Radiographic evaluation is an extremely powerful tool that allows the
orthopaedic surgeon to characterize and classify bone lesions by their
appearance and by the reaction of the surrounding bone and soft tissue.
Careful and analytical characterization of bone lesions on plain radiographs
often enables the surgeon to greatly narrow the differential diagnosis. The
goal of this exhibit is to provide the reader with a fundamental algorithm for
the radiographic evaluation of benign and malignant bone lesions. The
algorithm is summarized in the flow charts in Figures
1 and
2, which provide a step-by-step
process for the classification of lesions. Also shown in the charts are
important examples of each category.
Comparison of Radiodense and Radiolytic Lesions
The first step in this approach is to determine whether the lesion is
radiodense or radiolytic. At the cellular level, radiolytic lesions have
varying degrees of bone reabsorption, which is detected as a lack of calcium
on plain radiographs. Radiodense lesions at the cellular level, however, have
an overabundance of calcium compared with normal bone, producing their
appearance on plain radiographs. This overabundance of calcium may take the
form of bone, amorphous calcification, or
cartilage1-3.
Radiolytic Lesions
Radiolytic lesions can be broadly classified into four categories on the
basis of their radiographic characteristics: inactive, active, aggressive, and
ground glass. Inactive lesions are well circumscribed and surrounded by a
sclerotic rim of bone. Figure 3
shows an example of an inactive radiolytic lesion—in this case, subacute
osteomyelitis of the proximal aspect of the tibia. The lesion has central bone
reabsorption, which appears lytic on the radiograph; however, note the
sclerotic rim of bone surrounding the lesion. It is clear from this radiograph
that the lesion is not growing, and, in fact, the body has had time to create
a wall of new reactive bone around the lesion to further contain it.
Figure 4 shows an example of
an active lytic lesion of the proximal aspect of the tibia. This giant-cell
tumor of the proximal part of the tibia clearly exemplifies the typical
characteristics of an active lytic lesion. The lesion is still very well
circumscribed with a clear border delineating its margins within the bone.
However, unlike the inactive lesion in
Figure 3, this lesion has no
sclerotic rim. The lack of a sclerotic rim indicates that the lesion is still
growing and continuing to reabsorb bone at its borders, not giving the
surrounding bone a chance to wall it off.
Figure 5 shows an example of
an aggressive lytic lesion of the proximal part of the humerus. In this case,
the metastatic tumor shows a permeative process with no clear delineation of
its borders. There is also cortical expansion and destruction. This lesion is
growing fast and out of control. The body and surrounding bone has no way of
defending itself from this aggressive, uncontrolled growth. This is typical of
highly malignant bone lesions (such as Ewing sarcoma), acute osteomyelitis,
leukemia, and several primary malignant sarcomas of the bone.
Finally, the classic hazy "ground-glass" appearance of fibrous
dysplasia is placed in its own category. As shown in
Figure 6, fibrous dysplasia is
typically a long lesion in a long bone, with thinning of the cortices and
occasionally cortical destruction. However, it is distinguished from
aggressive lesions by its clearly demarcated borders.
Figure 1 is a flow chart
that provides a schematic representation of the way the orthopaedist should go
about characterizing and classifying lytic lesions.
Radiodense Lesions
Radiodense lesions can be broadly classified into three categories on the
basis of the structure of the material making up the lesion. These lesions can
be amorphous calcification, bone, or cartilage. Amorphous calcification, as
its name implies, is a collection of calcifications without a well-defined
matrix. It typically appears on the radiograph as fluffy "wet
cotton-ball" calcifications in the soft tissue and bone
(Fig.
7)4,5.
At the histologic level, bland calcium crystals without a matrix are seen.
Cartilage lesions appear with characteristic rings and stipples on plain
radiographs. These rings and stipples correlate histiologically to the
endochondral ossification at the core and periphery of the cartilage lobules.
Both benign and malignant cartilage processes have this radiographic pattern.
Cartilaginous lesions may be divided into intramedullary lesions and surface
lesions. As in most processes, benign lesions appear well circumscribed while
malignant lesions are ill defined. Figure
8 is an example of an enchondroma of the distal aspect of the
femur. An enchondroma is an example of a benign intramedullary cartilage
lesion. Note the clear rings and stipples present in the medullary canal.
Figure 9 is an example of an
osteochondroma, a benign extraosseous cartilage bone lesion. Again note the
rings and stipples and the well-demarcated nature of this osteochondroma. Also
note that the marrow of this lesion is continuous with the medullary canal of
the proximal aspect of the tibia from which it arises.
All radiodense lesions in the form of bone are characterized by the
presence of a matrix at the histiologic level, which on plain radiographs
appears as a trabecular pattern. Lesions composed of bone may be intraosseous
or extraosseous. Extraosseous bone lesions may be divided into heterotopic
ossification (well circumscribed) or tumor bone (ill defined). In
Figure 10, note the
well-circumscribed borders and trabecular pattern that are characteristic of
heterotopic ossification. The parosteal osteosarcoma seen in
Figure 11 is an example of a
malignant extraosseous bone lesion, demonstrating the poorly demarcated border
that is typical of such a malignancy.
Intraosseous bone lesions start in the medullary canal, but they also may
break through the cortex and extend into the surrounding soft tissue. These
lesions can be divided into three categories: normal bone, reactive bone, and
tumor bone, each with a distinct radiographic pattern. A bone island
(enostosis) is an example of dense normal bone that develops in the medullary
canal as a result of a remodeling error that occurs during skeletal growth
(Fig. 12). Its appearance in
every way looks like normal "misplaced" bone. It has a trabecular
pattern that is easily seen on radiographs and is always well
circumscribed.
Reactive bone is characterized by a zonal pattern of bone formation, with
smooth transitions from dense areas to normal bone areas.
Figure 13 is an example of
reactive bone. As the name implies, reactive bone is usually reacting to a
process such as infection, fracture, or osteoid osteoma.
Unlike reactive bone and normal bone, tumor bone is characterized by its
lack of a pattern. It usually has no clear distinct margins and may present as
a combination of lytic and dense blastic areas on plain radiographs. Tumor
bone is also noted for its cortical destruction and expansile nature.
Figure 14 is a classic example
of conventional osteosarcoma of the distal aspect of the femur. Note the
patternless density and cortical destruction seen on this radiograph. This
same image could also be an example of metastatic carcinoma, another form of
tumor bone.
The flow chart in Figure 2
provides an outline of the approach to radiodense lesions of the bone.
With the above information, one should now be aware of the radiographic
characteristics of the most common pathological bone lesions and should be
able to recognize these lesions on plain radiographs using an algorithmic
approach to radiographic diagnosis as outlined in Figures
1 and
2. It should be emphasized that
this approach is based solely on the plain radiographic appearance and is
intended to aid the surgeon in developing an initial differential diagnosis
when a lesion is first noted on radiographs. It must be emphasized that the
practitioner must combine this approach with an understanding of the common
locations of specific lesions, patient characteristics, other imaging
modalities, and biopsy results in order to arrive at a correct diagnosis.
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