Stereolithography model of a patient with a left mandibular tumor which has eaten away the bone.Surgery will involve removing almost half of the mandible and replacing with a large reconstruction plate and bone graft. Courtesy 3D Systems, Rock Hill, South Carolina, USA
The surgical removal of the left mandible has been simulated and a titanium reconstruction plate has been pre-bent before surgery.Performing the bending before surgery both saves time in surgery and provides for a better aesthetic outcome for the patient. Courtesy 3D Systems, Rock Hill, South Carolina, USA
In the last 5 years, the direct output of implantable parts produced using 3D printing has become more common (Hamid et al. 2016; Di Prima et al. 2016). When 3D printing is directly used for output of a patient-matched implant, it takes advantage of the fact that one-off designs are suited very well to this manufacturing technique. Another benefit of 3D printing is that “complexity is free,” and many times the more complex the design is, the faster and more economical the design is to actually produce (Fig. 9.3). This is a major shift in terms of design thinking, where biomedical engineers and others who have produced implants traditionally using subtractive machining need to reorient and expand their design thinking, which often adds constraints imposed by manufacturing processes.
Patient-matched acetabular cup produced by 3D printing shown during surgical insertion. Courtesy P. James Burn, MD and Paul Morrison, Ossis Ltd., Christchurch, New Zealand
In the 1990s, early uses for patient-matched implants centered around craniomaxillofacial (CMF) applications ; and these are still likely the most prevalent by percentage of total cases in any one anatomical area (Chepelev et al. 2017). Based on the intrinsic complexity of the face and the need for not only functional but aesthetic reconstruction, CMF applications continue to be solid users of patient-matched implant technology (Erickson et al. 1999; Powers et al. 1998; Müller et al. 2003). The technology matured in other areas of the body for large reconstructive surgery cases, many of which were oncology cases (Mulford et al. 2016). Over time, many more applications arose such as limb salvage procedures where complexity is created with defects that are not easily reconstructed with off-the-shelf sizes or shapes of implants. 3D printing is advantageous for the creation of patient-matched implants due to its accurate shape and scale, as well as the ability to print contralateral anatomy to use as a reference for anatomical reconstruction.
Currently, there is a major shift away from patient-matched implants being used solely for the extreme, massive reconstructive surgery cases toward these technologies being used for more “everyday” types of surgical cases. For example, one area that is now largely patient-matched is cranioplasty for repair of large cranial defects. For a defect over a couple of inches in diameter, a very large number of these neurosurgery cases worldwide involve prefabrication of a cranioplasty implant powered by 3D printing technology (Roberson and Rosenberg 1997; Eppley and Sadove 1998). Other even more common areas such as knee replacement are now also beginning to catch on, with patient-matched implant workflows being more commonly offered for partial or total knee arthroplasty (Slamin and Parsley 2012).
From a regulatory standpoint, the terminology used to describe a patient-specific implant is important. Historically, the word “custom” has been used to describe 3D printed implants made for a specific patient using medical image data. However, from the US FDA’s perspective, the term “custom” is closely affiliated with the Custom Device Exemption (FDA 2014), a very specific, defined regulatory path for use of a singular device in the treatment of a singular patient. Such devices have many restrictions, the most major of which is that no other commercially available device is available to treat the patient’s condition. Other major drawbacks to using the Custom Device Exemption for provision of an implant, from a device manufacturer’s standpoint, are related to the fact that there is a strict five units per year limit and that no marketing may be performed, both which severely hinder the ability to provide implants on a widespread basis under the Custom Device Exemption.
The FDA has recommended the use of the terminology “patient-matched” in an effort to make more clear the delineation between devices which go through a rigorous marketing clearance process such as a 510(k) or Premarket Approval (PMA) , patient-matched, and those which are used on more of a one-off basis for a truly unique surgical situation, custom devices (FDA 2016). Patient-matched implants going through the FDA’s traditional regulatory pathways for marketing clearance are much like regular, off-the-shelf-sized implants; however, instead of the FDA clearing the implant size, shape, etc., the FDA is clearing the “system” of design which leads to the final design. The system concept would talk about the inputs such as medical imaging studies and design constraints. The final design must fit into a bounding box that the company determines up front, allowing for testing at the extents of thickness, size, expanse, and material, among other considerations.
9.3 Medical Imaging and Digital Design of Patient-Matched Implants
Modern volumetric medical imaging studies can produce high-quality images that are usable for patient-matched implants. Most implants made for reconstruction of bony anatomy are designed with the aid of preoperative computed tomography (CT) scans. Typical workflows for medical image processing to extract the exact area of anatomy in question are performed by qualified technicians using specialized software tools. When the anatomy in question has been segmented, the workflow can proceed in a number of different ways depending on the patient-matched implant needs. This could look as simple as an anatomical model being 3D printed or as complex as a manufacturing mold being output or even direct output of the implant via 3D printing in a biocompatible material.
Although medical imaging has long been ready to support patient-matched implants, the software tools for digital design of the implants themselves have not always been robust enough for these tasks. It was only following the year 2000 that software tools which would allow for precise manipulation of very organic shapes became available. Many of those tools are still widely used today for implant design, tools such as Geomagic Freeform (3D Systems, Rock Hill, SC). Freeform is somewhat unique in that it combines organic manipulation software with haptic feedback, so the user can actually “feel” the model they are working on in digital space (Fig. 9.4). For many patient-specific implants which are anatomically designed (i.e., meant to mimic the shape of the anatomy they are replacing), this tool has been incredibly powerful. Other design tasks in different industries like the footwear industry also rely heavily on organic modeling software, which can be used to design very complex geometries for things like shoe soles. Digital design is most powerful in designing net-shape (final, perfect design) designs which can be directly built using digital fabrication techniques like 3D printing. In addition, digital design can also be used to design near-net-shape (close to final design) parts for surgeon input, further design, and rough design iterations.
An engineer uses Geomagic Freeform software to design a patient-matched cranioplasty. The tool in his left hand provides force feedback, giving the designer the sensation of “touching” the design he is working on. Courtesy 3D Systems, Rock Hill, South Carolina, USA
This is an exciting time for patient-matched implants from a design software standpoint. In the past, only very “one-off” implants were created with 3D printing, and these were primarily designed by hand, even when a designer would do this work digitally. Today the tools exist to almost totally automate many of these design tasks, taking what has been labor intensive and making it effortless once the system is developed. In addition to saving time and money on labor, other benefits of automation of design include reproducibility and standardization, both of which are much more predictable with automation of design. Watch this space for the coming 5 years to see automation totally change the economics and timeframes and accessibility of truly personalized, patient-matched design.
9.4 How 3D Printing Fits In
There is no single tool or method that fits the needs for all types of patient-matched implants. 3D printing supports the creation of patient-matched implants in a variety of ways including:
Anatomical model as a baseline for a design which is performed manually (i.e., with wax or clay)
Anatomical model as a template for preparing an off-the shelf implant by hand during or before surgery
Different types of models as manufacturing tools following digital design of the implant (molds for forming materials or sacrificial wax patterns)
Digital design and 3d printed fabrication of these implants directly in an implantable biomaterial
Anatomical Models as Baseline for Manual Design
In this scenario, the anatomical model is 3D printed and is used for the surgeon and engineer to develop an implant design. Many bone reconstructive, implantable devices have been designed in this way, allowing the surgeon to visualize the anatomy clearly in hand and to make needed modifications to the anatomy such as removing bone spurs and existing implants before design of a patient-matched implant (Fig. 9.5). The design of the implant could be as simple as creating a wax pattern of the implant on the model. Later this design could be investment cast into metal, machined by tracer mill, or digitized for computer numerical control (CNC) machining. Historically, without digital design tools, this has been the most common method to create a patient-matched implant; however, given the tools today available for digital design, this method has been surpassed by these more digital techniques.
Intraoperative or Immediately Preoperative Bending/Fitting by Surgeons
Many times models or templates are used to create patient-matched implants by the surgeon doing the fabrication using the model and the implant (think of a reconstruction plate being bent). This is also very common for personalizing implant hardware which is fairly straightforward and easy for the surgeon to modify in fitting to the patient’s anatomy.
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Models as a Manufacturing Tool/Pattern
If implant design is carried out digitally, there will be a need to output that design into physical form. Many methods exist, but the two main methods include (a) the digital design of the implant is produced as a sacrificial pattern for investment casting and (b) the digital design is subtracted from a box and output as a two-part mold for injection molding of the implant.
Sacrificial Pattern 3D Printing of the Implant Design. In this scenario, one could imagine a proximal total knee component being digitally designed with the target material being cobalt-chrome (Co-Cr) alloy. Co-Cr is typically investment cast for these applications using a sacrificial wax pattern invested in plaster. In this case, the digitally designed, patient-matched implant is 3D printed in wax or another investment casting-friendly material. Once printed, the pattern is used in the more traditional workflow for investment casting and subsequent finishing and polishing of the implant.
3D Printing of a Mold for Injection Molding. In this scenario, the implant may be polymeric and in a material that is not yet easy to directly 3D print. The net-shape designed implant would be digitally subtracted from a box, which would then be cut to form a two-part mold, with a cavity inside where the implant would be formed. Sprues and channels can be added to the digital model before being 3D printed in a material conducive to injection molding of the final implant material. Once the mold is 3D printed, the injection molding (i.e., injecting material into the mold to form the shape of the implant) is completed, and the implant is finished, packaged, and readied for use. This method is common for implant materials which are not yet suited for direct 3D printing.
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