Mechanical evaluation of external fixators used in limb lengthening.

Four external fixator systems (five configurations) used for limb lengthening were tested to determine the fixator stiffness and the fracture gap rigidity. There was a statistical difference between fixators in all modes of loading with respect to stiffness, shear, and axial motion. The fixators were graded to determine their relative stiffness, shear rigidity, and axial rigidity. The EBI Orthofix proved to be the most rigid fixator relative to the configurations tested with minimal shear or axial motion at the fracture site. The Ilizarov tibial configuration was the least rigid, demonstrating more shear and axial motion at the fracture gap. The Ilizarov femoral system combined excellent stability and shear resistance with preservation of axial dynamization. Fixators with a high stiffness provide less motion at the fracture site, which may cause stress shielding of the osteotomy. Fixators that provide more motion at the fracture gap are less stable. These data may be useful in determining which fixator may be ideal for a particular clinical situation.