Blocking screws (Poller screws) can be placed in the distal fragment to guide the nail and correct fracture deformity. These screws are placed from anterior to posterior to correct coronal plane deformities, and from medial to lateral to correct sagittal plane deformities. They can be left for stability or removed after nail insertion.
For a valgus deformity, a proximal Poller screw should be placed more laterally, to prevent the nail from migrating to the lateral cortex of the proximal segment and/or a distal Poller screw should be placed more medially to prevent the nail tip from migrating medially.
Conversely, for a varus deformity, a proximal Poller screw should be placed more medially and a distal Poller screw should be placed more laterally.
Alternatively, two screws can be placed (medially and laterally) to effectively centralize the nail in the distal fragment.
Blocking screws may be preplanned or used to correct intraoperative displacement. In the latter, the nail is withdrawn, and the principal blocking screw is placed to keep the nail from following its initial path and to redirect it towards the center of the distal fragment.
The fracture is manually reduced, and the nail is advanced past the blocking screw.