title: Can an Endplate-conformed Cervical Cage Provide a Better Biomechanical Environment than a Typical Non-conformed Cage?: A Finite Element Model and Cadaver Study.
abstract: OBJECTIVE
  To evaluate the biomechanical characteristics of endplate-conformed cervical cages by finite element method (FEM) analysis and cadaver study.
  
  
  METHODS
  Twelve specimens (C2 -C7 ) and a finite element model (C3 -C7 ) were subjected to biomechanical evaluations. In the cadaver study, specimens were randomly assigned to intact (I), endplate-conformed (C) and non-conformed (N) groups with C4-5 discs as the treated segments. The morphologies of the endplate-conformed cages were individualized according to CT images of group C and the cages fabricated with a 3-D printer. The non-conformed cages were wedge-shaped and similar to commercially available grafts. Axial pre-compression loads of 73.6 N and moment of 1.8 Nm were used to simulate flexion (FLE), extension (EXT), lateral bending (LB) and axial rotation (AR). Range of motion (ROM) at C4-5 of each specimen was recorded and film sensors fixed between the cages and C5 superior endplates were used to detect interface stress. A finite element model was built based on the CT data of a healthy male volunteer. The morphologies of the endplate-conformed and wedge-shaped, non-conformed cervical cages were both simulated by a reverse engineering technique and implanted at the segment of C4-5 in the finite element model for biomechanical evaluation. Force loading and grouping were similar to those applied in the cadaver study. ROM of C4-5 in group I were recorded to validate the finite element model. Additionally, maximum cage-endplate interface stresses, stress distribution contours on adjoining endplates, intra-disc stresses and facet loadings at adjacent segments were measured and compared between groups.
  
  
  RESULTS
  In the cadaver study, Group C showed a much lower interface stress in all directions of motion (all P < 0.05) and the ROM of C4-5 was smaller in FLE-EXT (P = 0.001) but larger in AR (P = 0.017). FEM analysis produced similar results: the model implanted with an endplate-conformed cage presented a lower interface stress with a more uniform stress distribution than that implanted with a non-conformed cage. Additionally, intra-disc stress and facet loading at the adjacent segments were obviously increased in both groups C and N, especially those at the supra-jacent segments. However, stress increase was milder in group C than in group N for all directions of motion.
  
  
  CONCLUSIONS
  Endplate-conformed cages can decrease cage-endplate interface stress in all directions of motion and increase cervical stability in FLE-EXT. Additionally, adjacent segments are possibly protected because intra-disc stress and facet loading are smaller after endplate-conformed cage implantation. However, axial stability was reduced in group C, indicating that endplate-conformed cage should not be used alone and an anterior plate system is still important in anterior cervical discectomy and fusion.
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Fan
authors.initials: F
authors.lastname: Zhang
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Hao-Cheng
authors.initials: HC
authors.lastname: Xu
authors.affiliation: Department of Clinical Medicine, Shanghai Medical College, Fudan University, Shanghai, China.
authors.forename: Bo
authors.initials: B
authors.lastname: Yin
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Xin-Lei
authors.initials: XL
authors.lastname: Xia
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Xiao-Sheng
authors.initials: XS
authors.lastname: Ma
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Hong-Li
authors.initials: HL
authors.lastname: Wang
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Yin
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Ming-Hao
authors.initials: MH
authors.lastname: Shao
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China. lufeizhou@hotmail.com.|Department of Orthopaedics, The Fifth People's Hospital of Shanghai, Fudan University, Shanghai, China. lufeizhou@hotmail.com.
authors.forename: Fei-Zhou
authors.initials: FZ
authors.lastname: Lyu
authors.affiliation: Department of Orthopaedics, Huashan Hospital, Fudan University, Shanghai, China.
authors.forename: Jian-Yuan
authors.initials: JY
authors.lastname: Jiang
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology / Q000601:surgery*; D005260:Female; D020342:Finite Element Analysis; D006801:Humans; D016268:Internal Fixators; D008297:Male; D008875:Middle Aged; D011897:Random Allocation; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing
keywords: Biomechanical; Cadaver study; Cage; Endplate-conformed; Finite element method