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Hospital Cristo Rei 05086-020 São Paulo. S.P., Brazil
The paper by Schmid et al. [1] is certainly interesting. The proper knowledge of the physiologic changes of the cross-sectional measurements of the spinal canal and intervertebral foramina helps the caring physician avoid a misdiagnosis of spinal canal stenosis when smaller measurements have been documented by means of imaging.
The changes in the capacity of the neural foramina are related to the displacement of the articular facets. The upright flexion of the vertebral column moves the superior articular facet downward whereas the inferior facet moves upward and the pedicles move away. The upright extension of the vertebral column leads to the reverse picture, and the capacity of the neural foramina decreases.
Nonetheless, I do not agree with the statements of Schmid et al. [1] indicating that the cross-sectional area of the spinal canal at the level of the pedicle is larger than the cross-sectional area at the level of the intervertebral disk. The reason for my disagreement is based on the shape of the vertebral canal. The shape of the vertebral canal is not exclusively triangular, as reported in the literature. At the pedicle level, the shape tends to be oval or circular, yet at the disk level, it is triangular [2, 3].
At the pedicle level, the epidural space is empty, the dural sac is in direct contact with the canal wall [2, 3], and the cross-sectional area of the dural sac represents the spinal area. On the other hand, the dural sac does not encroach entirely on the spinal canal wall in the triangular segment. This particular anatomic disposition leads to the emergence of three epidural compartments. One posterior (triangular) compartment contains fat tissue, and two lateral compartments contain fat tissue, blood and lymphatic vessels, and nerve roots. For this reason, the spinal canal cross-sectional area at this level should be larger, with a least the same area at the pedicle level added to the area of the two lateral and posterior compartments.
The length of the anteroposterior vertebral canal diameter [2] can confirm these data. It measures 15 mm at the level of the pedicle, whereas it measures 22 mm at the level of the disk. The triangular compartment may assume different areas depending on the level of the analysis. The anteroposterior diameter of the spinal canal is longer at the level of the intervertebral disk, and it decreases in length in adjacent spinal levels as depicted in figure 3 of the article by Schmid [1].
Figure 2 of that article [1] depicts the spinal canal cross-sectional area at the pedicle level (figure 2C) and the spinal canal area at the disk level (figure 2B). The drawings have been represented in different scales. Figure 4 depicts the spinal canal cross-sectional area at the disk level only. In conclusion, the authors developed an important study that should be extended and continued to further improve the understanding of the physiologic changes of the intervertebral and interlaminar spaces that occur with flexion, rotation, and lateral movements of the spine. With that understanding in mind, the caring physician can diagnose correctly true cases of spinal stenosis. Furthermore, the caring physician should always take into consideration the changes in the capacity of the neural foramina and the changes in the cross-sectional area of the epidural compartments. These changes can influence the dispersion of anesthetic, therapeutic, or diagnostic solutions instilled in the posterior epidural space.
Further studies must investigate the cross-sectional area of the spinal canal at disk level.
References
University Hospital Zurich CH-8091 Zurich, Switzerland
Orthopedic University Clinic Balgrist 8008 Zurich,
Switzerland
We thank Dr. Zarzur for his interest in our paper [1], for we are aware of his own merits in the investigation of the spinal canal.
Dr. Zarzur agrees with our assessment of the spinal foramina, but not of the spinal canal. He believes that the spinal canal is wider at the level of the disk than at the pedicle level. This opinion at first glance contradicts our quantitative results. However, the difference between his statement and our measurements is explained by different definitions of the spinal canal (defined by osseous structures by Dr. Zarzur, yet excluding the ligamenta flava in our case). Our definition can be debated. However, we have described both in words and with drawings the measurements we have obtained; therefore, the reader can follow our methods and results.
We would also like to emphasize that the comparison of the spinal canal area at different levels was not the scope of our paper. The pedicle level was used only as a control measurement that was intended to prove that no position-dependent measurement errors were present.
Therefore, we believe that no relevant differences exist between Dr. Zarzur's opinion and our own.
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