Non-Ossifying Fibroma (Fibrous Cortical Defect)

Non-ossifying fibroma is one of the commonest benign bone lesions in the growing skeleton: a developmental fibrous defect of the metaphyseal cortex whose radiographic pattern is usually diagnostic on its own.

The clinical skill is not making an exotic diagnosis. It is recognising the classic benign pattern, avoiding unnecessary biopsy, identifying the small subgroup at fracture risk, and knowing when multiple lesions imply a syndromic diagnosis.

Who. A child or adolescent with open physes, usually with no symptoms at all. The lesion is found on radiographs taken for something else: trauma, a limp, alignment assessment, or knee pain unrelated to it.

Where. The metaphyses of the long bones, often around the knee: distal femur, proximal tibia, distal tibia, proximal humerus, and other long-bone metaphyses.

Natural history. The lesion is active during growth and then ossifies as skeletal maturity approaches. It appears to migrate away from the physis, but it does not move: it stays at its original metaphyseal site while longitudinal growth carries the physis away from it, a benign natural-history clue rather than aggressive spread. It becomes sclerotic and usually disappears or leaves a healed cortical scar.

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Why it matters. Most lesions require explanation rather than intervention, and an unnecessary biopsy can create anxiety and morbidity. Two things change that: a large lesion in a weight-bearing bone can weaken the cortex, and multiple lesions may be associated with a Jaffe-Campanacci phenotype or neurofibromatosis type 1 features.

A developmental defect, not an aggressive tumour. NOF is best understood clinically as a benign developmental fibrous defect of metaphyseal cortex. Histology shows fibroblastic spindle cells in a whorled or storiform stroma, collagen, foamy histiocytes, haemosiderin-laden histiocytes and osteoclast-like giant cells, but biopsy is rarely required because the radiographic pattern is usually diagnostic.

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The molecular driver. "Developmental defect" describes the lesion without explaining it. NOFs are driven by activating variants in the RAS-MAPK pathway, with mosaic KRAS variants identified as the principal molecular cause.

Mosaic means the mutation is post-zygotic: it arises in one cell during development, so only that cell's descendants carry it.

Why one clone explains the whole clinical pattern. A late, small mutant clone produces one solitary lesion, which is the ordinary incidental NOF. An earlier or more widely distributed clone seeds several skeletal sites at once, which is the multifocal or polyostotic presentation, and it explains why multiple lesions cluster with extraskeletal findings rather than appearing as isolated bone disease. Because the mutation is mosaic rather than germline, these lesions are typically not inherited, which is a reasonable thing to be able to tell a parent.

The overlap with NF1. NF1 protein is a RAS GTPase-activating protein, so losing it and activating KRAS both drive the same downstream pathway. The two conditions are not coincidentally similar; they converge on one signalling axis.

Why the lesion looks scalloped. Slow growth lets the surrounding bone form a thin sclerotic margin, and the lobulated fibrous tissue produces a scalloped or multiloculated contour rather than a permeative aggressive pattern.

Why it heals with maturity. With physeal closure and remodelling, the fibrous focus progressively ossifies. The lesion may become densely sclerotic, shrink, or become invisible on later radiographs.

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The mechanical spectrum. The same biology produces three clinical situations:

• Fibrous cortical defect: a small focus of fibrous tissue in the cortex, usually less than 2 cm, incidental, mechanically insignificant and self-resolving
• Non-ossifying fibroma: a larger eccentric fibrous metaphyseal lesion that can thin and expand the cortex, so size and cortical support are assessed in weight-bearing bones
• Pathological fracture: loss of cortical cross-sectional strength presenting as pain, limp, deformity or acute fracture, with risk rising with large diameter and length

By radiographic maturation. Staging is useful because the active lucent stage is the period when fracture risk is most relevant.

Ritschl radiographic staging is the named NOF-specific scheme that formalises this natural history into four stages:

• Stage A: a small, round or oval cortical lucency lying close to the physis (the fibrous cortical defect end of the spectrum)
• Stage B: a larger, polycyclic or multilocular well-marginated lesion that may extend into the medulla as the bone grows
• Stage C: sclerosis begins, typically from the diaphyseal margin, as the lesion migrates away from the physis and starts to heal
• Stage D: complete sclerosis and ossification, the healed and often invisible residual lesion

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Fibrous cortical defect versus non-ossifying fibroma. Both belong to the same benign developmental spectrum. The NOF is larger and extends from cortex into the medullary cavity, whereas the fibrous cortical defect is smaller and confined to cortex; the terminology should describe morphology without provoking unnecessary biopsy.

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By clinical context. Four settings, each with its own key issue and treatment note.

Multiple lesions are not simply "more incidental lesions"; examine the child for syndromic features.

Jaffe-Campanacci syndrome. When multifocal or polyostotic non-ossifying fibromas occur together with café-au-lait macules and extraskeletal anomalies, consider Jaffe-Campanacci syndrome. The recognised features are:

• Multiple NOFs, causing bone fragility and recurrent pathological fractures
• Café-au-lait spots
• A variable combination of intellectual disability, hypogonadism or cryptorchidism, ocular anomalies, and cardiovascular malformations

It overlaps clinically with neurofibromatosis type 1, and both involve RAS-pathway mosaic mutations (see the KRAS evidence), so look specifically for neurofibromas and Lisch nodules and involve genetics and paediatrics. The orthopaedic priority is whole-skeleton fracture-risk assessment, because multiple large lesions weaken several bones at once.

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History. Start with why the imaging was done: trauma, knee pain, limp, deformity, or an incidental radiograph. True lesion pain is focal and activity-related, and many children have unrelated anterior knee pain. Sudden pain, inability to weight bear and swelling after low-energy trauma are fracture symptoms. The syndrome screen asks about café-au-lait macules, neurofibromas, developmental delay, jaw swelling, endocrine history, and a family history of NF1.

Examination. Look at limb alignment, swelling, deformity, gait, and the skin for stigmata of NF1. Feel for focal tenderness over the lesion; diffuse joint-line pain suggests another diagnosis. Move the adjacent joint for range, effusion and mechanical symptoms, and screen the hip and spine if pain is poorly localised. Test function with a single-leg hop, a squat, sport tolerance, and weight-bearing ability when safe.

The differential. The table holds the discriminators for a lytic metaphyseal lesion in a child. The NOF-versus-fibrous-dysplasia pair is the common exam trap because both occur in young patients and both are benign fibrous bone lesions.

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Plain radiographs. Orthogonal AP and lateral views of the involved bone, including the adjacent joints. The classic lesion is eccentric, cortically based, metaphyseal and well circumscribed, with a thin scalloped sclerotic rim and a narrow zone of transition. Its long axis usually parallels the bone, the matrix is lytic and multiloculated without aggressive mineralisation, and a large lesion may expand or thin the cortex.

If the child is pain-free and the pattern is classic, plain films are diagnostic; a ground-glass central matrix suggests fibrous dysplasia instead.

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Measuring the lesion. Measure it in two ways: maximum length, and maximum transverse diameter as a proportion of the bone diameter, on both the AP and the lateral view. The classic Drennan / Arata risk flags are length greater than 33 mm and occupation of greater than 50 percent of the bone diameter.

Two precisions usually lost in the retelling. Arata's lesions exceeded 50 percent on both the AP and the lateral film, so a lesion that looks large on one projection alone has not met the criterion, which is exactly why you measure orthogonally. And the 33 mm applied to non-fibular lesions; a long fibular lesion was the stated exception, because the fibula is not carrying the load.

What the numbers are not. Arata described 23 children who had already fractured, with no comparison group of intact lesions, so the flags identify what a fractured NOF looks like and cannot tell you the probability that any particular large lesion will break. Most large NOFs never fracture and never come to surgery. Use the measurements to decide how carefully to look at the cortex and how closely to follow, not as an operative indication standing on its own.

CT. For selected large lesions where cortical support or fracture risk cannot be judged on plain radiographs. It confirms the cortically based nature of the lesion, defines cortical breach, the residual cortical shell and the geometry before prophylactic curettage or fixation, and helps quantify lesion size.

A lesion that breaches or deeply scallops the cortex, leaving a mouth-like defect on the axial slice, is a high-risk feature for pathological fracture and should trigger formal assessment of lesion size and cortical integrity and consideration of activity restriction or prophylactic treatment. Avoid routine CT in classic small lesions; the radiation rarely changes management.

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MRI. For atypical margins, aggressive features, persistent unexplained pain, or a possible infection, aneurysmal bone cyst or malignant mimic. NOF can show variable signal and enhancement, so MRI is most useful to exclude alternative diagnoses, not to prove a classic NOF.

A soft tissue mass, marrow oedema out of proportion, cortical destruction, or a wide zone of transition is a referral trigger.

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The report. Four questions decide what happens next.

The incidental classic lesion. The goal is to reassure the family, avoid unnecessary biopsy, and confirm the lesion is mechanically safe.

At the first visit confirm that the radiographic pattern is classic, explain that this is a benign developmental lesion, not cancer, with no malignant transformation expected and usually resolving with maturity, and assess the risk by measuring width and length and examining for focal tenderness and gait change.

Low-risk lesion. Small, asymptomatic, no cortical breach, and below the fracture-risk thresholds: normal activities, reassurance, no biopsy, no immobilisation. Discharge, or one interval radiograph if diagnostic confidence or family anxiety requires it.

Larger lesion. A lesion approaching greater than 50 percent bone width or greater than 33 mm, particularly in the tibia or femur, is watched.

That "or" is deliberately more cautious than the source, where every fractured lesion exceeded 50 percent on both views and every non-fibular one exceeded 33 mm, so a lesion meeting only one flag sits in a group Arata never described, and is a reason to watch rather than to operate. Modify activity for high-impact sport if the lesion is symptomatic or the cortex is very thin, and repeat radiographs until sclerosis or reduced mechanical concern.

The large lesion at risk of fracture. The goal is to prevent avoidable fracture while avoiding prophylactic surgery for lesions that are simply large on a single projection. Stratify in clinic by measuring length, AP width, lateral width, cortical thinning and lesion stage. Focal pain, limp, or pain with loading increases concern, and distal tibial and other lower-limb weight-bearing lesions deserve closer attention.

Non-operative. Asymptomatic, stable, no fracture line, and sufficient cortex despite size: activity advice, avoiding high-risk impact while painful, and radiographic surveillance until progressive sclerosis or skeletal maturity.

Prophylactic surgery. Selective, for pain, progressive cortical thinning, a very large weight-bearing lesion, impending fracture, or unacceptable activity risk. The operation is curettage through a cortical window, filling of the defect with bone graft or substitute, and fixation only if structural support is inadequate. Counsel that surgery treats mechanical risk, not malignant potential.

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Pathological fracture. The goal is to heal the fracture, maintain alignment, and reassess the lesion after union.

Fix the fracture when the fracture needs fixation; do not operate merely because the background lesion has an alarming name.

• Undisplaced or acceptably aligned: immobilise in a cast or brace according to bone and age, restrict weight bearing until pain and radiographs permit progression, and after union decide whether the residual size still creates risk. Many fractures through NOF heal uneventfully with standard paediatric fracture care.
• Displaced, unstable, or load-bearing: reduction and fixation on standard paediatric fracture principles, curettage and grafting if the lesion compromises stability or remains a large void, and return to sport only after union and adequate cortical restoration.

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