Fretting Corrosion of Spinal Instrumentation Systems in Physiological Solutions Containing H2O2

Spinal instrumentation implant systems are increasingly used for stabilisation and correction in various spinal pathologies. However, failures related to fretting corrosion at modular interfaces remain a clinical concern. To date, there are no established testing methodologies for corrosion monitoring or the assessment of (fretting) corrosion on said modular rod-on-screw interfaces. Moreover, current standardised mechanical testing protocols (e.g. ASTM F1798-21 or F1717-21) do not fully replicate the complex biomechanical and biochemical environments found in vivo, particularly under peri-prosthetic inflammatory conditions.
To address this gap, this study presents a multi-level experimental framework for advanced investigation of wear and corrosion degradation on spinal instrumentation implant materials and implant assemblies. The framework comprised five experimental levels of increasing complexity, all conducted on the clinically prevalent low-carbon CoCrMo and Ti6Al4V alloy combination in phosphate-buffered saline (PBS, pH 6.4 and 7.4) solution with the addition of H2O2 to simulate inflammatory (SI) conditions. Novel pre-clinical assessment methodologies integrate real-time electrochemical monitoring and physiological loading to evaluate the fretting corrosion behaviour of rod-on-screw interfaces and bilateral spinal instrumentation systems.
Static immersion and crevice corrosion testing (Levels 1 and 2) deployed open circuit potential (OCP) and cyclic potentiodynamic polarisation to establish the baseline electrochemical behaviour of both alloys across a range of physiological and SI conditions. PBS+30 mM H2O2 was identified as the harshest SI condition and selected for all subsequent testing. Paradoxically, the co-addition of HCl and H2O2 inhibited rather than amplified the corrosive degradation. Crevice corrosion testing using artificial crevices (glass lenses) demonstrated that H2O2 outweighed both pH and crevice geometry as the dominant factor in CoCrMo’s passivity loss, with the corrosion current density increasing by about one order of magnitude in crevice presence. CoCrMo lost its passivity in every condition where H2O2 was present, whilst Ti6Al4V retained a compromised but intact passivity, demonstrating a fundamental asymmetry between the two alloys.
Pin-on-plate fretting corrosion testing (Level 3) utilised a bespoke fretting tribometer with an integrated 3-electrode cell, enabling OCP and intermittent linear polarisation resistance (LPR) measurements during short-term (6,000 cycles) and long-term (250,000 cycles) testing. The mechanical fretting response was electrolyte-independent. However, a previously unreported OCP double-step cathodic drop was observed exclusively in H2O2 presence, interpreted as sequential depassivation of the Ti6Al4V and CoCrMo oxide films. The total loss nearly quadrupled under SI conditions, with CoCrMo contributing up to 92% of the loss. ICP-MS analysis confirmed a considerable increase in ion release, with near-identical Ti concentrations after short- and long-term testing, suggesting a wear-in phenomenon.
Rod-on-screw implant subassembly fretting corrosion testing (Level 4) deployed an original purpose-designed corrosion cell, mounted on the tribometer, to test commercially available CoCrMo rods against Ti6Al4V pedicle screws over 250,000 cycles with in-situ OCP and LPR monitoring. The OCP double-step feature was reproduced with greater definition at the implant level. The corrosion current, which initially increased in H2O2 presence, declined gradually during fretting, attributed to H2O2 decomposition at the confined rod-on-screw interface. Despite the declining current response, the gravimetric material loss almost quadrupled, with CoCrMo rods accounting for about 91% of the loss. ICP-MS analysis revealed a 35-fold increase in chromium release under SI conditions. For the first time, metallic nanometre-sized wear debris was isolated and characterised from the dissimilar rod-on-screw interface.
Pre-clinical bilateral spinal instrumentation assembly testing (Level 5) employed a new purpose-designed testing setup combining ASTM F1717-compliant mechanical loading with real-time corrosion monitoring via OCP and potentiostatic polarisation measurements. Sequential loading profiles (flexion-extension FE, lateral bending LB, and axial rotation AR) and simultaneous physiological loading were applied in PBS and PBS+30 mM H2O2. The charge transfer followed a consistent hierarchy (FE < LB < AR) independent of the electrolyte. Simultaneous physiological loading dramatically exacerbated degradation compared to sequential testing, with charge transfer and material loss increasing by approximately one order of magnitude. The absence of measurable corrosion activity under pure axial compression confirmed that rotational and bending micro-motion determined the onset of fretting corrosion.
The novel pre-clinical testing platforms developed at Levels 4 and 5 advance beyond the scope of the current standards ASTM F1717, F1798, and F1875 by combining clinically representative implant geometries (sub- and bilateral assemblies) with electrochemical monitoring under SI conditions. The results show that H2O2 transforms the degradation regime at the CoCrMo-Ti6Al4V interface from a mechanically dominated process into one in which electrochemical and chemical dissolution become the primary causes of material loss. The methodologies and findings presented provide a foundation for future standardisation efforts and regulatory decision-making under the EU Medical Device Regulation.