Idiopathic Scoliosis as a Rotatory Decompensation of the Spine

Abstract

Many years of dedicated research into the etiology of idiopathic scoliosis have not led to one unified theory. We propose that scoliosis is a mechanical, rotatory decompensation of the human spine that starts in the transverse, or horizontal, plane. The human spine is prone to this type of decompensation because of its unique and individually different, fully upright sagittal shape with some preexistent transverse plane rotation. Spinal stability depends on the integrity of a delicate system of stabilizers, in which intervertebral disc stiffness is crucial. There are two phases in life when important changes occur in the precarious balance between spinal loading and the disc's stabilizing properties: (i) during puberty, when loads and moment arms increase rapidly, while the disc's "anchor," the ring apophysis, matures from purely cartilaginous to mineralized to ultimately fused to the vertebral body, and (ii) in older age, when the torsional stiffness of the spinal segments decreases, due to disc degeneration and subsequent laxity of the fibers of the annulus fibrosus. During these crucial periods, transverse plane vertebral rotation can increase during a relatively brief window in time, either as adolescent idiopathic or degenerative de novo scoliosis. Much more is known of the biomechanical changes that occur during disc aging and degeneration than of the changing properties of the disc during maturation.

The role of sagittal pelvic morphology in the development of adult degenerative scoliosis

“…The current and earlier studies again confirm the interaction between the sagittal spinal profile as dictated by the PI and the occurrence of scoliosis [1][2][3][4][5][6][7]. This strengthens the hypothesis of a causal relationship between sagittal spinal morphology and development of both idiopathic adolescent and adult degenerative scoliosis [7,[12][13][14].…”

“…The mechanical basis of scoliosis is becoming more and more understood. The human spine has a unique sagittal profile, that renders certain spinal segments prone to decompensate into a scoliotic deformity [2,4,5]. How, why and when this decompensation starts is still matter of continuing investigations, however, not yet fully understood mechanical properties of the maturing as well as the degenerating disc as the primary passive stabilizer of the spine likely play a role in the pathogenesis of scoliosis [4,25,26].…”

“…Linear regression analysis showed a weak but significant correlation between age and pelvic incidence in both the ADS group (R = 0.225; p = 0.024) and the sex-age matched controls (R = 0.197; p = 0.048), indicated by the dashed lines in figure cause or 'trigger', but a component cause, predisposing patients with a specific spino-pelvic alignment to develop scoliosis in different areas of the spine, dependent of the sagittal profile, during two separate phases of life. In both adolescence and in later adulthood, important changes to spinal biomechanics occur [4,25,26]. We propose that the 'trigger' is a relative loss of equilibrium between the passive stabilizers, mainly the intervertebral discs, on the one hand and the spinal loading on the other.…”

“…The unique upright sagittal profile of the human spine and its consequences for human spinal biomechanics provide an important mechanical basis for the development of spinal deformities [1][2][3][4][5]. Spinal alignment and consequent biomechanics are to a large extent determined by the pelvic incidence (PI), first described by Duval-Beaupère et al [6].…”

Ossification and Fusion of the Vertebral Ring Apophysis as an Important Part of Spinal Maturation

“…Recent studies have demonstrated the significant contribution of the intervertebral discs to the deformation of the spine in scoliosis [ 1 , 2 , 3 ]. For undisturbed and harmonious spinal development, rapidly increasing loads during the growth spurt require adequate maturation of the spine’s stabilizers, of which the disc is an essential component [ 4 ]. Skeletal maturity is traditionally assessed on X-rays of the iliac crest, the hand, and wrists, or other growth cartilages remote from the spine.…”