Orthopaedic Surgery - Fibrous Cortical Defect / Nonossifying Fibroma

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Basics

A nonossifying fibroma (NOF) is a common benign developmental bone lesion seen primarily in children and adolescents with open physes.

Despite its name, the lesion typically undergoes progressive mineralization and involution as skeletal maturity is reached.

When the lesion is very small and confined mainly to the cortex, it is commonly termed a fibrous cortical defect.

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Typical Location

NOFs arise eccentrically within the metaphysis of long bones.

The most common locations are the distal femur, proximal tibia, and distal tibia.

The lesion is characteristically associated with the growing skeleton and usually regresses or becomes incorporated into normal bone by adulthood.

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Staging

Like other benign bone lesions, NOFs can be described according to biologic activity.

Stage 1, or latent lesions, account for the great majority, approximately 96%.

Stage 2, or active lesions, represent roughly 2–3%.

Stage 3, or aggressive lesions, are very uncommon, accounting for less than 1%.

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Natural History

During childhood, some lesions may behave as active stage 2 abnormalities.

With skeletal maturation, they generally become latent and progressively ossify.

Ultimately, most disappear radiographically or leave only a residual area of sclerosis.

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Synonyms

Other names that have been used include benign metaphyseal cortical defect, metaphyseal fibrous defect, benign fibrous histiocytoma, and fibrous xanthoma.

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Epidemiology

NOF is one of the most common benign skeletal lesions of childhood.

It has been estimated to occur in approximately 35% of otherwise healthy children with open growth plates.

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Proposed Mechanism

The exact cause is uncertain.

One proposed explanation is a localized disturbance of cortical remodeling with increased periosteal resorption during growth.

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Associated Conditions

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Neurofibromatosis

Lesions resembling NOFs may occasionally occur in patients with neurofibromatosis, with reported frequencies of approximately 5%.

Their clinical context should therefore be considered when multiple or atypical lesions are present.

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Jaffe–Campanacci Syndrome

Jaffe–Campanacci syndrome is a rare congenital disorder characterized by multiple, often widespread and relatively symmetric NOF-like lesions.

Associated findings may include café-au-lait pigmentation, other nonskeletal abnormalities, and developmental or intellectual impairment.

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Diagnosis

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Signs and Symptoms

Most fibrous cortical defects and NOFs are completely asymptomatic.

They are typically discovered incidentally when radiographs are obtained for trauma or another unrelated complaint.

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Pain

Pain is unusual unless the lesion has weakened the bone sufficiently to cause an impending or completed pathologic fracture.

New weight-bearing pain therefore warrants further evaluation.

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Physical Examination

Most lesions are nontender and produce no visible abnormality.

There should generally be no swelling or pain with weight bearing unless the bone has become structurally compromised.

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Examination for Associated Syndromes

When multiple lesions are present, the patient should be examined for features of neurofibromatosis or Jaffe–Campanacci syndrome, including café-au-lait pigmentation and other systemic abnormalities.

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Imaging

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Plain Radiographs

Plain radiographs usually establish the diagnosis.

The classic appearance is an eccentric radiolucent lesion arising from the cortex of the metaphysis of a long bone.

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Common Sites

The distal femur, proximal tibia, and distal tibia are particularly common locations.

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Cortical Appearance

The lesion is cortical based.

The overlying cortex may become thinned, particularly in larger lesions, but it is usually preserved unless fracture occurs.

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Sclerotic Margin

A characteristic scalloped rim of reactive sclerotic bone surrounds the lesion.

This well-defined border is an important feature of its benign appearance.

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Multiloculated Appearance

Many lesions have a multiloculated or lobulated configuration, producing a characteristic “bubbly” appearance on radiographs.

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Size

Lesions may range from only a few millimeters to several centimeters.

A very small lesion is generally termed a fibrous cortical defect, whereas a larger lesion extending into the medullary cavity is more often called an NOF.

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Number of Lesions

Most NOFs are solitary, although more than one lesion may occur in the same patient or in multiple bones.

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Bone Scintigraphy

If bone scintigraphy is performed, an active lesion may demonstrate increased tracer uptake because of the reactive bone along its periphery.

As the lesion heals and becomes inactive, scintigraphic activity generally decreases and may return to normal.

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MRI

MRI is not routinely required when the radiographic appearance is characteristic.

It can, however, be useful when there is concern for a stress fracture, pathologic fracture, or atypical lesion.

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Pathological Findings

Histologically, the lesion consists predominantly of fibrous connective tissue arranged in a whorled or storiform pattern.

This appearance has sometimes been described as having a “starry-night” quality.

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Cellular Components

Additional findings may include multinucleated giant cells, lipid-laden or foamy histiocytes, and hemosiderin deposition.

True cystic spaces are not a typical feature.

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Differential Diagnosis

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Chondromyxoid Fibroma

Chondromyxoid fibroma can also present as a metaphyseal lytic lesion and may need to be distinguished from NOF when the imaging appearance is atypical.

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Giant Cell Tumor

Giant cell tumor generally occurs in skeletally mature patients and has a different distribution, often extending to the epiphysis.

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Fibrous Dysplasia

Fibrous dysplasia may produce an intramedullary lucent or ground-glass lesion and should be considered when the lesion is not classically cortical and eccentric.

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Treatment

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General Principles

Most NOFs require no treatment because they are benign and spontaneously involute with skeletal maturity.

Observation is appropriate for asymptomatic lesions with a typical radiographic appearance.

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Activity

Children with small, asymptomatic lesions may participate in normal activities without restriction.

Routine limitation of sports or weight bearing is unnecessary.

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New Weight-Bearing Pain

If a child develops new pain during weight bearing, AP and lateral radiographs should be obtained to look for an occult or stress fracture.

MRI is useful if radiographs are inconclusive but clinical suspicion remains high.

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Observation

Large asymptomatic lesions may be followed radiographically until sufficient healing or skeletal maturity occurs.

The exact frequency depends on lesion size and fracture risk.

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Follow-Up of Large Lesions

Children with larger lesions may be reviewed approximately every 6 months, particularly when a substantial portion of the cortex is involved.

Monitoring may continue until the lesion becomes sclerotic or growth is complete.

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Small Lesions

A small, asymptomatic lesion involving less than approximately 25% of the cortical width generally does not require routine serial surveillance when the diagnosis is secure.

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Fracture Risk

The principal clinical concern is pathologic fracture.

Risk increases when a lesion occupies a large portion of the bone diameter or substantially weakens the cortex.

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Lesions Involving More Than 50% of the Cortex

If more than approximately 50% of the cortical width is involved and the patient is symptomatic, the risk of pathologic fracture is increased.

Such lesions require closer observation and may warrant operative treatment.

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Protected Weight Bearing

When an impending pathologic fracture is suspected, the child should avoid unrestricted loading.

Non-weight bearing or protected weight bearing may be recommended until definitive management is determined.

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Surgery

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Indications

Surgery is generally reserved for lesions with impending pathologic fracture, substantial structural weakness, persistent symptoms, or selected completed fractures.

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Curettage

The lesion can be treated by curettage, removing the fibrous tissue from the involved bone.

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Bone Grafting

After curettage, the defect is usually filled with bone graft or another suitable bone substitute to restore structural strength.

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Internal Fixation

Internal fixation is usually unnecessary because most lesions heal readily after curettage and grafting.

Fixation may be considered only when fracture stability or lesion size makes additional support necessary.

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Follow-Up

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Prognosis

The prognosis is excellent.

Virtually all typical fibrous cortical defects and NOFs undergo spontaneous healing and sclerosis by skeletal maturity.

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Complications

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Pathologic Fracture

Pathologic fracture is uncommon.

It occurs primarily in large lesions involving more than approximately half of the bone diameter, particularly when the patient has pain or sustains significant trauma.

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Malignant Transformation

Typical NOFs do not undergo malignant transformation.

An aggressive or progressively destructive appearance should prompt reconsideration of the diagnosis rather than being interpreted as expected NOF behavior.

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Patient Monitoring

When follow-up is required, serial AP and lateral radiographs are used to document lesion stability, progressive sclerosis, cortical strengthening, and eventual involution.

Patients should be reassessed sooner if they develop new pain, swelling, difficulty bearing weight, or symptoms suggesting fracture.