Analysis of explanted MAGnetic Expansion Control (MAGEC
) growing rods.
To analyze explanted MAGEC
rods used in management of early onset scoliosis
and identify the mode of failure in such cases.
Summary of Background Data. Magnetically controlled
growing rods are increasingly used as the option of choice for early onset scoliosis
. However, being more complex than conventional growing rods they are perhaps more likely to succumb to multifarious failure modes. In addition, metallosis
has been reported around failed MAGEC
rods from seven UK spinal centers were obtained for independent analysis. Thirty-four MAGEC
rods, from 18 children, explanted for reasons including failure of rod lengthening and maximum rod distraction reached, were cut open to allow internal components to be evaluated and assessed.
Externally, all MAGEC
rods showed localized marks, which were termed “growth marks” as they indicated growth of the rod in vivo
, on the extending bar component. After cutting open, titanium wear
debris was found inside all 34 (100%) MAGEC
rods. Ninety-one percent (31/34) of MAGEC
rods showed measurable wear
of the extending bar, towards the magnet end. Substantial damage to the radial bearing was seen inside 74% (25/34) of MAGEC
rods while O-ring seal failure was seen in 53% (18/34) of cases. In 44% (15/34) of MAGEC
rods the drive pin was fractured but this was felt to be an effect of rod failure, not a cause.
The combination of high volumes of titanium wear
debris alongside O-ring seal damage likely accounts for the metallosis
reported clinically around some MAGEC
rods. Based on this explant
data, a failure mechanism in MAGEC
rods due to the natural off axis loading in the spine was proposed. This is the largest data set reporting a complete analysis of explanted MAGEC
rods to date.
Level of Evidence: 4