Fibrous Dysplasia
Fibrous Dysplasia (FD) is a disease that
strongly predominates in women in the first three decades of life. There are two basic forms of the disease that
include monostotic (meaning a single
bone) or polystotic (meaning multiple
bones). The polystotic version is
rare and tends to involve multiple bones on a unilateral side. It can strike
any bone but has a predilection for the proximal femur where it gives rise
to the “shepherd’s crook” deformity
(Figure #19). The disease process
accounts for this deformity. In FD,
the Gs protein is known to be defective.
This defect in some way prevents woven bone from becoming mature lamellar
bone. Woven bone is less structurally sound and weight
bearing regions such as the proximal femur are subject to the recurrent process
of fracture and limited remodeling. Over
years this process will dramatically deform bones.
|
Figure
#19
AP
plain radiograph of fibrous dysplasia in the proximal femur producing
the “shepherd’s crook” deformity. This
patient required internal fixation for a pathologic fracture many years
ago and several broken screws were left behind from the subsequent hardware
removal. |
Radiographically, this lesion has a
lytic effect on bones with sharp margins of sclerosis and what is termed “soap
bubbly” or “ground glass” osteolysis. Histologically, the unmistakable look consisting of large amounts
of woven bone encompassed within a fibrous stroma is often termed “alphabet soup” or “Chinese characters.” (Figure #18)
|
Figure
#18
micrograph of
fibrous dysplasia displaying the classic “Chinese Letters” made of osteoid. |
The Gs defect also has consequences
for the skin and endocrine systems. The classic skin lesion in FD is the “café-au-lait” discoloration. It shares a name with the lesion found in
neurofibromatosis (NF) but has a different look. In FD the lesion has a rough, irregular border often termed to
resemble the “coast of Maine.” In NF, the border is smoother and termed to
resemble the “coast of California.” There are five associated endocrine
disorders that include: McCune-Albright,
hyperthyroidism, acromegaly, Cushing’s Disease, and hypophosphatemic
osteomalacia. McCune-Albright
syndrome consists of fibrous dysplasia in the setting of dramatically precocious puberty. Five percent of patients with polystotic
disease will have McCune-Albright syndrome.
Mazabrand’s Syndrome describes polystotic FD with the presence of
multiple myxomas.
Treatment of FD is difficult. It will usually burn out in adulthood so most measures are conservative and aimed
at preventing and treating deformity.
The high recurrence rates
following resection make surgical intervention a poor choice. Instead, bisphosphonates are employed to help solidify bones. In adults, instability created by the lesion
will be approached with internal fixation and allogenic bone strut
grafting. The risk of secondary sarcoma
precludes the use of radiation therapy.
One percent of cases will
develop malignant sarcomatous
transformation within a lesion.
