Exploratory evaluation of the effect of axial rotation, focal film distance and measurement methods on the magnitude of projected lumbar retrolisthesis on plain film radiographs

Abstract

Objective: The purpose of this exploratory study was to evaluate the amount of error in retrolisthesis measurement due to measurement methods or projection factors inherent in spinal radiography. In addition, this study compared how accurately these methods determine positions of the lumbar vertebrae being studied and the expected projected size of the retrolisthesis.

Methods: Vertebral models were situated in a retrolisthesis position. Radiographs of the models were obtained in positive and negative y-axis rotations at 40- and 84-in focal film distances. The projected retrolisthesis was measured using the Gohl, Iguchi, and Lopes methods.

Results: At the 40-in focal film distance, the Iguchi method and Lopes methods were significantly more accurate than the Gohl method. At the 84-in focal film distance, the Lopes method was significantly more accurate than the Gohl method. Almost all measurements overestimated both the actual amount of retrolisthesis as well as the amount of trigonometrically calculated retrolisthesis that should have been present on the radiographs. Findings suggest that measurements were less accurate with vertebrae rotated more than 10°.

Conclusions: This study demonstrated that lumbar vertebral rotation, focal film distance, and measurement methods are potential sources of error in retrolisthesis measurement.

Keywords: Anatomic model; Chiropractic; Human; Lumbar vertebrae; Motion; Radiography; Spine.

Fig 1

Photograph of the models used in the study.

Fig 2

Radiograph of the models in the nonrotated position at the 40-in focal film distance. The end plates and margins of the vertebral bodies that were the points used for analysis of the retrolisthesis are clearly visible.

Fig 3

Radiograph of the models rotated 5° in the positive direction on the y-axis at the 40-in focal film distance. The end plates and margins of the vertebral bodies that were the points used for analysis of the retrolisthesis are clearly visible.

Fig 4

Radiograph of the models rotated 10° in the positive direction on the y-axis at the 40-in focal film distance. The end plates and margins of the vertebral bodies that were the points used for analysis are seen, but changes in the projected image are seen.

Fig 5

Radiograph of the models rotated 15° in the positive direction on the y-axis at the 40-in focal film distance. The end plates and margins of the vertebral bodies that were the points used for analysis are visible, but significant changes in the clarity and the projected image are seen.

Fig 6

Radiograph of models rotated 20° in the positive direction on the y-axis at the 40-in focal film distance. The end plates and margins of the vertebral bodies that were the points used for analysis are seen; but very significant changes in the clarity and the projected image are present, making analysis unreliable.

Fig 7

Gohl method. A line is drawn along the superior end plate of the fifth lumbar vertebra. Two lines are erected perpendicular to this line to pass through the adjacent posterior vertebral body corners. The distance between these 2 lines indicated by X is the amount of retrolisthesis.

Fig 8

Iguchi method. A line is drawn along the inferior end plate of the fourth lumbar vertebral body. Two lines are erected perpendicular to this line to pass through the adjacent posterior corners of the vertebral bodies. The distance between the points at which these 2 lines intersect the end plate line indicated by X is the amount of retrolisthesis.

Fig 9

Lopes method. A line is drawn through the middle of the disk space between the fourth and fifth lumbar vertebrae. Two lines are erected perpendicular to this line to pass through the adjacent posterior corners of the vertebral bodies. The distance between these 2 lines indicated by X is the amount of retrolisthesis.

Fig 10

Angle “B” is the angle between sides “a” and “c” that represent divergent rays from the x-ray source to the posterior edges of the fourth and fifth lumbar vertebrae and extending to the modeled x-ray film. They are offset because of the retrolisthesis of L4 on L5 in the vertebral model. Angle “C” is 90°, as there is no ± y-axis rotation of the model. Angle “A” is the angle between ray “c” and the side “b”. Side “b” represents the 5-mm-long modeled retrolisthesis.

Fig 11

Ɵy represents the amount the model is rotated (angle C is no longer 90°) on the y-axis. The y-axis rotation changes the projected length of side “b,” which represents the 5-mm modeled retrolisthesis. Angle “B” decreases as the y-axis rotation of the model increases. As the model's y-axis rotation increases, the length of the projected retrolisthesis decreases.

Fig 12

Representation of the A-P pelvic radiograph as viewed from above. Angle “B” is the divergence of rays: “c” from the x-ray source through the top dead center of the femur head to the film on one side and “a” from the x-ray source through the center of the nonrotated L4-5 vertebra model to the film on the other side. The x-axis of the pelvis is parallel to the film. Side “b” is the distance from one femur head to the center of the vertebra at L4-5. Ray “a” is the focal film distance of 84” (213.36 cm). The measured distance on the film between the top dead center of the projected images of the 2 femur heads is represented by “x” (19 cm in our example).

Fig 13

Representation of a lateral radiograph, showing the images of 2 femur heads offset because of rotation of the subject at the time of the exposure. “X” is the measured distance on the film between the top dead centers of the offset femur heads' projected images.

Fig 14

Representation of the lateral radiograph of a malpositioned patient as seen from above. Rays “a” and “c” pass through the center of the femur heads. Angle “B” is the angle formed between ray “c” and the central ray. “X” is the projected distance between the femur heads, whereas “b” is the actual distance between the femur heads. Angle “A” is the angle between ray “c” and the line between the femur heads. “C” represents the vertebral model.