[Principles of correct distraction device placement at lengthening of the femur to provide mechanical alignment of the extremity].

The authors has lengthened 16 femoral bones with good mechanical alignment. To maintain it through the lengthening process the distraction devise must be parallel to the mechanical axis of the limb. This can be accomplished if the axis is marked on the skin prior to surgery. The above mentioned rule should be observed, particularly in bone lengthening with semi-frame distraction device with no feature of progressive correction.