Neutral cervical sagittal vertical axis and cervical lordosis vary with T1 tilt.

OBJECTIVE: The authors conducted a study to determine whether a change in T1 tilt results in a compensatory change in the cervical sagittal vertical axis (SVA) in a cadaveric spine model. METHODS: Six fresh-frozen cadavers (occiput [C0]-T1) were cleaned of soft tissue and mounted on a customized test apparatus. A 5-kg mass was applied to simulate head weight. Infrared fiducials were used to track segmental motion. The occiput was constrained to maintain horizontal gaze, and the mounting platform was angled to change T1 tilt. The SVA was altered by translating the upper (occipital) platform in the anterior-posterior plane. Neutral SVA was defined by the lowest flexion-extension moment at T1 and recorded for each T1 tilt. Lordosis was measured at C0-C2, C2-7, and C0-C7. RESULTS: Neutral SVA was positively correlated with T1 tilt in all specimens. After increasing T1 tilt by a mean of 8.3 degrees +/- 2.2 degrees , neutral SVA increased by 27.3 +/- 18.6 mm. When T1 tilt was reduced by 6.7 degrees +/- 1.4 degrees , neutral SVA decreased by a mean of 26.1 +/- 17.6 mm.When T1 tilt was increased, overall (C0-C7) lordosis at the neutral SVA increased from 23.1 degrees +/- 2.6 degrees to 32.2 degrees +/- 4.4 degrees (p < 0.01). When the T1 tilt decreased, C0-C7 lordosis at the neutral SVA decreased to 15.6 degrees +/- 3.1 degrees (p < 0.01). C0-C2 lordosis increased from 12.9 degrees +/- 9.3 degrees to 29.1 degrees +/- 5.0 degrees with increased T1 tilt and decreased to -4.3 degrees +/- 6.8 degrees with decreased T1 tilt (p = 0.047 and p = 0.041, respectively). CONCLUSIONS: Neutral SVA is not a fixed property but, rather, is positively correlated with T1 tilt in all specimens. Overall lordosis and C0-C2 lordosis increased when T1 tilt was increased from baseline, and vice versa.