Osteonecrosis

Question 1

What is osteonecrosis?

Answer 1

Bone infarction (tissue death caused by an interrupted blood supply) near a joint
- osteonecrosis is different to bone infarction, which is applied to bone death that's not near a joint

Question 2

What is the significance of osteonecrosis?

Answer 2

Death of subchondral bone (bone right under the joint surface)
- can lead to collapse of the joint and end stage arthritis

Question 3

Where is osteonecrosis most common?

Answer 3

Hip

Shoulder

Question 4

What is the clinical presentation of osteonecrosis?

Answer 4

Can be asymptomatic
- incidental finding on imaging
Pain
- from either the arthritis or the infarction
When osteonecrosis is in the femoral head
- groin pain (worsens with weight bearing and motion)
- thigh and buttock pain
Rest pain
Night pain

Question 5

Once the joint function deteriorates, how can someone present?

Answer 5

Findings of arthrosis
- limp
- tenderness
- restricted motion 
In femoral head
- limitation in internal rotation and abduction

Question 6

Describe the epidemiology of osteonecrosis.

Answer 6

Accounts for 10% of total hip replacements
Most common in 40s and 50s
55% of cases are bilateral
Most common in the hip
10% of people with sickle cell anaemia have osteonecrosis

Question 7

Describe the pathophysiology of osteonecrosis.

Answer 7

Necrosis begins by involving the medullary bone (the cortex is spared because it has a collateral blood supply)
- articular cartilage receives nutrition from the synovial fluid and remains viable
The dead bone has empty lacunae and is surrounded by necrotic adipocytes that often rupture and release fatty acids
- these bind to calcium and form insoluble calcium salts

Question 8

Can the body heal necrotic bone?

Answer 8

Osteoclasts resorb the necrotic trabeculae
Remaining trabeculae serve as the scaffolding for deposition of new bone (creeping substitution)
However, this often happens at too slow a pace to be effective, and the necrotic trabeculae collapse before it occurs

Question 9

Name some causes of osteonecrosis.

Answer 9

Sickle cell anaemia
- causes mechanical blockade by rigid erythrocytes 
Vascular damage
Increased intraosseous pressure
Mechanical stresses

Question 10

How does osteonecrosis cause arthritis?

Answer 10

Bone is derived of blood supply, gets ischaemic and dies
Dead bone doesn’t remodel
- micro damage doesn’t get repaired and mechanical properties of the bone are impaired
If enough damage accumulates, the subchondral bone can be weakened to the point of collapse
- joint surface becomes irregular and rough
- if one side of the joint isn’t smooth, it will damage the other side

Question 11

Describe what is seen on X-Ray in osteonecrosis.

Answer 11

May be normal for months after onset of symptoms
- small density changes
- followed by sclerosis and cysts as it progresses
Pathognomonic crescent sign (subchondral radiolucency)
- precedes subchondral collapse
Late stages
- loss of sphericity and collapse of femoral head
- narrowing joint space
- degenerative changes

Question 12

Is doing an MRI useful?

Answer 12

Yes - 91% sensitivity

Question 13

What are some of the risk factor for osteonecrosis?

Answer 13

History of trauma (especially dislocation)
Corticosteriod use
Cushing's disease
Alcohol abuse 
Sickle cell disease/ haemoglobinopathies

Question 14

If someone is high risk, what can we do to help them?

Answer 14

Educate them about AVN and advise them to report any symptoms as soon as possible to allow treatment

Question 15

What are the treatment options for AVN?

Answer 15

Uncollapsed joint
- reperfusion and healing of infarcted area
- bisphosphonates
- core decompression with or without bone graft
- ESWT (relieves pain, improves function and induces regression of AVN)
Collapsed
- bone graft
- total hip arthroplasty

Question 16

What drug can be given to those on corticosteroids to reduce their risk?

Answer 16

Statins