What is new about growth-friendly implants for pediatric spinal deformity?

To the Editor: The treatment principle for pediatric spinal deformity is not only to control the spinal deformity but also to maintain the development of the spine and the chest and the mobility of the spine. To achieve this goal, various kinds of growth-friendly implants have been introduced over the last two decades, which can be divided into three categories, including distraction-based, compression-based, and guided growth systems. The aim of this article is to comprehensively review the currently available growth-friendly implants for pediatric spinal deformity, especially the advancements made in recent decades, to provide reference for clinical treatment and implant design. Distraction-based System,Traditional growing rods (TGRs) and magnetically controlled growing rods (MCGRs): The TGR techniques include single-growing rod and dual-growing rods. Although satisfactory outcomes are well described, complications are prevalent due to the repeated surgeries during treatment. The common complications include superficial infection, implant-related complications, alignment-related complications, and spontaneous fusion. MCGR is introduced to solve this problem and it has been approved by the US Food and Drug Administration (FDA) for the treatment of children (<10-year-old and immature skeleton) with progressive scoliosis (main curvature angle ≥30° and/or thoracic spine height <22 cm). Akbarnia et al[1] conducted a prospective multicenter study consisting of 24 patients (12 MCGRs and 12 TGRs) to compare the clinical outcomes of MCGRs and TGRs The results showed that there was no significant difference in curve correction, T1–T12 growth, T1–S1 growth, or incidence of revision surgery between the two groups. The number of total surgeries in the MCGR group was significantly lower than that in the TGR group. Although MCGR can decrease the incidence of wound complications and psychological problems caused by repeated surgery, mechanical complications, such as rod breakage, failure of lengthening, and anchor failure, still remain a concern to be solved. Distraction-based System,Vertical expandable prosthesis titanium ribs (VEPTR): VEPTR was first approved by the FDA for the treatment of thoracic insufficiency syndrome (TIS) in 2004, and currently, this technique has been approved for the treatment of moderate to severe (Cobb angle ≥60°) early-onset scoliosis (EOS) children (>6 months). According to the type of fixation, VEPTR can be divided into rib–rib fixation, rib–vertebra fixation, and rib–pelvis fixation. El-Hawary et al[2] conducted a multicenter study to evaluate the efficacy of VEPTR for the treatment of EOS. The deformity was corrected from 72° preoperatively to 47° at a 2-year follow-up. At the last follow-up, the T1–T12 height increased from 15.7 cm to 17.7 cm, and the T1–S1 height increased from 25.0 cm to 28.6 cm. However, there is no consensus on the effectiveness of VEPTR treatment. The incidence of complications after treatment with a VEPTR can be as high as 100%, and the most common complication is failure of proximal fixation. Therefore, surgeons need to be cautious when selecting VEPTR. Distraction-based System,Spring distraction system (SDS): The SDS was first proposed in 2020, and it has been approved by the FDA for the treatment of EOS in 2023. Currently, three generations have been introduced [Supplementary Figures 1–3, https://links.lww.com/CM9/C100]. One or multiple compressed springs were placed around a sliding rod to provide a constant distraction to stimulate the growth of the spine and to achieve further correction. The most common complication of first-generation SDS is implant prominence due to increased kyphosis. To prevent kyphosis, the design was improved by replacing 4.5 mm rods with 5.5 mm rods and adding an extra gliding connector. Moreover, a 50 N and 100 N springs were added to the portfolio, which allowed more spring placing strategies. The curve correction and spinal growth of the second-generation SDS were similar to those of the first-generation. However, the incidence of complications and unplanned surgery were decreased significantly. The third-generation SDS is currently in the preclinical stage. By combining an axial torsional spring and a concave distraction spring, the third-generation SDS can provide powerful three-dimensional deformity correction. Distraction-based System,One-way self-expending rod (OWSER): The OWSER (CE marked as Nemost® rod; Euros Company, SAS, La Ciotat, France) consists of two titanium alloy components: rod and domino connector [Supplementary Figure 4, https://links.lww.com/CM9/C100]. The rod has two sections, including a smooth section measuring 5.5 mm in diameter and a notched section measuring 6.35 mm in diameter. A ratchet in a domino connector makes it move in one-way. Proximal fixation uses hooks and distal fixation uses pedicle screws or iliosacral screws. The initial correction mainly comes from intraoperative traction. Patients underwent regular traction to expand the rod postoperatively. Gaume et al[3] reported the results of the OWSER in 21 neuromuscular scoliosis patients with a mean follow-up of 3.9 years. The average Cobb angle was 66° preoperatively, 38° postoperatively, and 32° at the last follow-up. The mean pelvic obliquity was 20° preoperatively, 8° postoperatively, and 6° at the last follow-up. The mean growth of T1–T12 was 0.8 mm/month, and it was 1.5 mm/month in T1–S1. The overall complication rate was 38%, including three cases of lack of rod expansion, two surgical site infections, two central venous catheter-related infections, and one pyelonephritis. As this technique is quite new, studies with a larger sample size, longer follow-up, and higher-level evidence are needed to further validate the effectiveness of OWSER. Distraction-based System,Posterior dynamic deformity correction (ApiFix): Posterior dynamic deformity correction (ApiFix Ltd., Misgav, Israel) has been approved by the FDA and is indicated for adolescent idiopathic scoliosis (AIS) patients with a Cobb angle of 40°–60° [Supplementary Figure 5, https://links.lww.com/CM9/C100]. The systems consist of a unidirectional expandable rod and two pedicle screws, which are inserted on the concave side of the curve. Although the expandable device is rigidly fixed, the polyaxial motion is maintained. The patient is encouraged to perform side bending after surgery, which can elongate the device. As the expandable rod includes a unidirectional ratchet, it will not shorten when the patient returns to an upright position. By repeating the above exercise, dynamic correction can be achieved. Floman et al[4] conducted a study including 22 patients with a minimum of 2 years follow-up to explore the effectiveness of ApiFix. The mean preoperative curve was 47° (range 40°–55°), and at the last follow-up, the mean Cobb angle of the major curve was 25°, representing a 46% correction. Four patients (18%) underwent revision surgery including one for pedicle screw backup, two for nut loosening, and one for ratchet malfunction. Compression-based System,Vertebral body stapling (VBS) and vertebral body tethers (VBT): VBS was first introduced in 1951. Currently, the FDA has not approved this technique for the treatment of EOS. The indications for VBS were: (1) idiopathic scoliosis; (2) Risser sign 0–2; (3) Cobb angle ranging from 25° to 40°; and (4) failure of brace treatment. VBT was first introduced in 2010, and it was approved by the FDA as a fusionless alternative in 2019. The indications for VBT were 30°–70° thoracic curves and 30°–60° thoracolumbar/lumbar curves in skeletally immature patients. Compared with VBS, VBT can theoretically preserve more spinal motion in directions apart from lateral bending and thus can have less negative impact on the overall health of intervertebral discs. Potential intraoperative complications of VBT consist of bleeding, lung parenchyma, and canal penetration, and among them, bleeding is the most common intraoperative complication. Overcorrection and tether breakage or loosening are the most common postoperative complications. Growth-Guided System,Shilla technique: The Shilla technique has been approved by the FDA for the treatment of moderate to severe (Cobb angle ≥60°) EOS. In 2014, McCarthy and McCullough[5] first reported the clinical results of the Shilla technique with a mean follow-up of 5 years. At the last follow-up, the mean Cobb angle of the curves was corrected from 69.0° preoperatively to 38.4° postoperatively, and the average improvements of the Campbell’s space available for lung (SAL) in the left and right hemithorax were 30.5% and 27.7%, respectively. Despite encouraging outcomes, the Shilla technique is also related to a high incidence of implant-related complications. The rods may slide off from the proximal and distal screws in patients with high growth potential and metal ions resulting from friction should also be considered. Growth-Guided System,Luque trolley technique: The Luque trolley technique was originally introduced in 1977 and consists of L- or U-shaped rods fixed segmentally to the spine by sublaminar wires without fusion. Due to the high incidence rate of complications, this technique was not widely used. Afterward, the modern Luque trolley technique (MLT) was introduced, where the proximal and distal ends of the deformity were fixed and fused, while the muscle-sparing technique was used to place gliding instrumentation in the remaining part of the deformity. Mehdian et al[6] reported the mid-term results (mean follow-up of 6.2 years) of MLT in 16 patients. There were six with idiopathic EOS, nine with neuromuscular EOS, and one with syndrome EOS. The average Cobb angle of the main curve was corrected from 66.1° preoperatively to 25.5° at the last follow-up. The mean growth of T1–T12 and T1–S1 were 7.8 mm/year and 10.5 mm/year, respectively. Six patients (five with idiopathic EOS, one with neuromuscular EOS) had rod breakage during follow-up. Currently, MLT has not been approved by the FDA for the treatment of EOS, and spontaneous fusion and wear debris remain concerns. In conclusion, the principle of the treatment for pediatric spinal deformity remains the same. However, with the development of techniques, various growth-friendly implants and techniques have been introduced over the past few decades which provide new methods for a better management of the pediatric spinal deformity in the future. Funding This work was supported by the National Natural Science Foundation of China (No. 82172382), Beijing Municipal Natural Science Foundation (Nos. 7222133 and L222096), and National High Level Hospital Clinical Research Funding (Nos. 2022-PUMCH-B-122 and 2022-PUMCH-A-045). Conflicts of interest None.