Meyerding grades III, IV and V.

see Meyerding classification.

In patients with HDDS, the alignment is always lost and the torque is of much greater force. The gravitational force passing through L5–S1 is counteracted by the ground reaction force passing through the centre of the femoral heads.

Pelvic retroversion, resulting in verticalization of the sacrum and anterior displacement of the centre of the hips, results in abnormal spinopelvic sagittal balance. As the distance between the opposing gravitational and ground reaction forces increases, an increasing torque results.

The severity index (SI) is a calculation of this torque and consequently of the pelvic retroversion. Unlike Vidal and Marnay’s Index C (couple-charnière) ¹⁾.