Page 339 - WSAVA2018
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R. Palmer1
1Colorado State University, Clinical Science, Fort Collins, USA
Intramedullary pins and cerclage are available in
most primary care veterinary practices and can be
used successfully to treat selected fractures in dogs
and cats. Unfortunately, when used improperly or in contraindicated scenarios, these fixation methods frequently cause severe complications including fracture disease, quadriceps contracture, mal-union, delayed or non-union, and/or infection; any of which can lead to limb amputation or euthanasia. Unexpected complications can negate their apparent affordability and, thus, lead
to client dissatisfaction. Therefore, the key to successful treatment using these fixation modalities is a thorough knowledge of their indications, proper application and limitations.
Intramedullary (IM) Pin Fixation
As with external coaptation, one must consider the ability of any internal fixation method to resist the disruptive forces acting upon the fracture to be treated. We will consider each of these disruptive forces individually.
Control of Bending with an IM Pin
Intramedullary pins, because they are placed in the central axis of the bone, are very good at resisting bending forces in all planes. An IM pin’s ability to resist disruptive bending forces is related to the pin’s radius raised to the 4thpower; thus, small changes in pin radius have a profound effect on its stiffness. In theory, a pin that completely fills the intramedullary canal would impart the greatest bending stiffness; in reality, it is
not feasible or advisable to completely fill the IM canal because of the irregular shape of the bone (bones are not uniform cylinders) and the relatively thin cortical bone typically has many grossly invisible “micro-cracks” that can easily propagate into fissures or fractures if the pin
is too large. As a rule of thumb, we typically select an IM pin that fills 60-75% of the IM canal at it’s narrowest portion (even smaller pins, ~ 30-50% canal fill, are used when combined with external skeletal fixation or bone plating). When deciding between two sizes of IM pins, it is typically better to initially choose the smaller pin; one can always “step up” in pin size if desired, but it is not possible to “drop down” to a smaller diameter pin if the first pin was too large. As we look at other disruptive forces, you will quickly note that an IM pin is a “one trick pony” that is only capable of resisting bending forces.
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Control of Rotation with an IM Pin
An IM Steinmann pin has no ability to control rotation whatsoever. Because most, if not all, long bone fractures are subjected to significant rotational forces, supplemental fixation (cerclage wire, ESF, and/or bone plate) is nearly always required with an IM pin.
Control of Axial Compression (Axial Collapse) with an IM Pin
An IM pin has little/no ability to resist axial collapse. Therefore, an IM pin either requires bony architecture (ie, a properly reduced transverse fracture configuration) or appropriate supplemental fixation to resist axial collapse.
Control of Tension with an IM Pin
IM pins have minimal ability to resist the pure tensile forces of muscle tendon unit insertions. Thus, whenever you see a patient with a fracture of a traction apophysis, either a figure of 8 tension band or a tension band plate will be required for adequate treatment of these fractures in most instances.
Other Relevant Factors for IM Pin Fixation
IM pin fixation is only beneficial when the pin can be inserted without disruption to articular surfaces and other important structures such as ligaments, nerves,
etc. The radius is not amenable to IM pin fixationbecause there are no extra-articular prominences for safe pin entry/exit; in addition, the IM canal of the radius is so small that an appropriately sized IM pin imparts very
little bending stability. The tibia must be pinned using
a normograde technique from the proximal endwith
pin entry in the extra-articular margin of bone between the medial collateral ligament and the tibial tuberosity (retrograde pinning disrupts the proximal articular surface and cruciate ligaments). The femur can be pinned using the normograde technique from the proximal end or retrograde pinning in the proximal direction though caution must be applied with the latter method. In normograde pinning, the pin is typically introduced into the lateral-most margin of the trochanteric fossa. In the retrograde technique, care must be used to direct the pin cranially and laterally as it is advanced proximally to try
to achieve an exit point within the lateral-most margin of the trochanteric fossa; the ability to direct the retrograde pin’s exit point is progressively less controllable by the surgeon in progressively more distal fracture locations. During retrograde pin insertion into the proximal femur, it is also important to position the hip in extension, neutral abduction/adduction and neutral rotation so that the proximal pin tip is directed away from the sciatic nerve as it exits the trochanteric fossa. The humerus can pinned normograde from the either the proximal or distal end
of the bone or retrograde in either directiondepending upon the fracture location and the desired position for pin seating in the distal segment.

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