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 the safe and secure application of a plate in a neutral mode in the reduced fracture.
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. The screw is 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.
Interfragmentary screws can achieve a lag effect in the bone in 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 a lag screw by making the drill hole size larger in the proximal fragment so that threads passing through it do not take hold in the proximal fragment. The screw holds in the distal fragment and causes compression.
The Technique of Interfragmentary Screw Placement
- Fracture is reduced and held temporarily with reduction forceps and or K-wires
- A hole is drilled in the near cortex called gliding hole
- Dill bit needs to be oversized (equal to the 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 cylinder 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 over-drilled cortex. Use an appropriately sized drill bit to drill the far cortex
- Tap the bone using an appropriately sized tapper
- Use a screw length so as to allow the screw to protrude 2 mm beyond the far cortex
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.
Following video shows the effect of interfragmentary compression.
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 interfragmentary screw accurately so that the forces they generate are evenly distributed across fractures, otherwise, distortion of reduction can occur
