The pathogenesis of osteonecrosis is commonly divided, artificially,
into four different phases, 1) the avascular phase, 2)
the revascularization phase, 3) the repair phase, and 4) the
deformity phase. Along with histologic changes, radiographic
changes correspond with each one of these phases. It is said
that the phases are divided artificially because the process
of osteonecrosis is on a continuum, with much overlap. This
is important to keep in mind due to the fact that many times
radiographic signs from two or more phases may be visualized
at one time.
1) Avascular Phase
This first step in the process of osteonecrosis begins as
an infarct to the bone from one of the many different causes
discussed on the previous page. This infarction typically
takes place within fatty marrow adjacent to the subchondral
cortex of the bone. Infarction within the medullary portion
of long bones can also happen and are termed intramedullary
infarcts which are not discussed in terms of different phases
of radiographic change. The infarction at the subchondral
portion of the bone leads to bone loss leading to bone death.
As the subchondral bone becomes necrotic, the overlying cartilage
hypertrophies. This is an attempt to convert itself to bone.
The radiographic appearance that corresponds to this first
phase is often, unfortunately, nothing at all. Minimal osteopenia
or subtle soft tissue changes may be appreciated if there
is a strong clinical suspicion. The reason why this is
so unfortunate is because only in the first phase of osteonecrosis
is treatment by far most effective. The osteopenia is
not due to the infarction itself, but hyperemia surrounding
the involved area as well as synovitis results in osteoclastic
activity which leads to osteopenia. The subtle soft tissue
changes are typically found around the proximal femur. One
may observe hip joint fascial plane changes or an increase
in the medial joint space between Kohler's teardrop and the
femoral head. With a properly positioned AP pelvis view, a
2 mm difference is significant, and should be followed up.
This increased medial joint space is termed Waldenstrom's
Waldenstrom's sign on the right.
Waldenstrom's sign is nonspecific also observed in hip trauma,
a joint space infection and a slipped capital femoral epiphysis.
2) Revasularization Phase
The second phase, the revasularization phase, is a result
of the area attempting to "clean up" the area via osteoclastic
and osteoblastic activity. The corresponding radiographic
appearance results in mixed areas of luceny and sclerosis.
Because of the cell death that occurs beneath the articular
surface of the bone, it is no longer able to withstand normal
stresses put on it resulting in fragmentation and collapse
of the overlying cortex. The first indication that collapse
is near is the crescent sign.
Crescent Sign of left hip
The crescent sign is a thin curvilinear lucency which occurs
in immediate subchondral bone and is located on the weight
bearing portion. Further collapse may result in a "step
sign" in which cortical offset is actually noted on the
film. The "Snow Cap Sign" refers to diffuse sclerosis
of femoral head or humeral head and is seen only if repair
is sufficient in revascularization phase.
cap" sign of the left femoral head
Cap" sign of the humeral head
3) Repair Phase
The repair phase actually begins when revascularization begins.
There is a variable degree of reconstitution and healing that
is dependent upon 1) the degree of blood loss and cell death
in the initial insult, 2) the patient's immune system and
healing response, and 3) whether the joint is weight-bearing
vs. non-weight bearing
4) Deformity Phase
The deformity phase is highly variable because it is dependent
upon severity of other phases
and is site specific. It is also dependent upon stresses in
the area (weight-bearing areas show increase in deformities).
The femoral head results in a classic "Mushroom deformity"
seen in Legg Calve Perthes disease.
"Mushroom" deformity of the left femoral head
with associated coxa plana and coxa magna
Healed Perthes disease with resultant mushroom deformity.
Note surgical wires from previous osteotomy
Early and severe degenerative joint disease
will occur due to incongruent joint surfaces often necessitating
a hip replacement by within the fifth decade of life.
The problem with osteonecrosis is that once
enough cell death occurs, the process will cascade itself
though all the stages of osteonecrosis with the final result
being severe and often debilitating degenerative joint disease.
The fortunate aspect of this process is that it is often
very treatable if discovered early.