Keeping a Chin Up in the Face of Adjacent Segment Pathology: A Biomechanical Analysis of Prophylactic Treatments for Proximal Junctional Kyphosis in Adult Spinal Fusions

Abstract

The use of prophylactic tethers for treatment of proximal junctional kyphosis has been gaining clinical interest in recent years. There is currently no clinical consensus on appropriate technique and little biomechanical data to provide initial guidance. The intent of this work is to provide an improved understanding of the basic techniques relevant to spinal reconstructive surgery and to provide initial biomechanical characterization of basic tethering techniques. Three primary goals are proposed: 1) complete a review of spinal tethering techniques to determine the current state of the art of spinal tethering, 2) conduct a series of mechanical characterizations of basic tethering parameters in order to demonstrate their effects on spine biomechanics, and 3) provide concise engineering commentary and perspectives that help tie study findings to relevant clinical concepts and concerns. A review of the literature on spinal tethering resulted in six common techniques, twelve devices, and only six publications to date focusing on tethering for prophylactic treatments in adult spinal deformity. The review indicated a severe lack in current understanding of biomechanical effects. The characterizations of basic technique parameters was done in a series of four biomechanical cadaveric studies which investigated the effects of tether tension, looping technique, and anchoring methods on segment range of motion, intervertebral disc pressures, spinous process loads, and failure modes. The primary results indicate that tether tension plays a significant role in the effectiveness/effect of a tethering technique and that increased spinous process loads are most critical at the uppermost tethered level. Additional findings indicate that the combination of varying multiple technique parameters allows for great flexibility in treatment strategies. While basic in nature, the results found in this work stand are the first of their kind and provide a basis upon which further investigations may better elucidate the relationship of tethering techniques to clinical outcomes.