Correcting torsional deformities with Ilizarov's apparatus.

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