JUNE 2022 | The Surgical Technologist | 255 images: magnetic resolution image (MRI), computerized tomography (CT), and ultrasound (US) in order to have a high impact on the health of patients [22]. 2. Additive Manufacturing Techniques With the advent of CT and MRI, the medical field achieves the ability to visualize in-body accurate geometries without surgery intervention [23, 24]. Using computer-aided design (CAD) software, healthcare professionals could model the anatomical topology from cardiac vasculature to the skeletal system [25, 26]. CT and MRI can provide a 16 bits map with 65.536 shades of gray approximately [27, 28], allowing medical specialists to diagnose and engineers to design models by the identification of the region of interests (ROI) [29]. The volumetric representation of the ROI is obtained by a format called Standard Triangle Language (STL), which is the first step to create the physical model using additive manufacturing techniques (AMT) [30]. Currently, the main AMT includes, but is not limited to fused deposition modeling (FDM), stereo-lithography (SLA), selective laser sintering (SLS), selective laser melting (SLM), electron beam melting (EBM), and direct energy deposition (DED). This section focuses on describing these 3DP principles and its application on the field. FDM is the most widespread AMT that uses a heated nozzle to melt engineering thermoplastic, such as lactic polyacid (PLA), acrylonitrile butadiene styrene (ABS), and polymethylmethacrylate (PMMA). The extrusion nozzle is built in an XYZ axis Cartesian robot platform to build layer-bylayer 3D parts [31]. FDM demands no special ventilation; however, high room temperature variation can affect the process [31, 32]. FDM was successfully used to fabricate patient-specific implants with varying densities for cranial defects and femur parts [33] and biocompatible nanocomposites for tissue engineering applications [34, 35]. One disadvantage of FDM, since it works with thermoplastics, is that it can only be sterilized using cold solutions [36]. Stereolithography (SLA), also known as photosolidification or resin printing, creates 3D parts layer-bylayer through photo-polymerization [37]. SLA uses optical light to scan over a reservoir filled with light-curable resin and induces the molecules to link and solidify specific resin surface regions. Printed parts by SLA method are especially good to recreate cavities, such sinuses and neurovascular channels [37]. The disadvantages of the SLA model structure include low mechanical strength and long manufactory time [38, 39]. Selective laser sintering (SLS) is based on the fusion of powder. It employs a high-power laser as a heat source to sinter powder material (usually nylon, polyamide, or metals) to build-up 3D parts layer-by-layer [40]. The powder material diameter should be in the order of 50 μm to improve the model’s mechanical properties and topological surface. Unlike SLA, this process is self-supporting, and each layer is deposited over another. SLS shows promising applications for bone tissue engineering [41] and other numerous biomedical applications such as oral, maxillofacial, neurological, and orthopaedics surgery [39, 40, 42]. Selective laser melting (SLM), a subcategory of SLS, is used to fully melt the powder material and bind them in layers, instead of only fusing the metal powder to bond specific regions. Electron beam melting (EBM) employs an electron beam in a high vacuum chamber at a very high temperature to melt the metal powder and fabricate metal parts layer-bylayer [26, 43, 44]. Typical materials used in the EBM process for surgery applications are commercially pure titanium (CP-Ti), titanium alloys, stainless steel, magnesium alloys, and nickel alloys [37, 43, 45, 46]. Directed energy deposition (DED) concentrated a heat source, such as an electron beam or a laser, to melt in situ delivery of powder to fabricate 3D objects [47]. Beyond manufacturing layer-by-layer 3D objects, DED can also restore existing parts or add material over the currently fabricated structure and perform surface modification [44]. DED processes have a better cooling effect and refabricating capability [48]. As a disadvantage, DED presents low fabrication efficiency compared to EBM and SLM [26]. Finally, SLS, SLM, EBM, and DED are attractive methods to manufacturing porous metallic structures with complex shapes, which are well desired for patient-specific surgical implants to improve bone-in-grown and reduce bone-metal elastic modulus mismatch, thereby allowing for long-term implant stability [26, 49, 50]. However, one disadvantage is the residual stress that may cause interlayer debonding or crack [26]. The most common materials used for this purpose are CP-Ti and titanium alloys [26, 45, 51]. 3. Surgical Applications 3.1. Craniofacial and Nervous System 3.1.1. Head and Neck. 3DP offers the possibility to understand complex structures, fractures, and malformations in craniofacial and head and neck surgery. Moreover, the biggest impact of this technology arises when merged with virtual preoperative planning. As a result, surgical CAD/CAM (computer-aided manufacturing) guides and patientspecific implants (PSI) can be created to improve surgical precision and reduce surgical time despite performing increasingly complex reconstructions [52, 53]. Due to its intricate anatomy and important cosmetic function, 3DP is having a relevant impact in this anatomical region (Figure 1). In fact, Pettersson et al. reported in 2019 that the craniomaxillofacial surgery department designed 73.5% of the 3D-printed implants used in Finland per annum [54]. The most common surgical application of this technology in craniomaxillofacial trauma is in orbital floor fracture reconstructions. The goal of this type of surgery is to preserve the shape and volume of the orbit, restore its function, repair any aesthetic impairment, and prevent future sequelae. This procedure involves placing a standard preformed mesh or a PSI. There are two main types of PSI: physically prebent in patient-specific 3D model implants and patient-specific manufactured implants. ORBITA III randomized multicentric clinical trial showed that PSI could restore orbital volume more precisely than standard preformed ones [55]. Further studies comparing those groups of PSI have reported that 3 BioMed Research International
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