Article: OsteoNecrosis or Bone Death

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OsteoNecrosis or Bone Death

With X-ray Pictures

http://www.rad.washington.edu/mskbook/osteonecrosis.html

OsteoNecrosis bespeaks bone death. Synonyms

include Aseptic

Necrosis, Bone Necrosis, Avascular Necrosis, Bone

Infarction

and Ischemic Necrosis. By convention, the terms

" Aseptic " or

" Avascular " Necrosis have been applied

to areas of Juxtaarticular

involvement and the term Bone Infarct is usually

applied to

Metaphyseal or Diaphyseal involvement. However,

this terminology

is not applied consistently throughout the

literature, and I feel

that these are largely artificial distinctions. I

prefer to simply

use the single term 'OsteoNecrosis' and to apply

it to areas of

bone infarction, no matter where in the bone they

occur.

Mechanisms of OsteoNecrosis

There are many paths to Satori, and likewise to

OsteoNecrosis.

OsteoNecrosis is multi factorial in Etiology, and

has a widely

differing Prognosis, depending upon the exact part

of the Bone

involved by the Necrosis. The exact mechanism of

OsteoNecrosis

depends primarily on the predisposing cause. This

is reflected

in the table below.

( view table of Diseases Affect with OsteoNecrosis)

http://www.rad.washington.edu/mskbook/osteonecrosis.html

It seems logical that the sequence of events in

developing

OsteoNecrosis should be as follows:

1. some phenomenon causing decreased blood flow to

bone

2. bone Ischemia, if the flow becomes low enough

3. bone infarction, if the flow becomes lower

still

OsteoNecrosis is generally thought of being an

irreversible process.

If the Necrosis occurs next to a Joint Surface, it

is generally

considered to cause Joint Deformity. However, no

one really knows

if this is true, and such conclusions are to some

extent Artifacts

of our current Diagnostic Methods for

OsteoNecrosis. With the advent

of MRI, we finally have a tool which can show

abnormalities within

about 2 weeks of the original insult. However,

current standard MRI

scanning cannot yet show abnormalities at the

instant of Infarction.

What is needed to answer this question is a

Diagnostic Technique that

is sensitive to Vascular Flow, and not just

intensity changes in the

Bone Marrow. Dynamic MRI scanning following a

Gadolinium Bolus may be

helpful in this regard, but definitive studies

have not yet been done

to prove this speculation.

There is some evidence in the literature that

suggests that in some

circumstances, OsteoNecrosis may be a reversible

process that can

resolve completely without Subchondral Collapse

and subsequent Joint

Arthrosis. This evidence is found in the

literature for Transient

Osteoporosis of the Hip, an Idiopathic Disorder

characterized by a

reversible Osteopenia of the Hip and marked Marrow

Edema. If this

definition sounds a bit circular, well, that's how

Idiopathic Disorders

are, isn't it. We don't know what causes them, so

their names pretty

well sum up most of what we know about them.

Anyhow, several studies

of TOH have been published in which the

characteristic Edematous

Marrow of TOH was Biopsied. These Biopsies have

shown the presence of

OsteoNecrosis in many cases, leading to the

suggestion that TOH may

actually be due to a low-grade and reversible

OsteoNecrosis.

Concave / Convex Joint Physiology

It turns out that, in part, the Shape of Joints

governs the distribution

of OsteoNecrosis. In particular, it is the Convex

Versus Concave Shape

of the opposing Joint Surfaces of most Joints that

determines this

distribution ( Simkin ).

On both sides of the Joint, the forces of

weight-bearing and other

loading are directed Tangential to the Joint

Surface. On the Convex

Side of the Joint, these forces converge to a

common center. On the

concave side, these forces diverge away from each

other. To see the

significance of this fact, one must first consider

the Phenomenon of

Hydraulic Resistance. The Human Body, including

the Bones are composed

of about 50 - 67% Water by Mass. One can think of

the Bones as

Calcium Balloons filled with Water. Water is not

compressible, so this

Internal Water provides some support to the Bones,

and this supporting

force is called Hydraulic Resistance. Nature is

thrifty, and the

presence of Hydraulic Resistance makes it possible

to provide the same

support with less Bone. Since the loading forces

converge to a common

center on the convex side of the joint, Hydraulic

Resistance becomes a

fairly efficient mechanism for resisting these

forces. On the concave

side of the joint, the divergent nature of the

forces makes this

mechanism much less efficient, and therefore,

Hydraulic Resistance

plays a much smaller role in bone support. The two

sides of the bone

are obviously supporting the same load, so more

bone is therefore

necessary on the concave side. This is seen in the

form of increased

thickness of the Subchondral Bone along the

concave side of the joint

in virtually every concave/convex joint pair in

the body.

The presence of this extra bone along the

Subchondral portion of

the concave joint surface has some important

ramifications for

OsteoNecrosis. Consider first the normal joint and

how Intramedullary

Pressure varies on both sides of the joint with

loading. Since

Hydraulic Resistance is used as an important

support on the convex

side of the joint, it is not surprising that the

Intramedullary

Pressure on this side of the joint will vary

widely with loading.

Due to the extra bone supporting the concave side,

there is less

variation of the Intramedullary Pressure on that

side of the joint,

as shown below.

Now, given this picture of normal variations in

Intramedullary

Pressure with loading, consider how the patient

developing

OsteoNecrosis. Specifically, consider a patient on

Steroids,

probably the most common cause of OsteoNecrosis.

Chronic Steroid

use causes Fat Cells to grow. This is the basis

for such well-known

physical findings such as the " Buffalo

Hump " or the " Moon Faces "

seen in these patients. Many people don't realize

that the

Intramedullary Fat Cells share in this growth. As

the patient

spends more and more time on Steroids and the

Intramedullary Fat

ells grow larger and larger, the Baseline

Intramedullary Pressure

begins to rise. Why? Because the Bone is a closed

and fairly stiff

cavity, and will not give with increases in

Intramedullary Pressure.

This is an Analogous situation to the Skull in

cases of Cerebral

Edema. However, in this case, the casualty is not

the Brain, but the

Intramedullary Veins. The Intramedullary Arteries

operate at Systemic

Systolic Pressure (90 - 140 mm Hg), but the

Intramedullary Veins

operate at a very low pressure (3 - 5 mm Hg).

Therefore, the

Intramedullary Pressure doesn't have to rise very

high to cause

Occlusion of the Intramedullary Veins. As the

Intramedullary Pressure

rises, this leads initially to Intermittent

Occlusion during loading

on the convex side of the joint.

However, as the pressure continues to rise, the

Intramedullary Veins

on the convex side of the joint finally become

occluded all of the time.

( See these graphs as a movie (101K)

http://www.rad.washington.edu/mskbook/osteonecrosis.html

The mechanism outlined here helps to explain the

overwhelming

distribution of OsteoNecrosis toward the convex

side of the joint.

No matter how much experience you've had looking

at OsteoNecrosis,

most of the cases of joint involvement that you've

seen involved the

convex side, whether you noticed it or not. In my

personal experience

with hundreds of cases, I've only seen joint

involvement on the concave

side in two or three cases. Your mileage may vary.

Methods of Diagnosis

Once OsteoNecrosis is pretty well established,

just about every

imaging method will reveal it. Other than MRI,

most of these

methods are not terribly sensitive, and the

OsteoNecrosis will

be well developed by the time that it is

diagnosed, as shown

in the table below.

http://www.rad.washington.edu/mskbook/osteonecrosis.html

Approach to OsteoNecrosis

Generally the diagnosis of OsteoNecrosis is not

too difficult. The

patient often has some known condition that places

them at high risk

for OsteoNecrosis, and the radiographic

requisition states something

like " R/O AVN " . In such cases, it is

usually not difficult to twig to

the idea that OsteoNecrosis belongs in the

differential.

Typical plain film findings

The earliest radiographic appearance of

OsteoNecrosis is zilch.

That is, the Radiograph looks completely normal.

After weeks to

a month or two, the patient may develop an

ill-defined Mottling of

the Trabecular Pattern as the earliest evidence of

OsteoNecrosis.

Early on, this is so ill-defined that most Radiologists

will miss it,

unless they have a lot of experience looking at

OsteoNecrosis. The

late findings of OsteoNecrosis depend upon its

location within the

bone. If the lesion occurs in the Medullary Space

well away from the

joint, one eventually may see the classic pattern

of dense, Serpiginous

Calcification. However, if the necrosis occurs in

the Subchondral

Bone, a different pattern usually emerges. Once

the OsteoNecrosis has

been present for months, Microfractures will

accumulate in the dead

bone to the point that one may see developing

Subchondral Fractures.

This may lead to a discontinuity in the

Subchondral line, or in some

cases, to the " Crescent Sign " , which

represents a Fracture between the

Subchondral Line and Adjacent Necrotic Bone. As

Living Bone reacts to

the presence of Adjacent Dead Bone, a thick

Sclerotic Zone may develop

along the " no-man's land " between the

Living and Necrotic Bone.

Tomogram of the right Hip showing a Segmental Zone

of Sclerosis

(arrows) in the Superior Portion of the Femoral

Head, consistent

with OsteoNecrosis (O).

AP Radiograph of the Hip in a patient with

OsteoNecrosis. Note the

Lucent Crescent Sign (white arrow) and the

discontinuity where the

Subchondral Bone has collapsed (black arrowhead).

Also note a Core

Decompression Channel drilled up through the

Femoral Neck (black arrows).

Typical MRI Findings

The classic MRI appearance of OsteoNecrosis is

that of a Segmental

area of low signal intensity in the Subchondral

Bone, bounded by a

low signal intensity border. This border may

sometimes appear as a

dark line adjacent to a bright line -- the

so-called " double line

sign " . Occasionally OsteoNecrosis will

present as an area of diffuse

low signal intensity, which may be difficult to

differentiate from

other entities, such as Osteomyelitis, stress

fracture, etc.

Coronal T1-weighted image of both Hips -- the low

signal intensity in

the superior weight-bearing area of the left

Femoral Head is typical

for OsteoNecrosis.

Sagittal T1-weighted surface Coil Image of the

Femoral Head -- this

Image demonstrates a Classic Segmental Area of

OsteoNecrosis with a

dark line denoting the border between Dead Bone

and Living Bone.

Sagittal T1-weighted image of the knee -- this

image demonstrates

multiple segmental areas of OsteoNecrosis in the

Distal Femur in this

patient with Gaucher's Syndrome.

Fortunately, the plain film and MRI findings of

OsteoNecrosis are

usually so typical that one does not often have to

offer a differential

diagnosis. However, sometimes, OsteoArthritis,

Subchondral Cysts,

Transient Osteoporosis of the Hip and other

entities may mimic

OsteoNecrosis on Flain Films or MRI. Sometimes

OsteoNecrosis is

unsuspected, and the alert Radiologist must

consider it whenever

unexplained Sclerosis or Lucency is noted adjacent

to a joint, or

whenever a patient presents with diffuse Skeletal

Sclerosis.

Besides confirming the diagnosis, another major

role for the Radiologist

in the workup of OsteoNecrosis is staging the

current state of the

disease. The usual plain film and MRI stages are

listed below.

Stage Findings

0 Asymptomatic, normal Radiographs

I Normal Radiographs (abnormal MRI)

II Radiolucency and Sclerosis

III Crescent Sign, Normal Contour

IV Subchondral Collapse, flattening

V Degenerative Joint Disease

Wise Sayings about OsteoNecrosis

1. Remember the Concave / Convex Joint Physiology

and its effect

on the distribution of OsteoNecrosis about a

Joint.

2. In a patient with Diffuse Sclerosis, consider

some Diffuse

cause of OsteoNecrosis, especially Sickle Cell

Disease.

References

1. Simkin PA, Graney DO, Fiechtner JJ. Roman

Arches,

Human Joints, and Disease: differences between

Convex and Concave Sides of Joints. Arthritis

Rheum 1980;23:1308-1311.

Hugs,

Deanna