Biomechanics Flashcards
Biomechanical functions of the spine (2)
- Protect the spinal cord while allowing mobility 2. Support loads during ADL’s
Biomechanical function of spinal orthoses (2)
- Protect spine in cases of instability 2. Facilitate healing Via restriction of gross triplanes motion which limits compression, strain, and shear
The motion segment / functional spinal unit is the smallest functional unit composed of _ with motion in _ planes.
Composed of a 3 joint complex which moves in three planes
Three joints of a FSU
- Vertebral bodies-intervertebral disc 2 & 3. Posterior facet joints
Compressive loads are directed on which structures and in which direction?
Directed axially through vertebral bodies, discs, and facet joints
Strain affects which structures?
Discs, ligaments
Shear affects which structures?
The joint line (disc-vertebral body endplate)
Gravity, external forces and moments, and muscle tension generates which type of force?
Compression
Axial rotation, flexion and extension, and lateral bending generate which type of force?
Strain
Flexion/extension and lateral bending generate which type of force?
Shear
What is the major anterior load bearing element and the major posterior load bearing element?
Anterior: intervertebral disc Posterior: facet joints
Flexion relieve compressive loading on which structures?
Facet joints
Extension relieves compressive loading on which structures?
Vertebral bodies and discs
Describe how the intervertebral discs maintain even pressure on the inferior endplate during loading?
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
What can amplify compressive loading of the spine?
Muscle tension which is generated to counter the torque created by a carried load
Function of elastic restraints
Maintain physiologic range of motion of the intervertebral joint
What is the benefit of annulus fibrosus fibers crossing at 30°?
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)
Which motions produce maximum strain and shear? At what location does this occur?
Combination of flexion and lateral bending At the posterior inferior vertebral endplate and posterolateral disc
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?
Axial rotation
Where are shear forces generated within the spine?
Along the joint line
Shear forces are resisted passively by which structures (2)?
Shear is resisted passively by: 1. Annulus fibrosus 2. Vertebral endplate
Shear is resisted actively by what?
Shear is resisted actively by muscle activation
Describe follower load mechanics.
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
What is the benefit of a follower load (2)?
- Minimizes segmental bending moments 2. Minimizes shear forces
What might disrupt normal follower load mechanics, resulting in increased shear forces and spinal instability (2)?
- Muscle imbalance or dysfunction 2. Impaired proprioception
Injury, degeneration, and surgery may alter what?
Load sharing within the spine due to abnormal motion response to loads (alters stiffness)
Stiffness is a measure of what for the spine?
Stability (or instability)
__ motion of the spine can tolerate __ loads.
Greater spinal motion can tolerate less loads
Instability of the spine can be described as a loss of __ and __ mobility of a functional spinal unit.
Instability = loss of stiffness and increased mobility
What are the two general types of spinal instability?
- Macroinstability 2. Microinstability
Microinstability is described as __, while macroinstability is described as __.
Microinstability = degenerative changes Macroinstability = gross disruption (eg. fracture, dislocation)
Describe how disc degeneration leads to spinal instability.
- 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.
Describe initial changes in spinal stability due to degeneration (microinstability).
- 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.
Describe chronic changes in spinal stability due to degeneration (microinstability).
- 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).
What model is used to assess macroinstability and what is the threshold for an unstable spine?
Denis three column model 2+ columns and/or the posterior longitudinal ligament disrupted indicates unstable spine
A spine classified as unstable is at a high risk for neuro damage due to what?
Excessive motion under load
What are the drawbacks to decompressive procedures (eg. discectomy, facetectomy, foraminotomy)?
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.
Patients who undergo spinal decompressive procedures are at risk for what?
Spondylolysis and spondylolisthesis
Why are patients who undergo spinal decompressive procedures more at risk for spondylolysis and spondylolisthesis?
Reduced shear stiffness of spinal segments, low bone mineral density, and lack of stabilizing osteophytes
Partial discectomy / removal or annulus due to disc herniation has what effect on motion?
Increased angular motion in all planes, especially rotation
What is the goal of spinal fusion?
Restore stability of the anterior (interbody fusion) or posterior (posteriolateral fusion) column of the spine
What options can provide stability to fused vertebrae in the perioperative period?
- Hardware (eg. plates, rods) 2. Orthoses
Describe how motion can affect bony fusion.
Too much stiffness (immobilization) impedes fusion due to lack of bone stimulation while too much motion doesn’t allow for healing/integration
Describe how orthoses and instrumentation stabilize a spine after fusion.
- 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)
