Week 6 - Osteology, fractures + fracture repair Flashcards
What are the 5 different types of bones in the body?
- Long
- Short
- Flat
- Irregular
- Sesamoid
What are the functions of bone?
↳ Protection
↳ Locomotion and support (through muscle attachment)
↳ Storage of mineral salts (e.g. calcium + phosphate)
↳ Production of red blood cells (RBCs)
↳ As a secretory endocrine organ
What is compact bone?
- Also known as cortial bone
- Gives bone smooth, white outer appearance
- Compact bone is densely structured and incredibly hard
- Responsible for protection, support and movement
What is cancellous bone?
- Also known as spongy bone
- Much less dense than cortial due to it’s open cell, porous network
- Typically found in short bones and the ends of long bones
- Despite it’s structure cancellous bone adapts along lines of stress known as trabeculae
- Excellent strength to weight ratio - strong in specific directions
What would a cross section through a long bone look like?
- Variable thickness along the length of the bone.
- Comprised entirely of compact bone in the shaft (diaphysis) → provides resistance to compression forces.
- Cancellous bone present towards weight bearing ends of bone (epiphysis) → aligns along lines of stress to provide multi-directional resistance to force.
What is the structure of a flat bone?
- Cancellous bone sandwiched between two relatively thick layers of compact bone that acts as protection
- Provides protection against impact while remaining relatively light.
What is the structure of short and irregular bones?
- Cancellous structure surrounded by a thin layer of compact bone
- Too small to have a shaft
- Contain trabeculae
What are trabeculae?
Thin sheets or layers of bone able to absorb compressive forces from a variety of directions. (means beam)
What is the epiphysis and what is it’s function?
“upon”
→End of long bones
→Part of the bone that forms articulation with another at a joint.
→Receives pressure and bears weight
What is the metaphysis and what is it’s function?
“after”
→ Neck of a long bone
→ Contains growth plate in children and young adults
What is diaphysis and what is it’s function?
“through”
→ Shaft of a long bone
→ Compact/cortial bone
→ Contains marrow cavity within
What is the periosteum?
Outer layer of connective tissue surrounding bones
2 layers:
→ outer fibrous layer
→ inner osteogenic layer
What are the circumferential lamellae?
- Deep to periosteum
- Bone is layered like an onion
What are haversion systems also known as in compact bone?
Osteons
What is a haversian canal?
Central canal that contains the neurovascular bundle within it.
What is an osteon?
Basic metabolic unit of bone
What are concentric lamellae?
Exist within an osteon around Haversion canals
The collagen fibres of each layer are arranged at different angles to create greater torsional strength.
What are circumferential lamellae?
Run just under the periosteum in parallel layers on bone surface
What are interstitial lamellae?
Run between osteons: remnants of osteons left behind during remodelling.
What does a haversion system contain?
Haversian canal + Concentric lamellae
What are volkmann’s canals?
Small horizontal channels in bone that transmit blood vessels from the periosteum into the bone and communicate with Haversian channels
What is the cement line?
Outer rim of haversion system
What are lacunae?
Black pitts within osteons.
→Within them is a mature bone cell or osteocyte
→This cell has participated in the bone making process and is then stuck to maintain the bone.
→They communicate through canaliculi (little canals) → means osteocytes are still able to communicate with other cells + receive nutrients and get rid of waste.
What is the inner circumferential layer?
Onion-like layers separating the Haversian systems from the endosteum and marrow cavity
What is the endosteum?
- Single layer of connective tissue
- Contains osteoprogenitor (stem) cells
What is the medullary (marrow) cavity?
- Children → contains “red” marrow of haematopoietic cells (produce RBCs)
- Adults → yellow marrow of adipose cells and cancellous bone
Only find haematopoietic cells as an adult in the bones of the trunk: vertabrae and areas of the pelvis and sternum.
What is woven bone?
- Haphazard arrangement of collagen fibres.
- Typically found in foetal bone.
- Also found in process of secondary fracture healing known as the callus formation.
What are the 3 main components that form bone (histology)?
Extracellular matrix → Type 1 collagen (90-95%) , Protein gel - Collagen gives resistance to tension.
Inorganic mineral salts → Hydroxyapatite (Calcium and phosphate) - gives bones resistance to compression.
Bone cells → Osteoblasts, osteocytes, osteoclasts, osteoprogenitor cells.
Function of osteocyte?
Live in lacunae in haversian systems.
Arms reach down canaliculi until they touch with another osteocyte meaning they can exchange information with each other e.g. if stress is detected
Function of osteoblast?
(Builders)
They form new bone and create haversian systems
Function of osteogenic cell?
(Stem cell)
Reside in endosteum and periosteum (inner and outer layers of bone)
Function of osteoclast?
(Resorbs bone)
Clast = destroying cell.
Resorb old bone that is not needed anymore.
First step in forming new haversian system.
Mature into Osteocyte cells.
How is bone tissue regulated?
- Bone - while 70% inorganic content - is not an inert tissue
- When bone is subjected to stress (compression or tension), small electrical signals stimulate osteocytes to signal bone growth .
- Stem cells are also sensitive to mechanical stress
- Cells respond to strengthen the region along that line of stress
- Similarly, a lack of detected stress in a region results in a resorption of bone
What is Wolffs Law of bone remodling?
“Every change in the form and function of bones, or of their function alone, is followed by definite changes in their internal architecture and equally definite secondary alteration to the external conformation, in accordance with mathematical laws”
(use it or lose it)
How did Frost add to wolff’s law?
It isn’t just about stress, but the level of stress as well as tension and other forces.
e.g. day to day level of stress =adapted state - not going to gain or lose bone
What is a fracture?
A loss of continuity within substance of bone.
Fractures can result from trauma, pathology (e.g. tumours) or prolonged exposure to high force (e.g. stress fracture).
What information are fractures classified by?
- mechanism
- open/closed
- joint involvement
- displacement
- pattern
- fragmentation
What mechanisms classify a fracture?
Traumatic → e.g. arising from high force, direct impact such as Road Traffic Collision (RTC)
Pathologic → e.g. arising from a smaller force, but suffered by a weakened bone e.g. in the presence of a bone tumour where bone content is less
What are open/closed fractures?
Closed - the skin is intact
Open - the bone fragment has penetrated the skin
What does joint involvement include?
Does the fracture enter into the articulating parts of a bone?
What information is displacement classified by?
Non-displaced → bone fragments remain in a relative anatomical position
Displaced → bones can be translated (moved sideways), rotated, angulated, or bone ends can overlap which shortens the limb
What does pattern of the fracture involve?
Descriptive of the fracture on radiograph. Can be linear, spiral, oblique, wedge/crush, avulsed, segmental
What does fragmentation involve?
→Incomplete fracture where bones remain in contact (e.g. a hairline fracture)
→ Complete - where bones are separated
→ Communited - where the bone has been fractured into multiple fragments.
What is the most difficult fracture to resolve?
An open, communited fracture is the most difficult due to the complexity of piecing the bones back together and the risk of infection.
(inevitably require surgery)
What is regeneration?
When tissue formed during the healing process is the same as the tissue that was damaged i.e. bone tissue replaces bone tissue.
What is repair?
When tissue formed during the healing process is different (and often inferior in function) to that which is damaged.
Is fracture healing regeneration or repair?
Regeneration
What are the two main types of fracture healing?
Primary
Secondary
What does primary fracture healing involve?
Occurs in the presence of a rigid fracture site, such as when surgical fixation has occurred.
Also known as “direct healing”
What does secondary healing involve?
Occurs when there is micromovement at the fracture site, such as when the fracture is fixed in Plaster of Paris (POP).
Also known as “indirect healing”
What are the 5 stages of secondary fracture healing in a long bone?
- Haematoma
- Cellular proliferation + cartilage model formation
- Callus formation
- Consolidation of callus
- Remodelling
What is fracture healing always dependant on?
The presence of other complications (such as infection, re-injury etc).
What occurs in stage 1 (Haematoma)
(week 0-2)
↳Trauma to the structure of bone disrupts the blood vessels causing a bleed
↳ Undamaged periosteum will prevent blood egress
↳ Clotting factors and platelets stimulate clot of blood
↳ Torn blood vessels lead to necrosis at bone ends
↳ New blood vessels, stimulated by acidic conditions of cellular breakdown sprout from undamaged vessels and grow towards fracture site.
↳ Cellular proliferation occurs - osteochondroprogenitor (stem) cells begin differentiation into osteoblasts and chondroblasts.
What occurs in stage 2 (Proliferation and cartilage model)
(week 1-2)
↳ Differentiation and proliferation of stem cells into chondroblasts and osteoblasts
↳ Cartilage laid down by chondroblasts, gradually replacing haematoma
↳ Chondrocytes proliferate and hypertrophy - releasing contents into ECM
↳Cartilage model - gel-like fusion between bone ends
What occurs in stage 3 (callus formation)
(week 2-6)
↳ New blood vessels bring chondroclasts and osteoblasts to the fracture site
↳ Osteoblasts lay down collagen and mineral salts to form woven bone
↳ Chondroclasts remove cartilage model
↳ External and internal “bridging” callus visible on X-ray
↳ Periosteum reformed
↳ Haematoma almost completely replaced by cellular tissue
↳ Necrotic bone end resorbed
↳ Fracture site is stabilised and some movement can be allowed
What occurs in stage 4 (consolidation of callus)
(6-12 weeks)
↳ Collagen fibres reorganise along lines of stress (when bone is being used functionally)
↳ Haversian systems start to reform in compact bone. This process is known as “cutting cones”.
↳ Osteoclasts lead the removal of old bone with osteoblasts following closely behind to form a new haversian system
↳ Outcome of this stage = sufficient bone strength to permit normal weight bearing
What occurs in stage 5 (remodelling)
(Up to 2 years)
↳ Gradual strengthening of bone by osteoblasts along lines of stress
↳ Surplus bone removed slowly by osteoclasts
↳ Marrow cavity and architecture reformed
↳ Bulbous collar will remain at fracture site in adults, but be removed in children
How does primary fracture healing occur in long bones?
↳ Occurs when there is a rigid fixation by metalwork to immobilise fracture site
↳ Still have haematoma and inflammation
↳ Stem cells differentiate into osteoblasts straight away (NOT chondroblasts) to produce new bone
↳ “Cutting cone” process occurs at fracture site without need for callus formation stage
↳ Result is much quicker healing and fracture recovery time
↳ HOWEVER can increase risk of things like infection as well as rejection to to the metal.
How does fracture healing in cancellous bone occur?
↳ Occurs in epiphysis of long bones and short/irregular bones.
Similar to secondary healing - 5 stages:
1. Haematoma
2. Cellular proliferation of osteogenic cells at fracture site
3. Intercellular collagen matrix laid down onto existing trabeculae by chondroblasts + osteoblasts
4. Matrix mineralised
5. Bone strengthened along lines of stress
Key points about cancellous fracture healing?
- Outer layer of cortial bone healing lags behind cancellous bone healing
- No cartilage model/callus formation is typically seen unless there has been significant fracture
-Typically these fractures heal quickly with few complications
What is the approximate timeline of fracture healing for callus formation?
3-4 weeks upper limb
6-8 weeks lower limb
What is the approximate timeline of fracture healing for consolidation of callus?
6-8 weeks upper limb
12-16 weeks lower limb
What is the approx. timeline of fracture healing for the completion of consolidation?
12 weeks upper limb
24 weeks lower limb
What is the approx. timeline of fracture healing for remodelling?
up to 2 years
What are local factors affecting fracture healing?
Fracture characteristics → excessive movement, misalignment, extensive damage, and soft tissues caught within fracture ends can lead to delayed or non-union
Infection → can lead to poor healing and delayed or non-union
Blood supply → reduced blood supply to the fracture site can lead to delayed or non-union
What are the systemic factors affecting fracture healing?
(the presence of any of these factors predisposes to poor healing)
→ advanced age
→ obesity
→ anaemia
→ endocrine conditions - diabetes mellitus, parathyroid disease and menopause
→ steroid administration
→ malnutrition
→ smoking
