Image intensification is used to confirm that the screw tip does not penetrate into the cartilage or the joint.
Typically, 5 mm of epiphyseal bone/cartilage should remain between the screw tip and joint surface. Three or four threads crossing the physis provides adequate fixation.
Pitfall: Screw penetration can occur, especially with eccentrically placed screws, even if not apparent on standard AP or lateral x-rays.
Real-time, dynamic image intensification, through a full range of internal and external rotation, and at different degrees of flexion, is useful to confirm the distance between the screw tip and the joint space.
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.