Non-Ossifying Fibroma
Dr. KS Dhillon
Introduction
Non-ossifying fibroma (NOF) and fibrous cortical defect (FCD) are common bone lesions. They are usually found in skeletally immature patients under the age of 15 years (1). They are generally asymptomatic. They are typically discovered incidentally. They are the most common lesions that are referred to orthopedic oncology clinics (2). Their radiographic features are usually very characteristic. If these lesions are detected incidentally a biopsy is not required.
The two lesions have different imaging characteristics. FCDs are small lesions primarily located in the bone cortex, larger NOFs are on the other hand located eccentrically in the medullary cavity (1,3,4). NOFs are large and usually symptomatic. FCDs are small and usually asymptomatic (3).
FCD and NOF are present in up to 30% of children during their skeletal growth period (1,5). However, it is difficult to draw definitive prevalence
rates. Larger lesions can cause pain (1). The association between lesion size and pain is not well known.
Non‑ossifying fibromas and fibrous cortical defects
NOFs and FCDs are benign, well-circumscribed radiolucent lesions that are present in the metaphyseal portions of long bones. The lesions are usually located in the distal femur, proximal and distal tibia, and proximal humerus (3). Usually, the diagnosis can be made from characteristic radiographic findings (6). NOF can diagnosed if there is an oval, rounded or polycyclic, sharply marginated, intramedullary radiolucency in the metaphyseal portion of the distal femur or proximal tibia, with or without a rim of sclerotic bone.
FCD can be diagnosed if there is an oval or rounded, sharply marginated, eccentric radiolucent zone in the medial metaphyseal cortex of the distal
femur.
The prevalence of NOF increases and that of FCD decreases with advancing age. About half of the patients with FCD complain of spontaneous pain. The lesion size and spontaneous pain, however, may not be associated.
NOF and FCD are metaphyseal fibrous defects. They have been considered to be synonymous because of similar pathologic findings, although they differ in size and primary locations in the long bones (4,9). NOF lesions are principally metaphyseal in location. The most common sites include the distal femur, proximal tibia, distal tibia, proximal humerus, fibula, and radius (3). About less than 5% of NOFs are multifocal. Most multifocal NOFs often develop sporadically. They also develop in patients with neurofibromatosis type 1 and Jafe-Campanacci syndrome (1).
More recently, NOF has been considered to be a neoplasm because of
Ras-MAPK activation by somatic mutation (10). Hence, these two lesions are different entities because of their differing clinical and biological characteristics. FCD maintains the same position (11), while NOF advances proximally or distally with age. FCD most likely occurs when tendons are inserted into the perichondrium of the epiphyseal plate (7,11,12). There is a male predominance (6,7,12). It has been estimated that NOF may be present in up to 30% of children (1,5). This prevalence may not be accurate because previous studies were retrospectively case series. Sontag and Pyle (11) examined the skeletal roentgenograms of healthy children until the age of 18 years. They found cystic lesions (consistent with FCD) in 22% of the girls and 53% of the boys. No similar research has been conducted on NOF. No ethnic prevalence has been described (3). The Ritschl classification of NOF is based on the clinical course of the healing process. According to Ritschl's classification stage A is the
most common (6).
FCD and NOF differ in terms of size and clinical course. These two lesions usually disappear with advancing age. NOFs are large and typically symptomatic, while FCDs are generally small and asymptomatic (3).
Pathologic fractures occur in up to 20% of NOFs (13). Fractures are likely to occur in NOFs that are more than 33 mm long (9). It is, however, unclear whether the lesion size is associated with the occurrence of fracture (14).
Absolute size parameters may be useful in predicting pathologic fracture rates. They however do not imply a requirement for prophylactic curettage and bone grafting. Patients with a stage B lesion have an increased risk of pathologic fractures (6). Thus, combining lesion size and Ritschl classification may be important for evaluating and predicting pathologic fracture.
Conclusion
Non-ossifying fibroma (NOF) and fibrous cortical defect (FCD) are common bone lesions. They are usually found in skeletally immature patients under the age of 15 years. They are often discovered incidentally. FCD and NOF are present in up to 30% of children during their skeletal growth period. These two lesions usually disappear with advancing age. NOFs are large and typically symptomatic, while FCDs are generally small and asymptomatic (3). Pathologic fractures occur in up to 20% of NOFs. Prophylactic curettage and bone grafting is not required.
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