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

• Loading Methods– Use of Material Testing System

• Axial Compression, Tension, and Shear in a continuous manner• Bending moments produced by eccentric application of axial or shear forces• Application of pure moments is impossible except for torsion• Resultant motion of the vertebra is constrained to the line of force application or within the loading

plane (not physiological)

– Use of Dead Weights• All loads can be applied in an unconstrained manner.• Pure moments can be achieved by the application of coupling loads.• It is impossible to apply the loads continuously

• Displacement Measurement – LVDT or RVDT: 1-directional displacement– Goniometer: 3-D relative motion– From the video or photo images obtained from one camera: 2-D motion analysis– Stereophotogrammetry: 3-D motion analysis

• Using set of photo or video images obtained from two or more cameras• optoelectronic motion analysis system (two or more cameras reading the infrared light)