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Lag screw principles

As the screw is tightened, the thread advances in the main tibial fragment and this pulls the head of the screw, and therefore the malleolar fragment, towards the main body of the tibia.

The smooth shaft of the screw prevents any significant hold between itself and the surrounding bone.

The length of the screw shaft must be chosen so that the threaded part of the screw lies fully within the opposite bone fragment, but does not penetrate beyond the denser epiphyseal cancellous bone.

To prevent the screw head from sinking into the thin malleolar cortex, the use of a washer is recommended.