Titanium wear from magnetically controlled growing rods (MCGRs) for the treatment of spinal deformities in children | Scientific Reports – Nature.com

MCGR devices have become a preferred treatment option for children with severe progressive spinal deformity2, thus avoiding repetitive surgical implant lengthening. Efficiency of these implants could be proven in several studies3,4,5, however, implant related complications still remain. MCGRs are usually implanted for several years during the critical pediatric growth period, therefore, the extent of metal wear and the potential hazards of titanium in the pediatric organism gains importance for the evaluation of the current standard treatment of EOS.

The presented data report on 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: Firstly, titanium abrasion was observed in the majority of analyzed rods. Secondly, duration of MCGR implantation time, number of external lengthening procedures, patients ambulatory status, gender, weight or height did not influence metal wear or titanium plasma values. Thirdly, material loss on the MCGRs showed a positive correlation to titanium blood plasma values.

Other studies could find titanium wear debris inside the rods for all observed cases8. It is suggested that off axis loading causes the extending bar of the rod to contact on the internal surface of the rod housing8. Obvious metallosis in the surrounding tissue of the implants can be explained either by growth marks on the extending bar of the rod as a result of high stress of the rods during the lengthening procedures18 or by leakage of titanium wear from inside the rod8. We could find a certain correlation between extent of metal loss on the rods and measured titanium values in the blood. However, blood plasma titanium values do not reflect local titanium debris in the surrounding tissue or titanium that may have been deposited in organs or excreted. Therefore, a direct correlation to the overall titanium content in the pediatric body cannot be made from the presented data.

In the present study, patients showed on average twice as high values of blood titanium compared to controls without a device implanted (with a high variance among individual patients). Other studies could also detect increased blood titanium values after implantation of titanium spinal implants. Li et al. determined three times higher values for patients with MCGRs (4.5ng/mL) than controls (1.5ng/mL)19. Yilgor et al. found four times higher values of titanium in patients with MCGRs (10.2ng/mL) than in controls (2.8ng/mL)7. Borde et al. found even higher values (15.9ng/mL) for patients with MCGRs20. Therefore, our measured values of 14.7ng/mL are in the range of the literature values.

In the control group, certain titanium plasma values were detected in some individuals, probably due to exposure to personal care products and cosmetics, such as sunscreen or toothpaste, as well as to food products (e.g. chewing gums and sweets)21, which are sources of titanium exposure unrelated to pediatric orthopedic titanium implants. A few authors tried to determine the range of normal titanium blood levels to establish a threshold value for titanium-induced implant failure22. However, individual ranges are wide and different methodological approaches to measure metals in blood rarely give the same values for the same sample.

Reliable methods to measure titanium in biological fluids are scarce. The most commonly used approaches are ICP-MS-based techniques. However, these methods require well-trained operators and have high running costs. Additionally, comparisons across laboratories are challenging, mainly due to lack of standardized sample preparation, instrument type and settings and analytical approach23. Therefore, absolute titanium blood values should be interpreted carefully.

To our knowledge, this is the first study to determine the volume of abrased material from MCGRs. Biases of visual scoring were excluded and we could determine width and depth of notches, and thereby volume of abrased material, by great precision (resolution in z 0.8nm, measuring point density was set at one measuring point per 0.25m). Measuring tactile traces furthermore revealed a rotating structure on the surface of the material that is invisible to the eye. In some areas, material appeared shiny, suggesting abrasion in form of a notch by visual impression, however the tactile trace revealed no notches, but only superficial abrasion of the rotating structure.

It was not possible to record some small marks of abrasion directly at the edges of individual segments, which occurred on some rods, therefore total abrasion from the segments may be slightly higher than our measured values. The rather regular patterns of notches imply that movement of the inner and outer cylinder against each other cause abrasion during the wear time in a certain position, which changes upon regular lengthening, additionally to stress during the lengthening procedure18. The observation that notches occur at only one or two, sometimes three adjacent segments in our study supports the hypothesis of off-axis loading causing one-sided abrasion16. However, further investigations with more parameters such as curve stiffness and coronal and sagittal balance are needed to define reasons for abrasion.

In our study, the leakage from the actuator of the MCGRs was not put into account as a source of titanium wear debris. However, this leakage was proven as a significant source of metallosis in a previous study where the MCGRs were cut open to allow internal components to be evaluated for metal wear8.

We could neither detect an influence of implantation time, number of elongations, nor of weight, height, gender or ambulatory status of the patient on metal implant abrasion or titanium plasma values. However, it cannot be excluded that influences of individual factors were overshadowed by the complex interactions of several factors for this study cohort, and that influencing factors may be found with a larger, more homogenous population.

In our study, linear regression analysis of metal abrasion and the values of titanium in plasma showed a positive correlation. Elevated levels of titanium in 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 may be used as a biomarker for orthopedic implant performance22,24,25,26. Measurements of serum cobalt and chromium can serve as 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, partly due to technical challenges and lacking comparability of results gained across laboratories23. Recently, a laboratory reference level for blood and plasma titanium in patients with well-functioning titanium hip implants has been proposed (2.2 and 2.56g/L for blood and plasma respectively)30. The authors suggested this to be a starting point for further studies to explore the clinical usefulness of blood titanium as a biomarker of orthopedic implant performance30. While technical challenges exist and elevated titanium levels may not be indicators for 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 fast and economically with our applied tactile technique, makes it appealing to use this in future larger scale studieswhere potentially influencing factors for abrasion may be detected. Also, given that levels of toxicity of titanium may be understood better in the future, potential clinical application of this method may be taken into accounti.e. measuring abrasion from an implant upon removal, either instead of or additionally to measuring blood-titanium levels, may help to assess titanium load within a patient and decide about treatment with further implants.

Limitations of this study are the small sample size and even more importantly the lack of data on blood titanium values before implantation to estimate the individual increase of titanium particles in the blood. Further studies on possible transport routes of titanium ions, their distribution in organs and therapeutic approaches against spreading of titanium within the childrens body would provide better understanding of the extent and long-term effects of metal wear by implants in children.

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Titanium wear from magnetically controlled growing rods (MCGRs) for the treatment of spinal deformities in children | Scientific Reports - Nature.com