Lecture 7 - Meniscus, Tendons and Ligaments Flashcards
What are menisci?
Fibrocartilaginous structures existing in a number of joints (e.g. knee)
Menisci in the knee: Morphology + where do they exist?
Two crescent-shaped wedges of fibrocartilage
–> wedge shaped in cross-section
Between femoral condyles and tibial plateau
What is the difference between the red zone and the white zone in knee menisci? What proportion is red zone?
Red zone = well vascularised
–> peripheral 10-30% medial, 10-25% lateral
White zone = avascularised, receives nourishment from synovial fluid
Composition of menisci?
- 60-70% water (higher in younger)
- 15-25% collagen (type I)
- 1-2% PGs
What is the layered structure of menisci?
- Superficial network
- surface layer
- random mesh-like woven matrix –> v smooth, fine fibrils - Lamellar layer
- rope-like collagen fibre bundles arranged circumferentially
- smaller radial fibres - Central layer
- Randomly arranged collagen and PG
- similar to hyaline cartilage
What are the menisci formed from and what is the timeline?
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
Role of the menisci (6)
- Distribute axial load
- hoop stresses generated
- axial load converted to tension in the circumferential fibres - Reduce contact stresses
- increase area - Increase joint stability
- acts as a wedge –> blocks tibial plateau
- circumferential fibres have multidirectional stabilising function - Contributes to joint lubrication
- may compress synovial fluid into cartilage - provides nutrition to articular cartilage
- system of microchannels through menisci to cartilage - Assists with proprioception
- mechanoreceptors
How do the kinematics vary between the lateral and medial menisci?
- Lateral meniscus is more mobile than the medial (11.2mm vs 5.1mm movement)
- radius decreases with flexion
Meniscal tears: occurence, symptoms and types?
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
Meniscal repairs:
- 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
What are tendons?
- dense connective tissues
- connect muscles to bone
- transmit tensile force from muscle to bone (allow you to move)
- store elastic energy
What are ligaments?
- dense connective tissue
- connect bone to bone
- stabilise joints
- prevent excessive motion
Tendon and ligament components:
Cells:
- fibroblasts (rod shaped, arranged in rows –> synthesise collagen and ECM)
Matrix:
- water
- collagen (1&3)
- ground substance (PGs bind fibrils together)
- elastin
How is the amount of elastin varied in tendons vs ligaments and why?
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
How do the collagen fibres arrange in ligaments vs tendons and why?
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
What are the three structures of a tendon?
How are tendons inserted?
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
Where does the blood supply come from in vascular and avascular tendons?
Vascular - surrounded by paratenon which supplies blood
Avascular - blood supply from synovial diffusion
–> compressive stresses - like cartilage
Ligament: what is the mechanism of direct and indirect insertion?
What zones exist with direct insertion?
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
Vascularity in the ligament
- very limited
- microvessels from insertion site provide nutrients
Force-deformation mechanics of the tendon/ligament:
3 regions
(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
Ligament and Tendon:
Elastic modulus?
UTS?
UTstrain?
Tendon is ___ times stronger in tension than muscle - what is the impact of this?
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
Factors affecting the biomechanics of tendons/ligaments (5):
- Specimen orientation
- properties = highly directionally dependent - Level of stress experienced - location/type
- E flexor > E extensor - Hydration (usually 60-80%)
- decreased hydration = increased stiffness - Temperature
- increase creep (e.g. isometric contraction - tendon stretches, muscle contracts)
- decrease stiffness - 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)
What is hysteresis?
Energy dissipation - the difference between the loading and unloading curve = the energy lost during loading
–> more energy required in loading than unloading
Club foot - what is it?
Tight tendons and ligaments prevent the foot from stretching into the right position
Casting + splinting for 3-4yrs
How does the loading rate effect failure of ligaments?
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
Quasilinear elastic theory:
What does it describe?
What is the function?
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
Effects of ageing on ligaments/tendons (4):
- 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)
Effects of mobilisation/immobilisation on ligaments/tendons:
Mobilisation:
- increase strength and stiffness
- increase in collagen fibre bundle diameter
- increase collagen cross-linking
Immobilisation:
- decrease in modulus / ultimate stress
Tendon injury (5)
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)
Ligament injury
Sprain
- overstretched, partially torn, completely torn
Repair vs Regeneration
Regeneration = normal tissue re-established
Repair = structure healed
- scar tissue
- abnormal composition and microstructure
- inferior mechanical properties
Extrinsic vs Intrinsic healing
Extrinsic
- tough scar tissue enveloping injury
- harmful effect of adhesions
Intrinsic
- formation of longitudinal aligned collagen fibres within the substance
- minimal adhesions
Challenges to repair for tendon/ligament (4):
- 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
Three effects of viscoelasticity
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
What is a meniscectomy (5)
- a surgical procedure wherein part of all of the meniscus is removed.
- Menisci decrease contact stresses by increasing the contact area of the tibia and femur;
- If they are removed the stress on the cartilage increases.
- Due to their wedge-like shape and assistance with proprioception, menisci increase joint stability; so, their removal may increase translation at the knee joint.
- Menisci contribute to joint lubrication and provide nutrition to the articular cartilage; therefore, their removal lessens support for the cartilage.
Role of Collagen in Bone
Type I Collagen serves to bear load.
It is stiffened with hydroxyapatite crystals, which allow the bone to withstand compression. Collagen fibres are arranged in arrays within the lamella, which lie at angles of approximately 30° to the axis of the bone, in alternating directions, providing torsional strength
Role of Collagen in Cartilage
predominately Type II Collagen serves to bear load.
Its arrangement varies with the zone of the cartilage. The collagen fibres at the articular surface are fine and provide a smooth bearing surface. The dense network of fibres in cartilage also forms a web in which proteoglycan molecules are trapped and through which water does not flow easily. Further, the collagen fibres retain the tissue structure against the pressures induced by joint loading and swelling pressure.
Role of Collagen in Meniscus
Type I with some Type II Collagen serves to bear load.
Similar to cartilage, the fibres on the surface provide a smooth interface for the joint. Collagen fibres also run circumferentially around the meniscus, which converts compressive forces on the joint into hoop stresses.
