Dear Pushpdant,

Simply, you can not calculate such a quantity, but only estimate. You may try to simulate a movement (flexion/extension, lateral bending etc.) in your model until you observe failure in ligaments. Of course, you need some assumptions in your model, for example, the location of the center of rotation. I would recommend you the following papers to start with:

1) BIOMECHANICAL PROPERTIES OF HUMAN LUMBAR SPINE

LIGAMENTS, J.Biomechanics. Vol 25 N.o. 11, pp.1351-1356, 1992.

2) A BIOMECHANICAL STUDY OF THE LIGAMENTOUS

STABILITY OF THE THORACIC SPINE IN MAN, Acta orthop. scand. 52, 315-326, 1981

Kind regards,

Riza.

I agree with Mr Bayoglu.  Assuming the location of the center of rotation for each vertebrae, and then using the axial line for each vertebrae individually you could calculate the degree of motion in f/e, lat bending until you observe failure in ligaments  (using ANSYS).

It has been a few years since I have been in the spine world.  

I will follow for alternate answers.