Lecture 4 - Tissue Mechanics: Tendons and Ligaments Flashcards
What do tendons and ligaments have in common?
- Both are composed of dense connective tissue
- Both are biologically active
What is the difference between tendons vs ligaments?
- Tendons connect muscles to bones
- Ligaments connect bones to bones
What are the functions of tendons?
- Force transmitter
- Macroscopic movements (create torque to rotate joint)
- Alter stress field within bone (compression, bending)
- Compliance
- Proprioception
- Stores elastic energy
What are the functions of ligaments?
- Microscopic movement
- Force transmission is a minor function
- Maintains optimal joint alignment
- Provides joint stability
- Compliance
- Proprioception
What are ligaments and tendons classified as?
Connective tissue
What are the three categories of connective tissue?
- Proper connective tissue:
a) Loose: within and between
muscle sheaths, delicate
b) Dense: less flexible,
resistant to stress (tendons
and ligaments) - Supporting connective tissue:
a) Bone
b) Cartilage - Specialized connective tissues:
a) Adipose tissue
b) Hematopoietic tissue
What are the components of tendons and ligaments?
- Cells: 20%
- Extracellular Matrix (ECM): 80%
What makes up the ECM?
a) Fibers
- Collagen
- Elastic
b) Ground substance
- Glycoproteins
- Protoglycans
- Inorganic components
- H20
What is the primary type of cell?
Fibrocyte/tenocytes
What are fibroblasts?
What fibrocytes are called when manufacturing proteins
What do fibroblasts manufacture?
Components of the ECM
What primarily makes up the ECM?
Collagen (gives white colour and provides tensile strength)
What do fibroblasts secrete?
Procollagen
What does procollagen assemble into?
Tropocollagen
What does tropocollagen assemble into?
Tendons and ligaments
What is the composition of tendons high in?
Collagen (75-85%)
greater than ligaments
What is the composition of ligaments high in?
- Collagen (70-80%)
- Elastin (5-15%)*
greater than tendons
What are the arrangements of collagen fibers in tendons and ligaments?
Tendons: Parallel
Ligaments: Less parallel
In composition, what are tendons and ligaments similar in?
- Water (~55-65%)
- Dry matter (30-45%)
What does the parallel arrangement of collagen fibers help with?
Withstanding high unidirectional loads
What does the less parallel arrangement of collagen fibers help with?
Sustain tensile stresses in one direction and smaller stresses in other
What are the two ways to measure tissue properties?
- Subjecting tissue preparations to uniaxial tensile loads to failure
- Measurement of in vivo forces
What are load deformation curves?
- Represents structural properties of the tissue
- Deformation produced by a load depends on the size of the structure
What is tensile stress?
Externally applied tensile load per cross sectional area
What is tensile strain?
the elongation/unit length of material in response to tensile load
What do mechanical properties depend on?
- Composition of the tissue
- Orientation of collagen fibers
- Number/type of collagen cross links
- Components of ground substance
What do the slopes of the stress-strain curves indicate?
Steep slope: high Young’s modulus (stiffness)
Gradual slope: low Young’s modulus (compliant)
What are the different parts of a stress-strain curve?
- Toe:
- Elongation
- Uncrimpling of collagen - Linear:
- Slippage b/w fibers
- Return to initial length - Plastic:
- Microfailures
- Collagen structure disrupted - Major failure
- Visible narrowing - Complete failure
What are some findings of comparing stress-strain curves?
- Differences between connective tissue types
- Cortical bone requires major stress
- Cartilage and trabecular bone deform easily
What are the characteristics of viscoelasticity?
- Creep
- Stress relaxation
- Recovery
- Rate effects
What tissues are viscoelastic?
Tendons and ligaments
What is viscoelasticity?
Time-dependent response of a material subjected to a constant load or deformation
What is hysteresis?
The amount of energy lost during loading
What makes up a hysteresis graph?
- The loading curve of a viscoelastic material
- The unloading curve of a viscoelastic material
What are the 3 modes of failure?
- Rupture (tearing)
- Failure through the enthesis (attch. point near bone)
- Avulsion failure (pulling away of bony attachment)
What affects the failure modes?
- Varies among different tendons/ligaments
- Affected by age, speed, structure, etc
What are the effects of rate and duration of force application?
- Tendons are more sensitive to strain rate
- Higher strain rates increase stiffness
- Strain rates may affect the mode of failure
- Avulsion: lower strain rate
- Substance rupture: higher strain rate - Tendons and ligaments are viscoelastic
- Rapid loads: stiffer and abrupt rupture
- Slow loads: fluid and elongate
What are the effects of temperature?
- Heat (>55-60 degrees): produce irreversible shrinkage
- Viscoelastic properties increase with increased temperature: increased stress relaxation/creep
What are the effects of maturation and aging?
- Maximal tissue strength achieved at skeletal maturity
- Very young/old: lower stress/strain and decreased stiffness
- Resistance training can reverse or slow effects of aging
What are the effects of hormones?
- Cortisol and glucocorticoids: reduce the synthesis of type I collagen
- Relaxin: softens and increases extensibility
- Growth hormone: increases collagen synthesis
What is tissue remodelling?
Adaptation of biological tissues to change in the mechanical stresses they are subjected to daily
What is specific adaptation to imposed demand?
Explains remodelling in response to alterations in external loading in soft tissues
What effects does joint immobilization have in normal connective tissue?
- Reduces the tensile forces
- Reduces the load to failure and stiffness
- Decreases the strain/elongation to failure
- Frequency of avulsion failures increases
What effects does joint remobilization have in normal connective tissue?
- Allows restoration of mechanical properties
What happens to healing connective tissue?
- Healing is slow
- Early mobilization (within limits) strengthens the unions, speeds healing and reduces scar tissue adhesions
What are the 4 stages of healing?
- Hemorrhagic
- Inflammatory
- Proliferative
- Remodelling and maturation
What is the response to stress enhancement?
- Increased loading can lead to tendon hypertrophy and increased strength/stiffness
- Exercise may protect against weakening effects of inactivity
- Exercise may increase the strength of some tissues
- Excessive loads can have harmful effects
