Stiffness of knee-spanning external fixation systems for traumatic knee dislocations: a biomechanical study

The purpose of this study was to compare the relative stiffness of four common external fixation (XF) configurations used to span and stabilize the knee after knee dislocation.

Synthetic composite femora and tibiae connected with cords were used to simulate a knee. Four configurations of external fixation were tested: anterior femoral pins with monotube (XF1), anterolateral femoral pins with monotube (XF2), anterolateral femoral pins with two connecting rods (XF3), and hinged ring fixator (XF4). Six specimens of each configuration were loaded nondestructively in varus/valgus, anterior-to-posterior shear, flexion/extension, axial compression, internal/external torsion, and failure in varus.

XF2 was stiffer than XF1 in varus, valgus, and axial loading (P < 0.01) demonstrating that anterolateral pins provided greater stiffness than anterior femoral pins. XF3 was stiffer than XF2 in varus, valgus, and anterior-to-posterior shear (P < 0.002), indicating that two connecting rods provided greater stiffness than the monotube. XF4 was similar to the other configurations in anterior-to-posterior shear and torsion, indicating the hinged frame provided adequate stability. The average load to failure in varus mode was 250 N-m, which was far beyond the nondestructive loading of all specimens. There was no statistical difference between the different constructs in load to failure.

The stiffest construct for external fixation of a knee dislocation was achieved when pins were placed anterior lateral on the femur and two connecting rods were used. A stiffer construct may provide a better clinical outcome and we therefore recommend this frame configuration.