The Biomechanics of Spinal Deformity in Adolescent Idiopathic Scoliosis

Spinal deformity biomechanics represent a complex topic, as many anatomic and physical variations exist that can affect the biomechanical behavior of the spinal column. Because the Homo sapiens species walks upright, the center of mass lies directly below the vertebral column, which stands in contrast to other vertebrate species [1,2]. In quadrupeds, the center of mass is typically anterior to the entirety of the thoracolumbar spine, and remains ventral to the base of the vertebral column [1,2]. In the human spine, because the center of mass lies directly below the vertebral column, axial compression remains the dominant force vector during ambulation, and variations in shear forces can be dramatic [3]. The normal anatomy of the human thoracolumbar spine has evolved as an adaptation to the resultant axial and bending forces, and the dorsally-directed shear forces that occur; the profound lumbar lordosis and associated thoracic kyphosis noted in normal human spinal anatomy is not observed in other vertebrates, including most closely-related primate species [3]. The anatomy of the human spine does not differ from other animals in the coronal plane as significantly as in the sagittal plane, and is less stable in rotation when placed under dorsally directed shear loading [2,3]. However, pathologic adaptations in functional spinal anatomy can develop, leading to major deformities that can significantly impact function, ambulation and quality of life. Adolescent Idiopathic Scoliosis (AIS) is a relatively common example, and the associated pathologic biomechanics—as well as the implications of such aberrant functional anatomy during surgical correction of AIS— are important to understand when considering treatment options for this condition.