Summary: We have utilized a finite element model to demonstrate the importance of the interspinous and supraspinous ligaments (ISL/SSL) in preventing pathologic intradisc pressure and excessive range of motion at the level above a scoliosis construct. When the ISL/SSL are absent, range of motion is increased by 29% and intradisc pressure by 32%.
Introduction: Proximal junctional kyphosis (PJK), a vexing complication of spinal deformity surgery, is defined as a focal kyphosis at the level immediately superior to a long instrumented fusion. It leads to deformity, disability, and extension of the instrumentation into the cervical spine. The factors that lead to PJK have not been clearly defined. It has been suggested that the interspinous and supraspinous ligaments play an important role in stabilizing the motion segment in flexion by acting as a tether against hyperflexion. In this study, we have attempted to quantify the effect of sectioning the interspinous and supraspinous ligaments in the upper thoracic spine by employing a finite element model of the spine with a long thoracic fusion from T2 to T12. We have quantified the differences in range of motion and intradiscal pressure with and without the presence of the interspinous and supraspinous ligaments.
Methods: The spine solid model was first altered to accurately simulate the non‐homogeneous structure of the vertebral bodies. A generic instrumented spinal fusion construct and ligaments were incorporated into the model according to published stiffness values. The model was run through a series of flexion tests while measuring the effect of the interspinous and supraspinous ligaments on the range of motion and stress levels in the adjacent structures. A 3000N‐mm moment was applied to the motion segment. Variations in sagittal range of motion and anterior disc compression were recorded.
Results: The flexion arc of T1 is 29.35% greater with the interspinous and supraspinous ligaments sectioned, while while pressure within the disc is nearly 32% greater. The stress along the anterior of the disc is much higher with the posterior ligaments severed (Figure 1).
Conclusion: It is important to preserve the interspinous and supraspinous ligaments in order to prevent proximal junctional kyphosis above a long instrumented construct ending at the base of the cervical spine.
Significance: When the ISL/SSL complex is compromised, the surgeon should consider extending the construct cephalad.