Whitmore, Amber R. MMS, PA-C
At the time this article was written, Amber R. Whitmore was an assistant professor in the PA program at the University of Florida in Gainesville. The author has disclosed no potential conflicts of interest, financial or otherwise.
A 10-year-old girl presents to a pediatric orthopedist with hip pain and limping with physical activity for the past 2 weeks. She denies any trauma, night pain, fever, or change in activity. Her pain is aggravated only after running in gym class. Her mother has noticed that the girl has a slight limp when she walks. On physical examination, the patient is in no distress and can bear weight without pain. Her right lower extremity has no obvious swelling, skin changes, or deformity. She is able to actively and passively move her hip without reproducing her symptoms. Neurovascular examination and muscle strength are grossly intact. She has a negative straight leg raise and no pain to palpation over the greater trochanter. What do the radiographs (Figures 1 and 2) show?
The radiographs reveal a well-circumscribed lesion in the right proximal femur without periosteal reaction (Figure 1). No fractures are identified, and the cortex is intact without a soft tissue component. The lesion involves the femoral neck and appears hazy with a classic ground-glass bone matrix.
Based on the location of the lesion, its appearance on radiographs, the patient's age, and her clinical presentation, the diagnosis is most representative of fibrous dysplasia. A biopsy of the lesion was sent to pathology and was confirmed as benign fibrous tissue proliferation of the bone.
Fibrous dysplasia is a benign bone disorder that can cause bone destruction, deformity, or fractures. The exact cause is unknown and genetic inheritance is not often seen except in cases involving the mandible or maxilla. Males and females have the same prevalence, with diagnosis most common in children and adolescents. Fibrous dysplasia is uncommon, accounting for 7% of all benign bone tumors.
Presentation is seen most often in patients ages 10 to 20 years, when bone growth leads to the expansion of the lesion and failure of normally developed bone. Lesions may be asymptomatic and found incidentally on imaging studies, or may be present with a pathologic fracture.
Symptoms from expanding lesions include pain, swelling around the affected bone, and neurologic symptoms from nerve entrapment. The cells of fibrous dysplasia have displayed increased hormone receptors, leading to the lesions expanding in women during pregnancy or becoming symptomatic with monthly menstrual cycles.
Classification The most common type of fibrous dysplasia, accounting for about 75% of cases, is monostotic, which affects only one bone. The solitary lesions commonly are found in the proximal femur, tibia, humerus, ribs, and craniofacial bones. Polyostotic fibrous dysplasia can occur as multiple lesions throughout the skeletal system or multiple lesions in adjacent bones of the same extremity. The polyostotic form is more common in children under age 10 years; early onset is associated with more severe disease.
Imaging Most cases of fibrous dysplasia are asymptomatic and found incidentally when another cause warrants a radiograph. A pathologic fracture through the lesion can also lead to the diagnosis. Fibrous dysplasia appears on radiographs as a hazy or ground-glass bone matrix. The lesion is classically well defined and located in the metaphysis or diaphysis without periosteal reaction. The appearance on plain films may consist of radiolucent areas with cystic lobulated areas of lucency within the lesion. If the bone becomes weakened enough to cause multiple fractures throughout the lesion (for example, when the lesion is located in the proximal femur), a characteristic shepherd's crook deformity can form. The angle between the neck of the femur and femoral shaft, normally between 120 and 135 degrees, decreases to less than 90 degrees, causing a bowing of the proximal femur that resembles the C-shaped curve on a shepherd's crook (Figure 2).
CT or MRI can be used in addition to plain films when identifying pathological fractures. Advanced imaging helps outline the lesion; the transition zone from dysplasia to healthy mature bone is difficult to detect on plain films.
Histopathology Tissue biopsy reveals benign immature bone that cannot undergo transformation into mature bone secondary to undeveloped osteoblasts. Lamellar or secondary bone is not found in fibrous dysplasia, which consists of much weaker woven primary bone. Most frequently, the solitary tumor mutates into abnormal tissue during episodes of growth in children, then enlarges over time.
After fibrous dysplasia is confirmed via a biopsy of the lesion, patient presentation guides treatment. Asymptomatic patients without risk of pathologic fracture can be monitored with imaging and clinical observation of the lesion. Patients whose lesions cause pain should have surgical curettage to remove the lesion. Patients with pain or risk of fracture through the lesion may be candidates for surgical curettage to remove the lesion. Bone grafting in addition to surgical removal of the lesion may be an option to fill the space occupied by the lesion or at sites of pathological fractures, nonunion, or malunion. When choosing treatment, tell the patient that the body can replace the graft with more fibrous tissue, causing recurrence of the lesion.
Additional options include long-term oral or IV bisphosphonates to reduce pain and increase cortical thickness; surgical stabilization with an intramedullary nail to prevent fracture and further deformity; and surgical osteotomy or a definitive hip replacement surgery if the lesion is in the proximal femur. Surgical correction of shepherd's crook or any deformity from fibrous dysplasia is a challenge. The ultimate goal is to obtain a normal gait and relieve pain.
Patients with polyostotic fibrous dysplasia and café au lait hyperpigmentation of the skin or precocious puberty should be referred to genetic counseling for possible McCune-Albright syndrome, a genetic mutation that causes polyostotic fibrous dysplasia and skin and endocrine disorders.
All patients should have follow-up imaging to observe for recurrent lesions, nonunion, fractures, and deformity during periods of growth. Once a patient has been assessed and the lesion becomes latent and asymptomatic, imaging is not necessary unless there is concern for recurrence or change.
Fibrous dysplasia is a benign lesion that is most often identified on radiographs. Providers should be familiar with the presentation on plain films when the lesion is accompanied by pathological fractures or incidentally found with routine imaging. Although lesions are benign, they can be destructive to the bone and cause fractures or permanent deformities in the adolescent hip, leading to long-term disability. Early identification and referral to a pediatric orthopedic surgeon can prevent irreversible hip deformity. Provide patients and their families with education about the treatment and prognosis of fibrous dysplasia.