Design and Evaluation of a Self-Adaptive Growing Rod for Early Onset Scoliosis

Introduction. Early Onset Scoliosis (EOS) is commonly defined as the development of an observable spinal curve that is diagnosed in children before age 10. Despite the relatively low incidence of EOS, the associated disabilities are often severe and with significant impact on the quality of life for the affected individuals. Children that failed to respond to conservative intervention such as brace and physical therapy treatment will often undergo corrective spinal surgeries. Clinically, instrumentations commonly employed to correct scoliosis can be divided into two systems: the forced growing rods system and the growth guidance system. Both systems have been found to demonstrate spinal alignment correction; however, both systems have its own shortcomings. The forced growing rods system required revision surgery to be carried out periodically in order to realign and lengthen the rods instrumentation as the children grow. The need for repeated open surgery increases the risk of complications such as wound infection and hardware failure as well as the detrimental effect on one’s quality of life and the subsequent psychological stresses. The growth guidance system was originally designed with the intention to avoid the need for revision surgery; however, due to the less restrictive nature of the implantation, some concerns regarding its efficacy in ensuring appropriate growth and to achieve the necessary spinal alignment correction have been raised.
Aim. Given the shortcomings and the disadvantages of the currently available implementation systems to treat EOS, the aim of the current study has two-folds: one is to design a novel self-adaptive growing rods system that would allow adequate spinal correction without the need for revision surgery and secondly, to validate and compare the biomechanical properties of the self-adaptive growing rods system to the traditional rigid-rods system in an in-vitro study.
Materials and methods. The design of the self-adaptive growing rods system, using the SolidWorks software, was centered on the development of a mechanism housed within a connector, which would allow a unidirectional extension of the connecting rods. The connector housing included a combination of a spring and a cylindrical inner sleeve and triangular slope ratcheted pawls. The inclusion of a reverse slope on the ratchet provided resistance against axial pressure and thus preventing the undesirable shortening of the system. The design focused on the maximal use of currently available systems with the addition of the described connector in order to prevent unnecessary deviation from current surgical procedures. For the biomechanical comparison of the self-adaptive growing rods system against the traditional rigid rods system, the biomechanical testing included comparison of the total Range of Motion (ROM) and Neutral Zone (NZ) of the instrumented as well as the adjacent levels between the two rods system, both in pre-extended and extended positions. The strains on the rods when performing the movements were also monitored and included. Moreover, the minimally required force for the extension of the self-adaptive growing rods will also be determined. Eight freshly harvested T1-T9 porcine spines were used in the study to create a scoliosis model. Wedge with 10 degrees of slope were inserted into T4, T5 and T6 respectively to create an overall scoliosis angle of 30 degrees. The biomechanical testing was then carried out using the simulated scoliosis model with and without the self-adaptive and rigid rods systems.
Results. A number of revisions were conducted and remodeled prior to the production of a prototype, which was then utilized in the next stage of biomechanical testing. It was also determined that a pull force of 2.78 Newton is required for the self-adaptive growing rod to be lengthened by a single scaled unit. In the pre-extended and extended positions, both systems demonstrated a significant decrease in total ROM and NZ when compared to the scoliosis model without the instrumentations but no significant differences were found between the two systems. In terms of the instrumented levels, the self-adaptive growing rods demonstrated a significant increase in ROM when compared to the rigid rods. The rods strain analysis revealed that for lateral bending, the self-adaptive growing rods generally demonstrated greater tensile and compressive strains when compared to the rigid rods system, especially for the growing rods placed on the convex side of the spine. For flexion and extension, a similar trend was also observed with the self-adaptive rods generally demonstrating a greater tensile and compressive strain than those measured from rigid rods.
Conclusion. The current study successfully designed and validated the development of a self-adaptive growing rods system, which possess a comparable biomechanical property to those of the traditional rigid rods system in terms of ROM and NZ. It is anticipated that such system will be useful in controlling the development of spinal curvature in EOS and more importantly, avoid the need for revision surgery for the young patients. The greater strain observed in the self-adaptive growing rods system is well within hardware failure tolerance and should not be of concern in the design of the next version of the prototype.