3D printing and virtual and augmented reality in medicine and surgery: tackling the content development barrier through co-creative approaches

Introduction

Information and Communication Technologies (ICT) defined healthcare interventions from their inception. Reduced costs, increased efficacy toward growth, social equality, diagnostic efficacy, and treatment effectiveness were the outcomes of these interventions. Contemporary education for healthcare, in particular, has greatly advanced toward widely varying educational resources and activities in the ICT area. The incentive behind this lies in the need for unconstrained time and place to access clinical skills worldwide. The incorporation of virtual, augmented, and, recently, mixed reality (VR/AR/MR) technologies was an approach that has greatly impacted the immersive medical education field. Defining these immersive technologies sometimes is difficult as there are, at points, overlaps. Nevertheless, a viable description of VR is the replacement of external sensory inputs (mainly visual and audio) with those generated by computers using a headset device. AR, instead, is the overlap of digital content over the real world using either 2D or 3D markers in the real-world environment. Finally, MR is similar to AR with one key difference. Instead of the real-world marker being a preprogrammed static item or image, the content overlay is done after completing a 3D mapping of the current environment. This way features can be used in intuitive ways such as table-positioned 3D models or 2D images and “hanging” notes on walls. There is evidence that such technologies significantly increase the educational impact of an episode of learning and subsequently can have a significant impact on educational outcomes. Realized examples include experiential world exploration, visualizations of high-impact physics and chemistry concepts, and even the incorporation of such modalities for VPs. It is this immediate capacity of engagement of these modalities that not only motivates the student but also allows the educational material to be internalized, thus avoiding conceptual errors.

Specifically in the field of surgery, there is a significant body of literature for both the efficacy and the specific technologies implemented for VR/AR visualizations. Additional features such as haptic technologies and suites of sensors (e.g., hand and finger tracking) have been implemented for all aspects of surgical training and education. Preoperative training, surgical anatomy, 3D visualizations for new approaches, and incision paths are the first and obvious ones. Real-time augmentation of the surgical field with MR content, or even the use of VR for relaxing the patient prior to surgery, is the less obvious ones. Preoperative patient VR presentations for better understanding of the specific surgical process have even impacted the overall litigation cost of healthcare institutions, since the visualizations prevented frivolous lawsuits against them by uneducated patients or relatives. These really exciting contemporary developments are mentioned here, only in passing since they are very well documented in the literature. For example, a contemporary, at the time of writing of this work, and rather succinct review of the overall applications and impact of VR/AR in the field of surgery is the one by Desselle et al.

Recent advances in technology have also moved the field of application for ICT technologies from the intangible (AR/VR/MR) to the realm of the tangible with easily accessible 3D print capabilities and applications.

This work aims to present the state of the art of the newly emergent 3D print technology in medical/surgical applications while moving past a simple description of the technologies for the maturing AR/VR/MR field and presenting a viable roadmap for participatory immersive content creation and standardized workflows and implementation pipelines. As such, it aims to become a useful tool for the medical/surgical educator and technologist in order to incorporate faster and easier immersive content in their curricula.

3D printing in surgery

The development of 3D printing (3DP) technology, also referred to as rapid prototyping (RP), has produced a more sophisticated method for an intuitive and realistic 3D-manufactured model that goes beyond basic 3D-shaped simulation on a flat screen. For its usage in medical fields, the immediacy between concept and final development is the most significant of the many benefits of 3DP technology. In a clinical environment, the prospect of one-stop development from medical imaging to 3DP intensified the current medical movement toward personalized, patient-specific care. Second, 3DP, as an additive manufacturing technique, demonstrates the characteristics of “zero constraint—zero skill” for 3D manufacturing, which are suitable for medical applications, since the form of 3D models produced from patient-specific medical photographs is typically too complicated to be created utilizing traditional manufacturing methods. In comparison to industrial approaches, 3D model architecture for 3DP medical applications is simpler since most can be obtained utilizing 3D surface reconstruction of medical images with the aid of postprocessed images. Thus, 3DP computers have been used in a number of medical applications since the early 2000s. The technique has been used primarily for rough tissue applications owing to the strength of most 3D-printable products. The advantage of such manufactures derives from the improved sensory perception that the sense of contact conveys to the user.

Technically, to achieve these goals, high-quality images must be collected from multidetector computed tomography (MDCT) or magnetic resonance imaging scans in order to create a valuable 3D virtual (3DV) reconstruction. The slicing width of the obtained photographs should not reach 2 mm, with an optimum value of less than 1 mm. ^, Picture preparation for 3DP begins with a segmentation process, the purpose of which is to reduce the complexity of the original image by choosing the anatomy to be printed, which is then extrapolated from the rest of the image.

The 3DV model is then exported as a surface triangulation language (STL) file that defines the spatial geometry of the object via a set of oriented triangular facets named mesh ^, software. The smaller the scale of these triangles, the more detailed is the surface of the 3DV model. At this point, surface smoothing is used to fix defects or sharp edges; in addition, further preparation of the STL file should be carried out in conjunction with the final purpose of the printed item, such as the production of interlocking parts to allow the assembly/disassembly of the model, which is then ready for 3DP.

The core idea in the actual 3DP production phase is the development of artifacts by a layer process: the 3DV model is broken down into a sequence of 2D layers that are deployed one after the other by a 3D printer. This “additive” method is the perfect way for 3D printers to handle extremely complicated geometries, such as anatomical templates. ^, ^,

3DP technology

3D printers may be differentiated by the method of deposition and curing method (e.g., content jetting, material extrusion), each requiring a broad variety of functional materials of various characteristics, such as clarity, stiffness or deformity, mechanical power, chromatic performance, and so on ^, ^, ( Figs. 4.2 and 4.3 ). In certain instances, a support frame or dedicated support content can be used to help the building and may be discarded or dissolved until the printing phase has been completed. ^, In rare instances, owing to the difficulties of cleaning and postprocessing complex anatomies, each configuration may be printed separately and then stuck together to reconstruct the final object; however, this technique is avoided due to potential misalignments during the assembly of 3D-printed components.

3DP technology used in medicine can be categorized according to the technique, substrate, or planned deposition process used. Scientific grouping covers stereolithography (SLA), polyjet printing, multijet printing (MJP), digital light processing (DLP), direct metal laser sintering (DMLS), selective laser sintering (SLS), color-jet printing (CJP or binder-jet), fused deposition modeling (FDM), laminated object manufacturing, and electron beam melting. Material classification covers titanium alloys, metal powder, eutectic metals, alloy metals, ceramic powder, photopolymer, paper, foil, plastic film, and thermoplastic.

SLA apparatus

The SLA device consists of a photosensitive resin tank, a model-building frame, and an ultraviolet (UV) laser for curing the resin ( Fig. 4.2 ). A computer-controlled mirror is used to concentrate the UV laser on the resin surface and to cure the resin on a slice-by-slice basis. This slice data are fed to the RP unit, which guides the exposure direction of the UV laser to the resin surface. The layers are cured sequentially and tie together to create a strong object, starting from the bottom of the model and building upward. Each fresh layer of resin is cleaned through the surface of the previous layer using a wiper blade until it is revealed and cured. The model is then extracted from the bath and cured for a longer time in the UV compartment. Generally, stereolithography (SL) is known to have the maximum precision and the best surface finish in any RP technology. The content of the model is durable, mildly brittle, and comparatively lightweight. ^,

Polyjet printing

Polyjet printing is achieved utilizing state-of-the-art, layer-by-layer, extrusion of photopolymer materials in ultrathin layers of 16 μm on a built-in tray before the model is finished. Each photopolymer layer is cured by UV light directly after it has been injected, providing completely cured versions that can be treated and used immediately without postcuring. A gel-like support layer that is specifically formulated to preserve intricate geometries and that is quickly withdrawn by hand and water jetting is used. Polyjet printing may take advantage of a range of materials, like rubberlike content, and the postprocessing period is short and easy. At present, this procedure is excessively time intensive and thus too costly to use in surgical applications.

Multijet printing

MJP's liquid-based 3DP technology uses a print head to spread acrylic photopolymer (part) and wax (support) simultaneously. Injected products are treated with UV light. MJP is the most accurate 3DP method. The opaqueness of the key acrylic photopolymer resins can be managed, but its intensity is relatively poor. In addition, the shape deformation arises at 65 degrees or higher.

Digital light processing

DLP's liquid-based 3DP approach uses the traditional method of a DLP projector as a light source. In principle, a 2D image is projected onto a light-curable resin in a vat sculpting the print. This system demonstrates an outstanding surface finish and the quickest printing operation. The mechanical properties of the material used are fine, but the form and color of the material are limited. In addition, the content and printing device are costly.

Direct metal laser sintering

Using a solid-state Yb fiber laser beam, the powder-based 3DP process of the DMLS methodology selectively produces several variants. Through SLS of various metal powders (e.g., aluminum, cobalt, brass, nickel alloy, stainless steel, and titanium) by the laser, guided through each layer of the 3D model, a variety of materials can be constructed. Since the metal powder used plays the role of a support in the model, postprocessing, including support removal, is not needed. Moreover, the printing efficiency is usually outstanding.

Selective sintering laser

SLS's powder-based 3DP processing utilizes the CO ₂ laser beam to selectively produce materials. 2D slice data are fed into the SLS unit, which guides the exposure direction of the laser over a thin layer of powder already accumulated on the baking tray and leveled with a roller. The laser heats the powder particles, fuses them to create a dense sheet, and then travels along the X- and Y-axes to design the structures according to the computer-aided design (CAD) results. After the first layer fuses, the create tray moves down, where a fresh layer of powder is placed and sintered. The method is replicated before the item is done. The prototype surface is postprocessed with sandblasting. The SLS prototype is opaque and has an abrasive surface. The production period of the prototype is long, spanning sometimes 15 h. The precision of the SLS model is reasonably high, with an overall standard error of 0.1–0.6 mm. Owing to the high cost of the components, multiple pieces are assembled concurrently. The equipment is costly, but, due to patent expiry, low-cost SLS devices are starting to emerge.

Color-jet printing

The CJP approach uses the print head to selectively spread the binder onto the powder sheet. Next, a thin film of powder is applied over a tray using a roller, identical to the one used in the SLS method. The print head scans the powder tray and delivers a continuous jet of solution, which fuses the powder particles when it touches them. No support systems are needed when the prototype is being assembled, as the surrounding powder supports the unconnected components. The leftover underlying powder is sucked until the process is complete. In the finishing process, the surfaces of the prototype are infiltrated with a cyanoacrylate-based substance to harden the framework. The printing technique allows the construction of complex geometric constructs, such as hanging partitions within cavities, without artificial support structures. Since it uses a CMYK color ink cartridge, as used in traditional 2D printers, a 3D molded model may be printed in about the same color spectrum. Printing and infiltration procedures take ∼4–6 h. The 3D printers used in this method are reasonably inexpensive, have short construction times, and are simple to manage. In addition, these 3D printers are cost efficient, produce minimal waste, and are precise (±0.1 mm in the Z-axis, ± 0.2 mm in the X- and Y-planes). They are also compact in scale and capable of manufacturing hard, soft, and versatile versions. This technology has lower costs than related methods.

Modeling of fused deposition

FDM's solid-based 3DP technology uses a similar concept to SL in that it creates models on a layer-by-layer basis. The key distinction is that the layers are dispersed as a thermoplastic, which is extruded from a fine nozzle. Acrylonitrile butadiene styrene is a widely used substance for this treatment. The 3D model is built by extrusion of the heated thermoplastic material onto the foam surface along the direction shown by the model details. When the coating has been deposited, the nozzle is lifted from 0.278 to 0.356 mm and the next layer is put at the top of the previous layer. This step is replicated until completion. As with SL, support systems for FDM models are necessary because time is needed for the thermoplastic to harden and the layers to fuse together. Supports may be extracted using simple mechanical tools, or dissolved with specific acidic solutions. While it is the most common 3D printer technology, the surface finish is reasonably poor. To improve that, several postprocessing options, including acetone fumigation, may also be used.

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