Biomechanics of prophylactic tethering for proximal junctional kyphosis: effects of cyclic loading on tether strength an
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BIOMECHANICS
Biomechanics of prophylactic tethering for proximal junctional kyphosis: effects of cyclic loading on tether strength and failure properties Damon E. Mar1 · Steven J. Clary2 · Brant Ansley2 · Joshua T. Bunch2 · Douglas C. Burton2 · Terence E. McIff2 Received: 5 December 2019 / Accepted: 25 March 2020 © Scoliosis Research Society 2020
Abstract Study design Biomechanical evaluation of woven polyester tethers. Objectives To quantify changes in tether elongation, stiffness, and failure characteristics after cyclic loading. Summary of background data Ligamentous augmentation is gaining interest as a technique to prevent proximal junctional kyphosis (PJK) in adult spinal fusions. There are a lack of data regarding the effects of cyclic loading on polyester tether mechanical properties. Tether stretch may lead to loss of stabilization and increased risk of tether failure. Biomechanical data are needed to determine the effects of cyclic loading on tether integrity. Methods Testing was done in two materials: (1) a synthetic cortical bone composite to determine baseline mechanical properties, and (2) nine cadaveric L1 spinous processes. 5 mm woven polyester tethers were looped through 2.5 mm holes drilled in each material. First, five tethers were tested directly to failure in the synthetic bone to establish baseline failure properties. Next, tethers were tested at one of the three cyclic load ranges [5%, 25%, and 50% (n = 5 each) of baseline failure] for 1000 cycles and then loaded to failure. Cadaveric tests were done at the 25% range and compared to synthetic bone tests at the same range. Cadaveric failure tests were classified as either tether failure or spinous process bone failure. Results Greater cyclic loading range had a significant effect on tether loop elongation, increased stiffness, and decreased ultimate tensile force. Among the cadaveric failure tests, 56% resulted in tether failure and the remaining 44% resulted in bone failure. Conclusions Polyester tethers stretch significantly when loaded to physiological ranges. Anticipation of tether stretch may be an important consideration for a tethering strategy to prevent PJK. Improved understanding of tether material properties can provide guidance for the evaluation of clinical outcomes associated with techniques to reduce the risk of PJK caused by ligamentous laxity. Level of evidence Biomechanical study. Keywords Tether · Biomechanics · Adult spinal deformity · Proximal junctional kyphosis
Introduction Adjacent segment pathology (ASP) is a common postoperative complication associated with spinal fusion [1]. Proximal junctional kyphosis (PJK) and proximal junctional failure * Douglas C. Burton [email protected] 1
Texas Back Institute, Plano, TX, USA
Department of Orthopedic Surgery, University of Kansas Medical Center, 3901 Rainbow Boulevard, Mail Stop 3031, Kansas City, KS 66160, USA
2
(PJF) represent two of the most common forms of ASP among adult spinal deformity (ASD) patients and have been the focus of numerous studies in rec
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