The 3D Sagittal Profile of Thoracic Versus Lumbar Major Curves in Adolescent Idiopathic Scoliosis

STUDY DESIGN

Retrospective.

OBJECTIVE

To compare the 3D sagittal profile of patients with main thoracic or thoracolumbar/lumbar adolescent idiopathic scoliosis (AIS) to a normal cohort.

SUMMARY OF BACKGROUND INFORMATION

Thoracic AIS is often associated with a loss of kyphosis. Classically, this measure has been made in 2D, which may underestimate the true sagittal deformity.

METHODS

Biplanar upright radiographs were obtained on 152 primary thoracic (TH: Lenke 1-4), 50 primary thoracolumbar/lumbar (TL/L: Lenke 5-6) curves, and 89 normal controls (NC). 3D spinal reconstructions were created using sterEOS software. MATLAB code was used to create segmental measurements of kyphosis/lordosis for each vertebral and disc segment from T1 to S1 in the local coordinate system of each motion segment. Comparisons were made between groups for the 3D summed segmental measures (T1-T5, T5-T12, T12-S1), pelvic incidence, sacral slope, and pelvic tilt.

RESULTS

Mean 2D Cobb was 57°±12° (range 40°-115°) for TH curves and 52°±9° (range 37°-75°) for TL/L curves. Significant differences in 3D sagittal measures were found between the 3 groups. Post hoc tests revealed significant differences at T1-T5, TH<NC, and TL/L<NC. All groups differed from each other from T5-T12, with the least kyphosis in TH curves. T12-S1 lordosis was significantly greater in TH and TL/L curves compared with NC. Lumbar lordosis extended proximally an average of one segment in AIS compared to normal spines (T11 vs T12). Pelvic incidence, sacral slope, and pelvic tilt were significantly greater for TH curves compared to NC.

CONCLUSIONS

There is a substantial average loss of thoracic kyphosis (∼15°-25°) for both primary thoracic and primary thoracolumbar/lumbar AIS curves compared to normal adolescents. Three-dimensional assessment of scoliosis allows the "true" deformity to be measured by correcting for error due to out-of-plane measurement associated with conventional 2D measurements.

LEVEL OF EVIDENCE

Level II, prognostic.

Thoracic Idiopathic Scoliosis Severity Is Highly Correlated with 3D Measures of Thoracic Kyphosis

BACKGROUND

Loss of thoracic kyphosis has been associated with thoracic idiopathic scoliosis. Modern 3-dimensional (3D) imaging systems allow more accurate characterization of the scoliotic deformity than traditional radiographs. In this study, we utilized 3D calculations to characterize the association between increasing scoliosis severity and changes in the sagittal and axial planes.

METHODS

Patients evaluated in a scoliosis clinic and determined to have either a normal spine or idiopathic scoliosis were included in the analysis. All underwent upright, biplanar radiography with 3D reconstructions. Two-dimensional (2D) measurements of the magnitude of the thoracic major curve and the thoracic kyphosis were recorded. Image processing and MATLAB analysis were utilized to produce a 3D calculation of thoracic kyphosis and apical vertebral axial rotation. Regression analysis was performed to determine the correlation of 2D kyphosis, 3D kyphosis, and apical axial rotation with the magnitude of the thoracic major curve.

RESULTS

The 442 patients for whom 2D and 3D data were collected had a main thoracic curve magnitude ranging from 1° to 118°. Linear regression analysis of the 2D and 3D T5-T12 kyphosis versus main thoracic curve magnitude yielded significant models (p < 0.05). The 2D model had a minimally negative slope (-0.07), a small R value (0.02), and a poor correlation coefficient (-0.14). In contrast, the 3D model had a strongly negative slope (-0.54), a high R value (0.56), and a strong correlation coefficient (-0.75). Curve magnitude also had a strong correlation with loss of 3D T1-T12 kyphosis and increasing apical axial rotation.

CONCLUSIONS

Segmentally calculated 3D thoracic kyphosis had a strongly negative correlation with the magnitude of the main thoracic curve. With near uniformity, 3D thoracic kyphosis progressively decreased as scoliosis magnitude increased, at a rate of more than half the increase in the main thoracic curve magnitude. Analysis confirmed a surprisingly strong correlation between scoliosis severity and loss of 3D kyphosis that was absent in the 2D analysis. A similarly strong correlation between curve magnitude and apical axial rotation was evident. These findings lend further credence to the concept that scoliosis progresses in the coronal, sagittal, and axial planes simultaneously.

CLINICAL RELEVANCE

The findings of this study suggest that 3D assessment is critical for adequate characterization of the multiplanar deformity of idiopathic scoliosis and deformity in the sagittal plane is linked to deformity in the coronal plane. Increasing severity of coronal plane curvature is associated with a progressive loss of thoracic kyphosis that should be anticipated so that the appropriate intraoperative techniques for correction of idiopathic scoliosis can be applied in all 3 planes.

Predicting 3D Thoracic Kyphosis Using Traditional 2D Radiographic Measurements in Adolescent Idiopathic Scoliosis

STUDY DESIGN

Retrospective.

OBJECTIVE

To develop and validate a prediction formula to estimate three-dimensional (3D) T5-T12 kyphosis in adolescent idiopathic scoliosis (AIS) from standard two-dimensional (2D) radiographic measurements.

SUMMARY OF BACKGROUND DATA

2D measurements of thoracic kyphosis in AIS patients overestimate 3D kyphosis; however, there is a lack of widespread availability of 3D imaging technology.

METHODS

Retrospective review was performed for AIS patients with right thoracic curves evaluated with EOS Imaging from January 2010 to June 2014. Standard 2D posteroanterior and lateral radiographic measurements, pelvic incidence, Nash-Moe grade, Perdriolle rotation, and "3D T5-T12" sagittal measures (reconstructed with sterEOS, analyzed with custom MatLab code) were input into a multivariate logistic analysis to create a prediction model for 3D T5-T12 sagittal alignment. An initial cohort of 66 patients (curves 14°-85°) was used to create a predictive model, and a separate cohort of 129 patients (curves 16°-84°) was used to validate the formula.

RESULTS

2D thoracic coronal Cobb and 2D T5-T12 kyphosis were the only significant predictors in the model. The prediction formula for estimating 3D T5-T12 sagittal measurement from standard 2D measurements, in degrees, was 18.1 + (0.81*2D T5-T12 sagittal Cobb) - (0.54*2D coronal Cobb), r² = 0.84. The average model error between predicted and measured 3D T5-T12 kyphosis was ±7°. The predicted 3D T5-T12 kyphosis (8.6° ± 12.1°) and measured 3D T5-T12 kyphosis (8.5° ± 13.0°) were not significantly different (p = .8). 3D kyphosis was less than standard measures of 2D kyphosis (8.5° ± 13.0° vs. 20.2° ± 12.6°, p < .001).

CONCLUSION

This simple validated formula to predict 3D T5-T12 sagittal alignment using routine 2D thoracic Cobb and T5-T12 kyphosis for thoracic AIS patients has great potential value in assessing historical data collected prior to the development of 3D imaging methods as well as understanding/planning surgical hypokyphosis correction in patients without access to 3D imaging.