Magnetically Controlled Growing Rods: The Experience of Mechanical Failure from a Single Center Consecutive Series of 28 Children with a Minimum Follow-up of 2 Years

Abstract

Study design: Retrospective observational study of a continuous series of 28 children.

Purpose: To determine the mechanical failure rate in our cohort of children treated with magnetically controlled growth rods (MCGRs).

Overview of literature: Previous studies report a MCGR mechanical failure rate of 0%-75%.

Methods: All patients with MCGR implantation between 2012 and 2015 were examined and followed up for a minimum of 2 years. A retrospective evaluation of contemporaneously documented clinical findings was conducted, and radiographs were retrospectively examined for mechanical failure. The external remote controller (ERC)-specified length achieved in the clinic was compared to the length measured on subsequent radiographs.

Results: Fourteen mechanical failures were identified in 28 children (50%) across a total of 52 rods (24 pairs and four single constructs). Mechanical failures were due to: failure to lengthen under general anesthesia (seven children), actuator pin fracture (four), rod fracture (one), foundation screw failure (one), and ran out of rod length (one). Of the 14 mechanical failures, six were treated with final fusion operations (reflecting limited further growth potential), and eight patients were treated with the intention for further lengthening. We therefore consider these eight patients to represent the true incidence of mechanical failure in our cohort (29%). The difference between the ERC length and radiographic length was found to be identical in 11% cases; 35% were overestimates, and 54% were underestimates. The median underestimate was 2.45 mm whereas the median overestimate was 3.1 mm per distraction episode. In total, 95% of all ERC distractions were within ±10 mm of the radiographic length achieved over a median of nine distraction episodes.

Conclusions: Our series is the most comprehensive MCGR series published to date, and we present a mechanical failure rate of 29%. Clinicians should be mindful of the discrepancies between ERC length and radiographic measurements of rod length; other modalities may be more helpful in this regard.

Keywords: Complication; Scoliosis; Spine.

Fig. 1.

Method of measuring rod length. Region of Interest Window to ensure adequate exposure of the radiographic area of interest. ^a)Bottom of the measuring aperture (greatest opacity). ^b)This is found just below the rod collar. ^c)Measure to top of the measuring aperture (greatest opacity). Express length in millimeters to nearest 0.1 mm, i.e., 18.9 mm in this figure.