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SURGERY
The Wonders of
External Skeletal
Fixation in
Small Animals
By Dr Ross Elliott
Bryanston Veterinary Hospital
Introduction Pins and Wires
In the past external skeletal fixators had a poor rep-
utation for repair of fractures in both human and
veterinary medicine. Although the theory behind the
concept is sound the complication rates were unac-
ceptably high. In terms of an ESF, the term pins is generally used for
both pins and wires placed in the fixator in a linear
fashion. These are always threaded pins or wires given
the high rate of failure of smooth pins/wires. The term
wires is generally used for wires and on the rare oc-
casion pins placed in a circular ring fixator. These are
always smooth pins placed in an orthogonal fashion
or at least 60 degrees to each other in an ideal world.
Given the versatility of ESF these terms are often ap-
plied loosely to the various implants.
This all changed dramatically once technology caught
up to the science behind the concept. With the ad-
vent of modern clamps and threaded pins the compli-
cation rate dropped dramatically and the true poten-
tial of external skeletal fixation (ESF) could be realised.
The greatest advantage of ESF is in the versatility of
the modality in both the degree of rigidity and the
construction of the frames to adapt to any fracture or
luxation. This is further enhanced by the advent of cir-
cular fixators and hybrid fixators, to the point that cer-
tain fractures can only be repaired using these con-
structs. The degree of rigidity increases as the type of
ESF increases: for example a type I is the least rigid
and a type III more rigid (Figure 1, 2).
There are three main components of an external fix-
ator:
• Pins or wires that make up the bone – pin interface
• Pin clamps that connect the pins to the side bar
• Sidebars that form the buttress support of the fixator
All three are vitally important to the function of the
fixator and the success of the fracture repair. A poorly
placed pin will loosen early and not transfer load. An
inferior clamp system will not grip the pin or side bar.
Poor sidebars will not take the weight of the patient
while the fracture heals leading to delayed union of
the fracture.
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As stated before, the pins used should always be
threaded pins and these should be positive profile
threaded pins or Duraface™. Positive profile have the
thread above the diameter of the shaft of the pin, the
thread does not cut into the shaft and create a stress
riser in the pin like a negative threaded pin. Negative
threaded pins will fail at the thread shaft junction due
to the cyclic loading. Duraface™ pins have a gradu-
al transition from thread to pin shaft and have been
shown to have similar, if not better, loading character-
istics to positive profile pins with the added advantage
of being smaller for each specific size as the thread is
above the shaft diameter.
Pins should always be pre-drilled when being placed
in bone. This limits micro-fractures of the bone and
thermal damage to the bone and is vital in prevent-
ing premature pin loosening. To pre-drill, an incision
is made into the soft tissue over the proposed pin site
and the tissue elevated away form the bone. The ad-
vantage of this is two-fold. It allows visualisation of the
bone to ensure the pin is optimally placed in the cen-
tre of the cortex and protects the overlying soft tissue
from being wound up by the pin during insertion.