Changes in intervertebral sagittal alignment of the cervical spine from supine to upright

Abstract

Cervical sagittal alignment is a critical component of successful surgical outcomes. Unrecognized differences in intervertebral alignment between supine and upright positions may affect clinical outcomes; however, these differences have not been quantified. Sixty-four patients scheduled to undergo one or two-level cervical arthrodesis for symptomatic pathology from C4-C5 to C6-C7, and forty-seven controls were recruited. Upright sagittal alignment was obtained through biplane radiographic imaging and measured using a validated process with accuracy better than 1° in rotation. Supine alignment was obtained from computed tomography scans. Coordinate systems used to measure supine and upright alignment were identical. Distances between adjacent bony endplates were measured to calculate disc height in each position. For both patients and controls, the C1-C2, C2-C3, and C3-C4 motion segments were in more lordosis when upright as compared with supine (all p < 0.001). However, the C4-C5, C5-C6, and C6-C7 motion segments were in less lordosis when upright as compared with supine (all p ≤ 0.004). There was an interaction between group and position at the C1-C2 (p = 0.002) and C2-C3 (p = 0.001) motion segments, with the controls demonstrating a greater increase in lordosis at both motion segments when moving from supine to upright. The results indicate that cervical motion segment alignment changes between supine and upright positioning, those changes differ among motion segments, and cervical pathology affects the magnitude of these changes. Clinical Significance: Surgeons should be mindful of the differences in alignment between supine and upright imaging and the implications they may have on clinical outcomes.

Keywords: biplane radiography; cervical sagittal alignment; computed tomography; lordosis; measurement.

Figure 1:

Data collection and processing workflow. (A) CT scans were collected and used to create subject-specific 3D models of each cervical vertebra. (B) Anatomic coordinate systems were established in each vertebra by manually placing 4 markers on each endplate. (C) Participants were seated upright and directed to look straight ahead while synchronized biplane radiographs were collected. (D) A validated volumetric model-based tracking technique matched digitally reconstructed radiographs, constructed from the bone tissue segmented from the CT scans, to the biplane radiographs. (E) After the registration process was completed for each bone, the orientation of each vertebra relative to the adjacent inferior vertebra was calculated by the angle between the respective green arrows in the bone-based coordinate systems, and the disc heights were calculated by the distance between the red anatomic landmarks placed on the anterior, central, and posterior endplates.

Figure 2:

Disc height measurement. Disc height was measured in the supine and upright positions using 3 points along the midsagittal plane of each vertebral body. The measurement points were identical in the supine and upright positions.

Figure 3:

Group average cervical lordosis during supine and upright imaging. (A) Overall, controls were in more lordosis than arthrodesis patients (p < 0.001). (B) The position by group interaction when moving from supine to upright (p = 0.005). Error bars indicate ± one standard deviation.

Figure 4:

Intervertebral orientation for each participant in supine and upright positions. Datapoints above (green shaded region) and below (yellow shaded region) the diagonal line indicate less or more lordosis, respectively, during upright imaging compared to supine.

Figure 5:

Group average intervertebral orientation in supine and upright positions. The C1-C2 through C3-C4 motion segments were in more lordosis when upright than when supine, however, the C4-C5 through C6-C7 motion segments were in more lordosis when supine than when upright. Note that all comparisons were made within subject, however, the error bars indicate inter-subject variability (± one standard deviation).

Figure 6:

Group average lordosis at the C6-C7 motion segment. Overall, controls were in more lordosis than arthrodesis patients at the C6-C7 motion segment. Error bars indicate ± one standard deviation.

Figure 7:

Group by position interactions in the upper cervical spine. Control subjects had a greater increase in lordosis at the (A) C1-C2 (p = 0.002) and (B) C2-C3 (p = 0.001) motion segments when moving from supine to upright than arthrodesis patients.