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Basic and Clinical Research

Effect of Steam Heat Sterilization on the Accuracy of 3D Printed Surgical Guides

Marei, Hesham F. BDS, MSc, PhD*; Alshaia, Alaa BDS; Alarifi, Sundus BDS; Almasoud, Naif BDS, MSc, PhD; Abdelhady, Adel BDS, MSc, DDS§

Author Information
doi: 10.1097/ID.0000000000000908
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Abstract

Computer-guided implant surgery refers to the use of computed tomography (CT) data to design and construct a surgical guide that directs the surgeon in reproducing the planned implant position and angulation.1 Computer-guided implant surgery provides more accurate results than freehand surgery, especially in the esthetic zone.2 However, the degree of accuracy, which is defined as the degree of matching between the virtual plan and the real implant positions, depends on the cumulative error that is incorporated with every step in the workflow from data acquisition to the surgical procedure.3

Multiple studies have investigated the effect of different variables such as cone-beam CT (CBCT) image acquisition, registration of the CBCT images with the jaw images, implant planning software, type of support offered by the oral structures to the surgical guides, and, finally, the surgical protocol, whether partially guided or fully guided, on the degree of accuracy of implant placement.3–7

One of the factors that might lead to inevitable error is the steam heat sterilization of the surgical guide.3 The surgical guide is a medical device that comes in contact with the blood, bone, and oral fluids of the patient during the implant procedure. It can cause serious infections if it is contaminated by any extraoral pathogens. Therefore, different regulations and guidelines have been developed from many countries to reduce the possible spread of infection effectively. The US Food and Drug Administration (FDA) and Centers for Disease Control and Prevention categorized the medical items into uncritical, semicritical, and critical.8–11

The surgical guide is considered a critical item that has to be sterilized by steam heat, according to the American Dental Association.8,9,11 However, different studies have claimed that owing to the thermosensitivity of the surgical guide, it should not be sterilized but immersed in FDA-approved sterilizing solution.11–13 Moreover, the availability of in-office 3D printers with different printing materials at reasonable prices has partially shifted the production of surgical guides from the well-known laboratory suppliers such as Simplant (DENTSPLY Implants NV, Hasselt, Belgium), NobelGuide (Nobel Biocare, Göteborg, Sweden), Facilitate (Astra Tech Inc., Waltham, MA), and Straumann (Institute Straumann AG, Basel, Switzerland) to the dental office, which invites even more lack of standardization over the sterilization protocols that should be followed.

Generally, the sterilization of critical items can be achieved by autoclaving (steam under pressure), dry heat, or heat/chemical vapor.11 Autoclaving is done under temperatures ranging from 121°C to 134°C, and the time varies depending on the size of load; however, there are 2 common techniques: normal cycle and fast cycle. The normal cycle heats up to 121°C for 15 to 20 minutes, whereas the fast cycle heats up to 134°C for 3 minutes.14 Simplant (DENTSPLY Implants NV) recommends steam heat sterilization of the surgical guide at 121°C for 20 minutes or 134°C for 4 minutes,15 whereas NobelGuide (Nobel Biocare) recommends the use of high-level disinfectants such as 70% diluted ethyl alcohol for 40 minutes.16 On the other side, Formlabs, which is a supplier for in-office printers, recommends steam heat sterilization of its surgical guide at 121°C for 15 minutes or 138°C for 3 minutes.17

Steam heat sterilization is superior to chemical disinfection in its ability to decrease the microbial load in the surgical device and thus decrease the number of microorganisms invading the patient's body during surgery, which ultimately contributes to reducing postoperative complications.18,19

Most surgical guides are formed of 2 materials, either polymethyl methacrylate (PMMA) or polyethylene copolymer resins.12 The effect of steam autoclaving on dimensional changes of heat-processed methyl methacrylate used in cranial implants was investigated by Rhonda (1991); the autoclaved specimens showed a statistically significant linear distortion of 1.21% compared with unsterilized ones; however, such distortion did not lead to any clinically significant difference.20 Furthermore, Consani et al21 reported a linear shrinkage as a result of residual polymerization in the acrylic resin used in the denture base when exposed to heat generated from microwave disinfection.

Shaheen et al conducted a pilot study to compare the effect of steam and gas plasma on the dimensional changes of 3D printed items. The changes were measured at volumetric and morphological levels. Volumetric analysis revealed no differences between presterilization and poststerilization objects, whereas morphological deformity was noticed with one of the objects after sterilization. It was concluded that fewer differences were associated with gas plasma sterilization than with heat sterilization, making it more reliable. However, the sample size was considered small, so the results were not conclusive.22

Therefore, the aim of this study was to investigate the dimensional changes of in-office printed surgical guides by comparing them with another gold standard of laboratory-manufactured surgical guides. The independent variable was steam heat sterilization; the dependent variable was the dimensional changes of both surgical guides. The study involved 2 research questions: (1) Would steam heat sterilization lead to significant dimensional changes in both surgical guides? (2) Would the dimensional changes of in-office–produced surgical guides be higher than laboratory-manufactured surgical guides when both are exposed to the same standard sterilization protocol?

Materials and Methods

This study involved the participation of 27 patients who planned to receive 65 implants. The selection of patients was limited to those who had enough teeth to provide a stable, tooth-supported surgical guide. The patients were randomly assigned to 2 groups. Group I involved 13 patients who had received 13 tooth-supported surgical guides for the placement of 19 implants (n1 = 19). The surgical guides of group I were constructed using in-office stereolithography (SLA) 3D printer (Form 2, Formlabs Inc., Somerville, MA). The guides were formed of Dental SG (Vertex-Dental B.V. Centurion Baan, the Netherlands), which is a class 1 biocompatible photopolymer resin made of a mixture of methacrylic esters and photoinitiators. Group II involved 14 patients, who received 14 tooth-supported surgical guides for the placement of 46 implants (n2 = 46). The surgical guides of group II were printed using SLA printers and delivered by Simplant (DENTSPLY Implants NV).

The study was approved by the ethical committee of the university, and all participants signed informed consent for participation in the study (IRB–2017-052).

Surgical Guide Production

Impressions of the dental arches were taken in both groups, using polyether impression, poured with hard rock stone (Whip Mix Corp., Louisville, KY); CBCT was taken and registered on the surface of the study models, using 2 kinds of the implant planning software. Group I surgical guides were virtually planned using an open-platform implant planning software (Blue Sky Plan 3; Blue Sky Bio, Grayslake, IL) and printed in-house by an SLA Form 2 3D printer (Formlabs Inc.). The printed guides were rinsed in a bath of 90% isopropyl alcohol for 10 minutes and then inserted in a bath of clean, unused, 90% isopropyl alcohol. The printed guides were left to dry for an additional 10 minutes and then exposed to blue ultraviolet light for 10 minutes for curing. The support material was removed after curing. Metal sleeves were inserted into guide holes corresponding to the implants' sites before sterilization.

Group II surgical guides were virtually planned by one operator, using Simplant implant planning software (DENTSPLY Implants NV) and then printed and delivered with the sleeves already in place by the Simplant manufacturer.

Optical Scanning Before and After Sterilization

Self-sticking reference points, using Spee-D mammography skin marker (1.5 mm in diameter), were attached to different areas of the study models (stone cast) to facilitate an optimum registration of the 3D surfaces of the study models before and after sterilization of the guide. The guides were fitted on the corresponding study models and rechecked for accurate fitting by an operator, blinded. The study model and the guide were scanned by a powder-free intraoral scanning device (TRIOS; 3Shape, Copenhagen, Denmark), and the image was exported in a Standard Tessellation Language (STL) file. The same process was repeated after sterilization of the surgical guide, and a second image was exported as a second STL file.

Sterilization Method

Following the standard sterilization protocol, all the surgical guides were sealed in separate pouches and autoclaved at 121°C for 20 minutes.

Analysis of Dimensional Changes

The volumetric dimensional changes were measured using the CAD interactive software (GOM Inspect; GOM mbH, Braunschweig, Germany) by 2 independent blinded operators. The 2 STL files for the digital models were imported into the software. The presterilization guide was set as CAD data, and the poststerilization guide was set as actual data. With the help of the metal markers, superimposition and alignment of the 2 STL files was achieved. Visual cylinders produced by the software were fitted into the implant sleeves (Fig. 1). The software determined the center of the cylinders so that a point and a plane at the top of the implant sleeve were also projected. The point intersects the center of the cylinder, and the plane projected from the presterilized guide was linked to poststerilized ones to allow the software to calculate deviations in x, y, and z axes and the Euclidian distance, dxyz (Figs. 2 and 3).

Fig. 1
Fig. 1:
The software determined points and planes at the center of the cylinders, which represent the sleeves of the guide. Blue and green geometries refer to presterilized and poststerilized surgical guides, respectively. Discrepancies between the centers of the cylinders in X, Y, and Z axes were evaluated and shown in the table.
Fig. 2
Fig. 2:
Surface comparison between presterilized and poststerilized surgical guides at the center of sleeves. Deviations represented in the scale by color ranges from blue to red. Measuring deviation at the center of one sleeve (point 2) of group I surgical guide. 0.00 represents no deviation; +0.20 and −6.66 represents high deviations at the red and blue sites of the guide.
Fig. 3
Fig. 3:
Surface comparison between presterilized and poststerilized surgical guides at the center of sleeves. Deviations represented in the scale by color ranges from blue to red. Measuring deviation at the center of the 3 sleeves (point 2, 4, and 6) of group II surgical guide. 0.00 represents no deviation; +0.24 and −6.19 represents high deviations at the red and blue sites of the guide.

Statistical Analysis

The data were collected and tabulated using Microsoft Excel, and then SPSS version 22 was used for data analysis. A significant correlation between the measurements was generated by the 2 independent operators. Therefore, the mean measurement was considered. The implant site was considered the unit of analysis (N = 65). Nonparametric analysis (Mann-Whitney U test) was applied to compare the mean deviation (dxyz) between groups I and II. A paired t test was applied to compare the mean measurements of x, y, and z axes (center of sleeves) before and after sterilization within the same group. A difference of less than 0.05% was considered statistically significant. An independent biostatistician has reviewed the methodology, statistical analysis, results, and conclusion sections.

Results

A total of 27 surgical guides were produced, and 65 implant sites were analyzed using the CAD interactive software (Table 1). There was a strong positive significant correlation (P < 0.00) between the measurements of the 2 blinded operators (Pearson correlation = 0.95).

Table 1
Table 1:
Mean and SD of the Point Deviation Between Presterilized and Poststerilized Surgical Guides in X, Y, and Z Axes

There was no significant difference between the mean x, y, and z axes, which represents the center of the sleeves when analyzed before and after sterilization (P values were 0.37, 0.24, and 0.29, respectively). Furthermore, nonparametric analysis showed no significant difference between the mean deviations of groups I and II surgical guides when both were exposed to the same standard steam sterilization protocol (P = 0.908).

Discussion

The current study investigated the influence of steam heat sterilization on the dimensional changes of 2 types of surgical guides. The study showed no significant influence of steam heat sterilization on the dimensional changes of the surgical guides of groups I and II. Such results were supported by the lack of changes at the center of the sleeves in x, y, and z axes before and after sterilization. The center of the sleeves represented the point of entrance of the first drill in different guided surgery protocols.

Our results are different from the study by Jacob and Collard, which showed a statistically significant linear distortion after sterilization of cranial implants.20 However, our results were consistent with the study by Shaheen et al. Such variations might be due to the method of measurements. Our study and the Shaheen et al's study used the 3D interactive software to calculate the distortion of 3D printed objects in 3 dimensions,22 whereas in the Jacob study, the authors relied on the linear measurement only.

Our study also showed no significant difference between the mean deviation in xyz at the center of the sleeves when the group I surgical guide was compared with the group II surgical guides. Such results might be because 121°C is a low temperature if compared with the thermal degradation temperature (>180°C) of PMMA, which is a point where deterioration of molecules takes place as a result of overheating.23

In our study, we used the CAD interactive software (GOM Inspect; GOM mbH), which was used before in a similar study by Matta et al.24 Furthermore, we followed the same protocol during measuring the point deviations in x, y, and z axes and dyxz. A significant advantage of the software is the complete visualization of the deviation point in a color map, which helps in detecting the site of deviation. Furthermore, the software provides automated options that aid in models' alignments, which eliminates any manual errors and biases.25

Our study involved teeth-supported surgical guides only. The main reason was to eliminate any deviations that might be invited as a result of lack of accurate positioning of bone- or mucosa-supported guides on the study models. Furthermore, we used a high-quality handheld scanner (TRIOS; 3Shape) instead of a desktop laboratory scanner to monitor the fitting of the guide directly on the study model during the scanning process. The 3D rotation of the axis of the desktop scanner might dislodge the guide from its place, leading to an error in scanning, which reflects on the deviation of the center of the sleeve in xyz axes. The Trios intraoral scanner was reported as the most accurate intraoral scanner, with an accuracy of (6.9 ± 0.9 µm) when compared with other well-known scanners.26

The main clinical implication of our study is that the steam heat sterilization method at 121°C for 20 minutes had no significant effect on the dimensional changes of the tested surgical guides. However, our study is not without limitations. The measurements in our study were limited to the deviations at the center of the sleeves in xyz axes before and after sterilization. Accuracy of guided implant surgery depends not only on the correct point of entry but also on the angulation during drilling. However, such angulation could be affected in vivo by other confounding factors such as the guided protocol that is followed and the type of support teeth, bone, or mucosa. Therefore, we recommend future research that investigates the deviations between the angulation plane of the sleeves before sterilization and the angulation plane of the inserted implants in vivo when a fully guided protocol is followed on dentate patients.

Conclusion

Steam heat sterilization has a nonsignificant effect on the dimensional changes of the tested surgical guides.

Disclosure

The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the paper.

Approval

The study was approved by the ethical committee of the university, and all participants signed informed consent for participation in the study (IRB–2017-052).

Roles/Contributions by Authors

H. F. Marei: Clinical interventions with patients, writing and reviewing the discussion. A. Alshaia: Measuring point deviations using the CAD interactive software, writing the introduction and methodology. S. Alarifi: Measuring point deviations using the CAD interactive software, writing the introduction and methodology. N. Almasoud: Statistical analysis of the result and proofreading. A. Abdelhady: Clinical interventions with patients and proofreading.

Acknowledgments

This work was supported by a grant from the Deanship of Scientific Research, Imam Abdulrahman Bin Faisal University, Saudi Arabia (Project no. Dent2017052). The authors acknowledge Dr. Kamel Mahmoud Mansi (PhD), University of Manchester, United Kingdom, who is currently working as assistant professor at Imam Abdulrahman Bin Faisal University.

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Keywords:

dental implants; computer-guided implant dentistry; guided surgery

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