as an excised area. Because reconstruction after total maxillectomy is difficult, only a small part of the maxilla will be a choice for beginners. In a clinical setting, otorhinolaryngologists will decide the area according to the cancerous region.
   2. Make a large plane and place it on the border of the area for removal in the object mode (Figures 1a and 1b). Follow that by placing a second plane (Figures 1b-1d) and continue to do so until the planes surround the whole area for removal. Unite these planes in the object mode.
   3. Select the vertices of all these planes and connect them to each other by making edges and faces (Figure 1e) in the edit mode to surround the areas for removal.
      NOTE: The excision planes should be copied and maintained because these original planes are used and discarded when solidifying the excision. In the present study, copying every plane and solid on every occasion is recommended in order to make it possible to reuse them.
   4. Subtract the resectable solid from the facial bone using a Boolean modifier in the edit mode. This results in a shaved facial bone (Figure 1f), which is the maxillary defect model.
2. Placing a fibula bone
   1. Place a fibula into the maxillary defect area (Figure 2a). Place small cubes at two points (8 cm distal from the fibular head and 5 cm proximal from the lateral malleolus) in the fibula as markers (purple small cubes are shown in Figure 2).
      NOTE: In clinical situations, a fibula can be used between 8 cm distal from the fibular head and 5 cm proximal from the lateral malleolus. By this marking, we can easily understand the areas that can be used.
   2. Link small cubes to the fibula as a parent setting in the object mode.
   3. Place small cubes as markers in several points in the maxillary lesion where reconstruction is necessary. With this marking, the visibility of the necessary reproduction points is increased.
   4. Fit the fibula to the front margin of the alveolar bone in the object mode, if the fibula is placed from the midline.
   5. Use the previous plane of the midline maxillary osteotomy as a first fibular osteotomy plane (Figure 2b).
   6. Place a new osteotomy plane where appropriate in the object mode (Figure 2c). Link this new plane to the fibula as a parent setting.
      NOTE: By setting the parent to the fibula, the relative orientation between this new osteotomy plane and the fibula is always maintained even if the fibula is moved into different places. The area of the fibula that is surrounded by these two cutting planes becomes the first fibular block.
   7. Copy the fibula and two planes of osteotomy as a parent setting in the object mode. Move this copied fibula, which has the first block area with two cutting planes on both ends, to the second area where reconstruction is necessary (Figure 2e) to plan the second fibula block.
   8. Place the second cutting plane by adding a new plane in the object mode.
      NOTE: The first and second cutting plane will become the ends of the second fibula block. If a third block is necessary, similar procedures are added. The appropriate length of the gaps between the adjacent fibular blocks should be maintained.
      NOTE: The gap between the first and second block is considered to be key to having a comfortable osteotomy. If this gap is wide, the osteotomy will be easy because of the wide working space, but the vascular length is somewhat wasted. By contrast, if the gap is narrow, osteotomy becomes troublesome, but the second or third block can be designed by eliminating the waste of the unused bone.
3. Designing the fibular cutting guides
   1. Visualize only the fibula and cutting planes for designing the fibula cutting guide in the object mode (Figure 3a).
   2. Make each cutting plane smaller to occupy only half the area of the fibula cutting section by sliding vertexes along the edges (Figures 3b-3d) in the edit mode.
      NOTE: The fitting side of the cutting guide is the lateral aspect of the fibula. Because the feeding vessels are located in the medial aspect, the guide is not designed in the medial aspect.
   3. Unite two planes of the ends to build a solid in the object mode (Figures 4a-4e). Select the vertexes of all these planes and connect them to each other by making edges and faces in the edit mode to form a rectangular solid.
   4. Subtract the fibula from this rectangular solid by using a Boolean modifier (Figures 5a-5c).
      NOTE: The surface of this subtraction completely fits the fibular lateral aspect. The same procedures are repeated in every necessary fibular block.
   5. Unite each subtracted solid in the object mode.
   6. Place a cube near the subtracted solids (Figure 5d). Extrude faces to make pillars (Figures 5e-5g). Unite these pillars to the subtracted solids. This is the fibular cutting guide (Figures 5h-5j).
4. Osteotomy cutting guide for the maxilla
   NOTE: To cut the maxilla, it is not necessary to design the guide for every cutting surface, because only limited areas are to be reconstructed using the fibula. Usually, two cutting guides, which cover the medial alveolar and lateral zygomatic areas, are designed.
   1. Prepare the maxillary and zygomatic planes that were the original remaining surface after the maxillary removal. A margin of 1 cm in width is sufficient (Figure 6a).
   2. Extrude the faces prepared in step 2.4.1 to thicken the plane and to solidify them in the edit mode using the solidify modifier (Figure 6b).
   3. Delete the thickened solid over the resection planes, which were decided in step 2.1, on both ends; this is how the maxillary cutting guides are designed.
      ​NOTE: If the fitting surface is jagged, a smaller fitting area is sufficient. If the fitting surface is apt to be flat, a large area is needed to avoid any slippage of the guide.
5. Fixation guide for the fibular segments
   NOTE: Fibular segments that are to be transferred to the maxilla are considered to be accurate in size and length, but the location of the transfer can deviate freely if the fixation guide is not used. The fibula and each cutting plane (as made in step 2.2) are used again in this segment.
   1. Construct each fibular block in the Boolean modifier by taking out the intersection area between the fibula and the solid with cutting planes on both ends (Figures 7a and 7b) in the edit mode.
   2. Extract half of the superficial surface of each fibular block.
   3. Unite all of these surfaces in the object mode (Figure 7c).
   4. Delete small faces in the edit mode by using a knife cut ( Figure 8a) to secure the spaces for fitting the metal plates.
   5. Thicken the surface by using a solidifying modifier in the edit mode (Figures 8b-8e).
      NOTE: A minimum 2–3 mm of thickness is necessary to stabilize the fixation guide and avoid warping. If the wing is designed on both ends, it will help the guide to the maxilla without using any metal plates.

3. 3-D Printing for Model Surgery and Real Guides

NOTE: The main purpose of this report is to show the method of designing surgical guides; the procedure described below is not necessary if 3-D printing is not needed.

1. Export the designs of the guides in .stl format, which can be 3-D printed.
2. Print all guides and bones.
   ​NOTE: In printing, rafts are considered to disturb smooth surface printing and lead to a jagged surface and poor fit to the bone, so the plane that needs to be smooth must be pointed upward.
3. Perform the model surgery as follows:
   1. Similar to actual surgery, fit the maxillary cutting guide to the facial bone model first (Figure 9a). Then, cut the facial bone along with the cutting plane using a saw.
   2. Attach the fibular cutting guide to the fibular bone model and cut it into pieces (Figure 9b). Attach the fibular blocks to the fixation guide (Figures 9c and 9d).
   3. Fix this fixation guide complex to the maxillary defect using screws and plates (Figure 9e). After fixing the fibular segments to the maxilla in the area where the fixation guide does not attach by using screws and plates, remove the fixation guide. This completes the reconstruction (Figure 9f).
4. Scan the 3-D-reconstructed image and record it in .stl format using a 3-D scanner²⁴.
5. Compare the post-model surgery .stl file and the CAD reconstructed design (Figure 10) using comparison software²⁵.
   NOTE: By comparing the virtual reconstruction design and the guided reconstruction model, actual accuracy is calculated. Because CAD/CAM accuracy is obtained within a 2.5 mm deviation in the mandibular reconstruction¹⁰, a similar precision should be required in this method. If the required accuracy cannot be obtained, redo the virtual design.

Results

Using the procedure presented here, the resection area was determined first. Using CAD software, the resection area was completely circumscribed by the faces. This area was subtracted from the facial bone by a Boolean operation. The fibula image was placed on the defect, and fibular cutting faces were placed in the appropriate reconstructed points. All fibular cutting faces were linked to the fibula in a parent setting. These faces were made smaller and were united to make solids. The fib...

Discussion

CAD/CAM reconstruction is considered to contribute to the attainment of an accurate osteotomy length, width, and angle in cutting bones while using cutting guides^{4,5,6,7,8,9,10,11,12,13}

Materials

Name	Company	Catalog Number	Comments
Information Technology Center, Renato Archer, Campinas, Brazil	InVesalius	Free software https://www.cti.gov.br/en/invesalius
The Blender Foundation, Amsterdam, Netherlands	Blender	Free software https://www.blender.org/
TurboSquid, Inc. 935 Gravier St., Suite 1600, New Orleans, LA.	Free 3D skeletal data file	Free3D https://free3d.com/3d-models/human
MakerBot Industries, LLC One MetroTech Center, 21st Fl, Brooklyn, NY.	MakerBot Replicator+	https://www.makerbot.com/replicator/
YouTube (Google, Inc.), 901 Cherry Ave. San Bruno, CA	video sharing website.	https://www.youtube.com/results?search_query=invesalius+dicom+to+stl
Artec 3D, 2, rue Jean Engling, Luxembourg	Artec Eva Lite	https://www.artec3d.com/portable-3d-scanners/artec-eva-lite
CloudCompare	CloudCompare	http://www.danielgm.net/cc/