ME 518
Lecture
7- Soft Tissues
TOPICS
Soft Tissues
- Besides the orthopedic soft tissues of ligaments, tendons,
and cartilage, a number of other soft tissues exist in the body
- These can be divided into collagen-rich tissues, elastic tissues,
and others
- The main function of collagen-rich tissues is load bearing
Other Soft Tissues
- The main function of elastic tissues is restoration of the
deformed state with minimum energy loss
- Examples of other tissues
- Ocular lens - epithelial cells
- Breast - fatty tissue
- Organs - various tissue structures, including muscle and specialized
cells
Comparison
of Behavior - Collagen and Elastic
- Collagen is a viscoelastic tissue
- Moderate hysteresis and stress relaxation
- High modulus at small elongations
- Characterized by tendon measurements
- Elastin is almost perfectly elastic
- Very small hysteresis and stress relaxation
- Low modulus
- Almost linear behavior
- Characterized by measurements on ligamentum nuchae from which
collagen has been removed
- See Figure 1
Skin
- Largest organ, by mass, in the body which acts to contain
all other structures
- Felt-like structure with a high proportion collagen fibres
randomly arranged in layers (See Figure 2)
- Largest organ, by mass, in the body which acts to contain
all other structures
- The mechanical behavior of skin is anisotropic, despite the
random arrangement of the collagen fibres
- Directional orientation of anisotropic properties is generally
repeatable between individuals for various locations on the human
body
- Deformation behavior of skin (See Figure 3)
- Skin is extremely extensible under small loads despite the
stiffness of collagen - why?
- Under high stresses, the modulus of elasticity approaches
that of tendon due to the collagen content
- Skin not only performs load bearing functions, it also:
- Skin is comprised of two layers - the inner dermis and the
outer epidermis
Injuries
to the Skin
- Burns
- Burns result in direct damage to skin cells
- Blistering occurs when plasma, the fluid portion of blood,
collects between the dermis and the epidermis in the damaged area
- If burns damage only the epidermis and the superficial portion
of the dermis, skin can regenerate completely
- However, if damage extends below the superficial layer of
the dermis then regrowth of the epidermis can only occur at the
perimeter of the burn. This is when a skin graft is required.
- Incisions and Lacerations
- Incision - cut
- Laceration - tear
- Result in a separation of skin tissue
- Generally heal by having the epidermis grow down into the
cleft and then fill in the space, with dermis and epidermis regenerating
- Involves deposition of substantial amounts of collagen
- Scars are formed by oriented, layered collagen which no longer
has the extension properties of healthy skin
Blood Vessels
- One of the most important elastic tissues
- Blood vessel walls consist of three layers (See Figure 4)
- Intima - longitudinally oriented structure
- Media - circumferentially oriented structure
- Adventitia - outer layer connected to surrounding tissue with
fascia (general connective tissue)
- The thickness and material components of each layer depends
on the type of blood vessel and its function
- Types of blood vessles
- Arteries
- Elastic arteries - maintain blood pressure between heart beats
- Distribution arteries - distribute blood throughout body
- Arterioles - transport blood to capillary beds
- Capillaries (See Figure 5)
- Veins
- Venules - collect blood from capillary beds
- Small and medium veins - transport blood from tissues
- Large veins - return blood to heart
- The major components of vessel walls are:
- Smooth muscle cells
- Elastin
- Collagen
- The proportion of each material depends on the type of vessel
- Ratio of collagen to elastin
- Elastic arteries - 1:2
- Distribution arteries - 2:1
- Veins - 3:1
- Blood vessels are lined with endothelial cells to provide
a smooth surface
- Comparison of veins and arteries
Arteries
- Carry blood from the heart to the tissues
- Elastic arteries -
- Largest arteries and closest to the heart (in network)
- Act to maintain blood pressure between heart beats
- Contain a large amount of elastin - how does this affect function?
- Substantial amount of collagen in the adventitia (outer layer)
may prevent the vessel from becoming overdilated on filling
- Distributing arteries -
- Large to medium sized arteries
- Contain a large amount of smooth muscle cells to control distribution
of blood to tissues - how does this affect function?
- Smooth muscle cells are arranged helically around the circumference
of the vessel in the media and longitudinally in the intima
Veins
- Venules
- Venules and the smallest veins (collecting veins) contain
increasing amounts of smooth muscle with increasing vessel diameter
- Small Veins
- Media consists mostly of a layer of smooth muscle fibres
- Intima consists mostly of endothelial cells
- Large veins
- Contain relatively little smooth muscle and instead has substantial
amounts of collagen with
some elastin
- How does this affect vasodilation in large vessels?
- Veins contain about 75 % of the blood supply at any one time
- The muscular control of vasodilation is very important in
determining such parameters as
cardiac output
- Blood which remains in the veins when they have dilated is
not returned to the heart as rapidly for recirculation
- Veins traveling through muscular regions of the body also
respond to skeletal muscle contraction which acts to pump blood
towards the heart
Mechanical
Properties of Blood Vessels
- Behave viscoelastically
- Elastic and viscoelastic properties of arteries vary along
the length of the arterial tree (See Figure 8)
- Composition and structural arrangement of fibres in walls
also changes along arterial tree
- Vessel walls are anisotropic with longitudinal and radial
deformation behavior differing significantly
- Due to the different arrangement of longitudinally, circumferentially,
and helically arranged fibres
- Blood vessels react under stress with a combination of the
behaviors of collagen and elastin depending on the degree of stretch
(See Figure 9)
- Smooth muscle contribution
- Smooth muscle is extremely deformable under load and does
not increase the resistance of the tissue to stretching
- Passive smooth muscle is has a high degree of viscoelasticity
and exhibits almost complete stress relaxation
- Active smooth muscle can resist deformation or further constrict
vessels, but with the use of energy
- The behavior of elastin dominates at low stresses and strains,
while at higher levels of deformation the collagen dominates and
vessels become much stiffer (See Figure 10)
Atherosclerosis
- Thickening and hardening of arterial walls characterized by
formation of plaque-like lesions
- Main underlying cause of
- Ischemic heart disease (heart attacks)
- Cerebrovascular disease (strokes)
- Aortic aneurysms (See Figure 11)
- Damage to the intima results in fatty deposits which then
become calcified and damage and weaken the entire vessel wall
- The surface of the plaque can cause clots to form which are
able to block arteries and result in tissue ischemia
- The plaques can also grow to such an extent that they close
of the passage through the vessel
- Primarily affects elastic and distributing arteries
Heart Valves
- The heart contains four valves, at the entrance and exit of
each of the two ventricles
- The valves consist of two or three leaflets of connective
tissue consisting mostly of elastin fibres attached to a fibrous
ring surrounding the orifice
- Diseases such as rhumatic fever can cause the leaflets to
become inflamed
- Healing of the leaflets involves deposition of substantial
collagen which changes the mechanical properties
- Valve leaflets become stiffer and shorter
- Resulting Problems:
- Inlet and outlet valves differ in several ways:
- Inlet (tricuspid and mitral valves) -
- Must withstand the full pressure of heart contraction without
turning inside-out (umbrella effect)
- Each leaflet is supported by chordae tendinae, cords of connective
tissue which connect the valve to the inside surface of the heart
(See Figure 12)
- Outlet (aortic and pulmonary valves) -
- Thinner leaflets than inlet valves
- Do not have to withstand pressure of heart contraction
- Valves open during contraction No chordae tendinae present
Muscle
- Three types of muscle exist
- Skeletal muscle - voluntary muscle including all muscles which
cause movement of the body
- Smooth muscle - involuntary muscle which lines organs such
as blood vessels and the
digestive tract
- Cardiac muscle - muscle of the heart
- Of most interest from a biomaterials standpoint, with the
possibility of replacement or interaction, is skeletal muscle
- All muscles are active tissues and have different properties
when they are and are not stimulated
- Muscle consists solely of muscle cells and intervening connective
tissue
- Cells contract individually as a result of electrical stimulation,
generally from the nerves
- The number of cells activated depends on the level of electrical
stimulation
- Contraction strength can be increased by
- Increasing the frequency of stimulation for a single muscle
cell to more than 10/sec
- In skeletal muscle, once a critical frequency is reached the
contractions cannot be distinguished from one another and the
muscle is tetanized - generates a maximal tension that is constant
in time
- Recruiting more muscle cells into the contracted state
Mechanical Behavior
of Skeletal Muscle
- Passive muscle behaves as a viscoelastic material with low
modulus
- The stress-strain relationship depends on the rate of loading
- Muscle will absorb energy during passive deformation
- Reduces the energy transmitted to other structures
- Actively contracting muscle has high stiffness and the ability
to shorten itself to varying degrees
- The passive and active components of muscle have been combined
into a simplified model (See Figure 13)
- Modification of traditional mechanical model with addition
of a contractile element
- The contractile element extends freely in the passive state,
so that behavior under these conditions is defined by the parallel
viscoelastic elements
- The series elasticity results from the intrinsic elasticity
properties of the components
Mechanical Behavior
of Smooth Muscle
- An knowledge of smooth muscle function is necessary for the
understanding of structures containing this material
- Some smooth muscle (intestine) contracts spontaneously while
the muscle in other structures (blood vessels) does not
- In spontaneously contracting muscle, it is not possible to
separate the active and passive mechanics of the structure
- Contractile mechanism is not completely freed when electrical
stimulation is removed
- In smooth muscle structures that do not spontaneously contract,
it is possible to separate these functions - more like skeletal
muscle (See Figure 14)
- All smooth muscle undergoes rapid and almost complete stress
relaxation in its "passive" state
- Exhibits plastic behavior - moldable
Structures of the
Eye
- Cornea
- Outer transparent portion at front of
eye
- Comprised of connective tissue, including
collagen
- Avascular
- Receives nutrients by diffusion from interior
region of the eye
- Receives oxygen directly from the outside
environment
- How is this of interest in materials design?
- Clarity depends on constant removal of
water from corneal tissue into the eye
- Lens
- Transparent structure further within the
eye (See Figure 15)comprised
of epithelial cells
- Relatively elastic bi-convex structure
that is held in place by ligaments which control its curvature
to allow both near and distant images to focus on the retina of
the eye
- With age, the elasticity of the lens is
reduced and the ability to change the focus length of the eye
(accommodation) is impeded
- What effect does this have?
REFERENCES
- Moore, K., Clinically Oriented Anatomy. 2nd Edition,
Williams & Wilkins, Baltimore, 1985.
- Fung, Y.-C., Biomechanics: Mechanical Properties of Living
Tissues, Springer-Verlag, New York, 1981.
- Guyton, A.C., Textbook of Medical Physiology, 7th Ed.,
W.B. Saunders, Co., Philadelphia, 1986.