Long Bone Fractures: Learning Intramedullary Pin & Wire Fixation From Practical Case Examples
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 4th power; 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 its 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.
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 (i.e., a properly reduced transverse fracture configuration) or appropriate supplemental fixation to resist axial collapse.
Control of Tension with an IM Pin
Intramedullary 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
Intramedullary 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 fixation because 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 end with 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 be pinned normograde from either the proximal or distal end of the bone or retrograde in either direction depending upon the fracture location and the desired position for pin seating in the distal segment.
IM Pin Summary
Intramedullary pins are uniquely capable of resisting bending forces in all planes, but this is the extent of their abilities. Therefore, IM pinning typically requires some form of appropriate supplemental secondary or primary fixation.
Cerclage Wire Fixation
To understand the ability of cerclage wire to resist these disruptive forces, the principles of proper cerclage wire application must first be discussed.
Only for Perfectly Reduced Long Oblique or Spiral Fracture Configurations
Full-cerclage wire fixation is an encircling wire that is capable of generating significant interfragmentary compression between the 2 bone segments, but only when properly applied to long oblique or spiral fracture configurations. The length of the fracture line must be at least twice the bone diameter; fracture lines that are 3 times the bone diameter permit greater interfragmentary compression (i.e., improved fracture stability). Sufficient interfragmentary compression is only achieved when the long oblique or spiral fracture configuration can be perfectly reduced (in this instance, “close” doesn’t count!). After closely studying your high-quality, orthogonal view radiographs, one simple rule kept in mind can save you a lot of problems: if you see multiple cortical fragments (especially small ones), “put down the wire and nobody gets hurt!”
One Cerclage Wire is Never Enough for Fracture Stabilization
A single cerclage wire is never sufficient to stabilize 2 main fracture segments because it concentrates all of the disruptive forces into this single site. When properly used to stabilize a long oblique or spiral fracture configuration, each cerclage wire adds to the interfragmentary compression that is achieved. When proper wire spacing is used (discussed below), insufficient room for a 2nd cerclage wire indicates that the fracture line is not sufficiently oblique for effective use of cerclage wiring.
Proper Cerclage Wire Spacing is a Must!
Proper wire spacing is required to permit vascular inflow and outflow from the cortical bone while achieving the needed interfragmentary compression. Adjacent cerclage wires are to be spaced 1 bone diameter apart from one another. Cerclage wires placed adjacent to the tips of long oblique/spiral fracture lines should be at least ½ bone diameter from the tip of the segment. Note that if a fracture line is less than 2 bone diameters in length, it would only be possible to place a single cerclage wire when the rules of proper wire spacing are applied. A fracture line that is 2 bone diameters in length will permit proper application of 2 cerclage wires and a fracture line that is 3 bone diameters in length will permit application of 3 cerclage wires.
Use the Correct Size of Wire
In small animal orthopedic surgery, 18-gauge wire is used for medium- and large-breed dogs, 20-gauge wire is used for medium and small breeds of dogs, and 22-gauge is used for most cats and toy-breed dogs. Braided wire is not used because it is not possible to generate sufficient wire tension with conventional tensioning and knotting techniques.
Cerclage Wire Knots
The twist knot is the most frequently used method to tighten and secure cerclage wires. In this technique, it is important that tension be applied as the wire is twisted in order to twist the wire ends uniformly around one another like a barber pole. On occasion, one end of the wire will twist upon the other like a snake on stick; this twist knot is not secure and the wire should be replaced. When an adjacent cerclage wire is placed, the additional interfragmentary compression that is produced often causes subtle, but relevant, loosening of the first wire. For this reason, when using twist knots, do not cut the twisted wire until all cerclage wires have been applied because additional tightening cannot be achieved once the wire is cut. Twist knots are usually cut between the 3rd and 4th twists. Bending the twist knot after the wire has been cut loosens the wire, so the twisted tip is usually left protruding into the soft tissues where a fibrous capsule will form around it. If there are critical neurovascular structures adjacent to the twist, the knot can be twisted as the wire is bent toward the cortical surface in attempt to minimize the loss of tension associated with bending. Single- and double-loop knots can also be used and have their own distinct advantages and disadvantages. While there are mechanical advantages (especially for double-loop knots), one of the key disadvantages is the requirement for purpose-specific instrumentation. Practically speaking, any these forms of cerclage wire fixation can be used effectively in most instances, provided the principles of proper use are strictly adhered to.
Preventing Cerclage Wire Loosening
Loose cerclage wires do not impart interfragmentary compression and, worse yet, they interfere with bone healing because they disrupt blood flow in and out of the bone. In short, “loose wires kill.” In order to minimize the risk of loosening, full cerclage wires should be oriented perpendicular to the long axis of the bone. As previously mentioned, application of an adjacent cerclage wire can loosen a wire that was previously tight; therefore, it is vital that all wires be checked for tightness immediately prior to surgical closure. For my pre-closure “wire- looseness” check, I like to use a periosteal elevator to try to shift each wire up or down the bone. Any loose wires are re-tightened (if possible) or replaced; a little extra work here can save hours of work and agonizing in the weeks to come. In conical segments of bone, cerclage wires are prone to migration toward the narrower region. A small-diameter transfixing K-wire can be placed adjacent to the cerclage wire on the narrower side, but this K-wire may encroach upon the IM pin in doing so. Alternatively, a triangular file can be used to etch a very subtle notch in the bone into which the cerclage wire can be tightened; this notch does not need to pass around the entire circumference of the bone and should not be deep into the cortex as this could increase the risk of fracture at this site; instead, the subtle notch needs only accept a small portion (∼5–10%) of the cerclage wire’s thickness at 1 or 2 sites around the bone’s circumference.
Control of Bending with Cerclage Wire Fixation
Cerclage wire is able to contribute to the control of disruptive bending forces assuming perfect anatomic reconstruction of a long oblique or spiral fracture with properly placed wires. Cerclage wire should never be used without supplemental fixation because its application is, by definition, restricted to the region of the fracture rather than distributed along the entire length of the bone.
Control of Rotation with Cerclage Wire Fixation
Properly placed cerclage wires provide good rotational stability, at least in the short term. When a long bone is subjected to repeated cycles of complex loading forces that include concurrent rotation, bending, and compression, the wires will tend to loosen, especially when supplementing an IM pin (remember, IM pins really only contribute to bending stability so the cerclage wire is heavily challenged in IM pin + cerclage scenarios). Thus, IM pin + cerclage wire fixation is typically reserved for cases in which rapid bone healing is anticipated (young patients, good soft tissue health, good systemic patient health, etc.), the number of cycles of load can be controlled (good patient/pet owner compliance) and the magnitude of the load cycles are not extreme (smaller patients, good compliance, 3 other healthy limbs, etc.).
Control of Axial Compression (Axial Collapse) with Cerclage Wire Fixation
Properly placed cerclage wires provide good resistance to axial collapse, at least in the short term. Once again, cerclage wires tend to loosen somewhat rapidly if subjected to repeated cycles of complex loading, especially if they are of larger magnitude (i.e., running, jumping, playing, stairs, falling, etc.). This tendency to loosen is particularly profound when cerclage wires are used in combination with IM pin fixation because IM pins have no ability resist rotation and compression; thus, the cerclage wires are heavily relied upon with IM pin + wire fixations. Thus, it only makes sense to restrict IM pin + wire fixation to rapidly healing scenarios with a controlled number and magnitude of cyclical loads as described in the paragraph above.