| Definitions |
- Functional spinal unit (FSU)
- made up of intervertebral disc, facet joints, and ligaments
- function is to provide physiologic motion and protect neural elements
- Spinal stability
- defined when, under physiologic loading, there is neither abnormal strain or excessive motion in the FSU
- maintained by
- FSU
- muscular tension
- abdominal and thoracic pressure
- rib cage support
- Sagital balance
- maintained by
- cervical lordosis
- thoracic kyphosis
- lumbar lordosis
- sacral kyphosis
|
| Cervical Kinematics |
| |
flexion/extension
|
rotation
|
lateral bending
|
| Occipitoatlantal joint |
13°
|
4°
|
8°
|
| Atlantoaxial joint |
10°/10°
|
45°
|
0°
|
| Lower cervical spine (C2/3 to C7/T1) |
C2/3 - 8°
C3/4 - 13°
C4/5 - 12°
C5/6 - 17°
C6/7 - 16°
C7/T1 - 9 °
|
50°
|
60°
|
| Total motion |
108°
|
99°
|
68°
|
- Rotation decreases caudally due to greater inclination of facet joints
- Spinous process rotates towards convexity
|
| Thoracic Kinematics |
| |
flexion/extension
|
rotation
|
lateral bending
|
| Thoracic spine |
75° (flexion > extension)
|
70°
|
75°
|
- Motion limited by facets and steep orientation of facet joints
- Rotation and lateral bending are coupled
- spinous process rotates towards convexity of curve in upper thoracic spine
- in middle and lower thoracic spine, direction of coupling is not consistent
|
| Lumbar Kinematics |
| |
flexion/extension
|
rotation
|
lateral bending
|
| Lumbar spine |
85° (flexion > extension)
|
10°
|
30°
|
- Rotation minimal due to sagital orientation of facet joints
- L5/S1 was least amount of rotation
|
| Disc Kinematics |
- Disc
- viscoelastic characteristics
- demonstrates creep (deforms) over time and hysteressis (ability to absorb energy decreases with time)
- Stresses
- nucleus pulposus
- highest compressive stresse
- annulus fibrosus
- Stability
- following subtotal discectomy, extension is most stable loading mode
- disc pressures lowest when lying supine
- disc pressures decrease when carrying objects close to body
|
| Spine Stability |
- Supporting Structures (Three coloumn theory)
- anterior column
- anterior longitudinal ligament
- anterior 2/3 of vertebral body and annulus
- middle column
- poterior 1/3 of body and anulus
- posterior longitudinal ligament
- posterior column
- ligamentum flavum
- pedicles
- facets
- spinous processes
- posterior ligaments (interspinous, supraspinous)
- Instability
- cervical facetectomy of > 50% causes loss of stability in flexion and torsion
- torsional load stability in lumbar spine made up of f
- 40% contribution from facets
- 40% contrbution from disc
- 20% resistance to torsional load
|
| Key Clinical Instability Examples |
- Occiput-C1 instability
- Basilar invagination
- Atlantooccipital dislocation
- C1/C2 Instability
- Atlantoaxial instability
|