Altered distribution of intervertebral disc degeneration in degenerative thoracolumbar kyphosis: A retrospective clinical study and finite element analysis

This study aimed to systematically characterize the unique topographic pattern of disc degeneration in degenerative thoracolumbar kyphosis (DTLK) and to elucidate its underlying biomechanical basis using combined clinical and finite element analyses. This is a retrospective clinical study combined with finite element analysis (FEA). This clinical study included 170 DTLK patients and 153 controls for analysis. Three finite element models were constructed for biomechanical simulation. We compared the 2 groups for both the grade of intervertebral disc degeneration and the distribution of degenerated segments across the T11–S1 levels. Finite element models of the T10–S1 segment were constructed for the normal, DTLK, and global kyphosis (GK) groups. The intervertebral disc pressure (IDP) from T11 to S1 were analyzed under standing, flexion, extension, lateral bending, and axial rotation. Regarding the overall degeneration grade, Grade IV and V degenerations were both significantly more prevalent in DTLK than in controls (Z = −2.232, P = .026 and Z = 4.504, P < .01, respectively), with grade V showing a more pronounced group difference. Grade IV and V disc degenerations occurred predominantly at the L1–L2 ( P < .001), L2–L3 ( P < .001), L3–L4 ( P < .001), and L4–L5 ( P < .001) segments in the DTLK group. FEA revealed that in both the DTLK and GK models, the IDP from T11 to L5 exceeded that of the normal model under standing, flexion, extension, lateral bending, and rotation conditions. The IDP at the apical L2–L3 segment in normal models were 0.21, 0.34, 0.15, 0.32, 0.32, 0.30, and 0.29 MPa during standing, flexion, extension, left bending, right bending, left rotation and right rotation. In the DTLK model, the corresponding values are 0.33, 0.45, 0.25, 0.42, 0.41, 0.39, and 0.40 MPa. In contrast, in the GK model, the IDP are 0.58, 0.76, 0.42, 0.64, 0.64, 0.63, and 0.61 MPa. DTLK exhibits a distinctive pattern of intervertebral disc degeneration predominantly localized to the L1–L4 segments, contrasting sharply with the L4–S1 predominance observed in normal aging. FEA indicates that the high-stress zone of the DTLK disc is located in the thoracolumbar kyphotic segment.