Introduction
The treatment of early onset scoliosis (EOS) is a complex matter. While braces and casts may be effective in controlling and preventing curve progression in smaller curves, larger curves may require surgery. It is important to note that the primary goals of EOS surgery are to correct the curvature and preserve the growth potential of the spine. Currently, there are three main categories of EOS surgery techniques: distraction-based techniques, growth guidance techniques, and hybrid techniques [
1]. The representative of distraction-based technique is traditional growing rods, which have the strong power to correct the curvature and preserve the growth potential [
2]. However, regularly scheduled distraction surgery under general anesthesia, poor control of apical curvature, and the high risk of implant failure are the main drawbacks of the traditional growing rods [
3,
4]. While magnetically controlled growing rods may avoid repeated surgeries under general anesthesia, they still have some disadvantages, including the high risk of implant failure, limited correction ability, and inability to be applied to severe EOS patients [
5].
McCarthy initially invented the Shilla technique, which is the most widely used growth guidance technique [
6]. The Shilla system was initially tested in goats to prove the ability to preserve the growth potential of the spine. The concept of the Shilla system is guiding spinal coronal alignment into a straighter alignment and utilizing the inherent growth potential of children’s vertebral column to allow the spine to grow. The core part of Shilla system is Shilla screws or sliding screws, which is a poly-axial pedicle screw with a locking cap that can only lock the top of the screw but capture the rod, allowing the screw can slide with the rod in a longitudinal direction. At the apex of scoliosis, traditional fixed-head pedicle screws are placed, and multiple levels of osteotomies are performed, to correct the curve maximally. The Shilla screws are placed at the two ends of the curve through the muscle layer, allowing the spine to grow and maintain the coronal and sagittal alignments.
According to the clinical reports from McCarthy, the Shilla system has demonstrated the abilities to correct curvature, allow the spine and lung cavity to grow, and significantly reduce the number of surgeries compared to the traditional growing rods [
7,
8]. However, the increasing use of the Shilla system in EOS patients has raised concerns among surgeons about its weaker ability to promote growth compared to traditional growing rods [
9,
10], as well as the potential adverse effects caused by the metal debris created by the friction of Shilla screws and rods in vivo [
11,
12].
In order to reduce the metal debris and improve the sliding ability, we modified the traditional Shilla system and named it as the novel growth guidance system. Our design was granted an utility patent in People Republic of China(CN202121171449.6). In this study, we described the design of the novel growth guidance system, and reported the preliminary in vitro experiments results.
Discussion
Growth guidance system was developed from the technique of Luque Trolley. Initially, the Luque Trolley technique employed sublaminar wires and stainless steel rods to correct scoliosis and allow for spinal growth [
13]. However, the implantation of sublaminar wires could strip the peritoneal of spine, caused interlaminar ankylosis and eventually autofusion [
14]. Additionally, sublaminar wires had limit corrective forces over the vertebra. With the advancement of pedicle screw technology and spinal correction techniques, particularly vertebral derotation techniques, McCarthy improved the Luque Trolley technique and invented the Shilla technique. Shilla technique corrected the apex of the scoliosis maximally with osteotomies and vertebra derotation, by using the fixed-head pedicle screw. At the two ends of the scoliosis, the Shilla screws were placed to allow the spine growing in a normal alignment with the inherent growth potential of the spine. The animal experiment result supported the use of the Shilla system in humans by allowing for continued guided growth. Subsequent clinical studies have confirmed that the Shilla technique can be widely used to treat various types of early-onset scoliosis, with a main curve correction rate reaching nearly 50% [
7,
8]. There was also significant growth observed in the height of T1-T12, T1-S1, and space available for the lung [
15,
16]. Meanwhile, throughout the entire treatment, patients treated with the Shilla technique undergo significantly fewer surgeries on average compared to those treated with the traditional growing rods. Additionally, the average treatment cost for patients treated with the Shilla technique was lower than for those treated with traditional growing rods or magnetically controlled growing rods [
16,
17].
However, the Shilla technique was primarily criticized for two major drawbacks, metal debris and the weak ability of the growth promotion [
18]. The metal debris was created by the sliding between the screws and rods, which may increase the concentration of metal ions in local tissues and blood. Actually, in the goat experiment, metallic wear debris was observed in the soft tissue and lymph nodes adjacent the Shilla screws [
6]. The metallic tissue staining was also observed in human patients population [
19]. In a clinical study, Lukina tested the content of Ti, AL and V metal ions in whole bloods and local tissues around the sliding instruments, found that the Ti and V ions in blood increased 2.8 and 4 time respectively, Ti ions in local tissues was more than 1500-fold higher than the control group [
12]. Metallic debris also can induce a large inflammatory response of the macrophages [
11]. Our novel growth guidance system addressed this issue by altering the friction interface between the screws and the rods to reduce metal debris. The UHMWPE gaskets fitted onto the rod can be perfectly positioned within the tulip of the screw, thus preventing direct metal-to-metal contact between the sliding screws and the rods. We chose UHMWP as the material for the gaskets because it was a highly biocompatible polymer with excellent wear resistance. It had been widely used in orthopedic and spinal surgery implants, such as artificial discs, sublaminar wires, and artificial joints [
20]. Fatigue tests confirmed that after 10 million cycles, the wear of UHMWPE gaskets was minimal, and they still effectively prevented direct contact between the screws and the rods. Therefore, we believed that the application of UHMWPE gaskets was an excellent method for improving the friction interface and avoiding metal debris.
The ability of the growth promotion was the second concern about the technique. According to the study conducted by the inventor’s institution, the Shilla patients had less T1-S1 height increase compare to the traditional growing rods [
15]. The study outside the inventor’s institution, showed that EOS patients treated with Shilla technique was approxiamately 1/3rd of predicted normal T1-S1 growth, less than 1/3
rd of growth reported in the inventor’s institution [
10]. The primary reasons for the limited growth-promoting capability of the Shilla technique are twofold: firstly, it lacks of external distraction force, and secondly, excessive friction between screws and rods restricts spinal growth. Based on the traditional Shilla technique, we improved the friction interface between screws and rods by applying UHMWPE gaskets and polishing the rod, to reduce the friction, facilitate screw sliding and minimize restriction on spinal growth. From our experimental results, it was evident that merely by polishing the sliding part of the rod surface can facilitate the sliding of the screws. Additionally, the use of UHMWPE gaskets significantly enhanced screw sliding.
Although the novel growth guidance system is a modification to the Shilla system, we hope our approaches to change the interface of sliding instruments can be also applicable to the all growth friendly techniques involved the sliding elements. Instead of using Shilla sliding screws, Agarwal modified the Shilla technique by using dominos as a sliding elements [
21‐
23]. Cody Bunger(CB) technique combined a single concave MCGR with a sliding rod on the convex side to control the apex, which also utilized dominos as sliding elements [
24,
25]. Same combination applied in the spring distraction system also [
21]. An fitted size UHMWP gasket can also be inserted into the holes of the domino. Additionally, the sliding part of the rod can be polished to minimize metal debris generation and decrease frictional forces.
This study is only a preliminary in vitro experiment by using the MTS system, which is the main limitation of the study. The efficacy of the system, including the metallic and UHMWPE debris created by the system, and the sliding ability, should be assessed in animal model in the future. Also in the future, we believe that the novel growth guidance system can be applied in human with a bright future.
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