DOI: 10.1097/BRS.0b013e31824c15c4
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Issn Print: 0362-2436
Publication Date: 2012/04/01
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Excerpt
We appreciate the opportunity to respond to criticisms of our article in the letter to the editor by Dr. Khandelwal.
Dr. Khandelwal criticizes our results and questions the validity of our measurements. He states that the center slice in sagittal cuts is likely to be in a different location for scans performed in 2 positions. It is true that it is unlikely that the center slice is at the exact location in 2 scans because the sequence planning for the scan has to be redone after each positioning of the patient. This, of course, affects automatic measurements like image subtraction, but this will not affect the measurement of angles or vertebral shifting unless severe lumbar scoliosis is present. It is much more likely that measurements performed on radiographs are inaccurate because of the spatial distortion due to the point-source geometry of the x-ray beam and because plain radiographs are summation images.
Furthermore, Dr. Khandelwal claims that measurements regarding transitional and angular instability can be easily performed on plain x-ray film. It is possible to perform these measurements on plain x-ray film if the central beam is centered on the affected segment; otherwise, an accurate measurement is not possible for the same reasons as stated earlier.
Dr. Khandelwal suggests that posterior instability might be diagnosed by supine magnetic resonance imaging (MRI). It is, of course, not possible to diagnose an unstable condition while imaging only 1 position. The characteristics of posterior instability such as cyst formation and flava ligament bulging are triggered by lumbar lordosis and weight bearing. Considering that the supine position is a position with maximized lumbar lordosis, stigmata suggesting posterior instability can be found. However, because the influence of weight bearing is lacking in the supine position, the mentioned characteristics are less pronounced as compared with weight-bearing extension, which made us exclude images acquired in supine position from the study.
We never concluded that instability is the only cause for nerve root compression and radicular symptoms, as wrongly interpreted from our data. Our data show that radicular symptoms are more common in patients with 1 or more forms of the presented instabilities than in patients without any instability.
Finally, we do not recommend, as suggested, evaluating all patients with spondylolysis and isthmic spondylolisthesis using positional MRI. We demonstrated that positional MRI adds more information on the disease and that it helps identify the underlying cause for nerve root compression. Therefore, we maintain that in patients with spondylolysis and isthmic spondylolisthesis presenting with radicular symptoms, a positional MRI study might be helpful.
