Use of magnetically controlled growing rods in the spine
Early Onset Scoliosis (EOS) is a spinal pathology that begins before the age of 10 and is defined as ≥10° curvature of the spine in the frontal plane. The most severe risk associated with EOS is the development of Thoracic Insufficiency Syndrome (TYS), which is defined as the inability of the thorax to support normal respiratory function and lung development, which can lead to increased morbidity and mortality.
Both surgical and non-surgical treatment options have been proposed. A brace and cast may be effective for less severe curves, but surgical intervention may be necessary in more acute cases. The most widely adopted surgical treatment is based on distraction-based structures, implants that correct the main curve while allowing spinal growth through extensible rods.
Magnetically Controlled Growing Rods (MBDs), used in the surgical treatment of severe early-onset scoliosis (EOS), are a distraction-based system that enables the ambulatory distraction procedure, while the alternative system, Traditional Growing Rods (TGRs), requires repetitive invasive surgeries. Perform implant lengthening with an increased risk of complications and burden for patients and their families.
MSDSs are a relatively new technology, with the first prospective patient series bars published in 2012. The commercially available MKBF system is the MAGnetic Expansion Control stick manufactured by NuVasive (Nuvasive Specialized Orthopedics, San Diego, CA). Since initial use, 7 design iterations have been commercially available, with the latest (still implanted) MAGEC 1.3, 2.0,2.1 and MAGEC X first deployed in mid-2017. The most significant design change introduced with the latest rod design has been the end cap component development aimed at improving the sealing system between the extension rod and the internal mechanism.
Concerns have been expressed recently about the risks associated with the use of IDBs. The latest design iteration (MAGEC X) was recalled in 2020 following an Emergency Field Safety Notice (FSN) describing a 0.5% post-implantation probability of an actuator end cap component detachment.
A Field Safety Notice (SGB) was issued in the UK on 1 April 2020 by the manufacturer voluntarily suspending the supply of all MAGEC rods. The same day, the Medicines and Healthcare Products Regulatory Authority (MHRA, UK) issued an MDA advising surgeons not to implant MAGEC rods in the UK and Republic of Ireland until further notice. In the EU, on April 5, 2021, NuVasive issued a company statement announcing the temporary suspension of CE marking due to evidence gaps for the MAGEC system. SGB states that failure of the implant in vivo can manifest as locking pin breakage, O-ring seal failure, metal wear debris, and rod distraction failure.
In the US, the FDA approved in July 2020 a modified version of the MAGEC Model X bar designed to reduce end cap separation incidents. Additionally, in early 2021, the FDA began receiving reports with MAGEC devices describing local tissue reactions potentially associated with end-cap separation events. MAGEC X-modified implants are currently implanted in the US while not in the EU.
Given the complex regulatory environment, post-market surveillance plays an important role in assessing the safety and effectiveness of IDBFs. At the Royal National Orthopedic Hospital (RNOH) Implant Science Centre, we collected over 200 explanted MBDs from the United Kingdom and the Republic of Ireland. Every implant received at our center goes through several testing steps, which we generally describe as 'retrieval analysis': 1) visual assessment of external damage; 2) plain radiographs to identify drive pin fracture in the internal mechanism; 3) functional tests to assess the distraction/force generation ability of the implants; 4) disassembly to assess the state of the internal mechanism. With the retrieval analysis of these implants,
Evaluation of the performance of MBDs includes the combined evaluation of surgery, implant, and patient risk factors. Surgical factors to consider include (but are not limited to): rod configuration, shaping, positioning, fixation technique, and lengthening protocol. These surgical choices are highly influenced by patient factors such as the patient's major curve size, age, or BMI. Several studies have been published on the evaluation of implant performance through rollback analysis, which analyzes the failure of the distraction mechanism due to breakage of a drive pin, wear patterns in the telescopic region of the implant. force produced and rod elongation at the explant.
We recommend that all new implant designs include take-back analysis in their post-market surveillance.
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