Orthopaedic Surgery - Fibrous Dysplasia

Basics

Fibrous dysplasia is a benign fibro-osseous disorder in which normal mature lamellar bone is replaced by immature woven bone and fibrous tissue.

The abnormal bone has reduced mechanical strength and may progressively deform under normal loading.

Clinical Characteristics

The condition can produce focal or multifocal defects in bone quality, progressive skeletal deformity, pain, and pathologic fracture.

Patients with polyostotic disease may also have associated endocrine abnormalities.

Café-au-Lait Pigmentation

Café-au-lait skin lesions are particularly associated with multifocal disease and McCune–Albright syndrome.

The lesions classically have irregular borders that have been compared with the “coast of Maine.”

Endocrine Manifestations

Endocrine abnormalities may appear early in childhood.

Precocious puberty may develop during the first years of life and can occasionally be evident as early as infancy.

Classification

Fibrous dysplasia is classified according to the extent of skeletal involvement.

Monostotic fibrous dysplasia involves a single bone.

Polyostotic fibrous dysplasia affects multiple bones and may show asymmetric involvement, often with greater disease burden on one side of the body.

Terminology

Fibrous dysplasia should not be confused with osteitis fibrosa cystica, which is a different skeletal disorder associated with hyperparathyroidism.

Epidemiology

Fibrous dysplasia is uncommon.

Males and females are affected approximately equally.

Risk Factors and Syndromic Association

McCune–Albright Syndrome

The classic McCune–Albright syndrome consists of the triad of:

Polyostotic fibrous dysplasia, café-au-lait skin pigmentation, and precocious puberty.

Other endocrine abnormalities may also occur.

Genetics and Etiology

Fibrous dysplasia results from a postzygotic activating mutation involving the GNAS gene, which encodes the alpha subunit of a stimulatory G protein.

This abnormality leads to excessive intracellular cyclic adenosine monophosphate (cAMP) signaling.

Mosaic Distribution

Because the mutation occurs after fertilization, affected patients have a mosaic distribution of abnormal cells.

The earlier in embryonic development the mutation occurs, the more extensive the disease is likely to be.

This explains why some patients have a single skeletal lesion while others have widespread skeletal and endocrine involvement.

Associated Conditions

Fibrous dysplasia may be associated with various endocrinopathies, particularly in polyostotic disease and McCune–Albright syndrome.

Rarely, malignant transformation may occur, most commonly to osteosarcoma and less often to fibrosarcoma or other sarcomas.

Diagnosis

Diagnosis is often suggested strongly by the characteristic radiographic appearance.

In typical lesions, plain radiographs may be sufficiently characteristic that biopsy is unnecessary.

Signs and Symptoms

Skeletal Deformity

Progressive deformity may develop in involved bones, particularly the proximal femur, pelvis, and craniofacial skeleton.

Because the abnormal bone is mechanically weak, repeated loading can gradually produce bowing.

Neurologic Compromise

Cranial or spinal lesions may occasionally cause neurologic symptoms through deformity or compression of neural structures.

Pain

Patients may experience a chronic dull ache in affected bones, particularly during weight bearing.

Approximately one-third of patients develop chronic pain.

Gait Abnormality

Lower-extremity deformity can produce a limp or waddling gait.

This may be especially apparent with bilateral proximal femoral or pelvic involvement.

Scoliosis

Spinal involvement or asymmetric lower-extremity deformity may contribute to scoliosis.

Physical Examination

Height

The patient’s height should be measured and followed over time, particularly when endocrine disease or extensive skeletal involvement is present.

Spinal Examination

The spine should be examined for scoliosis, kyphosis, or other progressive deformity.

Limb Length and Alignment

Limb lengths and angular deformities should be measured.

All four extremities should be inspected for bowing or asymmetry.

Hip Examination

Hip range of motion should be carefully documented, particularly in patients with proximal femoral involvement.

Loss of motion may reflect progressive deformity or associated pain.

Palpation

Areas of known disease should be palpated for tenderness, swelling, or increased warmth.

New or increasing tenderness warrants reassessment.

Gait

The patient’s gait should be observed for limping, waddling, limb-length inequality, or mechanical abnormalities caused by deformity.

Laboratory Tests

There is no single laboratory test that confirms isolated fibrous dysplasia.

Laboratory investigation is directed toward associated endocrine abnormalities when clinically suspected.

Endocrine Evaluation

Depending on the presentation, testing may include growth hormone, thyroid function, adrenal function, phosphate metabolism, and sex-hormone evaluation.

Further endocrine testing should be individualized.

Imaging

Plain Radiographs

The classic radiographic appearance is a relatively homogeneous “ground-glass” matrix.

This reflects the replacement of normal trabecular bone by immature woven bone and fibrous tissue.

Distribution in Long Bones

Fibrous dysplasia often extends over a long segment of the diaphysis and may involve nearly the entire shaft.

A traditional description is that fibrous dysplasia is a “long lesion in a long bone.”

Cortical Changes

The involved bone may demonstrate cortical thinning and expansion while maintaining a relatively smooth outer contour.

The lesion often blends gradually with surrounding normal bone rather than having a sharply defined border.

Shepherd’s Crook Deformity

A classic complication involving the proximal femur is the shepherd’s crook deformity.

This consists of progressive varus and bowing of the proximal femur caused by repeated loading of weakened dysplastic bone.

Consequences

The deformity increases bending stress across the proximal femur and may lead to worsening angulation, pain, shortening, gait disturbance, and fracture.

Bone Scintigraphy

Technetium bone scintigraphy may be used to identify additional skeletal lesions in patients with suspected polyostotic disease.

Fibrous dysplasia lesions are usually metabolically active and therefore often appear “hot”, although uptake can vary.

CT

CT is particularly useful for evaluating craniofacial lesions, spinal involvement, complex deformity, and surgical anatomy.

It provides excellent detail of cortical structure and osseous expansion.

Pathological Findings

Histologically, affected bone contains multiple small, irregular, disorganized trabeculae of immature woven bone within a fibrous stroma.

Alphabet-Soup Appearance

The irregular trabeculae are often described as resembling “alphabet soup” because of their curved and randomly arranged configuration.

Fibrous Marrow

Normal marrow is replaced by fibrous connective tissue.

The abnormal trabeculae fail to form the organized architecture required for normal mechanical strength.

Osteoblastic Rimming

Typical lesions lack prominent osteoblastic rimming around the abnormal trabeculae.

The bone forms through fibro-osseous metaplasia rather than normal lamellar remodeling.

Differential Diagnosis

Unicameral Bone Cyst

A unicameral bone cyst may produce a lucent lesion and pathologic fracture, particularly in younger patients.

Its central location and lack of a ground-glass matrix usually help distinguish it.

Fibrous Cortical Defect

Fibrous cortical defects are typically small, eccentric, cortical-based metaphyseal lesions rather than long intramedullary abnormalities.

Ollier Disease

Ollier disease produces multiple enchondromas with a characteristic cartilaginous appearance rather than the ground-glass matrix of fibrous dysplasia.

Treatment

General Principles

Treatment is determined by symptoms, deformity, fracture risk, location of disease, and associated endocrine abnormalities.

Many asymptomatic lesions can be observed.

Skeletal Deformity

Progressive deformity should be corrected when it causes pain, functional impairment, or increasing mechanical disadvantage.

Endocrinopathy

Associated endocrine disorders should be treated appropriately because hormonal abnormalities can worsen skeletal disease and contribute to deformity or fracture risk.

Craniofacial Reconstruction

Severe craniofacial deformity may require reconstructive surgery when there is substantial cosmetic deformity, functional impairment, or compression of adjacent structures.

Pain Management

Pain can be treated with appropriate analgesics.

Patients with persistent or complex pain may benefit from consultation with a pain-management specialist.

Bisphosphonates

Bisphosphonates have historically been used in selected patients with significant bone pain, although their effect on lesion progression or deformity is limited.

They should not be expected to restore normal bone architecture.

Activity

Increasing activity alone does not strengthen dysplastic bone.

Patients should generally use pain and mechanical symptoms as guides to activity.

High-Impact Activity

High-impact and prolonged endurance activities may increase the risk of fracture in significantly involved weight-bearing bones.

Activity recommendations should therefore reflect lesion severity and mechanical stability.

Physical Therapy

Physical therapy is most useful for postoperative rehabilitation, gait training, maintenance of mobility, and muscle strengthening.

Patients with involvement of several limbs may require individualized therapy because deformities can affect multiple segments simultaneously.

Surgery

General Orthopaedic Principles

The goals of surgery are to correct deformity, restore mechanical alignment, support weakened bone, and reduce the risk of fracture.

Correction of Bowing

Marked bowing should be corrected because deformity increases bending forces and promotes further progression.

Proximal Femoral Deformity

In the proximal femur, corrective procedures may include valgus-producing osteotomy or medial displacement osteotomy, depending on the pattern of deformity.

Mechanical Support

Dysplastic bone should be supported with durable fixation.

Metal implants or structural cortical grafts are generally more reliable than cancellous graft alone.

Intramedullary Fixation

An intramedullary device is usually preferred to a plate when feasible.

Because the implant lies near the mechanical axis and spans a long segment of bone, it provides more effective protection against bending forces and fatigue failure.

Plate Fixation

Plate constructs may be less advantageous in extensive disease because they protect only a localized segment and are exposed to greater bending stress.

Bone Grafting

Simple bone grafting alone often fails because the underlying dysplastic process can recur or replace the graft.

Complete removal of all abnormal tissue is generally unnecessary.

Cortical Allograft

Structural cortical allograft may be preferred when grafting is required because it provides mechanical support and remodels more slowly.

Cancellous Autograft

Autogenous cancellous graft is less useful because it may be rapidly resorbed and replaced by fibrous dysplastic tissue.

Operative Bleeding

Fibrous dysplasia lesions can bleed substantially during surgery.

Preoperative planning should therefore account for possible significant blood loss.

Follow-Up

Prognosis

Disease behavior varies considerably.

Some lesions remain stable after skeletal maturity, whereas others may continue to deform or become symptomatic in adulthood.

New lesions or progression can occasionally occur after skeletal maturity.

Chronic Pain

Approximately one-third of patients may experience chronic skeletal pain.

Polyostotic Disease

Patients with extensive polyostotic disease may have greater morbidity because of skeletal deformity, fractures, endocrinopathy, immobility, and associated systemic complications.

Complications

Fracture

Pathologic fracture is a common mechanical complication because dysplastic bone has reduced structural strength.

Chronic Pain

Persistent pain can substantially affect mobility, function, mood, and quality of life.

Depression

Chronic deformity and pain may contribute to depression and other psychological consequences.

Malignant Transformation

Malignant transformation is rare, occurring in roughly 1% or less of cases in most series.

The most common secondary malignancy is osteosarcoma, although fibrosarcoma and other sarcomas can occur.

Warning Signs of Malignancy

Concerning features include a sudden increase in pain, rapid enlargement of a previously stable lesion, new swelling, increased warmth, cortical destruction, or a new soft-tissue mass.

These findings warrant urgent reassessment.

Patient Monitoring

Patients should be followed periodically by clinicians familiar with the disorder so that progressive deformity and complications can be identified early.

Orthopaedic Surveillance

Patients with significant lower-extremity or spinal involvement may benefit from annual orthopedic review, particularly to monitor proximal femoral bowing and scoliosis.

Long-Term Monitoring

Follow-up should include assessment of pain, gait, limb alignment, limb length, scoliosis, fracture history, endocrine abnormalities, and functional status.

Patients should also be educated regarding the warning signs of malignant transformation and instructed to seek evaluation for any sudden change in a previously stable lesion.