Biomechanics

Question 1

Biomechanical functions of the spine (2)

Answer 1

1. Protect the spinal cord while allowing mobility 2. Support loads during ADL’s

Question 2

Biomechanical function of spinal orthoses (2)

Answer 2

1. Protect spine in cases of instability 2. Facilitate healing Via restriction of gross triplanes motion which limits compression, strain, and shear

Question 3

The motion segment / functional spinal unit is the smallest functional unit composed of _ with motion in _ planes.

Answer 3

Composed of a 3 joint complex which moves in three planes

Question 4

Three joints of a FSU

Answer 4

1. Vertebral bodies-intervertebral disc 2 & 3. Posterior facet joints

Question 5

Compressive loads are directed on which structures and in which direction?

Answer 5

Directed axially through vertebral bodies, discs, and facet joints

Question 6

Strain affects which structures?

Answer 6

Discs, ligaments

Question 7

Shear affects which structures?

Answer 7

The joint line (disc-vertebral body endplate)

Question 8

Gravity, external forces and moments, and muscle tension generates which type of force?

Answer 8

Compression

Question 9

Axial rotation, flexion and extension, and lateral bending generate which type of force?

Answer 9

Strain

Question 10

Flexion/extension and lateral bending generate which type of force?

Answer 10

Shear

Question 11

What is the major anterior load bearing element and the major posterior load bearing element?

Answer 11

Anterior: intervertebral disc Posterior: facet joints

Question 12

Flexion relieve compressive loading on which structures?

Answer 12

Facet joints

Question 13

Extension relieves compressive loading on which structures?

Answer 13

Vertebral bodies and discs

Question 14

Describe how the intervertebral discs maintain even pressure on the inferior endplate during loading?

Answer 14

Neutral posture: annulus fibrosus bulges radially and directs nucleus pulposus centrally to create even pressure Bending: annulus bulges on one side and undergoes tension opposite. The nucleus flows towards the side of tension to maintain even pressure

Question 15

What can amplify compressive loading of the spine?

Answer 15

Muscle tension which is generated to counter the torque created by a carried load

Question 16

Function of elastic restraints

Answer 16

Maintain physiologic range of motion of the intervertebral joint

Question 17

What is the benefit of annulus fibrosus fibers crossing at 30°?

Answer 17

1/2 the fibers can resist strain from torsion in one direction (eg horizontal) while the other 1/2 can resist strain from torsion in another direction (eg vertical)

Question 18

Which motions produce maximum strain and shear? At what location does this occur?

Answer 18

Combination of flexion and lateral bending At the posterior inferior vertebral endplate and posterolateral disc

Question 19

Compared to the well developed lumbar capsular ligaments of the facet joints, the thoracic spine has limited capsular reinforcement which permits more of what type of motion?

Answer 19

Axial rotation

Question 20

Where are shear forces generated within the spine?

Answer 20

Along the joint line

Question 21

Shear forces are resisted passively by which structures (2)?

Answer 21

Shear is resisted passively by: 1. Annulus fibrosus 2. Vertebral endplate

Question 22

Shear is resisted actively by what?

Answer 22

Shear is resisted actively by muscle activation

Question 23

Describe follower load mechanics.

Answer 23

Coactivation of trunk and paraspinal muscles direct the compressive force vector to follow the natural lordosis and kyphosis - results in compressive forces being directed through the instantaneous center of rotation of each motion segment

Question 24

What is the benefit of a follower load (2)?

Answer 24

1. Minimizes segmental bending moments 2. Minimizes shear forces

Question 25

What might disrupt normal follower load mechanics, resulting in increased shear forces and spinal instability (2)?

Answer 25

1. Muscle imbalance or dysfunction 2. Impaired proprioception

Question 26

Injury, degeneration, and surgery may alter what?

Answer 26

Load sharing within the spine due to abnormal motion response to loads (alters stiffness)

Question 27

Stiffness is a measure of what for the spine?

Answer 27

Stability (or instability)

Question 28

__ motion of the spine can tolerate __ loads.

Answer 28

Greater spinal motion can tolerate less loads

Question 29

Instability of the spine can be described as a loss of __ and __ mobility of a functional spinal unit.

Answer 29

Instability = loss of stiffness and increased mobility

Question 30

What are the two general types of spinal instability?

Answer 30

1. Macroinstability 2. Microinstability

Question 31

Microinstability is described as __, while macroinstability is described as __.

Answer 31

Microinstability = degenerative changes Macroinstability = gross disruption (eg. fracture, dislocation)

Question 32

Describe how disc degeneration leads to spinal instability.

Answer 32

1. Vasculature regresses around the nucleus pulposus leading to decreased proteoglycan synthesis. (disc dehydration) 2. This reduces the capacity to generate fluid pressure in response to compressive loads and loads get transferred asymmetrically. 3. Asymmetrical loading creates compression of anterior end plates leading to fractures while strain on posterior endplates leads to osteophyte formation.

Question 33

Describe initial changes in spinal stability due to degeneration (microinstability).

Answer 33

1. Initially, increased segmental motion/decreased stiffness (slack across a greater range of motion) 2. Decreased disc height and increased mobility leads to facet joint degeneration, increasing tension on joint capsules and increasing contact pressures which erodes articular cartilage, possibly leading to subluxation.

Question 34

Describe chronic changes in spinal stability due to degeneration (microinstability).

Answer 34

1. Remodeling occurs to reduce pain associated with increased mobility (increased instability) during initial changes. Includes stenosis (bony hypertrophy) and osteophyte formation. 2. Remodeling mitigates the hypermobility but also produces functional limitations. 3. May impinge neural elements (central canal and lateral neural foramen).

Question 35

What model is used to assess macroinstability and what is the threshold for an unstable spine?

Answer 35

Denis three column model 2+ columns and/or the posterior longitudinal ligament disrupted indicates unstable spine

Question 36

A spine classified as unstable is at a high risk for neuro damage due to what?

Answer 36

Excessive motion under load

Question 37

What are the drawbacks to decompressive procedures (eg. discectomy, facetectomy, foraminotomy)?

Answer 37

Procedures eliminate the homeostatic changes that occurred to increase stability. Removal ends up increasing segmental mobility and decreasing stability again which makes the functional spinal unit more susceptible to further degenerative changes.

Question 38

Patients who undergo spinal decompressive procedures are at risk for what?

Answer 38

Spondylolysis and spondylolisthesis

Question 39

Why are patients who undergo spinal decompressive procedures more at risk for spondylolysis and spondylolisthesis?

Answer 39

Reduced shear stiffness of spinal segments, low bone mineral density, and lack of stabilizing osteophytes

Question 40

Partial discectomy / removal or annulus due to disc herniation has what effect on motion?

Answer 40

Increased angular motion in all planes, especially rotation

Question 41

What is the goal of spinal fusion?

Answer 41

Restore stability of the anterior (interbody fusion) or posterior (posteriolateral fusion) column of the spine

Question 42

What options can provide stability to fused vertebrae in the perioperative period?

Answer 42

1. Hardware (eg. plates, rods) 2. Orthoses

Question 43

Describe how motion can affect bony fusion.

Answer 43

Too much stiffness (immobilization) impedes fusion due to lack of bone stimulation while too much motion doesn’t allow for healing/integration

Question 44

Describe how orthoses and instrumentation stabilize a spine after fusion.

Answer 44

1. Orthoses reduce segmental loading, shear, and strain. 2. Hardware increases rigidity of segments while reducing motion. (Rigidity depends on size/shape, number of vertebrae, and cross links)