Biomechanical Factors Affecting Fracture Stability and Femoral Bursting in Closed Intramedullary Rod Fixation of Femur Fractures

[+] Author and Article Information
A. F. Tencer

Division of Orthopedics, The University of Texas Medical Branch, Galveston, Tex. 77550

M. C. Sherman

Biomedical Engineering Program, The University of Texas at Arlington, Arlington, Tex. 76019

K. D. Johnson

Division of Orthopaedic Surgery, The University of Texas Health Science Center at Dallas, Dallas, Tex. 75235

J Biomech Eng 107(2), 104-111 (May 01, 1985) (8 pages) doi:10.1115/1.3138530 History: Received March 05, 1984; Revised August 01, 1984; Online June 15, 2009


Intramedullary rodding of femur fractures, although a safe and rapidly performed procedure, can result in several complications. If the rod fit is too loose, fracture instability, rod migration, and delayed union may result. If the rod fit is too tight, cracking of the femur may occur during rod insertion. These complications were investigated in terms of geometric and mechanical parameters of the bone-implant system. Results showed that rods of the same nominal size from different manufacturers showed more than twofold difference in flexural rigidity and a threefold difference in torsional modulus. These differences appear to be due to differences in cross sectional shape and wall thickness of the rods. Measurements of pushout force and hoop stress in cadaver femora showed a large difference in pushout force with different rods, and significantly lower forces in distal than in proximal femoral fracture components. Pushout force decreased with fracture component length proximally and dropped to zero in distal components less than 170 mm long. An increase in ream diameter in the distal components of just 1 mm was found to decrease the mean pushout force from 740N to 90N. The most significant variable was found to be anterior offset of the starting hole more than 6 mm from the centerline of the medullary canal which resulted in consistent lifting of the anterior cortex during insertion of the rod.

Copyright © 1985 by ASME
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