AO Principles

Strain affects the type of tissue that is laid down.




Lamellar bone


Hard Callus




Granulation tissue




Bone will remodel according to the loads under which it is placed. Sides of loading will lay down increased bone. Sites where loading does not occur will be resorbed.

Primary bone healing (direct)

  • Requires absolute stability
  • No callus
  • Cutting cones

Secondary bone healing (indirect)

  • Callus formation
  • Relative stability
  • 4 phases (strain is reduced at each phase)
    • Haematoma (2 weeks)
    • Soft callus formation (3 weeks)
    • Hard callus formation(3-6 weeks)
    • Remodelling (8 weeks – 2 years)
  • Anatomic reduction
  • Stable fixation
  • Preserve blood supply
  • Early active mobility

A device that converts torsional force to linear movement.

The linear distance a screw moves with one 360 degree rotation of a screw.

Distance between threads

Difference between thread diameter and core diameter of screw

Cortical screws have a finer pitch and lower effective thread depth.

Screws that engage into the plate creating a fixed angle construct. Increases the pull out strength.

  • Lag screw can be by design (lag screw) or by technique.
  • Must be perpendicular to the fracture site
  • Drill (3.5mm through first cortex)
  • Insert 2.5mm drill guide into proximal drill hole)
  • Drill far cortex
  • Countersink
  • Measure
  • Tap if not self-tapping
  • Insert screw (screw will only engage the distal bone leading to interfragmentary compression)
  • Dynamic compression
  • Locking plate
  • Reconstruction plate
  • Anatomical plate
  • Buttress plating
  • Compression plating
  • Tension band plating
  • Lag screw with neutralisation plate
Please log in to join the chat