Bone Flashcards
Functions of bone (5)
Support load Attachment site for muscles Protect organs (e.g. thoracic organs) Contains haematopoietic stem cells Calcium store
Composition of bone (% inorganic and organic)
65% Inorganic (Calcium hydroxyapatite)
35% Organic (Osteocytes, Protein matrix, Proteoglycans)
What mechanical properties do the inorganic & organic components of bone confer
Inorganic: Compressive strength
Organic: Tensile strength (collagen is like a rope)
What is the predominant collagen in bone
95% Collagen Type 1
What % of cortical bone is calcified
80-90%
What % of cancellous bone is calcified
15-25%
What are the 5 classes of bone
Flat bones: Scapulae, Sternum, Clavicle, Ribs, Skull, Pelvis
Long bones: Bones of the limbs
Irregular bone: Vertebrae, Sacrum & Coccyx, Base of skull
Short bones: Ossicles, Carpal & Tarsal bones
Sesamoid bones: Patella
What are the smallest functional units of cortical/lamellar bone?
Osteons
What are cement lines
The interface between the osteons and extra-osteonal bone matrix
What is the significance of cement lines
they are areas of low mineralisation - fractures/cracks may propagate along these lines
What 3 cells are in the Bone modelling Unit (BMU)
Osteoblasts
Osteoclasts
Bone lining cells (stem cells found on endosteum)
Recall the order of events in bone remodelling
After sensing damage, BLCs recruit ostoeclasts
Osteoclasts resorb bone
Osteoclasts die by apoptosis
BLC differentiate to Osteoblasts and secrete osteoid
Mineralisation
Maturation
Factors affecting mechanical properties of bone: Loading rate (3)
Increasing the loading rate increases the Youngs Modulus of bone –> bone is stiffer
Bone becomes more brittle –> More likely to see brittle failure
More energy is absorbed by bone
Factors affecting mechanical properties of bone: Loading orientation
Bone is anisotropic meaning that the stress response is dependent on the orientation of loading
Higher ultimate strength in Compression than tension, shear is lowest
Higher ultimate strength in Longitudinal than transverse
Factors affecting mechanical properties of bone: Age
Less stiff (lower E)
Less strong (lower ultimate stress)
More brittle
All increase fracture risk
Factors affecting cortical bone adaptation
Magnitude of loading
Frequency of loading
Number of loading cycles
Features that change in trabecular adaptation
Number
Spacing
Orientation
Thickness
Types of fracture (6)
Simple: only 2 fragments Comminuted: more than 2 fragments Open/compound: perforates the skin Closed: does not perforate skin Compression Displaced
Types of simple fracture (4)
Transverse: perpendicular to long bone axis
Oblique: fracture at an angle (usu. 45) to long bone axis
Spiral: fracture line is spiral/torsional
Greenstick: fracture doesn’t go all the way through the bone
Factors affecting mechanical properties: Creep
Bone is viscoplastic
Loading bone causes irreversible/plastic change (bonds break between collagen molecules)
Next time it is loaded, its material properties vary
Causes of fractures (2)
Traumatic
Non-traumatic
How does cortical bone sense loading to adapt to it
Change in stress
Change in strain - osteocytes are attached to stiff matrix, loads strains stretch receptors on cells
Change in fluid flow in canaliculi
Steps in fracture healing
Haematoma formations
Inflammation
Fibroblasts & Chondrocytes infiltrate and release fibrous tissue and collagen type 2 to form a soft callus
Callus is mineralised to form hard/bony callus
Bony callus is replaced by bone material - osteoblasts secrete protein matrix and mineralise
Remodelling
Complications of fracture healing (4)
Non-union
Unequal limb length
Infection
Compartment syndrome
