Ankle fractures are among the most common orthopedic injuries worldwide.1,2 The incidence of ankle fractures is estimated to range between 71 and 181 per 100,000 person-years.3,4 Recent evidence suggests the rate of ankle fractures has increased worldwide in the past decade.3,4 In the United States and other developed countries, orthopedic surgeons have a ready supply of available implants to treat ankle fractures surgically. However, in low- and middle-income countries, basic surgical items are in limited supply, and this lack of availability can limit patient access to surgical treatment.5
The World Bank has classified low- and middle-income countries as countries with a gross national income per capita less than $3895.6 It is estimated nearly a quarter of the world's disabilities are from conditions that require emergent access to surgical care.7 In addition, complication rates of surgical fixation of fractures vary widely in the literature, ranging from 1% to 40%, with surgical site infection being the most commonly reported complication.8,9 Globally, the rates of surgical site infection are significantly higher in low- and middle-income countries compared with the United States, 11.8 and 2.6 per 100 patients, respectively.10–12
These discrepancies highlight the critical need for readily available orthopedic implants that demonstrate clinical sterility in low- and middle-income countries for the surgical fixation of fractures. One possible solution that could provide a readily accessible, low-cost alternative to the current industry standards for emergent orthopedic treatment in low- and middle-income countries is 3-dimensional (3D) printing.
In the past decade, 3D printers have decreased dramatically in size and cost to the point that desktop printers are available at economical price points. Similarly, the types and cost of printing materials and time needed to complete prints have made the process more widely available.13 Currently, 3D printing is influencing many areas of medicine, including orthopedic surgery.13,14 Most of the advances in 3D printing in orthopedics have been in preoperative planning,15–18 surgical cutting or pin placement guides,15,19 rehabilitation devices,20 surgical simulation and training,21,22 prototype prosthesis development,14,23 and patient-specific treatments.18,24 The role of 3D printing in clinical implants has not been thoroughly explored because of limitations in biocompatible materials, costs, and printer resolution.
The current study was undertaken to determine whether open source software and a low-cost fused deposition modeling 3D printer using acrylonitrile-butadiene-styrene (ABS) polymer could produce sterile surgical implants. Surgical autoclave settings sterilize at 121°C for a 30-minute cycle or at 135°C for a 4-minute cycle.25 The heated extruder in fused deposition modeling 3D printers using ABS plastic material is set at temperatures between 220°C and 260°C.26 The current authors hypothesized orthopedic implants would be sterile through the process of printing, would not require postproduction sterilization, and would print in a time-efficient period.
Materials and Methods
Institutional review board approval was waived because no identifying patient information was used for this study. The printer, design software, and printing material were all open source to facilitate application in a rural environment. A free computer-aided design (CAD) software program, FreeCAD (version 0.17), was used to design the implant stereolithography (STL) files. Dimensions were obtained from a small fragmentation set (DePuy Synthes, Raynham, Massachusetts) to create STL files for 3.5-mm cortical screws and one-third tubular plates.
Implants were printed using a fused deposition modeling Lulzbot Mini 3D desktop printer (Alephi Objects Inc, Loveland, Colorado) modified with an acrylic enclosure for temperature control (TabSynth Design Works LLC, Madison, Wisconsin) at a total purchase price of $1370. This printer uses a melt-extrusion method to deposit polymer material via a nozzle heated to 240°C. The polymer is deposited in layer-by-layer additive manufacturing to create a 3D structure. The associated free open source printer software was Cura (version 3.4.0; Ultimaker, Utrecht, the Netherlands).
Implants were printed using ABS as the polymer material. Acrylonitrile-butadiene-styrene plastic is biocompatible based on International Standards Organization protocol 10993.27 The 3-mm diameter, 1-kg ABS spool was produced by eSun Industrial Co Ltd (Shenzhen, China) and cost $23.
Sterility was assessed using thioglycollate medium with dextrose broth (TMD) for culture (Remel; Thermo Scientific, Waltham, Massachusetts). This culture medium is used routinely by the current authors' hospital pathology laboratory for clinical cultures.
Prior to printing, the ABS plastic spool was handled by 4 investigators with ungloved hands and exposed to an unsterile environment with unpredictable airflow. The Lulzbot Mini 3D desktop printer, acrylic hood, and incubator were sanitized using 70% isopropyl alcohol between each print in a given study group. The printing plate and extrusion nozzle were sanitized prior to each study group using 2% chlorhexidine gluconate in 70% isopropyl alcohol. The TMD was prepared for inoculation by removing it from refrigeration and allowing it to reach room temperature.
Ten control samples were obtained by cutting 24-mm lengths of ABS from the plastic spool using sterile surgical scissors; the samples were immediately placed in TMD cylinders, closed with a loose cap, and incubated for 1 week at 37°C. After 1 week, the 10 control samples were sent to the hospital pathology laboratory for speciation of aerobic and anaerobic bacteria.
Figure 1 illustrates the experimental procedure. A total of 8 study groups were printed with 10 samples in each study group. Study group 1 consisted of ten 3.5-mm cortical screws, 14 mm in length, printed in the Z-axis (vertically). Study group 2 consisted of ten 3.5-mm cortical screws, 24 mm in length, printed in the Z-axis. Study group 3 consisted of ten 3.5-mm cortical screws, 14 mm in length, printed in the XY plane (horizontally). Study group 4 consisted of ten 3.5-mm cortical screws, 24 mm in length, printed in the XY plane. Figure 2A demonstrates the orientation of the screws in the Cura software.
Diagram showing experimental procedure. Study groups 1, 2, 5, and 6 were printed vertically. Study groups 3, 4, 7, and 8 were printed horizontally. Study groups 1, 2, 3, 4, and 7 were immediately placed in the thioglycollate medium with dextrose broth (TMD) cylinder (*). Study groups 5, 6, and 8 were submerged in a 70% isopropyl alcohol chemical cleanse (#) before being placed in the TMD cylinder.
Cura (Ultimaker, Utrecht, the Netherlands) print platform with the 24-mm screws showing the Z (vertical) orientation on the left and the XY plane (horizontal) orientation on the right (A). Cura Print platform with the 52-mm 5-hole plate showing the XY plane (horizontal) orientation (B).
For each print in study groups 1 to 4, one screw was printed at a time. Each screw then was removed from the printing platform with sterile forceps, placed directly in a TMD cylinder, closed with a loose cap, and incubated for 1 week at 37°C.
Study groups 5 and 6 involved chemical postproduction sterilization. Study group 5 consisted of ten 3.5-mm cortical screws, 14 mm in length, printed in the Z-axis that were removed from the printing platform postproduction with sterile forceps and immediately submerged in a 70% isopropyl alcohol bath for 10 seconds, placed directly in a TMD cylinder, closed with a loose cap, and incubated for 1 week at 37°C. Study group 6 consisted of ten 3.5-mm cortical screws, 24 mm in length, printed in the Z-axis that were removed from the printing platform postproduction with sterilized forceps and immediately submerged in 70% isopropyl alcohol bath for 10 seconds, placed directly in a TMD cylinder, closed with a loose cap, and incubated for 1 week at 37°C.
Study groups 7 and 8 involved one-third tubular plates printed in both standard and postproduction chemical sterilization technique, respectively. The culture broth tubes were too small to house the printed plates. Therefore, the broth was transferred to larger 15-mL sterile conical tubes before the 5-hole plates were printed. To control for contamination during the transfer process and to ensure sterility of the thioglycollate was retained, the broth was incubated in the 15-mL tubes for more than 48 hours to confirm no contamination before the introduction of the printed plates.
Study group 7 consisted of ten 1-mm 5-hole plates 52 mm in length printed in the XY plane. Figure 2B demonstrates the orientation of the printed plate in the FreeCAD software. One plate was printed, removed from the printing platform with sterilized forceps and placed directly in a TMD cylinder, closed with a loose cap, and incubated for 1 week at 37°C.
Study group 8 consisted of ten 1-mm 5-hole plates 52 mm in length printed in the XY plane. One plate was printed, removed from the printing platform with sterilized forceps, immediately submerged in 70% isopropyl alcohol bath, placed in a TMD cylinder, closed with a loose cap, and incubated for 1 week at 37°C.
In the event of a failed print, the print was discarded, the printer platform and extrusion nozzle were sanitized with 70% isopropyl alcohol swabs, and the sample was reprinted. The number of failed prints was recorded. Finally, due to the length of time of each print, the printing of each group was divided equally over 2 days. Five control samples and 5 samples from each study group were obtained on each day; therefore, no single area of the ABS spool contributed to any one study group.
Cultures were analyzed every 24 hours and photographed at 24 hours, 48 hours, and 7 days. If at any time a culture showed growth, the loose cap was secured to avoid any cross contamination. At the end of 7 days, any positive cultures were delivered to the authors' hospital pathology laboratory for speciation of aerobic and anaerobic bacteria.
Study groups of 10 were determined for cost and time of each print as determined by the investigators with the help of the hospital statistician because no similar prior studies were available for power analysis. All data were evaluated using chi-square tests. Significance was set at P<.05 using SigmaPlot software (Systat Software Inc, San Jose, California).
All 10 control samples were positive for bacterial growth. Figure 3 shows the positive growth visualized. Table 1 details the speciation of the positive control samples.
An unopened thioglycollate tube is demonstrated on the left with the 10 control samples showing positive growth at 1 week.
Speciation of Positive Controls
None of the samples from groups 1 to 8 (100%) demonstrated any signs of growth at 24 hours, 48 hours, or 7 days, and the samples were sterile at all time points. Figure 4A demonstrates a sample from groups 1 to 6 with no visual growth at 7 days. Figure 4B demonstrates a sample from groups 7 and 8 with no visual growth at 7 days.
An unopened thioglycollate tube (*) showing no growth, a control tube showing positive growth (C), and samples from study groups 1–6 (numbered) at 1 week showing no growth (A). A sample of 10 mL of thioglycollate broth on the left showing retained sterility after being transferred to a 15-mL conical tube, a sample from study group 7 in the middle, and a sample from study group 8 on the right at 1 week showing no growth (B).
The time of each print was recorded to determine whether the length of time on the printing platform influenced the construct's sterility. Table 2 details the print times for each study group.
Print Time for Study Groups
There were 18 failed prints during testing. Calibration error occurred 6 times, and an error in polymer extrusion or construct adherence to the build platform occurred 12 times. The most failed prints occurred in study group 4 (n=8 failed prints), 6 of which were due to an issue with adherence (P<.005).
This study demonstrated that a low-cost fused deposition modeling 3D printer and ABS plastic can create sterile implants with similar dimensions to orthopedic implants used in the treatment of fractures. An inherent sterilization process was found to occur during fused deposition modeling 3D printing.
Prior literature has demonstrated sterility associated with fused deposition modeling 3D printing of polylactic acid (PLA) plastic. Neches et al28 demonstrated PLA thermoplastic is sterile when extruded directly from the nozzle into a culture tube. However, no previous study has attempted to print orthopedic implants in different orientations or times, and then assessed the sterility of the printed implants. In addition, PLA is not biocompatible and has weaker mechanical properties than ABS. The current study found the intrinsic sterilization of fused deposition modeling 3D printing of ABS constructs was maintained through the entire process from printing to removal from the print bed. Time to print ABS and build platform contact area in limited (vertical) vs increased (horizontal) orientation did not influence the sterility of the prints.
Several methods to sterilize fused deposition modeling manufactured devices, which may change the mechanical properties of the plastic, have been described.29–31 ABS plastic is not capable of being placed in an autoclave for sterilization. The standard surgical autoclave temperatures cause deformation of the ABS implant.30 Ionizing radiation does not cause visual deformation; however, Demertzis et al32 demonstrated it does cause chemical and mechanical changes in polymer materials. In addition, the IPEX Chemical Resistance Guide of ABS plastic shows ABS is chemically resistant to ethylene oxide, sodium hypochlorite, and isopropyl alcohol; however, the results of the current study suggest chemical sterilization is not required.33
Low-cost, 3D-printed orthopedic implants could potentially improve orthopedic care in low- and middle-income countries through the increase in availability of sterile surgical implants. Meara et al34 concluded there is an unmet surgical need ranging from 301 to 5625 cases per 100,000, totaling nearly 143 million procedures per year, in low- and middle-income countries. Spiegel et al5 found most hospitals in low- and middle-income countries did not have all basic items for offering surgical services. The current authors' print times were short compared with the days associated with delivery of implants to low-income, rural environments. In the clinical scenario of a Weber C ankle fracture, a plate and 7 screws would be required to appropriately reduce and stabilize the fracture. Based on the results of the current study, the print time for a plate and 7 screws would be approximately 30 minutes and would have a total cost of less than $1 for the ABS material.
This study found the number of failed prints can be related to the length and orientation of the print. The 14-mm, horizontally printed screw yielded the highest number of failed prints due to displacement from the printing platform. One possible reason for this finding is the screws' contact area did not have time to properly adhere to the platform prior to the extrusion nozzle returning to the same position. This result suggests there is a limit to the length and orientation of a print that may determine the success of adhesion to the platform.
There were several limitations of this basic science study. Results of this study were limited to the printing of thermoplastics using fused deposition modeling manufacturing where the print filament is extruded at 240°C. This printing temperature is common for fused deposition modeling printing, and it is likely these results would hold true for other thermoplastics extruded at 240°C or higher. The sample size (n=10) of each group was chosen based on cost and time to print. With no growth observed in any study group, and 100% growth in the control samples, it is reasonable to generalize the process of printing as a sterilization technique despite the small sample size.
The contamination of the spool was not uniform across the entire length of ABS plastic. However, this effect was mitigated by the unpredictable airflow of the setup, the handling of the spool by 4 different investigators, spreading the sample collections across 2 days, and the length of time the spool sat on the printer during data collection. In addition, the speciated control samples helped to determine the limitations regarding which bacteria can be sterilized with this technique.
The in vitro environment of this study also limited the ability to generalize the results as being sterile and safe for long-term orthopedic implants. Samples were cultured for 1 week to better approximate the long-term sterility of the print. However, the study may have missed speciation in any bacterial growth if it occurred after 1 week. Also, the size of printing clinically relevant 6-hole or longer plates could not be cultured in the cylinder sizes available to this study.
Although estimating the cost of use and maintenance of the Lulzbot Mini Printer is beyond the scope of this study, no additional maintenance costs or repairs were needed during the study period. Per the user manual, the purchasing of new parts rarely is required because the printer is able to print replacements for almost all of its key components. This self-replicating process keeps the maintenance cost low.35 Finally, in the future, additional basic science and animal studies would need to be conducted on the mechanical and biologic properties of these implants before they could be considered for clinical applications.
There is an intrinsic sterilization process associated with fused deposition modeling 3D printing of ABS plastic. This study printed clinically relevant orthopedic cortical screws and one-third tubular plates with 100% sterility in a short time period and at lower cost than currently used implants. These results demonstrate the feasibility of using 3D printing to create sterile surgical implants and equipment for the management of orthopedic traumas in low- and middle-income countries.
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Speciation of Positive Controls
|8||Coagulase-negative Staphylococcus and Streptococcus viridans|
Print Time for Study Groups
|Study Group||Mean±SD Print Time, min|