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Ryan C. Hofler, Daniel M. Heiferman, Ayrin Molefe, Ryan LeDuc, Stephen J. Johans, Jordan D. Rosenblum, Russ P. Nockels and G. Alexander Jones

OBJECTIVE

Atlantoaxial instability is an important cause of pain and neurological dysfunction in patients with Down syndrome (DS), frequently requiring instrumented fusion of the upper cervical spine. This study provides a quantitative analysis of C2 morphology in DS patients compared with their peers without DS to identify differences that must be considered for the safe placement of instrumentation.

METHODS

A retrospective chart review identified age-matched patients with and without DS with a CT scan of the cervical spine. Three-dimensional reconstructions of these scans were made with images along the axis of, and perpendicular to, the pars, lamina, facet, and transverse foramen of C2 bilaterally. Two of the authors performed independent measurements of anatomical structures using these images, and the average of the 2 raters’ measurements was recorded. Pedicle height and width; pars axis length (the distance from the facet to the anterior vertebral body through the pars); pars rostrocaudal angle (angle of the pars axis length to the endplate of C2); pars axial angle (angle of the pars axis length to the median coronal plane); lamina height, length, and width; lamina angle (angle of the lamina length to the median coronal plane); and transverse foramen posterior distance (the distance from the posterior wall of the transverse foramen to the tangent of the posterior vertebral body) were measured bilaterally. Patients with and without DS were compared using a mixed-effects model accounting for patient height.

RESULTS

A total of 18 patients with and 20 patients without DS were included in the analysis. The groups were matched based on age and sex. The median height was 147 cm (IQR 142–160 cm) in the DS group and 165 cm (IQR 161–172 cm) in the non-DS group (p < 0.001). After accounting for variations in height, the mean pars rostrocaudal angle was greater (50.86° vs 45.54°, p = 0.004), the mean transverse foramen posterior distance was less (−1.5 mm vs +1.3 mm, p = 0.001), and the mean lamina width was less (6.2 mm vs 7.7 mm, p = 0.038) in patients with DS.

CONCLUSIONS

Patients with DS had a steeper rostrocaudal trajectory of the pars, a more posteriorly positioned transverse foramen posterior wall, and a narrower lamina compared with age- and sex-matched peers. These variations should be considered during surgical planning, as they may have implications to safe placement of instrumentation.

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Ryan C. Hofler, Muturi G. Muriuki, Robert M. Havey, Kenneth R. Blank, Joseph N. Frazzetta, Avinash G. Patwardhan and G. Alexander Jones

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° ± 2.2°, neutral SVA increased by 27.3 ± 18.6 mm. When T1 tilt was reduced by 6.7° ± 1.4°, 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° ± 2.6° to 32.2° ± 4.4° (p < 0.01). When the T1 tilt decreased, C0–C7 lordosis at the neutral SVA decreased to 15.6° ± 3.1° (p < 0.01). C0–C2 lordosis increased from 12.9° ± 9.3° to 29.1° ± 5.0° with increased T1 tilt and decreased to −4.3° ± 6.8° 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.