Rapid Prototyping : An Educational Extension of Anatomy Coloring

Access to cadaveric specimens, which is dependent on donation and heavy equipment, is still a limiting factor in the study of anatomy. As there are many spatially challenging structures throughout the human body for which physical support is beneficial, new strategies are always needed to implement new interactive teaching techniques to help students more efficiently grasp the complex organization of the human body. In this report, the author reveals the use of rapid prototyping technology to create an innovative and anatomically accurate teaching tool to engage students and help them build threedimensional (3D) mental maps of the human heart. The model consists of polylactic acid (PLA) thermoplastic deposited layer by layer on a platform in a pattern that corresponds to a digital Standard Tessellation Language (.stl) file. Printing a hollow human heart with a 1:1 ratio was rapid (<5 hrs) and affordable (approximatively US$5 per model). Given access to the appropriate printable digital files, students could print their own model and could subsequently paint them appropriately to give them an anatomically correct representation. This studentcentered activity is an extension of the current anatomy coloring textbooks, with the benefit of building an accurate 3D representation of the human body.


Introduction
From scientists to surgical residents and undergraduate students, laboratory users often compete for the same resources when studying anatomy.Adding to this stress, the number of people willing to donate their body could be low in some countries [1][2][3].New strategies are therefore necessary to complement cadaveric dissection with new interactive teaching techniques, as depicted recently with body painting [4,5] and clay modeling [6][7][8][9].Similar to dissection, [10,11] demonstrated that these hands-on teaching approaches also present the benefit of capitalizing on student's multimodal learning preferences as described by [12].By asking students to fill in blank drawings as the lecture advances, [13,14] were also able to involve students with visual (with the diagrams and the lecture slides), auditory (with lecture), reading/writing (with notes) and kinesthetic preference (with drawing and coloring).
For similar reasons, it is not surprising that commercially available anatomy coloring books are very popular among students.However, traditional 2D representations have posed problems for students to understand 3D relationships, [15] tried to overcome this limitation by asking students to paint muscles and bones on T-shirts.However, by their inherent properties, these T-shirts were not able to comprehensibly demonstrate anatomical structures in 3D.To remediate this problem, some anatomists redirected this approach by using new technologies of 3D images projection onto the surface of a living body [16].This exercise has proven effective to teach surface landmarks and the positions of organs in relation to bones.Nevertheless, this method is again inadequate to explain the 3D relationships between different anatomical structures, since it does not allow for manipulation by the students.3D computer-reconstructed models were therefore developed as they can better illustrate those 3D relationships and allow for manipulation by the users with a mouse.Once again, those models cannot offer what physical objects, such as cadaveric specimens with their haptic qualities do.When asked to compare 3D digital models to physical models, users reported a better learning experience with the physical models over 3D computer-aided design (CAD) models and textbooks [17].For those reasons, 3D printed anatomical models have already been used in training of surgeons [18][19][20][21][22][23][24][25][26][27][28][29].As the use of 3D printers in hospital will become increasingly integrated to medical imaging, introducing this technology early in undergraduate training could help students familiarize themselves with the conversion of preoperative imaging data to CAD files and the printing of personalized surgical models [30].
Rapid prototyping, which is a recent technology consisting of the automatized construction of working physical models from 3D digital models, could circumvent the aforementioned limitation of access to cadaveric specimens and the classical anatomy illustrations [31].In this report, a new model designed to help students construct 3D mental maps of the human heart was generated using rapid prototyping.

Generating 3D Printed Anatomical Models
3D printing, also known as additive layer manufacturing or rapid prototyping, was invented in the 1980s as a new technology to generate physical objects from 3D digital models.3D printing methods include selective laser melting, laser sintering, fused deposition modeling, stereo lithography, laminated object manufacturing and fused filament fabrication [32].The fuse filament fabrication technique, selected for this study, consists of generating physical models from ISSN: 2575-7563 polylactic acid (PLA, (C 3 H 4 O 2 ) n ) thermoplastic that is deposited layer upon layer on a platform in a pattern that corresponds to the digital input.Rapid advancements in recent years have allowed for more rapid and affordable prototyping.
Anatomically correct digital 3D models have been created in the last two decades from segmentation of images from the Visible Human Project dataset [33][34][35].3D atlases using Web 3D technology (www.zygotebody.com,www.primalpictures.com)were also introduced to the general public.Following segmentation of structures from a consecutive set of 200 micron thick sections, a 3D model can be generated by surface rendering and triangulation which describe the geometry of the 3D object.Converting this computation into digital Standard Tessellation Language (.stl) file, the model can then be processed for printing.Recent studies have shown that 3D printed model can be morphologically accurate [36,37].Already suggested that accurate physical models could be substitute for cadaver specimens [38].
In this study, a (.stl) file of a human heart (www.thingiverse.com)was processed by Makerbot Makerware software which prepares 3D files for printing on Makerbot Replicator 2 Desktop 3D printer (MakerBot ® Industries, LLC, Brooklyn, NY, USA).This printer is a high definition 3D printing system with a layer resolution of 100 microns.With a scale set at 1:1, the printing of a hollow human heart was rapid (<5 hrs).It was also affordable (approximatively US$5 per model), as the printed material (PLA) is not very expensive (costing approximatively US$45 per Kg) and the model weighs only 100 gms.

Coloring of 3D Printed Anatomical Models
In recent years, the introduction of innovative teaching methods changed the classic curricula from instructor-centered to studentcentered learning [39,40].Even yoga has recently been proposed as an activity to teach musculoskeletal anatomy [41].Providing students with .stlfiles could be the next step in student-centered learning of anatomy as students would be able to print the models themselves.They would subsequently be able to paint the models appropriately to give them an anatomically correct representation.
At McGill University, 600 science undergraduate students register each year to a systemic anatomy course with no prior exposure to gross anatomy.The course is distributed across 48 hours of lectures and laboratory sessions.With such constrained curriculum, simple models designed to reduce cognitive overload and help students conceptualize the 3D relationships of anatomical structures are deeply needed.Herein, the 3D printed model could not only offer the advantage of enhancing the understanding of the 3D relationships of the human body but also a complement to the study of prosections.
As presented in Figure 1A, the digital (.stl) model is an accurate representation a human heart which can enhance visual understanding of the heart when imported into the free open source Meshlab software (www.meshlab,sourceforge.net,Institute of the National Research Council, Italy).After printing with a scale set at 1:1 (Figure 1B), the model can be painted to reveal the different vessels (Figure 1C).The proposed teaching aid is a valuable replacement to any human specimen as students can use it to build their own mental image of heart without the costs and risks associated with wet specimens.In addition, the advancement in the printing material will soon allow the production of texture which will offer the possibility for surgical incisions to be made and sutures to be placed within the models [42].

Discussion
The 3D printed model presented in this report was selected to teach the complexity of human heart in a unique way.It can include all the anatomical structures of the human heart that are considered difficult to grasp spatially in order to help students understand the spatial relationship of the corresponding homologues of the human body.Each structure on the model represents accurately the absolute and relative sizes and shapes of the cadaveric equivalents.Ultimately, since this technology offers the potential of rapid production of multiple copies of any anatomical structure, this approach can ensure the standardization in teaching, which study on prosecutions cannot offer [43].In addition, unlike plastic models which are commonly used in high schools despite their "hypothetical" and "caricatured" nature, the 3D printed models offer great accuracy and great level of details.With the increasing use of 3D imaging modalities such as CT scans, magnetic resonance and ultrasound, the difficult exercise of understanding the 3D relationships of the human body has become increasingly important in clinical practice [44].Recently, it has been demonstrated that visual-spatial ability (VZ) of students greatly affects their performance in a gross anatomy course, with students scoring high on the Mental Rotation test performing better on spatially complex anatomy questions [45].Since students have different VZ, we, as educators, have the responsibility to consider this variability and appropriately implement pedagogical techniques that facilitate the acquisition of these skills [46][47][48].It has even been suggested that anatomy teaching should focus on developing those spatial skills that are susceptible to change with experience [49,50].
With the recent advances in image processing technologies, CAD models were able to be developed with the intent to help students build a better mental model of the complex spatial relationships found in the human body.Since the implementation of these high-fidelity digital 3D models developed from actual cross-sectional images, numerous studies have investigated their influence on the spatial anatomy comprehension of students [33][34][35].Recently, Nguyen et al. demonstrated that instruction with different CAD models could modulate the effect of visual-spatial abilities on the Spatial Anatomy Task [51].The authors indeed showed that dynamic visualization particularly benefits high VZ learners, therefore making 3D models unsuitable for every student.
At McGill University, we are using digital 3D models to teach complex anatomical structures such as the inner ear [52] and pelvis.However, with the reported model, the students have the advantages of being able go one step beyond and build the model themselves in order to manipulate it.This model could then be used on the long term to educate patients [53].
The study of both fresh and embalmed tissues has certain limitations.From specimens potentially carrying infectious agents, such as Mycobacterium tuberculosis and prions associated with encephalopathies, to the presence of formaldehyde which may be a potential carcinogen, the deliberated exposure of students to those risks need to be carefully considered.Since some students are avoiding cadaveric dissection due to moral, safety and religious grounds, options should be offered to undergraduate students who will not pursue careers in health care or to students who refuse to dissect.With the low cost of 3D printing a human heart, a user could print multiple organs and regions of the body for a total estimated cost comparable to many expensive anatomy textbooks [54].This strategy could help anatomy facilities to maximise the use of collective resources such as the limited amount of bodies to dissect [55].By embracing additional approaches emerging from new technologies and supplementing cadaveric dissection with independant hands-on activities which do not require access to human specimens, Anatomy departments could alleviate their economic constraints related to the availability of cadavers [56].As professional health care programs are seeing an increase in numbers of students, this pedagogical approach could also prove to be a logistical success.

Conclusion
This report demonstrates the ability to produce a physical and accurate 3D model of the human heart with potential benefits in education.While other approaches exist to teach anatomy, including plastic and wax models, cadaveric dissection, computer-based simulations and hands-on activities, the use of 3D printing offers an extension to the traditional coloring textbook while benefiting from 3D accuracy and student-centered approach.3D printing is a costeffective educational tool that allows for tactile and visual experience to improve understanding of anatomy.

Figure 1 :
Figure 1: Coloring of a 3D printed model of the heart.A. Screen capture of an interactive 3D model of the human heart (open source www.thingiverse.com).B. A white model of the human heart with 1:1 ratio is printed from .stlfile.C. By coloring the different structures such as arteries in red and veins in blue, the student can not only learn their location but also their three-dimensional relationships.