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This short guide to terms in spinal biomechanics is divided into five sections:
local, regional, spinal and global axis systems
(see figure 1)
Vector
a quantity that possesses both a magnitude and a direction (e.g. force; velocity; displacement).
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Force
an action that causes a body to displace or deform. (SI Unit of measure = Newton, i.e. N)
Tension force
a force that tends to elongate a structure or material.
Compression force
a force that tends to shorten a structure or material.
Moment or Torque
The sum of the forces applied to a structure multiplied by their perpendicular distance from a reference point or axis. (SI Unit of measure = Newtonmetre, i.e. Nm)
Bending Moment
at a point within a structure. (See Figure 2). The moment that tends to bend a structure. It is usually the sum of the moments due to several forces.
Couple
Two equal an opposite parallel forces separated by a distance, producing a torque.
3Point bending (See Figure 3)
a structure is loaded in 3point bending when a single force is applied on one side and two forces are applied on the other side acting in opposite directions.
4Point bending (See Figure 3)
a structure is loaded in 4point bending when two forces are applied on one side and two forces are applied on the other side acting in opposite directions.
Stress
the force per unit area of a structure and a measurement of the intensity of the force (SI Units are Newtons/m2 = Pascals. Hence 1 N/m2 = 106 N/mm2 = 1 mega Pascal = 1 MPa).
Normal stress
the intensity of force perpendicular to the surface on which it acts.
Shear stress
the intensity of force parallel to the surface on which it acts.
Compressive stress
a normal stress that tends to shorten material.
Tensile stress
a normal stress that tends to elongate material.
Principal stresses
the stresses normal to the principal planes of a material are called principal stresses. The principal planes are those where the stresses are maximum and minimum.
Stress concentration
A site of stress that is high compared to that of nearby sites in a structure or material. It is often caused by a sharp change in shape.
Center of gravity
the point in a body in which the body mass is centered.
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Absolute motion
Motion of a rigid body relative to the global axis system.
Relative motion
motion of a rigid body relative to the local axis system of an adjacent body.
Translation (Figure 4)
motion of a rigid body in which a straight line in the body always remains parallel to itself.
Rotation (Figure 4)
motion of a rigid body in which a certain straight line within or adjacent to the body remains motionless. (That straight line is the axis of rotation)
Plane motion
a motion of a rigid body in which the body moves in a single plane.
Degrees of freedom (Figure 5)
the number of independent translations and rotations that can occur in a mechanism (e.g. the spine and its instrumentation).
Instantaneous Axis of Rotation (IAR) (Figure 5)
when a rigid body moves at every instant there is a line in the body or some hypothetical extension of it that does not move. For plane motion the axis of rotation becomes the center of rotation. Note: The IAR can describe the absolute motion of a body, or its relative motion with respect to an adjacent moving body (e.g. an adjacent vertebra).
Bending
angular deformation of a structure, caused by a bending moment.
Neutral axis
line or axis within a beam or other structure about which bending occurs.
Strain (Figure 6)
Deformation (change in length) divided by the original length.
Normal strain
is defined as the change in length divided by the original length. Normal strain can be tensile or compressive.
Plastic Deformation (Figure 7)
Deformation that remains after the deforming load is removed.Strain  (Figure 6) Deformation (change in length) divided by the original length.
Shear strain
shear deformation divided by the thickness perpendicular to the shear.
Plastic Deformation (Figure 7)
Deformation that remains after the deforming load is removed.
Figure 6
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LoadDeformation and StressStrain Relations
Elastic Behavior:
Stiffness
force divided by the deformation it produces (i.e. the slope of the forcedeformation relationship).
Modulus of elasticity
Stress divided by the strain it produces (i.e. the slope of the stressstrain relationship). (e.g. Young's Modulus = normal stress divided by normal strain)
Torsional rigidity
Torque divided by the rotation that it produces.
Time Dependent Behavior:
Creep
Deformation produced over time by a constant load.
Viscoelasticity
Material behavior in which the resistance to deformation depends on the amount of deformation (elastic) and the rate of deformation (viscous).
Stress Relaxation
Loss of stress over time in a material while the strain is held constant.
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Yield Stress (Figure 7)
magnitude of stress on the stressstrain curve at which appreciable deformation takes place without any appreciable increase in the stress.
Ductility
property of a material in which there is a large amount of deformation possible after the yield point. This implies that a large amount of deformation energy is absorbed by the material before failure. (opposite of brittle)
Fatigue
Eventual failure after repeated cycles of subyield loading. This usually occurs as a result of the process of the growth of cracks in structures subjected to repetitive load cycles.
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State of a structure in which all forces and moments are balanced, hence it does not move.
Free body analysis (Figure 8)
A technique for determining the internal forces in a structure subjected to external loads. It involves an equilibrium analysis in which a system is split into real or imagined component parts (free bodies), in order to check that each part is in equilibrium.
Statics
the branch of mechanics that deals with the equilibrium of bodies at rest or in motion with zero acceleration.
Dynamics
The branch of mechanics that deals with motion of systems in which the accelerations of masses have significant effect.
Kinematics
The branch of mechanics that deals with motion alone.
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Behavior of a system whereby it returns to its equilibrium position after being disturbed. The stable equilibrium position is a position of minimum potential energy  any displacement of the structure requires a net input of energy. Although stiffness or rigidity of a structure can contribute to its stability, stiffness and stability are not the same thing. When referring to the rigidity of, for example an instrumentation construct, use the term stiffness or rigidity, not stability.
Buckling
A kind of instability in which a structure suddenly bends and collapses when a certain critical load is applied
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