Identifying complications and failure modes of innovative growing rod configurations using the (hybrid) magnetically controlled growing rod (MCGR) and the spring distraction system (SDS)

Abstract

Background: Magnetically controlled growing rods (MCGRs) offer non-invasive distractions in Early-Onset Scoliosis (EOS). However, implant-related complications are common, reducing its cost-effectiveness. To improve MCGRs functionality and cost-effectiveness, we often combine a single MCGR with a contralateral sliding rod (hybrid MCGR). Recently, we developed the spring distraction system (SDS) as an alternative, which provides continuous distraction forces through a helical spring. This study aims to identify complication rates and failure modes of EOS patients treated with either of these innovative systems.

Methods: This single-centre retrospective study included EOS patients treated with a (hybrid) MCGR or SDS between 2013 and 2018. Baseline demographics, and data regarding complications and implant growth were measured. Complication rate, complication profile, complication-free survival and implant growth were compared between groups.

Results: Eleven hybrid- and three bilateral MCGR patients (4.1-year follow-up) and one unilateral, eleven hybrid and six bilateral SDS patients (3.0-year follow-up) were included. Groups had similar age, sex, aetiology distribution, and pre-operative Cobb angle. Complication rate was 0.35 complications/patient/year for MCGR patients and 0.33 complications/patient/year for SDS patients. The most common complications were failure to distract (MCGR-group; 8/20 complications) and implant prominence (SDS-group; 5/18 complications). Median complication-free survival was 2.6 years, with no differences between groups (p = 0.673). Implant growth was significantly higher in the SDS-group (10.1 mm/year), compared to the MCGR-group (6.3 mm/year).

Conclusion: (Hybrid) MCGR and SDS patients have similar complication rates and complication-free survival. Complication profile differs between the groups, with frequent failure to distract leading to significantly reduced implant growth in (hybrid) MCGR patients, whereas SDS patients frequently exhibit implant prominence and implant kyphosis.

Level of evidence: III.

Keywords: Complications; Early-onset scoliosis; Growth-friendly; Implant failure; Magnetically controlled growing rod; Spring distraction system.

Fig. 1

Spring distraction system concept. The SDS consists of three parts that are added to a traditional screw-rod construct: (1) The side-to-side connector (green) with one oversized hole through which a rod can slide freely. (2) The spring (gold) can be compressed over the rod by (3) the buttress (blue) during surgery, and then provides a continuous distraction force

Fig. 2

Implant configurations. Different MCGR and SDS configurations, coloured rod outlines represent the parts of the rod that can freely slide. a Bilateral (offset) MCGR. b Unilateral concave MCGR combined with a convex sliding rod that is fixated to the apex for apical control (hybrid). The convex rod can freely slide through the proximal and distal side-to-side connectors. c Unilateral concave SDS combined with a convex sliding rod that is fixated to the apex for apical control. The convex rod can freely slide through the proximal and distal side-to-side connectors. d Bilateral SDS fixated to the pelvis with ilio-sacral screws. e Unilateral concave SDS without a convex rod

Fig. 3

Magnetically controlled growing rod complications. Examples of Magnetically Controlled Growing Rod complications. a Actuator rod that is broken and that is disengaged from the rest of the implant. b The actuator rod is disengaged from the actuator pin and radial bearing debris is present in the actuator portion of the MCGR. c Rod fracture close to the distal foundation after 1.5 years. d Anchor failure of the proximal hook and pedicle screws. e Proximal junctional kyphosis

Fig. 4

Spring distraction system complications. Examples of spring distraction system complications. a Fatigue failure of sliding side-to-side connector. b Post-operative radiograph showing the angle that the rods make in the coronal and sagittal plane. c After several years of follow-up, distraction caused kyphosis between the sliding and the static rod that resulted in prominence (dashed line). d Rod fracture near the apical screw. e Distal anchor failure. The iliosacral screw backed out of its original iliosacral trajectory

Fig. 5

Kaplan–Meier analysis of complication-free survival. Survival time to the occurrence of a complication was evaluated for all patients (left) and for the MCGR and SDS groups separately (right)

Fig. 6

Cumulative implant lengthening over time. Cumulative implant length over time is plotted for the MCGR a and the SDS b group. The measured values (dashed) of each patient are shown. With the cumulative implant length at latest-follow-up, a linear regression was performed for each group c. The intercept-free slopes and their 95% confidence intervals are shown