Authors of section

Author

Cassio Ferrigno

Executive Editor

Amy Kapatkin

General Editor

Noel Moens

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General considerations on external skeletal fixators

1. Advantages of external skeletal fixation

  • Less damage to blood supply to bone
  • Minimal interference with soft-tissue in the distal limb only
  • Useful for stabilizing grade 3 open fractures
  • Requires less experience and surgical skills than standard ORIF
  • Easy removal or staged disassembly
  • Closed approach to the fracture is easier

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2. Disadvantages of external skeletal fixation

  • Pin penetrating the soft tissues
  • Restricted joint motion
  • Pin tract complications in long-lasting external fixation that lead to loosening and loss of fracture reduction
  • Possible damage of vessels and nerves
  • Time demanding aftercare
  • Mechanical disadvantages: implant stability is related to the distance between implant and bone (x). The further the implant is away from the bone, the weaker the construct is.

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3. External skeletal fixator types and configuration

Type 1a: uniplanar and unilateral external skeletal fixator.

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Type 1b: uniplanar unilateral.

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Type 2: uniplanar bilateral external skeletal fixator. The connecting bars are placed parallel to the bone axis.

Note: ESF on the lateral side can be difficult to place because of the fibula. It must be avoided. Drilling through it with a large pin can fracture it.

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Type 3: biplanar bilateral external skeletal fixator.

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Circular

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4. Biomechanical aspects

The external fixator should provide enough stability to maintain reduction. The surgeon has to understand the biomechanical principles in order to correctly apply the device to achieve adequate stability. At least two pins have to be inserted into each main fragment through the safe zone and in each segment. For the construct to be stable, one pin should be placed close to the proximal and distal end of the bone and the other one near the fracture line in each segment.

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The stiffness of the frame depends upon the following factors:

  • Distance of the pins from the fracture line (x): the closer the stiffer (should not be closer than 0.5cm)
  • Distance of the pins inserted in each main fragment (y): the further apart the stiffer (not closer than 1 cm to another pin)
  • Distance of the longitudinal connecting bar from the bone (z): the closer the stiffer (should not be closer than 0.5cm from the skin edge)
  • Number of pins: the more the stiffer but stiffness does not increase after 4 pins per segment
  • Number of bars: two are stiffer than one
  • Configuration (low to high stiffness): unilateral/V-shaped/bilateral or triangular frame
  • Implant type: some clamps and bars are stiffer and have less loosening than others
  • Combination of limited internal fixation (lag screw) with external fixation; only rarely indicated because mixing an elastic with a stable fixation technique is not recommended!
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Comparative rigidity between number of pins per segment.

Note: There is no advantage in having more than 4 pins per fracture segment. Adding more pins violates some of the other rules of ESF placement and should not be done.

Bouby, B.M. et al. Ex vivo biomechanics of Kirschner-Ehmer external skeletal fixators applied to canine tibiae. Vet Surg. v.22, p.194-207, 1993

Comparative rigidity between constructs. The circular fixator has the highest rigidity.

Note: Although stiffness increases with each construct, a more flexible construct may be best in certain cases. Understanding the balance between stiffness and flexibility for each fracture is important.

Brinker, W.O. et al. Stiffness studies on various configurations and types of external fixators, J. Am. Anim. Hosp. Assoc., v.24, p.280-288, 1985