Authors of section

Author

Denis Marcellin-Little

Executive Editor

Amy Kapatkin

General Editor

Noel Moens

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Lag screw fixation

1. Screw direction

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. 

 

lag screw direction

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.

Hole drilling

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.

lag screw length measurement

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.

lag screw insertion
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.
lag screw fixation