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Review Article

Three-dimensional Printing Technology in Orthopaedics

Skelley, Nathan Wm. MD; Smith, Matthew J. MD; Ma, Richard MD; Cook, James L. DVM, PhD

Author Information
Journal of the American Academy of Orthopaedic Surgeons: December 15, 2019 - Volume 27 - Issue 24 - p 918-925
doi: 10.5435/JAAOS-D-18-00746

Abstract

Conrad Röentgen changed the field of medicine in 1895 with the description of X-ray radiographs. More than 100 years later, the field of orthopaedics continues to be influenced and directed by these two-dimensional images.1 Imaging technology has developed rapidly in the past century, and advanced imaging modalities including CT and MRI provide intricate two-dimensional anatomic detail.2 These modalities can be manipulated with unique software to create three-dimensional (3-D) representations of human anatomy using a computer interface.

Along with advances in 3-D imaging, 3-D printing technologies have also rapidly improved in recent years.3 These printers can transform computer images into tangible and functional objects. Because 3-D printers have become more accurate, affordable, and accessible, the technology is beneficially influencing the field of orthopaedics. Similarly, the types and costs of printing materials along with the time needed to complete prints have made the process more widely available and applicable to orthopaedic surgeons.4

The purpose of this article is to review types of 3-D printers readily available to practicing orthopaedic surgeons, how these printers function, regulation regarding orthopaedic 3-D prints, and current and future applications of 3-D printing in orthopaedics. With this article, the clinical orthopaedic surgeon will appreciate how this technology has advanced in recent years and ways that this technology can influence their current and future orthopaedic practice. Understanding 3-D printing technology and concepts is important to the practicing orthopaedic surgeon because, in orthopaedics, as in many fields, 3-D printers and 3-D printing are serving a larger and more impactful role in patient care, and this is likely to continue in the near future.3,5

Types of Three-dimensional Printers

Most 3-D printers can be described as functioning in either an additive or subtractive method for creating the 3-D structure. In additive manufacturing, material is applied layer-by-layer, whereas in subtractive printing, unwanted material is removed from the base material by the printer, leaving the 3-D object as the final structure.6 These 3-D printers have historically been high-end, expensive tools used for rapid-prototyping in engineering applications. The cost for high-end printers can range from $10,000 to over a million dollars.7

High-end printers have many benefits. They offer large-build platforms, which allow for production of large-scale, as well as more detailed, 3-D printed structures. They are also capable of printing using a wide variety of different materials (eg, plastic, clay, flexible polymer, nylon, metal, etc.). For orthopaedics, high-end machines can print in various biocompatible materials. Biocompatible prints allow for use in patients for devices such as cutting guides, pin placement guides, and alignment tools among others.6,8,9 Many orthopaedic products are printed using powder bed fusion printers in an additive method. A container of powder is heated below the material melting point and an energy source creates the structure layer-by-layer from the powder material.9 Given these abilities and properties, the high-end 3-D printers are typically operated by industry or academic centers, requiring a partnership to collaborate with the treating surgeon. For a surgeon to obtain one of these 3-D printed objects for clinical use, there is often a print turn-around time of several weeks to a few months.10

Desktop 3-D printers are more recent and affordable alternatives for clinicians exploring clinical applications for 3-D printing. Because 3-D printing technology has become more readily available to apply in practice, more surgeons are interested in using this technology to optimize patient care.11-13 The price point for desktop 3-D printers can range between $800 and $5,000.5,7 The two most common types of printing for desktop 3-D printers are fused deposition modeling (FDM) and stereolithography (SLA) (Figure 1).

Figure 1
Figure 1:
A, Photographs showing two different methods for desktop 3-D printing. FDM pushes a spooled polymer filament into a heated extruder. The heated extruder melts the material and as the extruder moves across a build platform, it deposits a 3-D structure. SLA submerges the build platform in a polymer resin. A UV light source crosslinks the polymer into a 3-D structure on the build platform. The build platform is raised as the construct is printed. B, Picture of a FDM (left) and SLA (right) desktop 3-D printers. 3-D, three-dimensional, FDM = fused deposition modeling, SLA = stereolithography

Both FDM and SLA methods create a 3-D structure using additive manufacturing, but they accomplish 3-D printing by different mechanisms. FDM involves extruding a polymer through a heated nozzle at a high temperature, typically over 200°C, and then rapidly cooling the polymer to have the material deposited layer-by-layer, making the 3-D structure.5 The extruder nozzle moves across the build platform during printing to create the structure (Figure 2). The SLA printing method uses a liquid resin, and then, photo-crosslinks the resin using, most commonly, ultra-violet light from a laser. The laser also moves across the build platform and it creates the structure in an additive fashion layer-by-layer from within the liquid resin.5 The SLA method can create greater print resolution and detail because of the laser focus, but it is also more expensive for desktop printing. Currently, most desktop printers use various polymer plastics such as polylactic acid and acrylonitrile butadiene styrene; however, many diverse and custom materials are being developed to improve the applications of desktop printers.5

Figure 2
Figure 2:
A and B, Photographs of the FDM printer demonstrating the additive layer-by-layer technique for creating a 1/3 tubular plate used in simulated fracture fixation. FDM = fused deposition modeling

How Three-dimensional Printers Work

Chuck Hall is frequently credited as the Father of 3-D Printing and was the first to develop the SLA or Standard Triangle Language (STL) file type in 1984.5 The STL file is the computer software language that informs a 3-D printer how to construct the desired structure.9 The STL file is used by the 3-D printer to generate a G-code. The G-code is a set of commands used by the printer to determine how to produce the 3-D structure in X-, Y-, and Z-dimensional planes.

STL files for orthopaedic 3-D printed structures are commonly made in one of two methods. One method involves creating the structure on a computer software program known as computer-aided design (CAD). A CAD program allows the surgeon to design the implant, guide, or prosthesis on the computer before sending it to the printer in STL format for printing.6 This process is frequently used in rapid prototyping of devices or tools. Many advanced CAD programs are available to engineers, architects, and advanced users. In addition, free open source platforms exist, such as FreeCad (Version 0.17, https://www.freecadweb.org/, release date April 22, 2018) that can be downloaded from the Internet.

The second method, which is more commonly used for making anatomic models from patient imaging studies, involves converting the advanced imaging modality data, such as CT or MRI, into a STL file for printing (Figure 3). The CT and MRI scans are composed of many DICOM files. DICOM files, known as Digital Imaging and Communications in Medicine (DICOM) files, are the standard for communicating and managing advanced medical imaging data.14 To isolate the desired anatomic structures for 3-D printing, such as bones for the orthopaedic surgeon, the user needs to upload the advanced imaging DICOM files into a slicing program and then perform a segmentation process.15

Figure 3
Figure 3:
This flowchart demonstrates the process of creating a 3-D printed structure from a patient's advanced imaging study. For more information and an example, please see Video, Supplemental Digital Content 1, https://links.lww.com/JAAOS/A356. 3-D = three-dimensional, DICOM = digital imaging and communications in medicine

3D Slicer (Version 4.8, http://www.slicer.org, Release Date 2018) is a free open source software platform for medical image processing, segmentation, and 3-D visualization. The program was created with support from the National Institutes of Health and a world-wide computer program developing community.16 Segmenting the body region of interest assigns codes to the various tissues in the viewing region and allows for removing tissues that are not wanted in the final printed structure. For orthopaedics, this commonly means removing or segmenting out soft tissues not identified as bone (see Video, Supplemental Digital Content 1, https://links.lww.com/JAAOS/A356). Because contrast between tissues is an important quality for segmentation, CT scans are most commonly used for this application. Once the anatomic region has been properly segmented, the file of the bone model can be exported as a STL file and sent to the 3-D printer for printing.

Several settings need to be made in the 3D printing software before starting the 3D print. This process is similar to establishing the properties for printing a paper document from an office printer, with certain special considerations for the 3-D material and print design. For example, in FDM printing, the settings for the generation of support structure material and the addition of a skirt need to be selected. Support structure is scaffolding that is generated beneath a region of a 3-D model, to support the model while printing, that has limited direct contact with the printing bed. This support is created with more airspace, making it more easily removed from the final print in the postprocessing of the print (Figure 4). A skirt is an outline that is created to surround the part being printed without touching the final print. The dual purpose of the skirt is to establish a smooth flow of filament before starting the deposition for the actual model and to observe any potential leveling or adhesion issues that may disrupt the proper completion of the print. The infill percentage of the 3-D structure can also be varied from 100% infill—which creates a completely solid piece with very little airspace. Decreasing this percentage would shorten the time of the print and decrease the amount of material needed, but it would also change the mechanical properties of the printed object. Once these parameters have been set, the 3-D printer can begin printing the desired structure.

Figure 4
Figure 4:
A, Photographs of a humeral head and scapula immediately after printing has completed demonstrating the scaffold material on the build platform used to support the 3-D printed objects during printing and the unique orientation needed to improve printing detail. B The same arthritic shoulder joint prints after manually removing the scaffold material. This patient had a previous anterior-inferior bony glenoid fracture treated nonsurgically, and the surgical team was planning glenoid implant positioning given the previous deformity. 3-D, three-dimensional

Clinical Applications

There have been numerous clinical applications of 3-D printing.4,17 This technology has affected all subspecialties of orthopaedic surgery. Most 3-D printing advances in orthopaedics have been in the clinical areas of preoperative planning,18 education,19 surgical cutting or pin placement guides,20 rehabilitation devices,21 surgical simulation/training,22,23 and prototype prosthesis development.17,24

3-D printed models can be useful in surgical simulation for transferring information to the surgeon in a more informative way for surgical planning. They can potentially help to illustrate intervention procedures to novice surgeons and patients and can be useful for testing the procedure on patient-specific anatomy through the use of printing materials able to resemble the biomechanical properties of bone (Figure 5). Current desktop 3-D printers allow for printing these individual anatomic-region models of shoulder, knee, or hip in approximately 4 to 7 hours depending on model and patient size and build platform limitations. These print times are highly variable depending on the type of print method used, print material selected, infill parameters, and detail selected for the print. However, most individual patient models can be printed within one day using desktop printers, markedly improving the turn-around time compared with using an off-site high-end 3-D printing service.6,25

Figure 5
Figure 5:
Photograph demonstrating superior-to-inferior view of a model of an acromioclavicular joint allows for the simulation of acromioclavicular joint reconstruction surgical techniques. The model is created from a patient's CT scan and is made of ABS plastic. This material allows for a bone-like consistency that can be cut, drilled, and fixed with numerous screws or suspensory button devices. ABS = acrylonitrile butadiene styrene

3-D printed models have been used across all fields of orthopaedics including arthroplasty, traumatology, sports-shoulder, hand, foot and ankle, oncology-orthopaedics, spine, and pediatrics to simulate joint replacements,26 pelvic reconstructions,27 shoulder instability,28 scoliosis correction,29 cubitus varus correction,30 osteotomies,22,23 and numerous other orthopaedic conditions (Figure 6). Vaishya et al,3 performed a 2018 analysis of publication rates in orthopaedic research using 3-D printing and found a dramatic increase within the last decade. In 2007, there were less than five orthopaedic publications using 3-D printed technology; however, in 2017, there were more than 80 orthopaedic publications using 3-D printed technology.3 These publications demonstrate the increasing trend and utilization for use of this technology in many different aspects of orthopaedic care. There is versatility in 3-D printing, which allows for a surgeon familiar with the printing methodologies to use this technology for many different orthopaedic indications and applications.

Figure 6
Figure 6:
Photograph of an FDM printer having a larger build platform for printing bone models as in pelvis fractures for trauma operative planning and patient education. FDM = fused deposition modeling

3-D printing has also had an impact outside of the operating room for application to custom braces, prostheses, and immobilization devices used in orthopaedic care. Wong et al have created 3-D printed mallet finger splints that can be used in remote locations and printed within an hour when needed.13 Patients with amputations have also benefitted from the customizability of 3-D printing to make prosthesis and limb instruments specific to their individual needs (Figure 7). These designs and prints have been shared on the Internet to allow for greater collaboration and exchange of 3-D printed applications. The National Institute of Health has created an online 3-D print exchange (https://3dprint.nih.gov/) to facilitate collaborations in 3-D design and printing.

Figure 7
Figure 7:
Photograph of a 3-D printed prosthetic hand made by researchers at the Food and Drug Administration to facilitate pediatric hand function for patients with partial amputations. The design of many 3-D printed prostheses, like this one, can be found online and shared by clinicians and patients around the world. 3-D, three-dimensional. (Courtesy of the U.S. Food and Drug Administration.)

The role of 3D printing in clinical implants has only recently been explored because of limitations in biocompatible materials, sterility, costs, and printer resolution.12 For many years, orthopaedic companies have used 3-D printing for rapid prototyping to quickly modify, implement changes, and test orthopaedic devices and tools for optimal usage before reaching the market. However, multiple research groups have become interested in studying 3-D printing for surgical instruments and implants, especially for use in remote locations and long duration space flights (Figure 8).31 Similarly, 3-D printing may be a solution for delivering surgical instruments and implants to hospitals in the developing world where the lack of these tools can be a challenge to providing needed orthopaedic care.32 The ability to deliver a 3-D printer to a remote location and print only the supplies needed instead of shipping and storing equipment could greatly change orthopaedic care in these locations. These indications and applications need further research and review before widespread clinical implementation, because as 3-D printing technology develops and improves, it is positioning itself as a possible solution for this resource utilization issue in orthopaedics.

Figure 8
Figure 8:
Photograph showing an example of various screws and a 1/3 tubular plate made of ABS plastic on an FDM printer. The material cost for all of these implants is less than $1 and all of the items printed within 1 hour. ABS = acrylonitrile butadiene styrene, FDM = fused deposition modeling

Regulation of Three-dimensional Printing

With so many advances and clinical applications emerging, 3-D printed technologies have come under notable regulatory scrutiny. In the United States, most orthopaedic devices are regulated by the Center for Devices and Radiological Health, which is a part of the Food and Drug Administration (FDA). Patient-specific instrumentation (PSI) has become a focus in many fields of orthopaedics. This instrumentation is created from patient's advanced imaging studies and segmented 3-D models to create a surgically defined correction or change in patient anatomy. This is currently most commonly used for prosthesis placement in arthroplasty procedures or spine deformity corrections.8,26,29

PSI and 3-D printed devices in orthopaedic surgery are typically considered class II devices, which denote a moderate to high risk to the patient and/or user. Therefore, most of the regulatory activity around these printed devices follows the 510(k) pathway.33 This pathway is described in Section 510(k) of the Food, Drug, and Cosmetic Act. This allows the FDA to determine whether the device is equivalent to a device already available on the market to improve time to clinical application. PSI 3-D printed guides are unique devices because they are custom made to that individual patient. Therefore, demonstrating equivalency or similarity of the technology presents new regulatory challenges.

The FDA released a guidance document in December of 2017 to clarify and publicize its expectations for 3-D printed devices titled, Technical Considerations for Additive Manufactured Devices.34 Importantly, the document explains that the FDA regulates manufacturer's claims regarding their orthopaedic devices. It does not, however, directly regulate the clinical practice of medicine or orthopaedics. Therefore, the treating physician must take an active role in understanding the technology to appreciate the utility and limitations for 3-D printed technology in patient care.

Technology diffusion is the widespread adoption of a new innovation. This diffusion applies to an innovation like 3-D printing. The clinician-scientist must understand the technology and its potential risks and benefits before applying the technology to patient care. For example, patient-specific custom cutting guides have had mixed clinical results in early follow-up outcome studies in total knee arthroplasty patients,8,35 and although the total shoulder PSI guides have consistently demonstrated better implant placement,14,26 no long-term clinical studies demonstrating a benefit in outcomes have been reported in the peer-reviewed literature.25 Further research is needed to understand the role of these 3-D printing technologies in patient care.

Future Applications for Three-dimensional Printing

The intersection between biologics and 3-D printed materials capable of transforming a material into a bioactive scaffold is an area of tremendous interest. Numerous types of 3-D printers exist, and 3-D printers capable of printing with living cells may be able to contribute to future cell-based therapies in orthopaedics. These may even offer truly regenerative alternatives that have eluded clinical application to date.36

3-D printing with living cells is often referred to as 3-D bioprinting. This type of 3-D printing is a novel and innovative method for the 3-D fabrication of living tissues and organ-like structures. Bioprinting is a form of additive manufacturing, which typically involves a scaffold structure that cells are printed onto or within.36 The scaffold offers structural support that creates an environment suitable for cell residence, growth, differentiation, and synthesis. In one application of this process, advanced imaging would be able to define an articular defect in a 3-D computer model and facilitate the creation of patient-specific 3-D bioprinted cell-based scaffold for tissue regeneration.37

Custom 3-D printers are being developed at academic and research institutions for printing specific biocompatible materials. For example, several research institutions are exploring bone graft substitutes created as 3-D printed scaffolds. These scaffolds are most commonly made from composite polycaprolactone, poly-lactic-co-glycolic acid, and β-tricalcium phosphate.38-40 3-D printed synthetic scaffolds have been attempted to be made more biologically compatible by attaching to them to a cell-laiden mineralized extracellular matrix that mimics the properties and qualities of bone.38 These 3-D printed bone substitutes could potentially offer a future treatment modality for grafting bone defects in trauma, orthopaedic-oncology, and revision arthroplasty cases. The ability to 3-D print tissue substitutes and osteochondral therapies will likely continue as a research focus for many orthopaedic institutions in the near future.

Summary

3-D printing technology is influencing many areas of medicine, including orthopaedic surgery. In the last decade, 3-D printers have dramatically decreased in size and cost to the point that desktop printers are available at cost-effective price points. Similarly, the types and costs of printing materials as well as time needed to complete prints have made the process more widely available and feasible for orthopaedic surgeons. Although clinical applications and research using 3-D printing technology is increasing, the technology is still in an early developmental phase with limited experience and evidence to support clinical use to date. With a greater understanding of this technology, orthopaedic surgeons will be better able to incorporate and understand the technology as it influences their orthopaedic practice and clinical care of their patients.

References

References printed in bold type are those published within the past 5 years.

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Supplemental Digital Content

Copyright 2019 by the American Academy of Orthopaedic Surgeons.