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Definition
History
Epidemiology
Anatomy
Aetiology-cause
Associated diseases
Clinical manifestations and investigations
Staging of the disease
Management

Definition:

Osteonecrosis is defined as cell death in components of bone - The marrow fat and mineralized tissue.
It represents the final common pathway of several disease entities, which result in impaired blood supply to the bone tissue - causing necrosis or death of the bone.

History

It was first described in 1794 by James Russel , who wrote a book on bone necrosis, and full description of the entity followed in 1930 by Phemister et al. In 1962 the entity was revived with the description of 5 cases by Mankin and Brower.
Since then thousands of cases have been described, and it is estimated that there are 15000 cases per year in the USA alone.

Epidemiology

It is more common in young males.
Male : Female ratio is 8:1
Usually the patient is < age 50.

Anatomy

Commonest sites are the hip, the knees and shoulders as well as the wrist bones an feet.
In the case of the femoral neck - there is a precarious blood supply with blood supply coming from above and below, leaving a watershed area at risk -between the two main areas of supply - in the femoral head.

The main blood supply is as follows:

Medial Epiphyseal artery from Obturator via ligamentum teres.
Medial circumflex femoral artery, supplying superior and inferior retinacular vessels and thence the inferior metaphyseal and lateral epiphyseal vessels.
Intramedullary vessels from the shaft and trochanteric regions.

Once there is an occlusion of blood supply, there is local oedema and stasis of blood flow. Because of the oedema, the pressure in the head of the femur increases and further reduces the blood supply. The marrow cells necrose and die, and there is lipocyte and reticular eosinophilic atrophy. The marrow then undergoes fibrosis. The bone Osteocytes also die and disappear - leaving cavities, and dead trabeculae, but retained architecture. The adjacent live bone cells attempt a repair..

Aetiology - Cause

The Cause of Avascular Necrosis is not certain and several mechanisms are hypothesized.
1. The compartment syndrome theory : Local oedema in the head causes raised pressure and reduces blood supply.
2. Occlusive vessel disease : That the blood vessels become blocked off as a result of disease in the blood vessel or they clot up and thrombose.
3. Fat Embolism theory: That the vessel blocks because of fat globules in the lumen of the vessel.
4. Hypertrophy of fat cells: That these fat cells expand and block the supply of blood.

Several conditions are known to cause it:

1. Definite known Cause.

a. Trauma : Fracture /Dislocation

b. Non Traumatic.

Caissons disease. - “the bends” - seen with deep water diving.
Gauchers disease.
Sickle cell anemia
Local Radiotherapy

2. Probable Cause.

a. Traumatic : Minor trauma

b. Non Traumatic

Occlusive vascular disease
Corticosteroids / Cushing's syndrome
Diabetes
Hyperlipidaemia
Alcohol
SLE
Thrombophlebitis
Carbon Tetrachloride poisoning
Hip dysplasia
Hyperuricaemia and gout
Osteomalacia
Tumors
Pancreatitis
Renal transplantation

Clinical features

The problem may occasionally be found incidentally on Xray of the hip, and may be entirely assymptomatic. The patient may present with pain developing in the hip area, often with pain radiating to the knee, over weeks to months. The patient may occasionally present with knee pain or even low back pain. The pain is often associated with a limp and examination early on shows nil or possibly restriction on internal rotation of the hip, and abduction and flexion. The greater the degree of AVN , the greater the restriction of joint movement.

The Xray findings :

Early on are completely normal, but with ongoing problem, one sees progressively :

Osteopenia
Central radiolucency with a sclerotic border
A Subchondral lucency - making up the “crescent sign”
Flattening of the head.
Collapse of the head.
Secondary changes of Osteoarthritis.
Joint space narrowing
Progressive joint destruction

Since the X-Rays may be completely normal - it is essential to be able to see the problem early.
Two main methods exist for early diagnosis.

Magnetic Resonance Scanning.
This was first used for AVN in 1983
One Requires T1 and T2 axial and coronal views.
Signs on MRI are:
Low Subchondral intensity (96%)
A margin of low signal (60-80%)
A double line sign (50-80%)
Joint fluid

The sensitivity is 75-100% before collapse of the head.
Both sides should be done as the opposite side may be involved asymptomatically, and also for comparison.

Radionuclide Imaging - Technetium 99m scan.
This shows increased uptake from increased metabolic activity
At the center of the uptake specific area there may be an area of reduced uptake.

Both sides should be done as the opposite side may be involved asymptomatically, and also for comparison. Sensitivity is 70% pre collapse.

Depending On The Symptoms and X-Rays - The condition Can be Staged :

The Staging of Avascular Necrosis

Stage	Clinical	Surgical potential
Stage 0	Theoretical stage: Assymptomatic. All diagnostic tests are normal. Biopsy from histology - is the only abnormality.	Nil Required : Theoretical stage only Not a clinical problem.
Stage 1	X-Rays and CT-scan are normal MRI and Tc99 and histology are abnormal. Symptoms may or may not be present.	Surgery is possible / desirable. Core - decompression possible.
Stage 2	X-Rays are abnormal - With linear sclerosis or cysts. There is no Subchondral lucency Head of the femur is still spherical.	Surgery is possible / desirable. Core - decompression possible.
Stage 3	The femoral head starts to fail mechanically - With Trabecular collapse. The radioluscent crescent sign is visible at the Subchondral endplate. The femoral head itself is still spherical	Surgery is possible Core - decompression possible. Success depends on degree of changes.
Stage 3a.	Crescent < 15%	Surgery still possible.
Stage 3b	Crescent 15-30%	Increasing risk of failure of preventative surgery.
Stage 3c	Crescent>30%	Increasing risk of failure of preventative surgery.
Stage 4	Flattening of the femoral head is now seen	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 4a	<15% of the surface is collapsed Depression <4mm	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 4b	15-30% collapse or 2-4mm depression	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 4c	>30% Collapse or >4mm depression.	Preventative surgery NO HELP Medical Management. Total Hip Replacement Required.
Stage 5	Any or all Xray features may be present There is a decrease in joint space. Secondary Osteoarthritis is present with: Sclerosis / Cysts / Osteophytes	Preventative surgery NO HELP Medical Management. Hip Replacement Required.
Stage 6	Extensive destruction	Total Hip Replacement - Where available / Possible.

Management

Management is dependent on the stage of the disease at diagnosis.

Prevention :
in Risk states are essential.
This is possible in :
Divers - prevent decompression hazards with decompression chambers.
Diabetics - Establish good control and control the lipid status of the patient.
Hyperlipidaemia - ensure good control.
Steroids - Ensure as low a dose as possible. (AVN has been associated even with short courses.)
Alcohol - control intake.

Surgery :
Up to stage 3, it is possible to perform a decompression procedure on the femoral head using a core decompression biopsy.
By taking a core of tissue out of the femoral head - in a relatively small procedure, there is a good possibility of preventing inevitable deterioration and averting the necessity of a hip replacement.
Core decompression has variable results - (34 - 100%) -depending on timing of the operation.

Alternative preventative surgical measures are:

Bone grafting
Transtrochanteric rotational Osteotomy. This requires expertise in this procedure, and Sugioka (Japan), quotes results of up to 95% success in preventing collapse of the head.

Medical therapy :
Initiated with the diagnosis, includes :
No weight bearing for 4-8 weeks.
Vaso-active drugs - These have been used with variable results, especially - naftidrofuryl oxalate.
Some claims are made, that pulsed electromagnetic fields may help.

Beyond stage 3, surgical prevention is not helpful, and therapy is essentially medical, with analgesics and judicial use of antiinflammatories, pending a need for total hip replacement.
The decision regarding total hip replacement is made depending on the age of the patient, fitness for surgery, degree of pain and lack of response to conventional analgesia and antiinflammatories, and on and the functional impairment of the patient.

Many patients are young, and an effort is made to delay surgery if possible, as there is a finite life of the surgical prosthesis. Delay is aimed at preserving the prosthesis and planning for any future revision surgery.

However - if surgery is required on functional and medical grounds,
then Age is not a contra-indication to surgery.