Cortical lag screws

The function of the cortical 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.

A screw has a specific core diameter and a thread diameter.

As the screw is tightened, the compressive effect occurs between the head of the screw on segment A and the threads at the far end of the screw in segment B.

To achieve this the screw must be able to move in the near hole (gliding hole) without the threads obtaining purchase, so a thread-diameter hole must be drilled in segment A.

For the screw threads to obtain purchase in the far cortex, a pilot hole of the core diameter must be drilled in segment B.

3D animation showing cortical lag screw fixation.

Note: The following animations show some separation of the fragments before the screw is tightened. This exaggerated situation is meant to illustrate the principle, but in clinical practice, it is best to reduce the fragments accurately and hold them with a clamp before drilling and inserting the lag screw.

The axis of the screw should be as perpendicular as possible to the plane of the fracture.

If the screw is far from perpendicular, as it is tightened, there will be a shearing force, which risks displacing the fracture.

Lag screw technique

Reduce and hold the fracture temporarily.

If a cannulated screw is to be used, insert the guide wire and use cannulated drill bits. Further steps are basically identical to what is described for fully and partially threaded screw insertion.

Drill the gliding hole in the near piece of bone using a drill bit with the same diameter as the threads of the screw.

To ensure that the pilot hole has the same axis as the gliding hole, insert a drill sleeve that has the outer diameter of the gliding hole and the inner diameter of the intended pilot hole. For the threads to engage in the pilot hole, the diameter of the distal hole must correspond to the core diameter of the screw.

Drill the pilot hole. The drill bit should penetrate the far cortex.

It is possible to drill both holes with the smaller drill bit first, then to use the larger drill bit to create the gliding hole in the near cortex.

To spread the contact area of the screw head and reduce the stress on the cortex, the gliding hole may be countersunk.

This also results in the head of the screw being less prominent, which may be desirable in subcutaneous bone.

An alternative to countersinking is to use a dished washer. This has a flat underside to apply the force evenly on the bone surface and a dished or concave surface on the top which matches the shape of the screw head.

The depth of the hole is measured.

It is better for the hook on the depth gauge to catch on the longer side of the drill hole, as shown.

Alternatively, if it is not possible to engage the hook, measure the shorter side of the drill hole, but this will tend to underestimate the required screw length. The surgeon must make an allowance for this.

To ensure full thread purchase the tip of the screw should protrude 1–2 mm through the opposite cortex.

When non-self-tapping screws are used, tap the distal hole.

As the near hole is a gliding hole, the tap should pass through this freely.

The screw is inserted at about 2/3 of the possible torque. Note that the screw protrude2 1–2 mm through the opposite cortex.

If self-tapping screws are used, this step can be omitted and the screw inserted directly.

As the near hole is a gliding hole, the screw should pass through this freely.

The tap portion of a self-tapping screw should lie outside of the screw hole when the screw is tightened, so a slightly longer screw is required.

However, too long a screw may cause soft tissue irritation.

The screw is inserted at about 2/3 of the possible torque.

When partially threaded screws are used, a gliding hole is not necessary. Drill the screw hole through both cortices using a drill bit with the same diameter as the core diameter of the screw.

The drill bit should penetrate the far cortex.

To spread the contact area of the screw head, and reduce the stress on the cortex, the screw hole may be countersunk.

This also results in the head of the screw being less prominent, which may be desirable in subcutaneous bone.

The depth of the hole is measured.

It is better for the hook on the depth gauge to catch on the longer side of the drill hole, as shown.

Alternatively, if it is not possible to engage the hook, measure the shorter side of the drill hole, but this will tend to underestimate the required screw length. The surgeon must make an allowance for this.

To ensure full thread purchase the tip of the screw should protrude 1–2 mm through the opposite cortex.

When non-self-tapping screws are used, tap both cortices.

As the near hole is a gliding hole, this would, in principle, not need to be tapped.

The screw is inserted at about 2/3 of the possible torque. Note that the screw protrudes 1–2 mm through the opposite cortex.

If self-tapping screws are used, tapping can be omitted, and the screw inserted directly.

The tap portion of a self-tapping screw should lie outside of the screw hole when the screw is tightened, so a slightly longer screw is required.

However, too long a screw may cause soft tissue irritation.

The screw is inserted at about 2/3 of the possible torque.

If the hold of lag screws alone is inadequate, a neutralization device such as a plate or an external fixator may be used to provide additional stabilization.

The plane of the fracture dictates the position 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 in a neutral (not dynamic compression) mode.

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 should be inserted outside the plate. The fracture should be reduced, and the lag screw