Biomechanics of the Spine

Biomechanics of the spine is a vast area of research that has generated numerous studiesin recent years on the part of doctors (mainly orthopaedic surgeons), bioengineers and physicists. This paper is a short introduction to some of the topics of major interest in spine biomechanics. The first topic is the development of the spine with a “mechanical” explanation of the physiological curvatures. As the spine is a multiarticular complex structure, understanding the mechanism responsible for its dynamics requires in-depth knowledge of the spine's basic components: the vertebrae and their architecture, the intervertebral joints, the ligaments and muscles. A short morphofunctional description of each of these anatomical parts is given mentioning their biomechanical features. Of particular interest, in relation to spinal trauma, is the architecture of the cancellous bone of the vertebrae responsible for most resistance to compressive weight loads. This specific characteristic is gradually lost over the years either due to disease or to a progressive resorption of the horizontal lamellae and thinning of the vertical columns typical of osteoporosis. Numerous studies have been conducted in vivo and in vitro to shed light on the mechanisms leading to particular traumatic lesions or degenerative arthrosis. These have given rise to various theories formulated to account for the distribution of loads and strength in the elements making up the functional spinal unit. These theories include the old “two vertical columns” theory, subsequently replaced by the “three columns theory”, and the latest “four columns” model which divides the spine longitudinally depending on carrying strength. The latest theory was formulated in the wake of in vivo CT studies using an axial loading device which allows axial loads to be applied even though patients are in a supine position. A short mention is made of the possible biomechanical applications of neuroradiological techniques, namely cine-MR scans, which allow detailed study of the ligaments, muscles and disc, especially the cervical spine under dynamic conditions. Lastly, the controversial concept of spinal stability and instability is discussed. Further studies are necessary to establish appropriate criteria for treatment of traumatic or degenerative lesions since an impairment to spine statics can result in permanent neurological damage.