Lag screw fixation
1. Screw selection
Screws with a large core diameter are recommended to counteract the sizeable forces at the femoral head.
This table can be used as a rough guideline for implant selection.
2. Screw direction
The screws are inserted in lag fashion. To achieve maximum interfragmentary compression, the screws should be directed perpendicular to the fracture plane. In most cases, the direction of the screw will be dictated by the orientation of the femoral neck and position of the plate hole.
3. Screw insertion
A glide hole (outer diameter of the screw) is first drilled into the cis (near) cortex followed by the thread hole (core diameter of the screw) into the femoral head/neck using the appropriate drill bits and drill sleeves.
The hole is started at a point just distal and slightly cranial to the third trochanter.
The drill sleeve for the thread hole must be inserted into the glide hole until it reaches the fracture to ensure proper placement of the thread hole.
If the cortex is thick, the hole may be countersunk to enlarge the contact area of the screw head to the bone. In soft bones countersinking is not performed; a washer may be used instead.
The screw length is measured with a depth gauge.
The thread hole is tapped and a screw of appropriate length is inserted. Although the screw length should be maximized, the screw should not penetrate the joint surface. The screw must be slightly shorter than the length of the hole to prevent the screw from bottoming out, preventing the lag screw from achieving compression.
