Lecture 7 - Meniscus, Tendons and Ligaments

Question 1

What are menisci?

Answer 1

Fibrocartilaginous structures existing in a number of joints (e.g. knee)

Question 2

Menisci in the knee: Morphology + where do they exist?

Answer 2

Two crescent-shaped wedges of fibrocartilage
–> wedge shaped in cross-section

Between femoral condyles and tibial plateau

Question 3

What is the difference between the red zone and the white zone in knee menisci? What proportion is red zone?

Answer 3

Red zone = well vascularised
–> peripheral 10-30% medial, 10-25% lateral

White zone = avascularised, receives nourishment from synovial fluid

Question 4

Composition of menisci?

Answer 4

• 60-70% water (higher in younger)
• 15-25% collagen (type I)
• 1-2% PGs

Question 5

What is the layered structure of menisci?

Answer 5

1. Superficial network
   - surface layer
   - random mesh-like woven matrix –> v smooth, fine fibrils
2. Lamellar layer
   - rope-like collagen fibre bundles arranged circumferentially
   - smaller radial fibres
3. Central layer
   - Randomly arranged collagen and PG
   - similar to hyaline cartilage

Question 6

What are the menisci formed from and what is the timeline?

Answer 6

Menisci are formed from mesenchymal cells –> cells arise from the perichondrium and cartilage

Week 8 - distinct structrues
Weeks 8-16 - distinct alignment of cells and beginnings of ECM
Further development - cell numbers decrease and collagen matrix becomes more dominant

Question 7

Role of the menisci (6)

Answer 7

1. Distribute axial load
   - hoop stresses generated
   - axial load converted to tension in the circumferential fibres
2. Reduce contact stresses
   - increase area
3. Increase joint stability
   - acts as a wedge –> blocks tibial plateau
   - circumferential fibres have multidirectional stabilising function
4. Contributes to joint lubrication
   - may compress synovial fluid into cartilage
5. provides nutrition to articular cartilage
   - system of microchannels through menisci to cartilage
6. Assists with proprioception
   - mechanoreceptors

Question 8

How do the kinematics vary between the lateral and medial menisci?

Answer 8

• Lateral meniscus is more mobile than the medial (11.2mm vs 5.1mm movement)
• radius decreases with flexion

Question 9

Meniscal tears: occurence, symptoms and types?

Answer 9

Occurence:

• most common knee injury
• 61 per 100,000 (medial tears 2x more common)
• twisting on a loaded flexed knee (younger), isolation/association with other injury, degenerative process

Symptoms:

• pain
• swelling
• clicking
• giving way
• locking

Types:

• partial / complete
• longitudinal (81%)
• flap
• degenerative
• radial (e.g. from operative tumour)
• horizontal
• bucket handle

Question 10

Meniscal repairs:

Answer 10

• structure never fully restored –> some are also difficult to repair

Partial meniscectomy:

• remove minimal tissue
• ensure smoothness and stability of remaining tissue –> stops tear from propagating
• done arthroscopically
• results in less wear

Meniscectomy:
- leads to progressive articular wear –> higher OA risk

Replacement:

• allograft transportation (cadaver)
• collagen meniscal implant (meniscal implant regrows into scaffold

Question 11

What are tendons?

Answer 11

• dense connective tissues
• connect muscles to bone
• transmit tensile force from muscle to bone (allow you to move)
• store elastic energy

Question 12

What are ligaments?

Answer 12

• dense connective tissue
• connect bone to bone
• stabilise joints
• prevent excessive motion

Question 13

Tendon and ligament components:

Answer 13

Cells:
- fibroblasts (rod shaped, arranged in rows –> synthesise collagen and ECM)
Matrix:
- water
- collagen (1&3)
- ground substance (PGs bind fibrils together)
- elastin

Question 14

How is the amount of elastin varied in tendons vs ligaments and why?

Answer 14

Elastin in ligament is variable

Very little elastin in tendon
- tendons need to be still as they are transmitting force from muscle –> don’t want muscle energy going into stretching

Question 15

How do the collagen fibres arrange in ligaments vs tendons and why?

Answer 15

Ligament:

• smaller diameter fibres
• collagen more randomly organised
• -> stress in multiple directions - stability function

Tendon:

• large parallel fibres
• uniform insertion into bone
• -> force from muscle to bone is a single vector - don’t need support in other directions

Question 16

What are the three structures of a tendon?

How are tendons inserted?

Answer 16

Endotenon:

• connective tissue - holds fascicles together
• allows longitudinal movement of fasicles

Paratenon:

• surrounds tendon
• vascular connective tissue

Epitenon:

• under paratenon at high friction locations
• produces synovial fluid

Insertion:
Endotenon continues into bone/periosteum and perimysium

Question 17

Where does the blood supply come from in vascular and avascular tendons?

Answer 17

Vascular - surrounded by paratenon which supplies blood

Avascular - blood supply from synovial diffusion
–> compressive stresses - like cartilage

Question 18

Ligament: what is the mechanism of direct and indirect insertion?

What zones exist with direct insertion?

Answer 18

Direct:

• superficial fibres join periosteum
• deep fibres join bone

Indirect:

• superficial fibres join periosteum
• v few deep fibres

Direct insertion zones:

• ligament
• fibrocartilage
• mineralised fibrocartilage
• bone

Question 19

Vascularity in the ligament

Answer 19

• very limited

- microvessels from insertion site provide nutrients

Question 20

Force-deformation mechanics of the tendon/ligament:

3 regions

Answer 20

(non-linear elastic behaviour –> different amounts of ‘crimp’ bear load at different points)

Toe region:

• stretches easily
• straightening of crimped collagen fibrils
• reorientation of fibres in direction of loading

Linear region:
- 95% elastic strain energy recovered –> efficient, do not lose energy

Yield and failure region:

• irreversible damage
• unpredictable

Question 21

Ligament and Tendon:

Elastic modulus?
UTS?
UTstrain?

Tendon is ___ times stronger in tension than muscle - what is the impact of this?

Answer 21

Ligament:
E = 400MPa
UTS = 80MPa
UTs = 12%

Tendon:
E = 1.2-1.8GPa
UTS = 50-125MPa
UTs = 9-35%

Tendon - v high tensile strength –> 3x stronger in tension than muscle ==> more likely to injure muscle

Question 22

Factors affecting the biomechanics of tendons/ligaments (5):

Answer 22

1. Specimen orientation
   - properties = highly directionally dependent
2. Level of stress experienced - location/type
   - E flexor > E extensor
3. Hydration (usually 60-80%)
   - decreased hydration = increased stiffness
4. Temperature
   - increase creep (e.g. isometric contraction - tendon stretches, muscle contracts)
   - decrease stiffness
5. Strain rate
   - viscoelastic
   - -> creep
   - ->stress relaxation
   - ->strain rate (higher = higher UTS))
   - ->hysteresis (continually decreasing stress with cyclic loading - protects ligament from fatigue failure - i.e. for same strain, get lower stress)

Question 23

What is hysteresis?

Answer 23

Energy dissipation - the difference between the loading and unloading curve = the energy lost during loading

–> more energy required in loading than unloading

Question 24

Club foot - what is it?

Answer 24

Tight tendons and ligaments prevent the foot from stretching into the right position

Casting + splinting for 3-4yrs

Question 25

How does the loading rate effect failure of ligaments?

Answer 25

Slow loading rate: - bony insertion = weakest - load to failure decreased by 20% Fast loading rate: - ligament is weakest component With increased loading rate, bone increases in strength more than ligament

Question 26

Quasilinear elastic theory: What does it describe? What is the function?

Answer 26

Describes time and history-dependent properties: Stress relaxation function describes stress as a function of time and elastic response in tendon/ligament: σ(t) = G(t) σ exp(ε) [MPa] G(t) = 0.86-0.05ln(t) --> component accounts for the time-dependent stress response (stress relaxation function) - decrease in stress with time σ exp(ε) = 9.7(exp(49.8ε) - 1) --> component accounts for the elastic response (dependent on strain and not on time) - creates increase in force as flexion occurs

Question 27

Effects of ageing on ligaments/tendons (4):

Answer 27

- Stiffness and elastic modulus increase up until skeletal maturity - # and quality of crosslinks increase - collagen fibre diameter increases - UTload decreases (1.3-3.3x higher in younger)

Question 28

Effects of mobilisation/immobilisation on ligaments/tendons:

Answer 28

Mobilisation: - increase strength and stiffness - increase in collagen fibre bundle diameter - increase collagen cross-linking Immobilisation: - decrease in modulus / ultimate stress

Question 29

Tendon injury (5)

Answer 29

Tendonosis (chronic tendonitis) - chronic degradation - damage at cellular level - no inflammation Tendonitis - inflammation due to acute injury Peritendinitis - inflammation of tendon sheath Direct: Laceration - e.g. cutting Indirect: tensile overload - e.g. ruptures at junction NB: most injuries occur at muscle-tendon junction or insertion to bone (rarely mid-substance)

Question 30

Ligament injury

Answer 30

Sprain | - overstretched, partially torn, completely torn

Question 31

Repair vs Regeneration

Answer 31

Regeneration = normal tissue re-established Repair = structure healed - scar tissue - abnormal composition and microstructure - inferior mechanical properties

Question 32

Extrinsic vs Intrinsic healing

Answer 32

Extrinsic - tough scar tissue enveloping injury - harmful effect of adhesions Intrinsic - formation of longitudinal aligned collagen fibres within the substance - minimal adhesions

Question 33

Challenges to repair for tendon/ligament (4):

Answer 33

- Difficult to restore normal mechanical function - relatively avascular - not just a process of restoring continuity --> also need healed tissue to glide in tissue - most injuries occur at muscle-tendon junction / insertion to bone --> rarely mid-substance

Question 34

Three effects of viscoelasticity

Answer 34

Creep: - increasing deformation under constant load - e.g. isometric contraction Stress relaxation: - reduction in stress under constant deformation Strain rate: - elongation depends on rate of force application - higher strain rate = higher UTS Hysteresis: - energy dissipation - difference between loading and unloading curve represents amount of energy lost during loading