Abstract
The role of the ligaments is fundamental in determining the spine biomechanics in physiological and pathological conditions. The anterior longitudinal ligament (ALL) is fundamental in constraining motions especially in the sagittal plane. The ALL also confines the intervertebral discs, preventing herniation. The specific contribution of the ALL has indirectly been investigated in the past as a part of whole spine segments where the structural flexibility was measured. The mechanical properties of isolated ALL have been measured as well. The strain distribution in the ALL has never been measured under pseudo-physiological conditions, as part of multi-vertebra spine segments. This would help elucidate the biomechanical function of the ALL. The aim of this study was to investigate in depth the biomechanical function of the ALL in front of the lumbar vertebrae and of the intervertebral disc. Five lumbar cadaveric spine specimens were subjected to different loading scenarios (flexion-extension, lateral bending, axial torsion) using a state-of-the-art spine tester. The full-field strain distribution on the anterior surface was measured using digital image correlation (DIC) adapted and validated for application to spine segments. The measured strain maps were highly inhomogeneous: the ALL was generally more strained in front of the discs than in front of the vertebrae, with some locally higher strains both imputable to ligament fibers and related to local bony defects. The strain distributions were significantly different among the loading configurations, but also between opposite directions of loading (flexion vs. extension, right vs. left lateral bending, clockwise vs. counterclockwise torsion). This study allowed for the first time to assess the biomechanical behaviour of the anterior longitudinal ligament for the different loading of the spine. We were able to identify both the average trends, and the local effects related to osteophytes, a key feature indicative of spine degeneration.
Highlights
The anterior longitudinal ligament (ALL) is a fundamental component of the spine
The ALL deeper fibers are solidly attached on the periosteum of the vertebrae and they continue in the external lamellae of the anterior part of the intervertebral discs (IVDs) [3]
A preliminary check of the bending moment-rotation plots from the spine tester confirmed that the difference between the two series of loading cycles (i.e.: those to measure the structural properties with the motion tracking system, and those to measure the strain distribution with the digital image correlation (DIC)) were smaller than 5 ̊ with a rotation of 33 ̊ at full load
Summary
The anterior longitudinal ligament (ALL) is a fundamental component of the spine. Much shorter intermediate fibers cover more intervertebral discs (IVDs) and insert onto the anterior aspect of the adjacent vertebrae, spanning 2–3 FSUs. The deepest layer covers longitudinally and obliquely (i.e.: alar fibers) a single IVD [2]. The ALL has an important role in stabilizing and limiting movements in the sagittal plane, and in confining the anterior wall of the intervertebral discs (IVD). The ALL prevents closure of the foraminal spaces and compression of the nerve roots. Such effects are even more important in case of disc degeneration. The ALL, like most spine ligaments, is rich with mechanoreceptors and plays a fundamental role in the neuromotor control [4]
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