The Study: 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
Today we're going to try something a little different because this is one of my own articles. In fact, as I write this, I have just received my copy of the Journal of Chiropractic Medicine a few hours ago. Many of you are familiar with a number of my other articles that have explored the projection of the vertebrae on radiographs. As you are well aware, the image on the radiograph is a projection, not a picture and is subject to a number of errors. Projection errors, patient positioning errors and measurement errors can all come together to be confounders to the clinician seeking to accurately measure the spine. However, the question is not if there can be errors but how large are those errors and how are they created. This exploratory study seeks to "evaluate the amount of error in retrolisthesis measurement due to measurement methods or projection factors inherent in spinal radiography". We also tried to find out how accurate the measurements were when compared to the real size of the retrolisthesis and when compared to the projected size of the retrolisthesis that was expected to be found on the film. The study looked at both 40 and 84 inch focal film distances and at the changes that occurred due to y-axis rotation of the vertebrae at 0, 5, 10, 15 and 20 degrees of rotation.
So now for the good stuff. While this is far too big an article to give all the answers here, our findings suggest that if you exceed 10 degrees of y-axis rotation when placing the patient in front of the film surface, your retrolisthesis measurements became less accurate. We also looked at three different measurement methods: Gohl, Iguchi and Lopes methods. In general, the Gohl method, which is the method most commonly used in chiropractic, did not appear to be as accurate as good as the other two. In addition there are pictures of radiographs with y-axis rotations of 0 to 20 degrees in 5 degree increments that show visible changes in the look of the vertebrae on the radiographs as they undergo rotation. This will give you a better idea of how rotated vertebrae appear on the film and may tip you off sometimes when you have poor patient placement or some other factor that has produced axial rotation on the film. We also produced tables showing the amount of error relative to the actual amount of retrolisthesis and the amount of projected retrolisthesis that was expected to be found on the film.
So do we have to quit measuring spinal displacement on radiographs? Of course not. But we do have to consider sources of introduced error and understand the amounts of error that can occur. This article is just another step in understanding what causes errors when radiographs are being measured and the magnitude of those errors. By knowing these things we can make better clinical judgments.
So here's the take home. X-rays are a tool and just like any other tool they have advantages and disadvantages. One major disadvantage is that axial rotation can significantly affect your ability to accurately measure retrolisthesis on the lateral radiograph. You should be aware of that and pay attention to patient placement to help reduce projection errors on your radiographs.
I personally deplore blanket attitudes that we either do or don't need x-rays to treat patients. It depends on the case, including what your goal is for a particular patient. Although I don't always agree with them, I like the idea of practice aids (notice that I didn't say guidelines) that help improve our clinical decision making. The more you understand about radiography the better you can decide how it works best in your practice. I hope our readers here at ScienceInBrief.com will take the time to read this article in its entirety and that you find it helpful in your understanding of factors which may affect your evaluation of retrolisthesis cases.
Reference: Coleman RR, Cremata EJ Jr, Lopes MA, Suttles RA, Rairbanks VR. 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. J Chiropr Med. 2014 Dec;13(4):247-259.
Link to Abstract: http://www.ncbi.nlm.nih.gov/pubmed/25435838
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