3D Planning and Printing of Patient Specific Implants for Reconstruction of Bony Defects

Summary

This protocol describes the use of 3D planning and printing for reconstruction of bony defects. We use segmentation tools to create 3D models followed by 3D design software to create patient specific implants for reconstruction purposes concomitant to ablative surgery or as a second stage.

Abstract

We are in the midst of the 3D era in most aspects of life, and especially in medicine. The surgical discipline is one of the major players in the medical field using the constantly developing 3D planning and printing capabilities. Computer-assisted design (CAD) and computer assisted manufacturing (CAM) are used to describe the 3D planning and manufacturing of the product. The planning and manufacturing of 3D surgical guides and reconstruction implants is performed almost exclusively by engineers. As technology advances and software interfaces become more user-friendly, it raises a question regarding the possibility of transferring the planning and manufacturing to the clinician. The reasons for such a shift are clear: the surgeon has the idea of what he wants to design, and he also knows what is feasible and could be used in the operating room. It allows him to be prepared for any scenario/unexpected results during the operation and allows the surgeon to be creative and express his new ideas using the CAD software. The purpose of this method is to provide clinicians with the ability to create their own surgical guides and reconstruction implants. In this manuscript, a detailed protocol will provide a simple method for segmentation using segmentation software and implant planning using a 3D design software. Following the segmentation and stl file production using segmentation software, the clinician could create a simple patient specific reconstruction plate or a more complex plate with a cradle for bone graft positioning. Surgical guides can be created for accurate resection, hole preparation for proper reconstruction plate positioning or for bone graft harvesting and re-contouring. A case of lower jaw reconstruction following plate fracture and nonunion healing of a trauma sustained injury is detailed.

Introduction

Personalized medicine is developing rapidly in many fields of medicine¹. Oncologic personalized treatment is a subject of much discussion and thus is well known to the general population. 3D printing was first introduced by Charles Hull showing 3D printing of objects using stereolithography². Since then, different technologies for 3D printing were developed. The method used is selected based on the purpose of the device.

The surgical field is rapidly embracing personalized medicine. Personalized treatment in the surgical field requires virtual planning using a computer-assisted design (CAD) software. The first stage always includes segmentation to create a 3D stl file. Computer assisted manufacturing (CAM) is referred as the manufacturing process of the 3D designed part. The first utilization of the technology was used in pre-operative model printing for surgical planning and mock surgery^3,4,5. With the development of technology, virtual planning of the surgeries followed by the planning and manufacturing of surgical guides to assist in the surgery itself and patient specific reconstruction implants fitted perfectly on the bone of the patient became more popular^6,7,8,9,10. The purpose of this protocol is to provide clinicians with the ability to create their own surgical guides and reconstruction patient specific implants. This method is more accurate than using stock plates because it fits perfectly and can be designed based on the characteristics of the specific defect. It also reduces the dependency on the surgeon's experience and reduces operation time.

Protocol

This study followed the Declaration of Helsinki on medical protocol and ethics and the Institutional Ethical Review Board approved the study.

1. Segmentation using a segmentation software

NOTE: The import process of the DICOM files requires the orientation of the axial, coronal and sagittal planes in the pop-up window to finish the setup.

1. In the Bone Segmentation menu, choose the General feature. Use the marker "-" for unwanted segments and "+" for the segment of interest. Add markers on the 3D reconstructed model or on the different cross sections when scrolling and moving throughout the scan.
2. Choose the Set button that demonstrates the segmentation. At this point, correct the markings and add new ones for better accuracy. Press Apply to create the new segment. Multiple segments can be created this way.
3. After segmentation is complete, export the files as stl 3D files for 3D printing or planning of 3D reconstruction implants in 3D design CAD programs.

2. Designing reconstruction implants using 3D design software

1. After preforming the bone segmentation using the segmentation software, import the stl files into the 3D design software (see Table of Materials).
2. If further separation is needed (e.g., if one part is intended to be moved separately), do so here. In the Sculpt Clay menu, use the shave tool to separate the bone into two parts. In the Select/Move Clay menu, select the clay and mark the part to work on. Then, copy this part and create a new identical object in the object list in order to manipulate its position as observed in the next stage.
3. Perform segment movement at this stage. Make sure that the axis of rotation is set on the part of the bone to stay in the same position. In the Select/Move Clay menu, select Reposition and set the rotation axis as planned.
4. As the human skull is mostly symmetrical, use the healthy side for guidance to obtain the right positioning/replacement of the missing/mal-positioned segment. Use a mirroring technique to create a mirroring image of the normal side. In the Construct Clay menu, use the Mirror Clay option and set the plane at the center of the skull.
5. Based on the mirrored half, perform segment rotation if needed and reconstruct the avulsed bony part using the Add Clay tool in the Construct Clay menu. This reconstruction is performed in order to build a patient specific reconstruction implant in the next stages, which will reconstruct the correct facial contour.
6. After reconstructing the bony segment, create the patient specific reconstruction implant. In the Curves menu, use the Draw Curve option and create a continuous outer shape of the desired implant.
7. At this stage, duplicate the bony segment as it will be needed to perform a Boolean function to separate the constructed implant. This is performed in the object list window by right clicking the segment and pressing the Duplicate option.
8. Work in the new duplicated segment. In the Detail Clay menu, use the Emboss With Curve option to create the volume of the reconstruction implant. Pick the outer form of the sketched implant, and then place the circle-shaped cursor inside the sketched implant, on the surface of the bone. Note that the emboss will work outwards or on the inside of the bone, depending on the placement of the cursor. Then, choose the desired parameters - most importantly, the distance option that controls the thickness of the implant.
9. Separate the implant from the bony segment. In the object list, choose the previously duplicated object from step 2.7, right-click and select Boolean → Remove From. Then choose the object containing the created implant.
10. In case holes for screw fixation or for allowing angiogenesis are required, select Planes Category → Create Plane to create a parallel plane in which the holes for the plate are designed. Using manual manipulation, place the planes in maximum parallelism to the implant. In the Sketch menu, choose a circle and create circles in the desired size and position. A second larger circle can be created, which will serve as the countersink for the head of the intendent screw.
11. In the Curves menu, use the Project Sketch option and choose the sketches designated to be transferred from the plane to the implant.
12. To generate the countersink for the screws, In the Detail Clay menu, use the Emboss With Curve option. Pick the outer circles of the sketch, place the circle-shaped cursor inside the marked circular area on the surface and enter the distance that controls the depths of the countersink (e.g., 0.3 mm). To complete the process, press Apply and Lower to make sure that the emboss is performed in a subtraction manner and not an additive one.
13. To complete the holes, in the SubD Surfaces menu, use the Wire Cut SubD option to create rods perpendicular to the implant based on the small circles created in step 2.10.
14. To create the holes using the rods, use Boolean > Remove From as in step 2.9. Choose one rod after the other, right click in the object list → Boolean → Remove From → Created Implant.
    NOTE: Alternatively, the desired screws can be created/scanned and the Boolean function can be used to create the desired holes.
15. To create a mesh in the implant (allowing for angiogenesis for instance), first generate a sketch (using the curve option) of the planned mesh as in step 2.6.
    1. In the Detail Clay menu, use the Emboss with Wrapped Image option. Choose an image according to which the mesh will be created (there are several templates which come with the program). The white parts of the image will be subtracted in the mesh, and the black parts will be spared.
    2. Using manual control, adjust the direction and size of the design. Set the distance that represents the thickness of the holes generated and press Apply. The patient specific implant is ready for production.

Representative Results

A 40 year old female patient with a broken, stock supplied, reconstruction fixation plate from a previous injury and a non-union fracture in the left body of her lower jaw presented to the department. Imaging shows the broken fixation plate and the mal-positioned left segment of the lower jaw (Figure 1). Using segmentation software, segmentation of the lower jaw was performed separating the broken fixation plate (Supplemental Figure 1 and Supplemental Figure 2). Using 3D design software, the left segment of the mandible was repositioned to the correct anatomical position (Supplemental Figure 3 and Supplemental Figure 4). Mirroring of the right healthy side was performed to allow for proper reconstruction of the missing bone (Supplemental Figure 5). The patient specific implant was designed, including holes for fixation screws (Supplemental Figures 6, 7, and 8). A mesh was designed to allow for additional bone graft placement according to the proper contour of the jaw, based on the healthy side, also enabling for superior angiogenesis through the holes in the mesh (Supplemental Figure 9).

The implant was sent for printing from titanium using selective laser sintering technology. Post-operative results can be observed in Figure 2. Notice the continuity of the lower jaw and the correct vertical position of the left lower jaw segment compared to the situation pre-operatively as observed in Figure 1. Also notice the symmetry in the bony contour that was reconstructed using the patient specific implant as the outer contour and an iliac crest bone graft for filling the voids.

Figure 1: Pre-operation imaging of a 40 years old patient with a broken reconstruction plate and non-union fracture of the left lower jaw. (A) Panoramic image, notice the broken fixation plate and upper position of the left mandibular segment compared to the right. (B) On the left, a posterior-anterior cephalometric image and on the right a front view of a 3D reconstruction from the patient's computed tomography image. Please click here to view a larger version of this figure.

Figure 2: Post-operative imagining. (A) Panoramic image; notice the continuity of the lower jaw, compared to the upper position of the left mandibular segment observed in Figure 1. (B) On the left, a posterior-anterior cephalometric image can be observed. On the right, a front view of the 3D reconstruction from a computed tomography image can be observed. Notice the continuity of the bone following repositioning of the left segment and filling the voids with an iliac crest bone graft. Please click here to view a larger version of this figure.

Supplemental Figure 1: Segmentation software. A view of the workspace and the bone segmentation process. On the left is the 3D reconstruction of the computed tomography image. On the right are the different views which allow for browsing through the different sections. The yellow circles are the "-" markers for removal of the marked piece and the orange ones are the "+" markers for the region of interest. Please click here to view a larger version of this figure.

Supplemental Figure 2: Segmentation process. Following the segmentation of the lower jaw. The previous broken reconstruction plate was removed from the region of interest. This segment is exported as a 3D stl file. Please click here to view a larger version of this figure.

Supplemental Figure 3: 3D design software. The 3D stl files were exported from a segmentation software and imported into 3D design software. (A) The workspace. (B) The imported 3D facial bones. Please click here to view a larger version of this figure.

Supplemental Figure 4: Further segmentation and repositioning. (A) Segmentation of the lower jaw into two different pieces. (B) The smaller part of the lower jaw was repositioned to its correct anatomical position. The hinge of the movement was set to the left mandibular condyle. Both the pre and post movement setups can be observed. Please click here to view a larger version of this figure.

Supplemental Figure 5: Mirroring function of the healthy side. (A) Defining the mid sagittal plane for the mirroring. (B) Merging the mirrored part (which allows for proper reconstruction of the lower border of the jaw) with the remaining segment in the patient and filling voids. Please click here to view a larger version of this figure.

Supplemental Figure 6: Creating the patient specific implant. (A) Creating the outer shape of the implant using the curve function. (B) Creating the thickness of the plate. This is created on the reconstructed mandible following the mirroring technique. Please click here to view a larger version of this figure.

Supplemental Figure 7: Separation of the plate and planning holes. (A) Following the separation of the patient specific implant using the Boolean function. (B) Creating holes for the screws and countersink using a perpendicular plane. Please click here to view a larger version of this figure.

Supplemental Figure 8: Holes and countersink preparation. (A) Following countersink preparation using the emboss function. (B) On the left, the rods created for hole preparation can be observed. On the right is the implant with holes following subtraction of the rods from the implant using the Boolean function. Please click here to view a larger version of this figure.

Supplemental Figure 9: Preparing a mesh. (A) Mesh preparation using the Emboss with Wrapped Image function. (B) Left and bottom views of the final patient specific implant on the existing lower jaw following repositioning. Note the implant will guide the surgeon for the correct repositioning during surgery. Please click here to view a larger version of this figure.

Discussion

With the constant developing use of computers for virtual planning of surgical procedures, the combination with another developing technology, 3D printing, led to a whole new era of surgical treatment. Accuracy is the goal of these technologies and patient specific care, as the future goal, is presented in the form of surgical guides and patient specific reconstruction implants. We discuss surgical guides as part of a different future protocol. In the current protocol, we discuss the segmentation of DICOM images into 3D stl files that can be 3D printed as a model. We also discuss the 3D virtual planning of a patient specific implant. The use of diverse functions to help with the reconstruction of missing or displaced bony fragments are presented. Re-positioning of a segment and mirroring of the healthy side are such functions. Construction of the implant using the planned contour of the bone is detailed. Holes for fixation screws and designing a mesh or a cradle for bone graft placement and enabling for angiogenesis is shown. Always remember that the design is limited in size to the available soft tissue for closure. Allow for proper angiogenesis and use large holes/mesh when possible for this purpose. Patient specific implants do not undergo physical manipulation during surgery for compatibility to the remaining bone, as opposed to regular reconstruction plates (which weakens them). Thus, thinner plates can withstand much larger forces. Take into account that if the production process of the 3D printed titanium implant includes smoothing of the external aspect (for avoiding soft tissue irritation), it removes additional material (approximately 0.3 mm). When preparing implants with a cradle, it is crucial to avoid angles that interfere with proper placement of the implant onto the bone.

With that said, 3D planning and printing of patient specific implants is already used in clinical practice, showing positive results. The limitations of the method are the cost and the need for an engineer for the planning, which results in time consuming web meetings and discussions during the planning phase.

We found that in-house planning of the implant reduces costs dramatically and also allows for the use of local companies to print the implant. With the development of technology, the software interface becomes more user-friendly and allows for the surgeon to plan his own surgical procedures and patient specific implants. This entitles great advantages, the surgeon enters the operating room with the implant he developed following movements and reconstruction procedures he planned and thus he is aware of each step, and knows how to deal with unexpected developments during surgery. This protocol is intended for this exact purpose, allowing the surgeon to plan his own surgeries and create his own patient specific implants.

Materials

Name	Company	Catalog Number	Comments
D2P (DICOM to Print)	3D systems		Segmentation software to create 3D stl files
Geomagic Freeform	3D systems		Sculpted Engineering Design