Pathology 4: bone growth and healing + metabolic bone disease

Question 1

Describe the anatomical regions of a normal tubular bone.

Answer 1

• A normal tubular bone has an epiphysis, physis (growth plate), metaphysis and diaphysis

Question 2

What are the differences between woven bone and lamellar bone?

Answer 2

Woven bone…
- made rapidly such as in the unborn child, a healing fracture or in some diseases
- type I collagen fibres are laid down and then mineralised as criss-cross woven bone which is able to withstand stress equally well in all directions
In lamellar bone…
- the collagen fibres are nearly parallel and this takes longer to make but is much stronger
- the collagen fibres run in opposite directions in alternating layers of lamellar bone, helping the bone to resist torsion forces
- normally the mature skeleton is composed solely of lamellar bone.

Question 3

A 12 year old boy falls on outstretched hand causing a fracture of the distal end of his right radius, if the fractured bone goes through the normal healing sequence, describe the typical histological features that a pathologist will see in the tissue at the following time points (less than 24 hours after fracture, one week after fracture, three weeks after the fracture)

Answer 3

Less than 24 hours after fracture…
- Haematoma (blood clot) fills the fracture gap (due to rupture of blood vessels)
- note: the blood clot provides a fibrin (protein involved in clotting blood) mesh which seals the fracture site and provides a scaffold for the influx of inflammatory cells, fibroblasts and support for new capillary growth (granulation tissue)

One week after fracture…
- soft callus forms (osteoclast and osteoblast activity stimulated)

Three weeks after fracture…
- bone remodeling occurs, woven bone first, then lamellar bone longer term, the medullary cavity is restored

Question 4

Describe three main steps by which a fracture is likely to heal normally and for each step describe the normal cellular response.

Answer 4

Note: fracture will heal by secondary healing
Inflammatory phase…
- bleeding from torn vessels, clotting cascade activated and inflammatory cells (platelets, PMNs, macrophages) brought into the area
- Haematoma forms in fracture gap
- (immediate from injury, peaks at 48 hrs, subsides by one week)

Reparative phase…
- mesenchymal stem cells become fibroblasts, osteoblasts, and chondroblasts at the fracture site
- callus formed by intramembranous ossification at cortical bone ends (hard callus) and by chondrogenesis at the periphery (soft callus)
- endochondral ossification converts callus to woven bone
- (starts within first few days and lasts a few weeks)

Remodeling phase…
- woven bone replaced by lamellar bone, and excess callus resorbed
- (begins a few weeks after injury and lasts years)

Question 5

What is osteoid, what is it composed of, and what is deposited to give bone its strength and rigidity?

Answer 5

• Osteoid = unmineralized bone matrix and is composed of type I collagen and glycosaminoglycans (GAGs)
• Calcium hydroxyapatite (a calcium salt crystal) is then deposited which gives bone its strength and rigidity.

Question 6

What are the two ossification methods by which bone normally forms?

Answer 6

1. Intramembranous ossification: bone develops directly from sheets of mesenchymal connective tissue (eg. flat bones of face, clavicles)
2. Endochondral ossification bone devleops by replacing hyaline cartilage, takes much longer than IO (eg. bones at base of skull and long bones)

Question 7

Describe the layers of a long bone beginning at the external surface and moving towards the centre of the bone.

Answer 7

The layers of a long bone, beginning at the external surface are:

1 (outer). Periosteum (yellow arrow)
2. Outer circumferential lamellae (green arrow)
3. Compact bone (Haversian systems) (green circle)
4. Inner circumferential lamellae
5. Endosteal surface of compact bone (white arrow)
6 (inner). Trabecular bone

Question 8

Describe the structure of an osteon/Haversian system.

Answer 8

• Compact bone is organized as parallel columns known as Haversian systems or osteons which run lengthwise down the axis of long bones
• these columns are composed of lamellae: concentric rings of bone surrounding a central channel or Haversian canal that contains nerves, blood vessels, and the lymphatic system of bone
• the parallel Haversian canals are connected to one another by the perpendicular Volkmann’s canals.

Question 9

Describe the process of bone remodelling.

Answer 9

• Mononuclear osteoclast-precursors arrive at a bone surface and differentiate into functional osteoclasts
• (remodeling is regulated by RANKL and OPG ratio)
• Mature osteoclasts secrete acid and proteases (matrix metalloproteinases) onto the bone surface, excavating a pit known as ‘Howship’s lacuna’
• Resorption phase ends with osteoclast apoptosis
• a reversal phase follows characterised by activation of osteoblasts to replace the excavated bone
• Osteoclast and osteoblast activity is tightly coupled, and the amount of bone synthesized matches the amount resorbed
• The newly secreted matrix (osteoid) becomes mineralised to form mature bone
• the remodelling cycle ends where new bone formation is complete and the osteoblasts are either incorporated into the new bone matrix (as osteocytes) or become dormant surface bone lining cells
• The net result of each cycle is a formation of a new osteon (a packet of bone in which the collagen fibres are aligned)

Question 10

Bone remodeling diagram…

Answer 10

Question 11

Describe the regulation of osteoclast activity by the Receptor Activator of Nuclear Factor Kappa Beta (RANK) and Osteoprotegrin (OPG).

Answer 11

• Osteoclasts are derived from the same mononuclear cells that differentiate into macrophages
• RANK-ligand binds to its receptor RANK located on the cell surface of osteoclast precursors (this interaction causes the precursor cells to produce functional osteoclasts)
• Stromal cells also secrete osteoprotegrin (OPG) which acts as a decoy for RANKL (preventing it from binding the RANK receptor on osteoclast precursors)
• Consequently, OPG prevents bone resorption by inhibiting osteoclast differentiation
• note: bone resorption or bone formation can be favoured by altering the RANK:OPG ratio
• note: PTH and steroids promote osteoclast differentiation and bone turnover
• note: in contrast, bone morphogenetic proteins and sex hormones block osteoclast differentiation or activity by favouring OPG expression

Question 12

List and define the various fracture types (7 types).

Answer 12

1. Simple - bone fracture without skin penetration
2. Compound - bone fracture with skin penetration
3. Comminuted - bone fracture with fragmentation
4. Displaced - bone fracture where the ends are not aligned properly
5. Stress - a fracture caused by increased physical activity and repetitive loads
6. Greenstick - a fracture seen in infants when the bone is soft, which extends only partially through the bone
7. Pathological - a fracture of a bone which is weakened by an underlying process such as a tumour

Question 13

What would you see if you examined a fracture under the microscope on day 1?

Answer 13

An organising haematoma

Question 14

What would you see if you examined a fracture after 2-3 weeks?

Answer 14

• Soft callus is transformed into bony callus
• the activated osteoprogenitor cells deposit woven bone
• in some cases the activated mesenchymal cells in the soft tissues and bone surrounding the fracture line also differentiate into chondrocytes that make fibrocartilage and hyaline cartilage
• the newly formed cartilage along the fracture line undergoes endochondral ossification forming a contiguous network of bone with newly deposited bone trabeculae in the medulla and beneath the periosteum
• the fractured ends are bridged

Question 15

What would you see after 12 weeks?

Answer 15

• As the callus matures and as it is subjected to weight-bearing forces, portions that are not physically stressed are resorbed
• this remodelling reduces the size of the callus until the shape and outline of the fractured bone are re-established as lamellar bone
• the healing process is complete with restoration of the medullary cavity

Question 16

List five factors which can impair fracture healing.

Answer 16

• Displaced/comminuted fractures - can lead to deformity.
• Inadequate immobilisation - prevents normal callus maturation causing delayed or non-union.
• Open fractures - can become infected.
• Malnutrition.
• Skeletal dysplasia (congenital abnormalities).

Question 17

Describe the features of osteonecrosis.

Answer 17

• Osteonecrosis = ischaemic necrosis of bone and bone marrow
• the vascular insufficiency can occur through injury to blood vessels, thromboembolism, or external pressure on vessels
• (in the setting of ‘the bends’, nitrogen which has previously been in solution within fatty bone marrow forms a gas as the pressure is reduced when a diver ascends. This gas can put pressure on blood vessels causing obstruction and necrosis)

Question 18

List the conditions which are known to be associated with osteonecrosis.

Answer 18

• Alcohol abuse
• Bisphosphonate therapy (jaw bones)
• connective tissue disorders
• steroid treatment, chronic pancreatitis, The ‘bends’, Gaucher disease, infection, pregnancy, radiotherapy, sickle cell disease, trauma and tumours

Question 19

List four factors which predispose to osteomyelitis.

Answer 19

• Trauma.
• Ischaemia.
• Presence of foreign bodies.
• Pressure ulcers.

Question 20

Which bacteria commonly cause osteomyelitis?

Answer 20

• Contiguous spread from adjacent infected tissue or opened bones causes about 80% of osteomyelitis
• Staphylococcus aureus is present in over half of patients and other common bacteria include streptococci, gram negative enteric organisms and anaerobic bacteria

Question 21

Which bones are more commonly affected in children compared to adults?

Answer 21

• In children gram positive infections are most common and these usually affect the metaphyses of the tibia, femur or humerus
• In adults haematogenously spread osteomyelitis usually affects the vertebrae

Question 22

List 5 risk factors for osteomyelitis in adults

Answer 22

• Older age
• Debilitation
• Haemodialysis
• Sickle cell disease
• IV drug use

Question 23

Which organisms can cause osteomyelitis in patients who are immunosuppressed?

Answer 23

• Fungi and mycobacteria can cause haematogenous osteomyelitis in patients who are immunosuppressed

Question 24

If treatment of acute osteomyelitis is only partially successful what is the likely outcome?

Answer 24

• Chronic osteomyelitis.

Question 25

What would you see if you examined a fracture at the end of the first week?

Answer 25

• A mass of predominantly uncalcified tissue called soft callus
• this would include granulation tissue and proliferating fibroblasts with collagen deposition

Question 26

Metabolic bone disorders quiz Q1…

Answer 26

Question 27

Metabolic bone disorders quiz Q2…

Answer 27

Question 28

Metabolic bone disorders quiz Q3…

Answer 28

Question 29

Metabolic bone disorders quiz Q4…

Answer 29

Question 30

Metabolic bone disorders quiz Q5…

Answer 30