Measurement of the wear volumes generated within retrieved magnetically controlled growing rods

Magnetically controlled growing rods are used for the surgical treatment of early onset scoliosis. However, one of the key concerns around the use of these rods is the extensive titanium alloy metallosis seen at revision surgery. To quantify the wear causing this metallosis, for the first time the internal wear volumes from magnetically controlled growing rods have been measured. A cohort of 30 retrieved rods was measured, from 18 cases (6 single rods and 12 dual rod constructs) after an average time in vivo of 1120 days. Another pair of rods explanted after 30 days due to infection served as a negligible-wear datum to validate the methodology. From each retrieved rod, the wear volume of the extending bar component and of the outer casing component was measured. This was achieved through geometric measurements of the components, with the geometry of the internal aspect of the outer casings determined using a replicating compound and then measuring the replica. The average wear volume of the retrieved rods was 71.8 (21.2-201.9) mm³ which equated to an average wear rate of 23.4 (6.9-65.8) mm³/year. Of the 32 rods measured, 21 were 90 mm in length and 11 were 70 mm in length. The 90 mm rods showed a statistically significant greater wear than the 70 mm rods (p = 0.007); this may be attributed to the greater extension length available in the 90 mm rods, and thus for greater wear under boundary lubrication. The wear volumes measured from the retrieved growing rods are substantial and of concern in this paediatric population.