Lag screw principles
As the thread advances in the main body of the tibia, the head pushes the medial fragment towards the tibia.
The smooth shaft of the screw prevents any significant hold between the screw and the surrounding bone.
The length of the screw shaft must be chosen so that the threaded part of the screw lies fully beyond the fracture line and within the epiphyseal cancellous bone of the distal tibia.
To prevent the screw head from sinking into the thin cortex, the use of a washer is recommended.
The function of a buttress plate is to neutralize any vertical shearing force.