Letter to the Editor Regarding Comments on "The Correlation Between Facet Tropism and Intervertebral Disk Herniation in the Subaxial Cervical Spine" by Wang et al.

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TO THE EDITOR: Letter to the Editor Regarding Comments on “The Correlation Between Facet Tropism and Intervertebral Disk Herniation in the Subaxial Cervical Spine” by Wang et al The article entitled “The correlation between facet tropism and intervertebral disk herniation in the subaxial cervical spine” written by Wang et al1 caught us with great interest. The authors performed a retrospective case-control study and found that the facet tropism on the sagittal, axial, and coronal planes are all associated with cervical disk herniation in the subaxial cervical spine. In addition, the side of greater facet angle did not affect the side of herniation. The manuscript is well organized. We appreciate their contribution to this important topic while we would like to communicate with the authors to address some concerns related to the methodology. The cervical facet joints play important roles in the spine motion and load transmission with complex anatomical structure.2 The shape and inclination of the facet joints are not regular and the surfaces are not genuinely planar.3,4 Thus, it may be difficult to describe the three-dimensional orientation of the facet joint planes accurately by drawing lines on two-dimensional images, especially considering the oblique position of the cervical facet joints.5 For all participants, the authors measured the facet angles on axial, sagittal, and coronal planes of computed tomography images. The facet angle was defined as the angle downward between the facet line and midsagittal line in the axial or coronal plane and the closed angle between the facet line and the endplate line in the sagittal plane. According to the definition, the two lines of the facet angle in the coronal and sagittal planes are in two different planes. Although this method is simple, it could be inaccurate. Given the anatomical feature of the facet joints, the orientation would be different if measured on different positions.4 As shown in Figure 1 (Supplemental Digital Content 1, https://links.lww.com/BRS/B916), the measurements are taken from three different positions on the reconstructed computed tomography scan images of the same facet joints. In the sagittal plane, the facet orientation is 58.63° (Figure 1B, Supplemental Digital Content 1, https://links.lww.com/BRS/B916), 51.74° (Figure 1C, Supplemental Digital Content 1, https://links.lww.com/BRS/B916), and 47.65° (Figure 1D, Supplemental Digital Content 1, https://links.lww.com/BRS/B916) in three consecutive images; in the axial plane, the facet orientation is 101.05° (Figure 1F, Supplemental Digital Content 1, https://links.lww.com/BRS/B916), 102.33° (Figure 1G, Supplemental Digital Content 1, https://links.lww.com/BRS/B916), and 100.35° (Figure 1H, Supplemental Digital Content 1, https://links.lww.com/BRS/B916) in three consecutive images; in the coronal plane, the facet orientation is 73.08° (Figure 1J, Supplemental Digital Content 1, https://links.lww.com/BRS/B916), 78.78° (Figure 1K, Supplemental Digital Content 1, https://links.lww.com/BRS/B916), and 80.25° (Figure 1L, Supplemental Digital Content 1, https://links.lww.com/BRS/B916) in three consecutive images. Our previous study4 adopted a three-dimensional method to calculate the facet orientation. The space vector was obtained after the reconstruction the three-dimensional model. Then, the facet angle was calculated by the formula of vector and cosine function. Accurately measuring the facet joint angle is clinically significant in elucidating the relationship between facet orientation and cervical spine degeneration. Hence, a more precise and comprehensive measurement is needed to avoid possible measurement bias. We analyzed the data from a cohort of 100 patients with cervical spondylosis.6 The preliminary results showed no significant relationship between facet tropism and cervical disk degeneration. Many factors might contribute to the degeneration of the intervertebral disk. We suspect the facet tropism might play a role in disk degeneration, but not in a significant way. A prospective cohort study might be a better way to elucidate the cause-and-effect relationship between facet tropism and disk degeneration. We are expecting to further interesting and instructive research about the cervical facet joints from the authors in the future.