title: A biomechanical study comparing polyaxial locking screw mechanisms.
abstract: OBJECTIVE
  Locking plates have become ubiquitous in modern fracture surgery. Recently, manufacturers have developed locking plates with polyaxial screw capabilities in order to optimise screw placement. It has already been demonstrated that inserting uniaxial locking screws off axis results in weaker loads to failure. Our hypothesis was that even implants specifically designed for polyaxial insertion would experience a drop-off in resistance when using non-perpendicular screws.
  
  
  METHODS
  Four different types (one monoaxial and three polyaxial locking plates) of readily available small fragment plates were tested. A biomechanical model was developed to test the screws until failure (defined as breakage and rapid loss of >50% force). Screws were inserted at 0, 10 and 15°.
  
  
  RESULTS
  The standard monoaxial locking mechanism sustained saw a 60% reduction in force (332N vs. 134N) when screws were inserted cross-threaded at 10°. Two polyaxial systems saw similar significant reductions in force of 45% and 34%, respectively at 15°. A third system utilizing an end cap locking mechanism showed highly variable results with large standard deviations. Polyaxial screws showed on average only limited reduction at 10 degrees of insertion angle.
  
  
  CONCLUSIONS
  Newer designs of locking plates have attractive properties to allow more surgical options during fixation. However this freedom comes at the price of reduced force. Our results show that the safe zone for inserting these screws is closer to 20°, rather than the 30° indicated by the manufacturers. Also, the various polyaxial locking mechanisms seem to influence the overall resistance of the screws.
authors.affiliation: University of Montréal, Hôpital du Sacré Cœur, 5400 Gouin Ouest, Local J-3245, Montréal, Québec, Canada H4J 1C5. jonah.davies@gmail.com
authors.forename: Jonah
authors.initials: J
authors.lastname: Hebert-Davies
authors.forename: G-Yves
authors.initials: GY
authors.lastname: Laflamme
authors.forename: Dominique
authors.initials: D
authors.lastname: Rouleau
authors.forename: Fanny
authors.initials: F
authors.lastname: Canet
authors.forename: Emilie
authors.initials: E
authors.lastname: Sandman
authors.forename: Ang
authors.initials: A
authors.lastname: Li
authors.forename: Yvan
authors.initials: Y
authors.lastname: Petit
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates; D001863:Bone Screws / Q000706:statistics & numerical data*; D005264:Femoral Fractures / Q000601:surgery*; D005593:Fracture Fixation, Internal / Q000295:instrumentation* / Q000379:methods*; D006801:Humans; D013314:Stress, Mechanical; D016474:Weight-Bearing
keywords: Angulation; Fractures; Load to failure; Locking plate; Mechanical testing; Polyaxial screws