Does the external remote controller’s reading correspond to the actual lengthening in magnetic-controlled growing rods?

Early-onset scoliosis (EOS) is an abnormal curvature of the spine diagnosed before the age of ten. The best treatment for EOS is unknown, with options ranging from bracing to surgery. Following failure of non-operative treatment, surgical treatment is usually the only option as these curves are generally progressive. Growth sparing spinal surgery is usually preferred as it allows correction of the curve while maintaining growth of the spinal column until the child is close to skeletal maturity when a final spinal fusion operation can be performed.

In June 2014, the National Institute for Health and Care Excellence (NICE) in the UK approved the use of magnetic-controlled growing rods (MCGRs) in EOS on the basis of efficacy and cost-effectiveness [1]. Prior to the introduction of magnetic-controlled growing rods (MCGRs), one of the main options for growth sparing spinal surgery was conventional growing rods. Conventional growing rods normally require 6-month operations to lengthen the rods under general anaesthesia throughout childhood and is associated with high complication rates, e.g. wound infections [2, 3].

However, in MCGR, non-invasive spinal lengthening is performed in the outpatient clinic without sedation or anaesthesia. The magnet within the actuator in MCGR is connected to a lead screw that can be rotated non-invasively by a magnetic field with a handheld device known as external remote controller (ERC), which also contains a permanent magnet. The amount of lengthening achieved is displayed on the ERC device in millimetres. The amount of lengthening can also be assessed radiologically using serial radiographs, fluoroscopy or ultrasound.

Radiograph is the gold standard for assessing the amount of lengthening obtained following a non-invasive spinal lengthening. As there is a radiation risk associated with radiographs, they are not always used after each distraction in some centres. This is particularly so for centres who perform more frequent distractions. At the moment, the optimal frequency of distractions and the amount of lengthening done at each outpatient appointment remain uncertain and this practice is evolving in many centres. There is an increasing trend towards the use of ultrasound as a method of assessing the amount of lengthening because there is no risk of radiation exposure. Ultrasound has previously been validated by two studies as a reliable method of measuring distractions with gold standard radiographs as controls [4, 5]. In our centre, distractions are carried out at 8–10 weekly intervals. Since 2014, we started using ultrasound in our centre in conjunction with our unit’s radiologists. We also took yearly interval radiographs as a routine in our centre. The experience within our unit suggests that there may be a discrepancy between the reported rod lengthening on the ERC and the actual rod lengthening when using the MCGR system. The aim of this study was to determine whether a genuine discrepancy exists between the reported rod lengthening on the ERC and the actual rod lengthening on both ultrasound and interval radiographs.

In total, 96 non-invasive rod lengthenings were undertaken on 11 patients with dual-rod constructs over this time period. This provided 192 sets of readings.

Our study indicates that only 8% of the ERC readings were accurate, while 74% of the ERC reading overestimated the actual rod lengthening. Previous studies have focused on assessing the reliability of using ultrasound rather than radiographs for assessing distraction in MCGR [4, 5]. A recent study has shown somewhat similar findings to ours [6]. They concluded that ultrasound can provide confirmatory information of non-invasive lengthening of MCGR but they tend to underestimate the achieved length as measured by radiographs only in primary cases. In their study, they found that revision cases demonstrate better concordance between ultrasound and radiographs. This was not observed in our study and could be due to a higher number of revision cases in their study. In our series, we found that ultrasound tends to overestimate the achieved length as measured by radiographs. The reason for this could be due to the difference in frequency of the radiograph taken in our series (3 months vs. yearly). As our unit adopted an ultrasound approach to confirming distraction based on other validation studies [4, 5], therefore we did not take radiographs every distraction like their study.

A prospective case series of magnetic-controlled growing rods in 2012 by Cheung et al. [7] reported an actual lengthening of the growing rods of 78.9% of the predicted lengthening (or 1.28 times more of the actual lengthening) when comparing pre- and post-lengthening on plain radiograph. The case series, which consisted of only two patients with three rods but with a total of 45 extensions, reported a good relationship between the predicted and actual distraction lengths and no difficulty with monthly distractions. In contrast, our study demonstrates 27 occasions (14%) when no distraction was achieved despite the ERC reporting a positive lengthening. The discrepancy between our results and the results of Cheung et al. is likely to be due to a difference in the number of patients and growing rods in the two studies. A large retrospective, consecutive, multicentre case series of 30 MCGR patients with a mean follow-up of 18 months reported that there was a 45.5% discrepancy between their intended total distraction and their total measured distraction on radiographs [8].

A recent study reported similar findings when they compared the ERC readings with fluoroscopy. In their study of 21 patients, true distraction (TD) was determined by measuring the expansion gap on dedicated fluoroscopic images of the actuator and this was compared to the intended distraction (ID) reported on the ERC. The true-to-intended distraction ratio was calculated as 0.33, and the difference between the ID and true TD varied from 2 to 44 mm [9]. In the follow-up study, Ahmad et al. have also shown a difference between true distraction (T) and intended distraction (I) in their series of 35 patients followed over 57 months treated with MCGRs. Patients were distracted at 3-month intervals and had fluoroscopic images taken immediately after the distraction. There was also a significant decrease in T/I ratio which was a gradual linear decline over time [10].

The traditional method of measuring the lengthening of MCGRs is using plain radiographs [6]. The use of ultrasound imaging is associated with no exposure to ionizing radiation for patients with MCGR. A previous study demonstrated that for every set of pre-lengthening and post-lengthening radiographs, MCGR patients are exposed to a mean effective radiation dose of 0.26 mSv and an additional mean attributable lifetime cancer risk of approximately 1 in 39,686 [5]. Radiation exposure in children is associated with radiation-induced cancer and as few as 25 whole spine radiographs have been reported to increase the risk of breast cancer by 70% in females [11]. Two previous studies have shown that distraction of the MCGR noted on radiographs can be reproduced on ultrasound with high inter-rater reliability and intra-rater reliability. Yoon et al. [5] reported an intra-rater reliability of 0.983 (95% confidence interval 0.956,1.000) and an inter-rater reliability of 0.987 (95% confidence interval 0.966, 1.000) between ultrasound and radiograph for rod length. Stokes et al. [4] reported that when a rod showed distraction on X-rays of 2 mm, the mean measurement on ultrasound was 1.7 mm (SD: 0.24 mm) while in scenarios where no distraction of the rod was noted on plain radiographs, the ultrasound measurements showed no significant difference. A paper by Cheung et al. [12] which looked at the learning curve in monitoring MCGR distractions with ultrasound found that correlation between radiograph and ultrasound measurements is reasonable to begin with but improves with time. In our series, all our measurements were made by two consultant musculoskeletal radiologists who are highly experienced, and we do not feel this would have played a factor in our results.

Schiedel et al. [13] reported similar problems with magnetic-controlled growing rods used as an intramedullary device in children for limb lengthening. In addition to reporting that 2/26 nails failed to extend with the use of an ERC, they also report a discrepancy between planned extension lengths and achieved length. The largest difference being 8 mm of achieved lengthening compared with 23 mm of planned extension, and they found ERC usage was problematic mostly in patients with femoral lengthening.

Although we adopted ultrasound to measure lengthening like many centres, we also continued with radiographs but done at yearly intervals. The reason for this is twofold: to ensure that metalwork is intact and to measure interval lengthening. One weakness of this study was we might not have detected an issue with lengthening at a particular distraction as we only did the radiographs yearly. However, if no lengthening is obtained on ultrasound, it was our department protocol to radiograph the child to ensure the metalwork is intact.

The National Institute for Health and Clinical Excellence (NICE) is a non-departmental public body of the Department of Health in the UK which makes evidence-based recommendations developed by independent committees, including professionals and lay members and consulted on by stakeholders. These appraisals are based primarily on evaluations of efficacy and cost-effectiveness. Section 3.27 of the NICE guidance for the use of MAGEC™ growing rods states that ‘the committee considered the potential impact of additional imaging associated with more frequent distractions. The committee was advised that X-rays were not always used after each distraction and that ultrasound may be an option for more frequent distractions’ [1]. Although there is now enough evidence in the literature to use ultrasound as the imaging of choice, we would like to draw caution to this. The ERC is wholly inaccurate, and the readings should be disregarded as reported in many studies. Our results suggest that radiographs still remain the gold standard. We would suggest that patients should have ultrasound to monitor each lengthening after distraction but also 6-month radiographs. This would be no different in patients who have been treated conservatively in brace or being extended 6 months for conventional growing rods.

This study suggests that the external remote controller is highly inaccurate at reporting the actual extension achieved. The ERC consistently over-reports the length of extension of the growing rod. The authors would therefore recommend that clinicians using the MCGR for the treatment of early-onset scoliosis include pre- and post-extension imaging (radiographs or ultrasound) to confirm extension lengths at each outpatient extension. To reduce radiation exposure, we would suggest a compromise and patients should have ultrasound to monitor each lengthening after distraction but also 6-month radiographs.