The mechanical stiffness of four configurations of the Adjustable hexagonal External Fixator (AHEF) was analyzed and compared with that of the same configurations conventional Ilizarov system in vitro.
Two types of experiments was performed. In the first experiment, the axial compression, torsional, anteroposterior(A-P) bending, and lateral bending stiffness were determined in both fixators. As compared with the Ilizarov fixator, the AHEF was
less
stiff in axial compression when the two pins were oriented at 90-90°regardless of type of wires used, and was also less stiff in lateral bending when the smooth wires were oriented at 90-90°. However, there were no significant differences
between
the
two fixators in other stiffnesses. When the olve wires were used, the AHEF was rather more stiff than the Ilizarov fixator regardless of wire configuration in all stiffnesses except axial compression of 90-90° configuration. Changing the pin
orientation 90-90° to 45-135° decreased all stiffnesses of the Ilizarov fixator. However, only torsional stiffness with smooth wire and A-P bending stiffness regardless of types of wires were decreased in the AHEF Changing the smooth wires to
the
olive wires increased the A-P and lateral bending stiffness in the Ilizarov fixator, while, increased all stiffnesses in the AHEF. In the second experiment, changes of axial and torsional vs. Numbers of pins, bending and shear stiffness vs. Angle
between pins were determined in AHEF. The stiffness of AHEF in axial and torsional loading is proportional to the number of wires utilized. Bending stiffness depends on the angle of wire convergence and the axis in which the bending occurs. It
decreases
dramatically when the angle between the wires is less than 60°and when bending occurs along the A-P axis. Shear stiffness also is dependent on the wire convergence angle and the direction of loading. It decreases rapidly when the wire
convergence
angle
falls below 60° and the load is being applied along the lateral axis.
We beliveve that the AHEF, which appears to be manufactured less expensively but provides not only adjustability in size but also similar or better biomechanical properaties compared to the Ilizarov circular fixator, will be an effective
alternative for
osteosynthesis, deformity correction, and lengthening of long bone.
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