Effect of Bacterial Infection By-Products on the Corrosion of Titanium Osteo-Implants
Authors: Ahmed Sheikh Imran Uddin , Usen Nazreen Mohamed Shahid , Sankarasubramanian Shrihari 
Published: August 2024

The use of titanium (Ti) in dental and orthopedic implant applications is extensive, owing to its remarkable resistance to corrosion, biocompatibility, low density, mechanical strength, and elastic modulus, which closely aligns with bone properties compared to alternative metals 1. There are numerous titanium grades (40-50), only a select few are officially recognized and specified by the American Society for Testing and Materials (ASTM) for use in biological applications. Biomedically compatible grades 1–4, classified as commercially pure (cp-Ti), contain <1% alloying elements, while others are considered Ti alloys with a higher percentage of alloying elements. Among titanium alloys, grade 5 Ti (Ti6Al4V) is predominantly utilized in orthopedic implants 2.

Approximately 800,000 knee replacement surgeries are performed annually in the United States alone 3. Projections indicate an anticipated four million joint replacements (hip, knee, and shoulder) annually in the US by 2030 4. Infection rates for orthopedic implants are reported to be below 2%, with the majority occurring within the initial 2 years post-operation 4. The ten-year survival rate for orthopedic implants stands at 85-90% 5 and only 58% of patients can expect their hip replacements to endure for 25 years 6. Implant failure often necessitates revision surgery, incurring high costs, a relatively low success rate, and considerable patient discomfort. Corrosion, arising from the dissolution of titanium and/or alloying elements from the implant, is a prevalent cause of orthopedic failure 7.

Surgical procedures, compromising the skin barrier and introducing foreign materials during joint replacement, predispose the body to infection, including bacteria that form biofilms. Gram-negative bacteria are a source of lipopolysaccharide (LPS) 8. Joint movements have the potential to rupture biofilms, releasing LPS into the synovial fluid surrounding the joint. This study aims to explore the impact of LPS in synovial fluid on implant corrosion.

The study utilized simulated synovial fluid (SSF) and LPS from Escherichia coli, employing electrochemical techniques to investigate corrosion behavior. Titanium grade 2, 4, and 5 were evaluated for corrosion under unpolished and polished conditions, with and without the presence of LPS in SSF at 37℃ (Figure 1). The unpolished Ti surface consist of titanium with an oxide protective layer on top, as a fresh titanium surface naturally develops a protective oxide layer of titanium oxides when exposed to air or aqueous environments 9. The corrosion resistance of this natural TiO2 layer was examined in the presence of LPS. Furthermore, the disruption of the native oxidation film on the implant surface due to joint movement was examined as a potential factor that may accelerate corrosion. Consequently, the self-healing process of titanium oxide layers emerges as crucial in mitigating failures in orthopedic implants. The influence of LPS on this self-healing process was explored by immersing polished titanium in SSF both with and without LPS for a duration of 80 days. The corrosion current density was measured for all cases using linear sweep voltammetry (LSV) and fitting the data to the Butler-Volmer equation. The native oxide layer was indeed found to be protective compared to the naked metal and the “self-healing” of this film overtime was found to significantly reduce corrosion rates over the 80-day test. Interestingly, LPS was found to both reduce the protective effect of the native layer and disrupt the self-healing process. Electrochemical oxidation of the Ti surface was found strengthen the corrosion resistance of the native film and found to offer a simple pathway towards enhanced corrosion resistance even in the presence of LPS. We propose a mechanism for the LPS effect and conclude that post-operative bacterial infections can significantly exacerbate the corrosion of Ti osteo-implants and increase the likelihood of implant failure.

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Figure 1