Cervical spine bone mineral density as a function of vertebral level and anatomic location

Abstract

Background context: Bone mineral density (BMD) measurements acquired from quantitative computed tomography scans have been shown to correlate with bone mechanical properties such as strength, stiffness, and yield load. There are currently no reports of BMD as a function of anatomic location within each vertebra.

Purpose: The overall objective of this study was to characterize BMD in the cervical spine as a function of level and anatomic location.

Study design: Cervical spine BMD was evaluated in vivo using a clinically relevant age group.

Patient sample: Twenty-two subjects (13 women and 9 men) were included with an average age of 48 ± 7 years (range, 35-61 years). Ten subjects were recently diagnosed with cervical radiculopathy (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; and three smokers, three quit smoking, and six nonsmokers).

Outcome measures: Physiologic measures included overall BMD for C3-C7, average BMD within 11 anatomically defined regions of interest for each vertebra, and density distribution (by volume) within each anatomic region and vertebral level.

Methods: Subject-specific three-dimensional bone models were created from high-resolution computed tomography scans of the subaxial cervical spine (C3-C7). Custom software calculated the average BMD within 11 anatomically defined regions of interest for each three-dimensional bone model. Bone mineral density values for each voxel of bone tissue were binned into 50 mg/cc ranges to determine the density distribution by volume. Repeated-measures analysis of variance was used to test for differences within subjects by level (C3-C7) and anatomic location. The correlation between BMD in the central vertebral body and the pedicle and lateral mass regions was tested using Pearson correlation.

Results: Average BMDs by level were 476, 503, 507, 473, and 414 mg/cm(3) for C3-C7, respectively. C3 and C6 BMDs were significantly less than those of C4 and C5 (p<.007). C7 BMD was significantly less than those of all other levels (all p<.001). Control and female subjects showed a trend toward higher BMD than radiculopathy and male subjects across all levels (p value: .06-.17). Wide variation in BMD was observed over anatomical regions, with the pedicles having significantly higher BMD than all other anatomic locations and the anterior portion of the central vertebral body having significantly lower BMD than all other anatomic locations. There was a significant positive correlation between central vertebral body BMD and lateral mass BMD at each level. Bone mineral density distribution by volume plots revealed women had a higher volume of very high-density bone than men but only in the posterior elements.

Conclusions: This study has characterized BMD in the cervical spine according to vertebral level and anatomic location within each vertebral level using live subjects from a clinically relevant age group. The results indicate significant differences in BMD according to vertebral level and among anatomical regions within each vertebra. The results suggest to the surgeon and device manufacturer that surgical procedures involving instrumentation attached to C7 may require a modification in instrumentation or in surgical technique to attain results equivalent to more superior levels. The results suggest to the basic scientist that computational models may be improved by taking into account the wide variation in BMD over different anatomical regions.

Figure 1

A: Anatomic regions of interest numbered from highest (1) to lowest (11) BMD. B: Superior (red) and inferior (blue) endplates and central vertebral body (green) regions of interest.

Figure 2

Overall BMD by vertebral level. C3 and C6 BMD were significantly less than C4 and C5 (p < .007) C7 BMD was significantly less than all other levels (all p ≤.001). Error bars on group averages are ± 1 SE.

Figure 3

Bone mineral density by anatomic location, averaged across C3–C7 levels. Symbols to the right indicate anatomic regions with BMD that are NOT significantly different from each other (e.g. pedicles have significantly higher BMD than all other locations; lateral masses and lamina are not significantly different, but are less than pedicles and greater than all other locations, etc).

Figure 4

Average volume of bone within each 50 mg/cm³ range for the entire C3 vertebra (A) the lateral masses (B) and the exterior-lateral portion of the vertebral body (C) for males (blue) and females (red). Note differences in the low-density ranges (Section 1) and high-density ranges (Section 2) and an overall shift in average density of highest volume ranges (blue and red ellipses) for the entire vertebra (A). For females, a larger volume of bone was present in the high-density ranges for the posterior elements (e.g. (B)) but not within any anatomical portions of the vertebral body (e.g. (C)). C3 data is representative of all other levels.

Figure 5

Average volume of bone within each 50 mg/cm³ range for the entire C3 vertebra (A) the lateral masses (B) and the exterior-lateral portion of the vertebral body (C) for radiculopathy patients (blue) and controls (red). Note differences in the low-density ranges (Section 1) and high-density ranges (Section 2) and an overall shift in average density of highest volume ranges (blue and red ellipses) for the entire vertebra (A). For controls, a larger volume of mid to high density bone was present throughout the entire vertebra, including the posterior elements (B) and all anatomic regions of the vertebral body (e.g. (C)). C3 data is representative of all other levels.