title: Anterior cervical fusion: a finite element model study on motion segment stability including the effect of osteoporosis.
abstract: METHODS
  Three-dimensional, nonlinear finite element models were used to evaluate the stability of the mid cervical spine after anterior fusion in patients with and without osteoporosis.
  
  
  OBJECTIVE
  The objective of this study was to compare the change in flexibility of C5-C6 after anterior discectomy with both loose-fitting and tight-fitting fusion graft.
  
  
  BACKGROUND
  Many factors such as surgical technique, osteoporosis, and excess neck motion during the postoperative period may contribute to fusion failure. Knowledge about changes in biomechanical properties after the surgical procedure is important for selection of grafts with appropriate strength and for guiding patients in postoperative care and rehabilitation.
  
  
  METHODS
  Analyses with anterior fusion models with both loose-fitting and tight-fitting graft were performed in a normal and an osteoporotic spine. The motion of the C5 vertebra in relation to the C6 vertebra were calculated, after multidirectional moment loads of 0.5 Nm combined with a compressive preload of 105 N.
  
  
  RESULTS
  Loose-fitting graft produced both an increase and a decrease in motion under various external moment loads, with graft compressive stress below the compressive strength of the graft material. A reduction in motion was observed under all moment loads when a tight-fitting graft was used. The compressive stress in the tight-fitting graft was higher than the strength of the graft material. Osteoporosis increased the principal motions with both the loose-fitting graft and tight-fitting graft. Maximum increase in motion with a loose-fitting graft construct was observed under extension and axial torsion moment loads.
  
  
  CONCLUSIONS
  Anterior discectomy and insertion of the loose graft resulted in increased motion. A tight-fitting graft is beneficial in reducing motion, but the stress within the graftincreases beyond the graft strength. The presence of osteoporosis was nominally significant when the graft was tight fitting.
authors.affiliation: Department of Orthopedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, Chicago, Illinois, USA. rnataraj@rpslmc.edu
authors.forename: R N
authors.initials: RN
authors.lastname: Natarajan
authors.forename: B H
authors.initials: BH
authors.lastname: Chen
authors.forename: H S
authors.initials: HS
authors.lastname: An
authors.forename: G B
authors.initials: GB
authors.lastname: Andersson
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002574:Cervical Vertebrae / Q000000981:diagnostic imaging / Q000503:physiopathology / Q000601:surgery; D017586:Diskectomy; D005260:Female; D020342:Finite Element Analysis; D006801:Humans; D010024:Osteoporosis / Q000000981:diagnostic imaging / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000379:methods*; D014057:Tomography, X-Ray Computed