Serum metal ion levels in adolescent idiopathic scoliosis (AIS) patients 25 years after treated with Harrington rod instrumentation or bracing

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 MoRe 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 the 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 0-1), 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 1-3) followed by a further reduction to very low steady-state rates <0.01 μg/cm² day during the third time interval (days 3-7), 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 a return to baseline ion release rates (days 2-4), with a further reduction to low steady-state rates of ∼0.01 μg/cm² day (days 4-8). 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) were far below the established FDA-permitted daily exposure (PDE) of 1,900 μg/cm² day for molybdenum and 4,400 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 MoRe 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.

Serum metal ion levels following spinal deformity surgery: a case-control study of 182 individuals

PURPOSE

The aim of this study is to evaluate the levels of chromium (Cr), cobalt (Co), and titanium (Ti) after instrumented fusion for scoliosis.

METHODS

Serum samples were collected at median 2.24 (range 0.1-38.8) years after fusion surgery for scoliosis in 91 individuals, of which 71 had been treated with steel implants and 20 with titanium implants. 91 sex and age-matched non-surgically treated individuals with scoliosis were used as controls. Levels of Cr, Co, and Ti were measured.

RESULTS

In the 91 surgically treated individuals median levels of Cr were 0.54 µg/l vs 0 µg/l in the 91 controls, p < 0.001. Corresponding results for Co were 0.29 µg/l vs. 0.24 µg/l, p = 0.19, and for Ti were 0 µg/l vs. 0 µg/l, p < 0.001. In the individuals with steel implants and their corresponding controls median Cr levels were 0.63 µg/l vs. 0.00 µg/l, p < 0.001 and Co levels 0.27 µg/l vs. 0.23 µg/l, p = 0.36. No Ti was detected. In the individuals with titanium implants, median Cr levels were 0 µg/l vs. 0 µg/l in their corresponding controls, p = 0.38. Corresponding results for Co was 0.39 µg/l vs. 0.31 µg/l, p = 0.27 and for Ti 4.31 µg/l vs. 0 µg/l, p < 0.001. In the individuals with steel implants a negative correlation between implant time in situ and levels of Cr was found ([Formula: see text] = - 0.52, p < 0.001) but not with Co ([Formula: see text] = - 0.14, p = 0.23). Ti was not detected. In the individuals with titanium implants, there was no correlation between implant time in situ and levels of Cr ([Formula: see text] = 0.36, p = 0.12), Co ([Formula: see text] = - 0.12, p = 0.60) or Ti ([Formula: see text] = 0.22, p = 0.35).

CONCLUSION

The use of stainless steel and titanium implants in spinal fusion surgery is associated with elevated metal ion concentrations several years after surgery.

Persistently Raised Serum Titanium Levels After Spinal Instrumentation in Children

STUDY DESIGN

Prospective study of patients undergoing elective spinal deformity surgery with repeated testing for circulating metal ions including preoperative levels acting as controls.

OBJECTIVE

The aim was to determine if levels of particular circulating metal ions are maintained to two years postsurgery including different implant systems and rods.

SUMMARY OF BACKGROUND DATA

Adults having hip replacements, especially metal-on-metal bearings, may develop high metal ion levels. Pediatric spinal implants are known to cause circulating metal ions, notably titanium, chromium, cobalt, and nickel.

MATERIALS AND METHODS

Fifty-six children having spinal deformity surgery were studied with repeated testing for circulating metal ions, using high-resolution inductively coupled plasma mass spectrometry. Linear mixed-effects models adjusting for repeated measurements over time were used to analyze levels of titanium, cobalt, chromium, and nickel.

RESULTS

Titanium levels showed a rapid increase by seven days and a peak at 30 days that was essentially maintained at the two-year assay. At two years, titanium levels were 5.14 times greater compared with the presurgery control level (P<0.0001). Cobalt levels were shown to gradually rise to a peak at 30 days and then slowly decline but remained 1.74 times above mean baseline level at two years (P=0.0004), with a declining trajectory. Chromium and nickel levels rose immediately postoperatively and then steadily declined to baseline by six months and remained at baseline at two years. The five implant systems tested had generally equivalent results.

CONCLUSION

The persistent and rising levels of titanium, in a predominantly female population, is concerning. Titanium is known to cross the placental barrier and enter the circulation of the fetus in rodents and humans, and to accumulate in solid organs especially the liver, spleen, heart, and lymph nodes in humans. This potentially exposes the offspring of mothers with spinal implants to titanium, with potential teratogenic effects.