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Magnetically Controlled Growing Rod

https://t.me/medicina_free

JacquesD.Müller-Broich

52.1 Introduction and Core Message

Magnetically controlled growing rods (MAGEC™) are intended to treat children with a progressive scoliosis during growth, when conservative treatment has already failed or missed and curve progression is expected not to be sufciently preventable with bracing or corset therapy. MAGEC™ therapy is mainly intended to treat, but not limited to, early- onset scoliosis (EOS), which is dened by a cobb angle of more than 10° under the age of 10 years [1]. MAGEC™ is an abbreviation for “Magnetic Expansion Control.” Patient selection and preoperative planning is crucial to reducing complication rates, requiring a thorough understanding of the technical features and capabilities of the device [2] as well as knowledge of child growth development. In comparison to traditional growth-modulating spinal systems, the MAGEC Rod System prevents frequent open surgical lengthening procedures, which are replaced by outpatient clinic interventions when the MAGEC™ construct is lengthened with an external, percutaneous, electromagnetic remote- control device. Those lengthening procedures of a few millimeters are very well tolerated by the children and their families and are almost pain-free [3]. Reducing the number of surgical procedures leads naturally to a decrease in a signicant number of surgical complications (e.g., infections). Still the MAGEC™ system has complications due to the complexity of spinal deformity, health status of the children, surgical technique, and also device-related adverse or even serious device-related adverse events [4]. Main advantage is that the frequent lengthening procedures are nonsurgical.

J. D. Müller-Broich (*) Department of Orthopedics (Friedrichsheim), University Hospital Frankfurt am Main, Frankfurt am Main, Germany e-mail: jdmbii@web.de

52.2 Indications andContraindications

52.2.1 Indications

The indication for a surgical procedure with the MAGEC™ rod system is correction and stopping of spinal deformity, enhancing the remaining expected spinal growth as well as the thoracic volume in early-onset scoliosis (EOS). Once acceptable spinal growth and thoracic capacity have been achieved, conversion to denitive scoliosis surgery/fusion should be considered.

• Child age>older than 2years.

• Severe and progressive disease with risk of progression of the deformity.

• Pulmonary insufciency (thoracic insufciency) with risk of respiratory failure.

• Failed conservative treatment (bracing/casting).

• Cosmetic deformation (relative indication).

The surgical technique and exact positioning of the

MCGR vary due to multiple factors such as etiology of the scoliosis, age, vertebral growth, segmental dysmorphism or malformation of vertebrae, thoracic deformities, presence of hyper- or hypokyphosis, and of course the nature of the scoliotic deformity. Hughes etal. demonstrated impressively in a survey that there is little consensus among pediatric spine surgeons regarding treatment of EOS [5]. Therefore, every case must be carefully assessed individually as long as common guidelines do not exist. The standardization of etiologies and description of implants by the International Congress on Early Onset Scoliosis and Growing Spine (ICEOS) has helped to simplify the scientific exchange [1, 6]. Rib-, spine-, and pelvis-based anchoring locations are available for proximal and distal fixation of the MCGR and are well known as they are the same as for TGR. Anchor methods (rib, spine, pelvis) [7], such as hooks, screws,

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and laminar bands/clamps, have been described in several technical descriptions.

Placement of the anchors is subject to the preferred surgical technique (freehand, percutaneous (MIS), navigation, robotics).

Choosing the level of the anchoring site might be inuenced by the fact that different senior pediatric surgeons use the principle of the last substantially touched vertebra (LSTV) as lowest instrumented vertebra (LIV) to avoid an increase in curvature below the level of instrumentation (distal adding on) [8, 9].

To avoid the rebound effect by an improperly contoured kyphosis in the proximal end causing proximal junctional kyphosis, the natural kyphosis of the spine should be mirrored. Still, this will only partially prevent PJK as it is multifactorial.

52.3 Contraindications

A major contraindication is the necessity of MRI postoperatively during further treatment (e.g., Tethered cord). While the MRI itself does not harm the construct or patient, the signals are disturbed/altered in the area of the actuators and MRI stronger than 1.5 Tesla should not be used due to the forces evolving with stronger electromagnetic elds. Reduced patient compliance for follow-up visits and continued treatment is a contraindication. Firm patient guidance is very important.

J. D. Müller-Broich

Fig. 52.1 Magec rod

52.4 Technical Prerequisites (Fig.52.1

Nuvasive)

Rod diameters available: 4.5 mm, 5.0 mm, 5.5 mm, and

6.0mm (Fig.52.1).

Actuator lengths available in 70mm/90mm with 28mm and 48 mm of distraction capacity. Total undistracted rod length is 470mm.

Number of rods used—single or double rod use possible. Variability of anchors proximal and distal to ribs, laminae, pedicels, sacrum/ilium via screw, hooks, bolts/ clamps.

External remote control (ERC—2)—allows for independent or simultaneous distraction mode (Figs.52.2 and 52.3).

Preoperative planning and denition of remaining growth potential, expected curve progression.

Screw length and size (small stature of children), cannulated/percutaneous versus open/freehand technique versus Navigation.

Fig. 52.2 External remote control 2

Mobile C-arm for 2D view at least or better for imaging control via uoroscopy during surgery when using freehand technique.

Navigation and 3D scan optional—Radiation exposure of the young children should be reduced to the minimum.

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Fig. 52.3 ERC 2 projecting magnetic eld to MAGEC rod

percutaneously

Neuromonitoring (if desired)—Proper planning and proactive interdisciplinary case discussion with anesthesiologists, neuromonitoring technicians/electrophysiologists, and surgeon are important and should be carried out several times prior to surgery.

Availability of bed at pediatric ICU should be cleared before surgery and reconrmed at the day of surgery prior to initiation of narcosis.

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52.5 Surgical Technique

• Patient is placed in a prone position with special attention

to correct padding avoiding soft tissue damage or skin

burns by uid dissipation under surgery.

• Surgery is carried out under general anesthesia.

• When neuromonitoring is desired, the use of narcotics

such as muscle relaxants or inhalation of narcotics needs

to be avoided as such substances may impede correct

signaling.

• Team-Time out is carried out. Special attention must be

paid to indication, patient history, comorbidities, esti-

mated blood loss, length of surgery, antibiotic regime and

expected problems during the course of surgery need to

be addressed. Technical issues such as neuromonitoring,

availability and sterility of instruments, and implants are

checked. Brief explanation of the surgical plan and fall-

back options should conclude the discussion.

• Skin incision sites are identied (under uoroscopy if

necessary). Anatomical landmarks are marked with a

water-resistant felt marker. (pelvis, spinous process,

c7-Sacrum plumb line) and the incision sites are checked

for plausibility.

Fig. 52.4 Open surgical approach: proximal and distal midline

incisions

• For a classic open surgical approach, skin midline incision is carried out. Coagulation of subcutaneous vessels with electrocautery reduces blood loss and improves visibility.

• The spinous process are identied and the fascia thoracolumbalis is opened longitudinally directly adjacent to the bone. Electrocautery is then used to prepare the pedicle entry points subperiosteally to reduce blood loss. Hooks or retractors are used as desired (Fig.52.4).

• Screw placement with the appropriate length and size is carried out. In our freehand technique, we prefer marking the screw slots with k-wires and check them uoroscopically prior to screw insertion.

• Neuromonitoring with dynamic pedicle probing and screw testing is also a regularly used technique to avoid negative outcomes.

• Adapting a no touch technique for the periosteum of the adjacent segments and facet joints as well as avoiding

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extensive soft tissue damage around the planned anchor locations will certainly foster a benecial outcome although reliable data is not available. (Frequent occurrence of segmental fusion adjacent to the targeted locations in our own experience has been iatrogenic and complicate the conversion to denitive scoliosis surgery).

• For a less invasive approach, we suggest the usage of cannulated screws. Skin midline incision is carried out as described above. Instead of direct incision of the fascia thoraco-lumbalis, a modied Wiltse approach incising the fascia 2–3cm lateral to the midline longitudinally is possible and marking of the pedicle entry points via K-wire or even Yamshidi needles for a transpedicular approach is feasible. The advantage is less soft tissue damage in the treated and adjacent segments. But when fusion is desired at the anchoring segment(s), the advantage is reduced (Fig.52.5).

• When the anchoring elements are in place, subfascial preparation from the two entry sites tunneling the fascia thoraco-lumbalis is carried out by a blunt surgical instrument (Fig.52.6).

• A silicone tube is attached at one site and pulled through the channel. It is later used to insert the MCGR at the desired entry and guides it subfascially avoiding soft tissue damage.

• When the rod is primarily inserted via the silicone tube, the appropriate length and curvature are marked and estimated. (If possible, the insertion of the rod is not needed when length and curve might be estimated sufciently by the surgeon).

• Usage of standard and offset rod need to be planned before surgery (Fig.52.7). Special attention needs to be taken to place the actuators at the correct level before cutting the rod (Fig.52.8).

• The rod is then taken out and cut to the appropriate length and bend with the proper curvature. I advise to add 1–3cm at both ends to allow for proper bending of the rods and account for the nal lengthening procedure. Contouring of the rod should be carried out with at least one centimeter of distance to the actuator in order to prevent intrinsic construct off-axis loading.

• Each rod must be tested prior to nal placement only after rod cutting and contouring has been completed. The manual distractor is mounted on the actuator, the rod is marked at the distraction position: 4 rotations to the left should be applied to distract/check the actuator and 3 rotations to the right will retract the rod to the starting position (Fig.52.9).

• For the nal tightening procedure after rod placement, we have an assistant pulling the patient at the armpits cranially and another assistant who xes the patient at the pelvis. With this technique, we objectively achieve an improved correction/lengthening of the spine prior to nal tightening of the screws. Appropriate team communication is important for this step, which has to be adapted to the individual patients’ situations. Application

J. D. Müller-Broich

Fig. 52.5 Surgical approach levels should be planned preoperatively

in order to reduce soft tissue involvement

Fig. 52.6 Rod placment via a subfascial tunneling technique. A sili-

cone tube is a powerful guiding tool for frequent insertions

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Fig. 52.7 Positioning of actuators at insertion planning with a saw

bone model

of a rod clamp and a distraction retractor facilitate the procedure (Figs.52.10 and 52.11).

• Also special attention to the orientation of the contoured rod ends prior to nal tightening of the set screws is of great importance.

• After nal tightening of the set screws, a MEP/SSEP check is carried out and compared to the baseline.

• Fascial, subcutaneous, and skin closure follow. In usual settings, a wound drain is not necessary. Sterile dressings are applied.

Fig. 52.8 Actuator location of standard and offset rod in x-ray

Fig. 52.9 Manual distractor for actuator mechanism testing prior to

nal rod insertion

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J. D. Müller-Broich

Fig. 52.12 Tools for rod distraction. (Felt marker, magnetic wand,

ERC 2 controller)

Fig. 52.10 Rod reduction tool to insert rod into screw head

Fig. 52.11 Distraction with aid of rod clamp as abutment

52.6 Revision Concepts

In double rod constructs, external distraction depends on the implanted rod types. While the standard rod has the magnet positioned at the caudal end, distracting the rod cranially, the offset rod has the magnet positioned at the cranial end of the actuator. Therefore, the external remote control (ERC) is

Fig. 52.13 Magnetic wand for magnetic actuator eld location

able to distract the rod independently from another, which might be helpful at curve control and correction. Finding of the magnetic areas through the skin is simplied by the use of a magnetic wand (Figs. 52.12 and 52.13). The skin is marked with a felt marker and the ERC-2 is directed onto this mark to properly couple with the actuator in order to distract either one or both rods (Fig.52.14).

Usually, the children are positioned in a comfortable lying prone position over a cushion or blanket or in a lateral decubitus position in order to relax the intrinsic tension of the rod system (Figs. 52.15 and 52.16), but they may also sit in a relaxed upright position, for example, on their parents’ lap (Figs. 52.17, 52.18, and 52.19). Two principal techniques to determine the amount of distraction

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Fig. 52.14 Marking of the skin over the magnetic actuator

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Fig. 52.16 Lateral decubitus position to off-load intrinsic forces of the

transport mechanism

Fig. 52.15 Positioning of child in comfortable prone position with

blanket or cushion with marks to position the ERC 2

Fig. 52.17 Parental assistance with child sitting on lap to off-load

transport mechanism

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Fig. 52.18 Rod distraction with ERC 2 device in prone position

J. D. Müller-Broich

Fig. 52.19 Rod distraction with ERC 2 device in prone position

are described in literature. One is the clunk technique with maximum distraction at each elongation procedure with the ERC-2in the outpatient clinic setting. Due to the axial overload of the actuator components, an audible noise (clunk) occurs and is also palpable through the skin. The second technique relies on expected spinal growth charts (DiMeglio et al.) for the calculation of the distraction parameters in comparison to a normal population using child age and weight. It is called tail-gating technique (TGT) [10]. Control of the lengthening is carried out via X-ray or ultrasound, and the length is documented (Figs.52.20 and 52.21).

Fig. 52.20 Ultrasound image for length control of distraction

Fig. 52.21 X-ray image for length control of distraction. Rod diameter

is used as conversion factor in rule of proportion.

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52.7 End ofMAGEC Therapy

At the end of the spinal growth phase, when the patients reach skeletal maturity, a growth-modulating rod technique such as the MAGEC™ rod system loses its relevance leaving the risk of severe complications if not removed.

Skeletal maturity can be judged by radiological signs of

the hand [11, 12] and pelvis. (Risser sign).

Balsano etal. [2] described disastrous developments after the end of the lengthening period with MAGEC treatment after primarily achieving acceptable results with an removal and observation strategy. The MAGEC rods were removed at skeletal maturity and within a very short time, the curves progressed almost doubling the Cobb angle at the time of removal of the rods. Consequently, the patients were treated with denitive scoliosis correction. Still, the available data on the management of EOS after MAGEC therapy upon reaching skeletal maturity is scarce and controversial. Our recommendation is to remove the rods and apply denitive scoliosis correction in a single stage approach.

Denitive scoliosis correction after MAGEC™ therapy should also be planned carefully. Due to autofusion processes, bony and soft tissue release, need for implant removal, and anchor revision where necessary, resources need to be allocated for complex revision surgery and not just normal idiopathic scoliosis surgery. The need for multilevel osteotomies to correct sagittal and coronal imbalances as well as rotational decits and thoracoplasties in order to regain segmental exibility are very frequent in my own experience.

52.8 Tips andTricks

• Protection of proximal instrumented vertebra—use of

universal clamps/ 6 screws /3 vertebra.

• Corset therapy for partial correction with MAGEC™

rods—shorter instrumentation/saving segments.

• Avoidance of PJK—increased bending of the proximal

part to reach estimated kyphosis (Avoidance of rebound

effect).

• Critical validation of the remaining growth potential—

when curve progression is expected but vertebral/spinal

growth is not expected to exceed 2years, the larger curve

progression might be accepted for a later surgery in order

to avoid a growth-modulating surgery with MAGEC™

rods. It may lead to preventable complications while the

procedure is not absolutely necessary and impeding later

denitive surgery.

• Outpatient organization—scheduling of the MAGEC™

expansion dates at a regular follow-up date (e.g., every

4 months) for every patient at the same day will show

children and their families that they are not alone with this

kind of therapy and may support psychological relief.

52.9 Complications

Due to the complexity of spinal deformity in EOS, several complications have been reported while using the EOS System. Complication rates vary in literature and are subject to high variabilities due to the heterogeneity of the etiology of EOS, age, surgical method/center, and rod type used.

A common complication in traditional growing but also MAGEC rods is proximal anchor failure with pullout of hooks or screws [13].

Single rod constructs are more susceptible to rod breakage than dual rod constructs and were more frequent with the early MAGEC rod types and though addressed by the manufacturer as also the frequent breakage of the actuator pin.

It also led to the development of a further 5.0mm rod with greater implant strength. Implant associated complications regarding failure to distract the system or breaking of components such as the rod itself or the actuator pin as well as abrasive circumferential markings, metallosis (titanium debris) with chronic inammatory soft tissue cellular inltration are frequently reported. Also, endcap sealing dysfunctions are under continuous investigation. In a laboratory investigation, Rushton etal. [14, 15] found a negative correlation of force produced to time in vivo and documented a relationship of off-axis loading leading to a asymmetric abrasion of the rods causative for titanium metallosis.

In a larger systematic review, Thakar etal. [16] reviewed clinical complications and radiological outcomes including 196 patients from 15 different studies. He noted a complication rate of 44.5% (rod or anchoring failure 10.6%, revision surgery 33%, implant failure 11.7%).

The design of the MAGEC™ rod with an unbendable actuator demands most frequently to place the actuator at the level of the at thoraco-lumbar junction or the thoracic spine. Its intrinsic forces atten out the exible spine and lead to a at back or hypokyphotic proximal thoracic spine. This is not always tolerable to the proximal part of the spine and kyphotic forces lead to an increasing proximal junctional kyphosis, a so-called rebound effect [17]. Actions to avoid this may include increased proximal rod end curving, usage of a smaller actuator, and placement of the actuator at the lowest possible position. Male, syndromic, hyperkyphotic, and younger (<5years of age at the time of surgery) children have a higher incidence of PJK [16]. In a larger study of proximal anchor xation by Meza etal. [13], it was documented that spine-based xation was superior to rib based xation with respect to deformity correction. A greater proximal anchor density led to improved major curve correction at 2 years. Higher anchor density (+5 anchors) in the proximal zone did not protect from proximal anchor complication. Helenius etal. suggested a lower risk of wound infections due to less surgical interventions in

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J. D. Müller-Broich

MCGR treatment compared to TGR treatment [5]. In an analysis by Teoh etal. [17], it was shown that MCGR had a lesser risk of deep wound and supercial wound infection than TGR.

References

1. Bai J, Chen K, Wei Q, etal. Selecting the LSTV as the lower instrumented vertebra in the treatment of Lenke types 1A and 2A adolescent idiopathic scoliosis: a minimal 3-year follow-up. Spine. 2018;43(7):E390–8.

2. Balsano M, Spina M. Idiopathic early-onset scoliosis treated with MAGEC rods: what to do after the lengthening period is over? Int J Spine Surg. 2020;14(5):847–51.

3. Cheung JP, Cahill P, Yaszay B, etal. Special article: update on the magnetically controlled growing rod: tips and pitfalls. J Orthop Surg. 2015;23(3):383–90.

4. El-Hawary R, Akbarnia BA.Early onset scoliosis- time for consensus. Spine Deform. 2015;3(2):105–6.

5. Helenius IJ. Standard and magnetically controlled growing rods for the treatment of early onset scoliosis. Ann Transl Med. 2020; 8(2):26.

6. Hughes MS, Swarup I, Makarewich CA, et al. Expert consensus for early onset scoliosis surgery. J Pediatr Orthop. 2020;40(7):e621–8.

7. Inaparthy P, Queruz JC, Bhagawati D, etal. Incidence of proximal junctional kyphosis with magnetic expansion control rods in early onset scoliosis. Eur Spine J. 2016;25(10):3308–15.

8. Joyce TJ, Smith SL, Rushton PRP.Analysis of explanted magnetically controlled growing rods from seven UK spinal centers. Spine. 2018;43(1):E16–22.

9. Kov ST, Bunger C, Li H, etal. Lengthening of magnetically controlled growing rods caused minimal pain in 25 children: pain assessment with FPS-R, NRS, and r-FLACC. Spine Deform. 2020;8(4):763–70.

10. Mardare M, Kieser DC, Ahmad A, etal. Targeted distraction: spinal growth in children with early-onset scoliosis treated with a tailgating technique for magnetically controlled growing rods. Spine. 2018;43(20):E1225–31.

11. Sanders JO, Browne RH, McConnell SJ, etal. Maturity assessment and curve progression in girls with idiopathic scoliosis. J Bone Joint Surg Am. 2007;89(1):64–73.

12. Sanders JO, Khoury JG, Kishan S, etal. Predicting scoliosis progression from skeletal maturity: a simplied classication during adolescence. J Bone Joint Surg Am. 2008;90(3):540–53.

13. Meza BC, Shah SA, Vitale MG, etal. Proximal anchor xation in magnetically controlled growing rods (MCGR): preliminary 2-year results of the impact of anchor location and density. Spine Deform. 2020;8(4):793–800.

14. Williams BA, Matsumoto H, McCalla DJ, et al. Development and initial validation of the classication of early-onset scoliosis (C-EOS). J Bone Joint Surg Am. 2014;96(16):1359–67.

15. Qin X, Sun W, Xu L, etal. Selecting the last “substantially” touching vertebra as lowest instrumented vertebra in Lenke type 1A curve: radiographic outcomes with a minimum of 2-year follow-up. Spine. 2016;41(12):E742–50.

16. Thakar C, Kieser DC, Mardare M, et al. Systematic review of the complications associated with magnetically controlled growing rods for the treatment of early onset scoliosis. Eur Spine J. 2018;27(9):2062–71.

17. Teoh KH, Winson DM, James SH, etal. Magnetic controlled growing rods for early-onset scoliosis: a 4-year follow- up. Spine J. 2016;16(4 Suppl):S34–9.