Biomechanics of the spine. Part I: spinal stability.

Biomechanics, the application of mechanical principles to living organisms, helps us to understand how all the bony and soft spinal components contribute individually and together to ensure spinal stability, and how traumas, tumours and degenerative disorders exert destabilizing effects. Spine stability is the basic requirement to protect nervous structures and prevent the early mechanical deterioration of spinal components. The literature reports a number of biomechanical and clinical definitions of spinal stability, but a consensus definition is lacking. Any vertebra in each spinal motion segment, the smallest functional unit of the spine, can perform various combinations of the main and coupled movements during which a number of bony and soft restraints maintain spine stability. Bones, disks and ligaments contribute by playing a structural role and by acting as transducers through their mechanoreceptors. Mechanoreceptors send proprioceptive impulses to the central nervous system which coordinates muscle tone, movement and reflexes. Damage to any spinal structure gives rise to some degree of instability. Instability is classically considered as a global increase in the movements associated with the occurrence of back and/or nerve root pain. The assessment of spinal instability remains a major challenge for diagnostic imaging experts. Knowledge of biomechanics is essential in view of the increasing involvement of radiologists and neuroradiologists in spinal interventional procedures and the ongoing development of new techniques and devices. Bioengineers and surgeons are currently focusing on mobile stabilization systems. These systems represent a new frontier in the treatment of painful degenerative spine and aim to neutralize noxious forces, restore the normal function of spinal segments and protect the adjacent segments. This review discusses the current concepts of spine stability.