DPT 530: Exam 2: Material Properties (Bone, Tendon/Ligament, Skin/Nerve, Articular Cartilage and Joints

Tissue Loading

Load: an externally applied force acting on a tissue

When a load is applied to a tissue, 2 things happen:

-Deformation (strain)

-Internal forces develop in tissue to resist deformation (stress)

When loads become excessive-> injury (failure) can occur

Tension (mode of force application)

Force tending to stretch or pull apart. Tissues get longer and narrower

Compression (mode of force application)

Force pressing together. Tissue gets shorter and wider

Bending (mode of force application)

Eccentric loading. Tissue experiences compression and tension

Shear (mode of force application)

Force applied parallel to a plane or surface within an object. Acts like cutting. Spondylolisthesis is an example

Torsion (mode of force application)

Twisting force

Combined loading (mode of force application)

Multiple loading mechanisms

Strain

When a load is applied to a tissue it tends to deform. Change in length of material expressed relative to its original length (deformation due to load)

Stress

When a load is applied to a tissue internal forces develop to resist the deformation. Intermolecular resistance within a body to the deforming action of an outside force (force per unit area)

Toe region (stress-strain curve)

Initial portion of curve where little resistance is offered to deformation. Not present in all materials. Removing "crimp" or taking up slack

Elastic region (stress-strain curve)

Elastic deformation is reversible. Material stretched will return to original length. Material stretched will return to original length. Stores energy

Stiffness (stress-strain curve)

Resistance to an external load by a material as it deforms. Slop of stress-strain curve. The steeper the curve, the more stiff and more load required to deform

Yield point (stress-strain curve)

Elastic limit of the material. Point where permanent deformation begins. Often point of injury

Plastic region (stress-strain curve)

Change in length that extends beyond the elastic capabilities, causing structural change. Permanent deformation

Failure point (stress-strain curve)

Point where material fails. Definite injury or complete failure of the tissue

Viscoelasticity

Display a stress-strain relationship that is dependent on the rate at which loads are applied

Relaxation

Stress (load) required to maintain a constant amount of deformation

Creep

Strain (deformation) that happens in response to a constant load

Ectoderm develops into...

CNS, PNS, ANS. Epidermis (skin, hair, nails) and epithelium of proximal and distal GI tract

Mesoderm develops into...

Notochord, connective tissues, muscle, blood vessels, blood cells, lymphatic system and majority of genitourinary system. Gives rise to mesenchyme

Endoderm develops into...

Epithelium of alimentary tract and organs that arise from it (liver, pancreas, thyroid, parathyroid, thymus) and epithelium of respiratory tract

Somites

Bilateral paired blocks of mesodermal segments. 42-44 pairs. 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8-10 coccygeal. Develops into vertebrae

Epithelial tissue

Covering or lining tissue. Arises primarily from ectoderm and endoderm. Specialized tissue which absorbs, secretes, transports, excretes or protects

Nervous tissue

Neurons. Arises from ectoderm. Functions to communicate and conducts impulses

Muscle tissue

Skeletal, smooth and cardiac muscle. Arises from mesoderm. Functions to contract and generate force

Connective tissue

Arises from mesoderm. Produces and maintains extracellular matrix. Extracellular component gives form and allows for transmission of force. Key for inflammation (mast cells in connective tissue)

Loose connective tissue

Areolar, reticular and adipose tissue

Dense connective tissue

Regular and irregular

Regular dense connective tissue

Extracellular matrix. Non-fibrous (GAG's, PG's and glycoproteins) and fibrous (collagen, elastin and reticulin)

Collagen

A protein formed of triple-helical chain. Secreted by cells and typically form fibrils. Arranged manner to provide functional integrity. Total of 19 types. Cross links help provide stability. Maintain crimped position

Type I collagen

Synthesized by fibroblasts, smooth muscle cells and osteoblasts. Main component of skin, tendon, dentin, bone and vessel walls. 80-99% total collagen in body. Primarily considered to resist tensile forces

Type II collagen

Main component of hyaline cartilage. Can be produced by chondrocytes. Found in other connective tissues. Considered to be able to oppose compressive forces

Type III collagen

10-15% of all collagen. Typically found with type I fibers. Very important in early wound development and deposits collagen rapidly. Responsible for normal tensile strength of skin, aorta and intestinal tract

Elastin

Fibers are more extensible than collagen due to more coil-like arrangement. Fibers can stretch to 150% of original length. Produced and deposited in tissue similar to collagen. Stabilized with cross-links

Reticulin

Type III collagen. Not arranged in fiber type formation like collagen, but more of a mesh. Produce supportive tissue to maintain spacing and provide stability

Functions of bone

Provides protection for brain and internal organs. Provide rigid kinematic links and muscle attachments. Provides a rigid support structure for maintenance of posture. Manufactures red blood cells. Serves as reservoir for many inorganic materials

Bone composition

Arises from mesoderm. Bone is a type of connective tissue. Very dynamic (not static). Distinguished by inorganic component (strength and rigidity). Organic component makes it flexible and resilient

Osteoblasts

Generate bone matrix

Osteocytes

In lacunae, support bone

Osteoclasts

Reabsorb bone matrix

Extracellular matrix (bone)

Collagen fibers (primarily type I) that resists stretch with minimal extensibility. Inorganic component (small amount of GAG's, makes bone somewhat flexible, 60-70% dry weight and provides rigidity/strength)

Cortical (compact) bone

Outer shell. 80% of bone. Very dense

Cancellous (trabecular) bone

Within cortical covering. Thin plates in mesh-like arrangement. Filled with red marrow

Formation of bone

Endochondral ossification (ossified from cartilage matrix, chondrocytes ossify) and intramembranous ossification (bone ossifies directly)

Development of long bones - endochondral ossification

Chondrocytes at ends of bone manufacture cartilaginous ECM. Ossification centers develop where matrix becomes calcified. Eventually ossification centers unite at epiphysis. Epiphyseal closure occurs slightly earlier in females than males

Diaphysis

Shaft surrounding bone marrow cavity

Metaphysis

Transition area, no medullary cavity

Epiphysis

End of bone, covered with articular cartilage

Physis

Growth plate

Biomechanical properties of bone

Bone acts like 2 composite materials (mineral content and collagen content). Each provides different characteristics. Cortical bone tolerates more compression. Cancellous bone tolerates more deformation

Order of forces that bone tolerates...

Compression>tension>shear

Viscoelasticity of bone

Bone is viscous (rate of loading matters. Under faster loading, a greater stress is tolerated. Under slower loading there is more strain and less stress tolerance

Wolff's law

Bone is laid down in areas of high stress and resorbed in areas of low stress. Bone remodels entirely over 10-20 years.

Immobilization effects

Immobilized bone has reduced density and strength as a function of Wolff's law. Immobilize 3 weeks is 12% reduction in bone density. Regain only 38-62% of that back in 3 weeks

Bone response to exercise

Increases density (increases bone strength). Magnitude of response depends on type and intensity of exercise workload, duration and frequency of exercise, length of training period and site of activity

Optimal stress for bone

Compression is optimal (force along line of stress). Response follows piezoelectric effects. Blood supply is critical to healing and remodeling

Osteoporosis

Marked by an accelerated rate of bone loss (decrease bone mineral density). Age is associated, not a cause. Risk factors include sex, physical activity level, corticosteroid use, nutritional status, alcoholism, estrogen deficiency, genetics, smoking and chronic diseases

Tendon

Attaches muscle to bone. Transmits forces to produce and control motion

Ligament

Connect bone to bone. Augment mechanical stability and prevent excess motion

What are the 3 ways muscles attach to bone?

Direct attachment (trapezius, coracobrachialis), aponeurosis (latissimus dorsi, palmaris longus) and through a tendon (biceps, triceps, tibialis anterior, gastrocnemius)

Capsular ligaments

Thickenings of the capsule. Surround entire joint (glenohumeral ligament)

Extracapsular ligaments

Outside the joint capsule (LCL of knee)

Intra-articular ligaments

Within the joint itself

Composition of tendons and ligaments

Made of dense connective tissue, containsignificant amount of collagen and are sparsely vascularized

Extracellular matrix (tendons and ligaments)

Large component of organic weight. Has supportive functions. Primarily type I collagen (resist tensile loads), 3 polypeptide chains combined to form triple helical molecule. Tendons tend to be more stiff

Biomechanical properties of tendons and ligaments

Both are viscoelastic. Provides mobility and stability

Typical vs. elastic ligament

Typical ligament has higher proportion of collagen. Elastic ligament has higher proportion of elastin

Muscle force

Tendons are adapted to tolerate the loads they experience, larger muscles produce larger forces and have stronger tendons. The amount of force depends on the type of contraction

Anatomic location (tendon and ligament biomechanical properties)

Tendons where forces are low are weaker than tendons that are constantly exposed to high forces

Maturation and aging (tendon and ligament biomechanical properties)

Number and quality of collagen fibers will diminish with aging. Fewer cross-links present. Leads to loss of strength, decreased viscoelasticity and less compliance

Pregnancy (tendon and ligament biomechanical properties)

Hormonal changes drastically affect tendon and ligament

Heat (tendon and ligament biomechanical properties)

Denatures cross-linking, use to help elongate tissues

NSAIDS (tendon and ligament biomechanical properties)

Can help increase tensile strength, affect proportion and content of collagen, cross-linking. Delays fracture healing

Steroids (tendon and ligament biomechanical properties)

Corticosteroids weaken connective tissues. Affects fibrocytes ability to lay down new collagen

Exercise (tendon and ligament biomechanical properties)

Tendons adapt to the loads (Wolff's law). Training enhances collagen synthesis (increases number and diameter of collagen fibrils, increases strength and stiffness, eccentric exercise elicits the greatest response)

Immobilization (tendon and ligament biomechanical properties)

Leads to resorption, reduces the number and diameter of collagen fibrils and decreases strength/stiffness. In a shortened position, decreases fiber length

Exercise and immobilization

Tendons and ligaments can be affected even when they are not the target tissue. Weaken during immobilization

Primary functions of skin

Provides a protective barrier against mechanical and chemical irritants, UV radiation and pathogens. Thermoregulation and prevention of fluid loss (large SA to dissipate heat, perspiration). Provides cutaneous sensation. Cosmesis (skin, hair, nails)

Epidermis

Forms a barrier, keeps fluid in and blocks UV. Avascular, keratinocytes and melanocytes. Basement membrane zone (separates dermis and epidermis)

Dermis

Supporting connective tissue, microvasculature, nerves, sensory organs, hair follicles and sweat glands. Highly vascular. Fibroblasts produce mainly type I collagen and significant elastin as well

Subcutaneous tissue

Predominately fat cells, connective tissue and blood vessels. Provides insulation, cushioning and adds mobility. Fascia separates from deeper tissues

Skin

Largest organ of the body. Variable thickness depending on location. Composition parallels to tendon and ligament. Very different arrangement of collagen fibers

Factors affecting strain

Skin affected by many factors (internal diseases, nutritional status, over-all well-being). Scar tissue in healing skin, age

Function of nerves

Sense changes in body and environment. Interpret changes. Respond to changes

Nerve

Axon with its sheath and schwann cells. Axons carry the impulses/action potential. Act as transport system (axonal transport, axoplasmic flow, rates of flow vary)

Axon coverings

Myelin sheath produced by oligodendrocytes in CNS and schwann cells in PNS. Nodes of ranvier interrupt myelin sheath. Myelin has high electrical resistance and low capacitance, acting as an insulator

Connective tissue around nerves

Epineurium (loose outer layer surrounding nerve bundle, cushion during movement, thick near bony prominences), perineurium (middle layer surrounding each fascicle), endoneurium (inner layer surrounding individual nerve fibers)

Vascular supply to nerves

Normal nerve function utilizes 20% of oxygen in the blood. Large supply of collateralization which is very beneficial in case of compromised supply. Compromise of blood supply is possible due to compression, common with stasis (edema)

Axoplasmic flow

Normal flow of axonal material through axon. Goes from body to distal end and back, supplies substances for maintenance of activity and repair

Double crush syndrome

Sub-clinical compression at one site can alter axoplasmic flow. Increases susceptibility to compression at another distal site in the nerve. Eventually compromise function

Factors influencing effects of compression

Blood and axoplasmic flow impacted. Stasis is often created. Duration (critical in terms of injury and recovery). Mode (direct vs. distributed throughout extremities, damage occurs at edges of compression)

Nerve regeneration

Following injury, nerve has little ability to regenerate. Better ability to regenerate if there is some "path" for nerve to follow. Slow

Articular cartilage

Thin, dense white covering on the ends of bones forming the articular surfaces of synovial joints. Highly specialized to tolerate compressive and shear forces. Mechanical properties determined by composition and structure. No blood vessels, lymph or nerve

Fibrocartilage

A lot more fibrous connective tissue. Includes intervertebral discs, menisci and interosseous membranes. Permits only limited motion

Function of articular cartilage

Distribute forces or joint loads, thickest where demands are greatest, allow motion with minimal friction

Composition of articular cartilage

Thickest where demands are greatest. Needs optimal stress. Cells (chondrocytes), water, extracellular matrix (collagen, proteoglycans)

Chondrocytes (articular cartilage)

Secrete organic component of matrix (collagen). Density of cells is less than in tendon

Composition of extracellular matrix (articular cartilage)

Proteoglycans and collagen

Proteoglycans (articular cartilage)

Protein chains with polysaccharides. Add structural rigidity to ECM. High affinity for water. Affects permeability. Help resist compression

Collagen (articular cartilage)

Mainly collagen fibrils with numerous cross-links. Type II collagen is 10-20% of tissue volume. Less than in tendon. Type 1 fibers present to resist tension (parallel to surface), type II fibers present to resist compression