The Influence of Passively Modeled Paraspinal Soft Tissues on Spinal Geometric Compensation: A Finite Element Analysis.

Alterations in the contributions of paraspinal soft tissues can influence the geometric profile of the spine. This study investigated the effects of passively modeled paraspinal soft tissues (i.e., paraspinal muscles and the thoracolumbar fascia (TLF)), on lumbar segmental mobility and geometric compensation, using a credible and previously validated finite element model (FEM) of the thoracolumbar spine. The model included the vertebrae, rib cage, intervertebral discs (IVDs), pelvis, ligaments, spinal and abdominal muscles, and the TLF. The model was subjected to 30 deg and 60 deg flexion rotation with a fixed pelvic support, and an applied follower load of 1175 N, increasing by 2.4% at each segmental level. Changes in lumbar L2-S1 intervertebral rotation (IVR), lumbar and thoracic range of motion (RoM), and curvature were analyzed for cases involving removal and increased stiffening of the paraspinal muscles and the TLF. Increasing TLF stiffness reduced lumbar RoM (5.1 deg) at 60 deg flexion relative to the validated model, with compensatory increases of 3.6 deg in thoracic RoM. Increases in lumbar lordosis (3.6 deg) were proportional to increases in thoracic kyphosis (3.3 deg). Similar effects of reduced magnitude were observed in 30 deg flexion. Inverse effects were observed following TLF removal. However, no changes were observed with changes in paraspinal muscle contribution. These findings suggest that changes in TLF stiffness influence lumbar segmental mobility and drive compensatory adjustments in the spinal geometric profile.