title: Biomechanical response of lumbar facet joints under follower preload: a finite element study.
abstract: BACKGROUND
  Facet joints play a significant role in providing stability to the spine and they have been associated with low back pain symptoms and other spinal disorders. The influence of a follower load on biomechanics of facet joints is unknown. A comprehensive research on the biomechanical role of facets may provide insight into facet joint instability and degeneration.
  
  
  METHODS
  A nonlinear finite element (FE) model of lumbar spine (L1-S1) was developed and validated to study the biomechanical response of facets, with different values of follower preload (0 N,500 N,800 N,1200 N), under loadings in the three anatomic planes. In this model, special attention was paid to the modeling of facet joints, including cartilage layer. The asymmetry in the biomechanical response of facets was also discussed. A rate of change (ROC) and an average asymmetry factor (AAF) were introduced to explore and evaluate the preload effect on these facet contact parameters and on the asymmetry under different loading conditions.
  
  
  RESULTS
  The biomechanical response of facets changed according to the loading condition. The preload amplified the facet force, contact area and contact pressure in flexion-extension; the same effect was observed on the ipsilateral facet while an opposite effect could be seen on the contralateral facet during lateral bending. For torsion loading, the preload increased contact area, decreased the mean contact pressure, but had almost no effect on facet force. However, all the effects of follower load on facet response became weaker with the increase of preload. The greatest asymmetry of facet response could be found on the ipsilateral side during lateral bending, followed by flexion, bending (contralateral side), extension and torsion. This asymmetry could be amplified by preload in the bending (ipsilateral), torsion loading group, while being reduced in the flexion group.
  
  
  CONCLUSIONS
  An analysis combining patterns of contact pressure distribution, facet load, contact area and contact pressure can provide more insight into the biomechanical role of facets under various moment loadings and follower loads. The effect of asymmetry on facet joint response should be fully considered in biomechanical studies of lumbar spine, especially in post structures subjected to physiological loadings.
authors.affiliation: Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent Control, Tianjin University of Technology, No. 391, Binshui West Road, Xiqing District, 300384, Tianjin, China.|School of Biological Science and Medical Engineering, Key Laboratory for Biomechanics and Mechanobiology of Ministry of Education, Beihang University, 100191, Beijing, China.
authors.forename: Cheng-Fei
authors.initials: CF
authors.lastname: Du
authors.affiliation: Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent Control, Tianjin University of Technology, No. 391, Binshui West Road, Xiqing District, 300384, Tianjin, China.
authors.forename: Nan
authors.initials: N
authors.lastname: Yang
authors.affiliation: School of Biological Science and Medical Engineering, Key Laboratory for Biomechanics and Mechanobiology of Ministry of Education, Beihang University, 100191, Beijing, China.
authors.forename: Jun-Chao
authors.initials: JC
authors.lastname: Guo
authors.affiliation: Department of Orthopaedics, First Affiliated Hospital of Fujian Medical University, 350005, Fuzhou, China.
authors.forename: Yun-Peng
authors.initials: YP
authors.lastname: Huang
authors.affiliation: Tianjin Key Laboratory for Advanced Mechatronic System Design and Intelligent Control, Tianjin University of Technology, No. 391, Binshui West Road, Xiqing District, 300384, Tianjin, China. zcqarticle@163.com.
authors.forename: Chunqiu
authors.initials: C
authors.lastname: Zhang
mesh_terms: D001696:Biomechanical Phenomena; D055119:Elastic Modulus; D020342:Finite Element Analysis; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology*; D008954:Models, Biological; D017711:Nonlinear Dynamics; D011312:Pressure; D014057:Tomography, X-Ray Computed; D054159:Torsion, Mechanical; D016474:Weight-Bearing; D021801:Zygapophyseal Joint / Q000000981:diagnostic imaging / Q000502:physiology*
keywords: Biomechanics; Facet joint; Finite element; Follower load; Lumbar spine