A Novel Mathematical Model to Calculate the Osteotomy in Ankylosing Spondylitis.

The authors developed a mathematical model to the sagittal vertical axis (SVA) change in ankylosing spondylitis whom PSO is planned. The mathematical model was developed using trigonometric equations. No significant difference exists between postop SVA change amount and SVA calculated. The mathematical model is reliable in restoring the global sagittal balance.