Basion–Cartilaginous Dens Interval: An Imaging Parameter for Craniovertebral Junction Assessment in Children

Discussion

The craniovertebral junction comprises the occiput, atlas, and axis with associated ligaments. These ligaments play an important role in the stability of the CVJ. The anterior atlanto-occipital membrane is the cranial extension of the anterior longitudinal ligament and extends superiorly to the clivus. The apical ligament extends from the dens to the clivus. The transverse band of the cruciform ligament is the main stabilizing ligament of atlantoaxial joint and passes behind the dens to attach to the lateral masses of the atlas. The ascending and descending bands of the cruciform ligament attach to the clivus and body of C2, respectively. The tectorial membrane is the cranial extension of the posterior longitudinal ligament and passes behind the cruciform ligament to attach to the clivus. The paired alar ligaments extend from the superolateral margins of the dens to the medial aspect of the occipital condyles. The posterior atlanto-occipital membrane is the cranial extension of the ligamentum flavum and extends from the anterior aspect of the posterior arch of the atlas to the posterior aspect of the foramen magnum.^9,10

It is important to be aware of the embryology of the CVJ to understand the anatomy. The craniovertebral junction is formed from the 4 occipital and 2 upper spinal sclerotomes. First, 2 occipital sclerotomes form the clivus below the spheno-occipital synchondrosis (basiocciput). The third occipital sclerotome forms the exoccipital bone, which forms the jugular tubercle. The fourth occipital sclerotome (proatlas) divides into cranial and caudal halves, with the cranial half forming the tip of the clivus, occipital condyles, and the margin of foramen magnum. The lateral mass and superior portion of the posterior arch are formed by the caudal division of the proatlas (fourth occipital sclerotome), and the posterior and inferior portions of the arch are formed by the first spinal sclerotome. The anterior arch is formed by the hypocentrum of the first spinal sclerotome. The centrum of the second spinal sclerotome forms the body of the axis, and the neural arch forms the facets and posterior arch of the axis. The centrum of the first spinal sclerotome forms the odontoid process.

In a 17-mm embryo (Carnegie stages 18 and 19, 44–46 days after fertilization), the odontoid process is a dense mass of mesenchymal tissue located close to the future anterior foramen magnum. The chondrification of the odontoid starts from the base in stage 21 (51 days after fertilization).^11,12 The odontoid ossification begins at the base from 2 ossification centers that fuse in the midline by the seventh gestational month. The terminal portion of odontoid arises from the proatlas (the fourth occipital sclerotome). The most inferior portion of the axis body is formed by the second spinal sclerotome.^13⇓–15 The body of C2 fuses with the odontoid by 3–6 years of age. A secondary ossification center (os terminale) at the apex of the odontoid process appears between 3 and 6 years of age and usually fuses by 12 years.^16,17 The cruciate and alar ligaments share the common mesenchymal origin in the tip of the primitive odontoid process.^12,18 The apical ligament is a functional vestige of the notochord and arises from either the notochord or its sheath.^12,19,20

Craniovertebral junction injuries are unstable, potentially fatal injuries and should be diagnosed promptly on the initial imaging studies. The basion-dens interval is an important imaging parameter for CVJ injuries, described by Wholey et al in 1958.²¹ Normal values of the basion-dens interval in the adult population have been well-described in the literature. In a study by Harris et al,²² the basion-dens interval was <12 mm in 95% of adult patients on lateral cervical spine radiographs.

However, BDI values on CT are different from those of radiographs due to better delineation of the anatomy. Gonzalez et al²³ reported that a BDI of >9 mm on CT is suggestive of injury to the craniovertebral junction. In a study by Rojas et al,²⁴ BDI values of <8.5 mm were seen in 95% of patients (of 200) on CT. Omercikoglu et al²⁵ also reported a BDI of 8.5 mm as the optimal upper limit of normal on CT in their study of 499 patients.

The literature on the BDI in the pediatric population is very limited. Bulas et al²⁶ mentioned 12.5 mm as an upper limit of normal on cervical spine radiographs in a study on 110 patients. Bertozzi et al²⁷ studied the basion-dens interval in 117 healthy children and found that the normal maximum value of the BDI is 9.5 mm if the os terminale is ossified and 11.6 mm if it is not ossified. Vachhrajani et al²⁸ reported 7.49 mm as the upper tolerance limit for the BDI in 42 pediatric patients. The basion-dens interval is highly variable in children due to the developing dens and the variable ossification of the os terminale (Fig 5).

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Fig 5.

Sagittal CT cervical spine images in 3 children of different ages show the variable appearance of the os terminale.

Recognizing that an inherent problem with the BDI in the pediatric population is that it does not include cartilage in the measurement, Birchansky et al⁸ were the first group to devise a new imaging parameter utilizing sagittal CT soft tissue window reformats to measure the distance from the basion to the readily observed cartilaginous dens tip. In their study, this novel distance was coined as the “basion-cartilaginous dens interval” (BCDI). They measured the BCDI in 86 children between 0–24 months of age and calculated the upper limit of normal to be 4.4 mm.⁸

Our results show that the BCDI varies with age, with the upper tolerance limit of 5.6 mm in children up to 6 years of age and 7.2 mm in children 6–10 years of age. Our results for the BCDI are smaller than those in prior studies that measured the BDI because the 2 measurements used different landmarks. The BCDI should serve as a substitute for the BDI in the pediatric population, especially in younger children in whom the dens is not completely formed because BCDI is measured from the clivus to the cartilaginous dens where ligaments attach. An example is shown in Fig 6 for comparing the BCDI versus BDI.

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Fig 6.

Comparison of BDI versus BCDI in a 6-month-old child. A, Sagittal CT cervical spine image in a bone window shows the BDI as 6.7 mm. B, Sagittal CT cervical spine image in a soft-tissue window shows the BCDI as 2.54 mm.

Some limitations of our study are due to its retrospective nature, such as selection bias; however, we tried to minimize this bias by prospectively selecting patients in the PACS from 2013 to 2016 after the careful application of exclusion criteria. The other limitation of this study is that we only looked at the BCDI values in the healthy pediatric population. We need to compare these data with data from patients with actual CVJ injury to validate these measurements.