Correction of Scoliosis

Abstract

Background: Scoliosis is a three-dimensional abnormal curvature of the spine, with Adolescent Idiopathic Scoliosis (AIS) being the most common type. Current fusion treatment is highly invasive and removes all mobility of the fused spinal segments. To tackle this problem, non-fusion techniques were developed, that try to correct the abnormal spinal curvature while maintaining healthy spinal mobility. The state of the art of non-fusion treatment includes techniques that have highly inefficient methods of correction, contain long and hefty structures leading to large surgical scars, or cause the need for revision surgery. Most importantly though, all of the current techniques have high complication rates. It was therefore deemed necessary to design a mechanism that facilitates minimal invasive and local, non-fusion correction in a mechanically effective way, by applying constant force in the appropriate direction.

Methods: The anatomy and mechanics of a thoracic spinal segment were analyzed, showing the challenges that are at hand. From there, lists of requirements and wishes were generated that initiated the conceptualization process. Concepts were then generated, categorized, and selected after which it was shown that the concept of a single buckled leaf spring with joints was the most suitable. Future vision figures were made showing how an implant could look like that contains this mechanism. The mechanism was then further developed and an experimental setup was designed and built. This setup was used to carry out four proof of principle experiments that examined the viability of the buckled leaf spring as functional mechanism. The relation between thickness and buckling force was tested, as well as the hysteresis of the mechanism. Furthermore, the effect of misaligning
the mechanism both translationally and rotationally has been examined. Finally, the mechanism was placed inside soft tissue phantoms to examine the effect of the presence of surrounding soft tissues.

Results: A buckled leaf spring that fits the size and material requirements was manufactured and the results of the proof of principle experiments show that it manages to create a constant buckling force of desired magnitude for correction. However, misaligning the implant did show effects on the force profile, with the effects of translational misalignment being the most significant. Finally, little hysteresis was perceived, which significantly increased when the leaf spring was placed within a soft tissue phantom.

Discussion and Conclusion: The mechanism is as of yet not suitable for clinical implementation. Further design steps must be conducted and an extensive analysis of the anatomical and mechanical effects of placing the implant against the spinous processes must be carried out. Further recommendations have been made to improve the design and functioning of the mechanism. This study has demonstrated the potential that the mechanism of a buckled leaf spring has in the constant force correction of scoliosis.