[Three-dimensional rigidity of the Ilizarov external fixator (original and modified) implanted at the femur. Experimental study and clinical deductions]. / Rigidité tridimensionnelle de l'appareil d'Ilizarov (original et modifié) implanté au fémur. Etude expérimentale et déductions cliniques.

The application of the Ilizarov device to the femur creates several problems: anatomical (transfixion of the thigh), mechanical (asymetrical assembly) and clinical (patient acceptability and duration of treatment). Geometric modifications of the original fixator are proposed. These comprise the use of threaded pins proximally and special connecting pieces. Two large fixators were tested under load and after the introduction of certain variables led the authors to experiment with 18 different assemblies. Four loads were used (compression, flexion in the sagittal and coronal planes and torsion) and stiffness calculated in three dimensions. Three linear and three angular displacements were defined for each type and magnitude of external load. The following sequence of analysis was followed to select the best type of assembly; increased ridigity in torsion; moderate displacement in flexion and axial elasticity. These considerations also apply to the mechanics of fracture healing. After testing under load it was concluded that the original assembly gave the best compromise. The modified assemblies gave a slightly inferior mechanical performance but their geometrical configuration should give better patient tolerance if the following items are used: a proximal arc of 120 degrees, 5 mm diameter threaded pins for the adult and 4 mm diameter for the child.