The most common location in the body for osteonecrosis occurs
of the hip. It is most commonly idiopathic and more often
seen in children than adults. When idiopathic osteonecrosis
is observed in children it is termed Legg Calve Perthes
disease (AKA Perthes or Legg Perthes disease). If a child
gets osteonecrosis of the hip from a known source it should
not be termed Legg Calve Perthes disease.
Legg Calve Perthes disease is the classic example of idiopathic
osteonecrosis and has well known and published radiographic
signs that may be observed at the hip. Early signs, as discussed
in the previous section, would include soft tissue swelling
and an increased Kohler's teardrop distance (Waldenstrom's
in an adult on the right - Increased distance between
Kohler's teardrop and the femoral head.
in a child of the left hip.
The "crescent" sign is one
of the earliest bony changes seen radiographically. This crescent
sign is best visualized on the Frog-leg lateral view,
therefore, if AVN is clinically suspected, this view should
definitely be taken.
It must be stressed that while a crescent
sign is regarded as an "early" sign of avascular
necrosis, the disease process itself is relatively advanced
and the prognosis at this point is relatively poor when compared
to the prognosis if discovered before any bony changes.
Other signs would include a small femoral head
(Fig. 4) presumably due to lack of blood flow resulting in
cessation of growth, and enlarged greater trochanter. The
enlarged greater trochanter is due to decreased femoral neck
growth and continued greater trochanter growth. The elevation
of the trochanter alters the efficiency of the attached gluteal
musculature and may result in a positive Trendelenburg test,
clinically. Later features would include, as in all osteochondroses,
fragmentation, mixed areas of lucency and sclerosis, and
Perthes disease on
the left. Observe the small femoral head, Waldenstrom's
sign, increase in density, and widened femoral neck.
Perthes disease on
the left. Note the fragmentation and the lateral displacement
of the femoral head.
Finally the femoral head will remodel into the
of the left hip. Also note the subchondral cysts most
likely due to secondary degenerative disease. Also note
the remodeling of the acetabulum.
The signs of the mushroom deformity of the femoral
head include coxa magna (enlargement of the femoral head)
and coxa plana (flattening of the femoral head). Other associated
radiographic features to look for include remodeling of the
acetabulum, coxa vara (femoral neck angle of less than 120
degrees) and the sagging rope sign (representing the
edge of the flattened femoral head).
Sagging rope sign
The average onset of Legg Calve Perthes disease
is between 4 and 8 years of age and affects males more commonly,
about 5:1. The patient typically will present with a limp
due to pain with insidious onset of symptoms. There may be
a positive Trendelenburg test as noted above related to an
enlarged greater trochanter. 90% of cases are unilateral.
The prognosis is important for doctors to know
so that they may implement proper treatment and accurately
inform their patients. The prognostic variables include gender,
age of onset, and most importantly which stage that the osteonecrosis
was diagnosed. Males, an early age of onset, and early diagnosis
all have a favorable prognosis.
Treatment for Legg Calve Perthes disease is
surgical and may include a correctional osteotomy stabilized
with plate and screws or an intramedullary rod to correct
lateral displacement of the femoral head and/or "core
decompression" that relieves the pressure inside of the
medullary cavity of the femoral neck.
fixation as treatment of osteonecrosis
In the adult, the most common known
cause is due to corticosteroids. As previously discussed,
however, there are many other causes. (Remember ASEPTIC?)
Idiopathic osteonecrosis in an adult is termed Chandler's
AVN of the femoral head in the
adult is often times much harder to detect, radiographically,
than in a child. This is because the subtle radiographic changes
observed in a child are not applicable in an adult, such as
a difference in femoral head size and fragmentation. Waldenstrom's
sign is also harder to appreciate in an adult. The early radiographic
signs in an adult are a patchy increase in density.
Subtle patchy increase in sclerosis
of the left femoral head.
This may be very difficult to appreciate
due to possible suboptimal technical factors and if it is
appreciated it may be misinterpreted as degenerative subchondral
sclerosis. An important finding that is often times overlooked
is lack of joint space narrowing. It is this lack of joint
space narrowing that should aid in this differential. Not
until there is secondary degenerative joint disease will there
be narrowing of the joint. Flattening of the weight bearing
portion of the femoral head is another early sign. As the
disease progresses, a crescent sign, then subarticular collapse,
resulting in a "step defect" may be observed.
Also due to altered stresses put on the medial portion of
the femoral neck, buttressing is often a result.
This is an subtle
example of a step defect. Also note the increased density
and subchondral cyst formation.
A more obvious example
of a step defect. Also note the buttressing-avascular
necrosis due to long term corticosteroid use
As the disease progresses, more
sclerosis, collapse and fragmentation of the weight bearing
portion of the femoral head may be appreciated.
After the femoral head continues
through the revascularization and deformity phases, severe
secondary degenerative joint disease in inevitable.
Endstage DJD secondary
to avascular necrosis. Note the bone-on-bone appearance
as well as the subchondral cysts and sclerosis of the
flattened femoral head.
As stated numerous times already,
if clinically suspected, insidious onset of progressive hip
pain that does not respond to conservative therapy should
be further investigated. The next imaging of choice would
be magnetic resonance imaging (MRI). There are different appearances
of avascular necrosis on MRI depending on the stage of the
disease. A classic appearance that has been described, however,
in the early stages is termed the "double line"
sign. This describes a focal area of high or intermediate
signal intensity (white) that is surrounded by a rim of low
signal intensity (black) on both T1 and T2 images.
MRI - Osteonecrosis
of the left hip. Double line sign
MRI - Double line
The stages and radiographic appearance
of avascular necrosis of the hip are very similar to those
seen in other body regions as will be discussed next.