95 - Bone Pathology Flashcards
Cells involved in bone mechanotransduction
Osteocytes
Appearance of osteoblasts
Resting - flat, look like fibroblasts
Active - large, densely basophilic
Location of RANKL
On surface of osteoblasts
Location of osteoprotegrin (OPG)
Released by osteoblasts
Role of PTH
Stimulates osteoblasts to increase RANKL, decrease OPG.
Doesn’t directly stimulate osteoclasts.
What is osteoid?
Organic matrix laid down by osteoblasts in cartilage.
Contains type 1 collagen, other proteins, some GAGs, growth factors, cytokines
Basic multicellular unit of bone
Module of remodelling.
Osteoclasts in front, osteoblasts behing.
Capillary in the middle.
Makes a tunnel of bone, rebuilds a new osteon (lamellar bone, not woven bone)
Do osteoblasts release calcium and phosphate into osteoid?
Not directly.
Release enzymes which increase retention of calcium and phosphate.
What can commonly cause a Colle’s fracture
Fall on outstretched hand
Comminuted fractures
When there are more than two pieces of bone
Stages of fracture healing
1 a, b
2 a,b
3
- Inflammatory phase:
a) bleeding and haematoma formation
b) vascular granulation tissue - Reparative phase:
a) Soft callus (mostly cartilage)
b) Hard callus (woven bone) - Remodelling phase
Inflammatory phase of bone healing
• Haematoma formation
– fibrin mesh creates a framework
– damaged matrix releases trapped growth factors and cytokines
– stem cells activated to start repair
– stem cells differentiate very early depending on local stability: osteoblast > chondroblast > fibroblast
• Granulation tissue formation
• Hours to days
What determines stem cell differentiation in a fracture?
Stability.
If the break is very stable, most will differentiate to osteoblasts.
If very unstable, most will become fibroblasts
Soft callus stage of fracture healing
1-3
• Cartilage formation
– Holds the fractured ends together but no structural rigidity (so don’t want to weight-bear with it)
• Periosteum repairs itself over the outside
• Days to weeks
Hard callus stage of fracture healing
1-3
• Osteoid formation and ossification
– woven bone
– endochondral ossification (same as in growth plates)
• A thickened area of woven bone (‘fusiform’, as bulges out from surrounding bone)
– rigid, but not as strong as it was when intact
• Weeks to months
Bone remodelling stage of fracture healing
1-3
• Bone remodelling – osteoclasts followed by osteoblasts – woven bone to lamellar bone – along lines of stress • Result: “completely reconstituted” • Months to years
Ways to promote union of fracture
1-3
– Minimise the gap (“reduction” of the fracture)
– Minimise the strain or movement (“fixation”)
– Minimise any other factors that slow healing
• e.g. poor blood supply, infection, smoking
Non-union
A compilation of fracture healing.
Can get pseudoarthrosis (cells differentiate into synovial sheath around non-union bone)
Malunion
Healing of a bone in an unacceptable position
Infection of fracture
Can lead to osteomyelitis. Very severe Disruption of vessels in periosteum/haversian canals lead to infected fragments of necrotic bone. Can cause sepsis. Often S aureus
Osteonecrosis
Fractures can interrupt the blood supply and leave part of the bone ischaemic.
EG: neck of femur, scaphoid
Examples of complications of bone healing
Non-union
Malunion
Infection of fracture
Osteonecrosis
Process by which soft callus changes to hard callus
Endochondral ossification
Which cells from normal bone form a soft callus
Osteoprogenitor cells
At which stage in fracture healing do osteoprogenitor cells decide which cell to differentiate into?
Inflammaotry phase - haematoma
