New insight into the mechanics of the lumbar interspinous ligament

Study Design. Repeated in vitro mechanical tests were performed on porcine and human interspinous ligament specimens with progressive disruption of the collagen fiber network to evaluate the existence of mechanical interactions between collagen fibers. Objective. To evaluate the existence of a load pathway in the interspinous ligament whereby loads are transmitted between collagen fibers. Summary of Background Data. Mechanical tests demonstrate that the interspinous ligament resists spinal flexion, but the collagen fibers are not oriented to oppose separation of the spinous processes. This seeming contradiction could be explained by the existence of mechanical interactions between collagen fibers of the interspinous ligament. Methods. In vitro mechanical tests were performed on porcine and human bone-interspinous ligament-bone specimens. The collagen network of the ligament substance was disrupted by passing a scalpel blade though the ligament substance. Repeated tests were performed with progressive disruption of the collagen fiber network. Results. The stiffness of the interspinous ligament specimens was reduced as the collagen fiber network was disrupted, but approximately half of the initial stiffness was maintained when the collagen fiber network was severely disrupted. In this case, no intact collagen fibers remained. Conclusions. Mechanical interactions exist between collagen fibers in the interspinous ligament. The mechanism of the interactions is unknown.