Ch. 4 - Soft Tissue Mechanics Flashcards
What are the major soft tissues encountered in the MSK system?
- Tendon
- Ligament
- Articular cartilage
- Intervertebral disc
- Muscle
What are the functions of soft tissue?
- Connect tissues - Ligaments provide connection btw bones. Tendons connect muscles to bone.
- Control relative joint motion - Ligaments.
- Lubricate joints - Articular cartilage ensures low friction to allow joint mobility.
- Actuate skeletal system - Muscles.
What is special about soft tissues as structural elements?
- Must withstand large loads
- Must provide kinematic restraint
- Fail at much higher strains than bone
- Show a non-linear elastic response, with increased stiffness at higher applied strain
- Show pronounced viscoelastic (rate or time-dependent) behaviour
What is viscoelasticity?
Time- or rate-dependent behaviour of stress-strain levels (assuming material is loaded to a level where it does not suffer any irreversible deformation).
What does viscous mean?
It means that the applied stress is proportional to the time rate of change (of strain). It is modelled with a dashpot.
What does elastic mean?
It means that it shows an instantaneous, reversible response upon loading. Linear elasticity is governed by Hooke’s Law. It is modelled using a spring.
State 2 common effects observed in viscoelastics solids.
- The material is stiffer at higher applied strain rate. The faster the material is loaded, the more relaxation mechanisms are not able to follow.
- When applying cyclic loading on a viscoelastic material, it does not follow the same path in both loading and unloading paths of the stress-strain curve. The paths are out of phase = Hysteretic behaviour. The area within the hysteresis loop is energy dissipated with one loading/unloading cycle.
What are the 2 most important behaviours shown by viscoelastic materials.
- Creep - Material is loaded with a constant stress. The instantaneous response of the material is elastic followed by a time dependent increase in strain, called creep.
- Stress Relaxation - Material is deformed with a strain step and held at a constant strain (deformation). Initial stress is governed by the instantaneous elastic behaviour, after which stress decreases gradually until an equilibrium stress level is reached.
What are the constitutive equations for:
a) an elastic spring
b) a viscous dashpot
a) sigma = E x epsilon
(stress is proportional to strain)
b) sigma = mu x time derivative of epsilon
(stress is proportional to strain rate)
Describe a Maxwell linear viscoelastic model.
The Maxwell model consists of a spring and dashpot in series. The same stress is displayed by both the spring and dashpot, but they can both deform independently of each other.
Describe the creep response of a Maxwell model.
We see instant elastic deformation of the spring, followed by linear increase of the strain in the damper. Upon unloading, the instant deformation of the spring is recovered but the strain of the damper stays.
Describe the stress-relaxation response of a Maxwell model.
There is instantaenous stress due to deformation of the spring. Over time, the damper elongates, resulting in reduction of stress over time. The dashpot will elongate until the stress reaches zero.
Describe a Kelvin-Voigt linear viscoelastic model.
The Kelvin-Voigt viscoelastic model consists of a spring and dashpot in parallel. Deformation of both elements is the same, but they can each carry different stresses.
Describe the creep response of a Kelvin-Voigt model.
There is no instantaneous response, as the load is immediately taken up by both elements. The creep rate decreases with time as more and more of the load is taken up by the spring. Deformation eventually finds an equilibrium where the load is totally take up by the spring, and the dashpot is unloaded. When the load is removed, the creep strain will be recovered completely if enough time is allowed.
Describe the stress-relaxation response of a Kelvin-Voigt model.
Initial stress is infinite, because the strain rate in the strain step is infinite, leading to infinite stress in the dashpot. As strain is held constant, the strain rate becomes 0 and stress in damper vanishes. Only spring (instant elastic) contribution to stress is kept.
Describe the Standard Linear Solid model.
The SLS model consists of a spring in series with a Kelvin-Voigt element.
Describe the creep response of an SLS model.
There is instantaneous deformation of the spring E1, followed by inverse exponential increase in strain of KV element. Upon unloading, the deformation of the spring E1 is recovered instantaneously, after which if enough time is allowed creep strain on KV element will be recovered.
Describe the basic role of tendons.
Tendons connect muscles to bones and are loaded in series with muscles. They are optimised to store elastic energy and transmit large forces on a regular basis.
Describe the basic role of ligaments.
Ligaments connect bones to bones. They guide and restrict joint motion, ensuring joint stability. Functional forces are usually small, as they should not hinder joint motion in the physiological range. Joint motion is restricted when a critical point is reached during an unusual event.
What is the composition of tendons? Why?
Tendons must be stiff and strong to be able to transmit large forces without mechanical loss from muscles to skeleton on a regular basis. Therefore, they need a higher amount of highly aligned collagen fibril. Composition: 60% water (wet weight) 75-85% collagen type I 1-3% elastin 1-2% proteoglycan
What is the composition of ligaments? Why?
Ligaments need to stretch considerably without failing to help guide large joint motions. Therefore, they feature high quantity of elastin and less aligned collagen fibrils, which both lead to increase in ultimate strain ligaments can reach before failure. Composition: 60% water (wet weight) 70-80% collagen type I 1-15% elastin 1-3% proteoglycan
State the hierarchical structure of tendon from smallest to largest element.
- Tropocollagen
- Microfibril
- Sub-fibril
- Fibril
- Fascicle
- Tendon
When does tendonitis occur? How does the healing process progress?
Tendonitis occurs when tendon becomes partially torn or inflamed.
Healing occurs more slowly than in bone since tendons are less vascularised than bones. It progresses over 3 phases:
1. Inflammatory - damages tissue removed
2. Proliferation - new ECM laid down with limited order
3. Remodelling - loosely oriented tissue gets remodelled to regain its original degree of order and therefore its full mechanical integrity
What are bursae?
Bursae are small fluid-filled sacs that help cushion the movement of certain tendons over bone.