A Comparison of Staphylococcus aureus Biofilm Formation on Cobalt–Chromium and Titanium Spinal Implants

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Introduction Infection after the implantation of spinal hardware is a costly problem that is associated with significant morbidity. Numerous measures are taken every day to minimize the risk of deep postsurgical spinal infection. Steps such as restricting operating room traffic during surgery, copious wound irrigation with antibacterial solution, and the use of vancomycin powder before wound closure all aim at minimizing the risk of infection. The formation of antibiotic resistant biofilm on implanted hardware provides a significant challenge with regard to modern-day spine surgery. Therefore, the objective of this study was to compare whether there is a difference in the propensity for biofilm formation between Titanium alloy (Ti) and Cobalt Chrome (CoCr) spinal implants. Materials and Methods Lyophilized pellets of Staphylococcus aureus subsp. Aureus (ATCC 6538) were obtained and incubated in brain–heart infusion medium for a period of 24 hours to allow for sufficient rehydration. A total of 24 well plates were prepared in which 12 smooth surfaced Ti and 12 smooth surfaced CoCr rods, each measuring 10 × 5.5 mm, were placed after being autoclaved. Each rod was then immersed in 5 mL of brain–heart infusion broth. To each of these well plates, 1 mL of overnight cultures of Staphylococcus aureus were added. The rods were then divided into two groups and incubated at 37°C for a period of either 48 or 96 hours. After the respectively designated incubation periods had elapsed, the rods were washed with a 0.25% trypsin solution to extract the biofilms. This bacterial solution was then collected and initially evaluated for biofilm formation using optical density measurement utilizing a spectrophotometer. Then, the respective samples were plated on brain–heart infusion agar plates at two different dilutions and incubated for 24 hours. The resulting number of colony-forming units (CFU) for each plate was evaluated. Results Two-way ANOVA analysis showed that both OD and CFU were significantly influenced by the duration of incubation and implant material. The CoCr rods yielded higher OD and CFU than the Ti rods ( p < 0.05). Higher OD and CFU values were encountered in 4-day incubation as compared with 2-day incubation ( p < 0.05). The mean ( ± SD) OD values for the biofilm for the Ti implants were 1.105 ± 0.096 and 1.040 ± 0.026 nm at 48 and 96 hours, respectively. In contrast, the OD values for the bacterial solution with the CoCr rods were 1.332 ± 0.161 and 1.115 ± 0.207 nm at 48 and 96 hours, respectively, which were significantly higher than Ti implants ( p = 0.025). Similarly, the CFU values were noted to be 1,481 ± 417 and 745 ± 159 at 48 and 96 hours, respectively, for the Ti group, which were significantly lower than 2,721 ± 605 and 928 ± 88 for the CoCr group ( p < 0.001). Conclusion Our findings, using both the OD values and CFU, indicated that CoCr implants had a higher proclivity toward biofilm formation than Ti alloy implants. In light of this finding, further investigation will be necessary to understand the reason behind this significant difference.