Biomechanical Role of the Thoracolumbar Ligaments of the Posterior Ligamentous Complex: A Finite Element Study

Objectives: To investigate the effect of sequential ligament failure on the range of motion (ROM) and location of the instantaneous axes of rotation (IAR) of the thoracolumbar spine (T12-L1) finite element (FE) model, and to verify the role of the supraspinous ligament (SSL) in maintaining the stability of the injured thoracolumbar spine.

Methods: An FE model of the fractured thoracolumbar spine was developed and validated against published data. The posterior ligamentous complex (PLC) in the fractured T12-L1 segment was then reduced in a sequential manner from the facet capsular ligament (FCL), part of the interspinous ligament (ISL), SSL, and entire ISL, to the ligamentum flavum (LF). The ROM and IAR of the T12-L1 segment were measured at the fracture and at each reduced ligament step under 4 directions of flexion, extension, lateral bending, and rotation, and 4 bending motions of 1.5, 3.0, 4.5, 6.0 Nm.

Results: The FE model showed a consistent increase in the ROM and location of the IAR as the ligaments were removed sequentially. Furthermore, failure of the SSL had the most significant influence on the change in the ROM and IAR in flexion. In extension, removal of the FCL caused the largest shift.

Conclusions: The SSL is a significant ligament that allows the PLC to maintain the stability of the thoracolumbar spine during injury.