Three-dimensional (3D) printing and liver transplantation

Introduction

Anatomical knowledge is a cornerstone and an essential part for the performance of successful surgical and invasive procedures. A number of studies suggested a relation between the volume of surgeries performed and the rate of complications, including mortality. This association has been attributed, at least in part, to the incomplete characterization of anatomical structures in a way to account for individual variations . Even from the medical-legal perspective, a substantial number of claims have directly attributed to anatomical errors leading to an unintended “damage” of nearby structures . In an effort to optimize outcomes, the surgeon used a wide range of preoperative planning techniques in order to improve efficiency, diminish operative time, and ultimately reduce the incidence of surgical complications. The widespread use of imaging for preoperative planning of high-risk surgical procedures provided accuracy and improved knowledge of anatomical variations . Additional approaches beyond standard imaging were more recently introduced to assist in surgical planning and for risk analysis of individual cases, including computer-assisted three-dimensional (3D) imaging and the use of surgical models. These techniques provided enhanced intraoperative orientation. 3D printing has recently been applied as an advanced tool with properties and potential advantages of both 3D imaging and physical surgical models where individual patient's imaging is used to replicate the authentic anatomy of the person to undergo surgery.

3D printing, also known as rapid prototyping or additive manufacturing, involves the implementation of various techniques in order to “translate” a computer-generated image into a 3D solid object by printing consecutive thin layers of a specific type of material . 3D printing became widely available after the expiration of patents in 2009 and the subsequent drop in printer prices . Converting the two-dimensional (2D) image into 3D is of paramount importance in medicine and particularly in surgical specialties, and consequently, the medical industry could not help but embrace this opportunity. Besides, the evolution of surgery over the past century has been closely associated with various technological advancements. Now, 3D printed anatomic models have already started to make valuable inroads into surgical planning and execution.

It is accepted that 3D imaging tools are superior to 2D in terms of orienting anatomical structures that help the surgeon create a solid preoperative plan. However, studying 3D images on a 2D screen imposes its limitations. The 2D screen entails the difficulty of accurately estimating the depth of the image, and hence the 3D printed object allows for the precise resemblance of the cutting planes and the intraoperative setting with a significant increase in spatial perception. In addition, surgeons can manipulate the organ and orient themselves, which makes it easier to identify critical anatomical landmarks and the most comfortable physical position in the operating room (OR), as well as understand how to achieve optimal exposure intraoperatively.

So far, the surgical fields witnessing most of the applications of 3D printing are oral and maxillofacial surgery and orthopedic surgery , neurosurgery , and cardiac surgery . Transplantation surgery, and liver transplantation, in particular, is a novel field of medicine that rapidly evolved technically after overcoming the many immunological hurdles inherent in transplantation. Arguably, solid organ transplantation, including liver transplantation, is a complex, multistep process that requires impeccable surgery from start to finish. Accordingly, greater preoperative preparation, including an anatomical understanding of the individual patient, will likely improve outcomes and decreases the likelihood for surgical complications.

Today, liver transplantation has become an everyday practice, primarily due to the reduction in contraindications and the expansion of transplantation criteria. Nevertheless, it is apparent that the largest challenge today in liver transplantation is the existing discrepancy between the shortage of donor organs and the ever-growing number of patients awaiting a graft. The new era of partial liver grafting, especially with living donor segments and lobes, has become a fertile ground for the development and application of 3D printing. Using 3D printed models has a special benefit for preoperative planning, intraoperative execution, and medical education.

3D printing technologies

3D printing is a term used to describe a series of technologies that are used to build functional parts for many different uses. The common feature among all of these technologies is that the part is built by adding horizontal layers of material sequentially in the vertical direction. It is similar in principle to building a part out of Lego blocks—typically the base layer is built, and additional blocks are added piece by piece in order to achieve the final shape. In both the example of the Lego block as well as every 3D printing technology, the build method is additive in nature. This may be contrasted with more traditional methods of manufacturing, such as machining (material is sequentially removed from a starting shape) or molding (a liquid material is forced into a mold and then cooled into a solid), in order to achieve the desired geometry.

Historically, 3D printing referred to one of several specific technologies. General terms to describe these technologies include solid freeform fabrication, rapid prototyping, and additive manufacturing. Of these, additive manufacturing best describes the term 3D printing as it is readily understood by the public.

3D printing got its start with Charles Hull, who invented the Stereolithography Apparatus, or “SLA.” He patented this technology in 1986 (Patent #US4575330); this patent was the technological basis for the company 3D Systems, which released the first commercially available SLA machine in 1988. In the following years, additional technologies were introduced, including Fused Deposition Modeling (“FDM”) in 1991 (Stratasys, Edina, Minnesota, United States), Selective Laser Sintering (“SLS”) in 1992 (3D Systems, Rock Hill, South Carolina, United States), and PBP (3D Systems) in 1996. In more recent years, the Polyjet (Stratasys) machines and several variations of metal laser sintering, including Direct Metal Laser Sintering (“DMLS”) (3D Systems; EOS, EOS GmbH, Krailling, Germany) were introduced. Each of these technologies has unique characteristics, and therefore potentially unique applications in medicine.

There are several advantages inherent in 3D printing technologies in comparison with traditional manufacturing methods. Perhaps the most important advantage of 3D printing is that virtually any shape, no matter the complexity, can be built. This feature is highly advantageous for medical applications given the complexity of anatomical structures of individual organs such as the human liver. Other key advantages of 3D printing technologies are the speed of fabrication and simplicity of the process. All 3D printing technologies rely on the operator supplying a digital file called an STL file (derived from the word “stereolithography”). The STL file contains coordinates that define triangles that, in turn, represent the geometry of the part to be built. Design engineers would typically use the Computer-Aided Design (CAD) software package they are using to design the prototype to generate an STL file for 3D printing of that prototype.