Biomechanical testing of rectangular humeral shaft prosthesis: higher torsional stability without increased fracture risk.
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.
Flury
Matthias P
MP
Orthopaedic Surgery, Schulthess Clinic, Zurich, Switzerland. matthias.flury@kws.ch
Schmoelz
Werner
W
Schreiber
Ulrich
U
Goldhahn
Joerg
J
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