Normal structure of bone Flashcards
What is the structure of bone?
Epiphysis - extends from articular surface to the growth plate.
Physis - growth plate
Metaphysis - extends from growth plate to where the diameter of the bone becomes narrow.
Diaphysis - shaft of bone. Extends from one metaphysis to the other.
Medullary canal - marrow cavity of a bone. The site of production of blood cells in children (red marrow). Red marrow is progressively replaced by inactive yellow marrow.
Periosteum - membraneous tissue that covers surface of bones.
Endosteum - membrane lining the inner surface of the bony wall.
NOTE- look at diagram on notes
What are the 2 types of bone?
- Woven bone
- Lamellar bone
- Includes cortical/ compact bone & Spongy/ cancellous bone
Describe the structure & formation of woven bone. When is the bone formed?
- Type 1 collagen fibres are laid down.
- Mineralised in a criss-cross woven pattern.
- forms rapidly & is able to withstand stress in all directions.
- Forms bones in the unborn child, in a healing fracture or in some diseases e.g. Paget’s.
Describe the structure & formation of lamellar bone. When is the bone formed?
- Type 1 collagen fibres are laid down nearly parallel
- The collagen fibres run in opposite directions in alternating layers (helps the bone resist torsion forces - twisting)
- Takes longer to make but is much stronger
- All adult bones are lamellar bones.
Describe the structure of cortical bone i.e. osteomyelitis
Found towards outer edge of long bone
- organised into parallel columns called Haversian Systems that run lengthways down axis of long bone
- Columns are formed by lamellae
- Lamellae surround central canal called Haversian canal- contain nerves, blood vessels & lymphatic system of bone
- Parallel Haversian canal are connected horizontally by perforating canals or Volkmann’s canals
Describe the structure of cancellous/ spongy bone
found deep in bone
The gaps between this bone contains red bone marrow
What is osteoid & which cell produces it?
Osteoid= unmineralised bone matrix
Made of type 1 collagen & glycosaminoglycans (GAGs)
Calcium hydroxyapatite (calcium salt crystal) is deposited= strength & rigidity
Produced by osteoblasts
How does bone form?
Bone formation= ossification
2 types of ossification:
- Intramembraneous ossification
- Endochondral officiation
Describe the process of intramembraneous ossification & give examples of bones that form by this method.
Bone develops directly from sheets of mesenchymal (undifferentiated) connective tissue.
Begins in utero & continues to adolescence
e.g. flat bones of face, most cranial bones (skull) & clavicles (collarbones)
Describe the process of Endochondral ossification & give examples of bones that form by this method.
Bones develop by replacing hyaline cartilage
cartilage acts as a template that is completely replaced by new bone.
Takes longer than intramembraneous ossification.
e.g. bones at base of skull & long bones
How is the structure of bone maintained?
bone remodelling
a balance between osteoblasts- produce bone cells & osteoclasts= remove bone cells
Process of bone remodelling?
- Activation:
- osteoclast precursors derived from haematopoietic stem cells arise at bone surface- differentiate into functional osteoclast - Resorption:
- mature osteoclast secrete acids & proteases onto bone surface
- this excavates a pit called Howship’s lacuna
- resorption phase ends w/ osteoclast apoptosis - Reversal:
-osteoblasts are activated to replaced excavated bones- derived from mesenchymal stem cells
- activity of osteoclasts & osteoblasts is balanced so amount of bone formed matches bone removed - Formation:
- osteoid is secreted by osteoblasts
- becomes mineralised w/ calcium hydroapittie to form mature bone - Termination:
- remodelling cycle ends when new bone formation is complete
osteoblasts are either incorporated into new bone matrix as osteocytes or become dormant surface bone lining cells
- end result of bone remodelling= osteon- a packed of bone in which collagen fibres are aligned
How is osteoclast & osteoblast activity regulated?
RANK
Osteoprotegrin (OPG)
How does RANK affect bone remodelling activity?
Encorugaes osteoclast activity
PTH stimulates osteoblasts & stromal cells to express RANKL on their cell membrane
RANkL bines to RANK receptor found on cell membrane of osteoclast precursor
In the presence of cytokine called M-CSF, causes osteoclast precursors cells to differentiate
How does osteoprotegrin affects bone remodelling?
Inhibits osteoclast activity
Stromal cells release osteoprgrin which stops cell from binding to RANK receptors on osteoclasts
It protects bone by inhitbit osteoclast differentiation= stops bone resportion
How can bone resorption or formation be favoured?
by altering the RANK: OPG ratio
Factors that affect this ratio:
- PTH- can promote osteoclasts differentiation & accelerate bone turn over by favouring RANK
- Vitamin D- favour RANK- stimulates bone resorption
- Sex hormones- testosterone & osterogen can block osteoclast differentiation by favouring osteoprotergin
- Inflammatory cytokines
How do fractures heal?
Will heal by secondary healing
Main cytokines- TGF-B, FGF, BMP
- Inflammatory phase:
-bleeding from torn vessels causes Haematoma in fracture gap
- clotting cascade activated & inflammatory cells (platelets, macrophages) move to site of fracture,
- haematoma forms in fracture gap
- occurs immediate from injury- 0-1 day - Reparative phase:
- Mesenchymal stem cells become fibroblasts, osteoblasts and chondroblasts at the fractures site
- callus formed by intramembranous ossification at cortical bone ends (hard callus) & by chondrogensis at periphery (soft callus)-
- starts w/in first few days & lasts few weeks (2-3) - Remodelling phase
- Wowen bone replace by lamella bone
- excess callus reabsorbed
- Begins few weeks after injury (3) & lasts years
Why is a haematoma or blood clot important for fracture healing?
Blood clot provides a fibrin mesh which seals fracture site & provides a scaffold for the inflammatory cells, fibroblasts & new capillary growth.
How is soft callus formed? Why is a soft callus important for healing?
Degranulating platelets & inflammatory cells release:
- Platelet derived growth factor (PDGF)
- Transforming growth factor beta (TGF-β)
- Fibroblast growth factor (FGF)
These growth factors activate osteoprogenitor cells in the periosteum, medullary cavity & surrounding soft tissues
Osteoprogenitor cells stimulate osteoclast & osteoblast activity
By end of first week, an uncalcified tissue mass has formed that provides anchorage btw the ends of the fractured bones
How is boney callus formed?
Occurs at 2-3 weeks after fracture when soft callus transforms into bony callus.
In some cases, activated mesenchymal cells in soft tissues & bone surrounding the fracture line also differentiate into chondrocytes that make fibrocartilage & hyaline cartilage
The newly formed cartilage along fracture line undergoes endochondral ossification forming a network of woven bone.
The fractured ends are bridged.
What occurs in the remodelling stage of fracture healing?
Occurs around 12 weeks after fracture.
As the callus matures, it becomes subjected to weight-bearing forces.
Areas of bone that are not physically stressed are resorbed by osteoclasts.
Remodelling reduces the size of the callus until the shape & outline of the fractured bone are re-established as lamellar bone.
Healing process complete w/ restoration of medullary cavity.
Factors that can lead to impaired healing
Open fractures- Infection
Older age- heals more slowly + poorer nutrition
Inadequate immobilisation - prevents normal callus maturation causing delayed or non-union
Avasular necrosis
Malnutrition
Displaced/comminuted fractures - can lead to deformity.
List complications of a fracture
Immediate:
- Haemorrhage
- Ischaemia
- Nerve palsy
Intermediate:
- Osteomyelitis
- Fat embolism from bone marrow
Late:
- Delayed union
- joint stiffness
- avascular necrosis
- malunion
What is osteonecrosis?
Osteonecrosis= avascular necrosis
Death of bone & bone marrow due to impairment of blood supply (ischaemia)
What causes osteonecrosis?
trauma- fracture or dislocation- injury to blood vessels
Steroids
alcohol
Thromboembolism
the bends- nitrogen which has previously been in solution w/in fatty bone marrow forms a gas as pressure is reduced when a diver ascends- gas can put pressure on blood vessels causing obstruction & infarction.
List conditions that are known to be associated with osteonecrosis
Alcohol abuse
Chronic pancreatitis
connective tissue disorder
bisphosphate therapy
sickle cell disease
Define osteomyelitis
Osteomyelitis is infection of the bone and bone marrow.
List four factors which predispose to osteomyelitis
- Trauma.
- Ischaemia.
- Presence of foreign bodies.
- Pressure ulcers
Which bacteria commonly cause osteomyelitis?
Contiguous spread from adjacent infected tissue or opened bones causes 80% of osteomyelitis- often polymicrobial.
Staphylococcus aureus is present in over half of patients
other common bacteria- streptococci, gram negative enteric organisms and anaerobic bacteria.
Haematogenously spread osteomyelitis usually results from a single organism
Which bones are more commonly affected in children compared to adults?
Children- gram positive infections are most common- usually affect the metaphyses of the tibia, femur or humerus.
Adults- haematogenously spread osteomyelitis usually affects the vertebrae
List 5 risk factors for osteomyelitis in adults
- Older age.
- Debilitation.
- Haemodialysis.
- Sickle cell disease.
- Injection drug use.
Which organisms can cause osteomyelitis in patients who are immunosuppressed?
Fungi & mycobacteria can cause haematogenous osteomyelitis in immunosupressed
If treatment of acute osteomyelitis is only partially successful what is the likely outcome?
Chronic osteomyelitis
What would you see if you examined a fracture at the end of the first week?
A mass of predominantly uncalcified tissue called soft callus.
would include granulation tissue & proliferating fibroblasts w/ collagen deposition.
