Air quality assessment of dissecting a J750 three-dimensional printed temporal bone and recommendations 2024

Introduction

Microsurgical competency in multiple temporal bone exercises is a requirement of Australasian Otolaryngology, Head and Neck surgery advanced training (1). This has traditionally required human cadaveric material or alternatively live supervised dissection (which can often be time-restrained). Access to cadaveric dissection in Australia unfortunately has become limited due to ethical considerations, cost, and availability of cadavers. Therefore, the use of a three-dimensional (3D) printed model which is transportable, has a low biohazard risk, and is readily available with a lower cost has attracted increasing interest for use as a comparable tool to train junior surgeons and surgical registrars (2-7). Advances in virtual reality simulation have also provided some benefits in training. However, this is not superior to practical dissection for trainees to learn the nuances and haptics of drilling (8). Printed models are also superior to virtual reality tools in that there are visual anatomical cues in dissection (such as thinned bone overlying the sigmoid sinus) (9).

The fabrication technology of 3D printing, which is a layer-by-layer construction of a model by a selected printer with a selected material, can be used to design a form from a digital image or model and print intricate shapes with various materials (10). This can be very useful as technologies improve in anatomical accuracy. Bone et al. (11) described the anatomical accuracy of 3D printed temporal bones by using fused deposition modeling (FDM) with the use of acrylonitrile butadiene styrene (ABS) plastics in a content validity study and compared the computed tomography (CT) scans of the printed products with the scans that the models were made from. They were found to be within 0.46 and 0.66 mm in difference at the cortical mastoid bone and external auditory canal compared to the scanned anatomy. There are various 3D printer technologies and materials that can be utilised with their own varying costs, properties, and restrictions (3,12). In medical use, 3D printing has multiple uses including simulation for training, biofabrication for patient-specific devices, and patient-specific surgical practice (7,13-18).

Aerosols produced during temporal bone dissection as a result of high speed drilling. Although some materials have been examined for fumes during dissection by Freiser et al. (19), our focus was on the aerosols generated by dissecting multi-material (Stratasys J750, Stratasys, Eden Prairie, MN, USA; Rehovot, Israel) printed temporal bone, with anatomically superior performance.

Methods

The study was approved by the Gold Coast University Hospital Human Research Ethics Committee (approval ID: HREC exemption EX/2022/QGC/88473/GCUH Ethics Committee/quality assessment activity).

Stratasys J750 multi-material printer used for temporal bone models

A literature search showed no results on measured air quality parameters or VOCs whilst drilling 3D-printed models with multi-material jet printers. The Stratasys J750 Digital Anatomy Printer (DAP) was selected for printing the temporal bone due to its multi-material printing qualities where a combination of different resin materials is jet printed in a layer-by-layer fashion and cured to represent various haptic qualities of anatomical regions. See Appendix 1 for details on modelling, printing and cleaning.

To compare the air quality and aerosolization of materials on drilling the Stratasys J750 model, three other material printers and models were selected to compare aerosol exposure. ABS and polylactic acid (PLA) temporal bone models were printed with FDM printers, the Ultimaker 2+ with a 6-mm infill. Support material was manually removed, and no caustic substances were required for this cleaning. A white resin-based SLA was printed using a Formlabs Form 2 SLA printer. The resin print was cleaned with isopropyl alcohol as per protocol. All models were printed at the same size of 110% larger than the stereolithiography (STL) file as seen in Figure 1. The printed models were mounted in a metal cadaveric temporal bone holder and stabilised using a silicone head ring.

Figure 1 3D printed temporal bones used for aerosolization study. From left to right: ABS FDM print, multi-material J750 print, acrylic resin SLA print, PLA FDM print. 3D, three-dimensional; ABS, acrylonitrile butadiene styrene; FDM, fused deposition materials; SLA, stereolithography; PLA, polylactic acid.

Laboratory setup and measurement protocol

The laboratory was set up similarly to the arrangements in a temporal bone dissection course, with a Medtronic MR8 or Anspach drill and a drilling speed of 80,000 rpm, a Zeiss Pentero operating microscope set up for otology dissection, silicone head ring and metal temporal bone holder beneath the microscope. One otolaryngology registrar dissected each sample for a cortical mastoidectomy. One sample was used of each material due to a limitation of material expense and the cost of hiring air quality testing services. The 6-mm cutting burr was used for the cortical mastoidectomy procedure on each sample. An occupational health and safety company (Cetec) with expertise in air quality testing was contracted with their equipment to perform air quality testing and to make recommendations with accordance of Safe Work Australia (SWA) occupational health and safety. Individual volatile organic compounds (VOCs), total respirable dust particles (dust particles small enough to enter nasal and respiratory mucosa of larger airways) and inhalable dust particles (dust particles small enough to enter gas exchange areas such as alveoli) were tested at the level of the collar of the training surgeon with three wearable air quality sensors (respirable particles: AS3580.9.3; inhalable particles: AS3580.9.3 and VOCs AS 2986/NIOSH1501) as seen in Figure 2. Particulates are measured in parts per million or micrograms per meter cubed over a selected time period and measured against the national standard (SWA) for these particles. Given that the Stratasys J750 multi-material printed model was printed with materials from varying quantities of each cartridge to create each ‘anatomical part’, it was difficult to predict which volatile organic substances may be encountered on heated drilling of these materials. Therefore, the chemical composition of each cartridge used for the model (Stratasys VeraWhite, Stratasys VeraBlue, and Stratasys Agyllis) were given to the occupational hygienist team (Cetec) which recommended analysis and testing for the following VOCs; acetone, acrylonitrile, methylethylketone, hexane, dichloroethane, cyclohexane, ethylacrylate, trichloroethane, toluene, tetrachloroethene, n-butylacetate, chlorobenzene, ethylbenzene, m-p-xylene, styrene, alpha-xylene, nonane, isopropylbenzene, diisobutylketon, alpha-methyl styrene, decane, benzylchloride, naphthalene.

Figure 2 Dissection of the J750 model with dust particles seen on drilling and the air quality samplers attached to the collar.

The ABS, PLA and SLA models were dissected (cortical mastoid dissection) for 10 minutes with rpm of 80,000 on the Anspach drill without irrigation (to simulate a worst case scenario of aerosilisation). This resulted in melting of each of these materials and overheating of the drill. These models did not contain small soft structures such as the facial nerve. The air quality was tested for the values of individual VOCs, total respirable dust particles and inhalable dust particles at the level of the collar of the dissecting surgeon with wearable vapour badges, and the total VOCs and total particulate (TP) count at 20 cm. The Stratasys J750 multi-material printed model was dissected (cortical mastoidectomy) with identification of the facial recess, facial nerve, lateral semi-circular canal, sigmoid sinus, tegmen and exploration of the middle ear by a novice training surgeon over a 30-min period using the Anspach drill without irrigation to simulate the stop and start actions of a training surgeon’s dissection. Although the drill did overheat, there was no melting of the materials. The study for the Stratasys J750 model was repeated with the same conditions, however with a Medtronic MR8 drill and with standard irrigation and suction to replicate a surgical trainee’s dissection in a temporal bone dissection laboratory. Local ventilation extraction was another variable factor and was not used in the study to identify the ‘worst-case scenario’ of a dissection in a room without air extraction. The door and windows of the laboratory remained closed during the study and the training surgeon wore a standard laboratory coat, eye goggles, an N95 respirator mask, and non-sterile gloves. The anylates of the VOCs, inhalable particles and respirable particles’ were tested on sampling equipment.

Statistical analysis was performed using IBM SPSS version 20.0. software. Data distribution was determined visually using scatterplots. Due to the limited number of repeated tests, meaningful distribution of data was not quantified.

Questionnaire

At the 2023 Australasian Society of Otolaryngology, Head and Neck Surgeons temporal bone dissection course, 14 otolaryngology training registrars were invited to complete a questionnaire after dissecting the J750 Stratasys multi-material printed temporal bone model. Particpants were asked to identify whether they noted a distracting odour during dissection on a Likert scale. One dissector did not complete the survey. The answers of the other 13 participants are referenced in the chart below.

Results

Table 1 summarises the findings of each analyte or VOC on the dissection of the Stratsays J750 multi-material printed model with and without irrigation and the recommended upper limit exposure to these compounds (SWA). Table 2 includes the inhalable and respirable particles tested on the Stratasys J750 model with and without irrigation and the recommended upper limit exposure to these compounds (SWA).

Questionnaire result

“I noted a distracting odour during dissection.”

The study showed that the air quality (inhalable and respirable particles, and individual VOCs) at the level of the collar of the surgeon or dissector of the model, using a high-speed drill over a 30-min period, within a closed space with no ventilation was within the limits of SWA and best practice guidelines. When the test was repeated with irrigation, some VOCs were detected at higher quantities (acetone, toluene, ethylbenzene, styrene, naphthalene, trichloroethane, and xylene). However, this made no difference in reaching the maximal limit of exposed VOC. The respirable dust and inhalable dust particulate count was also not repeated with irrigation or the EMR8 drill.

For the ABS, PLA and SLA models, the materials were deemed unsuitable materials for drilling without irrigation for a cortical mastoidectomy due to the melting of the materials and testing was not repeated with irrigation due to the superior characteristics of the multi-material printed model for anatomical accuracy in the dissection practice. However, the spot testing of TP count and total VOCs remained well within the SWA and best practice limits.

On answering a questionnaire on a Likert scale, as seen in Figure 3, 46% of participants strongly disagreed to noticing a distracting odour, however 8% of participants strongly agreed and 23% agreed to noticing a distracting odour during dissection. This highlights the variation in olfaction between dissectors and does not necessarily equate to harmfulness of substances.

Figure 3 Bar chart of response to participants noting a distracting odour during dissection.

Discussion

In otolaryngology (American Acadamy of Otolaryngology and Head and Neck Surgery), a recent review of published articles (2) found 36 studies that used 3D printing technologies to produce models for otology and temporal bone dissection for surgical training. These included various printing methods and materials, and various testing methods to identify the model’s suitability for training use. The anatomical similarity and accuracy of these models to CT imaging and cadaveric bones are of substantial quality to enable their use for surgical training and practice (20). Chauvelot et al. (20) validated the outcome of printing complex anatomy by the manufacturing process of bi-material printed models. The study analysed the STL files from the segmentation process and finalised printed models to the initial CT scan and revealed anatomical accuracy of 0.04 and 0.42 mm for soft tissues and 0.09 to 0.24 mm for hard tissues.

Printed models can also be used for patient or cadaver-specific surgical practice (21). For example, Freiser et al. (19) identified that surgeons were able to practice dissection on a printed model and create a mental-memory image of the arcuate eminence for a corresponding temporal bone in a middle cranial fossa approach (22). Frendø et al. identified the use of surgical practice for cochlear implantation and illustrated that the surgical practice of dissection on a temporal bone to be superior to virtual reality simulation when dissections of training surgeons were evaluated by senior surgeons (14).

Remaining filler materials in photopolymerization models remained an issue for the observance of the mastoid air space cells (23). However we employed the use of drainage holes, described by Takahashi et al. (24) to facilitate post printing removal of the filler materials and allow for voided mastoid air cells. Future developments will include allowing removal of this filler material from the otic capsule and allow for cochleostomy into a voided space.

In terms of air quality testing during drilling or dissecting these materials, few studies investigated or compared potential air quality hazards with temporal bone dissection. The largest comparative study of dissection of materials conducted by Freiser et al. (19) which compared FDM-printed PLA, polyreactive acrylic resin (PAR) and ABS models during dissection and measured the exposure of VOCs and TPs with a sampling badge worn by the surgeon at the level of the collar. The exposure of VOCs when these models were drilled continuously for 30 mins at 80,000 rpm, with irrigation was measured. The variables of local extraction or ventilation were excluded from the study. Though none of the materials tested had VOCs above the occupation health and safety standards, recommendation for local extraction ventilation was made due to the high volume of ‘bone dust’ produced by the dissection. In a simulated dissection, however, the registrar would intermittently stop drilling while identifying the anatomical landmarks which supports the hypothesis that in a surgically simulated dissection, the dissector would have less exposure to these particles and compounds.

Rose et al. (25) investigated multi-material printed temporal bones with an Objet350 for surgical simulation and evaluated the model’s performance as a training tool with a Likert scale questionnaire. Attendings and residents answered a single yes or no question of whether or not they experienced irritation to the mucus membranes, eyes, lungs, or skin. None of the participants described irritation during the dissection. However, this study did not quantify or measure aerosolised substances (VOCs or particles) which may not necessarily cause noticeable irritation to mucus membranes but could still be harmful on inhalation or as a result of exposure.

Some materials and printing methods require caustic substances for the post-processing or cleaning process of the prints prior to the final model’s use. As an example, acrylic resin prints require cleaning with isopropyl alcohol. Multi-material models such as the Stratasys J750 printed models require cleaning in sodium hydroxide. However, multi-material prints were not examined in the Freiser et al. study (19).

Another important finding when drilling printed materials, such as ABS and PLA, is that materials tend to overheat the drill and cause a melting effect rather than a dissecting through the materials and these materials are not suitable for high-speed drill dissection. When compared to the Freiser et al. study (19), the following analytes were similarly compared and found to be within the safe limits: methylethylketone, hexane, cyclohexane, toluene, ethylbenzene, styrene, naphthalene, trichloroethane, methyl-isobutyl ketone and xylene.

Of note, on the questionnaire given to otolaryngology surgical registrars for feedback on their dissection of the J750 multi-material model, most participants strongly disagreed to noting a distracting odour during the dissection. However almost a quarter of participants agreed to noticing a distracting odour. This reflects the wide varying perceptions of smell on dissecting these models but does not indicate whether participants were concerned about the odour or vapours. It is noted that participants also answered this questionnaire after they had dissected cadaveric materials which could have also affected their answer to this question.

Given the small sample size of two repeated tests of dissection of a multi-material Stratasys J750 print, and the short duration of time for measuring air quality at each test, there are limitations in conducting a meaningful statistical analysis of the data and future research would benefit from further funding and increasing the number of models dissected and measuring the air quality over a longer period of time, as most otolaryngology registrars would dissect a temporal bone over several hours of the day. The air quality testing was limited to 30 mins for this study. A complete temporal bone dissection with all procedures was not conducted beyond the cortical mastoidectomy due to a time limitation and cost with hired services aerosol testing services. It is expected that the aerosolization of particles would be greater at the start of the dissection where continuous and rigorous drilling are required to dissect the harder material parts, representing the mastoid bone during cortical mastoidectomy, compared to the slower exploration of the middle ear or the sharper dissection of the softer acoustic canal nerves.

This study was also reliant on the dissecting ability of a single otolaryngology trainee and not the wider cohort group of junior surgical trainees that this would be applicable for, who may drill faster, slower or with interruptions rather than continuous drilling. A future study could include multiple surgical training registrars in various levels of training (novice, intermediate and competent) which would give a broader average on dissecting speed and time to complete all the procedures on a model.

Although the concern for aerosolization of particulates and VOCs has been examined and considered for drilling 3D printed materials, these similar tests have not been tried on cadaveric temporal bones which are often prepared in preservatives such as formaldehyde which are known to have exposure concerns and require adequate room ventilation (26). This testing was outside of the limits of our expense for this study. A future similar study could compare the analytes and dust particles produced by cadaveric dissection and compare these with 3D printed materials.

Although smaller dust particles were not significant as to be identified on the spot test, an important finding was the significant larger particulate ‘bone dust’ identified on dissecting with a high-speed drill as seen on Figure 4. These were found to be of little annoyance to the dissectors and were large enough to not be aerosolised, however further analysis of this bone dust and its implications could be considered in a future study.

Figure 4 Bone dust during dissection of multi-material J750 model.

Conclusions

Our study revealed that the multi-material (Stratasys J750) model, which best replicates the temporal bone properties for dissection and anatomical identification, can be dissected with a high-speed drill within the safe limits of aerosolization exposure even with worst case scenario settings (no irrigation or ventilation) within a short period of time (30 mins). Large studies and further fundings are required to support this result over a longer period of time and to compare the aerosolised particles and substances produced dissection of a cadaveric temporal bone. Recommendations emanating from this study, similar to Freiser et al. (19), are to consider using personal protective equipment such as a surgical mask and goggles, the use of continuous irrigation with water or normal saline to cool the drill when dissecting materials and to consider soaking the model in water for 20 mins prior to dissection. We also recommend the utilisation of local ventilation/extraction, such as a Nederman arm, or room ventilation extraction to minimize exposure to particulates and odours in the air.

Acknowledgments

The authors would like to thank Dr. Michael Redmond, Director of the Craniofacial and Neurosurgical Division, Herston Biofabrication Institute, Metro North Hospital and Health Service for contributing valuable workforce resource to test and print the materials. The authors would also like to thank Dr. Andrew Lomas, Department of Otolaryngology Head and Neck Surgery, Royal Brisbane and Women’s Hospital for contributions to the dissection equipment and advice on the quality of the study. The authors would also like to thank Zeiss and Medtronic for contributing equipment used in this study.

Funding: None.

Footnote

Data Sharing Statement: Available at https://www.theajo.com/article/view/10.21037/ajo-24-25/dss

Peer Review File: Available at https://www.theajo.com/article/view/10.21037/ajo-24-25/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://www.theajo.com/article/view/10.21037/ajo-24-25/coif). E.K., J.B., D.F., R.F. and M.W. are receiving support from Herston Biofabrication Institute (HBI; Herston Biofabrication Institute, Metro North Hospital and Health Service, Brisbane, Queensland, Australia), which is the location of corresponding author’s University of Queensland MPhil Candidate placement. E.K. reported the use of Specialist Training Program Commonwealth grant funding for travel to temporal bone course in Brisbane, from trainees place of work (Townsville). J.B. is University of Queensland (UQ) Supervisor for UQ Masters of Philosophy Candidate. D.F. is a Herston Biofabrication Institute Senior Biomedical Engineer and Industrial Designer, and an Adjunct Research Fellow of the University of Queensland. R.F. is an HBI Senior Biomedical Engineer. J.M.B. provides knowledge of skull base anatomy advised on designing model as part of fellowship position, Princess Alexandra Hospital, Brisbane, at Queensland Health. M.W. is Clinical Director of HBI. HBI provides support with printing and drilling materials and equipment as well as engineering and design expertise. HBI received equipment on loan from Zeiss and from Medtronic. There is potential for HBI to produce 3D Printed models commercially in the future. Project intellectual property (IP) is owned by Metro North Hospital and Health Service and The University of Queensland as tenants in common in shares proportionate to their respective inventive contribution to the development or creation of that Project IP. Payment for materials, labour and testing was made by the Craniofacial Unit at the Royal Brisbane and Women’s Hospital, Brisbane, QLD, Australia. Payment for production of models was made to Herston Biofabrication Institute by the Fiji group of OHNS for dissection of this model on subsequent study. Cetec was hired by Herston Biofabrication Institute to conduct Air Quality Testing. All authors received the equipment on loan from Zeiss, Medtronic and Herston Biofabrication. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was approved by the Gold Coast University Hospital Human Research Ethics Committee (approval ID: HREC exemption EX/2022/QGC/88473/GCUH Ethics Committee/quality assessment activity).

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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