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What are the different types of bone cells? And what are their functions?
- Osteoblasts - form bone matrix, initiate bone mineralisation and bone resorption
- Osteocytes - detect changes in the mechanical environment and signal these changes to osteoblasts
- Osteoclasts - resorb bone
What often proceeds 'stress fractures'?
In what domestic species is exercise induced micro damage common?
What is pseudoarthrosis?
When the bony ends move into a pocket of fibrous tissue to become a false joint
Give examples of other factors that can complicate fracture repair
Malnutrition, bacterial osteomyelitis, interposition of large fragments of necrotic bone or soft tissue, age
What are the two main types of fractures?
Traumatic (excessive force) and pathological (abnormal bone broken by minimal trauma or weight bearing)
What area of bones is commonly fractured in immature animals, rather than mature animals?
The growth plate - the area of transition between bone and cartilage is a relatively weak point
Using the Salter-Harris classification system, describe type I-V fractures
- Type I and II fractures tend to heal quite well and have few complications because the physis is undisturbed.
- Type III and IV fractures cross the growth plate and so these often result in DJD due to the fracture changing the alignment of the joint.
- Type V fractures crush the growth plate.
What is an infarction?
A fracture involving trabecular bone only without cortical bone deformation
Explain the difference between a simple/open and compound/closed fracture?
A simple/open fracture involves cortical bone where the fracture is inside the skin and has not been exposed to the environment. Whereas a compound/closed fracture involves cortical bone where bone has escaped and been exposed to the environment.
What is a comminuted fracture?
A fracture that results in several small fragments
What is an avulsed fracture?
A fracture caused by the pull of a ligament
What is a greenstick fracture?
A fracture where one side is broken and the other side is only bent.
What is a transverse or spiral fracture?
A fracture involving orientation of the fracture line
Describe the events involved in a stable fracture
- Immediate - the periosteum tears, bone fragments are displaced, soft tissue is traumatised and haematoma forms. At the broken ends there can be necrosis of bone and marrow. Growth factors are released by macrophages and platelets in the clot and from the dead bone.
- 24-48hrs - proliferation of undifferentiated mesenchymal cells and neovascularisation penetrate the haematoma. Together these form a loose collagenous tissue.
- 36 hrs - the first woven bone is visible.
- 4-6 weeks - the primary callus forms, this is composed of woven bone and possibly hyaline cartilage.
- Months to years - the woven bone is replaced by strong mature lamellar bone. This is the secondary callus.
What is a good indication of fracture healing?
A reduction in callus size
What forms the internal and external parts of a callus?
- External - formed by the periosteum
- Internal - between ends of fragments and in medullary cavity
What may a callus contain if the blood supply is less than optimal?
Cartilage - this is not as strong but eventually undergoes endochondral ossification
What is rigid fracture repair?
Rigid fracture repair involves the surgical application of a device.
What is contact healing? How small must the gap be between bone ends to allow contact healing to occur?
Direct osteonal bridging with no callus formation. The gap should be less than 1mm to allow bone cells to migrate from the ends of the fracture.
What happens if there is a gap greater than 1mm between the ends of a fracture?
Woven bone forms and must be remodelled into osteonal bone
Why is instability bad for fracture repair?
Excessive movement and tension favours development of a fibrous tissue callus. Fibrous tissue does not stabilise the fracture and, unlike cartilage, does not act as a template for bone formation.
Give definitions for the following terms a) osteitis b) periostitis c) osteomyelitis d) sequestrum
- a) inflammation of bone
- b) inflammation of periosteum
- c) involves the medullary cavity
- d) fragment of dead bone isolated from blood supply and surrounded by a pool of exudate
How can pathogens enter bone directly?
Through the periosteum and cortex. This can either be through trauma or direct extension.
What are the different ways bone can become infected?
- Haematogenous - haematogenous bacterial osteomyelitis is common in foals, neonatal ruminants and pigs
- Bacterial - purulent (suppurative)
- Perinatal - umbilicus or oropharangeal origin
Why is haematogenous infection more common in immature animals?
The metaphyseal growth plate is where capillaries make sharp bends to join the medullary veins. In this area there is slow flow and turbulence of blood in larger descending limbs, lower phagocytic capacity discontinuous endothelial cells and no anastomoses (so thrombosis results in infarction and favours bacterial localisation). As the metaphyseal growth plate is still active an has not closed off
What is embolic osteomyelitis?
An embolus lodges in a capillary loop at the metaphysics. The inflammation causes lysis of metaphyseal bone and growth plate cartilage. This can cause mechanical instability and so the periosteum responds by producing reactive (woven) bone.
What can happen when the exudate in embolic osteomyelitis lyses the cortex? At what point of the cortex does it do this?
- The exudate can extend into the periosteum and cause periostitis. Exudates can also extend into the joint and onto the surface of the skin. Exudate in the joint cavity can potentially result in a septic joint.
- It lyses the cortex at its thinnest point - the metaphyseal cut back zone
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