title: Biomechanical analysis of a synthetic femoral spiral fracture model: Do end caps improve retrograde flexible intramedullary nail fixation?
abstract: BACKGROUND
  Elastic Stable intramedullary Nailing (ESIN) of dislocated diaphyseal femur fractures has become an accepted method for the treatment in children and adolescents with open physis. Studies focused on complications of this technique showed problems regarding stability, usually in complex fracture types such as spiral fractures and in older children weighing > 40 kg. Biomechanical in vitro testing was performed to evaluate the stability of simulated spiral femoral fractures after retrograde flexible titanium intramedullary nail fixation with and without End caps.
  
  
  METHODS
  Eight synthetic adolescent-size femoral bone models (Sawbones® with a medullar canal of 10 mm and a spiral fracture of 100 mm length identically sawn by the manufacturer) were used for each group. Both groups underwent retrograde fixation with two 3.5 mm Titanium C-shaped nails inserted from medial and lateral entry portals. In the End Cap group the ends of the nails of the eight specimens were covered with End Caps (Synthes Company, Oberdorf, Switzerland) at the distal entry.
  
  
  RESULTS
  Beside posterior-anterior stress (4.11 Nm/mm vs. 1.78 Nm/mm, p < 0.001), the use of End Caps demonstrated no higher stability in 4-point bending compared to the group without End Caps (anterior-posterior bending 0.27 Nm/mm vs. 0.77 Nm/mm, p < 0.001; medial-lateral bending 0.8 Nm/mm vs. 1.10 Nm/mm, p < 0.01; lateral-medial bending 0.53 Nm/mm vs. 0.86 Nm/mm, p < 0.001) as well as during internal rotation (0.11 Nm/° vs. 0.14 Nm/°, p < 0.05). During compression in 9°- position and external rotation there was no statistical significant difference (0.37 Nm/° vs. 0.32 Nm/°, p = 0.13 and 1.29 mm vs. 2.18 mm, p = 0.20, respectively) compared to the "classic" 2-C-shaped osteosynthesis without End Caps.
  
  
  CONCLUSIONS
  In this biomechanical study the use of End Caps did not improve the stability of the intramedullary flexible nail osteosynthesis.
authors.affiliation: Department of Paediatric Surgery, Medical Faculty of the University of Luebeck, Ratzeburger Allee 160, Luebeck, 23562, Germany. kaiser@uni-luebeck.de
authors.forename: Martin M
authors.initials: MM
authors.lastname: Kaiser
authors.forename: Gregor
authors.initials: G
authors.lastname: Zachert
authors.forename: Robert
authors.initials: R
authors.lastname: Wendlandt
authors.forename: Marion
authors.initials: M
authors.lastname: Rapp
authors.forename: Rebecca
authors.initials: R
authors.lastname: Eggert
authors.forename: Christine
authors.initials: C
authors.lastname: Stratmann
authors.forename: Lucas M
authors.initials: LM
authors.lastname: Wessel
authors.forename: Arndt P
authors.initials: AP
authors.lastname: Schulz
authors.forename: Benjamin J
authors.initials: BJ
authors.lastname: Kienast
mesh_terms: D001696:Biomechanical Phenomena; D001858:Bone Nails; D004867:Equipment Design; D005264:Femoral Fractures / Q000601:surgery*; D005269:Femur / Q000000981:diagnostic imaging / Q000601:surgery; D005471:Fluoroscopy; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation* / Q000379:methods*; D006801:Humans; D008954:Models, Biological