An Evaluation of the Bacterial Adherence to Casting Materials

Introduction: The purpose of this study was to evaluate bacterial adherence to common casting materials including plaster of Paris (plaster), fiberglass, three-dimensional (3D) printed plastic, and silicone-coated 3D printed plastic. Methods: The minimal inhibitory concentration of a phosphate-free detergent (Palmolive) needed to achieve total bacterial kill off was determined. 3D printed polylactic acid plastic samples were coated with silicone. Plaster, fiberglass, plastic, and silicone-coated plastic samples were inoculated with Staphylococcus aureus. After bacterial inoculation, scanning electron microscopy of the samples was performed to visualize bacterial adherence to the materials' surface. Using either sterile water or a 5% detergent solution, the materials were subjected to washings. Each material was run in 30 replicates: 6 without washing, 6 with sterile water for 1 minute, 6 with detergent for 1 minute, 6 with sterile water for 3 minutes, and 6 with detergent for 3 minutes. The replicates that did not undergo a washing trial represented the initial bacterial inoculation. Samples were then rinsed and sonicated in polysorbate to isolate the remaining adherent bacteria on the materials’ surface. The sonicated solutions were plated, incubated, and counted for quantification of colony forming units (CFU) of bacteria. This protocol was repeated for a total of four trials. Results: During inoculation, there were significantly less bacteria that adhered to silicone-coated 3D printed plastic (58879 CFU) compared to plastic (217479 CFU), plaster (140063 CFU), and fiberglass (550546 CFU). Silicone coating showed further superiority. Silicone-coated 3D printed plastic was able to be decontaminated as demonstrated by significantly fewer remaining bacteria (9.3%) on its surface after being washed with a 5% detergent solution (1797 CFU) compared to sterile water (19321 CFU). The mean remaining bacteria on silicone-coated 3D printed plastic was significantly less than that remaining on all other materials when washed with either sterile water or a detergent solution for both durations of 1 minute and 3 minutes. Conclusions: The current study demonstrates that significantly less bacteria adhere to the surface of 3D printed plastic with silicone coating showing added protection and that this material can be decontaminated to a greater degree with washing than conventional casting materials. These results provide evidence that 3D printed casts can be washed and successfully decontaminated during a patient’s period of immobilization, which is advantageous especially during an infectious crisis such as the coronavirus disease 2019 (COVID-19) pandemic.