Tissue Mechanics - Bone Flashcards
What is the base material for bone?
Collagen Fibrils
What is the 2 structures collagen fibrils can form that create the basis of bone structure?
Lamellar: stacked, thin sheets with unidirectional fibrils
Woven: random orientation fibrils, common in growing organisms and initial stages of fracture healing
What are the different structures of bone? (0.5-1mm)
Primary lamellar: concentric rings of lamellae
Haversian (Osteonal): 10-15 lamellae arranged in concentric cylinders about the Haversian canal (for blood vessels, nerves, bone cells)
Laminar: Constructions of layered lamellae around blood vessels, contain woven bone
Woven
What are the 2 types of bone and how do their structures differ?
Cortical: tightly packed lamellae, Haversian, Laminar, and Woven
Trabecular: highly porous cellular solid, lamellae arranged in packets with rods and plates with marrow spaces
What is an osteon and what type of bone is it found in?
Primary unit of cortical bone: substructure of 10-15 concentric lamellae
- Represent the primary unit of adult cortical bone
- Can be removed which are then filled over time
What is the process of breaking down patrons and replacing them by different types of bone cells referred to as?
Remodelling
What is a cement line?
Weak interface between osteons surrounding interstitial lamellar bone thought to help dissipate energy during crack propagation
What are osteocytes?
Osteoblasts (cells) trapped between lamellae with processes reaching through to other osteocytes through canaliculi
What surrounds osteocytes?
Lacunae: Thin layer of fluid surrounding osteocytes
What is the purpose of osteocytes?
Sense mechanical deformation and transmit signals to other cells which can start remodeling
What is an osteoclast?
A cell which dissolves bone by secreting acid
What is an osteoblast?
A cell which secretes osteoids that mineralized to form bone
What is a bone lining cell?
Dormant osteoblasts which line trabecular bone surfaces, periosteal surfaces and Haversian canals, thought to start repair processes
What is a BMU?
Basic Multicellular Units, which coordinate the formation of secondary osteons
What are the processes in BMU’s and how long do they take on average?
Activation: osteoclasts are signalled by osteocytes - 3 days
Resorption and Reversal: bone is dissolved - 30 days
Formation: osteoblasts lay down osteoid - 90 days
Mineralization: 10 days for sufficient calcium and phosphorus, 60% occurs in 24hrs, rest over 6 months - 180 days
How does remodeling differ for trabecular bone?
Less organized than cortical bone
Osteoclasts attach themselves at the surface and eat a small region, then osteoblasts follow
What is Frost’s mechanical theory?
- Stipulates that bone remodeling responds to strain at specific biological setpoints
What happens to bone if it it’s usage is trivial (low usage of the bone)?
The body is thought to remodel the bone, decreasing its mass
What happens if the Repair MES is exceeded?
The bone is repaired with woven structure which is more susceptible to fracture
What are the 2 mechanical properties of bone that differentiate it from steel and other metals?
Anisotropic: bone strength varies with direction
Heterogenous: strength varies with spatial location
What is typically defined as the longitudinal axis?
The diaphyseal axis of long bone
Is the strength of bone similar in tension and compression?
No
Why is the transverse tensile strength of bone so low?
Has not adapted due to rarely occurring during normal physiological loading
What is the typical failure criteria for bone?
Excessive strain
Why does drilling into the femur as part of a hip replacement cause fracture of the femur?
Transverse tensile strength is quite low due to rare occurrence in normal physiological loading
What are mechanical properties of bone typically attributed to?
Ash density differences, not age
What is the conversion from bone volume fraction to apparent density?
1.8g/cm3
What is the relationship between ash density and yield strain and what are its implications?
No correlation suggesting that it controls failure
Does bone have a greater resistance to fatigue and creep in tension or compression?
Compression
How is fatigue for bone typically plotted?
Strain vs time (as opposed to stress vs time for steels and other materials)
What is an important caveat regarding our understanding of fatigue in bones?
It is based on cadaveric bone, so high cycle fatigue actually occurs
What happens to bone post-yield?
The elastic modulus decreases - a biological response is triggered
How is the elastic modulus and strength of cortical bone related to the stress rate?
Proportional, it is a visco-elastic material
Why can bone be assumed to behave elastically?
Normal loading is within a narrow range of strain rates
How is the ultimate strength affected by aging? How is the ultimate strain?
Strength: unaffected
Strain: Reduces with age
What difference is noteworthy between anisotropy in trabecular bone when compared to cortical bone?
The main longtudinal axis exhibits various positions throughout the structure
What are mechanical properties of trabecular bone characterized as?
A function of density (usually apparent)
Is the fatigue behaviour of trabecular bone similar to cortical?
Yes, it exhibits fatigue and experiences a reduction in elastic modulus and strength when loaded beyond yield
How does the anisotropy of trabecular bone change with age?
The degree of anisotropy increases with age
What is the primary failure mode for trabecular bone at later ages?
Buckling
