Tendons Flashcards
What is a tendon?
A range of morphologically diverse structures that connect muscle to bone
Transmits force created within the muscle to the bone
What are the 2 categories of musculoskeletal injuries?
- Acute injuries:
o Single macro traumatic event in which the tissue is acutely overloaded and fails - Chronic (overuse) injuries:
o Tissue eventually becomes painful when it is subjected to multiple repetitive stresses that it is not able to withstand.
What are the common causes of musculoskeletal injuries?
- Participating in physical activity
2. Specific activities in the workplace
What is tendon rupture?
Acute tendon injury
What is Tendinopathy?
Chronic overuse injury
What is the Iceberg Theory?
- Model of Tendinopathy pathogenesis
- Has 3 stages
- Healthy exercise -> healthy load (within physiological range)
o Net increase in collagen synthesis
o Tendons becomes larger, stronger, more injury resistance
o Increased tensile strength & elastic stiffness
2. Relative Overload -> micro ruptures o Collagen fibers begin to: - Slide past one another - Break at crosslinks - Causing tissue denaturation o Cumulative microtrauma
- ECM Degradation
What is the Tendon Pathology Continuum Model?
- Model of Tendinopathy pathogenesis
- Has 2 stages
- An appropriate load within a physiological range lead to positive adaptation (e.g. increased proteoglycan production)
o Called reactive tendinopathy
o Is reversible if load is reduced - If load exceeds healthy levels -> matric/collagen disruption by increased PG’s -> opportunity for vascular growth
o Tendon disrepair
o Not reversible
o Can lead to degenerative tendinopathy
What are 2 models of tendinopathy pathology?
- The iceberg Theory
2. The Tendon Pathology Continuum Model
What are the predominant causes of chronic tendinopathy?
- Collagen disarray and degeneration
- Fiber disorientation and thinning
- Increased ground substance (Aggrecan)
- Neovascularization
- Areas of increased or decreased prominent of tenocytes
- Failed healing response
What are the mechanical properties of Tendons?
- Non-linear
- Viscoelastic:
a. Has both elastic & viscous properties
b. When force 1st applied, it stretches and returns to original length (elastic).
c. If force is continually applied, it stops returning to original length (viscous) - Anisotropic:
a. Properties are directionally dependent - Heterogeneous:
What is the general makeup of tendons?
- ± 2/3 of tissue = water (Not liquid, bound in collagen structure)
- 1/3 is tightly packed parallel bundles of predominately Type I Collagen fibrils
- Fibrils aggregate into larger structural units tendon
What is the hierarchical structure of a tendon?
- Collagen molecule
- Collagen fibril
a. Smallest structural unit - Collagen Fiber
a. Endotenon: thin layer of loos connective tissue that surrounds the fiber which also has tenocytes
b. Tenocyte: cells that make up the tendon - Fascicle
a. Innervated and has blood vessel supply - Tendon
a. Surrounded by peritendon (Paratenon & Epitenon)
What are the structural components of a tendon?
- Extracellular Matrix: o Ground Substance: a. Proteoglycans (PG's) b. Glycosaminoglycans (GAGs) c. Inorganic components o Protein Fibers: a. Collagen b. Elastin o Non-fiber Proteins: a. Glycoproteins - Cells: o Tenocytes
What is the Extracellular Matrix?
- Part of any tissue in the body
- Proteins + ground substance
- Space that surrounds the cell
- Much higher amount of ECM in tendons than in other tissues
- Is produced and maintained by cells (e.g. fibroblasts)
What are glycoproteins? And what are their role in tendons?
- Non-fiber proteins, part of ECM
- Don’t form fibers
- Variety of shapes and functions & sizes:
o Adhesive proteins: act as cement, holding fibers and cells together
o Regulatory cell-matrix interactions:
a. Repair & adaptation
What is Ground Substance?
- Saline gel-like medium in which the protein fibers and cells are embedded
- Consisting of:
o Water (60-80% total weight)
a. Therefore highly hydrophilic
o Maj. Macromolecular components:
a. Proteoglycans
b. Glycosaminoglycans (GAGs)
What does Ground Substances contribute to?
- Contributes to: o Lubrication o Protection o Diffusion of mols & metabolites o Strength & elasticity
What is the basic structure of Proteoglycans?
- Forms a highly hydrated, gel like substance, in which fibrous proteins are embedded
- Basic structure:
o Core protein
o Carbohydrate chains = glycosaminoglycans
Aggrecan
o Large hyalectan Proteoglycan
o Has lots of GAG’s so will bind lots of water – so lots in tissues that need to withstand lots of compressor forces
What are Glycosaminoglycans?
- Long unbranched (linear) polysaccharide chains formed from repeating disaccharide units
- Disaccharide unit sugars:
o Uronic acid
o Either N-acetylglucosamine or N-acetylgalactosamine - 1 or both sugars contain sulphate groups (negative charge)
o Therefore GAGs are HIGHLY negatively charged
o Therefore they bind water – giving it resistance to deformation
What is a Tenocyte?
Mature tendon cell – maintains tissue
What is a Tenoblast?
Immature, synthetically active cell
What are integrins?
- Cell surface receptors
- Part outside cell can bind to matrix
- Part inside can bind to cytoskeletal components
o Resulting in a physical link between ECM & cell - Any force applied to the matrix can be transmitted through integrins & cytoskeleton into the cell
- Mechanical loading triggers fibers in the matrix to stretch
- This stimulates the cell through the integrins
- Cell-cell communication
- The cell responds through protein synthesis
What is Collagen?
- Family of 28 proteins defined by structural component of amino acids not function
o Therefore family has very diverse functions - Building blocks of several connective tissues
- Type I fibril = basic building blocks of tendons, ligaments, bones
- Type II fibril = basic building blocks of cartilage
Describe Collagen alpha-chains
- The basic structural unit of the collagens
- Consists of repeating uninterrupted (1 repeating unit) or interrupted (>1 unit) Gly-X-Y sequence
- 3 alpha-chains form a left handed triple helix
- Every 3rd amino acid = glycine:
o Gly has simplest side chain (-H) allows it to twist gently into chain - X is often proline and Y is often hydroxyproline (Add -OH)
- Some lysin residues are also hydroxylated
- Molecule is glycosylated
Type I Collagen
o Major fibrillar collagen in bone, tendon, ligament, muscle
o Most abundant type
o Confers rigidity
o Heterotrimer: 2x alpha-1 chains; and 1x alpha-2 chain
Type II Collagen
o Major fibrillar collagen in cartilage and fibrocartilage
o Homotrimer
Type III Collagen
o Frequently found with Type I
o Forms reticular fibers which crosslink to form a fine meshwork
o Confers elasticity
- Changing collagen type (e.g. from Type I to III) can change properties of tissue
o Found particularly in tissues with elastic property (blood vessels, lungs etc.)
o Homotrimer
How are collagen fibrils formed (Type I)?
- Alpha-chains produced from cellular mRNA -> wrap into triple helix
- Triple helix starts where Gly-X-Y sequence starts and continues till it ends
- This forms Procollagen – ‘pro-‘ means it’s a precursor to collagen that must be cleaved
a. Has a collagen domain AND 2 globular domains on either end - Procollagen is exported out of cell and the globular domains are cleaved -> collagen
How are fibrils assembled?
- Fibrillogenesis:
o Assembly of collagen molecules to form fibrils - The quarter-staggered array:
o The last ¾ of the top molecule aligns with the 1st ¾ of the next molecule - Originally non-covalent interactions hold the molecules together
- Over time they are replaced by covalent interactions
- Lysyl oxidase forms covalent crosslinks between lysine and hydroxylysine
o Crosslinks important for tensile strength, energy absorption & resistance to proteases
What are the stages of the Tendon Stress-Strain Curve?
- 0-1% stretch = crimping. The crimps straightening out
- 1-3% stretch = Physiological region:
o Elastic properties, will return to original length
o Straight line graph of stress vs strain % - 3-5% stretch = partial ruptures:
o Microtrauma – intermolecular cross-links start to fail - 5-8% stretch = overuse /chronic injury:
- > 8% stretch = Tendon rupture
