Fig. 1

Roussouly classification of the sagittal alignment of the spine based on sacral slope. a Type 1 (sacral slope < 35°), a long thoracolumbar kyphosis and a short lumbar lordotic curve; b type 2 (sacral slope < 35°), a flat lumbar spine having a fat back appearance; c type 3 (35° < sacral slope < 45°), an almost equal length of the kyphotic and lumbar lordosis curves; and d type 4 (35° < sacral slope), a long lumbar lordosis and a shorter kyphosis