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AAOS Atlas of Orthoses and Assitive Devices > Chapter six: Biomechanics of the spine > Flashcards

Chapter six: Biomechanics of the spine Flashcards

1
Q

What are the three loads on the human spine produced by?

A

Gravitational forces due to the mass of body segments
External forces and moments induced by a physical activity
Muscle tension

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2
Q

The loads on the spine are shared by what?

A

The osseoligamentous tissues

Muscles of the spine

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3
Q

What do the tensile forces in the paraspinal muscles do?

A

Balance the moments created by gravitational and external loads.

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4
Q

What is the range of the compressive force on the human lumbar spine?

A

200 to 300N during supine and recumbent postures to 1400N during relaxed standing with the trunk flexed 30 degrees.
It is substantially larger when a person is holding a weight.

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5
Q

What is the compressive preloads on the cervical spine?

A

It’s 3X the weight of the head
It increase during flexion, extension, and other ADLs.
It is estimated to reach 1200N

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6
Q

Without muscle forces can the osteoligamentous spine support vertical compressive loads of in vivo magnitude?

A

No

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7
Q

What kind of loads are individual spinal units or functional spinal units (FSUs) subjected to?

A

Pure compressive loads in vivo.

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8
Q

What changes with increased trunk inclination?

A

The compressive force on the lumbosacral disc increased with the amount of weight lifted, whereas the maximum anterior posterior shear force remained small.

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9
Q

Why is there less shear force on the FSUs during loaded trunk inclination?

A

The obliquity of the short lumbar extensor muscles allows them to share anteior shear forces with the FSUs when lifting a load.

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10
Q

The coactivation of the trunk muscles alters what, so that it what?

A

The compressive force vector so the path follows the lordotic and kyphotic curves of the spine passing through the instantaneous center of rotation of each segment.

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11
Q

What does the coactivation of the trunk muscles minimize?

A

Bending moments and shear forces induced by the compressive load, allowing the ligamentous spine to support loads that otherwise would cause buckling and providing a greater margin of safety against both instability and tissue injury. This is called a follower load.

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12
Q

What does muscle dysfunction of the spine cause?

A

Abnormal shear forces at the lumbar FSU, leading to segmental instability in the presence of disc degeneration.

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13
Q

What can help treat muscle dysfunction of the spine?

A

Muscle conditioning and therapy to strengthen the muscles that provide compressive force to cause the load to go over the follow load area of the spine.

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14
Q

What is the smallest functional unit of the osteoligamentous spine?

A

A spinal motion segment.

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15
Q

What does an FSU unit consist of?

A

Two vertebral bodies connected by an intervertebral disc, facet joints, and ligaments (except C1-2, they have no intervertebral disc present.)

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16
Q

In a healthy spine, load transmission from vertebra to vertebra occurs primary through what?

A

The disc’s nucleus pulposus and distributed equally in all directions from within the nucleus, placing the annulus fibers in tension.

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17
Q

The intervertebral disc of the spine provides most of what?

A

The motions segement’s stiffness in compression

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18
Q

The ligaments and facets of the spine provide what?

A

Resistance to bending moments and axial torsion.

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19
Q

Facet joints provide what and have an important role in determining what?

A

A posterior load path

Have an important role in determining the limits of motion in FSU.

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20
Q

Facet joints in the lumbar spine carry how much of the compressive load?

A

10-20% when standing upright.

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21
Q

Maintence of what help to reduce the load on the disc,?

A

Cervical and lumbar lordosis

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22
Q

What increase load on the intervertebral disc?

A

Flexion

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23
Q

The contribution of the facet joints to the stability of an FSU is dependent on what?

A

The capsular ligament and the level withint the spine.

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24
Q

How is instability quantified in terms?

A

Loss of stiffness or increase in flexibility of an FSU.

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25
Q

Stiffness of an FSU is a measure of what?

A

How much load is required to produce a given motion.

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26
Q

Flexibility of an FSU is a measure of what?

A

The motion produced by a given load.

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27
Q

When is an FSU unstable?

A

When the stiffness is too small and the flexibility is too large.

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28
Q

What are the two terms of FSU instability?

A

Macroinstability

Microinstability.

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29
Q

What is macroinstability?

A

Gross disruption of the spinal column, such as that caused by a fractre leading to disruption of load transmission from one vertebra to another.

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30
Q

Macroinstability can lead to what?

A

Progression of the deformity at the injury site and neurologic deficit.

31
Q

What are examples of macroinstability?

A 
Instability caused by injuries of the thoracolumbar Spine such as compression fracture, 
Fracture-dislocation, 
Traumatic spondylolisthesis
Burst fracture
Tumors
Infections, 
Iatrogenic causes.
32
Q

What are the three columns of the spine?

A

Anterior column
Middle column
Posterior column

33
Q

What makes up the anterior column?

A

The anterior longitudinal ligament
Anterior annulus fibrosis
Anterior part of the vertebral body.

34
Q

What makes up the middle column?

A

The posterior longitudinal ligament
Posterior annulus fibrosis
Posterior wall of the vertebral body.

35
Q

What makes up the posterior column?

A

The posterior arch
Supraspinous and interspinous ligaments
Facet joints
Ligamentum flavum.

36
Q

A compression fracture involves which spinal column?

A

Anterior column with the middle column totally intact.

37
Q

A burst fracture involves which columns/

A

The anterior and middle column

38
Q

A seat-belt-type injury involves which columns?

A

Middle and posterior columns.

39
Q

A fracture-disolaction injury involves which columns?

A

All three

40
Q

Loss of load-carrying capacity of the spine is influenced by what?

A

The number of spinal columns disrupted.

41
Q

What are microinstabilities?

A

Instability associated with degenerative disorders of the lumbar spine.

42
Q

What is thought to precede all other changes within the aging FSU?

A

Disc degeneration

43
Q

Degenerative changes in a disc are associated with a loss of what?

A

Proteoglycans in the nucleus, which leads to a decrease in the ability of the nucleus to generate fluid pressure.

44
Q

What occurs to the annular fibers of the disc with dehydration and narrowing of the disc space in disc degeneration?

A

They are no longer subjected to the same tensile stresses as they would be in a healthy disc with the hydrated nucleus. So the annulus is more likely to bear the axial load.

45
Q

Facet facet degeneration is most commonly a result of what?

A

Segmental instability.

46
Q

what happens with narrowing of the disc space as a result to degernation?

A

The facets begin to undergo subluxation until the tips of the inferior facets impinge on the lamina below, causing the facets to increase their share of load transmission.

47
Q

In patients with facet syndrome, symptoms are aggravated by what?

A

Extension maneuver because facet loading increases in extension.

48
Q

Increased peak pressure with in the facets of patient’s with facet syndrome, may give rise to what?

A

Degeneration of the joint cartilage. Thinning of the cartilage may cause capsular ligament laxity and allow abnormal motion or hypermobility of the facet joints.

49
Q

Cartilage degernation further increases what?

A

Segmental movements that already were increased by disc degeneration.

50
Q

What is the final stage of the degenerative cascade?

A

Attempted stabilization.

51
Q

Attempted stabilization in disc degeneration gives rise to what?

A

Formation of body hypertrophy and steophytes and a decrease in segmental mobility.

52
Q

Uneven collapse of the disc space in disc degeneration can occasionally cause what?

A

Acute angular deformities within the three-joint complex.

The patient will present with complaints of both neurogenic as well as low back pain.

53
Q

What treatment might be necessary for disc degeneration?

A

Surgical intervention.

54
Q

What are some of the surgical intervention that can be used during disc degeneration?

A

Discectomy
Facetectomy
Foraminotomy
Laminectomy

55
Q

What are the options for surgical stabilization?

A

Uninstrumented fusion

Fusion aided by the combination of anterior and posterior instrumentation.

56
Q

An uninstrumented fusion fusion can only be done in what mechanical environment?

A

A greater degree of immobilization of adjacent vertebrae enhances the chances of obtaining a solid bony fusion.

57
Q

Spinal instrumentation increases, what?

A

The rigidty of segments at the fusion site, reducing the relative motion between the vertebrae during the biologic healing process.

58
Q

Posterior spinal implants typically consists of what?

A

Two longitudinal components and segmental attachments to the vertevrae, forming a solid construct.

59
Q

The size and shape of the longitudinal components of a posterior spinal fusion determines what?

A

The rigidity of the construct

60
Q

What also determines the rigidty of a posterior spinal fusion?

A

The number of vertevrae spanned by the implant, the method of their attachment to the vertevrae, and the cross-links between the longitudinal components.

61
Q

Implants that use wires or hooks to attach the longitudinal components to the vertebrae can resist what? But can’t resist what?

A

tensile forces, but cannot resist angular deformity caused by loads experienced during activities of daily living in the presence of compromised load-bearing ability of the anterior column.

62
Q

Spinal instrumentation that uses what, has the ability to resist both compressive and tensile forces, as well as bending moments.

A

Transpedicular screws.

63
Q

Transpedicular screws span which columns?

A

All three

64
Q

What are the problems with using transpedicular screws?

A

There is a high possibility of screw failure (loosening and breakage.)
Micromotion in the intervertevral space, causing continued back pain.

65
Q

When have spinal orthoses been used in regards to providing spinal stability?

A

As a nonoperative alteranative to spinal fusion in some cases of microinstability and macroinstability
Also as a postoperative adjuncts to protect the surgical construccts used for stabilizing macroinstability in the thoracolumbar spine.

66
Q

The postoperative orthosis should limit what?

A

The gross motion o the trunk during ADLs, thereby protecting the surgical construct from large loads created from torso motion until solid biologic fusion occurs.
It should also protect the surgical construct from the planes of motion in which the construct may be vulnerable to failure. Usually flexion and torsion.

67
Q

A TLSO is most likely to reduce intervertebral motion in which spinal section?

A

Upper lumbar levels

68
Q

What might need to be added to help restrict motion in the lower lumbar levels of the spine?

A

A thigh extension.

69
Q

Cervical orthoses are pirmarily used to limit what motions?

A

Flexion-extension
Roation
Lateral bending
Translational motion in the cervical spine.

70
Q

What orthosis is used for upper cervical spine (occiput-C2)?

A

Four-poster orthoses

71
Q

Two poster orthoses are effective in reducing which motions? Which ones can it not reduce.

A

Flexion, but is limitied in its effectiveness in lateral bending and rotation

72
Q

What orthosis is used to immobilize odontoid injuries?

A

Halo orthosis

73
Q

Rigid collars provide what type of immobilization?

A

Moderate amount of immovilization at the midcervical levels but lose efffectiveness at the upper (occiput-C2) and the lower cervical segments (C6-7).

74
Q

What orthosis is the most effective in achieving immobilization in flexion-extension at the lower cervical segments?

A

The four-poster.
Two-poster is effective for only flexion.
Halo is the best for all planes in the upper and lower cervical spine.

AAOS Atlas of Orthoses and Assitive Devices (40 decks)

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