Tissue Biomechanics Flashcards
What morphological feature causes the so-called ‘toe’-region in tendons and ligaments?
Some highly elastic tissues exhibit a toe or exponential region where the crimp in collagenous fibres straightens and fibres become taut (ie. go from a relaxed state to a fully elongated position before stretching begins).
What are the main differences in material properties between ligaments and tendons?
The matrix of ligaments consists of water (66%), collagen, proteoglycans, fibronectin, elastin, and actin. Collagen type I and III predominate in ligaments and makes up 70-80% the dry weight; proteoglycans make up only 1% of the dry weight. Elastin provides elasticity (ability to return to original proportions) to ligaments but is in relatively low proportions (approximately 1.5%) compared to collagen. In elastic ligaments such as the ligamentum flavum however, elastic fibres are twice as common as collagenous fibre
Water makes up 58-70% of a tendon’s net weight, with collagen making up the majority of the remainder (30%) with small amounts of elastin, proteoglycan and fibronectin.
Why is the Elastic modulus greater in tendons than ligaments?
The attachment sites of a tendon to bone (osteotendinous junction) consist of fibrocartilage; whilst the transition between the tendon and muscle tissue belly (myotendinous junction) displays a number of longitudinal infoldings to increase the surface area of the junction. The staggered tissue densities at a tendon insertion allows increased stiffness and reduced stress concentration
Discuss the differences in the material behaviour in the elastic and plastic regions during loading of a material.
Elastic behaviour is characterized by the ability of a material to deform under an applied load and return to its original shape when the load is removed.
In the plastic region, the material undergoes permanent deformation when subjected to an increasing load beyond a certain point. It does not return to its original shape when the load is removed.
By what mechanism are ligaments able to sustain higher forces during rapid actions compared to slow movements?
Due to the viscoelastic component of ligaments, they will appear stiffer under rapid force than under force applied more gradually.
How to calculate stiffness
Stiffness = Force Change/Length change = ΔF/Δl
How to calculate stress. What is it a measure of?
stress = Force/Cross-sectional Area = σ = F/A (N/m2 or Pa)
Material and structural strength of tissue
How to calculate strain
strain = Change in length/ original length = Δl/l (Dimensionless)
How to calculate elastic modulus. What is is a measure of?
Elastic Modulus = E = Change in stress/ change in strain = Δσ/ε (N/m2 or Pa)
Material strength of tissue
How to calculate yield load
Point between elastic and plastic phases
How to calculate ultimate load
Highest load achieved
How to calculate safety factor
Safety factor = ultimate load/ externally applied load [safety factor < 1 is unsafe and will lead to material failure]
What is the difference in meaning between Elastic modulus and stiffness?
“elastic modulus” (Young’s Modulus) is a specific material property that quantifies the material’s resistance to elastic deformation and is related to the stress-strain behaviour in the elastic region. “Stiffness,” on the other hand, is a broader term that can refer to a material’s overall resistance to deformation, encompassing both elastic and inelastic behaviours
Consider a ‘sprained ankle’. What has actually happened to the tissue and which phase of the load- deformation curve characterises this tissue change?
During a sprained ankle, the ligaments that stabilize the joint are subjected to excessive mechanical loads. This can occur due to sudden twists, impacts, or other traumatic events. The mechanical forces applied to the ligaments can lead to overstretching and tearing of the ligament fibres.
In terms of the load-deformation curve, the tissue changes associated with a sprained ankle occur in the plastic region of the curve.
Consider the stress-strain curves for cortical and trabecular bone tissue below. Which tissue would fracture first under conditions of high stress (100N) and short duration? Would the same tissue fracture first under maximum stress conditions for each tissue when loaded for a long duration? Explain
Under conditions of high stress and short duration (such as an impact or sudden loading), cortical bone is more likely to fracture first. This is because cortical bone is denser and has higher tensile and compressive strength than trabecular bone. It can withstand higher loads in a shorter duration before reaching its failure point. The increased density of cortical bone contributes to its ability to resist rapid, high-stress impacts.
Under conditions of long-duration loading, the outcome might be different. Trabecular bone is better at handling repeated, cyclic loading because of its porous and spongy structure. It has the ability to absorb energy and distribute loads more effectively, which makes it more suitable for situations involving prolonged, cyclical stresses. In such cases, trabecular bone may resist fracture better than cortical bone.
