Titanium Wear of Magnetically Controlled Growth Rods (MCGR) for the Treatment of Spinal Deformities in Children

MCGR devices have become a preferred treatment option for children with severe progressive spinal deformity2, thus avoiding repetitive lengthening of surgical implants. The effectiveness of these implants has been proven in several studies3,4,5, however, complications related to the implant remain. MCGRs are typically implanted for several years during the critical period of pediatric growth, therefore the extent of metal wear and the potential hazards of titanium in the pediatric body become increasingly important for treatment evaluation. current EOS standard.

The data report presented on the titanium wear analysis of 23 pediatric patients with scoliosis both in blood samples and on the explanted MCGR implant itself. Three main observations could be found: First, titanium abrasion was observed in the majority of the rods analyzed. Second, MCGR implantation time, number of external lengthening procedures, patient ambulatory status, gender, weight, or height did not influence metal wear or titanium plasma values. . Third, material loss on MCGRs showed a positive correlation with blood plasma titanium values.

Further studies may find titanium wear debris inside the rods for all observed cases8. It is suggested that off-axis loading causes the rod extension bar to contact the inner surface of the rod housing8. Obvious metallosis in the surrounding tissues of the implants can be explained either by growth marks on the rod extension bar due to high stress of the rods during lengthening procedures18 or by wear leakage of the titanium from the inside of the rod8. We were able to find some correlation between the extent of metal loss on the rods and the titanium values ​​measured in the blood. However, titanium values ​​in blood plasma do not reflect local titanium debris in surrounding tissues or titanium that may have been deposited in organs or excreted. Therefore, a direct correlation with the overall titanium content in the pediatric body cannot be established from the data presented.

In the present study, patients showed on average twice as high blood titanium values ​​compared to controls without an implanted device (with high variance between individual patients). Further studies may also detect an increase in blood titanium values ​​after implantation of titanium spinal implants. Li et al. determined values ​​three times higher for patients with MCGR (4.5 ng/mL) than for controls (1.5 ng/mL)19. Yilgor et al. found titanium values ​​four times higher in patients with MCGR (10.2 ng/mL) than in controls (2.8 ng/mL)seven. Border et al. found even higher values ​​(15.9 ng/mL) for patients with MCGR20. Therefore, our measured values ​​of 14.7 ng/mL are within the range of literature values.

In the control group, some plasma titanium values ​​were detected in some individuals, probably due to exposure to personal care and cosmetic products, such as sunscreen or toothpaste, as well as food products (e.g. , chewing gum and sweets)21, which are sources of titanium exposure unrelated to pediatric orthopedic titanium implants. A few authors have attempted to determine the range of “normal” titanium blood levels to establish a threshold value for titanium-induced implant failure.22. However, individual ranges are wide and different methodological approaches to measuring metals in blood rarely yield the same values ​​for the same sample.

Reliable methods for measuring titanium in biological fluids are rare. The most commonly used approaches are ICP-MS based techniques. However, these methods require well-trained operators and have high running costs. Additionally, comparisons between laboratories are difficult, primarily due to the lack of standardized sample preparation, instrument type and settings, and analytical approach.23. Therefore, absolute titanium blood values ​​should be interpreted with caution.

To our knowledge, this is the first study to determine the volume of abraded material from MCGRs. Visual scoring biases were ruled out and we were able to determine the notch width and depth, and therefore the volume of abraded material, with high accuracy (z resolution 0.8 nm, measurement point density was fixed to a measuring point by 0.25 µm). The measurement of the tactile traces further revealed a rotating structure on the surface of the material which is invisible to the eye. In some areas the material appeared shiny, suggesting abrasion in the form of a notch by visual impression, however the tactile trace revealed no notch, only superficial abrasion of the rotating structure.

It was not possible to register small abrasion marks directly on the edges of the individual segments, which occurred on some rods, therefore the total abrasion of the segments may be slightly higher than our measured values. The rather regular notch patterns imply that the movement of the inner and outer cylinders against each other causes abrasion during wear time in a certain position, which changes during regular elongation, in addition to stress during the lengthening procedure18. The observation that notches occur on only one or two, sometimes three adjacent segments in our study supports the hypothesis of off-axis loading causing unilateral abrasion.16. However, further investigations with more parameters such as curve stiffness and coronal and sagittal balance are needed to define the reasons for the abrasion.

In our study, MCGR actuator leakage was not considered as a source of titanium wear debris. However, this leak was proven to be a significant source of metallosis in a previous study where MCGRs were cut to allow internal components to be assessed for metal wear.8.

We could not detect any influence of implantation time, number of lengthenings, or patient weight, height, gender, or ambulatory status on metal implant abrasion or titanium plasma values. . However, it cannot be excluded that the influences of individual factors were overshadowed by the complex interactions of multiple factors for this study cohort, and that influential factors could be found with a larger and more homogeneous population.

In our study, linear regression analysis of metal abrasion and plasma titanium values ​​showed a positive correlation. Elevated levels of titanium in the blood have also been observed in patients with implant failure24 or implant loosening25 and it has been proposed that titanium levels in blood, serum or plasma can be used as a biomarker for the performance of orthopedic implants22,24,25,26. Measurements of serum cobalt and chromium can serve as a biomarker for wear of metal joint implants27.28 and reference levels are available for well and poorly functioning hip implants29. However, neither guidelines nor normal or abnormal blood values ​​have been established for titanium, in part due to technical challenges and the lack of comparability of results obtained between laboratories.23. Recently, a laboratory reference level for blood and plasma titanium in patients with functional titanium hip implants has been proposed (2.2 and 2.56 µg/L for blood and plasma respectively)30. The authors suggested this was a “starting point for further studies to explore the clinical utility of blood titanium as a biomarker of orthopedic implant performance”30. Although technical challenges exist and high titanium levels may not be indicative of implant performance for all patients, it is worth further exploring the possibility of using titanium levels in plasma or other body fluids as potential biomarkers for implant performance.

The fact that we have established a protocol to measure small-scale abrasion reasonably quickly and economically with our applied tactile technique, makes it attractive to use in future larger-scale studies, where potentially influential factors for abrasion can be detected. Additionally, since the toxicity levels of titanium may be better understood in the future, the potential clinical application of this method could be considered, i.e. the measurement of the abrasion of an implant upon removal, either instead of or in addition to measuring titanium blood levels, can help assess a patient’s titanium load and decide on treatment with other implants.

The limitations of this study are the small sample size and, more importantly, the lack of data on pre-implantation blood titanium values ​​to estimate individual increase in blood titanium particles. Further studies on the possible transport pathways of titanium ions, their distribution in organs and therapeutic approaches against the spread of titanium in children’s bodies would provide a better understanding of the extent and long-term effects of the metal wear by implants in children.

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