title: An in vitro biomechanical comparison of the insertion variables and pullout mechanical properties of ao 6.5-mm standard cancellous and 7.3-mm self-tapping, cannulated bone screws in foal femoral bone.
abstract: OBJECTIVE
  To compare screw insertion variables and pullout mechanical properties between AO 6.5-mm cancellous and 7.3-mm cannulated bone screws in foal femoral bone.
  
  
  METHODS
  A paired, in vitro mechanical study.
  
  
  METHODS
  Seven pairs of femora from immature (1-7 months) foals.
  
  
  METHODS
  The 6.5 cancellous and 7.3-mm cannulated screws were inserted at standardized proximal and distal metaphyseal, and mid-diaphyseal locations. Insertion torque, force, and time to drill, tap (6.5-mm cancellous), guide wire insertion (7.3-mm cannulated), and screw insertion were measured. Screw pullout properties (yield and failure load, displacement, and energy, and stiffness) were determined from mechanical tests. The effects of screw type and location on insertion variables and pullout properties were assessed with repeated measures ANOVA. Pairwise comparisons were examined with post hoc contrasts. Significance was set at P<.05 for all comparisons.
  
  
  RESULTS
  Insertion torques for the 7.3-mm cannulated screws were significantly greater than for the 6.5-mm tap, but significantly lower than for the 6.5-mm cancellous screws. Total screw insertion times were similar. Pullout properties of both screws were similar at each femoral location. The holding power of both screws was significantly greater in the mid-diaphysis than in either metaphyseal location. Pullout failure occurred by bone shearing at the bone-screw interface in all specimens.
  
  
  CONCLUSIONS
  The 6.5-mm cancellous and 7.3-mm cannulated screws vary in insertion properties, but have similar pullout properties in the mid-diaphysis, proximal, and distal metaphysis of foal femora. Both screw types have greater holding power at the mid-diaphyseal location compared with metaphyseal locations. Based on overall similar holding powers of 6.5-mm cancellous and 7.3-mm cannulated screws, it is unlikely that increasing the screw diameter beyond 6.5 mm will provide increased holding power in foal femoral bone.
  
  
  CONCLUSIONS
  Use of the 7.3-mm cannulated screw should be considered for foal femoral fracture repair when greater accuracy is needed, or when bone threads for the 6.5-mm cancellous screw have been stripped.
authors.affiliation: JD Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California-Davis, Davis, CA, USA.
authors.forename: Nicole L
authors.initials: NL
authors.lastname: Johnson
authors.forename: Larry D
authors.initials: LD
authors.lastname: Galuppo
authors.forename: Susan M
authors.initials: SM
authors.lastname: Stover
authors.forename: Ken T
authors.initials: KT
authors.lastname: Taylor
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000662:veterinary*; D005260:Female; D005264:Femoral Fractures / Q000601:surgery / Q000662:veterinary*; D005594:Fracture Fixation, Intramedullary / Q000379:methods / Q000662:veterinary*; D006736:Horses / Q000293:injuries* / Q000601:surgery; D008297:Male; D016896:Treatment Outcome