3D printing: shedding light into the surgical education

Technical background

Before reporting applications of rapid prototyping for training purposes, it is prudent to lay out an overview of the consecutive manufacturing steps of these procedures. Data preparation, selection of printing materials and technology according to the final purpose are some of the key elements to take into account.

Steps of rapid prototyping process:

• 1.

  Collection of medical imaging data is a crucial step toward extracting reliable and accurate 3D models. High-quality imaging can guarantee time-saving and decent fidelity level of the final outcome stemming from multidetector computed tomography, ultrasound imaging, magnetic resonance imaging, angiography, cone beam computed tomography, X-rays, positron and single photon emission computed tomography. High contrast between structures of interest and adjacent areas is essential for better outcome . All data collected are saved in DICOM format .

• 2.

  Afterward, the segmentation procedure follows. It includes the identification of the anatomical landmark of interest by selecting the corresponding voxels and, thus, marking a region of interest (ROI). Segmentation is conducted by software programs in automatic, semiautomatic, or manual way, and files containing the ROI can be turned into a 3D format through conversion to an STL file .

• 3.

  Additional model designs and modifications have to be done before final printing, which include optimizations such as surface smoothing. For example, “hollowing” technique is useful for fabrication of cardiac and vascular models, creating empty spaces through capturing and removing blood volume voxels from file during segmentation .

• 4.

  Undoubtedly, appropriateness and final utility of fabricated models rely mainly on accurate representation of anatomical structure or pathologic entities. Meeting this fundamental need, quality assurance and control of this multistep process have to be fulfilled .

• 5.

  Furthermore, postprocessing procedures will refine the 3D model as long as final control of anatomical accuracy is conducted, usually after evaluation by experienced professionals . In this way, models can gain acceptance as training tools.

• 6.

  Final step is the printing process, in which many aspects have to be taken into consideration, such as optimal materials, multiple colors, overall cost, and time requirements .

In 1980s, Charles Hulk reported the first rapid prototyping technique, which was called stereolithography (SLA) . Nowadays, 3D printing technologies have rapidly evolved and new laser techniques such as selective laser sintering are used. Nevertheless, even nowadays, SLA printing remains a valuable choice for medical use ( Figs. 3.1–3.4 ).

The following table contains basic characteristics of the most common 3D printing technologies implemented in recent years.

General surgery

Without any doubt, rapid prototyping has contributed in various teaching procedures of general surgery, particularly in minimally invasive processes. Steep learning curves in this field signify the need for 3D printing as an educational tool in this field.

Hepatobiliary surgery is a fertile ground for use of 3D printing as an alternative solution for visualization and hands-on training for the needs of medical students' education . Nevertheless, concerns regarding complexity of liver parenchyma/venous/biliary anatomy and the need for anatomical landmarks have to be addressed. Laparoscopic and robotically assisted hepatectomies in neoplasias and in living donor liver transplantations (LDLTs) are some of the training applications of 3D printing . Two-dimensional (2D) imaging and intraoperative ultrasound are useful tools for hepatic surgical procedures, but there are limitations which can be effectively overcome by the use of 3D hepatic models in order to help surgeons acquire a better understanding of liver anatomy and develop hand-eye coordination .

An application of this state-of-the-art technology has been the recognition of hepatic segments according to Couinaud classification, which is a confusing classification system for many students and novice surgeons. A study fabricated three discrete liver models, particularly type 1: 3D-printed hepatic segments without parenchyma, type 2: hepatic segments with transparent parenchyma, type 3: hepatic segments with corresponding hepatic ducts. Each segment was colored in a different way, facilitating the visualization of liver segment architecture in three dimensions and also accurately replicating spatial relationships with crucial adjacent structures. Students' tests scores demonstrated the statistically significant impact of 3D hepatic segment models in teaching procedure, especially of the replica type 3, rending them useful to traditional anatomy teaching . Deeper perception of liver anatomy and tumor characteristics through the 3D models could help eliminate some of the limitations of 2D radiographic methods . Taking into consideration the significance of proper and in-depth understanding of hepatic vasculature for achieving successful surgical maneuvers and minimal complications intraoperatively, Watson et al. fabricated low-cost physical models of portal and hepatic veins of about $100 per model, abrogating the barrier of high cost. This study implied that these models can be introduced in weekly conferences, aiming to teach operative techniques to young residents and students, on the basis of following elective surgeries .

A great amount of 3D printed liver models reported in literature replicate liver and biliary tract structure with accuracy as well as common situations such as tumors, and thus, are used for stepwise surgical resection simulation . Up to date, 3D solid models have been utilized for helping young surgeons and clinicians acquire proficiency in the anastomosis procedure during LDLT . Javan et al. designed a novel solid 3D hepatic model of blood vessels, biliary tract, and pathological structures permitting the performance of hepatobiliary procedures. Via this, trainees were encouraged to practice procedures such as tumor embolization and biopsy, transjugular intrahepatic portosystemic shunt (TIPS) positioning, abscess drainage catheter placement, percutaneous biliary drainage and percutaneous cholecystostomy tube placement, acquiring confidence in dealing with hepatobiliary emergencies. Skills and simulation opportunities incorporated in training through 3D printed models so far also include laparoscopic gallbladder excision and hepatoblastoma management , while, Kagaki et al. were the first to fabricate a 3D printed hepatic model with replication of a perihilar cholangiocarcinoma, which allows simulation-training for the management of hepatobiliary malignancies .

Surgical resection is the most effective solution for hepatic metastasis from colorectal cancer. The advent of laparoscopy has initiated multiple pathways for hepatectomies resulting in a need for sophisticated and up-to-date hands-on training. Witowski et al. described the fabrication of a low-cost 3D printed hepatic model based on radiographic data of a patient, that was used as a guide for preoperative planning for a laparoscopic right hemihepatectomy for colorectal cancer metastases. Authors support that this model also has great potential as an adjunct of the educational process for medical students due to lifelike size and accurate spatial relationships .

Additive manufacturing has opened up new horizons in the simulation of biliary system interventions, since 3D printed models can represent specific disorders or anatomical variations such as common bile duct obstruction. Techniques requiring high level of expertise, such as EUS-guided biliary drainage (EUS-BD) of malignant obstructive jaundice due to gastrointestinal, pancreatic or peripapillary diseases after unsuccessful ERCP can also been taught on 3D printed models. Opportunities for training in basic maneuvers of this technique on real patients are sparse, hindering a continuous sequence of the learning process. Aiming to mitigate this challenge, Dhir et al. manufactured a 3D printed model of a dilated biliary tree by polycarbonate incorporated and replicas of blood vessels filled with aerated water in an animal liver tissue and examined the performance of clinicians experience in interventional EUS in four discrete maneuvers of EUS-BD, including needle puncture, guidewire manipulation, tract dilation, and stent adjustment. Τrainees had the opportunity to practice on the most challenging steps of the procedure, maximizing educational profit. Materials were chosen so that the model would provide lifelike EUS and radiographic data. Each participant performed both the antegrade procedure and the choledochoduodenostomy. Consequently, participants were asked to evaluate the experience of practicing on the model for each one of the four steps and the quality of the radiographic data, but also the simulator's accuracy and feasibility to be adapted into the training curriculum. Results emphasize that this kind of novel seventeen bile duct replica would be a cornerstone in mastering ultrasound-guided interventions in biliary system, even without any further modification and also an inspiration for further models of EUS-guided interventional procedures .

Another interventional procedure with a key role in the management of hepatopancreatobiliary diseases is choledochoscopy, which is considered a challenging process even for experienced surgeons. Previous endeavors included 3D printed models of the biliary tract and especially its spatial relationships with the hepatic parenchyma. Based on these attempts and on lack of previous 3D printed choledochoscopy simulators, Li et al. used CT data from two patients with biliary system dilation to fabricate two biliary tree models using 3D printing technology for educational purposes. Simulators were evaluated by four experienced surgeons regarding accuracy and utility and then used by group A of junior residents for anatomy comprehension and choledochoscopy maneuvers practice, while group B had to study the same data through a virtual 3D image on computer. Outcomes have shown this choledochoscopy simulator to be a realistic and valuable tool for acquiring basic principles of the procedure, as it enhanced biliary anatomy and variation complexity and boosted manipulation dexterity and confidence of participants. In conclusion, similar innovations involving endoscopic procedures in the biliary system will be an important part of training curriculum of hepatobiliary surgery residents .

Apart from hepatobiliary surgery, the adoption of 3D printing technology for fabricating teaching and simulation models also constitutes an excellent option for other general surgery subspecialties. Transabdominal preperitoneal (TAPP) inguinal laparoscopic hernia repair consists nowadays the state-of-the-art strategy in management of bilateral or recurrent hernias; yet today it presents a steep learning curve and demands mastered surgical skills and techniques. Nishihara et al. noticed the physical inadequacy of current teaching and practicing status quo based on the apprenticeship model of training and tools such as cadaveric specimens and virtual reality platforms to provide sufficient proficiency to general surgery residents. To examine the impact of 3D printed models in training novice residents in fundamental but challenging procedures, Nishihara et al. developed an original TAPP laparoscopy inguinal hernia repair simulator composed of a 3D printed replica of human trunk and abdominal wall layers under pneumoperitoneum and handmade models of organs of the inguinal region, representing a realistic and reusable simulation station for repair of all types of inguinal hernias. Fifteen participants rehearsed in coordination with an endoscopist the basic steps of the repair procedure including management of trocars and hernia sac, mesh placement, and peritoneal flap creation and closure. Statistical analysis of answers to a standardized questionnaire revealed that incorporation of this simulator in resident training curriculum would be of great benefit, both for skill acquiring and maintenance.

An additional advantage regarding the use of similar simulators for laparoscopic procedures is the feeling of stress positions of surgeon's wrist, corresponding with accuracy to difficulties otherwise faced only in the operating room . Apart from TAPP laparoscopic inguinal hernia repair, 3D printed models are gaining importance as a first contact tool of novice surgeons with principles of laparoscopic and robot-assisted surgery, rendering necessary the incorporation of minimally invasive techniques training in residency curriculum. For this purpose, UCI Trainer (UCiT) laparoscopic simulator, which is a laparoscopic simulator in connection with a PC or tablet device that collects training data on a platform, was designed and used by Parkhomenko et al. to assess basic laparoscopic surgical skills, such as peg transfer and knot tying. This type of simulation would be helpful for novice doctors who do not have the ability to perform and polish up their laparoscopic skills regularly, mainly due to lack of time or equipment .

Education on upper gastrointestinal system procedures is also a section of great interest, where 3D printed models are a sophisticated alternative option. Examples are gastroscopies and stomach biopsies, techniques mainly acquired through hands-on practice under the supervision of an expert. Lee et al. conceived and produced a silicone-based 3D printed stomach simulator with ten lesions for training in endoscopic biopsy. Parameters analyzed include total time required for taking tissue biopsies from all ten lesions, simulator accuracy to real anatomy and procedure circumstances and, last but not least, potential use of this model as part of stepwise training program. Trainees, including residents, first- and second-year fellows and faculty members, were called to perform endoscopic biopsy five times. Results highlight a decrease in time needed to complete the procedure, noted in all levels of experience and especially among residents, enhancement of basic skills needed, such as coordination with the assistant and equipment handling .

Another point of interest for further training opportunities of the upper gastrointestinal tract is thoracoscopic esophageal atresia (EA) and tracheoesophageal fistula (TEF) repair, which is considered an advanced endoscopic procedure with a steep learning curve. Barsness et al., after having fabricated a novel EA/TEF simulator repair with high cost and need for animal tissue, proceeded in producing a low-cost repair simulator of a C-type EA/TEF, which includes proximal EA with distal TEF, through 3D printing of the molds, filling them with silicone and placing them in a model of neonatal thorax. Trainees, both experienced and novice surgeons, claimed this simulator to be a flexible solution and a new promising tool in field of thoracoscopic EA/TEF repair training, especially for less experienced surgeons .

Furthermore, 3D printing could be highly useful in education of surgeons regarding colorectal and anal disorders. Arguably, anal fistulas can be a challenge in colorectal surgery, even for experienced surgeons, since they are accompanied by complex anatomy and route, close relation with sphincters, all of which may lead to severe complications and high rates of recurrence, even after state-of-the-art primary treatment. In the field of Coloproctology, Bangeas et al. was the first to endeavor to evaluate the impact of ten diverse 3D printed models of anal fistulas based on MRI images on anal fistula understanding by final year residents both pre- and postoperatively. Participants were divided into two groups, based on studying MRI images or the 3D printed replicas. After completing the fistula assessment test, residents who had studied fistula replicas achieved higher scores. Positive answers regarding other parameters, such as enjoyment, educational effectiveness and utility, originality and ethical issues were also more common among participants who studied 3D printed models. It is worth mentioning that the cost of each single model was approximately 3–5 Euros, enabling use of additive manufacturing technology for training clinicians in developing countries .

In the field of endocrine surgery, advances in radiographic imaging, mainly ultrasound and CT and ultrasound or CT-guided cytology examinations, have led to a significant increase in detection of patients who need to be treated. With regard to the field of thyroid pathology, fine-needle aspiration cytology (FNAC) contributes critically in the therapeutic decision and treatment and is necessary for nodules larger than 1.0 cm or with suspicious US findings and thyroid cysts. Herein, it is strongly advisable to assure accuracy, confidence, and constant skill maintenance of the technique, since it presents a high rate of complications due to close spatial relationships of the thyroid gland with the jugular veins, carotid arteries, and the trachea. Baba et al. used CT data to construct a cervix. The originality of this study lies in the combination of mold-based fabrication and 3D printing procedures, since the group constructed 3D printed models and filled them with polymers, mainly agar, so that organs of region developed after mold removal. This strategy assures that once templates have been fabricated, replicas can be easily produced, resulting in markedly lower cost and repeated practicing. Models produced were used by 45 medical students, residents, fellows, and thyroid experts for performing FNAC. The evaluation questionnaire revealed that all participants found the FNAC simulation model excellent for educational purposes and, also, a large number of medical students gained interest in thyroid diseases, resulting in a more profound learning procedure .