Treatment of Facial Deformities using 3D Planning and Printing of Patient-Specific Implants

Summary

As technology develops and becomes more user-friendly, planning of operations and patient-specific surgical guides and fixation plates should be performed by the surgeon. We present a protocol for 3D planning of orthognathic skeletal movements and 3D planning and printing of patient-specific fixation plates and surgical guides.

Abstract

Technological advancements in surgical planning and patient-specific implants are constantly evolving. One can either adopt the technology to achieve better results, even in the less experienced hand, or continue without it. As technology develops and becomes more user-friendly, we believe it is time to allow the surgeon the option to plan his/her operations and create his/her own patient-specific surgical guides and fixation plates allowing him full control over the process. We present here a protocol for 3D planning of the operation followed by 3D planning and printing of surgical guides and patient-specific fixation implants. During this process we use two commercial computer-assisted design (CAD) software. We also use a fused deposition modeling printer for the surgical guides and a selective laser sintering printer for the titanium patient-specific fixation implants. The process includes computed tomography (CT) imaging acquisition, 3D segmentation of the skull and facial bones from the CT, 3D planning of the operations, 3D planning of patient-specific fixation implant according to the final position of the bones, 3D planning of surgical guides for performing an accurate osteotomy and preparing the bone for the fixation plates, and 3D printing of the surgical guides and the patient-specific fixation plates. The advantages of the method include full control over the surgery, planned osteotomies and fixation plates, significant reduction in price, reduction in operation duration, superior performance and highly accurate results. Limitations include the need to master the CAD programs.

Introduction

3D printing is an additive method based on gradual placement of layers from different materials, thus creating 3D objects. It was originally developed for rapid prototyping and was introduced in 1984 by Charles Hull, who is considered the inventor of the stereolithography method based on solidifying layers of photopolymer resin¹. Technological advancements in virtual planning of surgeries and planning and printing of patient-specific implants are constantly evolving. Innovations arise both in the field of computer assisted design (CAD) software and in 3D printing technologies². Simultaneous to developments in technology, the software and printers become more user-friendly. This shortens the time required for planning and printing and allows the surgeon the option to plan his/her operations and create his/her own patient-specific surgical guides and fixation plates in a field that was exclusively an engineer’s “playground”. These developments also allow for surgeons and engineers to introduce new applications and designs of patient-specific implants^3,4,5.

One of these applications is 3D planning of orthognathic surgeries followed by 3D planning and printing of surgical guides and patient-specific fixation plates. Historically, orthognathic surgeries were planned using articulators. A facebow was used to register the relationship of the upper jaw to the temporomandibular joint thus positioning the patient’s casts in the articulator. Later, the surgical movements were performed on the casts and an acrylic wafer was prepared to help with proper positioning of the jaws during surgery. This method was used for many years and is still used nowadays by most, but the utilization of cone beam computed tomography (CT) together with intra-oral scanners and CAD software allowed for accurate planning, sparing the need for facebows or casts and moving towards creation of digitally planned wafers⁶. This method reduced the inaccuracy of manual manipulation and measurements but still had flaws including using the instable lower jaw as a reference point for positioning the upper jaw and lack of control over the vertical positioning of the upper jaw⁷. Thus, a new method was introduced. This method is called the “waferless” surgery and is based on repositioning of the jaws anatomically using surgical cutting guides and patient-specific fixation titanium plates⁸. This method resolves the disadvantages of the digital wafer method described before. We will describe this method, which allows the surgeon complete freedom in planning these surgeries in a patient-specific manner, with minimal possible errors and inaccuracies. This method allows for a “waferless” surgery, which means there is no need for using the opposed jaw as reference for repositioning the bones, thus decreasing the inaccuracies derived from this reliance⁹.

Protocol

1. Repositioning of the jaws

NOTE: This section is performed using the imaging software (i.e., Dolphin).

1. Load the facial bones CT image DICOM files of the patient (Figure 1A) into the software by selecting the 3D button on the left and clicking Import New DICOM (Supplemental Figure 1). Enter the 3D editing mode by clicking 3D | Edit.
2. Orient the 3D image using the orientation button on the left. Create a panoramic image using the build X-rays button on the left (Supplemental Figure 2).
3. Go to Tools | Orthognathic Surgical Planning | Start New Workup.
4. Position the segments in the panoramic image. Crop each segment to contain the area of the corresponding bone.
   NOTE: The cleaning stage is useful when, for accuracy, a scanned dental arch and a CT scan are superimposed to create a wafer. This is not indicated in a “waferless” surgery as presented here and thus at this stage one can clean CT imperfections if they exist.
5. Choose the appropriate osteotomy for the patient on the left pan under osteotomies (such as LeFort I, sagittal split, etc.). Mark the exact location of the osteotomy lines by moving the yellow circles (Supplemental Figure 3).
   NOTE: It is extremely important to note the root apexes of the teeth as the location of the osteotomy decided here will be the one performed later based on the surgical guides. Always avoid the roots and maintain a 5 mm distance.
6. Mark different landmarks by left clicking on the right location for each suggested landmark.
   NOTE: This is important for measurements and movement purposes in the next stages.
7. Perform movements of bone segments. Drag the bone to the right location, or for accuracy, right click and choose Input Movements Using Keyboard.
8. In order to track the movement of key landmarks, press Treat Options Button on the left and choose Show Landmark Offset and Measurement Tables.
   NOTE: In the next tab the pre and post virtually planned operation can be observed (Supplemental Figure 4).
9. Export the stl files of the two different positions of bone segments, one in the pre-operative stage and one in the post-operative stage, using the slide bar on the left and the Export Segments in stl button on the left.

2. Preparation of patient-specific fixation plates and surgical guides

NOTE: This section is performed using the 3D design software (i.e., Geomagic Freeform).

1. Click File | Import Model (Supplemental Figure 5A) to import the stl files obtained from step 1.9 showing the position of the upper jaw and midface following the osteotomy but prior to the repositioning in the final position.
2. Start with planning the patient-specific fixation plates in the final position of the upper jaw. In the tool palette on the left under the Planes category, select Create Plane (Supplemental Figure 6A). Here the initial design of the plates will be performed. Manually move the plane parallel to the bone where the plate will be placed.
3. Under the Sketch category (Supplemental Figure 6B), choose a circle shape and create circles with a size appropriate for the screws to be used later. Create a second circle around the previous one 3 mm larger in diameter to outline the fixation plate.
   NOTE: The size of the circles is determined based on the fixation sets used in each institute. The circles are placed above and below the planned surgical osteotomy (decided already in section 1).
4. Project the design from the plane to the bone. Under the Curves category (Supplemental Figure 7), use the project sketch tool and choose the circles that will be transferred from the plane to the bone.
5. To connect the outer circles for the outer border plate design, choose under the Curves category the split tool and define the part of the circle that will be removed to allow for a connection to the adjacent circles. Using the select option, choose the defined part of the circle and delete it. Under the Curves category, use the draw curve tool and connect the outer circles to create a continuous outer shape of the patient-specific plate.
6. Before creating the fixation plate, duplicate the upper jaw by right clicking and selecting Duplicate from the object list (Supplemental Figure 7A). This will allow the use of the Boolean tool in the next stages to create the fixation plate.
7. Under the Detail Clay category, use the emboss with curve tool. This creates the volume of the fixation plate based on the curves previously projected. Choose the outer shape curve and then place the circle-shaped cursor inside and on the surface of the shaped plate (note that the cursor should be placed on the side to be embossed). At the bottom, choose the parameters of the function, mainly the Distance option that controls the thickness of the future fixation plate.
8. Separate the plate from the upper jaw. At this stage the Boolean option is performed. Choose the original upper jaw, right click from the object list and click Boolean | Remove from | Upper Jaw with Plate.
9. To create the holes for the screws, either draw the screws/scan them and then use the Boolean option or use the SubD tool. Under the SubD Surfaces category (Supplemental Figure 8), use the wire cut SubD tool to create rods perpendicular to the plate in the size of desired holes, which is performed based on the circles created in step 2.3 originating from the perpendicular plane.
10. Next, subtract the rods from the plate using the Boolean | Remove from technique.
    NOTE: At this stage the final fixation plate is ready (Supplemental Figure 9). Appropriate surgical guides need to be planned for the osteotomy in order for the plates to fit perfectly.
11. To create the guides, reposition the upper jaw to its original location but with the screw holes marked in the bone according to the fixation plate created in the final position of the jaw (note the holes in the midface do not change position as the midface stays in the same position).
    1. To perform this, reposition the jaw with the curves for the holes used for the final fixation plate to the original location of the jaw prior to the movement. Under the Select/Move Clay category, use the Register Pieces option. choose the Source (upper jaw post movement) and the Target (upper jaw and midface prior to the movement). Use a large number of fixed points on both objects for accuracy in the repositioning.
12. Based on newly positioned holes create the surgical guides in a similar way as described for the fixation plates (steps 2.3−2.10).

Results

To observe the clinical use of the method, we present a case of a 23 year old female. She suffered from condylar hyperplasia at a younger age in the right condyle resulting in asymmetry of both jaws. Figure 1A shows the retrognathic upper jaw and prognathic lower jaw exhibiting the discrepancies between the jaws. In the frontal view, the severe asymmetry can be observed as detailed using the yellow and red lines. Using the imaging software (Supplemental Figure 1), a surgical...

Discussion

3D planning and printing is one of the most rapidly evolving methods in the surgical field. It is not only a promising tool for the future, but a practical tool used nowadays for highly accurate surgical results and patient-specific solutions. It allows for highly accurate results and reduces the dependency on the surgeon’s experience¹⁰. It solves many of the disadvantages of previous old fashion surgical methods, but the costs delay the full implementation of the method^10<...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Dolphin imaging software	Dolphin Imaging Systems LLC (Patterson Dental Supply, Inc)		3D analysis and virtual planning of orthognathic surgeries
Geomagic Freeform	3D systems		Sculpted Engineering Design