title: Loosening at the screw-vertebra junction in multilevel anterior cervical plate constructs.
abstract: METHODS
  An in vitro biomechanical study of one-level and three-level corpectomy and anterior cervical plate models.
  
  
  OBJECTIVE
  To investigate the failure of the screw-vertebra interfaces in one- and three-level corpectomy models.
  
  
  BACKGROUND
  Although there are several biomechanical studies of strength and stability of anterior cervical plating, there has been no investigation into clinically observed failures.
  
  
  METHODS
  One- and three-level models (corpectomy, strut graft, and anterior plate) were constructed from eight cadaveric specimens (C2-T1). Multidirectional flexibility tests (1.0 Nm moments) performed before and after fatigue (1000 cycles, 1.0 Nm flexion-extension, 0.14 Hz) documented the screw-vertebra motions at upper and lower ends. Ranges of motion and neutral zones were determined. Analysis of variance was used to evaluate significant differences between the upper and lower ends of the plates and changes caused by fatigue loading (P < 0.05).
  
  
  RESULTS
  Extension motion at the lower ends was more than at the upper ends in both models. Fatigue increased three-level model ranges of motion at the lower end by 171% in flexion, 164% in extension, 153% in lateral bending, and 115% in axial rotation. Similar increases were observed in neutral zones. Fatigue loading produced no significant changes in one-level models.
  
  
  CONCLUSIONS
  There was excessive screw-vertebra motion caused by fatigue at the lower end of the three-level corpectomy model. These findings of loosening may explain clinically observed failures at the caudal end of long anterior cervical plate constructs. Longer screws, larger diameter screws, and supplemental posterior fixation may decrease screw-vertebra loosening.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut, USA. manohar.panjabi@yale.edu
authors.forename: M M
authors.initials: MM
authors.lastname: Panjabi
authors.forename: T
authors.initials: T
authors.lastname: Isomi
authors.forename: J L
authors.initials: JL
authors.lastname: Wang
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws; D002102:Cadaver; D002574:Cervical Vertebrae / Q000601:surgery*; D006801:Humans; D008297:Male; D008875:Middle Aged; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D013997:Time Factors