title: External fixation design evolution enhances biomechanical frame performance.
abstract: BACKGROUND
  External fixation has become a quick and easy application for fracture stabilisation of the extremities and/or pelvis to maintain the reduction and provide stability while sparing the soft tissues. Over the last years, enhanced construct stiffness has become an essential requirement to preserve fracture reduction, particularly in active and overweight patients. This study was performed to determine whether the advancement of design features enhances the external fixation construct stiffness. The stiffness of the recently developed Hoffmann 3 external fixation system was determined and its characteristics compared with the widely clinically accepted Hoffmann II MRI fixation system.
  
  
  METHODS
  A synthetic fracture model was used. Two carbon tubes with a fracture gap of 20 mm were appropriate to determine the stiffness of three different configurations: the basic frame configuration (group H 3, representing Hoffmann 3 with a rod diameter of 11 mm) using a double rod construction with 6 mm Apex pins, was compared with the Hoffmann II MRI fixation system using two 8.0 mm diameter rods with 6 mm (group H II-6 mm) and 5 mm (group H II-5 mm) Apex pins. Each group was tested five times under anterior-posterior bending (N/mm), medio-lateral bending (N/mm) and axial torsion loading directions (Nm/deg). The stiffness results of each construct were compared statistically.
  
  
  RESULTS
  The basic frame construct (group H 3) showed consistently higher stiffness properties compared with the other configurations. The anterior-posterior-bending loads resulted in a mean value of 31 N/mm, which was significantly higher compared with the other groups (p=0.008) at 16 N/mm. The medio-lateral-bending test revealed a mean stiffness of 59 N/mm in the H3 group, compared with 43 N/mm in the H II-6 group and 31 N/mm in the H II-5 group. The axial torsion measurements of the Hoffmann 3 group yielded significantly higher results (1.03 Nm/°) compared with group H II-6 (0.61 Nm/°) and group H II-5 (0.56 Nm/°).
  
  
  CONCLUSIONS
  The Hoffmann 3 construct showed the highest stiffness properties under bending and torsion loads. The enhanced stiffness of the Hoffmann 3 device may be helpful in maintaining fracture reduction and soft tissue compromise. This investigation showed the advancement of Hoffmann design features may be effective in enhancing frame stiffness.
authors.affiliation: Harald Tscherne Research Laboratory for Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany; Department of Orthopaedic Trauma, Sana Klinikum Offenbach, Starkenburgring 66, 63069 Offenbach am Main, Germany. Electronic address: Richard.Sellei@sana.de.
authors.forename: Richard Martin
authors.initials: RM
authors.lastname: Sellei
authors.affiliation: Harald Tscherne Research Laboratory for Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany; Department of Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Kobbe
authors.affiliation: Harald Tscherne Research Laboratory for Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany; Department of Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany.
authors.forename: Azad
authors.initials: A
authors.lastname: Dadgar
authors.affiliation: Harald Tscherne Research Laboratory for Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany; Department of Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany.
authors.forename: Roman
authors.initials: R
authors.lastname: Pfeifer
authors.affiliation: Stryker Trauma & Extremities, Bohnackerweg 1, 2545 Selzach, Switzerland.
authors.forename: Markus
authors.initials: M
authors.lastname: Behrens
authors.affiliation: Stryker Trauma & Extremities, Prof. Künscher Str. 1-5, Schönkirchen, 24232 Kiel, Germany.
authors.forename: Geert
authors.initials: G
authors.lastname: von Oldenburg
authors.affiliation: Harald Tscherne Research Laboratory for Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany; Department of Orthopaedic Trauma at Aachen University Medical Centre, Pauwelsst. 30, 52074 Aachen, Germany.
authors.forename: Hans-Christoph
authors.initials: HC
authors.lastname: Pape
mesh_terms: D001696:Biomechanical Phenomena; D001858:Bone Nails; D003365:Costs and Cost Analysis; D004867:Equipment Design / Q000639:trends; D016267:External Fixators; D005592:Fracture Fixation / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D008422:Materials Testing; D013314:Stress, Mechanical; D016474:Weight-Bearing
keywords: Double stacking; External fixation system; Hoffmann 3; Stiffness