The Study: A Proposed Mathematical Method to Quantify y-Axis Pelvic Rotation on the Anteroposterior Radiograph.
O.K., the format for this review will be a lot different. This paper is one of mine, published last year in the Journal of Chiropractic Medicine. Simply stated when you measure the pelvis on the anteroposterior radiograph the measurements will be different if there is any positioning error caused by rotation around the y-axis. We’ve known about this problem for at least 36 years but the only recommended solution was to take the x-ray correctly in the first place i.e. with the symphysis pubis in line with the second sacral tubercle of the sacrum. Unfortunately, in the real world that doesn’t always happen.
This study was funded by the Gonstead Clincial Studies Society and came out of a genuine interest in better understanding how to deal with y-axis rotation distortions on the A-P film. So how much does rotation affect the Gonstead measurements of the pelvis? Well in 2005 Dr. Weinert, the Provost at Palmer University, wrote an article that correlated the changes in the Gonstead measurements to the degree of rotation of the pelvis on the y-axis. (https://www.ncbi.nlm.nih.gov/pubmed/15800511) While there were some limitations to that article, it does give us some idea of the magnitude of changes relative to the degree of rotation. Remember that because y-axis rotation changes the projection of all structures on the film, it is subject to affect all radiographic measurements as well. The problem was that in a clinical setting we had no good way of knowing how much positioning error had actually occurred so that we could more accurately determine how much of any measured misalignment might be due simply to error in patient placement.
Our article gives you a formula to figure that out. If the amount of rotation is so small that it would have minimal effect on the measurements then great, but if the rotation is large enough to skew your measurements then you can know it without guessing. Are the computations difficult or time consuming to do? Yeah, they are. So we are working on an app for your phone or tablet to do that and we plan for it to be free. (Thank the good folks at GCSS people who fund my work.) Keep checking the Gonstead Clinical Studies Society site (Gonstead.com) as we plan to put a link to it on that site when it is ready. The beta version should be available in just a few weeks so give us a few months to get the final version ready. In the meantime, the entire article is available for FREE so you can be getting up to speed. See below for links to this and other articles I have written on Radiographic projection.
As always, more research is needed. This article would not have been possible without my co-authors Mark Lopes, DC and his son Derek Lopes, BS and the financial support of the Gonstead Clinical Studies Society who make it possible for me to eat while I work on a host of projects. Also Mark Payne, DC and his company Matlin Manufacturing Inc. who pay me to review and write about the chiropractic literature in Science In Brief. To support our work, please contact the Gonstead Clinical Studies Society (888-556-4277) or contact the folks at Matlin Manufacturing (334-448-1210) to learn more about how their products can help you improve your corrective care outcomes. If you’d like to understand more about projection on the x-ray read this article or my previous articles, some of which, including this one are free in PubMed.
Reviewer: Roger Coleman DC
Reference: Coleman RR, Lopes MA, Lopes DA. A Proposed Mathematical Method to Quantify y-Axis Pelvic Rotation on the Anteroposterior Radiograph. J Chiropr Med. 2017 Sep;16(3):204-210. doi: 10.1016/j.jcm.2017.08.002. Epub 2017 Sep 28.
Link to Abstract: https://www.ncbi.nlm.nih.gov/pubmed/29097950
Links to two more of my free projection articles in the Journal of Chiropractic Medicine: https://www.ncbi.nlm.nih.gov/pubmed/25435838