Lag screw technique

The function of the lag screw is to compress one piece of bone against another. This technique is commonly used to achieve absolute stability, which promotes direct bone healing, in contrast to healing with callus.

Because the lag screw technique compresses the fracture fragments together, the use of this technique is contraindicated in comminuted fractures.

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

During screw tightening, the compressive effect occurs between the undersurface of the screw head on segment A and the threads at the far end of the screw in segment B

To achieve this:

• The screw hole diameter in the proximal segment (A) needs to be larger or equal to the thread diameter of the screw to prevent screw purchase in the proximal segment (A).
• The screw hole diameter in the distal segment (B) needs to be equal to the core diameter of the screw, to allow screw purchase in the distal segment (B).

If the near cortex is not over drilled, the screw threads will engage in both the near and far cortices, preventing compression.

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

If the screw is far from perpendicular to the fracture plate, there will be a shearing force during screw tightening, which risks displacing the fracture.

When drilling obliquely to the bone's surface, the point of the drill can easily slide along the bone. It is helpful to first orient it perpendicular to the near cortex to start the hole before gradually reorienting the drill perpendicular to the fracture plane.

Drill the gliding hole in the near cortex using a drill bit with the same diameter as the thread diameter of the screw.

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

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

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

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

The gliding hole should be countersunk to spread the contact area of the screw head and reduce the stress on the near cortex.

Countersinking also makes the screw-head less prominent, which may be desirable in the subcutaneous bone.

To avoid this, it is essential that countersinking is performed manually and not with a power tool.

If too much bone is removed during countersinking, a single-hole plate may be used as a washer.

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 may underestimate the required screw length. The surgeon must make an allowance for this.

The screw's tip should protrude 1-2 mm through the opposite cortex to ensure full thread purchase.

As the near hole is a gliding hole, the screw threads will not engage and the screw will pass freely.

When a self-tapping screw is used, the flute should lie outside of the screw hole when the screw is tightened. Thus, a slightly longer screw is required.

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