Evaluation of Corrosion Mechanisms at the Bone-Metal Interface of Hip Implants

Total hip replacements (THR) are considered to be a successful surgical intervention in orthopedic community. In a hip joint environment, the orthopedic metal implants are exposed to a severe degradation process that takes place at the implant-bone interface. The harsh chemical-biological milieu caused by the biological fluids surrounding the contact area may result in severe corrosion damages of the metallic alloys. Such effects were found to be one of the sources of early fracture, inflammation, metal ion release to the host, further loosening of the hip implant, thus limiting the durability of the implant. In this way, the proliferation and differentiation of osteoblastic precursor cells in Ti6Al4V alloys may be compromised, interfering with the continuous osseointegration processes of bone formation around the implant. The aim of this work is to investigate the corrosion behavior of Ti6Al4V alloys with the osseointegration process of osteoblastic cells (bone-forming cells) cultured on Ti-based materials, commonly used in uncemented femoral stems. The influence of these organic materials on the performance of these alloys is evaluated using an electrochemical approach.

Ti6Al4V discs were mechanically polished using a series of SiC grinding papers up to a mirror finish (Ra < 40 nm). Then samples were sterilized in 70% alcohol during 1h. The non-colonized samples were tested with the sterilized polished surface. Other samples were then cultured with MG-63 osteoblastic-like cells in a culture medium: alpha-MEM supplemented with 10% fetal bovine serum and 1% penicillin-strepsin, pH of 7.4, at 37ºC in a humidified atmosphere of 5% CO₂/air. Cell cultures on the colonized materials and control groups (polystyrene culture dish) were characterized through cell viability/proliferation, alkaline phosphatase activity, gene expression and respective morphology after 6 days of culture. All samples were analyzed by scanning electron microscopy (SEM), fluorescence microscopy and white light interferometry before and after the corrosion tests. Corrosion tests were performed using a three-electrode electrochemical cell configuration with a saturated calomel electrode (SCE) as the reference electrode (RE), a graphite rod as the counter electrode (CE), and the metallic alloy as the working electrode (WE). Three electrolytes were used for comparison: a common saline solution, phosphate buffer solution (PBS); and the culture media (CM) used for the cell cultures. All tests were performed inside an incubator in a controlled environment of temperature (37⁰C) and CO₂ (5%). The open circuit potential was monitored during 1 hour, followed by an electrochemical impedance spectroscopy (EIS), in a frequency range from 10 KHz to 0.01 Hz with a ± 10 mV synovial amplitude, at a rate of 10 frequency per decade. The potentiodynamic scan was performed from -0.5 V to 1.5 V. The data was acquired with a Gamry Interface 1000 potentiostat.

The results showed that when the surface of the Ti6Al4V alloy was exposed to an electrolyte containing organic material (culture media), the corrosion potential (E_corr) decreased to -0.32 V, when compared to the surface exposed to a saline solution rich in phosphates (PBS), -0.22 V. Consequently, the passive current density (I_pass) also decreased, from 3.06 X 10^-6 A/cm² in PBS to 1.95X 10^-6 A/cm² in culture media.  On the other hand, when the Ti6Al4V was colonized with osteoblastic cells, maintaining the electrolyte rich in organic material (culture media), apparently, this values were always higher than former conditions. The colonized surfaces have shown a higher corrosion potential of the metallic surface with a quicker formation of the passive film with higher polarization resistance. The simultaneous presence of osteoblastic cells and serum proteins in the media might induce a higher passivation rate of the oxide film, promoting a higher resistance to corrosion of the surface of the material. These findings bring a better understanding on the correlation between the corrosion mechanisms happening at the surface of the metallic implants and the cellular responses to its products.