Introduction
Three-dimensional (3D) printing, also known as rapid prototyping or additive manufacturing, is the new frontier of personalised medicine and surgery. The manufacturing process involves the construction of models layer-by-layer, allowing for the production of intricate structures that would otherwise be too complex by traditional means. Within the last decade, the ability of 3D printing to easily produce patient-specific 3D models has empowered surgeons in a vast field of applications, from treating life-threatening tracheobronchomalacia with a bio-resorbable airway splintReference Zopf, Hollister, Nelson, Ohye and Green1 to pre-operative plate bending in acute midface trauma.Reference Dorrity and Odland2 Whilst maxillofacial and orthopaedic surgery lead the field in implant and pre-operative applications,Reference Martelli, Serrano, van den Brink, Pineau, Prognon and Borget3 within otolaryngology the principal utilisation of 3D printed models to date has been in the production of surgical simulators.
The traditional Halstedian apprenticeship model of ‘see one, do one, teach one’ faces significant challenges in ensuring trainees gain adequate experience in a specialty's full case mix through purely opportunistic encounters.Reference Shaharan and Neary4 Thus, increasingly, surgical educators have turned to a complement of other training modalities to achieve cost-effective means of training confident surgeons without compromising patient safety.Reference Forgione and Guraya5 Live animal and cadaveric models offer the highest level of fidelity in simulation, but are restrictive in cost and accessibility in light of ethical, legal and biohazardous storage issues.Reference Forgione and Guraya5 Hence, the use of procedural simulators has become a crucial part of modern surgical education. The adoption of this modality of teaching allows for the efficient development and assessment of trainee skills in a diverse range of clinical scenarios.Reference Forgione and Guraya5
Otolaryngologists have recognised that 3D printing technology is uniquely positioned for the production of surgical simulators, offering the potential to create models with any anatomical or pathological variation of adult or paediatric size, for any surgical task. Furthermore, the manufacturing of these models on a consumer scale is increasingly affordable, because production costs continue to decrease as the technology matures.Reference Coles-Black, Chao and Chuen6
This article provides an overview of the current state of play and the future of such models’ applications, as otolaryngology develops tools to enhance the surgical skills of its trainees.
Materials and methods
A literature search of the Medline and Embase databases was performed using the terms ‘3D printing’, ‘otolaryngology’ and ‘simulation’. Articles available in English language, published within the past 10 years, which met the inclusion criterion of describing 3D printed models utilised for surgical education, were appraised. Articles that did not report the results of the usage of such models as simulators were excluded, as were 3D printed models used for other purposes such as pre-operative planning. Our last search was conducted on 16 July 2019.
Results
Twenty-two papers were identified for inclusion in the study (Figure 1).Reference Da Cruz and Francis7–Reference Ha, Morrison, Green and Zopf28 Of these articles, eight (36 per cent) were prospective cohort studies.Reference Nguyen, Mamelle, De Seta, Sterkers, Bernardeschi and Torres11,Reference Barber, Kozin, Dedmon, Lin, Lee and Sinha12,Reference Ding, Yi, Jiang, Xu, Yan and Zhang14,Reference Yoshiyasu, Chang, Bunegin, Lin, Aden and Prihoda16,Reference Alrasheed, Nguyen, Mongeau, Funnell and Tewfik17,Reference AlReefi, Nguyen, Mongeau, Haq, Boyanapalli and Hafeez21,Reference Gauger, Rooney, Kovatch, Richey, Powell and Berhe23,Reference Ha, Morrison, Green and Zopf28 The cohort studies objectively measured surgical skill (utilising a septoplasty model, a functional endoscopic sinus surgery model, a transcanal endoscopic ear surgery model, an otosclerosis prosthesis model, a needle cricothyroidotomy model and a costal cartilage airway grafting model), and demonstrated varying levels of construct validity. The other 14 articles (64 per cent) were cross-sectional studies.Reference Da Cruz and Francis7–Reference Rose, Kimbell, Webster, Harrysson, Formeister and Buchman10,Reference Monfared, Mitteramskogler, Gruber, Salisbury, Stampfl and Blevins13,Reference Hsieh, Cervenka, Dedhia, Strong and Steele15,Reference Chang, Lin, Bowe, Bunegin, Weitzel and McMains18–Reference Cote, Schwartz, Arbouin Vargas, Canfarotta, Kavanagh and Hamdan20,Reference Al-Ramahi, Luo, Fang, Chou, Jiang and Kille22,Reference Barber, Kozin, Naunheim, Sethi, Remenschneider and Deschler24–Reference Chari and Chan27 These reported subjective measures demonstrating largely positive outcomes in terms of anatomical fidelity, haptic feedback, and value in the translation of surgical skill to the operating theatre. Several studies reported unanimous interest for integration into the curriculum.Reference Barber, Kozin, Dedmon, Lin, Lee and Sinha12,Reference Chang, Lin, Bowe, Bunegin, Weitzel and McMains18,Reference Barber, Kozin, Naunheim, Sethi, Remenschneider and Deschler24
Fig. 1. Flowchart of the literature search.
Discussion
Printing workflow
In order to generate an end-product of a 3D printed simulator, medical imaging datasets, such as from computed tomography or magnetic resonance imaging, are processed using medical image processing software. This process utilises both automated and manual methods to outline the anatomical structures of interest that form the basis of the 3D model. Computer-aided design software can be used to further refine the model, after which it can be physically printed.
Printing materials and cost
Three-dimensional printing encompasses several different technologies and materials, with dramatically different implications for mechanical properties and cost.
Stereolithography was the first 3D printing process developed. It boasts favourable cost and resolution; however, the resin models produced are not widely used in otolaryngological simulators because of their undesirable haptic feedback profile when drilled.
Conversely, fused deposition modelling uses thermoplastics such as acrylonitrile butadiene styrene and polylactic acid to produce extremely low cost models. These include the acrylonitrile butadiene styrene temporal bone for USD$1.92, created by Mowry et al.,Reference Mowry, Jammal, Myer, Solares and Weinberger9 at the expense of resolution.
Inkjet and Polyjet printing technologies are favoured amongst the reviewed studies. These offer high-resolution models made from photopolymers, such as the Rose et al.Reference Rose, Kimbell, Webster, Harrysson, Formeister and Buchman10 temporal bone model (USD$400), which feature good haptic feedback and a multi-coloured model at the price of high-end initial printer and material costs.
Silicone moulding is a technique commonly combined with 3D printed negative moulds to create realistic soft tissue structures at extremely low costs. These include the costal cartilage models created by Ha et al.Reference Ha, Morrison, Green and Zopf28 for USD$0.60 each, and the endoscopic nasal surgery simulator created by Chang et al.Reference Chang, Lin, Bowe, Bunegin, Weitzel and McMains18 for USD$21.
Simulator types
Over the past decade, the application of 3D printing in otolaryngology has produced a significant variety of simulators, as detailed in Table 1.Reference Da Cruz and Francis7–Reference Ha, Morrison, Green and Zopf28
Table 1. Existing 3D printed simulators in otolaryngology
* Cost included if described in article. 3D = three-dimensional; ABS = acrylonitrile butadiene styrene; FDM = fused deposition modelling; FESS = functional endoscopic sinus surgery; PLA = polylactic acid
Temporal bone models
One of the earliest models in otolaryngology was a temporal bone simulator produced by Suzuki et al.Reference Suzuki, Ogawa, Kawano, Hagiwara, Yamaguchi and Ono29 in 2004, which has since become the most widely reported simulator.Reference Da Cruz and Francis7–Reference Nguyen, Mamelle, De Seta, Sterkers, Bernardeschi and Torres11,Reference Kasbekar, Narasimhan and Lesser30,Reference Kozin, Barber, Wong, Kiringoda, Kempfle and Remenschneider31
As 3D printing technology advanced and educators became more creative in their simulator designs, models have demonstrated gains in anatomical fidelity and function. Rose et al.Reference Rose, Kimbell, Webster, Harrysson, Formeister and Buchman10 created a multi-material and multi-coloured temporal bone model that revealed facial nerve and carotid arteries clearly if improperly dissected, displaying the potential for trainer model realism to be further refined. Nguyen et al.Reference Nguyen, Mamelle, De Seta, Sterkers, Bernardeschi and Torres11 modified commercial 3D printed temporal bones with force sensors attached to 3D printed ossicular bones for augmented stapes fixation training, showing the capacity of simulators to teach highly specialised skills through the creative modifications of existing models. Kozin et al.Reference Kozin, Barber, Wong, Kiringoda, Kempfle and Remenschneider31 coupled a 3D printed temporal bone with a commercial virtual reality skull base navigation system, demonstrating the role of physical simulators in the developing field of virtual reality in surgical teaching.
Whilst the aforementioned simulators have all featured temporal bones, the diversity of their applications exemplifies the versatility of being able to create a model to suit any teaching need of the surgical educator.
Other simulators
Other simulators that have been created and assessed for their utility in surgical teaching include: skull base models,Reference Hsieh, Cervenka, Dedhia, Strong and Steele15,Reference Narayanan, Narayanan, Rajagopalan, Karuppiah, Rahman and Wormald19,Reference Chan, Siewerdsen, Vescan, Daly, Prisman and Irish32 endoscopic nasal surgery simulators,Reference Hsieh, Cervenka, Dedhia, Strong and Steele15,Reference Chang, Lin, Bowe, Bunegin, Weitzel and McMains18,Reference AlReefi, Nguyen, Mongeau, Haq, Boyanapalli and Hafeez21 cleft palate surgery models,Reference Cote, Schwartz, Arbouin Vargas, Canfarotta, Kavanagh and Hamdan20 endoscopic ear surgery simulators,Reference Barber, Kozin, Dedmon, Lin, Lee and Sinha12,Reference Monfared, Mitteramskogler, Gruber, Salisbury, Stampfl and Blevins13 airway simulators with difficult anatomy for bronchoscopy,Reference Al-Ramahi, Luo, Fang, Chou, Jiang and Kille22 models for laryngoscopy and microlaryngoscopic procedures,Reference Kavanagh, Cote, Tsui, Kudernatsch, Peterson and Valdez25 tracheoesophageal puncture with prosthesis placement models,Reference Barber, Kozin, Naunheim, Sethi, Remenschneider and Deschler24 needle cricothyroidotomy models,Reference Gauger, Rooney, Kovatch, Richey, Powell and Berhe23 and costal cartilage models for airway graft carving.Reference Ha, Morrison, Green and Zopf28
Model validation
Given the scarcity of time allotted for training, it is imperative that new teaching methods are proven to be effective. If simulators are to be used to complement surgical education, their validity should be demonstrated. The validity of a simulator can be expressed in terms of face validity, content validity, construct validity and predictive validity.Reference Gallagher, Ritter and Satava33 These forms of validity help to confirm whether a simulator truly does train or assess the skills it claims to.
Face validity is ‘validity that is assessed by having experts review the contents of a test to see if it seems appropriate’.Reference Gallagher, Ritter and Satava33 It is a highly subjective measure, assessed with surveys that simply express whether the evaluators think the simulator is an accurate facsimile. The surveys focus on anatomical fidelity and haptic feedback in comparison to current ‘gold standards’ of teaching or to procedures in the operating theatre. It was the most common validity type measured in the articles reviewed (64 per cent) because of its ease of assessment. Whilst simulators received mostly positive feedback regarding anatomical fidelity, finer structures like trabecular bone in the mastoidReference Hochman, Rhodes, Wong, Kraut, Pisa and Unger8 have been found to be lacking. Deficiencies in anatomical accuracy may represent a limitation of the imaging datasets from which models were derived, the segmentation process or the type of 3D printer used.
Content validity is ‘an estimate of the validity of a testing instrument based on a detailed examination of the contents of the test items’.Reference Gallagher, Ritter and Satava33 This remains a subjective measure, often determined using post-simulation survey data that describe whether participants felt the model improved task-specific surgical skills and increased confidence. Content validity was assessed simultaneously with face validity in post-simulation Likert surveys in the articles reviewed (64 per cent), with highly positive results for all simulators. Studies by Barber et al.Reference Barber, Kozin, Dedmon, Lin, Lee and Sinha12,Reference Barber, Kozin, Naunheim, Sethi, Remenschneider and Deschler24 and Chang et al.Reference Chang, Lin, Bowe, Bunegin, Weitzel and McMains18 even report unanimous support for the inclusion of simulators into the existing curriculum. However, trainees were reluctant to endorse the replacement of existing teaching methods such as cadaveric temporal bones with 3D printed models.Reference Hochman, Rhodes, Wong, Kraut, Pisa and Unger8
Construct validity is ‘a set of procedures for evaluating a testing instrument based on the degree to which the test items identify the quality, ability, or trait it was designed to measure’.Reference Gallagher, Ritter and Satava33 This is an objective measure that only a few of the reviewed studies attempted to assess (36 per cent), but it provides a stronger indication of the utility of 3D printed simulators than subjective measures. Articles that assessed construct validity are detailed in Table 2.Reference Nguyen, Mamelle, De Seta, Sterkers, Bernardeschi and Torres11,Reference Barber, Kozin, Dedmon, Lin, Lee and Sinha12,Reference Ding, Yi, Jiang, Xu, Yan and Zhang14,Reference Yoshiyasu, Chang, Bunegin, Lin, Aden and Prihoda16,Reference Alrasheed, Nguyen, Mongeau, Funnell and Tewfik17,Reference AlReefi, Nguyen, Mongeau, Haq, Boyanapalli and Hafeez21,Reference Gauger, Rooney, Kovatch, Richey, Powell and Berhe23,Reference Ha, Morrison, Green and Zopf28 Primarily, construct validity has been shown by observing a difference in task completion, through comparing time taken and/or error rates, between different groups with varying experience (trainees vs consultants). Seven out of eight studies that attempted to show construct validity succeeded. However, Ha et al.Reference Ha, Morrison, Green and Zopf28 were unable to show a significant difference between experts and trainees, citing heterogeneity of the participant groups (amongst other confounding factors) as a potential reason.
Table 2. Construct validity of 3D printed simulators in otolaryngology
3D = three-dimensional; FESS = functional endoscopic sinus surgery; PGY = post-graduate year
Predictive validity is ‘the extent to which the scores on a test are predictive of actual performance’.Reference Gallagher, Ritter and Satava33 This form of validity is objective; it provides the most conclusive support that a simulator will result in improved clinical outcomes for trainees’ patients and is therefore considered the gold standard method of evaluating a training method before implementation into training programmes. However, it is also the most difficult type of validity to assess, requiring significant follow-up time. Given the relative infancy of 3D printed simulators, to date no publication in the field has attempted to assess predictive validity.
It seems intuitive that physical surgical simulators will be beneficial to clinical outcomes, giving trainees more opportunities to practise procedural tasks before attempting them on a patient. However, there remains a clear need for objective evidence, which is currently lacking in the otolaryngological literature, to support the adoption of these training tools.
Limitations
Barriers to adoption
Three-dimensional printed simulators face several practical barriers before they become more widely adopted in surgical training. The segmentation of the 3D models from medical imaging data and subsequent refinement requires specific skills in 3D modelling software that may be inaccessible without detailed instruction. Also, there may be large initial costs to 3D printing technology; printer prices currently range from a few hundred dollars to tens of thousands of dollars. These issues may be eased by the co-operation of multiple hospitals in a shared 3D printing facility operated by specialists.
Research methodology
The over-reliance on face and content validity types as subjective measures of the utility of 3D printed simulators in otolaryngology training is an issue faced by the field of surgical education as a whole.Reference Kostusiak, Hart, Barone, Hofmann, Kirollos and Santarius34 Whilst considerations must be made as to whether certain study designs are feasible, objective evaluations of skill acquisition will supersede subjective evaluations. The gold standard of randomised, controlled trials will be difficult to accomplish for multiple reasons, including heterogeneity in the experience levels of the participants and the inability to blind participants when comparing against the simulators currently used in the curriculum.Reference Kostusiak, Hart, Barone, Hofmann, Kirollos and Santarius34
Therefore, single-subject designs that expose each subject to interventions and make comparisons amongst subjects are most preferred, with participants acting as their own control.Reference Kostusiak, Hart, Barone, Hofmann, Kirollos and Santarius34 Participant performance can be objectively assessed using quantitative measurements such time taken, error rate, number of corrective manoeuvres, and scores on task-specific checklists or global rating scales, amongst other validated assessment tools.Reference Kostusiak, Hart, Barone, Hofmann, Kirollos and Santarius34 Nevertheless, there remain merits in utilising subjective measurements; confidence-based marking and self-marking are strongly correlated with test performance,Reference Kostusiak, Hart, Barone, Hofmann, Kirollos and Santarius34 which may be confirmed using correlation analyses.
Conclusion
The early-phase adoption of 3D printed simulators in otolaryngology has seen the production of a wide variety of simulators, with enthusiastic reception from trainees and experts alike. As models become more refined and the barriers to 3D printing lowered, their use in surgical simulation will continue to expand and become commonplace in surgical skills acquisition.
Acknowledgements
The authors would like to thank the staff at Austin Health Services Library, Melbourne. This research was made possible by grant funding received from the Harold and Cora Brennen Benevolent Trust.
Competing interests
None declared
