Tendons, ligaments and bursae

What are tendons and ligaments?

Tendons (sinews) and ligaments (and fascia) are tough bands of dense fibrous connective tissue that withstand tension

What do a) tendons b)ligaments and c)fascia connect?

a) tendons connect muscle to bone

b) ligaments connect bone to bone

c) connect muscles to muscles

Briefly outline the role of tendons and ligaments

Tendons: transmit muscle force to boon in a controlled manner to effect movement at joints and to control the direction of the muscle force (some act as springs, storing and releasing energy).

Ligaments limit and control direction of motion at a joint.

Outline function of tendons

• transmit muscle force to bone to allow joint movement

• passive springs, store energy and reduce work of muscles

• tendons reduce weight over lower limbs by keeping muscle mass near the body.

What are benefits of tendons in limbs

• reducing weight

• keep neuro-vascular bundles as short as possible

• permits rapid and efficient limb movement

Outline tendon morphology

• most are narrow and oval/round in cross section

• some are wide and flat - called aponeurosis

Give an example of aponeurosis

abdomen

• no bones in the abdominal wall therefore the tendon is flattened out allowing a wide attachment to other structures.

• Acts as a sheath (called the rectus sheath)

How does tendon insertion work?

Muscles have origin and enthuses (insertion poitns) where they attach to bone.

Tendons of origin vs tendons of insertion

• connect into bone via Sharpeys fibres

• 1. Indirect (fibrous) - simple

• 2. Direct (fibrocartilaginous) - complicated (see image)

What is an example of a specialised tendon?

Manica Flexoria - a specialisation of the superficial tendons of digits

Outline manica of superficial ditigal flexor tendon (manica flexoria) of dog

• Superficial ditigal flexor tendon forms a sheath (tube) to allow the deep digital flexor tendon to run through it forming the manica flexoria

• allows DDFT to swap positions with SDFT

• Controls the position of DDFT over digit

• distal in limbs

How does the proximal manica flexora differ in the horse?

• only 1

• a small ring of SDFT which wraps around DDFT in proximal part of digital flexor sheath

what’s a risk of proximal manica flexoria in the horse?

• often missing

• can be torn

What cells do we find in tendons, ligaments and fascia

• specialised fibroblasts that make the ECM

• called tenocytes in tendons

What is in the ECM of tendons, ligaments and fascia

Varies however generally compromises:

• Collagen type 1 fibres

• Elastic fibres, amounts vary (allow stretch and elastic properties)

• Proteoglycans (absorb and retain water)

• Ground substance (material found b/w the above materials, some components inter, others potentially active.

Outline the cells found in tendons

Mechanocytes

• Tenocytes are located b/w and associated with collagen fibrils

• orientated to the long axis of fibrils

• have a 3D network of processes that enable cell signalling via gap junctions - enables detection and response to mechanical loading

• sense tensile loads transmitted by tendon or ligament through their contact with fibres and cells response by altering quantity and composition of ECM.

Tenocytes: detect changes in load by changing length - strain receptors

Why are tendons slow healing?

• low cellularity

• poor vascularity

• difficult to rest and injured/repairing tendon

What is the composition of tendons?

55-70% water

Dry mass includes:

• few cells

• 60-85% collagen, mostly type 1, some minor types too (2,3,4,5,6 and 10)

  15-40% non collagen ECM

• 3% cartilage oligomeric matrix protein (glycoprotein) COMP

• 1-5% proteoglycans retains water to resist compression

• 1-2% elastin protein fibres

• 0.2% inorganic components (Cu2+, Mn2+, Ca2+)

When may tendons appear more yellow?

When they have more elastin

What role do elastin protein fibres have in tendons?

enable return to a normal size after stretching

Why are there so many kinds of collagen?

Amino acid variations

How is collagens steady turnover controller

• metallo-proteases (MPs) are enzymes that break down collagen cross-links

• MPs are controlled by Tissue Inhibitors of Matrix Metallo-Proteases (TIMPs)

How is collagen synthesised

1. Procollagen is a triple helical protein made INSIDE fibroblasts

2. Outside the cell, procollagen is trimmed to a soluble triple helix collagen molecule helps elasticity.

3. Molecules cross link to become insoluble

4. Collagen molecules self-assemble into fibrils outside cells

5. bunches of fibrils arrange into a collagen fibre

6. these are wrapped up in a fine connective tissue sheath

what bonds hold collagen together

• Initially by hydrogen bonds

• Covalent inter- and intra-molecular links provide strength

• Proteoglycans placed longitudinally and transversely strengthen them.

How do the molecules lay themselves out in collagen?

A staggered pattern with regular gaps, cuases bands

Outline gross anatomy of collagen

1. tenoctyes produce and secrete tropocollagen (precursor molecules) - self assemble into microfibrils then outside the cell into fibrils.

2. Bundles of fibrils are organised to form fibres

3. Bundles of fibres are organised parallel to each other with tenocytes and organised into fascicles (bundles) bound and separated by endotenons

4. bundles of fascicles form tendons - covered by an epitenon

5. tendon is enclosed by fascia loosely held on by a fatty areolar tissue (paratenon)

what feature do collagen fibres have and why

• wavy pattern/crimp

Important in shock absorption, crimp straightens out taking the shock of the load, preventing bone damage)

What is the epitenon

an outer sheath of dense irregular connective tissue on the tendon

from which endotenons extend between fascicles

what is the paratenon

fatty areolar tissue that holds the fascia.

How are collagen arranged in tendons vs ligaments

tendons - parallel rows

ligaments - criss-crosses (so that forces can be applied in different directions)

what kind of stress to collagen fibrils resist

tensile stress

Outline elastin properties and function

• fibroblasts make fibrillin that makes a scaffold for elastin to be deposited on.

Elastin:

• hydrophobic = tight coils and loops

• very elastic (150% stretch)

variations in amount alter mechanical properties

• elastic vs positional tendons (nuchal ligament vs collateral ligaments)

Outline proteoglycans (PGs)

• resist compressive stress

• interconnect collagen fibres and bind water - forms a water bag

• consist of a protein core bonded to GAG

examples

• decorin

• aggrecan

What are decorin and aggrecan

Decorin promotes fibril slippage, found in bones

Aggrecan binds water, resists compression and is a major PG of cartilage tendon

what GAG components in tendons are involved in collagen fibril assembly

1. Dermatan sulphate GAG - organises collagen fibrils (forms associations b/w them, causing parallel alignment and separates them whilst forming interfibril bridges)

2. Chondroitin sulphate GAG - involved with occupying volume b/w collagen fibrils - keeps separated and helps withstand deformation.

Outline proteoglycan structure

• GAG side chains attached to a core protein (hyaluronan molecule)

Large PG:

• form aggregates with hyaluronan, large swelling potential, bind cations and water, resist compression

Small PG:

• bind to ECM molecules, fibril diameter and cross-linking, tensile strength, binds growth factors.

Look like toilet brushes

Outline GAG (glycosaminoglycan as special ECM molecules

• repeats of CHO and amino acid as a chain

• bind with collagen to create a meshwork: ECM

• able to bind water - creates the gel-like qualities of tissues (important for tendon function)

Outline the anatomical location of sheaths and bursae

Synovial bursae and tendon bags

• tough connective tissue bags

• sheaths secrete and contain synovial fluid and a lubricant therefore are closed bags

• analogous to synovial wall of joints

located mainly under tendons/ligaments near the insertion at joints to reduce friction on the tendon as it passes across a bone/under a ligament

Outline tendon neurovascularity

• blood vessels run in endotenons, parallel to collagen fibres with occasional branching transverse anastomoses - poorly vascularised though

• aneural in tendons - epitenon and paratenon do contain nerve endings. Golgi tendon organs are present at myotendinous junction b/w tendon and muscle

What is the clinical relevance to a tendon sheath injury

• can be inflamed by repeated movements/strains e.g. tendonitis/tenosynovitis

note: blood supply in sheathed tendons is conveyed by the mesotenon n.v.b.

• Tendon healing w/in a sheath will be delayed as only the internal tendon blood supply is available

• If the tendon sheath is torn, healing will be accelerated (vessels have easier access) but may lead to adhesions which impair gliding functionD

Outline the gross anatomy and role of ligaments

Role: support and control movement

Skeletal ligaments join bone to bone/cartilage

• dense collagen fibre bundles - tough thickened parts of joint capsule, dense bundles of elastin fibres (multiple directions but not irregular), lined by synovial membrane

• similar composition to tendons, 10% elastin content, more type 3 collagen and more cells.

Briefly outline splanchnic ligaments (organ related)

1. peritoneal - a fold of the peritoneal membrane supporting and separating abdominal organs

2. remnants of a foetal tubular structure - should close up and shrivel after birth, becoming a cord-like structure.

3. peridonteal ligament - attaches tooth cement to alveolar (socket) bone.

Outline some features of skeletal system ligaments

1. reinforce joint to limits its direction of motion

2. viscoelastic

3. provide stability at the price of mobility

4. collagen fibres are criss-crossed

what are the 3 locations formations of ligament attachment

1. form the joint capsule (capsular)

2. form outside the capsule reinforcing it (extracapsular)

3. form inside the capsule (intracapsular)

what does the viscoelastic property of ligaments enable

deformation when strained and tension when under stress but return to normal shape as long as it’s not deformed past strain threshold.

what is a potential consequence of a weakened ligament

joint becomes unstable and may dislocate

Label this diagram

Label this diagram

What are the names of the 6 ligaments found in the spinal column

• supraspinous (above spinous processes)

• interspinous (b/w spinous processes)

• intertransverse (b/w transverse processes)

• interarcuate (b/w vertebral arches, online one that lies inside dorsally)

• Intervertebral (b/w vertebral bodies)

• longitudinal (run longitudinally along vertebrae)

what is the function of the nuchal ligament

1. support head and neck

2. prevents head/neck descending on every stride, storing energy that helps neck muscle mass to pull head back up at each stride, preventing muscle fatigue

Outline some features of the nuchal ligament in different animals

• better developed in animals with large heads and/or long necks

• can have 2 parts (lamellar and funicular e.g. horse) - most only have funicular (e.g. dog)

• pigs have no nuchal ligament

what enables the nuchal ligament to stretch and store energy?

dense elastin

Outline the nuchal ligament in the horse - structure.

outline the structure of the 3rd interosseous muscle (TIOM) aka suspensory ligament

Key point - not actually a ligament, it’s a muscle however it contains so few muscle fibres it looks like a ligament

outline the gross anatomy of retinaculae and what are they

• straps of tissue similar to ligaments that cross over the long axis of joints (don’t directly serve the joint e.g. cross the entire wrist rather than b/w individual bones)

• binds down the tendons close to the bone - preventing bow-stringing away from the bone

what will happen if the retinaculum is ruptured?

tendons will displace (bow-string) and ‘tent’ the skin

what are fascia

• membranous sheets, similar to ligaments

what kinds of fascia are there

• superficial: connect/anchor skin to deep fascia - loose adipose, vascular

• deep - wrap up muscles, less adipose, more elastic, many thin blood vessels

• visceral - connect/anchor serous membranes to organs and inside of body wall

what do fascia form?

• fascial compartments separated by inter-fascial compartments

• enable fluid to be drained

what is the clinical use of (inter-) fascial compartments

• can predict the spread of infections

• administering IM injections - keep the muscles in place.

what are 5 functions of fascia

1. reduce friction of muscular force - provide a movable wrapping, supporting and separating muscle groups and for structures that pass through/b/w muscles e.g. nerves and blood vessels

2. support muscle - provide attachment, for muscle fibres to exterior of muscle

3. stabilise and connect structures- distribute tensional forces across several joints in a network-like manner

4. store and release elastic potential energy

5. proprioceptive ability - due to the innervation of sensory nerve endings

what is the clinical significance of fascia

• problems if it loses stiffness, too stiff, decreased shearing ability.

• inflammatory fasciitis or trauma may cause fibrosis and adhesions (where fascia fail to separate adjacent structures effectively (post surgery for example)

• increase in intra-fascial compartment pressure (compartment syndrome) may require fasciotomy

What are the mechanical properties of tendons

Vary, matched to functional requirements

• energy storing tendons tend to be flexors, more elastic/less stiff, so store/absorb energy - fail at higher strains (12-15%)(horse SDF can stretch in excess of 20% when galloping)

• stiffer ‘positional’ tendons tend to be extensors, less elastic, so can provide finer control of movement. Guide limb movement.

tendons respond to changes in mechanical loading with growth and remodelling processes, disuse tendon results in a decreased average thickness of its collagen fibre bundles

What’s responsible for the crimp of tendons and where do we see it

• tenocytes are contractile and responsible for the crimp

• seen in secondary bundles (wavy appearance when relaxed, disappears when first loaded)

• degree of crimp reduces with age - leads to a more rigid structure so more prone to failure.

what are 4 properties of tendons and what enables these properties

1. cross linking - GAGs

2. ability to stretch - related to water content and proteoglycans

3. elasticity due to elastin and helical collagen molecules

4. shock absorption due to crimp

Outline the stress-strain curve

• tendons have viscous and elastic behaviours (viscoelastic) so their mechanical behaviour is dependent on RATE of mechanical strain

1. SS curve starts with a very low stiffness ‘toe’ region (shock absorbing) as the crimp straightens and the collagen fibres align

2. after ‘toe’ region, linear part of SS curve where tendon becomes significantly stiffer and collagen molecules slide past one another (Inter-molecular sliding) - tendon can revert back to original length.

3. When intercollagen cross links fail get irreversible plastic deformation and failure.

Outline the mechanical properties of tendons with 2 diagrams