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Spinal Biomechanics

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Tom Young

on 25 March 2015

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Transcript of Spinal Biomechanics

Spinal Biomechanics
Spinal Joint Composition
The material a structure is made of dictates its functional properties
Spinal Mobility
Now we look at spinal mobility as it relates to the structures we've discussed.
Coupled Motions
The body relies upon coupled motions to create fluidity
Deformation
Vertebral Structure
Understanding structure
will often clarify why regions function they way they do
In a standing individual, the COG is located 4cm anterior to S1

Postural syndromes will affect the location of the COG, as do body positions

COG in the Human Body

Definition:
Where the mass of a body is centered.

If a body is suspended at this point, it will not
rotate unless acted upon by a force

Sum of all torque is zero

Center of Gravity

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Vertebral Function

Two adjacent vertebrae

Intervertebral disc

Soft tissues that join them

*This motion segment that allows us to impart specific motion with the adjustment.

Motion Segment

Cortical Bone

Cancellous Bone

Vertebral Body


Layers of cartilage between disc and body

In early life, composed of Hyaline & Fibrocartilage,
later becoming mostly Fibrocartilage

Vertebral End Plates

AKA Facet Joints
Articulation b/w superior (concave) and inferior (convex) articulating facets
Guide motion
Transverse plane (rotation)
Frontal plane (lateral flexion)
Orientation of facets determines predominant motion in the region

Zygapophaseal Joints

Fluid in nucleus allows rocking enabling
movement of motion segment

Composition resists compressive forces
to aid in shock absorption

Allow increased mobility by increasing space between vertebrae

IVD Function

Compose 25% of the spinal column length

Increase & Decrease height throughout day

Avascular structures relying on diffusion
Through end plates for nutrition

Only outer 1/3 -1/2 innervated by sinuvertebral nerve

Intervertebral Discs

70-90% water

Collagen is 15-20% dry weight

PG makes up 65% of dry weight to attract water content

Resists compressive forces


Nucleus Pulposus

60-70% water contributing to viscoelasticity

Collagen makes up 50-60% of dry weight
Type I is predominant
Type II also present

Designed in concentric lamellae to resist tensile forces (10-12 layers)

Annulus Fibrosis

Serves as exit point for spinal nerve root

Diameter determined by disc height & pedicle shape

Decreases by 20% in extension & Increases by 24% with flexion

Intervertebral Foramen

Resist motion
Cervical
Axial rotation
Lateral bending
Little flexion/extension
Thoracic
No primary restraint
Lumbar
Axial Rotation
Flexion
Strongest at curve transitions (stress points)

Facet Joint Capsule

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.



In Flexion:
Posterior subjected to tensile force
Anterior subjected to compressive force
In Rotation
Some fibers subjected to increased tension, opposing subjected to decreased tension

Vertebral End Plates

The nucleus pulposus

The annulus fibrosus

Inter-body Joints
Symphysis
Facet/Zygapophyseal Joints
Diarthrodial

Spinal Motion Segment

Loading along the long axis of a motion segment

Most compression resisted by disc and body

As density (trabecular bone) decreases with age, cortical bone takes on greater stress

Z Joints and posterior elements may take on up to 33% of compressive load, depending upon posture


Axial Loading



Highest ratio is in Cervical region
Lowest ratio is in Thoracic region

Disc Thickness
Cervical 3 mm
Thoracic 5 mm
Lumbar 9 mm

Ratio of disc height to vertebral body height determines mobility

Intervertebral Discs

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Annulus
Anterior Compression
Posterior Tension
Nucleus
Anterior compression
Posterior displacement
Canals
IVF increased 24%
Central canal widened
Tension applied to
Ligamentum Flavum
Z Joint capsule
PLL


Spinal Flexion

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Superior vertebra tilts, translates, & rotates over inferior vertebra
Tension on intertransverse ligaments and annular fibers limit motion



Lateral Flexion & Rotation

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Annulus
Posterior Compression
Anterior Tension
Nucleus
Anterior negative pressure draws forward
Superior Vertebra tilts and glides posteriorly
IVF narrowed 20%
Central canal narrowed
Motion limited by facet joints
ALL is only ligament limiting extension


Spinal Extension

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Cervical
45 Degrees from frontal & horizontal planes
Favors rotation

Thoracic
Frontal plane
Favors lateral flexion

Lumbar
Sagittal plane
Favors Flexion & Extension

Facet Orientation

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Six degrees of freedom
Flexion/Extension
Lateral Flexion (R & L)
Rotation (R & L)

Amount & direction of motion is determined by disc height & facet orientation

Spinal Motion

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Water is expressed from NP & AF through pores in
end plates in loading, and imbibed when unloaded

Nucleus is displaced laterally exerting tension
through annular fibers and end plates

Disc pressure my be up to 5x greater than exerted load

Equal & Opposite force is applied by annulus & end plates working together

Axial Loading

Joint Structure and Function, Levangie, PK, FA Davis, 2005 - Chapter 4

Typical Coupling Patterns

Coupled motion
Association of motion about one axis with another movement about another axis


In rotation & lateral flexion
Extension in ipsilateral facet
Flexion in contralateral facet


Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Lateral translation
S-I tranlation
Compression/distraction
A-P tranlastion
Lateral tilting
Axial rotation
A-P tilting

Spinal Accessory Motion

Herzog. Clinical Biomechanics of Spinal Manipulation.

Reflects the spine’s ability to withstand multi-directional forces applied as the spine provides stable base for posture & movement

Dependent upon
Type, degree, duration of loading
Integrity of structural elements
Integrity of nervous system

Body utilizes fibrosis & osteophytosis to maintain stability (stiffness) with age


Spinal Stability

Creep & Recovery

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

The spine is continually subjected to multiple forces in many directions

Axial loading
Bending
Torsion
Shear

Postural variation will change magnitude of forces

Segmental Loading

Levangie, P. Joint Structure & Function: A Comprehensive Analysis. 4th Ed.

Failure Strength of Spinal Ligaments

Ipsilateral compression
Ipsilateral shearing
Contralateral Tension

Occurs with
Flexion
Extension
Lateral flexion

Bending & Torsion

Herzog. Clinical Biomechanics of Spinal Manipulation.

Grade 0 - Non-degenerative disc
Grade 2 - Mild degenerative disc
Grade 3 - Severe degenerative disc

Creep
Deformation over time when constantly loaded
Recovery
Gradual return to original length when load is removed

Tendons & Ligaments will lengthen under a constant load (stretching), and return to original length when the load is removed unless the point of plastic deformation is achieved.

Disc Creep
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