Spinal Biomechanics Flashcards
What is the role of the spine?
- Tolerates & transfers weight to pelvis
- Protects spinal cord
- Allows flexibility in all directions
- Key role in posture
What is the smallest functional unit of the spine?
FSU: 2 vertebrae + IVD
What components contribute to the spinal axial load?
- Gravitational forces due to mass of body segments
- External forces & moments
- Muscle tension (provides equilibrium)
List the factors that affect axial loading of spine:
- Type, rate & duration of loading
- Age
- Posture
- Condition of anatomical structures
- Integrity of nervous system
What posture has least amount of pressure on spine?
Supine (25lbs)
Almost any motion increases loads on spine.
What posture causes the greatest pressure on spine?
Flexion while lifting (>600lbs)
Almost any motion increases loads on spine. Nachemson studied pressure in L2-L3 (150lb man).
Ideal lifting posture is:
Lifting w/knees bent & back straight, with object close to body.
What is intracavitary pressure?
As it relates to spinal axial load.
The pressure within the abdomenal cavity, created by muscles.
The rigid-walled cylinder supports a portion of spine loading.
What is the implication of weakened abdominal muscles on the spine?
Weakening of the abdominal muscles causes increased spine loading & the risk of lower back pain.
What is a major anterior compressive load bearing element?
Intervertebral Disc (IVD)
What is the load carrying capacity of the spine?
Maximum load the axial spine can tolerate w/o collapse.
Influenced by: muscle & bone health
What model demonstrates the contribution of orthotic devices to the load carrying capacity?
Euler Model
1 end fixed: 8x; 2 ends fixed: 16x; Milwaukee Brace is an orthotic illustration.
What load increases LCC?
LCC = load carrying capacity
Transverse Load
All orthoses provide some form of transverse laod; braces increase LCC.
True or False: Most spinal orthoses provide some sort of intracavitary pressure.
True
What is the consequence of lifting weights?
Lifting imposes significant load on spine; contraction of back extensors produces large compressive forces, which may cause failure/fracture.
Especially true in pts w/Osteoporosis
How are shearing forces generated?
Flexion, extension and lateral bending
How are shearing forces resisted?
They are passively resisted by bonds btwn annulus fibrosus & vertebral endplates.
How are shearing forces minimized?
Active stabilization by axial musculature.
What are the zones of the Stress-to-Strain Curve?
- Neutral zone
- Elastic zone
- Plastic zone
- Failure zone
What injuries occur in the Failure Zone?
Subluxation, fracture or other traumatic injuries
Which zone has damaging deformation?
Plastic Zone
What zones are within the Physiological range?
Neutral & Elastic zones
Physiologic range: safe; neutral zone has the load or stress.
What zones are within the Traumatic range?
Plastic & Failure zones
Plastic: change; failure: rupture
Compression & tension are caused by:
Bending
Prime mover muscles initiate bending, esp. weight of upper body.
Widening of the intervertebral foramen occurs during:
Flexion
IVD widens and narrows anteriorly and/or posteriorly when there is flexion and/or extension.
How do antagonistic muscles control motion?
By resisting gravitational pull (eccentric contraction*)
*When a muscle lengthens while under tension.
What spinal structures influence ROM?
- Facet joints
- IVDs - annulus fibrosus
- Ligaments
- Thoracolumbar fascia
- Abdominal fascia
Tension in both 4 & 5
What is the ROM in various spinal levels?
Cervical: +++ in 3 planes
Thoracic: limited
Lumbar: resists rotation
Sagittal plane motion occurs:
50% of cervical motion
Occiput & Atlas
Transverse plane motion occurs:
50% of cervical motion
Atlas & Axis
Articulating facets in cervical vertebrae face 45 degrees to transverse plane, and lie parallel to frontol plane.
Facet joint orientation in Thoracic spine is:
- 60 degrees to transverse plane
- 20 degrees to frontal plane
Least mobile portion of spine due to restriction of rib cage & spinous processes.
Facet orientation of Lumbar spine is:
- Right angle (90 degrees) to transverse plane
- 45 degrees to frontal plane
- Favors: flexion, extension, lateral bending
Additional ROM from pelvic tilt.
How is spinal instability measured?
Stiffness - the spine’s ability to resist loads
Loss of stiffness = increased mobility
Instability
Fractures & Dislocation cause:
Macroinstability
Instability
Disc degeneration, facet joint degeneration & subluxation cause:
Microinstability
What is decompression?
Surgical decompression is the opening or removal of bone to relieve pressure and pinching of spinal nerves.
Lateral stenosis (root); central stenosis (top of cord); foraminal (nerve)
What are the types of decompression?
- Discectomy
- Facetectomy
- Foraminotomy
- Laminectomy
Laminectomy: more common, w/possible spinal fusion (which is surgical stabilization).
