Additive manufacturing (three-dimensional [3D] printing) has many applications in orthopaedics, beyond its use in building models for preoperative planning and patient and resident education. For example, with the use of computer-aided design (CAD) software, 3D printing can be used to build custom instruments such as retractors and drill/saw guides. The process involves importing a surface model file of the anatomy or object in stereolithography (STL) format, based on processing an imaging (CT or MRI) dataset. The 3D CAD design of the surgical tool, instrument, or implant can be superimposed on the anatomy.
Several companies offer custom (patient-specific) saw guides for total knee arthroplasty (TKA). An acetabular component guide to facilitate positioning of a Kirschner wire with the desired anteversion and abduction during total hip arthroplasty is in the experimental stages. This guide has been shown clinically to place components within 2 degrees to 4 degrees for abduction and anteversion.
Custom pedicle screw drill guides are also being developed. One randomized clinical trial for lumbar and sacral pedicle screws found that using custom drill guides significantly lowered the risk of cortical perforation and resulted in less deviation and displacement than traditional techniques. However, other studies have raised concerns about placement and accuracy. One report found that 26 percent of pedicle screws were misplaced, despite the use of custom guides. Another study using custom drill guides to place cervical pedicle screws demonstrated a much lower misplaced screw rate than previous studies that used fluoroscopy alone.
The quality (resolution) of the original CT or MRI imaging data, as well as the design process, manufacturing, and thermal stability of the material, influence the accuracy of custom guides. Overall, the key questions are whether custom guides improve placement relative to conventional approaches and whether they result in improved clinical outcomes or reduced surgical/fluoroscopy time. If effective, these guides may provide intraoperative guidance faster and less expensively than more sophisticated navigation systems.
Fit is a common concern for currently available extremity and spine braces. Additive manufacturing can build custom braces at a lower cost and faster speed than alternative approaches. One enterprise is developing 3D-printed scoliosis braces designed to be more comfortable and fashionable. Another is working on quickly fabricating a waterproof cast that also allows easy skin visualization.
Additive manufacturing can be used to create custom grafts and implants. Synthetic bone grafts can be fabricated with controlled bone porosity, size, and shape to optimize bone healing. A custom TKA implant, approved by the Food and Drug Administration (FDA), is available commercially. The custom mold is based on 3D images of the knee and fabricated using additive manufacturing. This mold is then used to cast a cobalt-chrome component. However, the custom mold is more expensive than off-the-shelf knee implants, and further study is needed to determine its clinical success.
Other FDA-approved orthopaedic implants constructed through additive manufacturing processes and commercially available include custom spinal interbodies for the cervical spine and anterior lumbar interbody fusions and spacers for calcaneus osteotomies.
Around the world, additive manufacturing has been used to create a variety of other orthopaedic implants for special clinical cases. Custom mandible, skull, spine, calcaneus, and even hemipelvis implants have been described in case reports from outside the United States. Another potential use for additive manufacturing is to fabricate custom antibiotic spacers for periprosthetic joint infections. Even antibiotics that cannot be used with cement spacers due to heat stability issues can be incorporated with additive manufacturing techniques.
Although using rapid prototyping to fabricate tissue engineering scaffolds to place stem cells and growth factors may be possible in the future, nearer term clinical applications will likely involve the seeding of custom-built scaffolds with intraoperatively collected bone marrow cells or other cell or growth factors. Current technology is restricted to manufacturing sterile tissue constructs with a resolution of 100 µm. Tissues thicker than 100 µm to 200 µm do not get adequate nutrition and oxygen via diffusion alone. Researchers are developing ways to facilitate diffusion of nutrients and to manufacture vascular supplies as part of the constructs.
Researchers in Australia are attempting to miniaturize 3D printing to a handheld device, which could build layers of scaffold and cells directly onto bone or cartilage defects. This would avoid any problems from inaccurate preoperative sizing.
Hurdles for clinical utilization
Current commercially available products have been cleared by the FDA through the 510k pathway. Future products that can claim to be equivalent to commercially available products (produced either by additive manufacturing or other means) will need premarket approval. A clear demarcation of additive manufacturing products that fall into each category has not yet been determined. The FDA has established workgroups to evaluate the issue and clarify the pathway for future products.
New products using additive manufacturing will have to demonstrate reliability, accuracy, and safety prior to widespread utilization. An important consideration is the quality of the imaging from which the models are built, and this could be affected by patient motion or old implants. Although the printed model may accurately represent the imaging study with all its artifacts, some level of discrepancy may exist between the imaging study and the actual patient.
In cases involving the design of a custom implant or guide, the design process itself will come under regulatory scrutiny. Indeed, a draft document from the International Organization for Standardization (ISO) is being developed to standardize the procedure for imaging, segmentation, reconstruction, and parameterization (eg, dimensioning) of the process for designing safer and more efficient custom patient-specific implants. The ISO would welcome any ideas for improving this early effort; information on how to contribute can be found in the online version of this article.
In other cases, the sterility of manufacturing and mechanical properties will need to be evaluated. The very nature of the “additive” process may physically allow porosity and types of cavities to occur. These can be sealed during manufacturing and sterilization; however, they may open due to implant wear or other factors. The risks of infection are still unknown.
Tissue engineering applications also require additional studies on the interactions of these products in vivo and in the long-term. These studies will be difficult because the performance of biologic implants may vary greatly from patient to patient. Another factor is that the long-term survival of cells that have been through additive manufacturing processes is not known. Some processes use ultraviolet light or other curing methods that can result in genetic or structural damage that could influence the effectiveness or safety of the cells. The time from fabrication to clinical use will be important to avoid degradation of growth factors and cells.
The key hurdles will be to demonstrate reproducibility and to achieve clear improvements such as better patient outcomes (ie, fewer revisions, superior outcome scores), shorter surgery times, or reduced fluoroscopy time in a cost-effective manner. In most settings, these additive technologies will be more expensive than conventional approaches. Their success will depend on their equivalency or superiority to current standards, while delivering cost-savings in other aspects of patient care (such as length of stay, complications, or revisions).
Given the rapidly decreasing costs of 3D printers, some hospitals have already adopted the technology. In the future, every hospital may have its own 3D printer. But whether hospitals or surgeons can become implant manufacturers is unknown. In today’s litigious climate, hospital-based additive technologies will likely be limited to creating models rather than to customizing implants.
The scientific and regulatory hurdles may delay the clinical applications of additive manufacturing for tissue engineering. Custom models, braces/casts, instruments, and implants are already here and will make a larger impact in the coming years, especially if and when they can demonstrate significant clinical or cost advantages to the current standard of care.
Karthikeyan E. Ponnusamy, MD; Hani Haider, PhD; Paul A. Anderson, MD; Ahman Nassr, MD; and Nicholas John Giori, MD, are members of the AAOS Biomedical Engineering Committee. One or more of the authors report potential conflicts of interest; for more information, visit www.aaos.org/disclosure
Editor’s Note: This is a follow-up article to “One Layer at a Time: Rapid Prototyping in Orthopaedics,” which appeared in the April 2015 issue of AAOS Now and discussed additive manufacturing technology, techniques, and materials, as well as the use of models for preoperative planning and resident education.