Using Two X-Ray Images to Create a Parameterized Scoliotic Spine Model and Analyze Disk Stress Adjacent to Spinal Fixation-A Finite Element Analysis

Abstract

Posterior instrumentation is used to treat severe adolescent idiopathic scoliosis (AIS) with a Cobb angle greater than 40 degrees. Clinical studies indicate that AIS patients may develop adjacent segment degeneration (ASD) post-surgery. However, there is limited research on the biomechanical effects on adjacent segments after surgery, and straightforward methods for creating finite element (FE) models that reflect vertebral deformation are lacking. Therefore, this study aims to use biplanar X-ray images to establish a case-specific, parameterized FE model reflecting coronal plane vertebral deformation and employ FE analysis to compare pre- and postoperative changes in the range of motion (ROM), endplate stress, and intervertebral disk stress of adjacent segments. We developed an FE model from biplanar X-ray images of a patient with AIS, using ANSYS software to establish pre- and postoperative models. The shape of the preoperative model was validated using computed tomography (CT) reconstruction. A flexion moment was applied to C7 of the spine model to achieve the same forward bending angle in the pre- and postoperative models. This study successfully developed a case-specific parameterized FE model based on X-ray images. The differences between Cobb angle and thoracolumbar kyphosis angle measurements in X-ray images and CT reconstructions were 6.5 and 5.4 mm. This FE model was used to analyze biomechanical effects on motion segments adjacent to the fixation site, revealing a decrease in maximum endplate and disk stress in the cranial segment and an increase in stress in the caudal segment.

Keywords: X-ray image; adjacent disk; biomechanics; finite element model; scoliosis.

Figure 1

X-ray image parameters were obtained with a 3D Slicer in PA view and lateral view. Note: Pink markers indicate sampling points.

Figure 2

Adjustable spine parameters: (a) Coordinates of the superior posterior point and the vertebral body lengths along the three axes. (b) Sagittal tilt angle of the vertebral body and transverse tilt angle of the vertebral body. (c) Coronal tilt angles of superior and inferior endplates. (d) Calculation of angle and length from nodal locations. Note: 1. Sampling points included A, B, C, D, E, F, and G; 2. X, Y, Z indicate the coordinates.

Figure 2

Adjustable spine parameters: (a) Coordinates of the superior posterior point and the vertebral body lengths along the three axes. (b) Sagittal tilt angle of the vertebral body and transverse tilt angle of the vertebral body. (c) Coronal tilt angles of superior and inferior endplates. (d) Calculation of angle and length from nodal locations. Note: 1. Sampling points included A, B, C, D, E, F, and G; 2. X, Y, Z indicate the coordinates.

Figure 3

Posterior views of the case-specific FE model: (a) pre-op, (b) post-op, (c) instrumentation.

Figure 4

Comparison between FE- and CT-based reconstructed 3D model.

Figure 5

Calculation of FE model’s (a) Cobb angle and (b) TLK angle. Note: A, B, C and D indicate the sampling point to measure the Cobb’s and TLK angles.

Figure 6

Differences in vertebral locations from the bottom vertebral body L5 to upper vertebral body C7 between FE model and CT images. (a) X-axis coordinates (ML direction), (b) Y-axis coordinates (AP direction), (c) Z-axis coordinates (vertical). Note: Arrows indicate the vertebrae with the most significant discrepancies. LX, LY, and LZ represent the X-, Y-, and Z-coordinates of the vertebral bodies.

Figure 7

Stress distribution at the superior endplate of T10 during flexion preoperatively (PRE) and postoperatively (POST). Note: Black arrows indicate areas of high stress.

Figure 8

Stress distribution of the spine model during flexion: (a) pre-op; (b) post-op (fixed at T10-L4). Note: Red arrows show the pre-op stress concentration on the concave side of the curve; black arrows indicate reduced stress in the surgical segments; and green arrows indicate higher stress at T10.