title: The effect of different insertion points on the security of pedicle screw fixation in bovine vertebrae.
abstract: METHODS
  Cyclic loading of pedicle screws inserted into bovine vertebrae was applied and the security of fixation was assessed as a function of the point of insertion of the screws.
  
  
  OBJECTIVE
  To determine which point of insertion is associated with more secure fixation using 2 commonly selected insertion points.
  
  
  BACKGROUND
  The strength of pedicle screw fixation depends on several factors, some of these have been studied. The present study looked at the difference in fixation security resulting from the use of 2 different insertion points to obtain a more stable fixation and to reduce the risk of loosening.
  
  
  METHODS
  Pedicle screws were inserted through a different insertion point on each side of the vertebra, one lateral to the superior articular process and one through the superior articular process and into the pedicle. They were tested by applying cyclic loading in a craniocaudal direction and the vertical displacement was measured. Permanent migration of the screw through the bone was compared using repeated measures analysis of variance.
  
  
  RESULTS
  Screws inserted lateral to the process were associated with significantly less displacement than those placed through the process (P = 0.02).
  
  
  CONCLUSIONS
  Using bovine vertebrae, different points of insertion of pedicle screws were associated with different security of fixation. However, further tests using human vertebrae are recommended to confirm the above results.
authors.affiliation: Musculoskeletal Surgery Group, Imperial College London School of Medicine, London, UK.
authors.forename: Maan
authors.initials: M
authors.lastname: Al-Hadithi
authors.forename: Sean P F
authors.initials: SP
authors.lastname: Hughes
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Amis
mesh_terms: D000818:Animals; D015519:Bone Density; D001863:Bone Screws; D002417:Cattle; D019544:Equipment Failure Analysis; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D023421:Models, Animal; D018583:Pliability; D011475:Prosthesis Failure; D011859:Radiography; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013314:Stress, Mechanical; D013904:Thoracic Vertebrae / Q000000981:diagnostic imaging / Q000601:surgery*; D016474:Weight-Bearing