CT and radiographic analysis of sagittal profile changes following thoracoscopic anterior scoliosis surgery

Abstract

Background: Previous studies report an increase in thoracic kyphosis after anterior approaches and a flattening of sagittal contours following posterior approaches. Difficulties with measuring sagittal parameters on radiographs are avoided with reformatted sagittal CT reconstructions due to the superior endplate clarity afforded by this imaging modality.

Methods: A prospective study of 30 Lenke 1 adolescent idiopathic scoliosis (AIS) patients receiving selective thoracoscopic anterior spinal fusion (TASF) was performed. Participants had ethically approved low dose CT scans at minimum 24 months after surgery in addition to their standard care following surgery. The change in sagittal contours on supine CT was compared to standing radiographic measurements of the same patients and with previous studies. Inter-observer variability was assessed as well as whether hypokyphotic and normokyphotic patient groups responded differently to the thoracoscopic anterior approach.

Results: Mean T5-12 kyphosis Cobb angle increased by 11.8 degrees and lumbar lordosis increased by 5.9 degrees on standing radiographs two years after surgery. By comparison, CT measurements of kyphosis and lordosis increased by 12.3 degrees and 7.0 degrees respectively. 95% confidence intervals for inter-observer variability of sagittal contour measurements on supine CT ranged between 5-8 degrees. TASF had a slightly greater corrective effect on patients who were hypokyphotic before surgery compared with those who were normokyphotic.

Conclusions: Restoration of sagittal profile is an important goal of scoliosis surgery, but reliable measurement with radiographs suffers from poor endplate clarity. TASF significantly improves thoracic kyphosis and lumbar lordosis while preserving proximal and distal junctional alignment in thoracic AIS patients. Supine CT allows greater endplate clarity for sagittal Cobb measurements and linear relationships were found between supine CT and standing radiographic measurements. In this study, improvements in sagittal kyphosis and lordosis following surgery were in agreement with prior anterior surgery studies, and add to the current evidence suggesting that anterior correction is more capable than posterior approaches of addressing the sagittal component of both the instrumented and adjacent non instrumented segments following surgical correction of progressive Lenke 1 idiopathic scoliosis.

Figure 1

Typical standing sagittal radiographs of an AIS patient before and after thoracoscopic anterior spinal fusion surgery illustrating the poor definition of vertebral endplates on radiographs.

Figure 2

Method of reformatting CT scan dataset to obtain sagittal section by tracing the path of the mid-sagittal plane to account for the scoliotic curvature in the coronal plane.

Figure 3

Resulting reformatted sagittal images for three patients in the study showing the paired sagittal CT reconstructions before and after surgery for each patient. Note the superior endplate definition compared to standing radiographs.

Figure 4

Changes in T5-T12 kyphosis Cobb angle before and two years after surgery for all patients in the study measured from ethically approved low dose CT for research purposes.

Figure 5

Comparison of T5-T12 kyphosis between supine CT and standing radiographs both before and two years after surgery. Linear regression lines and equations are given (bold = before surgery).

Figure 6

Comparison of T12-S1 lordosis between supine CT and standing radiographs both before and two years after surgery. Linear regression lines and equations are given (bold = before surgery).