Locomotor - Fracture Physiology Flashcards
Describe the functional cells and their involvement in the biology of fracture repair.
RBCs - formation of haematoma
Macrophages - remove necrotic tissue in medullary region of fracture
Fibroblasts - formation of granulation tissue (eg., proud flesh) & bone matrix
Chondroblasts - formation of connective tissue and fibrocartilage
Osteoclasts - derived from fusion of monocytes; multinuclear; remove necrotic bone in fractures
Osteoblasts - differentiated from inner cells of the soft callus near the fracture fragments; lay down periosteal callus (woven/cancellous bone) & compact bone (lamellar)
Describe how the pathway of functional-cell involvement in fracture repair.
Ruptured blood vessels → haemorrhage and clot formation by RBCs, some ischaemic necrosis → dead marrow liquefies and is phagocytosed by macrophages → dead bone is removed by osteoclasts → continues long after bone has been united by callus
Periosteum tears, fragments displaced → haematoma formation by RBCs, necrosis of any isolated fragments → mesenchymal cells proliferate in haematoma - granulation tissue formed by fibroblasts → invading cells: endothelial, fibroblasts, osteoprogenitor → loose collagenous tissue **(primary callus) **→ mesenchymal cells differentiate to osteoblasts and chondroblasts → Woven bone (secondary callus)
What is the difference between internal and external secondary calluses?
Secondary callus:
External callus
- from periosteum
- Periosteal growth eventually bridges the gap between the fracture ends
- Blood supply is outstripped
- Cartilage is produced instead of osteoid
- Blood vessels invade cartilage → endochondral ossification
- Osteoid becomes ossified
Internal callus (aka medullary callus)
- from endosteum
- Rarely forms cartilage
- May occlude the medullary cavity
- Osteoid remodelled by osteoclastic resorption
- Mature lamellar bone
What are the four main stages of fracture repair?
1. Vascularisation →
2. Organisation Differentiation (stem cells recruited into site & differentiating) →
3. Osteogenesis (bone development, depends on amount of movement and oxygen avail at fracture site) →
4. Remodelling
What are the two types of fracture healing?
- Indirect aka Secondary union- natural process that involves formation of callus, ie., specific alignment of tissue
- Direct union - normally requires surgical intervention; two types: contact healing & gap healing
How does indirect aka secondary union work?
Haematoma → granulation tissue → **connective tissue **→ fibrocartilage and cancellous bone → bone.
- Will occur even with large gap eg., >1 mm
- Tolerates some movement
- Occurs in incompletely reconstructed fractures, casts & in absence of intervention
What are the three overlapping phases of indirect aka secondary union?
- Inflammation
- Callus formation
- Remodelling
What is faster: Indirect/Secondary Union or Direct/Primary Union?
Indirect/Secondary Union
Indirect/Secondary Union, which involves formation of a callus, is stronger than Direct/Primary Union, which doesn’t involve the formation of a callus.
True or false?
True.
The three overlapping phases of indirect aka secondary union are inflammation, callus formation and remodelling. What happens with inflammation and how long does it occur?
** Inflammation:**
- formation of haematoma from bleeding in medullary portion of bone
- haematoma holds continued release of growth factors, which act as beacon for stem cells to site (chemotactic)
- medullary vessels re-form in five days
- formation of medullary callus
- bone resorbed from fractured ends
- extra-osseous blood supply develops → osteoblast layer thickens to form start
of periosteal callus
Inflammation takes place in about 48 hours
The medullary blood vessels can re-form in 5 days
The three overlapping phases of indirect aka secondary union are inflammation, callus formation and remodelling. What happens with callus-formation phase and how long does it occur?
Medullary callus & periosteal callus grow/fuse together across fracture to form final callus
- unlike normal/original bone in which ~70% of blood supply comes from medullary vessels, callus derives 60-80% of blood from extra-osseous supply
This can take several weeks.
The three overlapping phases of indirect aka secondary union are inflammation, callus formation and remodelling. What happens with remodelling phase and how long does it occur?
Remodelling is very efficient but it can take weeks, months & years.
- osteoclasts resorb parts of trabecular/cancellous bone, replaced by compact bone laid by osteoblasts; but no direct cortical/compact-bone repair.
What are the two types of direct union/primary union?
Contact healing and Gap healing.
What is contact healing? How does it work?
Contact healing is a type of direct fracture healing/primary union:
- essentially skips callus formation, straight to internal, cortical remodelling
- absolute stability required
- fragments in contact over very tiny fracture gaps of <0.01 mm (hairline fracture)
- normally forced together by plate compression
- direct migration of osteones (Haversian systems) across fracture lines
What is gap healing?
Gap healing is a type of primary fracture healing/direct union:
- absolute stability required
- small fracture gap <1 mm
- normally relates to implant reconstruction
Layered bone forms in fracture gap along line of shear (eg. weak joint in which fibres are in right angles to bone)
→ osteones then expand across gap
- stronger than contact healing because fibres formed along line of bone add reinforcement
