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Chapter 9

Biomechanics of the Human Spine

Vertebral Column

• Curved stack of 33 vertebrate divided into

5 regions

• Cerivcal Region – 7

• Thoracic Region – 12

• Lumbar Region – 5

• Sacrum – 5 fused

• Coccyx – 4 fused

Motion Segment

• 2 adjacent vertebrae and the associated soft tissues

• Functional unit of the spine

Vertebral Joints

• Anterior

– intervertebral symphysis joints

• Posterior

– Gliding diarthrodial facet joints

Facet Joint Functions

• Channel and limit ROM in the different regions of the spine

• Assist in lad bearing, sustaining up to 30% of the compressive load on the spine– Especially in hyperextension

Intervertebral Discs

• Fibrocartilaginous discs that cushion the anterior

spinal symphysis joints

• Composed of a nucleus pulposus surrounded by

the annulus fibrosus

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Spinal Curves

• Lordosis

– Exaggerated lumbar curve

• Kyphosis

– Exaggerated thoracic curve

• Scoliosis

– Lateral spinal curvature

Spinal Movements

• All three planes

• circumduction

Cervical Flexors

• Rectus capitusanterior

• Rectus capitislateralis

• Longus capitis

• Longus colli

• 8 pairs of hyoid muscles

Abdominal Flexors

• Rectus abdominis

• Internal obliques

• External obliques

Cervical Extension

• Splenius capitis

• Splenius cervicis

• Assisted by:

– Rectus capitisposterior major/minor

– Obliques capitissuperior/inferior

Thoracic/Lumbar Extensors

• Erector Spinae– Iliocostalis– Longissimus

– Spinalis

• Semispinalis– Capitis

– Cervicis

– Thoracis

• Deep Spinal Muscles– Multifidi

– Rotatores– Interspinales

– Intertransversarii– Levatores cotarum

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Lateral Cervical Flexors

• Sternocleidomastoid

• Levator scapulae

• Scalenus

– Anterior

– Posterior

– Medius

• Cervical flexors/extensors

– Unilaterally

Lateral Lumbar Flexors

• Quadratus Lumborum

• Psoas Major

• Lumbar Flexors/Extensors

– unilaterally

Forces Acting on the Spine

• Body Weight

• Ligament Tension

• Muscle Tension

• External loads

Anatomical Load

• Body weight acts anterior to the spine

– Causing forward bending (moment)

Spinal Loading

• Because the spine is curved, body weight, acting

vertically, has components of both compression (FC)and shear (FS) at most

motion segments.

Lifting Loads

• Compression

• Anterior Shear

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Lumbar Hyperextension

• Bending load (moment) in posterior direction

• Anterior Tension

• Posterior Compression

Lumbar Hyperextension

• Produces compressive forces on facet joints

• Supporting up to 30% of

force

Spinal Rotation

• Generates shear stress in the intervertebraldiscs