Interfragmentary Screw or Lag Screw

Interfragmentary screw or lag screw is used in fixation of spiral or oblique fractures of the shaft of a bone. Interfragmentary screw or lag screw is used as an adjunct to plating and often as initial fixation holder so as to safe and secure application of plate in neutral mode in reduced fracture.

[Read more on Bone Screws]

The procedure involves  placement of one or more screws across a fracture site so that fractured fragments are apposed and compressed to each other, called interfragmentary compression ( See image below). The interfragmentary screw can also be used for fixation after osteotomies.

Interfragmentary screw is put in a direction perpendicular to the fracture. Therefore in case of a spiral fracture, the screw direction may vary as the fracture plane changes.

Image Credit:Muller ME, Allgower M. Schneider RW, 30. et al: Manual of Internal Fixation. New York. Springer- Verlag, 1979.

Lag Screw Principle

Compressing two objects together is sometimes termed lagging. Screw are a means of compressing objects together. In bone fracture fixation, stabilization of fragments provides a basis for early mobilization of the limb and will prevent excessive movement of metal implants which could lead to failure through metal fatigue.

Interfrag screws can achieve a lag effect in bone of two ways:

• They can be designed specifically as lag screws, which are partially threaded only, at the end nearest the tip.

• A fully threaded screw can be used as lag screw by making the drill hole size larger in the proximal fragment so that threads passing through it do not make hold in the proximal fragment. Screw holds in distal fragment and causes compression.

Technique of Interfrgmentary Screw Placement

• Fracture is reduced and held temporarily with reduction forceps and or Kwires-
• A hole is drilled in the near cortex called gliding hole. This drill bit needs to be oversized (equal ot outer diameter of screw) than the drill in far cortex so that screw just glides and compresses two surfaces of fracture. Far cortex should not be drilled with this drill bit.

[The bone is like  a hollow cylinder.The shell of the cyllinder is cortex and empty hollow is medulla. When drilling one would encounter  bone [or near cortex)followed by relatively empty feeling of medulla followed by striking the bony surface from within on the other side {far cortex} in the path of the drilling.

• Insert  a drill sleeve into the overdrilled cortex. Use appropriately sized drill bit to drill the far cortex
• Tap the bone using appropriately sized tapper.
• Use a screw length so as to allow screw to protrude 2 mm beyond far cortex a

Lag screw fixation of an oblique or spiral fracture requires neutralization by a plate if sufficient stability is to be achieved to allow immediate rehabilitation without external protection.

Lagging as a means of fixing bone is a very powerful technique and great forces of compression can be generated. It is essential to position interfrag screw accurately so that the forces they generate are evenly distributed across fractures, otherwise distortion of reduction can occur