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Questions and Answers
What percentage of compressive load bearing is assumed by the vertebral bodies of the lumbar spine?
What is the main orientation of the trabeculae in lumbar vertebrae?
What is the function of the paired pedicles in the neural arch?
What percentage of compressive load is carried by the facet joints?
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What is the shape of the vertebral foramina?
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What are the components of the functional unit or spinal motion segment?
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What is the function of the facet joints?
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What is the purpose of the paired laminae in the neural arch?
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What is the main component of the nucleus pulposus?
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During bending, what happens to the nucleus pulposus?
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What is the function of the annulus fibrosis?
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What is the main reason for the intervertebral disc's inability to tolerate torsional forces?
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What is a risk factor for injuring the intervertebral disc during lumbar flexion?
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What is the purpose of the vertebral end plate?
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Study Notes
Lumbar Spine Biomechanics
Vertebral Body
- Designed to support mainly compressive loads
- Composed of a thin shell of cortical bone surrounding a core of porous cancellous bone
- Cancellous bone is formed from vertical and horizontal trabeculae
- Not solidly filled with bone, allowing for 'springy' and 'cushion' effects when loaded suddenly
Neural Arch
- Paired pedicles formed from cortical bone to resist tension and compression during spinal movements
- Paired laminae that enclose the posterior portion of the vertebral foramen
- Give rise to the spinous process
- Superior and inferior articular processes resist lumbar rotation and protect the intervertebral disc from excessive torsional stress
Facet Joint
- Guides motion between adjacent vertebrae
- Orientation changes between spinal segments
- Shares load transmitted between vertebrae with the intervertebral disc, carrying 18-20% of compressive load
Vertebral Foramina
- Typically triangular, bordered by vertebral body, intervertebral disc, pedicle, and lamina
- Functions as a passageway for spinal nerves
Functional Unit or Spinal Motion Segment
- Components:
- Two adjacent vertebrae
- Intervertebral disc
- Associated ligaments
Intervertebral Disc (IVD)
- A viscoelastic material composed of:
- Annulus fibrosis (10-20 layers of collagen fibers)
- Nucleus pulposus (mucopolysaccharide gel with 70-90% water content)
- Vertebral end plate connects the disc to adjacent vertebrae
Mechanical Properties
- During compression: Hoop phenomena with radial expansion
- During bending: Nucleus pulposus deforms in the opposite direction of motion
- During rotation: Disc is less able to tolerate torsional forces, designed to resist compressive loads
Clinical Relevance
- Smoking, exposure to vibration, and prolonged immobilization can impair proteoglycan synthesis
- Most lifting injuries to the low back occur in the morning
- Flexion avoidance in the morning may help reduce pain
- Elevated disc fluid volume early in the morning predisposes the disc to injury during lumbar flexion
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