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Distortion Control in Radiographs of External Fixators

Clinician's Brief (Capsule)

Imaging

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August 2015

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Circular external fixators are composed of rings or arches interconnected by threaded rods to create a rigid frame. Small, tensioned transfixing wires secure the frame to the bone. Correct anatomic location of the transfixing wires is important to ensure proper fixation and a good surgical outcome. Understanding the effects of radiographic distortion is critical. In this study, 10 radiographic images were taken of 3 circular external fixators with differing intersecting angles (30°, 60°, and 90°) for transfixing wires. The 10 images were obtained by rotating the device through varying degrees from parallel with the central x-ray beam (10°-80°). For all 3 devices, distortion was greatest when transfixing wires were at smaller angles of rotation or farthest from the x-ray table. Likewise, at all angles of rotation, the 30° device created the greatest distortion. The authors demonstrated that angle and distance from the table affected distortion. This could mislead the interpreter and lead to unnecessary surgical adjustments, anesthesia time, and repeated radiography. Placement of the fixator in the center of the x-ray beam with the wire intersection as close to the table as possible can best reduce distortion artifact.

Commentary

External skeletal fixators (ESFs) are versatile devices used in orthopedic practice to treat conditions such as fractures and limb deformities. The components are metallic, which makes radiographic assessment of bones and the fixation extremely challenging. It is important to consider the position of the device and angle of the radiographic beam because distortion can be substantial, as this study shows. The radiographic beam should be centered on the area of interest and positioned 0° or 90° orthogonal to the fixation in order to minimize artifacts. This may require several different views along various aspects of a long bone or fixator, at various obliquities, to fully appreciate each fixation element in an ESF construct.—Jason Bleedorn DVM, DACVS

References

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