Fibrous Cortical Defect and Non-ossifying Fibroma Imaging

Introduction

Fibrous cortical defects are small (less than three centimeters), non-aggressive, asymptomatic, eccentric cortical defects that usually disappear; however, sometimes, they increase in size and develop into fibroxanthoma. Non-ossifying fibroma is eccentric, larger (more than three centimeters), intramedullary lesions that form a superficial and scalloped pattern in the nearby cortex.

Non-ossifying fibroma or fibroxanthoma is a benign bone cell tumor that commonly affects children. Non-ossifying fibroma differs from fibrous cortical defects in their natural history or the size of the lesion. However, non-ossifying fibroma and fibrous cortical defects (FCD) are non-aggressive and developmental defects commonly found within the metaphysis of the long tubular bones of children, often in the knees. Sometimes the non-ossifying fibroma is associated with neurofibromatosis (noncancerous nervous system tumor) or Jaffe-Campanacci syndrome, which is usually found along with mental retardation hypogonadism, and cafè-au-lait spots (flat, light brown colored birthmark).

What Are the Symptoms of Non-ossifying Fibroma?

Non-ossifying fibroma is usually asymptomatic. However, sometimes they show the following symptoms:

• Mild soreness or swelling in the affected area of the bone.

• Even during rest, there may be mild pain.

• Decreased strength of the bone sometimes results in a fracture.

What Are the Imaging Techniques Used in the Diagnosis of Non-ossifying Fibroma and Fibrous Cortical Defect?

The imaging techniques used in the diagnosis of non-ossifying fibroma are:

Plain Radiograph:

1. Non-ossifying Fibroma:

• In plain radiography, non-ossifying fibroma appears eccentric, round or oval, meta-diaphyseal, and intramedullary radiolucency.

• The margins of the reactive bone appear as a sharp, sclerotic rim with a scalloped contour.

• The medullary rim of the lesion seems thicker than the cortical rim.

• Non-ossifying fibroma starts appearing in the physeal plate and later appears inside the diaphysis as the normal growth pattern.

• Larger lesions appear multiloculated with dimensions of about four to seven centimeters, along with the erosion of the endosteal surface of the cortex.

• The epiphysis of the bone is never involved.

• Additional imaging techniques are not required until the lesion becomes symptomatic.

2. Fibrous Cortical Defect:

• Focal cortical defects appear as small, round, or oval geographic radiolucencies that measure about one to three centimeters.

• The margin of the reactive bone appears sharp and is identified in the areas where the mass impinges the medullary cavity.

• It does not affect the soft tissue and is located inside the distal femoral cortex along their posteromedial aspect and proximal to the adductor tubercle.

• In plain radiography, focal cortical defects are visualized better in the AP (anteroposterior) oblique projection of the femoral bone, with the limb rotated externally to 20 to 40 degrees.

Computed Tomography (CT) Scan:

• In CT scans, fibroxanthoma appears eccentric, intramedullary, sclerotic, and well-marginated, along with central lucency.

• In the later stages, reactive sclerosis and heterogeneous lucency are identified.

• The fibrous component of fibroxanthoma has less soft tissue attenuation value, whereas, in the later stages, they show high attenuation.

• CT scans can depict bone involvement and cortical thinning that may result in pathologic fracture.

• If the fibroxanthoma lesion involves more than 50 percent of the width or is found to be more than 33 millimeters, the tubular bone is at higher risk for fracture.

• In case of fracture, the margins are well demarcated, with mild periosteal reaction and mild soft tissue swelling. Soft tissue components are not seen in fibroxanthoma.

Magnetic Resonance Imaging (MRI) Scan:

• Fibroxanthoma appears as low signal intensity on T1 and T2 weighted spin echo MRIs (magnetic resonance imaging)which is similar to that of non-ossified lesions.

• Low signal intensity T2 weighted images represent hemosiderin deposits and increased collagen. Usually, MRI is not taken for focal cortical defects. The intracortical lesion appears as low signal intensity on T1-weighted images and high signal intensity on T2 and proton density-weighted images due to the presence of heterogeneous fibrous areas.

• The low signal intensity rim represents the sclerotic border. The appearance of fibroxanthoma in T2-weighted images often varies, depending on the phase of healing.

Nuclear Imaging:

• A bone scan, also known as skeletal scintigraphy, is a diagnostic tool that uses nuclear medicine in which a special dye called radioactive substances (radiotracers or radiopharmaceuticals) is used to detect bone diseases.

• The appearance of the lesion depends on the phase of the lesion and is usually negative.

• The radioactive substance technetium-99m has minimal to mild accumulation in fibroxanthoma, which indicates a benign lesion. The intensity of the uptake is less when compared to the active bone lesion. Nevertheless, moderate uptake and mild hyperemia are present, and it refers to the healing phase.

• In children or adults, the eccentric region of uptake is located closer to the physis of a tubular bone.

• In children, the findings in nuclear imaging are often masked by a distal femoral epiphyseal plate which shows marked uptake of the tracer.

• The focal cortical defects do not uptake the radioactive substances on bone scans.

What Are the Treatment Options for Non-ossifying Fibroma?

The treatment options for non-ossifying fibroma include:

• Non-ossifying fibroma usually requires no treatment. It usually resolves on its own once the child completes its growth phase. However, at a later stage, the fracture needs to be stabilized with medical help.

• In rare cases, surgical options such as intralesional curettage and bone grafting may be required.

• Intralesional curettage involves the scraping out of the complete lesion, and a bone graft is performed to replace the missing bone with artificial graft material. This surgery has an excellent recovery rate, and the children can return to regular activities within three to six months.

• A periodic follow-up is required after the surgery in which a radiograph is taken to confirm the bone healing and to check whether there is a recurrence of the lesion after the surgery.

Conclusion

The plain radiograph is an effective imaging technique used alone to detect the presence, extent, and location of the non-ossifying fibroma. However, as most of the non-ossifying fibromas are asymptomatic, they are diagnosed accidentally on X-rays taken for other medical conditions. A computed tomography scan is effective in showing the extent of the bone involvement and the degree of cortical thinning. Non-ossifying fibroma is more common in boys than in girls, usually between the age of 10 to 15, and gradually disappears by the age of 25.