M4 Spinal Biomechanics Flashcards
Failure of the Anterior Fontanel occurs mostly with:
Hydrocephalus trisomy 13 Cleidocranial dysplasia Hypothyroidism Hypophosphatasia Down’s Syndrome Osteogenesis Imperfecta
3 primary functions of the vertebral column
- Support the trunk and transmit the weight to the pelvis and lower extremities.
- Protect the spinal cord and membranes
- Provide a central axis for the thorax.
4 main curvatures of the spinal column.
Cervical (Lordotic) Secondary Curve
Thoracic (Kyphotic) Primary Curve
Lumbar (Lordotic) Secondary Curve
Sacral (Kyphotic) Primary Curve
The frequency of:
Cleft Posterior arch
3% to 4%
Frequency of Cleft Anterior arch
1%
Normal variants throughout the body occur about how often?
~5%
The standard deviation of bilateral asymmetry of the vertebral column.
2.5 mm
Elongated mastoid or something covering C1 TP
15%
Mechanics -
the study of forces and their effects.
Biomechanics
is the application of mechanical laws to living structures
Kinematics
branch of mechanics that deals
with the geometry of the motion of objects,
including displacement, velocity, and
acceleration, without taking into account the forces that produce the motion.
Kinetics
the study of the relationships
between the force system acting on a body
and the changes it produces in body motion
Displacement
The change in position
of a body.
Linear – in one direction
Angular – multiple directions at once.
Spinning
ArcingLever
a rigid bar that pivots about a
fixed point, or axis or fulcrum, when a
force is applied to it.
Force (or resistance)
applied by muscles at some point along the lever to move the body part.
A push or pull exerted on a body
producing acceleration.
Newton’s second law, a force acting on a
the body causes an acceleration in the direction
of the force.
Velocity
The change in position over
time.
Includes magnitude and direction.
Acceleration
The change in velocity
over time. (m/s2)
Can be constant, increase or decrease.
Negative acceleration = deceleration.
Mass
The quantity of matter within a
given object.
Intertia
The property of a body to remain at rest, or in a uniform motion unless acted upon by another force. -Newton’s first law. -The mass of a body determines the magnitude of this resistance.
Mass moment of Inertia
Rotating bodies that move at a constant angular velocity. Bodies at rest have a fixed axis. These bodies tend to stay at rest or in motion unless acted upon by an external force. Resistance to change is determined by the mass of the body.
Momentum
Mass x velocity An amount of motion Increasing the mass of the body or the velocity will increase the momentum “Moment of force” The product of force and distance through which the force acts. Example is using a wrench. A twisting around an axis of rotation.
Center of Mass
the point at which the
entire mass of the body is equally
distributed.
Often termed “center of gravity.”
Work
Force x displacement
Force acting over a distance.
Impulse
The force which two colliding
bodies exert on each other.
First Class Lever
the axis (fulcrum) is located between the force and the resistance, like a teeter-totter.
Second Class Lever
the resistance is between the axis and the force. Wheelbarrow.
Third Class Lever
the force is between the axis and the resistance. snow shovel
X-axis
Coronal (flexion and extension)
Y-axis
Longitudinal (axial rotation)
Z-axis
sagittal (lateral flexion)
Cartesian coordinate system hand
right hand
Sagittal Plane
Y and Z axes
Horizontal Plane
X and Z axes
Frontal Plane
X and Y axes.
X-axis rotation
Flexion = +OX
Extension = -OX
Sagittal plane
Y-axis rotation
Right rotation = -OY
Left rotation = +OY
Horizontal plane
Z-axis rotation
Right lateral bend = +OZ
Left lateral bend = -OZ
Vertical plane
Coronal Axis
X-Axis
Flexion and Extension through the sagittal plane
Flexion is motion in the anterior direction for
joints of the head, neck, trunk, upper
extremity, and hips
Flexion is motion in the posterior direction for
joints of the of the knee, ankle, foot, and toes
Sagittal Axis
z-axis
Movements of abduction and adduction of the extremities, as well as lateral flexion of the spine, occur around this axis and through the coronal plane.
Lateral flexion is a rotational movement and is used to denote lateral movements of the head, neck, and trunk in the coronal plane.
Abduction and adduction are also motions in a coronal plane.
Sagittal definition
Latin for “like an arrow” as in the spine
Lateral flexion
a rotational movement and is used to denote lateral movements of the head, neck, and trunk in the coronal plane. usually combined with some element of rotation.
Longitudinal Axis
Y-axis
This axis is vertical, extending in a head-to-toe direction.
Movements of the medial (internal) and lateral (external)
rotation in the extremities, as well as axial rotation in the spine, occur around it and through the transverse plane.
Axial rotation is used to describe this type of movement
for all areas of the body except the scapula and clavicle.
Rotation
Rotation occurs about an anatomic axis.
In the human extremity, the anterior surface of the
extremity is used as a reference area.
Rotation of the anterior surface toward the midsagittal
plane of the body is medial (internal) rotation, and
rotation away from the midsagittal plane is lateral
(external) rotation.
Rotation of the head, spine, and pelvis is
described as rotation of the anterior surface
posteriorly toward the right or left.
Rotation of the scapula is movement about a
sagittal axis, rather than about a longitudinal
axis.
Because the head, neck, thorax, and pelvis
rotate about longitudinal axes in the
midsagittal area, rotation cannot be named in
reference to the midsagittal plane.
The terms clockwise or counterclockwise are used.
Translational movements
Lateral-to-Medial glide and Medial-to-Lateral glide
(laterolisthesis) translate along the x-axis.
Distraction and Compression translate along the y-
axis.
Curvilinear motion combines both rotational and
translational movements and is the most common
motion produced by the joints of the body.
joint movement
The potential exists for each joint to exhibit three
translational movements and three rotational
movements, constituting 6 degrees of freedom.
The extent of each movement is based more or less
on the joint anatomy and, specifically, the plane of the
joint surface.
synovial joints
The most common joints of the human appendicular skeleton. The components of a typical synovial joint include bony elements, subchondral bone, articular cartilage, synovial membrane, fibroligamentous joint capsule, articular joint receptors.
Joint types of the spine
Synarthrotic Symphysis—fibrocartilage Intervertebral discs Diarthrotic Trochoid (pivot) Atlantoaxial joint Plane (nonaxial) Posterior facet joints in the spine
Boney elements of the joints function
The bony elements provide the
supporting structure that gives the joint
its capabilities and individual
characteristics by forming lever arms to
which intrinsic and extrinsic forces are
applied.
Articular Cartilage
Articular cartilage covers the articulating bones
in synovial joints and helps to transmit loads
and reduce friction.
It is bonded tightly to the subchondral bone
through the zone of calcification,
the end of bone visible on x-ray film.
The joint space visible on x-ray film is
composed of the synovial cavity and non-
calcified articular cartilage.
Articular cartilage composition
In its normal composition, articular cartilage has four
histologic areas or zones.
Gliding zone
Transitional zone
Radial Zone
Zone of calcified cartilage
The outermost zone of cartilage is known as the
gliding zone, which itself contains a superficial layer
(outer) and a tangential layer (inner).
The gliding zone also has a role in protecting the deeper elastic cartilage.
Gliding Zone of cartilage
contains a superficial layer
(outer) and a tangential layer (inner).
The gliding zone also has a role in protecting the deeper elastic cartilage.
The outer segment is made up solely of collagen
randomly oriented into flat bundles.
The tangential layer consists of densely packed layers
of collagen, which are oriented parallel to the surface
of the joint.
This orientation is along the lines of the joint motion, which
implies that the outer layers of collagen are stronger when
forces are applied parallel to the joint motion rather than
perpendicular to it.
Providing a great deal of strength to the joint in normal motion.
Transitional zone of cartilage
lies beneath the gliding zone.
It represents an area where the orientation of
the fibers begin to change from the parallel
orientation of the gliding zone to the more
perpendicular orientation of the Radial Zone.
Zone of calcified cartilage
where the articular cartilage meets the Subchondral plate
Articular cartilage nutrition
Articular cartilage is considered mostly
avascular.
Articular cartilage must rely on other sources
for nutrition, removal of waste products, and
the process of repair.
Therefore intermittent compression (loading)
and distraction (unloading) are necessary for
adequate exchange of nutrients and waste
products.
The highly vascularized synovium is
believed to be a critical source of
nutrition for the articular cartilage it
covers.
The avascular nature of articular
cartilage limits the potential for cartilage
repair by limiting the availability of the
repair products on which healing
depends.
Degeneration of the articular cartilage
depends on:
the size and depth of the lesion the integrity of the surrounding articular surface the age and weight of the patient associated meniscal and ligamentous lesions other biomechanical factors
Continuous passive motion and articular cartilage injury
Continuous passive motion has increased the ability of full-thickness defects in articular cartilage to heal, producing tissue that closely resembles hyaline cartilage.
ligamentous elements of spinal synovial joints
The primary ligamentous structure of a
synovial joint is the joint capsule.
Throughout the vertebral column, the joint
capsules are thin and loose.
The capsules are attached to the opposed
superior and inferior articular facets of
adjacent vertebrae.
spinal joint capsules have how many layers
Outer layer - composed of dense fibroelastic connective tissue made up of parallel bundles of collagen fibers. Middle layer - composed of loose connective tissue and areolar tissue containing vascular structures. Inner layer - consists of the synovial membrane.
synovial fluid
The exact role of synovial fluid is unknown,
it is thought to serve as a joint lubricant or at least to interact
with the articular cartilage to decrease friction between joint
surfaces.
This is of clinical relevance because immobilized joints
have been shown to undergo degeneration of the
articular cartilage.
Synovial fluid is similar in composition to plasma, with
the addition of mucin (hyaluronic acid), which gives it a
high molecular weight and its characteristic viscosity.
3 models of joint lubrication
The Hydrodynamic Model
The Elastohydrodynamic Model
The Boundary Lubrication Model
The Hydrodynamic Model of joint lubrication
Synovial fluid fills in spaces left by the
incongruent joint surfaces.
During joint movement, synovial fluid is
attracted to the area of contact between the
joint surfaces, resulting in the maintenance
of a fluid film between moving surfaces.
This model was the first to be described and
works well with quick movement.
The Elastohydrodynamic Model of joint lubrication
Considers the viscoelastic properties of articular
cartilage where deformation of joint surfaces occurs
with loading, creating increased contact between
surfaces.
This would effectively reduce the compression
stress to the lubrication fluid.
Although this model allows for loading forces, it
does not explain lubrication at the initiation of
movement or the period of relative zero velocity
during reciprocating movements.
The Boundary Lubrication Model of joint lubrication
Here, lubricant is adsorbed on the joint surface,
which would reduce the roughness of the surface
by filling the irregularities and effectively coating the
joint surface.
This model allows for initial movement and zero
velocity movements.
The boundary lubrication model, combined with the
elastohydrodynamic model, create a mixed model,
which meets the demands of the human synovial
joint
Elasticity –
The tendency of tissue, under
load, to return to its original size and
shape after removal of the load.
No energy is lost to deformation.
Plasticity –
the property of a material that
instantly deforms when a load is applied to
it and does not return to its original shape
when removing the load.
Viscosity –
The property of a material that
does not deform instantaneously when a
load is applied.
Stress will develop but the deformation is
delayed.
Deformation is, therefore, relative to time.
Viscoelasticity –
A combination of viscosity
and elasticity.
The property of a material to deform slowly
and nonlinearly when a load is applied.
Also, the property of the material to return to
its original shape and size, slowly and
nonlinearly when the load is removed.
Examples include articular cartilage and
interverterbral disks.
Creep
When a constant load is applied to a
ligament, it will first elongate to a given length.
Left at a constant load, it will continue to
elongate over time in an exponential fashion up
to a finite maximum.
Creep is this elongation over time.
Expressed as the percent elongation relative to its
length immediately after the load was applied.
An increase in strain that occurs during a constant
stress from loading.
A body undergoing creep may or may not
return to its original shape.
Returning to its original shape will depend on
the load, and whether the structure underload
is damaged.
A damaged disk can deform faster under load than
a normal intervertebral disk.
tension–relaxation phenomena
observed when ligaments are subjected to a stretch and hold overtime The tension in the ligament increases immediately upon the elongation to a given value. As time elapses, the tension decreases exponentially to a finite minimum while the length does not change.
strain rate
Strain Rate - The tension developed in a ligament also
depends on the rate of elongation or Strain Rate.
Slow rates of elongation are associated with the
development of relatively low tension,
where as higher rates of elongation result in the development of
high tension.
The fast stretch of ligaments, such as in high-frequency
repetitive motion (sports activities) are known to result in
high incidents of ligamentous damage or rupture.
Fast rates of stretch, may exceed the physiological loads
that could be sustained by a ligament safely, while still
within the physiological length range.
Hysteresis -
The inability to track the same length–
tension curve when subjected to a single stretch–release
or load–unload cycle, is termed Hysteresis.
Hysteresis is also associated with repetitive motion when
a series of stretch–release cycles are performed
overtime.
When the ligament is stimulated repetitively with
constant peak load, the hysteresis develops along the
length axis,
i.e., the ligament length limits increase with each cycle reflecting
the hysteresis associated with the development of creep
Conversely, when cycles of constant peak
stretch are applied, the peak tension decreases
in sequential cycles, reflecting the on going
development of tension–relaxation.
The impact of progressive hysteresis, is
manifested by:
gradually decreasing tension in the ligament,
development of joint laxity,
reduced joint stability
increased risk of injury.
Clinical:
Repetitive sports and occupational tasks should be
limited in duration and allow sufficient rest periods to
facilitate recovery of normal ligament function.
Frequency of Cyclic Motion
Ligament behavior is also dependent on
the frequency of load application and
unloading.
Cyclic loading of a ligament with the same
peak load, but at a higher frequency,
results in larger creep development and
longer period of rest required for the full
recovery of the creep
Occupational and sports tasks requiring
repetitive motion at high frequency, and
induce larger creep in the ligaments.
This requires longer rest time to recover, and
may increase the risk for cumulative creep
from one session to the next, in the same day
and from day-to-day.
Larger creep results in increased laxity of
the joint as the activity goes on, and the
associated risks as discussed above.
Ligaments as sensory organs
Anatomical studies demonstrate ligaments in the
extremity joints and the spine are endowed with
mechanoreceptors consisting of:
Pancinian,
Golgi,
Ruffini
bare nerve endings
Flexion and extension at the atlantoaxial
articulation are limited by
the transverse ligament and tectorial membrane, respectively.
Lateral bending at the atlantoaxial articulation is restricted by
the contralateral alar ligament and some very minimal anteroposterior translation may occur at this joint.
AO flexion limitation
Flexion was limited by
impingement of the odontoid
process on the foramen magnum
AO extension limitation
tectorial membrane
AO lateral flexion limitation
Contralateral alar ligament.
In vivo –
experimentation relating to the
study of the whole living subject in a
natural environment.
In vitro (ex vivo) -
experimentation relating
to the study of the whole living subject
outside its natural environment.
In silico –
experimental studies which
simulate the living system.
Osteokinematic Movement-
The physiologic movement
which occurs at the joint when muscles contract or when
gravity acts on bone to cause motion.
describes how each bony joint partner moves relative to the
other.
Arthrokinematic Movement-
The specific movements
that occur at the articulating joint surfaces.
considers the forces applied to the joint
include the accessory motion present in a particular articulation
(coupled motion)
Instantaneous Axis of Rotation (IAR)-
Term denotes the location point of the axis around
which motion occurs
In addition, when an object moves, the axis around
which the movement occurs can vary in placement
from one instant to another.
Asymmetric forces applied to the joint can cause a
shift in the normal IAR.
This concept is designed to describe plane
movement, or movement in two dimensions.
Helical Axis of Motion (HAM)-
The axis of motion when three-dimensional motion
occurs between objects
a screw axis of motion
“The nature and extent of individual joint motion
are determined by the joint structure and,
specifically, by the shape and direction of the
joint surfaces.”
Harmony Medical
“Joint play”
is an accessory movement of the
joint that is essential for normal functioning of
the joint. Present in open-packed position.
Resting Position of a joint (Neutral Position)-
occurs when the joint capsule is most relaxed and the
greatest amount of play is possible.
When injured, the joint will move to its maximum
loose-packed position to allow for swelling.
Close-Packed Position-
when the joint capsule and ligaments are maximally
tightened.
In the Close-Packed Position, there is maximal
contact between the articular surfaces, making the
joint very stable and difficult to move or separate.
Compression-
occurs when a joint moves
toward its close-packed position.
The spine is more
susceptible to compressive load injury.
Distraction-
occurs when a joint moves toward its open-packed position. Distractive and Tensile Loading injuries are less common but do occur during whiplash type injuries.
Flexion –
COMPRESSION of anterior
structures and TENSILE LOADING of
posterior structures.
Extension -
COMPRESSION of posterior
structures and TENSILE LOADING of
anterior structures.
Rotation (Torsional Loading) –
occurs
when the body of a moves in concentric
circles or an arc.
Rotation is potentially more damaging to the
vertebra because it involves shearing, tensile
and compressive forces combined with
rotation.
Stress –
measured per unit area. Force
involves internal stress within the body
that arises as a result of external loads
applied to the body.
Most common place for a compression fracture
T11-T12
Hooke’s Law –
deformation of a body
increases in proportion to the load that is
applied.
Strain increases in proportion to the body’s
internal stress that is resisting the applies
load.
Functional Spinal Unit –
Two adjacent
vertebrae and the joint that links them,
with the skeletal muscle that moves the
articulation.
Joint motion consists of five qualities of
movement that must be present for normal
joint function.
Joint Play Active Range of Motion Passive Range of Motion End Feel Paraphysiologic Movement
Paraphysiologic Movement
is the small amount of
movement past the elastic barrier
typically occurs after cavitations
Movement of the joint beyond the Paraphysiologic
Barrier takes the joint beyond its limit of anatomic
integrity and into a Pathologic Zone of Movement.
When the joint enters the pathologic zone, there is
damage to the joint structures, including osseous and
soft tissue
The individual coupled motions are governed by
the architecture of the vertebrae (smooth,
rough, etc.),
their joint surface inclination,
the associated ligaments,
the interactive functioning of the paraspinal
muscles
and the physiologic anteroposterior curvature
of the spine in the sagittal plane.
Bogduk and Mercer describe the function of
the cervical anatomy as follows:
AnatomicalFunctional Atlas, Cradle The Axis, Axis The C2-3 Junction, The Root Typical Cervical Vertebrae,The Column
The Cradle
The atlas vertebra serves to cradle the occiput.
The articulation consists of the occipital condyles
joining with the superior articular surface of the
atlas lateral masses.
The articulation of the atlanto-occipital joints is
strong, and allows mainly for the nodding
movements between the two structures.
In all other respects the head and atlas move
and function essentially as one unit.
The stability of the atlanto-occipital joint comes mainly
from the depth of the atlantal lateral mass superior articular surfaces.
The sidewalls of the lateral mass prevent the occiput from sliding sideways;
The front and back walls prevent anterior and posterior
gliding of the head
The only physiological movements possible at this
joint are Flexion and Extension
Occ-C1 Axial Rotation
Axial rotation and lateral flexion are not physiological movements of the atlanto-occipital joints.
They cannot be produced in isolation by the action of
muscles. But they can be produced artificially by forcing the head into these directions while fixing the atlas.
Axial rotation is prohibited by impaction of the
contralateral condyle against the anterior wall of its
socket and simultaneously by impaction of the ipsilateral
condyle against the posterior wall of its socket.
For the head to rotate, the condyles must rise up their
respective walls.
Paradoxical Tilt of Atlas
Occiput and dorsal part of Atlas Arch approach each other rather than moving away from one another at the end of flexion • Reverse is also true on extension
AO flexion and extension degree of motion
Flexion CO/C1 (3.5 degrees)
Extension CO/C1 (21.0 degrees)
Flexion of C0-C2 Stage 1
Forward movement of occiput in relation to atlas 8
degrees (nutation) (+OX)
All other segments are neutral
Flexion of C0-C2 Stage 2
C1-C2 tilt forward (+OX)
C2-C3 to C6-C7 undergo flexion
Axis tilts forward 45 degrees with respect to C7
Occ-C1 moves into extension (-ox), preventing
an abnormal position of the spinal cord.
AO rotation degrees
Dvorak reports occipitoatlantal rotation around
the x-axis between 13-50 degrees.
AO lateral flexion
takes place around
the sagittal axis
Amounts to approximately 5 deg.
(Penjabi et al, 1988; White and Panjabi, 1990; Penning, 1976)
Greater when the head is slightly flexed
Resisted by the alar ligaments
Mean maximum lateral bending of the cervical
spine to one side was 1.6oto 5.7o at each
level.
AO rotation
Axial rotation has been reported to be between 2.4°– 8°
that can take place at this joint, as well as minimal lateral
and axial rotation and anteroposterior translation. (N.
Bogduk, S. Mercer)
The lower levels were reported from live patients
The higher levels were reported from cadaver studies
Authors such as Panjabi, White Penning and Fielding
have reported axial rotation between the occiput and
atlas to be nonexistent.
Depreux and Mestdagh (1974) report approximately 5o
of motion.
Measurements are higher with atlantoaxial fussion.
Gutmann (1981) reports the occiput and atlas will rotate
together, with respect to the axis.
Dvorak and Hyek (1986), using cadaver spines recorded
axial rotation between 4.5oand 5.9oto the right and left
respectively.
Atlanto-axial joint cartilage structure
The articular cartilages of both the atlas
and the axis joint are convex, thus, the
articulation is biconvex.
The spaces formed anteriorly and
posteriorly, where the articular surfaces
diverge, are filled by intra-articular
meniscoids.
Explanation for Atlas paradoxical motion at end range flexion and extension
At full flexion of the neck, the atlas can extend.
This arises because the atlas, rests between the head and axis, and is balanced on the summits of the lateral atlantoaxial facets, and thus is subject to compression loads.
If the net compression passes anterior to the contact point in the lateral atlantoaxial joint, the lateral mass of the atlas will be squeezed into flexion.
Conversely, if the line of compression passes behind the contact point, the atlas will extend; even if the rest of the cervical spine flexes.
If during flexion, the chin is tucked backwards, the paradoxical extension of the atlas is virtually assured, because the retraction of the chin favors the line of weight-bearing of the skull to fall behind the center of the lateral atlantoaxial joints.
