Characterization of ion release from a novel biomaterial, Molybdenum-47.5Rhenium, in physiologic environments

Background Context Metals from spinal implants are released into surrounding tissues by various mechanisms. Metal ion release has been associated with clinical implant failure, osteolysis, and remote site accumulation with adverse events. Significant corrosion and associated metal ion release has been described with currently used spinal implant alloys. A novel metal alloy, Molybdenum-47.5Rhenium Alloy (MoRe), was approved for use in medical implants in 2019 by the FDA. Purpose To evaluate the metal ion release profile of Molybdenum-47.5Rhenium alloy after immersion in both a stable physiologic, as well as in an inflammatory environment. Study Design In vitro study Methods The ion release profile of MoRe alloy was comprehensively evaluated in-vitro after prolonged immersion in physiologic and inflammatory environments. Ion concentration analyses were then conducted using inductively coupled plasma - mass spectrometry (ICP-MS) methods. Comparative testing of titanium (Ti-6Al-4V) and cobalt chromium (Co-28Cr-6Mo) was also performed. Results Under baseline physiologic conditions, the MoRe alloy demonstrates very low molybdenum and rhenium ion release rates throughout the 30-day test period. During the first time interval (Day 0 to 1), low levels of molybdenum and rhenium ions are detected (<0.3 μg/cm2 day) followed by a rapid reduction in the ion release rates to <0.05 μg/cm2 day during the second time interval (Days 1 to 3) followed by further reduction to very low steady state rates <0.01 μg/cm2 day during the third time interval (Days 3 to 7), which were maintained through 30 days. In the inflammatory condition (H2O2 solution), there was a transient increase in the release of molybdenum and rhenium ions, followed by return to baseline ion release rates (Days 2 to 4), with further reduction to low steady state rates of ∼0.01 μg/cm2 day (Days 4 to 8). The measured molybdenum and rhenium ion release rates in both steady state (<0.01 μg/cm2 day), and inflammatory environments (0.01 μg/cm2 day) was far below the established FDA Permitted Daily Exposure (PDE) of 1900 μg/cm2 day for molybdenum and 4400 μg/cm2 day for rhenium. In contrast, titanium and cobalt chromium approached or exceeded their established PDE values in an inflammatory environment. Conclusions The novel biomaterial Molybdenum-47.5Rhenium demonstrated a lower metal ion release profile in both a physiologic and inflammatory environment and was well below the established PDE. Comparative testing of the cobalt chromium and titanium alloys found higher levels of ion release in the inflammatory environment that exceeded the PDE for cobalt and vanadium. Clinical Significance Stainless steel, cobalt chromium, and titanium alloys cause elevated levels of metal ions in patients who have undergone spinal fusion. The known metal toxicities, in-vitro results, and clinical experience with metal-on-metal hip arthroplasty raise concern of potential harmful effects with long-term exposure. A metal alloy such as Molybdenum-47.5Rhenium, which demonstrates a superior metal ion release profile, may represent a valuable alternative to currently widely utilized materials for spinal fusion implants.