title: Offset laminar hooks decrease bending moments of pedicle screws during in situ contouring.
abstract: METHODS
  A biomechanical study was conducted using cadaver spines to determine the influence of supplemental offset laminar hooks on pedicle screw bending moments and migration during in situ contouring of short-segment pedicle instrumentation.
  
  
  OBJECTIVE
  To determine the effects of offset laminar hooks on short-segment pedicle instrumentation constructs during in situ contouring. It was hypothesized that the screw bending moments and screw migration would decrease when offset laminar hooks were used with short-segment pedicle instrumentation.
  
  
  BACKGROUND
  Clinical studies have implicated screw bending or breakage at the screw hub as failure mechanisms in short-segment pedicle instrumentation constructs used to stabilize thoracolumbar fractures, particularly when rods are contoured in situ.
  
  
  METHODS
  Cadaver spines were instrumented using short-segment pedicle instrumentation or short-segment pedicle instrumentation with supplemental offset laminar hooks. The instrumentation was contoured in situ, and screw bending moments were measured at the hub of the screws. Screw migration was measured from lateral radiographs. Comparisons of screw bending moments and migration were made between the two instrumentation configurations.
  
  
  RESULTS
  The addition of offset laminar hooks significantly reduced screw bending moments and screw migration during in situ contouring. The mean screw bending moments decreased approximately 30% at the maximum bending angle of 30 degrees (P < 0.05), and the mean screw migration during contouring decreased from 8 degrees to 2 degrees (P < 0.05).
  
  
  CONCLUSIONS
  Addition of offset laminar hooks to short-segment pedicle instrumentation decreases screw bending moments and migration of the screws during in situ contouring of the rod. The authors speculate that decrease in loading of the screw will improve durability of the constructs clinically.
authors.affiliation: Orthopaedic Research Laboratories, University of California, Davis, Sacramento, USA.
authors.forename: S A
authors.initials: SA
authors.lastname: Yerby
authors.forename: J R
authors.initials: JR
authors.lastname: Ehteshami
authors.forename: R F
authors.initials: RF
authors.lastname: McLain
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D004867:Equipment Design; D005069:Evaluation Studies as Topic; D005548:Foreign-Body Migration / Q000517:prevention & control; D006801:Humans; D016268:Internal Fixators; D008422:Materials Testing; D011859:Radiography; D013131:Spine / Q000000981:diagnostic imaging / Q000601:surgery