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Spine biomechanics

The application of the physical laws of force and motion to the body is termed biomechanics. When you are finished with the biomechanics section, you will understand:   

Four elemental forces of the spine: compression, tension, torque, and    

The normal ranges of motion for different regions of the back

Distribution of forces on the spine

Spinal balance



In physics, a force is any external agent that causes a change in the motion of a free body or that tries to deform a fixed body (create internal stress). Gravity is a force that we battle from infancy. Our "first steps" are a landmark in our development. In this battle against gravity, we use muscles, ligaments, and bones.

When external forces are applied to the FSU, internal stress is produced in the ligaments, bones, and disk.



Muscle actions

Muscles can contract and they can relax. Thus they can only apply tension, like the strings of a marionette. Muscles pulling on the vertebrae cause them to move, pivoting about the disks and the two articulating processes.

When one muscle contracts, it applies a force on the attached bone. When the opposing muscle contracts, force is applied in the opposite direction.

(Exaggerated movement.)



Compression and tension

Forces acting along the axis of the spine can apply compression or tension. As implied, compressive forces act to flatten the disks and vertebrae. Tension acts to elongate them. Forces develop from many internal and external sources, not just gravity, although gravity is a persistent force.

We start with Jessica standing. The FSU is in its normal position, under the persistent load of gravity. The yellow mesh exaggerates the deformation of the disk.

Jessica is beginning her jump. As she accelerates upward, her spine experiences a compressive force.

As she hangs freely from the bar, gravity exerts tension on her spine.



Shear forces

Forces acting perpendicular to the axis of the spine apply a shearing force that tries to slide the components away from their normal axis. Stresses develop in the interior of the structure. If the shear forces are great enough, ligament and disk tears may result as well as shear fractures of the vertebrae.

As Michael leans from side to side, gravity develops shear stresses on his spine. If he were holding weights in his hands, the forces would be greater.
(Movement exaggerated.)



Torsional forces

Forces acting perpendicular to the axis of the spine, but not in the plane of the spine, apply a rotational force that tries to rotate the components around their normal axis. This rotational force is formally called torque.
Stresses develop in the interior of the structure. If the torsional forces are great enough, ligament and disk tears may result, as well as torsional fractures of the vertebrae.



Range of motion

Muscular forces cause bones to move, but the motion is limited by structural constraints. The physically possible limits for each motion define the range of motion (ROM) for that structure. In a clinical examination, measuring the range of motion can reveal disorders of the spine.

We will examine the normal ranges of motion for the cervical and the thoracolumbar regions of the spine.
(With aging, we typically lose a few degrees of ROM per decade.)

Cervical spine ROM

Cervical degenerative disease is less common than degeneration in the lumbar spine. The neck is subjected to far less torque and force, but a fall or a twisting injury can spur degeneration. Accumulated wear and tear on the discs over time can also lead to neck pain caused by disc degeneration.

Thoracolumbar spine ROM

The ROMs for the trunk of the body vary quite a bit throughout the population and also change with age. The ranges are measured using anatomical landmarks.



Load balancing

What systems help distribute the wide variety of loads experienced by the spine?

The three columns of the spine

The load-bearing structures of the spine are described as the anterior column, middle column, and the posterior column. They provide resistance to compressive forces.

The anterior column includes the anterior longitudinal ligament and the anterior portion of the vertebral bodies.

The middle column includes the posterior portion of the vertebral bodies and the posterior longitudinal ligament.

The posterior column includes pedicles, facets, spinous processes, and the associated ligaments.

Posterior tension band

Lucky Skinner! He has found a golden egg. But he is not stable in this position. (Why?)
Move the slider to make him more stable.

Yes, the spine needed to extend to bring the center of gravity of Skinner plus egg centered over the feet. What structures of the spine enabled the spine to extend?  Hint: the same structures keep the body upright,  resisting the gravitational load of the organs and all other structures anterior to the vertebral column.

The muscles and ligaments of the back form the posterior tension band.  The PTB acts to stabilize the compressive loading carried by the spinal vertebrae. Think of a high tower (resisting gravity) that is stabilized in a vertical positon by guy-wires that are under tension.

A crane was used (T.E. Whitesides) as an analog to spine stabilization. An anterior load (golden egg) is resisting gravity. The force of gravity is supported by the rigid tower (vetebral column). The tower is stabilized by the tension band. If the tension band is weakened or damaged, The entire structure would tend to fall forward (kyphosis).


Spine biomechanics recap

Spine biomechanics is a combination of forces and resistance to forces.

Four types of force on the spine:
Compression – forces, like gravity, which compress vertebrae and discs.
Tension – forces that tend to elongate the vertebrae and discs.
Torque – forces perpendicular to and off the axis of the spine that try to             
rotate the vertebrae and discs around the axis.
Shear – forces perpendicular to and coplanar to the axis of the spine that   
try to slide the vertebrae and discs away from the axis.

Range of Motion (ROM) defines the normal ranges of motion for each region of the back. This is useful in determining the presence and extent of damage.
The vertebral column provides the compressive resistance of the spine.
The posterior tension band allows the vertebral column to remain upright by applying tension posterior to the axis of the spine.
Spinal balance is maintained by the interplay of the vertebral column, the muscles of the back, and the posterior tension band.

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