Mechanical performance of the Monticelli-Spinelli external fixation system.

Six configurations of the Monticelli-Spinelli external fixator were tested for axial compression, bending, and torsion. The ring wire clamp unit was separately tested under repeated cyclic loading and retightening of the clampnuts. The configurations studied were the following: 3 standard (4-ring and 3-ring with 5-mm between the wires and 3-ring with 40 mm between the wires); and 3 hybrid (3-ring and 1 pin, and 2-ring and 2 pins with the rings either placed closer or distant to the fracture). For each configuration, 5 frames were constructed. Axial stiffness was nonlinear for both fixator types. Four-ring fixators and 2-ring 2-pin fixators had similar stiffness, yet were stiffer than 3-ring configurations. Anteroposterior bending stiffness was highest in the 4-ring configurations, whereas oblique bending for standard fixators was similar to lateral bending stiffness for the hybrid configurations. Torsion was higher in the 4-ring construction and in hybrid frames compared to the standard 3-ring frames. For the ring wire clamp unit, stiffness increased and hysteresis diminished after each retightening of the clampnuts because of the increase of contact between clamps and ring. The hybrid configurations provide an alternative that enables more simple configurations with less transfixing elements and sufficient stiffness.