External fixation in arthrodesis of the ankle. A biomechanical study comparing a unilateral frame with a modified transfixion frame.

Arthrodesis of the ankle was performed on eight fresh-frozen human specimens, and four external fixator configurations were applied: a unilateral external fixator, a Calandruccio clamp alone, a Calandruccio clamp with two anterior struts, and a Calandruccio clamp with four struts (two anterior and two posterior). Each specimen was subjected to 4.0 newton-meters of internal-external tibial torque and 4.0 newton-meters of plantar flexion-dorsiflexion bending moment. The unilateral frame permitted the least tibiotalar motion during plantar flexion-dorsiflexion testing. The Calandruccio clamp allowed more than twice the motion allowed by the unilateral fixator in this testing mode, but the rigidity of the fixation with the Calandruccio clamp was improved with the addition of two interlocking struts, which connected fixator pins proximal and distal to the site of the arthrodesis. The addition of two more struts, to make a four-strut construct, resulted in a small (0.8-degree) additional decrease in the mean tibiotalar motion. There were no detectable differences in the mean tibiotalar motion due to torsional loading between the four fixation configurations.