A lag screw provides compression across a fracture by use of a gliding hole in the proximal fragment so tightening the screw draws the distal fragment towards the screw head.
The lag screw is not a specific ‘screw’ but it is the name given to a technique whereby compression can be obtained between two bony fragments. This is done by drilling an oversized hole in the proximal fragment (cortex) and a normal sized hole in the distal fragment (cortex), so that when the screw is inserted, the threads glide through the proximal fragment without biting, but bites into the distal fragment and draws it in. This leads to compression between the head of the screw (abutting the cortex of the proximal fragment) and the distal fragment.
However, if a normal sized hole is drilled in the proximal fragment, the threads will bite in the proximal fragment as well as in the distal fragment and compression will not be achieved.
Note: The same effect can be achieved with a half-threaded screw, or with a screw with two different pitches within the length. However, for practical purposes for fixing hand fractures, normal screws are used.
Step 1 – Reduce the fracture with fracture reducing forceps.
Step 2 – The 2.0 mm screw will be used (in the course). Take the 2 mm drill (single blue collar) and the drill guide (blue flash present). The drill guide has a centred and off-centred guide on the same handle. Use the concentric circles (centred) end as shown. The off-centered guide end is used only in compression plates.
Note: The key to this technique is the correct use of the drill guide which must be held absolutely still in the non-dominant hand. For the purposes of this Manual, we assume the operator is right handed.
To maximize the compression, place the screw perpendicular to the fracture line. Line up the drill guide accordingly. This will give you the maximum inter-fragmentary compression.
If instead you wish to position the screw in a way to maximize the resistance to axial loading, place the screw transversely, perpendicular to the cortical surface. In practice, this is not often done as the lag screw is usually only part of a fixation strategy, and the lag screw specifically chosen to provide compression across the fracture.
Taking the core drill, go through the drill guide and drill across both cortices.
Step 3 – Remove the drill and, without moving the left hand, replace the single collared drill with the double collared drill which is for the glide screw. You will need assistance for this. Drill the first cortex only.
Step 4 – Still keeping the left hand in place, now take the depth gauge and line it up with the angle and orientation of the drill guide. Then, remove the drill guide and place the depth gauge now that you know the orientation and angle at which to insert it. Measure the depth. (14 mm here).
Note: Depending on the clinical setting, you may choose to counter-sink the screw head. This reduces the surface profile of the head and increases the contact over which the head can compress the fragment. However, there is a risk in very thin bones that you may go all the way through and this is a DISASTER!
Step 5 – Select the appropriate screw. The correct screwdriver is identified by the coloured band. Apply the screwdriver to the head perpendicular to the tray and push home. The screw will be secure. Always check the screw length as shown here before using the screw.
Note: In the clinical setting, single inter-fragmentary lag screw has insufficient strength to resist compression and torsional forces and so an additional neutralisation plate is recommended to offset these loads.