| The Surgical Technologist | APRIL 2022 162 manufacturing and made its way into personal homes with the advent of desktop 3D printers. With compatible software and appropriate materials, consumers can witness the transformation from starting material to finished product of their own designs. Within the field of orthopaedic surgery, 3D printing has impacted patient care and education in several orthopaedic subspecialties.1-3 Three-dimensional printed anatomic models are commonly used in preoperative planning and have become a useful educational tool for patient instruction and trainee teaching. For many orthopaedic procedures, including arthroplasty and complex reconstructions, the use of 3D-printed patientspecific instrumentation (PSI) has become commonplace. The excitement around 3D printing continues to build as the fusion of 3D printing and biomedical science has shown early promise. This review article summarizes the fundamentals of 3D printing, discusses its utility within orthopaedic surgery, and highlights its potential future impact. Basics of Three-dimensional Printing in Medicine Twomain types of product manufacturing exist: additive and subtractive. Additive manufacturing fuses successive layers of solids, liquids, or powders to generate the finished product.4,5 In contrast, in subtractive manufacturing, the beginning material is cut, milled, or molded from a base product to create the final structure. Various methods of 3D printing exist, but each involves a common stepwise process (Figure 1). First, a digital representation of the end product is generated through a de novo design or by processing cross-sectional imaging fromCT and/orMRI scans saved in the digital imaging and communications in medicine format. This approach enables software to refine these images in the segmentation process to precisely define the shape of the object to be printed with regions of interest, which differentiate between tissues and surrounding anatomical structures.6,7 The contours of segmented regions of interest are computationally transformed into an standard triangle language file. In 2011, the additive manufacturing file was approved by the American Society for Testing and Materials, allowing users to integrate additional features of the 3D-printed object into the design (eg, surface texture, color, and material properties).6 The next step translates the standard triangle language or additive manufacturing file into a code, typically the G-code, which enables the printer to transform the digitally supplied coordinates of the file into a sequence of two-dimensional cross-sections. These cross-sections are essential as they form the base of each layer, which the printer fuses together to create the final 3D object.8 Once the final product is ready for printing, several methods from which to choose are available, which include material extrusion, material jetting, binder jetting, powder bed fusion, directed energy deposition, stereolithography, sheet lamination, and vat polymerization.Material extrusion, or fused depositionmodeling, has become one of the most common printing methods and uses solid-based starting materials. In this process, tiny beads or streams of material exit an extruder in a heated liquid or semiliquid form that is rapidly cooled, forming a hardened layer.9 For metal-based products, powder bed fusion-based methods have proven to be successful and are commonly used for orthopaedic implants. A thin layer of powder is deposited on the building platform of the printer, where a thermal energy source, either laser or electron beam, fuses the appropriate region as indicated by the original design. This process is repeated for each layer or the slice of the structure until each has been fused properly, resulting in the desired final product (Figures 2 and 3). Traditional (subtractive) manufacturing relies on a base product that is milled or cut away to obtain the desired structure, resulting in waste and production of scrap. In contrast, additive manufacturing results in decreased amounts of raw material waste with reported rates less than 5%.10,11 This advantage has made additive manufacturing a popular and efficient alternative; in addition, customized products are typically more expensive and time-consuming when traditional methods are used.12,13 Although 3D printing is accompanied by its own set of limiting factors, its growing popularity and expansion across industries has substantially decreased costs, increased access, and led to increasing applications in several industries, including medicine.9,12,14 Orthopaedic Applications An overview of cited literature is provided in Table 1. Anatomic Models Three-dimensional printed anatomic models are useful both for preoperative planning of complex cases and for teaching purposes. Surgeons can see and feel what they will encounter in the operating room with an accurate 2 Journal of the AAOS Global Research & Reviews® ----- April 2021, Vol 5, No 4 ----- © American Academy of Orthopaedic Surgeons Three-dimensional Printing in Orthopaedic Surgery
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