Effect of tribolayer formation on corrosion of CoCrMo alloys investigated using scanning electrochemical microscopy

Metallic Implants Used in Lumbar Interbody Fusion

Over the last decade, pedicle fixation systems have evolved and modifications in spinal fusion techniques have been developed to increase fusion rates and improve clinical outcomes after lumbar interbody fusion (LIF). Regarding materials used for screw and rod manufacturing, metals, especially titanium alloys, are the most popular resources. In the case of pedicle screws, that biomaterial can be also doped with hydroxyapatite, CaP, ECM, or tantalum. Other materials used for rod fabrication include cobalt-chromium alloys and nitinol (nickel-titanium alloy). In terms of mechanical properties, the ideal implant used in LIF should have high tensile and fatigue strength, Young's modulus similar to that of the bone, and should be 100% resistant to corrosion to avoid mechanical failures. On the other hand, a comprehensive understanding of cellular and molecular pathways is essential to identify preferable characteristics of implanted biomaterial to obtain fusion and avoid implant loosening. Implanted material elicits a biological response driven by immune cells at the site of insertion. These reactions are subdivided into innate (primary cellular response with no previous exposure) and adaptive (a specific type of reaction induced after earlier exposure to the antigen) and are responsible for wound healing, fusion, and also adverse reactions, i.e., hypersensitivity. The main purposes of this literature review are to summarize the physical and mechanical properties of metal alloys used for spinal instrumentation in LIF which include fatigue strength, Young's modulus, and corrosion resistance. Moreover, we also focused on describing biological response after their implantation into the human body. Our review paper is mainly focused on titanium, cobalt-chromium, nickel-titanium (nitinol), and stainless steel alloys.

Alumina and tricalcium phosphate added CoCr alloy for load-bearing implants

Cobalt-chromium (CoCr) alloys are used in load-bearing implants due to their excellent wear resistance. However, poor tissue-material interactions can originate due to the release of wear and corrosion-induced Co and Cr ions, motivating the use of surface modification to reduce such phenomena. Premixed feedstock powders of CoCrMo + 2 wt.% tricalcium phosphate (TCP, CoCr_TCP) and CoCrMo + 2 wt.% tricalcium phosphate + 4 wt.% Al₂O₃ (CoCr_TCP+Al) were used to surface coat CoCr alloy via Laser Engineered Net Shaping (LENS™) with the objective of increasing CoCr alloy's wear resistance. Electron micrographs of the microstructure revealed the dissociation of intergranular globular carbide phases and reprecipitation into a finer network-like microstructure with homogeneous distribution of Co and Cr. X-ray diffraction (XRD) spectra revealed texturing or preferential crystallographic orientation amongst the LENS™ processed materials, with the TCP added CoCr displaying some ε-phase stabilization. Tribological testing resulted in an 82.3% and 71.6% decrease in wear rate and wear coefficient, respectively, for CoCr_TCP when compared to commercially available CoCr alloy. Additionally, in situ tribofilm development was observed for the fabricated samples via an increase in contact resistance. The current study resulted in not only a decrease in wear volume but also a decrease in the overall degradation of the coated CoCr alloy.

Titanium-Silicon on CoCr Alloy for Load-Bearing Implants Using Directed Energy Deposition-Based Additive Manufacturing

Cobalt-chromium (CoCr) alloys offer outstanding wear resistance when compared to other biocompatible metallic materials and are extensively used in articulating surfaces of total hip and knee arthroplasty. However, CoCr alloys' biocompatibility is known to be inferior to titanium (Ti). Wear- and corrosion-induced metal-ion release from CoCr alloys has been reported to cause cancer and negative physiological impacts. In this study, CoCr alloy was coated with commercially pure Ti (CpTi) and CpTi-Silicon (CoCr_Ti-Si) with the specific objective of reducing Co and Cr ion release during articulation, without degrading the excellent wear resistance of the CoCr alloy. Directed energy deposition (DED), a blown powder-based laser additive manufacturing technique, was utilized to process CpTi- and CpTi-Si-based coatings on Stellite 6B commercial CoCr alloy. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analyses, and hardness testing found that refined carbides and titanium silicides increased the hardness from 321 ± 13 to 758 ± 48 HV_0.5. Tribological studies determined a comparable wear rate between Stellite 6B alloy and CoCr_Ti-Si in DI water but a statistically significant reduction in Dulbecco's Modified Eagle Medium (DMEM). The wear rates for Stellite 6B were 8.5 ± 0.8 × 10^-5 and 12.9 ± 0.4 × 10^-5 mm³/Nm in DI water and DMEM, respectively. While the wear rates for CoCr_Ti-Si were 9.1 ± 0.5 × 10^-5 and 8.9 ± 0.8 × 10^-5 mm³/Nm in DI water and DMEM, respectively. Contact resistance acquisition displayed the presence of a passive film formation during tribological testing. ICP-MS results for Stellite 6B and CoCr_Ti-Si concluded a reduction of Co ions release in DI water from 149.8 ± 66.7 to 17.5 ± 0.7 ppb and a reduction in Cr ions release from 66.7 ± 32.4 to 18.0 ± 0.5 ppb, respectively. In DMEM media, Co ion release for Stellite 6B and CoCr_Ti-Si reduced from 10.1 ± 1.4 to 4.1 ± 0.2 ppb and Cr ion release for Stellite 6B and CoCr_Ti-Si reduced from 8.7 ± 0.2 to 5.0 ± 0.7 ppb, respectively. The current study revealed a new mode of manufacturing for CoCr alloy-based load-bearing implants that can reduce toxic metal ions release due to wear- and corrosion-induced damages.

Additively manufactured calcium phosphate reinforced CoCrMo alloy: Bio-tribological and biocompatibility evaluation for load-bearing implants

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in load-bearing implants; specifically, in hip, knee, and spinal applications due to their excellent wear resistance. However, due to in vivo corrosion and mechanically assisted corrosion, metal ion release occurs and accounts for poor biocompatibility. Therefore, a significant interest to find an alternative to CoCrMo alloy exists. In the present work we hypothesize that calcium phosphate (CaP) will behave as a solid lubricant in CoCrMo alloy under tribological testing, thereby minimizing wear and metal ion release concerns associated with CoCrMo alloy. CoCrMo-CaP composite coatings were processed using laser engineered net shaping (LENS™) system. After LENS™ processing, CoCrMo alloy was subjected to laser surface melting (LSM) using the same LENS™ set-up. Samples were investigated for microstructural features, phase identification, and biocompatibility. It was found that LSM treated CoCrMo improved wear resistance by 5 times. CoCrMo-CaP composites displayed the formation of a phosphorus-based tribofilm. In vitro cell-material interactions study showed no cytotoxic effect. Sprague-Dawley rat and rabbit in vivo study displayed increased osteoid formation for CoCrMo-CaP composites, up to 2 wt.% CaP. Our results show that careful surface modification treatments can simultaneously improve wear resistance and in vivo biocompatibility of CoCrMo alloy, which can correlate to a reduction of metal ion release in vivo.

Scanning electrochemical microscopy: an analytical perspective

Scanning electrochemical microscopy (SECM) has evolved from an electrochemical specialist tool to a broadly used electroanalytical surface technique, which has experienced exciting developments for nanoscale electrochemical studies in recent years. Several companies now offer commercial instruments, and SECM has been used in a broad range of applications. SECM research is frequently interdisciplinary, bridging areas ranging from electrochemistry, nanotechnology, and materials science to biomedical research. Although SECM is considered a modern electroanalytical technique, it appears that less attention is paid to so-called analytical figures of merit, which are essential also in electroanalytical chemistry. Besides instrumental developments, this review focuses on aspects such as reliability, repeatability, and reproducibility of SECM data. The review is intended to spark discussion within the community on this topic, but also to raise awareness of the challenges faced during the evaluation of quantitative SECM data.