Radiographic Distortion Artifact of Circular External Fixators

Introduction

Circular external fixators are made up of a series of complete rings or arches that are interconnected by threaded rods to create a rigid frame. The frame is then secured to the patient by small, tensioned, transfixing wires. Circular external fixators have been used to treat various fracture types (e.g., acute, chronic, open, infected), correct angular limb deformities, and perform arthrodesis and distraction osteogenesis. Anatomically, they are best suited to treat conditions distal to the distal femur and humerus. The advantage of circular external fixation includes superior mechanical properties, such as greater bending, lower axial stiffness, the ability to adjust the frame, and augment healing after fixation to the patient.^1–3 Circular external fixators were initially introduced to human medicine by the Russian physician Dr. Gavriil Ilizarov in the early 1950s.⁴ It wasn't until 1984 that the Ilizarov method was first used clinically in veterinary medicine by Dr. Antonio Ferretti.⁵ Since then, the Ilizarov method has been documented in dogs, cats, equids, cattle, goats, and camelids.^6–17

Correct identification of the location of transfixing wires is important for a number of reasons. The circular external fixator and transfixing wires are often manipulated either during the acute postsurgical period or progressively throughout healing to obtain better alignment at the surgical site. This includes improving fracture fragment reduction as well as correcting any angular or rotational abnormalities. The circular external fixator is also manipulated to optimize osteogenic and vascular activity in cases of either distraction osteogenesis or complicated fractures. Misinterpretation of the true anatomic location of the transfixing wires may either complicate or negatively impact patient care/healing due to additional surgery, anesthesia time, and repeat radiographs, which increases radiation dose to the patient and diagnostic imaging personnel.

In the study authors' clinical experience, the location of the transfixing wires can be projected incorrectly on postoperative radiographs in relation to their true anatomic position. This represents a distortion artifact, which could mislead the interpreter and lead to a variety of negative possibilities. Distortion is defined as unequal magnification of different parts of an object. One axis of an image is magnified while the other, which is parallel to the X-ray beam and perpendicular to the X-ray table, is not altered. Distortion is determined not only by the shape of the object but also by the angle of the beam and the relationship/distance of the object to the film/detector.¹⁸ Depending on how the object is oriented, distortion can alter the size and shape of the final image.¹⁹ The degree of magnification that occurs will be different in different parts of the beam. Distortion of relative position occurs when two objects are different distances from the film. That distortion will be reduced as the object gets closer to the film.

The purpose of this study were to (1) document that distortion artifact is associated with circular external fixators; (2) determine the angle at which the artifact occurs; and (3) determine what angle of transfixing wire placement is most prone to the artifact. This information could then be used to assist the practitioner with appropriate positioning of the patient/circular external fixator in relation to the primary X-ray beam and ensure accurate interpretation of the radiographic study.

Materials and Methods

Three circular external fixator apparatuses were constructed using a full ring, 84 mm internal diameter ring fixator.^a Three pieces of polyvinyl chloride (PVC) pipe with an internal diameter of 1.7 mm were used to simulate a long bone. Two through and through holes were drilled in each piece of PVC pipe. The holes were drilled at 30, 60, and 90° from one another, perpendicular to the long axis of the PVC pipe. Two 1.6 mm transfixing wires were then placed through the cannulated wire fixation bolts in the circular external fixator and PVC pipe and tightened in place with a nut. A ruler (in mm) and protractor were used to verify the wires crossed in the center of each PVC pipe at their respective angles.

The 30° circular external fixator apparatus was placed in the center of a noncollimated X-ray field using the calibrated markings in the lighted field (Figure 1). A 10 cm reference marker was placed the same distance away from the table as where the transfixing wires crossed within the PVC pipe and orthogonal radiographs were obtained. Additional radiographs were made with the circular external fixator and wires parallel to the central X-ray beam and the PVC pipe parallel to, but not touching, the table. The apparatus was then rotated horizontally in 10° increments with the portion of the circular external fixator in contact with the table, fixed in location (Figure 2). Radiographs were made with the circular external fixator and wires 10, 20, 30, 40, 50, 60, 70, and 80° from parallel with the central X-ray beam. The 10 cm reference marker was placed in the field at the level of where the transfixing wires crossed within the PVC pipe in each image. That was repeated with both the 60 and 90° apparatuses. A protractor was used to verify all angles in this study.

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Results

Ten radiographic images, including the initial orthogonal views, were made of the 30, 60, and 90° circular external fixator apparatuses for a total of 30 images. On the 30° circular external fixator apparatus, the distance the transfixing wires were projected from the center of the PVC pipe (distortion) was 2 cm when rotated 10° from parallel to the central beam. At 20° the distance was 1.4 cm, 0.7 cm at 30°, 0.5 cm at 40°, 0.4 cm at 50°, 0.3 cm at 60°, and 0.2 cm at both 70 and 80° (Figure 3).

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In regards to the 60° circular external fixator apparatus, the distance from the center of the PVC pipe to the point where the transfixing wires crossed was 0.7 cm at 10°, 0.4 cm at 20°, 0.2 cm at 30°, 0.1 cm at 40 and 50°, and 0 cm at 60, 70, and 80°. The following measurements were obtained for the 90° circular external fixator apparatus: 0.4 cm at 10°, 0.3 cm at 20°, 0.2 cm at 30°, 0.1 cm at 40 and 50°, and 0 cm at 60, 70, and 80°. When looking at the 30, 60, and 90° circular external fixator apparatuses separately, distortion was greatest with smaller angles of rotation. Comparing the three apparatuses, distortion was greatest for the 30° apparatus at all angles of rotation. The 60 and 90° circular external fixator apparatuses had the same distortion at angles of 30–80°, with distortion being greater in the 60° apparatus at angles of 10 and 20°. No distortion artifact was present in the 60 and 90° circular external fixator apparatuses at 60, 70, and 80° of rotation.

Discussion

The purpose of this study was to document distortion artifact associated with circular external fixators, determine at what angle it occurs, and determine what angle of transfixing wire placement had the most severe distortion artifact. These study results show that not only does distortion artifact occur with circular external fixators but is also more severe with smaller angles of rotation from the central X-ray beam (transfixing wires farther from the table/detector) and smaller angles of transfixing wire placement.

Distortion artifact appears to be inversely proportional to the angle of rotation (i.e., greatest in circular external fixators with smaller angles of rotation from parallel to the central X-ray beam). In all three apparatuses, distortion was greatest with 10° of rotation then directly decreased with increasing angulation. As stated previously, one of the variables that effects distortion is the relationship of the object to the film/detector with distortion being greater when the object is farther away. As the fixator is angled farther from parallel, the point where the wires intersect gets closer to the X-ray table, thus reducing distortion. Clinically, distortion artifact can be reduced/prevented by placing the circular external fixators in the center of the X-ray beam with the intersection of the transfixing wires located as close to the table/detector as possible.

The degree of distortion also appears to vary with the angle at which the wires within the circular external fixator intersect. When comparing the three apparatuses to one another, distortion artifact was most severe in the 30° apparatus with the wires projected farther from the center of the PVC pipe than the 60 and 90° apparatuses at every angle of rotation. In fact, distortion artifact was present at all angles of rotation in the 30° apparatus. The 60° apparatus had greater distortion than the 90° apparatus with angles of 10 and 20°; however, distortion was the same between the two apparatuses at the remaining angles of rotation. This would suggest that when the transfixing wires are in closer proximity to each other (at smaller angles), the point where they intersect is distorted to a larger degree. As the angle of intersection increases, the degree of distortion decreases, therefore, the angle of intersection is inversely proportional to distortion. The 60 and 90° apparatuses had the same measurements of distortion from 30 to 80°, including no distortion artifact present at 60, 70, and 80° of rotation. That is likely due to a combination of both the effects of decreasing object film distance and the shape/angle of intersection. Clinically, that means that circular external fixators with smaller angles of intersection of the transfixing wires have greater distortion artifact. In addition, distortion artifact can be reduced/prevented in circular external fixators with transfixing wires placed at 60 and 90° if properly positioned in the center of the X-ray beam with the transfixing wires placed as close to the table/detector as possible.

In this study, apparatuses that would be similar to those used in clinical practice on a long bone of a larger dog were evaluated. Surgical guidelines for circular external ring fixator placement include using the smallest ring possible to maintain a minimum of 2 cm of space between the skin and the fixator.²⁰ This was obtained in this study by using PVC pipe with an internal diameter of 1.7 mm and a full ring fixator with an internal diameter of 84 mm, leaving approximately 31 mm between the PVC pipe and the fixator. In addition, 1.6 mm transfixing wires were used because wires 1.5 mm or greater are standard in patients >20 kg.²⁰

Thirty, 60, and 90° angles of intersection of the transfixing wires were used in this study because 60 and 90° angles are commonly used at the authors' institution. In general, wires intersecting at 90° provide maximum stability while minimizing shear forces.²⁰ As the angle decreases, so does the axial and bending strength of the fixator. In a clinical setting, it is not always possible to place the transfixing wires at a 90° angle either because of fracture configuration or normal neurovascular structures; however, angles <45° should be avoided.²⁰

Conclusion

This study indicates that distortion artifact does occur with circular external fixators and is greatest when the transfixing wires are at smaller angles of intersection and smaller angles of rotation from parallel (i.e., farther from the film/detector). Practitioners should be aware of this artifact and try to position the circular external fixator parallel to and in the center of the primary X-ray beam and as close to the table as possible to avoid/reduce distortion artifact and inaccurate interpretation of the radiographic study.