Techniques used for Functional and Multi- segmental Spinal Unit •Functional Spinal Unit (Motion Segment) – the smallest segment of the spine that exhibits biomechanical characteristics similar to those of the entire spine – It consist of two adjacent vertebrae and the connecting ligamentous tissues •Purpose of In vitro Mechanical Tests – To determine the load-displacement behaviors of • intact specimens • specimens with a particular element dissected – To estimate relative contributions of the spinal elements in resisting various load types •General Experimental Protocols – Lower vertebra is fixed to the base frame while the loads are applied to the upper vertebra. Resultant linear and/or rotational displacement of the upper vertebra is measured – Types of Loads: • axial compression, tension, and shear • bending moments in flexion, extension, lateral bending, or axial rotational directions • Combination of force and moments – Loading Methods: • Material testing machine (continuous loads) • Using dead weights (discrete loads) – Output • Linear or rotational stiffness (or flexibility) obtained from the load-displacement curve