title: Biomechanical comparison of polyaxial and uniaxial locking plate fixation in a proximal tibial gap model.
abstract: OBJECTIVE
  Lateral locked plating for proximal tibial fractures with metaphyseal disruption provides a biomechanically stable and biologically favorable alternative to conventional medial/lateral plate fixation. New polyaxial screw technology incorporates expanding screw bushings, allowing variable angle screw placement, while providing angular stability. We hypothesize that polyaxial locking plates will exhibit comparable stiffness, strength to failure, and resistance to plastic deformation to conventional locking plates in a proximal tibial gap model.
  
  
  METHODS
  We stabilized extra-articular metaphyseal gap osteotomies in synthetic composite tibiae with dual medial and lateral plating, Less Invasive Stabilization System (LISS) plates, 4.5-mm proximal tibial lateral locking plates with (LP+) and without (LP-) angled screws, and 4.5-mm polyaxial locking plates with (PA+) and without (PA-) angled screws. All were tested with cyclic, ramped, and axial loading to failure.
  
  
  RESULTS
  No plates demonstrated screw failure before plate failure. Dual-plate constructs did not fail. All lateral plates failed at the osteotomy. LP- failed at low load. PA+ was significantly stiffer (165 +/- 17 N/mm) with greater load to failure (711 +/- 23 N) than all other constructs (PA-: 56 +/- 6 N/mm, 617 +/- 33 N; LP+: 137 +/- 23 N/mm, 488 +/- 39 N; LISS: 76 +/- 5 N/mm, 656 +/- 39 N). PA+ had significantly less plastic deformation (12.1 +/- 0.8 mm) than LP+ (13.4 +/- 3.7 mm), but more than PA- (5.8 +/- 1.2 mm) and LISS (3.9 +/- 0.6 mm). PA- did not differ significantly from LISS in any parameter.
  
  
  CONCLUSIONS
  This study demonstrates that this unique polyaxial locking plate mechanism, when tested in various constructs, exhibits similar biomechanical performance regarding stiffness, strength to failure, and resistance to plastic deformation when compared with uniaxial locking plates. The polyaxial locking plate with an angled screw was stiffest and had the greatest load to failure. The polyaxial locking plate alone tested similar to the LISS. In addition, the benefit of the angled screw for biomechanical stability is demonstrated.
authors.affiliation: Department of Orthopaedic Surgery, University of California, Davis Medical Center, 4680 Y St, Sacramento, CA 95817, USA. aaron.cullen@ucdmc.ucdavis.edu
authors.forename: Aaron B
authors.initials: AB
authors.lastname: Cullen
authors.forename: Shane
authors.initials: S
authors.lastname: Curtiss
authors.forename: Mark A
authors.initials: MA
authors.lastname: Lee
mesh_terms: D001860:Bone Plates; D001863:Bone Screws; D055119:Elastic Modulus; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D017759:Fractures, Malunited / Q000503:physiopathology* / Q000601:surgery*; D017276:Friction; D006244:Hardness; D006801:Humans; D013718:Tensile Strength; D013978:Tibial Fractures / Q000503:physiopathology* / Q000601:surgery*