I am a passionate and driven academic who is committed to multidisciplinary working (health/clinical). I strive to ensure that the interrelationship between research informed teaching and enterprise informed teaching is maintained to enhance the delivery of undergraduate and postgraduate curriculums. I have a particular interest in the role of spinal biomechanics & spinal orthotics.

1. Glossary of Spinal Deformity
Biomechanical Terms
Hasan A R
Calcutta

2. Title and content layout with list
• (Selected and adapted from Panjabi MM and White AA: Appendix:
Glossary, in:
Clinical Biomechanics of the Spine. (2nd Edition)
by White AA and Panjabi MM, Philadelphia: J.B. Lippincott,
1990)Proposed by the SRS Terminology Committee, 1999

3. This short guide to terms in spinal biomechanics is
divided into five sections:
• Axis systems, etc.
• Loading
• Displacement and deformation
• Load-deformation and stress-strain relations
• Failure
• Equilibrium
• Stability

4. Axis systems
• Axis systems, etc.
• local, regional, spinal and global
axis systems
• Vector
a quantity that possesses both a
magnitude and a direction (e.g.
force; velocity; displacement).

5. Loading
• 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 = Newton-metre, i.e.
Nm)

6. • Bending Moment
at a point within a structure. 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.

7. • 3-Point bending
a structure is loaded in 3-point
bending when a single force is
applied on one side and two
forces are applied on the other
side acting in opposite directions.
• 4-Point bending
a structure is loaded in 4-point
bending when two forces are
applied on one side and two
forces are applied on the other
side acting in opposite directions.

8. • 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.

9. • Center of gravity
the point in a body in which the
body mass is centered.
• Center of pressure
• Center of pressure (fluid
mechanics)
• The center of pressure is the point
where the total sum of
a pressure field acts on a body,
causing a force to act through that
point. The total force vector acting at
the center of pressure is the value
of the integrated
vectorial pressure field.

10. Displacement and Deformation
• 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
motion of a rigid body in which a straight line in the body always
remains parallel to itself.
• Rotation
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.

11. • Degrees of freedom
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)
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).

12. • 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
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.

13. • Normal strain
is defined as the change in length divided by the original length.
Normal strain can be tensile or compressive.
• Plastic Deformation
Deformation that remains after the deforming load is removed.
Strain - Deformation (change in length) divided by the original
length.
• Shear strain
shear deformation divided by the thickness perpendicular to the
shear.
• Plastic Deformation
Deformation that remains after the deforming load is removed.

14. Load-Deformation and Stress-Strain Relations
• Elastic Behavior:
• Stiffness
force divided by the deformation it produces (i.e. the slope of the force-
deformation relationship).
• Modulus of elasticity
Stress divided by the strain it produces (i.e. the slope of the stress-strain
relationship). (e.g. Young's Modulus = normal stress divided by normal
strain)
• Torsional rigidity
Torque divided by the rotation that it produces.

15. 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.

16. 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.

17. • Failure
• Yield Stress
magnitude of stress on the stress-strain 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 sub-yield loading. This
usually occurs as a result of the process of the growth of cracks
in structures subjected to repetitive load cycles.

18. Equilibrium
• State of a structure in which all forces and moments are
balanced, hence it does not move.
• Free body analysis
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.

19. • Stability
• 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

20. Hasan A R