The use of magnetically controlled growing rods in the spine

Authors Martina Tognini, Harry Hothi, Sean Bergiers, Holly Morganti, Johann Henckel, Anna Di Laura, John Skinner, Alister Hart, of The Royal National Orthopaedic Hospital, discuss the use of magnetically controlled growing rods in the surgical treatment of severe early onset scoliosis.

Early Onset Scoliosis (EOS) is a spinal pathology defined as a curvature of the spine ≥10° in the frontal plane with onset before 10 years of age [1]. Among the risks associated with EOS the most severe one is the development Thoracic Insufficiency Syndrome (TIS), defined as the inability of the thorax to support normal respiratory function and lung development in growing children [2], which can lead to increased morbidity and mortality.

Both surgical and non-surgical treatment options have been proposed. Bracing and casting may be effective for less severe curves, yet surgical intervention may be necessary in more acute cases [3]. The most commonly adopted surgical treatment is based on distraction-based constructs, implants correcting the major curve while allowing for spinal growth thanks to extendable rods.

Magnetically Controlled Growing Rods (MCGRs), used in the surgical treatment of severe early onset scoliosis (EOS), are a distraction-based system enabling outpatient distraction procedure, while the alternative system, Traditional Growing Rods (TGRs), require repeated invasive surgeries to perform implant lengthening, with an increased risk of complications and burden to patients and families.

MCGRs are a relatively recent technology, the first prospective patient series rods being published in 2012 [4]. The commercially available MCGR system is the MAGnetic Expansion Control rod, manufactured by NuVasive (Nuvasive Specialised Orthopaedics, San Diego, CA). Since the first use, 7 design iterations have been commercially available, the latest (still implanted) being MAGEC 1.3, 2.0,2.1 and MAGEC X, first used in mid-2017 [5]. The most important design modification introduced with the latest rod design has been the end-cap component development, aimed at enhancing the sealing system between lengthening rod and internal mechanism.

Recently, concerns regarding the risks associated with the use of MCGRs have been raised. The latest design iteration (MAGEC X) was recalled in 2020 following an Urgent Field Safety Notice (FSN) describing a 0.5% probability of post-implantation separation of an actuator end cap component.

In the UK, on 1st April 2020, a Field Safety Notice (FSN) was issued by the manufacturer, voluntarily suspending the supply of all MAGEC rods. The Medicines & Healthcare products Regulatory Agency (MHRA, UK) on the same day released an MDA advising surgeons not to implant MAGEC rods until further notice in the UK and Republic of Ireland. In the EU, on 5th April 2021, NuVasive published a company statement communicating the temporary suspension of the CE mark due to evidence gaps for the MAGEC system. The FSN states that implant malfunctioning in vivo can manifest as locking pin breakage, O-ring seal failure, metal wear debris and failure of the rod to distract.

In the US, the FDA in July 2020 cleared a modified version of the MAGEC Model X rod, designed to mitigate endcap separation events. In addition, the FDA began receiving reports in early 2021 describing local tissue reactions potentially related to endcap separation events with the MAGEC devices. MAGEC X modified implants are currently being implanted in the US, while they are not in the EU.

Considering the complicated regulatory landscape, post-market surveillance plays an essential role in the evaluation of MCGRs’ safety and efficacy. At the Royal National Orthopaedic Hospital (RNOH) Implant Science Centre, we’ve collected over 200 explanted MCGRs from around the UK and Republic of Ireland. Each implant received at our Centre undergoes several steps of testing, which overall we define as “retrieval analysis”: 1) visual assessment of external damage; 2) plain radiographs to identify drive-pin fracture in the internal mechanism (Figure 1); 3) functional testing to evaluate the implants’ ability to distract/generate force; 4) disassembly to assess the internal mechanism’s state. Together with performing retrieval analysis of these implants, we collect clinical and imaging data (Figure 2) to get a deeper understanding of the causes of eventual early failure.

The assessment of MCGRs’ performance involves the evaluation of surgical, implant and patient risk factors [3] altogether. Surgical factors to be taken into consideration include (but are not limited to): rod configuration, contouring, positioning, anchoring technique and lengthening protocol. These surgical choices are highly influenced by patient factors, such as patient’s major curve magnitude, age, or BMI [6]. Various studies on the assessment of implant’s performance by means of retrieval analysis have been published, analysing failure of the distraction mechanism due to the fracture of a drive pin (Figure 1) [7], wear patterns in the telescopic region of the implant [8], force produced [9] and rod lengthening [10] at explant. The comparison of retrieval findings with comprehensive clinical and imaging data will offer deeper insight into further studies in this direction are required.

We recommend that all new implant designs should incorporate retrieval analysis in their post-market surveillance.

References:

1. Yang S, Andras LM, Redding GJ, Skaggs DL (2016) Early-onset scoliosis: A review of history, current treatment, and future directions. Pediatrics 137: . https://doi.org/10.1542/peds.2015-0709
2. Campbell RM, Smith MD (2007) Thoracic insufficiency syndrome and exotic scoliosis. J Bone Jt Surg – Ser A 89:108–122 . https://doi.org/10.2106/00004623-200701001-00013
3. Tognini M, Hothi H, Dal E, Masood G, Colin S, Stewart N, Henckel J, Hart A (2021) Understanding the implant performance of magnetically controlled growing spine rods : a review article. Eur Spine J. https://doi.org/10.1007/s00586-021-06774-8
4. Cheung KMC, Cheung JPY, Samartzis D, Mak KC, Wong YW, Cheung WY, Akbarnia BA, Luk KDK (2012) Magnetically controlled growing rods for severe spinal curvature in young children: A prospective case series. Lancet 379:1967–1974 . https://doi.org/10.1016/S0140-6736(12)60112-3
5. Hothi H, Tucker S, Shafafy M, Nnadi C, Cheung KMC, Dal Gal E, Tognini M, Henckel J, Skinner J, Hart A (2020) Management of patients with magnetically controlled growth rods amidst the global COVID-19 pandemic. Eur Spine J 29:2409–2412 . https://doi.org/10.1007/s00586-020-06516-2
6. Abdelaal A, Munigangaiah S, Trivedi J, Davidson N (2020) Magnetically controlled growing rods in the treatment of early onset scoliosis: a single centre experience of 44 patients with mean follw-up Of 4.1 years. Bone Jt Open 1:405–414 . https://doi.org/10.14531/SS2020.1.25-41
7. Panagiotopoulou VC, Tucker SK, Whittaker RK, Hothi HS, Henckel J, Leong JJH, Ember T, Skinner JA, Hart AJ (2017) Analysing a mechanism of failure in retrieved magnetically controlled spinal rods. Eur Spine J 26:1699–1710 . https://doi.org/10.1007/s00586-016-4936-z
8. Wei JZ, Hothi HS, Morganti H, Bergiers S, Dal Gal E, Likcani D, Henckel J, Hart AJ (2020) Mechanical wear analysis helps understand a mechanism of failure in retrieved magnetically controlled growing rods: a retrieval study. BMC Musculoskelet Disord 21:1–11 . https://doi.org/10.1186/s12891-020-03543-4
9. Rushton PRP, Smith SL, Forbes L, Bowey AJ, Gibson MJ, Joyce TJ (2019) Force Testing of Explanted Magnetically Controlled Growing Rods. Spine (Phila Pa 1976) 44:233–239 . https://doi.org/10.1097/BRS.0000000000002806
10. Rushton PRP, Smith SL, Kandemir G, Forbes L, Fender D, Bowey AJ, Gibson MJ, Joyce TJ (2020) Spinal Lengthening with Magnetically Controlled Growing Rods: Data from the Largest Series of Explanted Devices. Spine (Phila Pa 1976) 45:170–176 . https://doi.org/10.1097/BRS.0000000000003215

Staff Writer