Avascular Necrosis (Osteonecrosis) of the Hip
Avascular Necrosis (AVN) of the hip occurs when the blood supply to the
bone of the femoral head (the ball part of the hip joint) is disrupted. This
typically leads to death of the bone cells (osteocytes) in a localized area of the
top of the femoral head/ball leading to collapse of the affected bone and its
associated joint surface. This irreversible damage generally leads to a
progressive arthritis (often quite rapid) of the hip with pain, stiffness and loss
of function for walking, bending etc.
Normal femoral head on left and bone changes in an avascular head on the
right.
AVN is a much less common condition affecting the hip than either
osteoarthritis or inflammatory arthritis. In saying this AVN can affect the bone
around other joints such as the knee and shoulder, but the hip is the most
common joint with this condition.
In most cases, the cause of this loss of blood supply is unknown or termed
“idiopathic”. There are however a number of risk factors that make it more
likely for someone to develop this condition:
A. Traumatic Injury – Hip dislocations and certain types of hip fracture can
damage the blood supply to the femoral head.
B. Alcohol Intake – Excessive alcohol consumption over a long period of time
can be a causative factor. It is not known how this occurs.
C. Corticosteroid Medication – Cortisone/Prednisolone medications for
treatment of airways disease or inflammatory arthritis/bowel disease or other
conditions. Long term use carries a higher risk of developing AVN but even
short term relatively high dose administration has been associated with this
condition. Again it is not known how these drugs lead to AVN. Chemotherapy
agents (used to treat various cancers) can lead to the development of AVN in
a similar manner.
D. Other Medical Conditions – Blood disorders like Thalassaemia and Sickle
Cell Disease and Metabolic conditions like Gauchers disease can cause small
clots to form in the tiny arteries supplying blood to the femoral head thereby
blocking blood flow to part of the bone. An unusual condition called Caisson’s
Disease (seen in deep sea divers experiencing “the bends”) causes the
release of nitrogen bubbles into the arterial circulation which can block the
blood vessels of the femoral head in a similar manner. There are many other
conditions which can be associated with AVN of the Hip but they are quite
rare.
AVN of the hip usually occurs in adults in the range from 50-70 years but can
occur at any age, even in children occasionally. AVN of the hip develops in
stages and early on there are often no symptoms with the condition only being
picked up incidentally on a scan or other radiological investigation. As the
condition progresses and there are bone changes, most patients will
experience pain which is usually felt in the groin area but may radiate into the
thigh or very occasionally into the buttock. Once the bone of the femoral head
starts to collapse, the pain usually starts becoming more severe affecting all
weight bearing activities but also occurring with leg movements and bending.
With end stage AVN, the hip is often painful at rest, movement is restricted
and activity is severely limited. AVN can progress through these stages quite
rapidly over a period of just a few months or it may take 12 – 18 months. This
is in contrast to osteoarthritis of the hip which is a generally slowly progressive
condition that takes years to develop.
The different stages of avascular necrosis.
The diagnosis of AVN of the Hip is usually made after obtaining xrays or
scans of the hip but sometimes can be quite difficult. The diagnosis may be
suspected in a patient with a painful hip with restricted movement who may
have one of the recognized conditions or risk factors (outlined above)
sometimes associated with AVN. Plain xrays of the hip will usually show bone
changes (increased density or whiteness of the bone in a localized segment
of the femoral head, early collapse – a crescent sign or more advanced
collapse) once the condition is established. Nuclear bone scans will
sometimes pick up this condition but are often non specific. MRI scan is
probably the gold standard investigation for this condition if it is suspected but
plain xrays of the hip are normal. MRI has the capability to pick up very early
stage AVN before symptoms have started as well as more advance disease
and is useful for determining how much bone is involved by the condition. It is
also extremely useful for evaluating the opposite hip for this condition as AVN
can develop in both hips, particularly in those individuals where a systemic or
general risk factor is present ie alcoholism, steroid use, blood conditions etc.
An MRI scan of the hips showing a normal femoral head (left) and typical
bone changes of AVN (femoral head on right side of the scan).
The treatment of avascular necrosis depends on a number of factors –
individual factors, how much bone is affected and most importantly the stage
of the condition. Whilst non operative or conservative measures may be
appropriate for some patients with AVN, most will require some form of
surgical intervention. Painkilling/analgesic and anti-inflammatory medications
and the use of crutches can reduce the symptoms and possibly slow down the
progression of AVN, but rarely result in resolution. Sometimes these
measures are used when the diagnosis is not clear – it can sometimes be
difficult to differentiate AVN of the hip from transient idiopathic osteoporosis
(TIO) or stress/insufficiency fractures of the femoral head. Follow up with
serial MRI scans is usually required to ascertain the correct diagnosis.
Surgical treatment involves either a procedure called Core Decompression
(also known as a “forage” procedure) or Total Hip Replacement.
Core Decompression is a procedure that involves the drilling of one large or
several small holes into the affected area of the femoral head to relieve
pressure within the bone and to create channels for new blood vessels to
grow into the “avascular” bone in an attempt to re-establish a new blood
supply. This procedure is only relevant when AVN is diagnosed in its very
early stages – often before symptoms and certainly before changes are seen
on plain xray. Unfortunately this procedure is not successful in all cases.
Sometimes bone grafting is undertaken in addition to decompressive surgery
to try and lessen the chances of bone collapse but so far this has not
conclusively been shown to improve outcomes in AVN.
Core decompression for avascular necrosis of the femoral head.
Unfortunately for the majority of patients diagnosed with AVN of the hip, Total
Hip Replacement (THR) remains the only treatment that is able to
satisfactorily deal with the damaged bone and joint surface of the femoral
head. Similar to end stage osteoarthritis, it involves replacing the ball and the
socket of the joint with artificial implants which have bearing surfaces which
provide for generally very satisfactory pain relief, restoration of movement and
significant improvements in function (walking, bending and activities of daily
living/simple recreations etc.). THR is regarded as one of the most successful
operations in all of medicine.