title: The effects of muscle weakness on degenerative spondylolisthesis: A finite element study.
abstract: BACKGROUND
  Whether muscle weakness is a cause, or result, of degenerative spondylolisthesis is not currently well understood. Little biomechanical evidence is available to offer an explanation for the mechanism behind exercise therapy. Therefore, the aim of this study is to investigate the effects of back muscle weakness on degenerative spondylolisthesis and to tease out the biomechanical mechanism of exercise therapy.
  
  
  METHODS
  A nonlinear 3-D finite element model of L3-L5 was constructed. Forces representing global back muscles and global abdominal muscles, follower loads and an upper body weight were applied. The force of the global back muscles was reduced to 75%, 50% and 25% to simulate different degrees of back muscle weakness. An additional boundary condition which represented the loads from other muscles after exercise therapy was set up to keep the spine in a neutral standing position. Shear forces, intradiscal pressure, facet joint forces and von Mises equivalent stresses in the annuli were calculated.
  
  
  RESULTS
  The intervertebral rotations of L3-L4 and L4-L5 were within the range of in vitro experimental data. The calculated intradiscal pressure of L4-L5 for standing was 0.57MPa, which is similar to previous in vivo data. With the back muscles were reduced to 75%, 50% and 25% force, the shear force moved increasingly in a ventral direction. Due to the additional stabilizing force and moment provided by boundary conditions, the shear force varied less than 15%.
  
  
  CONCLUSIONS
  Reducing the force of global back muscles might lead to, or aggravate, degenerative spondylolisthesis with forward slipping from biomechanical point of view. Exercise therapy may improve the spinal biomechanical environment. However, the intrinsic correlation between back muscle weakness and degenerative spondylolisthesis needs more clinical in vivo study and biomechanical analysis.
authors.affiliation: Spine Division of Orthopaedic Department, Tongji Hospital, Tongji University School of Medicine, 389 Xincun Road, 200065 Shanghai, China; Department of Histology and Embryology, Tongji University School of Medicine, 1239 Siping Road, Shanghai 200092, China.
authors.forename: Rui
authors.initials: R
authors.lastname: Zhu
authors.affiliation: Spine Division of Orthopaedic Department, Tongji Hospital, Tongji University School of Medicine, 389 Xincun Road, 200065 Shanghai, China.
authors.forename: Wen-Xin
authors.initials: WX
authors.lastname: Niu
authors.affiliation: Spine Division of Orthopaedic Department, Tongji Hospital, Tongji University School of Medicine, 389 Xincun Road, 200065 Shanghai, China.
authors.forename: Zhi-Li
authors.initials: ZL
authors.lastname: Zeng
authors.affiliation: Institute for Biomedical Engineering and Nano Science, Tongji University School of Medicine, Chifeng Road 67, Shanghai 200092, China.
authors.forename: Jian-Hua
authors.initials: JH
authors.lastname: Tong
authors.affiliation: Department of Histology and Embryology, Tongji University School of Medicine, 1239 Siping Road, Shanghai 200092, China.
authors.forename: Zhi-Wei
authors.initials: ZW
authors.lastname: Zhen
authors.affiliation: Department of Histology and Embryology, Tongji University School of Medicine, 1239 Siping Road, Shanghai 200092, China.
authors.forename: Shuang
authors.initials: S
authors.lastname: Zhou
authors.affiliation: Spine Division of Orthopaedic Department, Tongji Hospital, Tongji University School of Medicine, 389 Xincun Road, 200065 Shanghai, China.
authors.forename: Yan
authors.initials: Y
authors.lastname: Yu
authors.affiliation: Spine Division of Orthopaedic Department, Tongji Hospital, Tongji University School of Medicine, 389 Xincun Road, 200065 Shanghai, China. Electronic address: chlm.d@163.com.
authors.forename: Li-Ming
authors.initials: LM
authors.lastname: Cheng
mesh_terms: D000328:Adult; D064131:Back Muscles / Q000503:physiopathology*; D001696:Biomechanical Phenomena; D005081:Exercise Therapy; D020342:Finite Element Analysis; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology; D008297:Male; D018908:Muscle Weakness / Q000503:physiopathology*; D011187:Posture / Q000502:physiology; D013168:Spondylolisthesis / Q000503:physiopathology* / Q000534:rehabilitation; D021801:Zygapophyseal Joint / Q000502:physiology
keywords: Back muscle; Degenerative spondylolisthesis; Exercise therapy; Finite element analysis