Image intensification is used to confirm that the screw tip does not penetrate into the joint.
Typically, 5 mm of epiphyseal bone/cartilage should remain between screw-tip and joint surface. Three, or four, threads lying within the epiphysis provides adequate fixation.
Pitfall: Screw penetration can occur, especially with eccentrically placed screws, even if not apparent on AP or lateral x-rays.
Dynamic, real-time image intensification, with a full range of internal and external rotation at different degrees of flexion, is useful to see how close the screw-tip is to the joint surface of the head.
Observing an approach/withdrawal of the screw tip helps in judging its position. Some surgeons supplement this examination with 3-D image intensification for confirmation.
Arthrography is useful to confirm correct wire and screw placement in the younger patient.