Topics in Diagnostic Imaging
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

https://doi.org/10.1016/j.jcm.2014.09.001Get rights and content

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.

Introduction

Diagnoses and clinical decision making for a variety of orthopedic conditions depend heavily upon radiographic studies; therefore, measurements derived from radiographs must be accurate.1 This is especially true in the chiropractic profession with the existing controversy over the historical usage of radiographic measurement of the spine.2 Gonstead manipulative technique was estimated to be used by approximately 58% of the chiropractic profession in 1998,3 although it is unknown how many use the Gonstead Method, which includes measurement of misalignment in its analysis system. Although the value of assessing relatively small spinal misalignments has been seriously questioned,4 some providers who use the Gonstead Method attempt to measure the amount of retrolisthesis seen on radiographs.5, 6, 7 Given that some practitioners use this technique, it would be important to know if the measurements derived from the radiographs accurately reflected the position of the vertebrae being studied.

Because of distortion related to pelvic rotation, the use of radiography in measurement of the pelvis has been questioned.8, 9 As well, other issues have been raised, including a number of general measurement problems related to lumbar vertebrae such that x-axis translation of a vertebra results in projected y-axis rotation and that the irregular shape of the vertebrae can have unexpected effects on the projected image when they are rotated on the y-axis.10, 11, 12, 13 These types of projection errors can lead to inaccurate analysis. Wall and Oppenheim14 have also noted that the determination of the progression of spondylolisthesis, a condition that commonly occurs in conjunction with retrolisthesis,15, 16 may be hindered by projection errors. Given these findings, radiography is not an accurate tool for the assessment of spinal position; and different measurement methods have not been adequately tested. Clinicians could be using faulty information in their decision-making process. It is therefore important to determine if retrolisthesis measurement on radiographs is subject to significant error.

The purpose of this exploratory study was to evaluate if measurement of retrolisthesis, using the Gohl,6 Iguchi,17 and Lopes measurement methods, may have errors in the measurement methods themselves or the projection factors inherent in spinal radiography. Additionally, this study compared how accurately these measurements reflect the actual positions of the vertebrae being studied and the projected retrolisthesis expected to be found on the radiographs.

Section snippets

Methods

Plastic models of a fourth and fifth lumbar vertebra were used to demonstrate the effects of y-axis rotation and changes in focal film distance on projected retrolisthesis (Fig 1). Holes were drilled in the inferior body of the fourth lumbar vertebral model and the superior body of the fifth lumbar vertebra. A copper wire was inserted into these holes such that the relative positions of the vertebrae could be changed and maintained by the bending of this wire. An angle of 17° was then formed

The Magnitude of the Projected Retrolisthesis

To determine the magnitude of the projected retrolisthesis that should have been found on the radiographs, trigonometric calculations were made. A triangular expression of the angles and lengths involved in the equations is shown in Fig 10. Angle B represents the angle of the divergent rays at the source of the x-ray beam that contact the posterior aspect of the L5 vertebral body and the posterior L4 vertebral body, given the 5-mm retrolisthesis, used in our model. Angle C is 90° in this

Results

The magnitude of error of the measured projected retrolisthesis using the 3 measurement methods compared with the actual modeled size of 5 mm for the radiographs taken at 40 and 84 inches along with the mean and standard deviations are seen in Table 1. The person performing the measurements was able to accurately locate the points needed for analysis on only half of the radiographs in which the models had been rotated 15° and on none of the radiographs in which the models had been rotated 20°.

Discussion

Although other appropriate distances for the placement of the models could have been chosen, we determined the distance from the bucky by using the measurement of one of the authors who at a height of 179 cm and weight of 70 kg had a pelvic width measurement of 34 cm. Therefore, a 17-cm placement of our model from the bucky was determined to be a reasonable distance. Five millimeters of slippage for the modeled retrolisthesis was chosen because it exceeds the 3 mm Iguchi17 uses to determine

Limitations

There are a number of limitations to this study. Only 1 vertebral model was used. The radiographs were only measured once at each rotation and focal film distance and only by one person. We have previously noted the supporting factors for method 2, but it has not been proven to be reliable. The reliability of method 3 has not been established either. Finally, this is an exploratory study; and the number of methods we investigated was limited to three that are used to measure retrolisthesis.

Conclusions

The findings of this study suggest that either method 2 or method 3 will generally provide smaller errors than method 1 when measuring retrolisthesis. Results also suggest that attempts to measure retrolisthesis beyond 10° of rotation may not be reliable.

Funding Sources and Conflicts of Interest

Dr Mark Lopes is the Chairman of the Research Committee, Gonstead Clinical Studies Society, which is a nonpaid position. The Gonstead Clinical Studies Society funded this study; however, the Gonstead Clinical Studies Society had no role in conducting or reporting the study.

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