Mechanical wear analysis helps understand a mechanism of failure in retrieved magnetically controlled growing rods: a retrieval study

Abstract

Background: To assess the relationship between mechanical wear and the failure of the internal lengthening mechanism in retrieved MAGnetic Expansion Control (MAGEC) growing rods. Methods: This study included 34 MAGEC rods retrieved from 20 patients. The state of the internal mechanism and mechanical wear were assessed in all the rods using plain radiographs and visual inspection. Metrology was then performed to assess the topography and mechanical wear of the telescopic bars, using a Talyrond 365 (Taylor Hobson, Leicester, UK) roundness measuring machine. Results: Plain radiographs showed evidence of a broken internal mechanism in 29% of retrieved rods. Single-side wear marks were found in 97% of retrieved rods. Material loss was found to significantly increase in rods with a damaged internal mechanism (p < 0.05) and rods with longer time in situ (r = 0.692, p < 0.05). Conclusions: We found an association between damage to the internal mechanism of the rods and (1) patterns of single-side longitudinal wear marks and (2) increased material loss. As the material loss was also found to increase over time of rod in situ, we emphasise the importance of early detection and revision of failed MAGEC rods in clinical practice.

Titanium wear from magnetically controlled growing rods (MCGRs) for the treatment of spinal deformities in children

“…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 procedure 18 . 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 abrasion 16 . However, further investigations with more parameters such as curve stiffness and coronal and sagittal balance are needed to define reasons for abrasion.…”

“…Also wearing marks as a sign of metal abrasion from MCGRs have been observed 8 , 16 , 17 , but only few studies have used metrological techniques to characterize the extent of metal abrasion 16 . Measuring abrasion from implants is challenging, especially due to the small scale of changes on the material and the individual shapes in which abrasion may display.…”

Rod Fracture in Magnetically Controlled Growing Spine Rods

“…MCGRs are usually explanted due to an implant failure, infection, or after the end of treatment. Implant failure may occur for several reasons such as actuator pin fracture, 18 , 19 internal wear and damage, 12 rods pullout from the fixation devices, and rod fracture. The complication profile and failure rate for these implants still remain unclear and further studies analyzing the overall reasons for failure are needed, combining retrieval, clinical, and imaging data.…”

“…MCGRs have been shown to experience implant failure in several ways, including an inability to distract due to corrosion or internal pin fracture issues. 12 A fracture of the MCGR itself is estimated to occur in 10.6% of the total number of complications, 13 however, the risk factors for this are poorly understood. 14 …”

The magnetic field strength and the force distance dependency of the magnetically controlled growing rods used for early onset scoliosis

“…By combining the limited information provided by the manufacturer (Nuvasive Inc.) with the literature 13 , 14 , it is possible to outline the construction of the MCGR, the lengthening mechanism, and the interaction between ERC and MCGR: An internal dipolar magnet mounted on a screw mechanism is brought into rotation, which leads to an extension of the rod that drives the distraction. The rotation is initiated by an interaction with an external rotating magnetic field generated by the ERC.…”