Musculoskeletal Pathology - Bone

Question 1

Examination of bone - methods

Answer 1

◼ Diagnostic imaging
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◼ Macroscopic
> Debone and saw
> ‘bug’ box (sarcophagus beetles)
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◼ Histological
> Requires demineralization (aka decalcification) and thus longer processing time.

Question 2

bone anatomy from middle to end

Answer 2

diaphysis
metaphysis
physis (gowth plate)
epiphysis
articular - epiphyseal cartilage (AEC) complex

Question 3

cortical vs trabecular bone

Answer 3

Cortical bone is dense and solid and surrounds the marrow space, whereas trabecular bone is composed of a honeycomb-like network of trabecular plates and rods interspersed in the bone marrow compartment.

Question 4

subchondral bone

Answer 4

Subchondral bone refers to the bony layer beneath the hyaline cartilage and cement line

Question 5

lamellar bone

Answer 5

Histology:
Lamellar bone is a mature bone that results from the remodeling of immature woven bone

Question 6

Histology of Bone: Bone formation mechanisms

Answer 6

Endochondral ossification
◼ Growth plates
> Metaphyseal = physis
> Epiphyseal = AEC
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Intramembranous ossification
◼ Any membrane
◼ Periosteum mainly

Question 7

Histology of Bone; constituents of bone - cells and matrix

Answer 7

Cells of bone
◼ Bone lining cells
◼ Osteocytes
◼ Osteoblasts
◼ Osteoclasts
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Matrix
◼ Mineral
◼ Osteoid

Question 8

Bone lining cells - where are they, what do they do?

Answer 8

◼ Line the endosteal surface forming a functional barrier between interstitial fluid and bone.
◼ Activate bone formation and resorption
◼ Source of osteoblasts
◼ Haematopoietic functions

Question 9

Osteoblasts - what do they do?

Answer 9

◼ Form bone on the surface of bone
◼ Respond to bone lining cells, osteocytes and parathyroid hormone
◼ Stimulate osteoclasts

Question 10

Osteocytes - where do they come from, where are they found? what do they do?

Answer 10

◼ Derived from osteoblasts
◼ Are surrounded by bone and are in lacunae
◼ Connect to each other with cytoplasmic processes in bone channels.
◼ Couple bone formation and resorption

Question 11

Osteoclasts - what are they, what do they do? inhibition?

Answer 11

◼ Monocyte–macrophage lineage (excavators)
◼ Respond to parathyroid hormone, and paracrine inflammatory cytokines
◼ Inhibited by calcitonin

Question 12

Histology: Bone; matrix - what is it? components?

Answer 12

Bone Matrix (osteoid)
◼ Type 1 collagen
◼ Ground substance
> Water
> Proteins
> Proteoglycans
> Glycosoaminoglycans (GAG)
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◼ Mineral

Question 13

lamellar bone vs woven bone? organization?

Answer 13

Lamellar bone
◼ Collagen arranged in lamellae
◼ Takes time to form
◼ Examples
> Trabecular bone
> Cortical bone (arranged in osteons)
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Woven bone
◼ Rapidly produced new bone
◼ Reactive bone
◼ Periosteal or endosteal new bone

Question 14

what is bone remodelling?

Answer 14

◼ Gradual replacement of bone
◼ Repairs microcracks from bone strain/fatique. Microcracks lead to microfractures
◼ Results in production of lamellar bone.
> not to be confused with modelling! (eg. fracture repair is modelling)

Question 15

what is bone modelling?

Answer 15

◼ Change in size and shape of bone with use - implies quicker change
◼ Mechanical forces change bone
> fluid movement through bone channels,
> piezoelectrical forces.
> Race horses running clockwise have larger bones of left side!

Question 16

Case Example:
‘Powder’, Great Pyrenees, 5 yr old
◼ Sudden onset of forelimb lameness
◼ Pain of distal radius.
◼ Radiographical osteolysis and new bone formation.
Ddx?

Answer 16

◼ Osteosarcoma
◼ Osteomyelitis
◼ Osteoarthritis (degenerative joint disease)
◼ Other

Question 17

Biopsy of Bone: Best technique

Answer 17

◼ Do not biopsy if referring for limb sparing surgery
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◼ Bone biopsy instrument (Jamshidi biopsy needle, Michelle’s
trephine)
◼ Deep biopsies
> Aggressive – through one cortex to other
> 3 different sites
> Radiograph when finished to ensure sample is representative
◼ Increases probability of successful diagnosis.

Question 18

Reactions of bone to injury

Answer 18

◼ Replay of bone formation
> Endochondral ossification
> Membranous bone formation
◼ Archetype is a fracture

Question 19

Reaction of Bone to Injury: Fracture repair
- primary vs secondary

Answer 19

Primary fracture repair
> when two ends of bones are held together > fixation
◼ Osteoclastic removal and bone formation to reform osteons
◼ Occurs with bone forced on bone – surgical fixation
> this fracture heals by remodelling > the normal physiologic process, as though they are repairing a normal microfracture
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Secondary fracture repair
◼ Hematoma
> beneath periosteum > periosteum lifts away from bone > fracture, produces new bone
◼ Reparative phase and Callus formation
> Fibrous callus > granulation tissue that matures
> Primary callus (woven bone) > removed over time, and replaced with…
> Secondary callus (lamellar bone)
◼ Modelling and remodelling > ‘clean up’, to look the way it was before

Question 20

Reaction of Bone to Injury: Blood flow, Pressure, Movement relationship and resulting tissues

Answer 20

◼ Blood + pressure – movement = bone
◼ Pressure – blood – movement = cartilage
◼ Movement = fibrous tissue

Question 21

Bone lesions and diseases broad categories

Answer 21

• Metabolic bone disease
• Bone necrosis
• Inflammation

Question 22

Metabolic bone disease

Answer 22

◼ Osteoporosis
◼ Rickets (young) and osteomalacia (adult)
> Vit D deficiency
> Phosphorous deficiency
◼ Fibrous osteodystrophy (FOD, ODF) > bone replaced by fibrous tissue
> Excess phosphorous
> Renal failure

Question 23

Osteoporosis - causes

Answer 23

Humans
◼ Overactive osteoclasts
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Animals
◼ Disuse atrophy
◼ Senile atrophy
◼ Starvation/Malnutrition > mostly production animals > lack of protein in particular
◼ Calcium deficiency

Question 24

Rickets and osteomalacia
- what age do they affect?
- how it arises

Answer 24

◼ Rickets – young
◼ Osteomalacia - adult
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◼ Bone fails to mineralize, osteoid forms but doesn’t mineralize
◼ Hypovitaminosis D (rare in animals > they produce their own vitamin D)
◼ Phosphorous deficiency (more commonly)

Question 25

Fibrous osteodystrophy
- when do we see it? types?

Answer 25

Primary hyperparathyroidism
◼ Neoplasia
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Nutritional hyperparathyroidism
◼ Phosphorous excess + calcium deficiency
> Dogs and cats fed meat only diet (BARF)
> Horses on bran diet
> Pigs on grain diet
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Renal hyperparathyroidism
◼ Chronic renal disease
◼ hyperphosphatemia and vitamin D deficiency (needed to absorb Ca from intestine) and resistance

Question 26

Bone Disease: Diseases with dead bone

Answer 26

• Sequestrum
• Necrosis of femoral neck
• Marrow necrosis

Question 27

Sequestrum in the bone - what is it? how it arises?

Answer 27

dead tissue with liquid surround
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Secondary to a primary problem
◼ Trauma and or fracture
◼ Lymphoma
◼ Osteomyelitis

Question 28

Necrosis of femoral head
- how it arises?

Answer 28

blood supply is through the neck, or synovial membrane
> if we get fracture of the neck, the head dies
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◼ Femoral neck fracture
> Trauma
◼ Avascular necrosis of femoral head (Legg Perthes disease, small dogs) > young dogs (<18 months)
◼ Physeal dysplasia with slipped capital epiphysis (cats) (overweight neutered males > slip their physis)

Question 29

bone marrow necrosis often due to what condition

Answer 29

Lymphoma

Question 30

Inflammation of Bone - called what?

Answer 30

Osteitis
Osteomyelitis - if it is in the marrow

Question 31

osteitis vs periostitis vs osteomyelitis definitions?
- where we see bone inflammation in young vs older animals? origins?

Answer 31

◼ Osteitis – inflammation of bone
◼ Periostitis – inflammation of periosteum
◼ Osteomyelitis – inflammation of medulla of bone.
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Young - Metaphyseal region.
◼ Bacteremia, localisation, proliferation, neutrophilic inflammation, bone lysis and osteomyelitis
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Older – localized
◼ Fractures
◼ Local injection of bacteria – lumpy jaw

Question 32

Osteomyelitis of metaphysis - can lead to what?

Answer 32

Osteomyelitis of metaphysis > formation of sequestrum