Flashcards in Bone And Fracture Repair Deck (20):
What is intramembranous ossification?
• Takes place within condensations of mesenchymal tissue
• Not by replacement of a pre-existing hyaline cartilage template
• Mesenchymal tissue = capable of differentiating into various tissues (bone, cartilage, blood)
• Contributes to the thickening (but not lengthening) of long bones at their periosteal surfaces by appositional growth
Which bones develop by intramembranous ossification?
Flat bones develop by this process
- Clavicle (lateral end only)
- Pelvic bones
Describe the stages of intramembranous ossification
1. A small cluster of mesenchymal stem cells (MSCs) form a tight cluster of cells (a nidus).
2. The MSCs become osteoprogenitor cells (each developing more Golgi apparatus and rough endoplasmic reticulum).
3. The osteoprogenitor cells become osteoblasts and lay down an extracellular matrix containing dense, interwoven type I collagen (osteoid).
4. The osteoid mineralises to form rudimentary bone tissue spicules, which are surrounded by osteoblasts, and contain osteocytes.
5. The spicules join to form trabeculae, which merge to form woven bone, which is finally replaced by the lamellae of mature compact bone.
What are the types of bone?
1. Cancellous/ Spongy/ Trabecular bone
- A network of fine bony columns or plate.
- Combine strength with lightness
- Spaces filled with bone marrow
- Diagonal struts forming triangles
2. Cortical/ Compact bone
- Forms external surfaces of bones
- 80% of skeletal mass
What are the Haversian and Volkmann's canals?
Canals that carry blood vessels, lymph vessels and nerves
What are the physical differences between immature and mature bone?
- Immature bone has randomly arranged osteocytes
- Mature bone osteocytes arranged in the concentric lamellae of osteons.
- Resorption canals in mature bone run parallel with the osteons long axes
Describe the structure of trabeculae
• In cancellous bone
• No Haversian or Volkmann’s canals
• Not thick enough to need internal blood supply
• Obtain oxygen and nutrients from the bone marrow they’re lying in
• Each trabeculum consists of numerous osteocytes embedded within irregular lamellae or bone
• Osteoblasts and osteoclasts on their surface act to remodel them
Describe the process of bone remodelling
• Cutting cone bores a tunnel through the bone by the action of osteoclasts
• Osteoclasts release H+ ions and lysosomal enzymes
• Cut their way through the bone parallel with the plane of orientation of osteons
• Trailing osteoblasts begin to build new lamellae behind this
Describe the composition of bone
• 65% = mineral (calcium hydroxyapatite crystals)
• 23% = collagen (type 1)
• 10% = water
• 2% = non-collagen proteins
How do bones resist fracture?
• Bone resists fracture because it has great tensile and compressive strength, but also because it has a degree of flexibility.
• The lamellae are thought to be able to slip, relative to each other, before excessive load causes fracture.
Describe the process of fracture repair
1. Blood clot (haematoma) is formed in which granulation tissue arises
2. The procallus of granulation tissue is replaced by a fibrocartilagionous callus in which bony trabeculae develop
3. Endochondral and intramembranous ossification give rise to a bony callus of cancellous bone
4. Cancellous bone is replaced by compact cortical bone until remodelling is complete
How does a haematoma form from a fracture?
• Blood vessels in bone and periosteum break.
• A mass of clotted blood (haematoma) forms.
• Bone cells at the fracture edge die (no blood supply).
• Swelling and inflammation occur.
• Phagocytic cells and osteoclasts begin to remove dead and damaged tissue.
• Macrophages will eventually remove the blood clot.
Describe how the fibrocartilaginous callus forms during bone repair
• New blood vessels infiltrate the fracture haematoma = Angiogenisis
• A procallus (soft callus) of granulation tissue (i.e. tissue rich in capillaries and fibroblasts) develops.
• Fibroblasts produce collagen fibres that span the break. Others differentiate into chondroblasts that give rise to a sleeve of hyaline cartilage. An externally bulging, fibrocartilaginous matrix thus splints the broken bone.
• Concurrently, and more centrally, osteoblasts from the nearby periosteum and endosteum, (and multipotent cells from the bone marrow) invade the fracture site and begin bone reconstruction by forming spongy/ trabecular bone.
Describe how the bony callus forms during bone repair
• Bone trabeculae begin to appear in the fibrocartilaginous callus.
• The trabeculae develop as the former fibrocartilaginous callus is converted to a hard (bony) callus of cancellous bone.
• Ultimately endochondral ossification replaces all cartilage with cancellous bone, but intramembranous ossification also produces new cancellous bone in the area.
• These processes begin as soon as two days after fracture in young people.
• Bony callus formation continues for about two months until a very firm union is formed.
What can be done if a fracture involves a loss of bone fragment ?
• If a fracture involves loss of bone fragments, then bony union and callus formation is not possible.
• Since the 1970s, bone banks have become available to supply viable bone for grafting purposes. Bone fragments are frozen and used by orthopaedic surgeons.
What are the three types of bone donation?
• Autograft: donor is the recipient (most successful)
• Homograft: donor is a different human (may be rejected as ‘foreign’)
• Heterograft: donor is of a different species (least successful, though calf bone loses antigenicity with refrigeration)
What is osteoporosis?
• Metabolic bone disease (associated with age)
• Mineralised bone is decreased in mass to the point that it no longer provides adequate mechanical support
• Increased susceptibility to fracture
• Outer surfaces of the bony trabeculae of cancellous bone are regularly remodelled by osteoclasts resorption and osteoblasts deposition, In osteoporosis osteoclast resorption bays are incompletely filled
What is type 1 osteoporosis?
• Occurs in postmenopausal women
• Due to increased osteoclasts number
• Result of oestrogen withdrawal
What is type 2 osteoporosis?
• Occurs in elderly people of both sexes (senile osteoporosis)
• Generally occurs after age 70
• Reflects attenuated osteoblast function