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 1/3rd 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 [21,22,23]. 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.