Note: In this technique, the proximal tibia is shown as a model for the application of a cancellous bone screw. For other cancellous bone regions, the illustrated technical details need to be adjusted accordingly.
The function of the lag screw is to compress one piece of bone against another. This improves the stability of a reduction, so it is commonly used to achieve absolute stability, leading to direct bone healing, as opposed to healing with callus.
There are various screw types available, and details of the lag screw technique will depend on the type of screw.
3. Screw axis
The axis of the screw should be as perpendicular as possible to the plane of the fracture.
Metaphyseal fractures occur close to joints. A large fragment may include a significant portion of the joint surface. If this is loaded, there will be a tendency for the fragment to displace.
To counter this shearing effect, the orientation of a lag screw may be adjusted so that it is not quite perpendicular to the fracture plane but lies more perpendicular to the long axis of the bone.
If the screw is far from perpendicular, as it is tightened, there will be a shearing force, which risks displacing the fracture.
The compressive effect occurs between the head of the screw in segment A and the threads at the far end of the screw in segment B as the screw is tightened.
To achieve this the screw must be able to move in the near hole without the thread obtaining purchase. The screw with the appropriate unthreaded shaft length should be selected.
The cortex tends to be very thin near the ends of the bone, particularly in osteoporosis. As the screw is tightened, there is a risk that the head will sink. This can be prevented by using a washer.
Cancellous screws have a greater difference between the thread diameter and the core diameter than cortical screws.
This would require a much larger diameter gliding hole, so a partially threaded screw is used.
4. Reduction and temporary fixation
The fracture is reduced, and the reduction temporarily held with, eg, reduction forceps.
5. Pilot hole
Drill the pilot hole perpendicular to the fracture plane, with a diameter corresponding to the core diameter of the screw. In cancellous bone, it is often not necessary to drill a pilot hole once the cortex is penetrated.
As there is minimal cortical bone in the metaphyseal region, and the threads of the screw are holding in the cancellous bone, there is no need for the screw to penetrate the far side of the bone.
In addition, there are often important soft tissue structures adjacent to the bone in the metaphyseal region, which may be damaged or irritated by a protruding screw.
6. Screw length
Measure the depth of the hole, taking into account the thickness of the washer, if one is to be used.
7. Screw selection
A partially threaded screw is selected. The length of the unthreaded shaft should only hold in the far cancellous bone but without threads remaining in the near fragment.
8. Screw insertion
The torque must not exceed the holding power of the threaded segment of the screw in the cancellous bone. Therefore, care must be taken to avoid maximal torque. When using a screwdriver, the surgeon may choose to use a tripod finger grip instead of a power palmar grip.
9. Lag screw with neutralization device
If the hold of lag screws alone is inadequate, a neutralization or buttressing device such as a plate or an external fixator may be used to provide additional stabilization.
The plane of the fracture dictates the position and direction of the lag screw. Anatomic considerations will influence the optimal positioning of the plate.
If the plate position and lag screw insertion site are aligned, the lag screw may be inserted through the plate.
The plate should be contoured and positioned appropriately. Then, with the fracture reduced, the lag screw is inserted through the plate first, as described below. The remaining screws are then inserted through the plate.
If the fracture plane necessitates that the lag screw is inserted from a position where it would be awkward to apply a plate, the lag screw can be inserted outside the plate. The fracture should be reduced and the lag screw inserted first. The neutralization plate is then applied.