title: Biomechanical effects of fusion levels on the risk of proximal junctional failure and kyphosis in lumbar spinal fusion surgery.
abstract: BACKGROUND
  Spinal fusion surgery is a widely used surgical procedure for sagittal realignment. Clinical studies have reported that spinal fusion may cause proximal junctional kyphosis and failure with disc failure, vertebral fracture, and/or failure at the implant-bone interface. However, the biomechanical injury mechanisms of proximal junctional kyphosis and failure remain unclear.
  
  
  METHODS
  A finite element model of the thoracolumbar spine was used. Nine fusion models with pedicle screw systems implanted at the L2-L3, L3-L4, L4-L5, L5-S1, L2-L4, L3-L5, L4-S1, L2-L5, and L3-S1 levels were developed based on the respective surgical protocols. The developed models simulated flexion-extension using hybrid testing protocol.
  
  
  RESULTS
  When spinal fusion was performed at more distal levels, particularly at the L5-S1 level, the following biomechanical properties increased during flexion-extension: range of motion, stress on the annulus fibrosus fibers and vertebra at the adjacent motion segment, and the magnitude of axial forces on the pedicle screw at the uppermost instrumented vertebra.
  
  
  CONCLUSIONS
  The results of this study demonstrate that more distal fusion levels, particularly in spinal fusion including the L5-S1 level, lead to greater increases in the risk of proximal junctional kyphosis and failure, as evidenced by larger ranges of motion, higher stresses on fibers of the annulus fibrosus and vertebra at the adjacent segment, and higher axial forces on the screw at the uppermost instrumented vertebra in flexion-extension. Therefore, fusion levels should be carefully selected to avoid proximal junctional kyphosis and failure.
authors: Department of Mechanical Engineering, Kyung Hee University, Yongin, Republic of Korea.
authors: Won Man
authors: WM
authors: Park
authors: Department of Mechanical Engineering, Kyung Hee University, Yongin, Republic of Korea.
authors: Dae Kyung
authors: DK
authors: Choi
authors: Department of Applied Mathematics, Kyung Hee University, Yongin, Republic of Korea.
authors: Kyungsoo
authors: K
authors: Kim
authors: Department of Orthopaedic Surgery, Columbia University College of Physicians and Surgeons, New York, NY, USA.
authors: Yongjung J
authors: YJ
authors: Kim
authors: Department of Mechanical Engineering, Kyung Hee University, Yongin, Republic of Korea. Electronic address: yoonhkim@khu.ac.kr.
authors: Yoon Hyuk
authors: YH
authors: Kim
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D004868:Equipment Failure; D020342:Finite Element Analysis; D006801:Humans; D007403:Intervertebral Disc / Q000601:surgery; D007738:Kyphosis / Q000209:etiology / Q000503:physiopathology*; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery; D065289:Pedicle Screws; D016059:Range of Motion, Articular / Q000502:physiology; D016103:Spinal Fractures / Q000209:etiology / Q000503:physiopathology; D013123:Spinal Fusion / Q000009:adverse effects* / Q000379:methods; D013312:Stress, Physiological / Q000502:physiology; D013718:Tensile Strength / Q000502:physiology; D013904:Thoracic Vertebrae / Q000503:physiopathology* / Q000601:surgery
keywords: Biomechanics; Finite element analysis; Proximal junctional failure; Proximal junctional kyphosis; Spinal fusion; Thoracolumbar spine