Biomechanical comparison of bending and torsional properties in retrograde intramedullary nailing of humeral shaft fractures.
OBJECTIVE
To establish whether the bending and torsional stiffness of an implanted nail are influenced by nail design and nail-bolt interface, this study compared two implanted retrograde nail systems: the AO/ASIF unreamed humeral nail (UHN) and the Russell-Taylor (RT) nail.


METHODS
Pair randomization.


METHODS
Mechanical laboratory testing.


METHODS
Twelve pairs of freshly harvested cadaveric humeri.


METHODS
Transverse fractures were simulated with a standardized midshaft osteotomy and a three-millimeter gap. Both nails were proximally and distally 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 design of the RT nail features slots, whereas the UHN has round screw holes.


METHODS
Anteroposterior and mediolateral bending stiffness and torsional stiffness.


RESULTS
The RT nail showed higher bending stiffness in anteroposterior and mediolateral bending. Large differences were seen in the torsional characteristics: for the first 30 degrees, the RT nail showed a much lower resistance 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 both nails.


CONCLUSIONS
The torsional differences between the two nail systems are attributable to the nail-bolt interface of the RT nail. This dynamic system allows a clinically relevant degree of movement. The greater resistance to rotatory forces of the UHN is explained by the fact that the interlocking at its tip and base creates a static rather than a dynamic system.
Blum
J
J
Department for Trauma Surgery, University Clinics Mainz, Germany.
Machemer
H
H
Baumgart
F
F
Schlegel
U
U
Wahl
D
D
Rommens
P M
PM
D001696:Biomechanical Phenomena; D001858:Bone Nails; D002102:Cadaver; D004867:Equipment Design; D004869:Equipment Safety; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation* / Q000379:methods; D006801:Humans; D006810:Humeral Fractures / Q000601:surgery*; D011897:Random Allocation; D012680:Sensitivity and Specificity