Cortical lag screws

1. Principles

The function of the lag screw is to compress one piece of bone against another. This improves the stability of a reduction, so is commonly used to achieve absolute stability which then leads to direct bone healing, as opposed to healing with callus.

Compression of oblique fracture

There are various screw types available, and details of the lag screw technique will depend on the type of screw.

Various lag screws

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

Screw details

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.

Lag screw compressing fracture

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

Screw axis

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

Screw inserted outside screw axis
Pearl: If the bone is likely to be loaded in its long axis, there will be a tendency for an oblique fracture to displace.
To counter this shearing effect, the orientation (a) of a lag screw may be adjusted so that it is not quite perpendicular to the fracture plane (b), but lies more perpendicular to the long axis of the bone (c).
Screw axis in case of bone loading

AO Teaching Video

Lag screw technique

2. Reduction and temporary fixation

Reduce and hold the fracture temporarily.

Reduction and temporary fixation

3. Insertion of cannulated screws

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.

Insertion of cannulated screws

4. Insertion of fully threaded screws

Drilling

Gliding hole

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

Pitfall: Care must be taken not to accidentally pass the larger drill bit too far and over-drill the distal hole as well.
Drilling of gliding hole
Pearl: When drilling obliquely to the surface of the bone, the point of the K-wire or 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 K-wire / drill perpendicular to the plane of the fracture.
Pearl for drilling obliquely to the surface of the bone
Pilot hole

To ensure that the pilot hole has the same axis as the gliding hole, insert a drill sleeve which 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.

Drilling of pilot hole
Alternative drill technique

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.

Note: As the hole in the bone surface is oblique, it is possible that the larger drill bit may not create a hole with an identical axis to the distal hole (as illustrated).
Alternative drilling teqhnique

Countersinking

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.

Countersinking
Pitfall: When the cortex is thin, it is imperative that countersinking does not remove all the cortical bone around the circumference of the head of the screw. Otherwise, as the screw is tightened its head will enter the medullary space and provide no compression of the fracture fragments.
Pitfall when countersinking

Use of a dished washer

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.

Use of a dished washer

Determination of screw length

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.

Determination of screw length

Insertion of non-self-tapping screws

Tapping

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.

Tapping
Screw insertion

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

Insertion of non self tapping screw

Insertion of self-tapping screws

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.

Insertion of self tapping screw

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.

Self tapping screw inserted

5. Insertion of partially threaded screws

Drilling

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.

Drilling for partially threaded screw
Pearl: When drilling obliquely to the surface of the bone, the point of the K-wire or drill can easily slide along the bone. It is helpful to first orient it perpendicular to the near cortex to create an initial hole before reorienting the K-wire / drill perpendicular to the plane of the fracture.
Pearl for drilling obliquely onto bone surface

Countersinking

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.

Countersinking
Pitfall: When the cortex is thin, it is imperative that countersinking does not remove all of the cortical bone around the circumference of the head of the screw. Otherwise, as the screw is tightened its head will enter the medullary space and provide no compression of the fracture fragments.
Use of dished washer

Determination of screw length

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.

Determination of screw length

Insertion of non-self tapping screws

Tapping

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.

Tapping
Screw insertion

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

Screw insertion

Insertion of self-tapping screws

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

Insertion of self tapping screw

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.

Protrusion of self tapping screw

6. Lag screw with neutralization device

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

Lag screw with neutralization device