The screws are inserted in lag fashion. To achieve maximal interfragmentary compression and avoid shear, the screws must be directed perpendicular to the fracture plane.
Note: Ideally, two screws should be placed. However, the size of the patella or fragments may only allow placement of one screw.
2. Screw insertion
A glide hole (outer diameter of the screw) is drilled first into the cis (near) cortex followed by the thread hole (core diameter of the screw) in the trans (far) cortex using the appropriate drill bits and drill sleeves. The drill sleeve for the thread hole must be inserted completely through the cis cortex to ensure proper placement and compression.
The screw length is measured with a depth gauge. At least two measurements in opposite directions should be taken. The longest measurement should be used.
The thread hole is tapped, and a screw of appropriate length is inserted. The screw should protrude minimally (1 mm or less) from the patella to minimize interference with the patellar ligament.
Note: If two lag screws are placed, take care not to place them too close to each other. Safe distance is at least 1 screw head diameter away from any fracture line and the screws should be separated by 2 x their diameter.