Lag Screw
Introduction and Definition: A lag screw is a specialized type of cortical or cancellous screw used in orthopaedic surgery to achieve interfragmentary compression across fractures or osteotomies. Its primary function is to provide stable fixation by pulling one bony fragment towards another, thereby promoting direct bone healing through absolute stability. The lag screw technique utilizes differential thread engagement: threads engage only in the far fragment, and the near fragment is overdrilled to allow smooth passage of the screw shaft, producing compression when the screw is tightened.
Anatomy and Biomechanics
The success of a lag screw construct depends on understanding bone anatomy and biomechanics. Bone consists of an outer cortical shell with dense lamellar bone and an inner cancellous (trabecular) network. Cortical bone has higher density and mechanical strength, suitable for rigid fixation, whereas cancellous bone offers less thread purchase.
- Bone Anatomy: The near and far fragments in fractures (e.g., distal fragment vs. proximal shaft in a long bone) must be considered for screw placement.
- Biomechanical Principle: The screw acts as an axial tension device that converts rotational torque into longitudinal compression between fragments.
- Screw Design: The lag screw's differential thread engagement: threads only grip the far fragment, while…