title: A biomechanical comparison of static versus dynamic lag screw modes for cephalomedullary nails used to fix unstable peritrochanteric fractures.
abstract: BACKGROUND
  The gamma nail has an option to statically lock its lag screw (static mode) or to allow its lag screw to move within the nail to compress the intertrochanteric fracture (dynamic mode). The purpose of this study was to compare the biomechanical stiffness of static and dynamic lag screw modes for a cephalomedullary nail used to fix an unstable peritrochanteric fracture.
  
  
  METHODS
  Unstable four-part peritrochanteric fractures were created in 30 synthetic femurs and fixed with Long Gamma 3 Nails. Mechanical tests were conducted for axial, lateral, and torsional stiffness with intact femurs, femur-nail constructs with static lag screw mode,and femur-nail constructs with dynamic lag screw mode. A paired Student's t test was used for all statistical comparisons between test groups.
  
  
  RESULTS
  Axial and torsional stiffness of intact femurs was significantly greater than femur-nail constructs (p < 0.01 all comparisons),whereas lateral stiffness was significantly less (p < 0.01 all comparisons). Axial stiffness of the femur-nail construct was significantly greater (p < 0.01) in static mode (484.3 N/mm 80.2 N/mm) than in dynamic mode (424.1 N/mm 78.0 N/mm).Lateral stiffness was significantly greater (p < 0.01) in static mode (113.9 N/mm 8.4 N/mm) than in dynamic mode (109.5N/mm 8.8 N/mm). Torsional stiffness was significantly greater (p = 0.02) in dynamic mode (114.5 N/mm 28.2 N/mm) than in static mode (111.7 N/mm 27.0 N/mm).
  
  
  CONCLUSIONS
  There is a 60 N/mm (12.4%) reduction in axial stiffness when the lag screw is in dynamic mode. Given the statistically significant reduction in axial and lateral stiffness with use of the dynamic mode, static lag screw mode should be further explored clinically for treatment of unstable peritrochanteric fractures.
authors.affiliation: Division of Orthopaedics, Department of Surgery, University of Toronto, Mount Sinai Hospital, 600 University Avenue, Toronto, ON, Canada. paul.kuzyk@utoronto.ca
authors.forename: Paul R T
authors.initials: PR
authors.lastname: Kuzyk
authors.forename: Suraj
authors.initials: S
authors.lastname: Shah
authors.forename: Rad
authors.initials: R
authors.lastname: Zdero
authors.forename: Michael
authors.initials: M
authors.lastname: Olsen
authors.forename: James P
authors.initials: JP
authors.lastname: Waddell
authors.forename: Emil H
authors.initials: EH
authors.lastname: Schemitsch
mesh_terms: D001696:Biomechanical Phenomena; D001858:Bone Nails; D001863:Bone Screws; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation / Q000379:methods*; D006620:Hip Fractures / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D066298:In Vitro Techniques; D013314:Stress, Mechanical; D019415:Torque