Basic Science Trauma

Tension band wiring and external fixators

Tension band principle and lever shift, knee-spanning external fixator construction and uses

15 questions 2 source pages

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Q1What is a tension band?▸

A device that converts a tensile force into a compression force
Shifts the fulcrum so the system changes from a 1st class lever to a 2nd class lever

Q2List the prerequisites for a tension band construct.▸

Q3Which implants can act as a tension band?▸

Q4How does a tension band alter loading across the fracture?▸

Shifts the neutral axis from the mid axis to the plate-bone junction
Axial loading produces pure tension in the plate and pure compression across the fracture line
This bending configuration is used to produce bending close situation to reduce the working length of the plate

Q5What is an external fixator and what are its advantages and disadvantages?▸

External device stabilising the fracture with pins and rods coupled by clamps
Can be used for temporary or definitive fixation
Advantages: minimally invasive, quick application, flexible during construction
Disadvantages: pin tract infection and neurovascular injury during insertion

Q6What is the goal of external fixation?▸

Q7What are the four ways to improve the stability of an external fixation?▸

Q8How can pins improve the stability of an external fixator?▸

Place pins close to the fracture (near-near) to decrease working length
Span a longer length (far-far) to decrease the moment arm of bending force and bending stress
More pins; larger pins (not to exceed 1/3 of cortical diameter)
Radial preload decreases micromotion-induced loosening and bone resorption
Bicortical purchase, Material (HA coated), reduce thermal necrosis (predrill, cooling)

Q9Explain the near-near far-far principle.▸

2 pins are placed close to the fracture to achieve a short working length and better stability
2 far pins over both fragments share out the stress and increase bending and torsional stability

Q10How can rods improve the stability of an external fixator?▸

Q11How does the construct improve the stability of an external fixator?▸

Q12How does reduction improve the stability of an external fixator?▸

Reduction is listed as one of the four ways to improve stability of the external fixation

Q13How is stability improved in a ring external fixator?▸

Decrease ring diameter, increase number of rings, decrease ring spacing, inner rings closer to fracture
Increase wire diameter and pretension (up to 130kg); add wires or half pins
Wire crossing at 90 degrees to bone surface (at least >30 degrees)

Q14What are the ways to decrease infection in external fixation?▸

Q15What are the safety corridors for external fixation pins (right side)?▸

Femur: proximal 7-10 o'clock, distal 7-10 o'clock
Tibia: proximal 10-11 and 1-3 o'clock, middle 12-3 o'clock, distal 1-3 o'clock
Humerus: proximal 9-11 o'clock, distal 7-8:30 o'clock (Distal: 7-830 OC)

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