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Brian J. Park, Colin J. Gold, Royce W. Woodroffe, and Satoshi Yamaguchi

OBJECTIVE

The ability to utilize the T1 slope is often limited by poor visibility on cervical radiographs. The C7 slope has been proposed as a reliable substitute but may have similar limitations of visibility. Herein, the authors propose a novel method that takes advantage of the superior visibility on CT to accurately substitute for the radiographic T1 slope and compare the accuracy of this method with previously reported substitutes.

METHODS

Lateral neutral standing cervical radiographs and cervical CT scans were examined. When the T1 slope was clearly visible on radiographs, the C3–7 slopes and T1 slope were measured. In CT method 1, a direct method, the T1 slope was measured from the upper endplate of T1 to the bottom edge of the CT image, assuming the edge was parallel to the horizontal plane. In CT method 2, an overlaying method, the T1 slope was calculated by superimposing the C7 slope angle measured on a radiograph onto the CT scan and measuring the angle formed by the upper endplate of T1 and the superimposed horizontal line of the C7 slope. A Pearson correlation with linear regression modeling was performed for potential substitutes for the actual T1 slope.

RESULTS

Among 160 patients with available noninstrumented lateral neutral cervical radiographs, the T1 slope was visible in only 54 patients (33.8%). A total of 52 patients met the inclusion criteria for final analysis. The Pearson correlation coefficients between the T1 slope and the C3–7 slopes, CT method 1, and CT method 2 were 0.243 (p = 0.083), 0.292 (p = 0.035), 0.609 (p < 0.001), 0.806 (p < 0.001), 0.898 (p < 0.001), 0.426 (p = 0.002), and 0.942 (p < 0.001), respectively. Linear regression modeling showed R2 = 0.807 for the correlation between C7 slope and T1 slope and R2 = 0.888 for the correlation between T1 slope with the CT method 2 and actual T1 slope.

CONCLUSIONS

The C7 slope can be a reliable predictor of the T1 slope and is more accurate than more rostral cervical slopes. However, this study disclosed that the novel CT method 2, an overlaying method, was the most reliable estimate of true T1 slope with a greater positive correlation than C7 slope. When CT studies are available in patients with an invisible T1 slope on cervical radiographs, CT method 2 should be used as a substitute for the T1 slope.

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Brian J. Park, Colin J. Gold, Royce W. Woodroffe, and Satoshi Yamaguchi

OBJECTIVE

The ability to utilize the T1 slope is often limited by poor visibility on cervical radiographs. The C7 slope has been proposed as a reliable substitute but may have similar limitations of visibility. Herein, the authors propose a novel method that takes advantage of the superior visibility on CT to accurately substitute for the radiographic T1 slope and compare the accuracy of this method with previously reported substitutes.

METHODS

Lateral neutral standing cervical radiographs and cervical CT scans were examined. When the T1 slope was clearly visible on radiographs, the C3–7 slopes and T1 slope were measured. In CT method 1, a direct method, the T1 slope was measured from the upper endplate of T1 to the bottom edge of the CT image, assuming the edge was parallel to the horizontal plane. In CT method 2, an overlaying method, the T1 slope was calculated by superimposing the C7 slope angle measured on a radiograph onto the CT scan and measuring the angle formed by the upper endplate of T1 and the superimposed horizontal line of the C7 slope. A Pearson correlation with linear regression modeling was performed for potential substitutes for the actual T1 slope.

RESULTS

Among 160 patients with available noninstrumented lateral neutral cervical radiographs, the T1 slope was visible in only 54 patients (33.8%). A total of 52 patients met the inclusion criteria for final analysis. The Pearson correlation coefficients between the T1 slope and the C3–7 slopes, CT method 1, and CT method 2 were 0.243 (p = 0.083), 0.292 (p = 0.035), 0.609 (p < 0.001), 0.806 (p < 0.001), 0.898 (p < 0.001), 0.426 (p = 0.002), and 0.942 (p < 0.001), respectively. Linear regression modeling showed R2 = 0.807 for the correlation between C7 slope and T1 slope and R2 = 0.888 for the correlation between T1 slope with the CT method 2 and actual T1 slope.

CONCLUSIONS

The C7 slope can be a reliable predictor of the T1 slope and is more accurate than more rostral cervical slopes. However, this study disclosed that the novel CT method 2, an overlaying method, was the most reliable estimate of true T1 slope with a greater positive correlation than C7 slope. When CT studies are available in patients with an invisible T1 slope on cervical radiographs, CT method 2 should be used as a substitute for the T1 slope.

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Brian J. Park, Colin J. Gold, David Christianson, Nicole A. DeVries Watson, Kirill V. Nourski, Royce W. Woodroffe, and Patrick W. Hitchon

OBJECTIVE

Adjacent-segment disease (ASD) proximal to lumbosacral fusion is assumed to result from increased stress and motion that extends above or below the fusion construct. Sublaminar bands (SBs) have been shown to potentially mitigate stresses in deformity constructs. A similar application of SBs in lumbar fusions is not well described yet may potentially mitigate against ASD.

METHODS

Eight fresh-frozen human cadaveric spine specimens were instrumented with transforaminal lumbar interbody fusion (TLIF) cages at L3–4 and L4–5, and pedicle screws from L3 to S1. Bilateral SBs were applied at L2 and tightened around the rods extending above the L3 pedicle screws. After being mounted on a testing frame, the spines were loaded at L1 to 6 Nm in all 3 planes, i.e., flexion/extension, right and left lateral bending, and right and left axial rotation. Motion and intradiscal pressures (IDPs) at L2–3 were measured for 5 conditions: intact, instrumentation (L3–S1), band tension (BT) 30%, BT 50%, and BT 100%.

RESULTS

There was significant increase in motion at L2–3 with L3–S1 instrumentation compared with the intact spine in flexion/extension (median 8.78°, range 4.07°–10.81°, vs median 7.27°, range 1.63°–9.66°; p = 0.016). When compared with instrumentation, BT 100% reduced motion at L2–3 in flexion/extension (median 8.78°, range 4.07°–10.81°, vs median 3.61°, range 1.11°–9.39°; p < 0.001) and lateral bending (median 6.58°, range 3.67°–8.59°, vs median 5.62°, range 3.28°–6.74°; p = 0.001). BT 50% reduced motion at L2–3 only in flexion/extension when compared with instrumentation (median 8.78°, range 4.07°–10.81°, vs median 5.91°, range 2.54°–10.59°; p = 0.027). There was no significant increase of motion at L1–2 with banding when compared with instrumentation, although an increase was seen from the intact spine with BT 100% in flexion/extension (median 5.14°, range 2.47°–9.73°, vs median 7.34°, range 4.22°–9.89°; p = 0.005). BT 100% significantly reduced IDP at L2–3 from 25.07 psi (range 2.41–48.08 psi) before tensioning to 19.46 psi (range −2.35 to 29.55 psi) after tensioning (p = 0.016).

CONCLUSIONS

In this model, the addition of L2 SBs reduced motion and IDP at L2–3 after the L3–S1 instrumentation. There was no significant increase in motion at L1–2 in response to band tensioning compared with instrumentation alone. The application of SBs may have a clinical application in reducing the incidence of ASD.