Biomechanical comparison of bending and torsional properties in retrogradeintramedullary nailing of humeral shaft fractures

Objective: To establish whether the bending and torsional stiffness of an i mplanted nail are influenced by nail design and nail-bolt interface, this s tudy compared two implanted retrograde nail systems: the AO/ASIF unreamed h umeral nail (UHN) and the Russell-Taylor (RT) nail. Design: Pair randomization. Setting: Mechanical laboratory testing. S Specimens: Twelve pairs of freshly harvested cadaveric humeri. Methods: Transverse fractures were simulated with a standardized midshaft o steotomy and a three-millimeter gap. Both nails were proximally and distall y interlocked. The RT nail has a single interlock at its base and tip. The UHN has double interlocking both proximally and distally. The screw hole de sign of the RT nail features slots, whereas the UHN has round screw holes. Main Outcome Measures: Anteroposterior and mediolateral bending stiffness a nd torsional stiffness. Results: The RT nail showed higher bending stiffness in anteroposterior and mediolateral bending. Large differences were seen in the torsional charact eristics: for the first 30 degrees, the RT nail showed a much lower resista nce against torsion than the UHN. Analysis of variance of stiffness at four , six, and eight newton-meters showed statistical significance (p < 0.0001) . Torsional stiffness, defined as the slope of a straight line approximated to between 75 and 100 percent of the maximum torque, was very similar in b oth nails. Conclusion: The torsional differences between the two nail systems are attr ibutable to the nail-bolt interface of the RT nail. This dynamic system all ows a clinically relevant degree of movement. The greater resistance to rot atory Forces of the UHN is explained by the fact that the interlocking at i ts tip and base creates a static rather than a dynamic system.