Biomechanical role of lumbar spine ligaments in flexion and extension: Determination using a parallel linkage robot and a porcine model

Study Design. Porcine lumbar spine specimens were cyclically loaded in flexion and extension with a pure moment. The resistance to flexion and extension of the individual spinal elements was quantified using a sequential resection procedure. Objective. To determine the biomechanical contribution of the individual posterior spinal ligaments, including the interconnections between the supraspinous and interspinous ligaments, to resisting flexion-extension moments applied to the lumbar spine. Summary of Background Data. Numerous experimental studies have performed repeated tests with sequential resection to determine the role of spinal ligaments. Most studies have not maintained the kinematics across trials, and therefore these studies have assessed the kinematics and kinetics of injured spines rather than delineating the mechanics of individual ligaments. Methods. The L4-L5 motion pathway for pure moment loading (between in 16 Nm of flexion and 12 Nm of extension) of porcine spinal specimens was learned and replayed using a parallel-linkage robot. Each specimen was tested 6 times, with sequential resections performed as follows: intact, cut interconnections between the supraspinous and interspinous ligaments, supraspinous ligament removed, interspinous ligament removed, ligamentum flavum cut, and facet joints removed. The kinematic pathway was repeated for each of these tests, and, accordingly, the changes in loads between trials reflected the mechanics of the cut/removed structures. Results. The interaction between the supraspinous and interspinous ligaments is a significant contributor to the flexion resistance of the porcine lumbar spine, resisting 10.5% of the peak flexion moment. This contribution is similar to the interspinous ligament (11.3%) and almost as large as the supraspinous ligament (14%) and facet joints (14.2%). The supraspinous/interspinous ligament complex was the largest contributor to the resistance of flexion motion (35.9%), followed by the intervertebral disc (25.2%) and the ligamentum flavum (24.7%). The only structures involved in resisting extension were the facet joint complexes, intervertebral disc, and possibly the spinous processes. Conclusions. The supraspinous/interspinous ligament complex is the largest contributor to resisting applied flexion moments in the porcine lumbar spine.