In the present study, we assessed the effect of the sterilization process on the dimensional stability of FDM 3D printed anatomical models and cutting guides and accuracy to the original design. The mean differences in dimensional stability after sterilization with VH2O2 in both groups, models, and guides, were under ± 0,5 mm and ± 0,05 mm, respectively. The analysis revealed that the low-temperature hydrogen peroxide sterilization effect over the medical-grade ABS models and guides was sub-millimetric, assuring the dimensional stability after the process. Likewise, the mean differences in the accuracy of the models and guides after sterilization to the original design were under ± 1 mm and ± 0,25 mm, respectively. These findings indicated that even though the 3D-printing and posterior sterilization process could have caused dimensional errors in the models and guides, the fidelity to the original design was maintained. The shape and dimensional accuracy remained high.
The stereolithographic anatomical models are generally used for surgical preparation, training, and educational and consultation purposes. Yet, they are often needed in the operating room as a visual aid for the surgeon and are often in contact with the patient (11–14). Cutting guides are used in both the surgical simulation and the actual procedure. These guides come in contact with the bone, blood, and fluids of the patients during the surgeries (11–13, 15).
FDM printed models and guides are prone to contraction and distortion during the thermoplastic cooling process leading to geometric inaccuracies (16). Several studies have assessed the dimensional accuracy of FDM pieces, biomedical and non-biomedical, manufactured in ABS after the 3D-printing process. Popescu et al. (8), as mentioned above, evaluated several dimensions of a non-biomedical ABS test part, measuring and comparing it to the nominal values in different sections. They found divergence values of +/−0.27 mm with mostly positive deviations in comparison with the nominal part. On the other hand, E-Katatny et al. (14) and Hsu et al. (17), using anatomical models of a mandible and a canine fibula, respectively, found surface deviations of 0,159 mm and 0,121 mm to the original design. In our study, the mean differences between the printed pieces and original design were within the 95%CI of -0,096 to -0,094 mm for models and 0,140 to 0,141 mm for guides. It showed that our manufacturing parameters are accurate and very close to those reported in the other studies.
FDM 3D printing is likely to produce devices with some degree of sterility, given the high temperatures used during the manufacturing process (18). However, handling in non-sterile conditions contaminate the devices and lead to an intraoperative infection (7). Therefore, the sterility of those models and guides is critical during surgery but without sacrificing the accuracy of the devices.
The sterilization methods adequate for different 3D-printing materials have been tested in terms of infection rate, mechanical performance, and dimensional stability, or, on the contrary, geometrical deformation (7, 9, 15, 19–21). Low-temperature hydrogen peroxide gas plasma has been an optimal sterilization procedure for printouts produced by FDP in ABS. They show a low infection rate with the preservation of the geometrical dimensions (7, 8, 17). In a recent publication, Shea et al. (7) evaluated the infection rate of 124 pieces (59 models and 55 guides) 3D-printed in ABS and reported an overall infection rate of 7%, mostly associated with age and long surgical times. The authors stated the infection rate was comparable to that reported by others using traditional surgical techniques and sterilization processes (7). On the other hand, Popescu et al., in two different publications (7–9), have demonstrated that low-temperature gas plasma sterilization does not influence the tensile and flexural strength of ABS specimens (9) nor the dimensions of the geometrical features remained stable (8). However, it has been indicated that for more multifaceted structures, mostly containing large surfaces of low depth, sterilization with VH2O2 could significantly impact the accuracy (22).
Two factors are essential when considering the use of a model or guide after the sterilization process. First, the dimensional stability in which the mean surface deviations of the pieces does not considerably alter the proportions, making them suitable for their use in the operating room and patients. Second, the devices' accuracy with a high level of conformity concerning the original design being truthful to the patient's anatomy (17, 22, 23).
The structural variations of VH2O2 sterilized FDM pieces produced in ABS have been previously addressed in various publications. Popescu et al. (8), as mentioned above, assessed a non-biomedical ABS part for dimensional accuracy following the printing and sterilization processes. After the latter, the dimensional changes were +/−0,20 mm, leading the authors to conclude that the sterilized part's dimensions were closer to the nominal design than the pre-sterilized one. Likewise, Kuczko et al. (22) evaluated the effect of VH2O2 sterilization over non-biomedical ABS 3D-printed pieces finding an average dimensional error of 0,036 mm (22). Hsu et al. (17) also tested the effect of low-temperature sterilization on the canine fibula model, getting a mean deviation of 0,043 mm (17).
We have shown that our 3D-printed models and cutting guides in ABS also maintained the dimensional stability after VH2O2 sterilization. First, the mean deviations on the 3D-printed pieces resulting from the low-temperature sterilization were low, with a 95%CI of -0,011 to -0,010 mm in models and of 0,002 to 0,003 mm in guides. Our values agree with those previously published, indicating that our sterilization process did not significantly affect the devices, making them suitable for intra surgical use. Second, the dimensional errors of the sterilized pieces compared to the original design were within a 95%CI of -0,083 to -0,081 mm in models and 0,126 to 0,128 mm in guides. Those values were smaller than those of the non-sterilized pieces, similar to what Popescu et al. (8) reported in their study.
In agreement, as shown in Fig. 4, the fidelity of the original design is preserved even after the 3D-printing and sterilization processes with significant correlations of 0,339 for models and 0,106 for guides.
In our ABS models and guides, the dimensional analysis indicated that errors that could occur during the manufacturing and sterilization processes were negligible. These results also indicate that the data acquisition procedure, the design, and manufacturing processes are reliable due to the final piece's accuracy.
A limitation of our study resides in that the results are only valid for FDM 3D printing technology and ABS devices. Therefore, there may exist other conditions under which these results are not reproducible. Identifying an FDM 3D printing material that deforms the least under the sterilization treatment is essential for clinical applications. The dimensional analysis was performed by one engineer, which could have introduced some bias on the measurements. However, the software provides programmed alternatives that assist in the device's alignments, eliminating manual errors and biases.