Biomechanics of the Lumbar Spine

Introduction
Human spine complex structure.

Major function are to protect spinal cord and transfer load from head and trunk to pelvis
Providing the mechanical linkage between the upper and lower extremities
Each of the vertebra articulate with the adjacent ones to permit motion in three planes.
The lumbar region of the spine is of particular interest because low back pain is a major medical and socioeconomic problem in modern times.
Spinal stability
spine gains stability from the intervertebral discs
and from the surrounding ligaments and muscles;
Intrinsic stability
Discs and ligaments provide intrinsic stability
Extrinsic stability
Muscles provide extrinsic support
STRUCTURE OF THE SPINE
Vertebral Column
The spine consists of a curved stack of 33 vertebrae divided structurally into five regions.
Proceeding from superior to inferior, there are 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 fused  sacral vertebrae, and 4 small, fused coccygeal vertebrae.
Cervical vertebras
The first two vertebrae are specialized in shape and function.
The first, known as the atlas, provides receptacle for the condyles of the occiput of the skull.
The atlantooccipital joint is extremely stable
Allow  flexion/extension of about 14–15°
but no motion in any other plane.
A large range of axial rotation is provided at atlantoaxial joint .
Motion at the atlantoaxial joint  includes 75° of rotation, 14° of extension, and 24° of lateral flexion
Lumber vertebra
vertebral size increases from the cervical region down through the lumbar region .
The lumbar vertebrae, are larger and thicker than the vertebrae in the superior regions of the spine.
This is for  functional purpose, when the body is in upright position each vertebra t support the weight of not only the arms and head but all the trunk positioned above it.
Increased surface area of the lumbar vertebrae reduces the amount of stress to which these vertebrae would otherwise be subjected.
The weight-bearing surface area of the intervertebral disc also increases with the weight supported .
Intervertebral Discs
fibrocartilaginous discs that act as cushions.
Healthy intervertebral discs in an adult account for approximately one-fourth of the height of the spine.
When the trunk is erect, the differences in the anterior and posterior thicknesses of the discs produce the lumbar, thoracic, and cervical curves of the spine.
Structure of discs
Two functional structures:
A thick outer ring composed of fibrous cartilage called the annulus fibrosus, or annulus, surrounds a central gelatinous material known as the nucleus pulposus, or nucleus
Mechanical properties
Annulus acts as a coiled spring whose tension holds the vertebral bodies together against the resistance of the nucleus pulposus
Nucleus pulposus acts like a ball bearing composed of an incompressible gel.
What happened during movement.

Flexion and extension
vertebral bodies roll over the nucleus while the facet joints guide the movements. spinal flexion, extension, and lateral flexion produce compressive stress on one side of the discs and tensile stress on the other.
spinal rotation creates shear stress in the discs.
Stress higher with flexion than rotation approximately 450 times greater than the twisting.
During daily activities, compression is the most common form of loading on the spine.
Annulus
In Bending
◦Increased tensile force posteriorly
◦Increased compressive force anteriorly
In Rotation
◦Reorientation of collagenous fibers
◦Tightening of fibers traveling in one direction
◦Loosening of fibers traveling in opposite direction
Effect of loading on disc
When loaded in compression, it simultaneously lose water and absorb sodium and potassium til its internal electrolyte concentration is sufficient to prevent further water loss.
At this point internal disc pressure is equal to external pressure .
Continued loading over several hours results in slight
decrease in disc hydration
For this reason, the spine undergoes a height decrease of 2 cm over the course of a day, with 54% of this occurring during the first 30 minutes after an individual gets up in the morning
Effect of unloading
Once pressure relieved, the discs quickly reabsorb water, disc volumes and heights are increased .
On earth, disc height and volume are typically greatest when a person first arises in the morning. Increased disc volume also translates to increased spinal stiffness, so greater risk of disc injury early in the morning.
Effect of injury and aging
Irreversibly reduce the water-absorbing capacity as well as shock-absorbing capability.
Results of MRI degenerative changes to be the most common at L5-S1
Fluid content of all discs begins to diminish around the second decade of life
Geriatric disc has a fluid content that is reduced by approximately 35%
As a result of degenerative change, abnormal movements occur between adjacent vertebral bodies
Effect of injury and aging
More compressive, tensile, and shear loads on the spine.
Reduced height of the spinal column.
Normal lordtic curve of the lumbar region may be reduced as an individual attempts to relieve compression on the facet joints by maintaining posture of spinal flexion
Factors affecting disc health negatively and positively.
Negative factor
Habitual smoking
Exposure to vibration can negatively affect disc nutrition.
Positive factor
Regular exercise can improve it.