Principles of Internal Fixation
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Direct answer
Internal fixation holds a fracture with screws, plates and wires, and the construct is chosen by whether it provides absolute or relative stability. Absolute stability uses interfragmentary compression and no motion, so the bone heals directly without callus, whereas relative stability allows controlled micromotion and heals through callus. The lag screw, neutralisation, buttress and tension band constructs are the standard options, each matched to the forces acting on the fracture.
What you must remember
- Absolute stability: rigid fixation with compression and no motion at the fracture, allowing primary bone healing without callus.
- Relative stability: controlled micromotion across the fracture, allowing secondary healing through callus formation.
- Lag screw: provides interfragmentary compression, with the near cortex over-drilled so that only the far cortex is engaged.
- Neutralisation plate: protects the lag screw from bending and torsion, and carries little of the load itself.
- Buttress plate: resists axial load by supporting the fragment against the direction in which it tends to displace, preventing collapse.
- Tension band: converts distractive tensile forces into compression at the fracture, classically in the olecranon and patella.
- Dynamic compression plate: eccentric screw holes in the plate create additional compression across the fracture on tightening.
Choosing a construct from the forces at the fracture
Every fixation construct is chosen by asking which force the fracture must resist, and whether the bone should heal directly or through callus. The steps below move from the forces to the construct.
- Identify the forces: tension, compression, bending and torsion act differently, and the construct must match the dominant force.
- Decide the stability: simple, anatomically reduced fractures aim for absolute stability, whereas comminuted fractures accept relative stability through a bridging plate.
- Apply the lag screw: a screw placed at right angles to the fracture, with the near cortex over-drilled, compresses two fragments together.
- Add neutralisation: a plate applied over the lag screw protects it from bending and torsion, so the fixation survives loading.
- Use a buttress for support: a plate placed against the direction of fragment displacement resists collapse, which matters in metaphyseal and articular fractures.
- Apply a tension band: wires or K-wires on the tension side convert distraction into compression, as in the olecranon and patella.
Where the exam tries to catch you
Fixation questions usually test whether the student matches the device to the force, not to habit.
- Trap: using a lag screw alone across a comminuted or unstable fracture Correct: a lag screw alone cannot resist the loads, so a neutralisation or buttress plate is added.
- Trap: applying a tension band where the failing force is compressive Correct: tension bands convert tensile distraction into compression, so they suit fractures in which the tension side is the failure site.
- Trap: believing absolute stability is always best Correct: comminuted fractures often heal well with relative stability and callus, so rigid compression is not required everywhere.
Frequently asked questions
What is the difference between absolute and relative stability?
Absolute stability is achieved by rigid fixation with interfragmentary compression and no motion, allowing direct bone healing. Relative stability permits controlled micromotion across the fracture and heals through callus formation.
How does a lag screw compress a fracture?
A lag screw compresses a fracture when the near cortex is over-drilled so that the thread engages only the far fragment. Tightening then pulls the two fragments together, creating interfragmentary compression.
When is a buttress plate used in fracture fixation?
A buttress plate is used when a fracture needs support against axial load, typically in comminuted metaphyseal or articular fractures. It is placed against the direction of fragment displacement to prevent collapse.
What is a tension band wire used for?
A tension band wire converts distractive tensile forces into compression at the fracture site, most commonly in the olecranon and patella. Wires or K-wires placed on the tension side allow muscle pull to close the fracture.