Physicians have been using external fixation to treat fractures for more than 2000 years after being first described by Hippocrates as a way to immobilize the fracture while preserving soft tissue integrity. The fixator design and biomechanics have changed dramatically over the years, but the principles remain the same. The primary goal of external fixation is to maintain the length, alignment, and rotation of the fracture. External fixation can serve as provisional fixation or definitive fixation purposes. Both methods can be performed in conjunction with partial internal fixation if necessary. It is important for orthopedic surgeons at a trauma center to be familiar with the techniques and principles of external fixation for various fractures of the upper extremity, lower extremity, and pelvis.
Fracture healing physiology largely depends on the mode of fixation and level of stability. With absolute fracture stability such as compression plating, the bone will undergo primary intramembranous bone healing. On the other hand, relative fracture stability, such as external fixation, results in secondary enchondral bone healing. There are also several ways to alter the external fixation construct to make the fracture more or less stable.
Influential Factors and Variables for Construct Stiffness and Stability
One method of changing the stability is to alter the pin configuration. Placing pins closer to the fracture site, adding more pins and increasing the spread of the pins will all add to the stiffness of the construct. However, one must also place the pins out of the field of future surgical approaches during definitive fixation. Any increases in pin diameter will strengthen the construct to the fourth power and reduce the stress at the bone-pin interface. Increasing pin diameter has the greatest influence on the stability of unilateral frames. That said, larger pins increase the risk of a potential stress riser and can ultimately lead to fracture. For example; a 5 mm pin is 144% stiffer than a 4 mm pin.
Other variations in pin morphology include self-drilling pins, trocar tip pins, and hydroxyapatite-coated pins. Another way to change the strength of the construct is to increase the diameter of the rods or secure it closer in proximity to the bone. One can also add multiple bars to the same pins to enhance stability. Bars get secured to the pins by clamps. The most common material for bars today is carbon fiber, which is 15 % stiffer than stainless steel bars.
External Fixator Types
External fixator types divide into several different subcategories, including uniplanar, multiplanar, unilateral, bilateral, and circular fixators. By adding pins in different planes (i.e., placed perpendicular to each other), one can create a multiplanar construct. Uniplanar fixation devices are fast and easy to apply but are not as sturdy as multiplanar fixation. Bilateral frames are created when the pins are on both sides of the bone and can also add additional stability. Circular fixators have gained popularity with limb lengthening procedures but are especially effective at allowing the patient to weight bear and maintain some joint motion during the treatment. They are more difficult to apply and use smaller gauge pins and more of them to distribute the weight.
There are many different ways to change and enhance the external fixation construct. To complicate things further, there are also hybrid frames which are a combination of any of the previous constructs described. The surgeon must create a level of stability that is appropriate for optimal healing. It is essential also to have a good understanding of basic fracture principals because stiffer is not always better when it comes to external fixation.
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