Osteomyelitis occurring in long bones has atypical clinical manifestations and radiographic changes due to the low virulence of the infecting bacteria, older age of onset, and the use of antibiotics, which makes diagnosis difficult and even misdiagnosis and incorrect treatment, causing great undue damage to patients.
The x-ray manifestations of atypical osteomyelitis of long bones are diverse and sometimes need to be differentiated from benign tumors and tumor-like disorders such as osteoid osteoma, osteoid osteomalacia, non-ossifying fibroma, and adjacent joint cysts, and especially from osteosarcoma according to their different changes.
(1) Bone like osteoma
When the lesion of osteomyelitis long bone is limited in scope, it can be limited to a small cystic area of destruction less than 1 cm within the cortex, accompanied by obvious hyperplasia and sclerosis of the inner and outer membranes of the bone, or sometimes even just the hyperplasia and sclerosis of the outer membranes of the bone to the thickening of the bone cortex, which is very similar to the x-ray manifestation of osteoid osteoma lesions located in the cortex, with small nests of cystic bone destruction, and thickening and sclerosis of the bone cortex by periosteal hyperplasia.
The main points of difference between osteoid osteoma and osteoid osteoma are as follows.
The clinical manifestations of osteoid osteoma are different from those of osteomyelitis. The former has characteristic pain, which increases with the course of the disease with severe pain at night, and there are no local changes of inflammatory reactions such as redness, swelling and heat.
In osteoid osteoma, the low-density change area of the nest is surrounded by an opaque shadow of increased density, typically resembling a “bird’s egg” or “bull’s eye”. In osteoma, the hyperplastic sclerotic changes surround the nest and form a sharp contrast in density, while in myeloma, the cystic bone destruction is not as regular as the nest, and the hyperplastic sclerotic changes do not necessarily surround the destruction area. CT scans can further differentiate between the cystic bone destruction and the digital subtraction angiographic changes in osteoid osteoma. In essence, this feature reflects the interconnectedness of bony sinus tracts within the infected bone marrow cavity and the spread of pus within the marrow cavity.
② Abnormal proliferation of bone fibers
Osteomyelitis occurring in the cancellous bone of the long bone epiphysis and in the stem cortex, sometimes presenting as multiple cystic bone destruction with irregular endosteal hyperplasia and sclerosis, thickened and dense medial bone cortex, and narrowing of the medullary cavity, has similarities to the X-ray changes of osteofibrillar abnormal proliferation.
Those occurring in proximal joints are similar to adjacent joint cysts. The cystic hypodense areas of bone in atypical osteomyelitis of long bones are characterized by
Multiple cystic bone destructions are interconnected with each other by a follower-like translucent shadow or a single cyst, and the area of bone destruction is connected to the follower-like shadow.
The second feature of osteomyelitis is that dead bone can be seen in the bone cortex and in the medullary cavity in the form of striated, hyperdense shadows.
Abnormal proliferation of bone fibers and adjacent joint cysts are often accompanied by crustal reactions within the focal area. Cortical swelling and thinning, and areas of cystic destruction in osteochondrodysplasia are often together with hairy glass-like changes. Hairy glass-like changes are the direct metamorphosis of fibrous tissue into immature bone, and the X-ray shows the disappearance of normal structures of the cortex and medullary cavity, replaced by a hairy glass-like density area with uniform, non-normal bone pattern structures below the bone cortex and above the cancellous bone of the medullary cavity, which is a characteristic X-ray change for the diagnosis of osteochondrodysplasia.
In osteochondrodysplasia, cartilage tissue is sometimes mixed with the lesion, so there may be a coexistence of circumferential calcification and ossification.
(3) Non-ossifying fibroma
The former is a low-density cystic expansion with uniform density along the long axis, sometimes with thin anterior cortex and thickened posterior cortical mass protruding into the medulla, causing narrowing of the medullary cavity. In multicystic cases, sometimes there are slender bony gaps and sometimes there are rough and thickened bony ridges, which are different from the chronic infection of bone that is surrounded by cortical thickening and osteosclerosis.
Non-ossifying fibromas occurring in the epiphysis are also central cystic changes extending along the long axis. The uniform density within the cystic changes reflects the fibrous connective tissue composition of the tumor, surrounded by only a mildly hyperplastic and sclerotic reaction layer. In osteomyelitis, the proliferation and sclerosis are obvious and exist in an enveloping pattern around the cystic bone destruction area, and the cystic bone destruction area can also be seen with the characteristic of channel-like translucent shadowing through the connection.
④ Osteogenic sarcoma
Atypical osteomyelitis of long bones occurs with bone destruction, sometimes worm-like, sieve-like erosive bone destruction, or with lamellar, irregular osteolytic bone destruction similar to that of osteosarcoma.
In most cases of osteomyelitis, bone destruction is accompanied by bone proliferation and sclerosis, sometimes resembling the osteosarcoma bone. The periosteal reaction in osteomyelitis is more active, sometimes with radiating needle-like periosteal hyperplasia, Codman’s triangle, and intermittent discontinuity of the periosteum, similar to the periosteal hyperplasia common in osteosarcoma. Osteomyelitis is often associated with soft tissue swelling, some of which is confined without clear borders and sometimes not easily distinguished from the soft tissue masses of osteosarcoma.
The above changes make the differentiation of osteomyelitis and osteosarcoma quite difficult.
However, in the early stage of bone tumor, the diagnosis is not easy when it is atypical.