Biomechanical contribution of spinal structures to lumbar spine – novel biomechanical insights

Goal of the Study?

The goal of this study 1 is to understand to function and load distribution of each of the passive structures of the lumbar spine using a displacement-controlled transection study. 


Why are they doing this study?

The passive structures of the lumbar spineanterior longitudinal ligament (ALL), ligamentum flavum (LF), intertransverse ligaments (ITL) and the interspinous and supraspinous ligaments (ISL & SSL), facet joints (FJ), posterior longitudinal ligament (PLL) – contribute to stability during certain movements and under different loading conditions. However, there is little understanding of the particular function and load distribution of these passive structures in the spine’s stability. This knowledge is important to diagnose better and treat spinal pathologies. 



What was done?

The researchers used fifty spinal segments from 22 fresh frozen cadavers. After thawing, they used CT, MRI, and X-rays to exclude any cadavers with defects or other spinal injuries, using only cadavers that had little to moderate spinal degeneration. All specimens were dissected, and the segments were mounted with specific clamps to allow for testing. 

The fifty lumbar spinal segments were biomechanically tested in flexion, extension, axial rotation (AR), lateral bending (LB), with anterior (AS) posterior (PS) and lateral shear (LS). They used a displacement controlled, stepwise reduction approach. This means that they examined the load distribution first with the spinal segments intact and then gradually removed (transected) the remaining segments while examining the load distribution. Using a displacement-controlled biomechanical approach, the researchers determined the load distribution of the spinal structures by measuring the reduction in load required for the same motion. 


What did they find?

The study found that the load distribution of passive spinal structures varied widely. For example, about 67% of the main load in LB and SL is the responsibility of the IVD, but it plays a less important role in flexion, extension and AR. In comparison, the facet joints (FJ) and facet joint capsules (FJC) are the main stabilizers in axial rotation (AR) with 49% but provide only 10% stability in extension. The ALL’s main role is during extension (40%), but it also contributes 15% to LB.

Interestingly, while the intertransverse ligaments (ITL) were thought to be important for LB, their contribution to passive stability was not observed in this study.


Why do these findings matter?

Understanding the load distribution between the passive lumbar structures has important clinical implications. In lumbar spine surgery, spinal structures are often cut or dissected, resulting in a loss of stability. For example, in midline decompression with laminotomy, the SSL, ISL and LF are resected, which would result in the loss of about 28% of the passive stabilizers in flexion. This lack of stability can result in pain and malfunction, ultimately impacting a patient’s quality of life. For this reason, it’s important to understand the role of each segment in spinal stability.


This research provides new knowledge of how load distribution occurs for the different passive structures and the whole spine. This knowledge will assist healthcare providers in their diagnosis and treatment of patients.