Fibrous dysplasia

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Fibrous dysplasia is a disorder of bones which may occur with or without endocrinological and skin disorders. It may cause bony pain, deformity, fracture and / or entrapment of nerves, It is basically the acquired mis-sense mutation of gene coding for the α-subunit of the stimulatory G-protein, Gs, in the guanine nucleotide binding, alpha stimulating (GNAS) complex locus in chromosome 20q13. It leads to immature or poor differentiation of body tissue at the time of ossification and replacement of bone by fibrous tissue. Diagnosis depends on radiology and biopsic specimen.

Historical Perspective

• Fibrous dyspepsia was first observed in bone radiography by Von Recklinghausen in 1891.
• It was then entitled as separate entity by american pathologist Dr. Louis Lichtenstein in 1938 as fibrous dyspepsia polyostotic.
• In 1942, Dr Lichtenstein and Dr. Jaffe together labeled syndrome named McCune-Albright syndrome (fibrous dysplasia-café au lait spots-endocrine dysfunction) or Mazabraud syndrome (fibrous dysplasia-myxomas). 

Classification

• Fibrous dyspepsia may be classified according to number of bony sites involved into two groups:

• Monoostiotic
• Polyostiotic

• Other variants of FD include McCune-Albright syndrome, and Mazabraud syndrome.

Pathophysiology

The pathophsiology of FD is based on mechanism mutation of Gs protein.

Genetics

• The somatic mutation in GNAS1 gene located on the long arm (q) of chromosome 20 (20q13.2)  has been associated with the development of FD, involving the gain-of-function mutation pathway.
• As it is somatic mutation that is why it is not heritable, post fertilization mutation occurring due to unknown reason.
• Mutation of GNAS1 gene results in the overproduction of this G-protein, which in turn increases cyclic adenosine monophosphate (cAMP).
• cyclic adenosine monophosphate (cAMP) is important regulatory molecule of differentiation modulating osteoblasts and osteoclasts while ossification and remodeling of bones.
• Improper differentiation of osteoblasts due to mutation of the GNAS1 gene is the cause of FD.
• Osteoclasts removes ossified bones and creates space for immature osteoblasts to produce fibrous tissue and occupy space which may entrap nerves causing pain or nurological deficit depending on the site involved.
• On gross pathology, Well circumscribed, intramedullary,Tan-white-yellow, gritty, Large lesions distort bone, Cortical bone often thin and expanded are characteristic findings of FD.
• On microscopic histopathological analysis :
  • Curvilinear trabeculae (Chinese letters) of metaplastic woven bone (never matures) in hypocellular,
  • fibroblastic stroma
  • No osteoblastic rimming (due to maturation arrest), 20% of cases have cartilaginous nodules (particularly in femoral neck region)
  • Also myxoid areas, rapidly growing secondary aneurysmal bone cysts, hemorrhage, foamy macrophages, calcified spherules (similar to cementifying fibromas), cellular areas, focal hyaline cartilage or cystic areas
  • Usually abrupt transition of normal to abnormal bone
  • No/rare mitotic figures, no atypia (rarely is degenerative)
  • Overall resembles endochondral ossification in skull are characteristic findings of FD.

Clinical Features

There are four varriets of FD and the clinical features are pertaining to that specific feature.

• Monoostiotic
• Polyostiotic
• McCune-Albright syndrome
• Mazabraud syndrome

Differentiating Fibrous dyspepsia from other Diseases

• Fibrous dyspepsia must be differentiated from other diseases that cause bone pain, deformity, and extra-skeletal involvement, such as:

Disease	Bone involvement	Bone pain	Fever	Fractures	Mechanisum	ALK levels	Diagnosis
ossifying sarcoma	single	yes	no	yes	neoplasm	normal	radiology and biopsy
paget's disease	multiple	yes	no	yes	malfunction of osteoblast	high	biopsy
osteosarcoma	single	yes	no	yes	neoplasm	normal	radiology and biopsy
cherubism	single	yes	no	no	malfunction of osteoblas	high	radiology and biopsy
hyperparathyroidisum	multiple	yes	yes	yes	high PTH	normal	hormone workup
solitary endocytosis	single	yes	no	no	neoplasm	normal	radiology and biopsy
osteoblastoma	single	yes	no	yes	neoplasm	high	radiology and biopsy
osteomyelitis	single	yes	yes	no	infection	normal	radiology and biopsy
brodie's abscess	single	yes	yes	no	infection	normal	radiology and biopsy

Epidemiology and Demographics

• The prevalence and incidence of FD is not known exactly.

Age

• FD is more commonly found in age group from 3 -15 years of life.
• Polyostiotic does not become symptomatic before age 10 years.
• Monostiotic is asymptomatic until age of 20-30 years of life.

Gender

• FD affects men and women equally.

Race

• There is no racial predilection for FD.

Risk Factors

• Common risk factor in the development of PD is gain in function mutation.

Natural History, Complications and Prognosis

• The majority of patients with FD remain asymptomatic for first decade of life.
• Early clinical features include bone pain, bony deformity, fever, pathological fractures, and skin/endocrine/malignancies depending on the variant.
• Common complications of FD include neurological deficit, endocrine abnormality, and in rare cases soft tissue tumors.
• Prognosis is generally good, and the patients with polyostetic form may have frequent fractures.

Diagnosis

Diagnostic Criteria

• The diagnosis of FD is made when on the basis of history, radiology findings and genetic testing to verify presence og GNAS mutation.

Symptoms

• FD is usually asymptomatic.
• Symptoms of FD may include Early clinical features include bone pain, bony deformity, fever, pathological fractures, and skin/endocrine/malignancies depending on the variant. Common complications of FD include neurological deficit, endocrine abnormality, and in rare cases soft tissue tumors

Physical Examination

• Patients with FD usually appear normal unless the is any bony deformity or skin is involved.
• Physical examination may be remarkable according to the classification of disease:

Monostetic

• Single bone involved
• Mostly affects rib, femur, and tibia.
• Carinofasical bone and humorus bone
• Fracture at age of 10 years
• Bone defrmity less sever
• painless swelling of jaw
• swelling involve buccal and labial plate
• Protuberance of inferior border of mandible

Polyostetic

• Multiple bones involved
• Involve rib, femur, clavicle, femur, lumber spine and tibia.
• Asymmetrical involvement
• Mainly maalignment of bones
• Tenderness over involved bone
• Bowing of weight bearing bone and increased curvature of femur and tibia

Cranofasical

• Involvement of maxilofascial bones
• Maladjustment
• Tappering
• Daisplacemnt
• Intact over lesion
• Maxillary sinus and Zygomatic bone involved mostly
• Floor of orbit may extend to skull
• No extracranial involvement
• Hypertelorisum, facial asymmetry, visual impairment, expothalmus and blindness
• Vestibular dysfuntion, hearing loss and tinnitus.

McCune-Albright syndrome

• Involves all the bone of body
• Cafe au lait spots
• Endocrine abnormality of parathyroid hormone, gonadotropins Mazabraud syndrome
• Involve variable number of bones
• Cafe au lait spots

Laboratory Findings

• There are no specific laboratory findings associated with FD however we may find normal PTH, Ca levels, and increased levels of ALK phosphate due to increased bone turn over and increased BMR.

Imaging Findings

• Radiology is the imaging modality of choice for FD.
• On Xray, FD is characterized by
  • Network of fine bone trabeculae
  • Increased trabeculation
  • Opaque trabeculation forming ground glass appearance
  • Cortical thinning
  • Root of teeth separated

Other Diagnostic Studies

• Work up of FD may also be involve using MRI and CT.
• Bone biopsy will revel the histological findings of as:
  • Curvilinear trabeculae (Chinese letters) of metaplastic woven bone (never matures) in hypocellular,
  • fibroblastic stroma
  • No osteoblastic rimming (due to maturation arrest), 20% of cases have cartilaginous nodules (particularly in femoral neck region)
  • Also myxoid areas, rapidly growing secondary aneurysmal bone cysts, hemorrhage, foamy macrophages, calcified spherules (similar to cementifying fibromas), cellular areas, focal hyaline cartilage or cystic areas

Incidental finding

Dull, aching pain and subsequent radiographs

Full-body 99Tc-methylene diphosphonate bone scan

Typical thinning of the cortex without periosteal reaction with a matrix appearance Ground glass

If Yes no further studies

If no Bone Biopsy

Treatment

Treatment of FD is mostly symptomatic and it can be either medical or surgical depending on severity and presentation of disease.

Medical Therapy

• The mainstay of therapy for FD is Bisphosphate which help in improving pain, slow down process of bone turnover and also lower the fracture risk factor.
• Bisphosphate acts by inhabiting osteoclasts.

Surgery

• conventional surgical procedures can only be performed for patients with symmtomatic FD patients in conjugation of over all evaluation of disease and the age of child.