Original research
A Proposed Mathematical Method to Quantify y-Axis Pelvic Rotation on the Anteroposterior Radiograph

https://doi.org/10.1016/j.jcm.2017.08.002Get rights and content

Abstract

Objective

Researchers have identified potential errors in the Gonstead method’s analysis of pelvic alignment resulting from y-axis rotation of the pelvis on the anteroposterior (A-P) radiograph. The purpose of this article is to propose a method that can be used to determine the magnitude of y-axis rotation of the pelvis present on the A-P radiograph.

Methods

In this proposed method, measurements are obtained from the patient and from the lateral and A-P radiographs. With a mathematical method, these measurements are used with the focal film distance to calculate the degree of pelvic rotation present on an individual A-P radiograph.

Results

This method may help with the accuracy of measurement of pelvic y-axis rotation on the A-P radiograph.

Conclusion

The method proposed can be used to calculate the magnitude of pelvic y-axis rotation on an A-P radiograph.

Introduction

The image produced by a radiograph is not a picture but a projection and is subject to many distortions. This concept is recognized within the chiropractic community and has been the subject of much discussion. It is understood that vertebral rotation can produce projection distortion that may result in errors in spinal analysis.1, 2, 3 These image distortions affect not only spinal vertebrae, but also the pelvis and its measurement analysis. Researchers have identified potential errors in the Gonstead analysis of pelvic alignment.4, 5, 6 Schram et al stated, “Postural changes can introduce significant errors in pelvic spinography x-ray analysis.”5 Weinert discussed areas of error in the Gonstead pelvic analysis of the anteroposterior (A-P) radiograph and correlated and quantified the errors produced by axial (y-axis) rotation in a single pelvic model.6 Some chiropractic clinicians utilize portions of the Gonstead method in their practices.7 For those who use the Gonstead method of pelvic analysis, the errors produced by y-axis rotation can be important.

At present, there is no method used by Gonstead practitioners that accounts for the distortion of pelvic rotation. The purpose of this article is to propose a method to determine the degree of axial pelvic rotation that has occurred because of patient positioning on the A-P radiograph. This method explores the effect that different axes of rotation within the pelvis may have on calculations for determining y-axis rotation of the pelvis and presents methods of analysis.

Section snippets

Methods

With measurements obtained from the patient, the focal film distance, and A-P and lateral radiographs of the pelvis, the clinician can calculate pelvic y-axis rotation on the A-P radiograph. The image on the radiograph is magnified; therefore, a portion of the analysis necessitates determining the amount of magnification that has occurred in some of the areas utilized for the calculations.

Results

By use of the parameters established for this example, the projected length of the horizontal line from MFH to S2 on the lateral lumbopelvic radiograph was 100 mm, whereas the true horizontal distance between these 2 structures and the distance on the z-axis between MFH and S2 when both are on the central ray was 93.5 mm. If S2 had been used as the axis of rotation, the magnification factor would be 1.12. The projected horizontal offset between MFH and S2 on the A-P radiograph was 10 mm,

Discussion

As this proposed mathematical method has been used to quantify y-axis pelvic rotation on the A-P radiograph to better understand its effect on Gonstead measurements of the pelvis, it has not been tested for validity, repeatability, or whether it has any effect on clinical outcomes. It provides a clinical tool that, when coupled with present and future research, may improve our understanding of the magnitude of effect that y-axis–induced projection distortion has on Gonstead measurements of the

Conclusion

Pelvic rotation on the y-axis can serve as a confounder to those clinicians utilizing radiographic pelvic analysis, such as those using the Gonstead method. The method proposed in this article is used to calculate the magnitude of pelvic rotation on the A-P radiograph.

Funding Sources and Conflicts of Interest

Funding for this study was provided by the Gonstead Clinical Studies Society, Santa Cruz, California. Roger R. Coleman, DC, is the Director of Research for the Gonstead Clinical Studies Society, which is a paid position. Mark A. Lopes, DC, is the volunteer Chair of the Research Committee of the Gonstead Clinical Studies Society, which is an unpaid position. Derek A. Lopes, AA, received payment for services on this article and a previous article from the Gonstead Clinical Studies Society. He was

Contributorship Information

Concept development (provided idea for the research): R.R.C., M.A.L., D.A.L.

Design (planned the methods to generate the results): R.R.C., M.A.L., D.A.L.

Supervision (provided oversight, responsible for organization and implementation, writing of the manuscript): R.R.C., M.A.L., D.A.L.

Data collection/processing (responsible for experiments, patient management, organization, or reporting data): R.R.C., M.A.L., D.A.L.

Analysis/interpretation (responsible for statistical analysis, evaluation, and

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