title: Biomechanical testing of rectangular humeral shaft prosthesis: higher torsional stability without increased fracture risk.
abstract: BACKGROUND
  Rectangular cementless femur shaft prostheses have a higher primary stability than round shafts. A novel rectangular humeral shaft design was tested with two questions: does the rectangular design cause a higher fracture risk during implantation than round designs, and does it increase the torsional stiffness?
  
  
  METHODS
  Two series with six paired human humeri (total 24) were tested on one side with the rectangular shaft and on the contralateral side with a round shaft. In the first series, the shaft implantation was carried out with a constant speed of 100 mm/min and the maximum force was measured when the fracture occurred. In the second series, the implants were preloaded with 50 N and then rotated at 2° per second with monitoring of the torsional torque.
  
  
  RESULTS
  The maximum force at fracture showed no significant difference for the two designs (p = 0.34). Higher age and low bone density reduced the force required for fracture. The rectangular shaft showed significant higher torsional moments (p < 0.05).
  
  
  CONCLUSIONS
  In biomechanical testing, the rectangular shaft had a significantly higher primary torsional stability than the round shaft without a higher risk of fracture during cementless implantation. Fracture risk and torsional stability are influenced by age and bone density.
authors.affiliation: Orthopaedic Surgery, Schulthess Clinic, Zurich, Switzerland. matthias.flury@kws.ch
authors.forename: Matthias P
authors.initials: MP
authors.lastname: Flury
authors.forename: Werner
authors.initials: W
authors.lastname: Schmoelz
authors.forename: Ulrich
authors.initials: U
authors.lastname: Schreiber
authors.forename: Joerg
authors.initials: J
authors.lastname: Goldhahn
mesh_terms: D000328:Adult; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D006810:Humeral Fractures / Q000453:epidemiology*; D006811:Humerus; D008297:Male; D008875:Middle Aged; D057068:Periprosthetic Fractures / Q000453:epidemiology*; D019736:Prostheses and Implants; D011474:Prosthesis Design; D012306:Risk; D054159:Torsion, Mechanical