Cervical spondylosis encompasses a variety of motor and sensory abnormalities, such as radiculopathy, myelopathy, and myeloradiculopathy. However, the radiological findings in many patients do not match well with their symptoms [5]. A better understanding of the anatomic basis of cervical spondylosis may help to uncover the reasons for this mismatch. Veleanu [9] initially divided the nerve groove into two portions—the radicular portion and the anterior ramus of the transverse process—but did not provide quantitative data. Ebraheim et al. [10] described the cervical nerve groove according to the different anatomic features along its length and divided the nerve groove into three zones: medial (pedicle), middle (vertebral artery), and lateral. The medial zone was suggested to play an important role in the etiology of cervical spondylotic radiculopathy [8]. However, this zone division was focused on the cervical nerve groove and did not include the spinal cord and ventral nerve rootlets within the spinal canal.
To better understand the etiology of cervical spondylosis, which is important for obtaining an accurate diagnosis and decompressing the site, we defined three zones outward from the midline and two zones from cranial to caudal on the coronal plane. The coronal widths of zone I did not differ significantly at C4-C8 levels. Since zone I contains only spinal cord and anterior spinal artery, localized compression in zone I may cause myelopathy without radiculopathy. Zone II contains cervical ventral nerve rootlet origins and the lateral border of the spinal cord. Compression in zone II may cause both myelopathy and radiculopathy. Ebraheim et al. [10] illustrated the importance of the nerve root groove, but not the preforaminal area, in the etiology of cervical spondylotic radiculopathy. Osteophytes of the posterolateral corner of the vertebral body, uncinated process, facet joints, and lateral intervertebral disc herniation are often observed in zone III [11]. In the preforaminal area, compression in zone III can be missed by clinical examination, leading to misdiagnosis and inadequate decompression.
Cervical spondylotic amyotrophy (CSA) is characterized by weakness and wasting of upper limb muscles without sensory or lower limb involvement [12-14]. The underlying mechanisms of CSA may involve damage to the ventral nerve rootlets and vascular insufficiency to the anterior horn cells. Accurate diagnosis of CSA and precise surgery require a detailed characterization and definition of different anatomical zones. Compression of zone I may cause anterior spinal artery impingement that leads to dysfunction of anterior horn cells, which may partially explain why a moderate compression of the central canal sometimes leads to CSA [13]. Compression at zone II will impair the ventral nerve rootlets and anterior horn of the spinal cord. As a result, such compression may cause motion dysfunction without sensation loss. The compression at zone III defined in our study may only impair the ventral nerve rootlets. Since zone III at C6/7 contains ventral nerve rootlets of C7 and C8, compression in this region may cause CSA at both levels (Figure 3). Thus, the definition of zone division is important to correct diagnosis of CSA and decompression of the ventral nerve rootlets.
The relationship between disc level and LV is important for estimating the corresponding disc level in neurological diagnosis. In this study, LV lengths at C4 and C8 were shorter than those at C5, C6, and C7. Although the difference appears subtle, it may have a clinical impact because the cervical canal area is very small. The distance between the superior margin of the pedicle and the uppermost ventral rootlet (PN) was also very important in the diagnosis. In our study, the uppermost ventral rootlet at C4 was at about the same height as the C3 pedicle, and the C8 uppermost ventral rootlet was at the same level as the inferior part of the C6 pedicle. This spatial relationship may explain why some patients with compression at C6/7 show symptoms of C8.
The degree of spondylosis would affect the IP value. We should note, however, that the cadaveric cervical spines that we used had no cervical metastasis or gross deformities (e.g., scoliosis, kyphosis). We also avoided cadavers from patients with a history of cervical surgery or cervical spondylosis. Nevertheless, since the average age of this cohort was 67 ± 12.1 years, cervical degeneration may have occurred that could have affected the accuracy of the measurements. Early surgery is recommended to CSA patients who have not been helped by conservative treatments. The zone definition that we established may help in the diagnosis of CSA for early surgical intervention.
To our knowledge, no study had examined the relative position of segmental ventral rootlet and pedicle. Here, we defined two zones in the coronal plane from cranial to caudal. At the C3 pedicle level, the segmental spinal cord and ventral nerve rootlets of C4 were at zone P. From C3 to C7, the position of segmental ventral rootlet and spinal cord became more rostral to the corresponding pedicle. For example, the segmental spinal cord and ventral nerve rootlets of C8 were near to the inferior part of the C6 pedicle. This finding may help to explain why patients with disc herniation at C6/7 could have symptoms from C8 nerve root compression. A previous study suggested that C8 nerve root does not contact the C7–T1 disc. Hence the chance of a direct C8 nerve root compression by the C7-T1 disc is very small [8]. However, our findings suggest that the chance of compression of C8 ventral nerve rootlets by C6/7 disc is rather large. Furthermore, compressions of zone I and zone II at C6/7 may also cause dysfunctions of C8 spinal cord and ventral nerve rootlets. However, compression at zone III may lead to radiculopathy of both C7 and C8 ventral nerve rootlets.
Anatomic studies have been conducted to evaluate the width of the spinal segment [15-17]. Shinomiya et al. [17] investigated the width of the spinal segment from C5 to C8 using cadavers and reported that C6 was the widest and C8 the narrowest. Kobayashi et al. [18] also confirmed that C8 was the narrowest, and this characteristic continued to the entry of the root in the foramen. Karatas et al. [16] measured the widths from C2 to C8 and found that the longest longitudinal length was at C5, whereas the shortest was at C2. We found that the longitudinal length of segmental ventral rootlets at C4 and C8 were shorter than those at C5, C6, and C7.
As shown in the study by Tanaka et al. [8], intradural intersegmental connections were rare in ventral rootlets (two connections out of 36 intersegments, 6%) and much thinner. Both connections were found between C4 and C5 ventral rootlets. We found three connections (two specimens) out of 20 intersegments (15%) (Figure 5). One specimen had bilateral connections and the other had unilateral connections. Both connections were found in the C5/6 segment between ventral rootlets of C5 and C6 and were relatively thinner than other bundles of the rootlets. These anatomic findings may also help to explain why some patients with cervical radiculopathy show diffusive symptoms with blurred nerve root location. Our study had some limitations. First, only a small number of cadavers were available. Additionally, those that were available were formalin-fixed, which may affect the accuracy of the measurements. Findings from our study will need to be confirmed in future studies that include a larger sample size and unfixed tissues.