Basic ScienceCervical spine bone mineral density as a function of vertebral level and anatomic location
Introduction
The complex geometry of the cervical spine provides multiple locations where load may be applied to each vertebra either through articulating joint surfaces (eg, vertebral body and facets) or through muscle and ligament attachments (eg, lamina and spinous processes). The asymptomatic neck is capable of a wide variety of movements, ranging from simple single-plane motion (eg, flexion-extension) to complex multiplane motions. Thus, it is clear that the anatomical structures that comprise the cervical vertebrae are routinely subjected to diverse combinations of load. Furthermore, the multiple joint articulations and ligament and tendon attachment sites on each bone lead to variation in the type and magnitude of loading applied to different anatomic locations within each vertebra (eg, pedicles are loaded differently than vertebral bodies). Finally, because of the natural curvature of the cervical spine, it is probable that vertebral loading is different among cervical levels. The bone tissue, following Wolff's Law, adapts to these various loading patterns both within an individual vertebra and among vertebra. Therefore, it follows that bone mechanical properties are likely to differ among anatomic locations within an individual vertebra and among vertebrae depending on level.
Bone mechanical properties may be estimated by bone mineral density (BMD) measurements acquired from dual-energy X-ray absorptiometry (DXA or DEXA) or quantitative computed tomography (QCT) scans. For example, DEXA scans of the cervical spine have shown a correlation between BMD and pullout strength [1], [2]. Similarly, BMD measured using QCT has been shown to be related to mechanical properties, such as elastic modulus, strength, stiffness, and yield load [3], [4], [5], [6], [7]. From the clinical perspective, a clear relationship has been established between BMD and surgical instrumentation performance, such as screw pullout strength [8], [9], [10] and disc replacement subsidence [3]. Previously, BMD measurements at specific anatomic locations within the cervical spine have been performed almost exclusively on the central vertebral body, by measuring either trabecular BMD in a single computed tomogaphy (CT) slice [5], [11], [12] or a large volume of trabecular bone in multiple CT scan slices [13], [14]. In addition to the central vertebral body, the only other BMD measurements of specific anatomic regions within the cervical spine were performed on the end plates [3], [11]. The relationship between site-specific bone characteristics and bone mechanical properties has been investigated to a limited extent in the thoracic and lumbar spine. In the thoracic spine, bone mass and thickness at specific anatomic locations within the posterior elements have been related to the strength at these sites [15], whereas in the lumbar spine, regional measures of bone density have been correlated to whole-bone fracture load [16].
In the case of the cervical spine, it would be beneficial to know the extent to which BMD, and therefore bone mechanical properties, vary in relation to vertebral level and anatomic location. However, this information does not currently exist. Knowledge of BMD variation according to vertebral level and anatomic location would be beneficial to surgeons who must secure instrumentation to the vertebrae during surgery, to those in the medical device industry who must design instrumentation such as screws and disc replacements, and to basic scientists who assign mechanical properties to bones in computational models of the spine.
The overall objective of this study was to characterize BMD in the cervical spine as a function of vertebral level and anatomic location. The first specific aim was to determine the overall BMD of cervical vertebrae C3–C7. The second specific aim was to determine the BMD in distinct anatomic regions within C3–C7, and the third specific aim was to correlate BMD in the central vertebral body to BMD in anatomic regions commonly used for screw fixation. The three null hypotheses tested were the following: BMD does not change with vertebral level; BMD is not significantly different across anatomic regions; and BMD in the central vertebral body is not correlated with pedicle and lateral mass BMD.
Section snippets
Methods
High-resolution CT scans (0.29×0.29×1.25 mm voxels) of the subaxial cervical spine (C3–C7) were collected from 22 subjects after institutional review board approval (overall average age 48±7 years; range 35–61 years; and 13 women, and 9 men). Ten subjects were diagnosed with cervical radiculopathy within 30 days of data collection (age 49±8 years; six women and four men; and two smokers and eight nonsmokers), and 12 subjects were asymptomatic controls (age 46±6 years; seven women and five men;
Results
Average BMD varied significantly by level. C3 and C6 BMDs were significantly less than those of C4 and C5 (all p≤.007). C7 BMD was significantly less than those of all other levels (all p≤.001) (Table 1, Fig. 2). Control subject average BMD was higher than radiculopathy subject BMD at each vertebral level; however, this difference did not reach significance at any level (p value range: .06–.17). Female average BMD was higher than male BMD at each vertebral level; however, this difference
Discussion
Differences in BMD by level for the entire C3–C7 vertebrae were consistent with previous reports based on single-CT slices [5], [11], [20] and volumetric measurements [13], [14] of trabecular vertebral body bone density. These previous reports indicated that cervical vertebral body BMD was highest at C5 and decreased in the direction of C3 and C7 [5], [11], [13], [14], [20], [21]. Significantly lower BMD of C7 in comparison to C4–C6 has also been reported in cadaver specimens (mean age 71, 9
Acknowledgment
The study was funded by NIH/NIAMS, Grant R03-AR056265, and the Cervical Spine Research Society 21st Century Development Grant.
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FDA device/drug status: not applicable.
Author disclosures: WJA: Nothing to disclose. EDT: Nothing to disclose. JYL: Nothing to disclose. WFD: Nothing to disclose. JDK: Research Support (Staff/Materials): Stryker (E, Paid directly to institution/employer); Grants: J & J (C, Paid directly to institution/employer).
The disclosure key can be found on the Table of Contents and at www.TheSpineJournalOnline.com.
The authors WJA and EDT contributed equally to this article.