Biological Materials

Question 1

Describe the composition/structure of bone.

Answer 1

Bone contains

• 20-30% water
• Collagen (30% dry weight) resistance to tensile loading, ductility, toughness
• Minerals (70% of dry weight) mainly calcium phosphate/carbonate. Resist compression and provide rigidity.

Question 2

Bone composition/structure, two points.

Answer 2

Bone is capable of withstanding loads 6 times those experienced in normal life.
Bone is a specialised form of connective tissue, bone consists of many cells (osteocytes) which are: Light, in a collagen matrix and packed with minerals.

Question 3

What are the two types of bone?

Answer 3

Cortical and Trabecular. The two types of bone should be considered as one material with a structure whose strength to weight ratio exceeds that of its constituents. So, the whole structure is greater than the sum of parts.

Question 4

Bone injuries, which bone is more susceptible?

Answer 4

Bone is stronger in resisting compressive forces than tensile forces. However, Trabecular bone is much weaker than cortical bone in resisting either type of force.

Question 5

What is the main form of bone damage?

Answer 5

Fractures are the main form of bone damage whether the fracture be sustained traumatically or fatigue induced.

Question 6

What are the bone fracture types and how are they induced?

Answer 6

Diaphyseal Impaction - Axial compression
Transverse - Bending
Spiral - Torsion
Oblique transverse (butterfly) - Axial compression and bending
Oblique - Axial compression, torsion and bending

Question 7

Bone Transverse Fractures Information…

Answer 7

Induced via bending load
Cortical bone is weaker during tension than compression, so the bone withstanding tension fails first.
Failure mechanism is break propagation perpendicular to bone’s long axis, from surface layer inwards.

Question 8

What is Cartilage Composition/Structure?

Answer 8

Cells (chondrocytes) account for less than 10% of the tissue volume. The organic matrix consists of collagen fibrils (10-30%) within a proteoglycans solution (3-10%).
Remainder of the tissue (60-87%) is made up of water, inorganic salts and small quantities of matrix proteins, glycoproteins and lipids.
A cartilage’s mechanical properties are load dependant, the structure reacts to how the load is applied (Anisotropic).

Question 9

What are the biomechanical properties of Cartilage?

Answer 9

Cartilage shows viscoelastic behaviour.

• Loading leads to rapid initial deformation followed by more gradual increase.
• When load is removed the structure returns to its initial shape (hysteresis)

Question 10

Articular Cartilage Injuries, How do they occur? What are their properties?

Answer 10

Articular Cartilage is capable of deformation which allows for reduction of stress through the joints as a result of increasing load-bearing area.

• It’s avascular and tends to have very few nerve endings, this means that injured cartilage/damaged shows little external symptoms (pain) and also repairs poorly.
• Overuse can lead to degeneration which in turn leads to Osteoarthritis.

Question 11

Examples of Dense Connective Tissue?

Answer 11

Viscoelastic tissues…

• Tendons, tough cords of closely packed collagen fibres
• Aponeuroses, sheet-like tendons. They attach muscle to bone and transmit tensile loads so that movement can occur.
• Deep fascia, between muscles, enveloping individual muscles and binding them in to groups (TFL).
• Intramuscular Septa, separating muscle groups passing from the deep fascia to the bone.
• Ligaments, like tendons in structure however contain more elastic properties and fibres.
• Ligamentous Joint Capsule, Connect bone to bone, guide joint motion, enhance mechanical stability, and prevent excessive motion.

Question 12

What are the characteristics of Tendons?

Answer 12

• Ultimate failure strain of tendons is between 8 and 10%
• Limited Viscoelastic behaviour, with only 2.5 to 20%
• Main site for high energy return during elastic recoil in the muscle-tendon unit

Question 13

What are the characteristics of Ligaments?

Answer 13

• Ligaments are designed best to cope with tensile forces
• Stiffness increases with force applied, preventing bones from pulling apart
• Ligaments have mechanoreceptors which may initiate tension in nearby muscles in order to assist in maintaining joint stability

Question 14

Ligament Injury Characteristics

Answer 14

• Sprains caused by excessive joint motion
• Direct blows result in stretching and permanent deformation
• Failure occurs as a result of bending and torsional load

Question 15

What are the characteristics of muscles?

Answer 15

• Most commonly injured form of connective tissue, because of its low stiffness
• Quickest to recover due to its large blood supply
• Muscle stiffness is related to the force produced in relation to its length.
• Muscle is capable of stretching up to 160% before rupture.
• Maximal stiffness is encountered during eccentric contraction, up to 200 times that in concentric and is probably under reflex control.
• Muscle and tendon are to be considered a single unit where neither connective tissue acts independently of the other.