CORRECTING TORSIONAL DEFORMITIES WITH ILIZAROVS APPARATUS

Patients undergoing limb lengthening with circular fixators often have rotational deformities requiring correction as well. At the end of le ngthening, special rotation constructs are added to the Ilizarov frame to correct the rotational deformity. Unfortunately, the bone is usual ly eccentrically positioned in the rings, leading to secondary transla tional deformities after derotation. A trigonometric equation can pred ict the amount of inadvertent translation (t) that occurs with a given amount of derotation. Factors in this equation include the distance f rom the center of the ring to the outer edge of bone (r) and the amoun t of planned derotation (theta): t = (r sin theta)/sin (90 - theta/2). This sinusoidal function for derotation angles of less than 45 degree s can be approximated as a simple equation: t = 0.017 theta r. Armed w ith this information, the surgeon may calculate how much translation w ill be induced by the derotation maneuver. If the amount of predicted translation is deemed significant, then a more complex derotation fram e with outriggers can be constructed to change the center of rotation to the middle of the eccentric bone. Alternatively, the translation in duced by rotation can later be corrected with a translation construct.