The biomechanical understanding of increasing anterior column load with progressing kyphosis leading to subsequent vertebral compression fracture (VCF) established the basic rationale for kyphoplasty. The lumbar spine can support an effort of 500 kg in the axis of the vertebral body, and a bending moment of 20 Nm in flexion. Consequently, if this effort is forward deviated of only 10 cm, the acceptable effort will be reduced to 20 kg so it is important to restore the vertebral anterior wall after a VCF: the authors describe the biomechanical modifications in the spine after kyphoplasty.