The evaluation of the musculoskeletal system is difficult due to the numerous soft tissues as well as the bone structures
involved. Rapid assessment of the bone structure is routinely performed using radiographs; however, the subtlety of disease
and joint compared to bone pathology can be confusing. The purpose of this lecture is to cover the identification of aggressive
compared to non-aggressive bone lesions as well as erosive compared to non-erosive joint pathology.
When evaluating the skeletal system, the first thing to determine is if the lesion is aggressive or non-aggressive. A non-aggressive
lesion diagnoses include callous, malunion fractures, bone cysts, osteomas, osteochondritis dessicans, panosteitis, fragmented
medial coronoid process, osteoarthritis or metabolic disorders. Aggressive lesions are due to neoplasia or osteomyelitis.
When deciding about aggressive lesions, there are 6 radiographic signs that are used: bone lysis, periosteal reaction, rate
of progression, zone of transition, cortical lysis. Bone lysis has three different appearances, geographic (focal) moth-eaten
and permeative. The difference between the degree of lysis is mainly on the rate of progression. It requires approximately
50% of the bone per unit area to be destroyed before it is visible on radiographs. This is because the bone is a three dimensional
object viewed from two dimensions. Because of this, bone is superimposed on itself, making subtle lesions hard to detect.
The more lysis that is present, the easier it is to see on radiographs. Also, by the time lysis is seen on a radiograph,
the lesion is quite severe.
Periosteal reaction can either be smooth (continuous) or interrupted. The easiest way to determine this is if you could trace
the outline of the periosteal reaction with a pencil and never have to lift the pencil from the radiograph. Smooth periosteal
reactions are generally associated with trauma whereas interrupted periosteal reactions are due to an aggressive process.
Rate of progression is probably the most overlooked method to assess an aggressive lesion. By the time a questionable aggressive
lesion is seen on a radiograph, the lysis is quite substantial. Therefore, the rate of progression in 2-4 weeks will also
be dramatic. If a question exists between an aggressive and non-aggressive lesions, supportive medical management for 2-4
weeks then repeat radiographs to look for progression can aid in determining if the lesion is aggressive.
Zone of transition is a more nebulous sign, but the idea is that if a clear-cut demarcation between normal and abnormal bone
is seen, then the lesion is more likely non-aggressive. If there is a long zone of transition, the difference between normal
and abnormal bone is blurred and the lesion is more likely to be aggressive. In addition, cortical lysis as opposed to overall
bone lysis can be used to determine aggressive bone lesions. If the cortex is thin, but no lysis is present, then it is more
likely that the lesion is non-aggressive.
After determining these radiographic signs, the next clue is based on the location of the lesion. If the lesion is generalize
in that it effects all bones equally, then the primary differential diagnosis is a metabolic or nutritional abnormality.
If only one bone is involved, this is a focal or monostotic lesion and a primary bone tumor or soft tissue tumor with secondary
bone involvement is considered most likely. If multiple bones in the same region (locally extensive), different bones that
are not in close proximity or multiple areas in the same bone are involved, this generally indicates a hematogenous spread
disease as bacterial osteomyelitis or metastatic neoplasia. A soft tissue tumor with secondary bone involvement is possible
with locally extensive lesions, such as aggressive lesions that cross a joint.