Pathologic Hip Fracture in a 4-Year-Old Boy

A 4-year-old boy was transferred from another hospital to the authors’ institution for evaluation and treatment of a pathologic fracture of the left femur. The child had a 5-month history of mild limp and pain in his left proximal thigh. He was knocked over by a dog on the day of admission and subsequently was unable to ambulate. The patient was healthy otherwise with no previous injuries, major illnesses, or surgeries. Developmental, social, and family histories were noncontributory. On physical examination, the patient was a healthy appearing boy who reported pain in his left hip region. He was unable to bear weight on his left lower extremity. There was shortening and external rotation of the left lower extremity. The neurovascular examination was normal. Plain radiographs and magnetic resonance imaging (MRI) scans were taken and are shown in Figures 1 through 4.Fig 1.: Anteroposterior plain radiograph of the left proximal femur obtained preoperatively.Fig 2.: The long axis STIR image of the left proximal femur.Fig 3.: Axial T1-weighted image of the left proximal femur.Fig 4.: Axial T2-weighted image of the left proximal femur.Based on the history, physical examination, and imaging studies, what is the differential diagnosis? RADIOGRAPHIC INTERPRETATION The anteroposterior (AP) radiograph (Fig 1) of the proximal femur shows a pathologic fracture through a septated, lytic lesion that markedly expands the femoral neck. There is overriding and relative rotation of the fracture fragments and varus angulation of the distal fragment. The long axis short-tau inversion recovery image (Fig 2) shows high signal intensity material at the fracture site, and extensive edema and fracture hematoma in the surrounding muscles. The axial T1-weighted image (Fig 3) shows low and intermediate signal material with fluid-fluid levels within the expanded medullary cavity of the proximal femur. The axial fast spin echo T2-weighted image (Fig 4) more clearly shows the fluid-fluid level with the high signal superior component and low signal inferior component. The overlying bone is intact but thinned and there is no associated soft tissue mass. DIFFERENTIAL DIAGNOSIS Pathologic fracture through: Aneurysmal bone cyst Unicameral bone cyst Langerhans cell histiocytosis Osteosarcoma Fibrous dysplasia with cystic component An incisional biopsy with a frozen section was done, and photomicrographs are shown in Figure 5.Fig 5A.: Photomicrographs of the lesion (stain, hematoxylin and eosin); (A) magnification, ×10 (continues)Fig 5B–C.: (continued) (B) magnification, ×20, and (C) magnification, ×40.Based on the history, physical findings, radiographic studies, and histologic pictures, what is the diagnosis and how should this lesion be treated? HISTOLOGY The hematoxylin and eosin stained sections show fragments of bone, some devitalized, incorporated into reactive fibroplasia and fragments of bone marrow (Fig 5A). Blood-filled spaces are surrounded by spindle cells with subjacent connective tissue incorporating osteoid and multinucleated giant cells (Fig 5B–C). DIAGNOSIS Displaced pathologic fracture through aneurysmal bone cyst of proximal femur TREATMENT Based on the history, physical examination, imaging studies, and histologic appearance, the diagnosis of pathologic fracture associated with aneurysmal bone cyst was confirmed. Surgical intralesional extended curettage (with high-speed burring and phenol cauterization) and bone grafting, and open reduction internal fixation of the left proximal femoral fracture was done at the same surgical setting as the incisional biopsy and frozen section. Through the fracture and cortical window, the cyst was curetted and the membrane around the cyst was eliminated with a high-speed burr. Phenol cauterization was used. Under fluoroscopy control, open reduction and internal fixation of the fracture was obtained by inserting a pediatric hip screw (Howmedica, Rutherford, NJ) and a side plate. Great care was taken for placing the screw away from the growth plate. Morselized iliac crest and allograft cancellous cubes were packed into the cyst cavity. A unilateral spica cast allowing free ankle motion was applied for 6 weeks postoperatively with toe touch weightbearing with crutches. The patient gradually progressed to full weightbearing after the cast was removed. At 20 months after the initial operation, plain radiographs (Fig 6) revealed a completely healed fracture, so the hardware was removed.Fig 6A–B.: (A) Anteroposterior and (B) lateral plain radiographs of the left proximal femur obtained 20 months postoperative.At 28 months followup (after hardware removal), plain radiographs revealed no evidence of recurrence of the aneurysmal bone cyst (Fig 7). On physical examination, the patient had a normal gait, full range of motion of the left hip, no limb length discrepancy, and no angular deformity in the left proximal femur.Fig 7A–B.: (A) Anteroposterior and (B) lateral plain radiographs of the left proximal femur obtained 28 months postoperative.DISCUSSION Aneurysmal bone cyst was recognized and described as a distinct clinicopathologic entity by Jaffe and Lichtenstein in 1942. 13 It was defined as a hyperplastic process reactive to an intraosseous or subperiosteal hemorrhage. 5 The pathogenesis of the hemorrhage still is unclear. Although the etiology remains uncertain, the lesion behavior has been well-documented. It has a well-differentiated benign histologic feature but can be locally aggressive. 25 Approximately 30% of aneurysmal bone cysts occur in the presence of other lesions, including giant cell tumor, chondroblastoma, osteoblastoma, and osteosarcoma, and less frequently in fibrous dysplasia, eosinophilic granuloma, nonossifying fibroma, and unicameral bone cyst. 1,2,4 Histologic examination of secondary aneurysmal bone cysts usually shows the underlying primary lesion. 26 Aneurysmal bone cysts account for approximately 1% to 2% of all biopsied primary bone tumors and are distributed equally among males and females. 18 Most aneurysmal bone cysts are seen in patients who are in the second decade of life, and the lesion is considered rare in patients who are younger than 5 years. 15,18 Most patients are seen within 6 months of onset with localized pain. 4 Pathologic fracture is an infrequent event, only occurring in approximately 10% to 11% of patients with aneurysmal bone cysts. 4,5,23 Aneurysmal bone cysts have been observed in almost every skeletal site. 27 More than 50% occur in the long bones, 12% to 30% occur in the spine, and 12% occur in the pelvis. 4,12,15 In the long bones, aneurysmal bone cysts tend to involve the metaphysis and metadiaphysis, whereas in the vertebrae, they tend to involve posterior elements. 4 The proximal 1/3 of the femur is one of the most common locations for benign bony lesions and aneurysmal bone cysts. Most of the lesions are small and silent; occasionally they may be large with progressive growth and weaken the supporting framework in the proximal femur resulting in a pathologic fracture. 21 On conventional radiography, aneurysmal bone cysts typically are septated, lytic lesions that expand the bone. An aneurysmal bone cyst may start in an eccentric location, but can expand to involve the entire width of the bone. An aneurysmal bone cyst can destroy the cortex, but grows slowly enough that there usually is a rim of overlying calcified periosteum that stimulates a thinned, but intact bony cortex. A rapidly growing aneurysmal bone cyst, however, may break through the periosteum to cause a soft tissue mass. Campanacci et al 4 classified aneurysmal bone cysts based on radiographic appearance into aggressive, active, or inactive lesions. Aggressive aneurysmal bone cysts have no signs of reparative osteogenesis, no periosteal shell, and ill-defined margins. Active aneurysmal bone cysts have an incomplete periosteal shell with well-defined margins. Inactive aneurysmal bone cysts have a complete periosteal shell with defined sclerotic margins. Magnetic resonance imaging can be a valuable tool for the assessment of aneurysmal bone cysts. The margins of the lesions usually are well seen because of the presence of calcified or noncalcified periosteum that surrounds the lesion and appears as a thin rim of low signal intensity (on all imaging sequences). The cystic tissue usually is mixed intermediate and low signal on T1-weighted images and mixed high and low signal on T2-weighted images. Aneurysmal bone cysts frequently have internal septations that are low signal on all imaging sequences and contain fluid-fluid levels. Sullivan et al 22 evaluated the MRI findings in patients with aneurysmal bone cyst and unicameral bone cyst and reported a statistical difference between the lesions with the presence or absence of double density fluid levels and septations. The fluid-fluid levels usually reflect the presence of hemorrhage and often are seen best on the T2-weighted images, as in the current patient. Intralesional fluid-fluid levels are a nonspecific finding and can be seen in various benign and malignant bone tumors. 24 However, in a child with a bone cyst, the presence of fluid-fluid levels and internal septations on MRI scans strongly suggests the diagnosis of aneurysmal bone cyst. 22 Aneurysmal bone cyst has the macroscopic aspect of a blood-filled sponge limited by a thin fibrous periosteal membrane. 5 Histologically, it consists of blood-filled cavernous spaces lined by thin septae of variable cellularity, composed of stromal cells, foci of immature bone or osteoid and multinucleated giant cells. The blood in the spaces is not clotted and the erythrocytes are intact. Hemosiderin deposits may be found in the septae. 3 In the differential diagnosis, the histologic appearance of aneurysmal bone cyst can be difficult to distinguish from unicameral bone cyst after pathologic fracture because of the presence of hemorrhage, fibrin, reactive fibroplasia, and granulation tissue. Pathologic fracture also can cause a fluid-fluid level in a simple cyst that can mimic aneurysmal bone cyst, but the presence of fluid-fluid levels and internal septations on radiographic studies made the diagnosis of aneurysmal bone cyst more likely (Figs 1, 3, 4). Osteosarcoma was ruled out based on a combination of a benign appearing radiographic picture (well-delineated lytic lesion with neither soft tissue extension nor a significant degree of periosteal reaction), and a lack of an irregular, pleomorphic underlying cellular pattern with malignant cells. The lack of mononuclear histiocytes and eosinophilic leukocytes, and a negative immunohistochemistry with CD1a staining, made Langerhans cell histiocytosis unlikely. Although occasionally having an expansile radiographic appearance, fibrous dysplasia has a characteristic ground glass radiographic appearance, which was not seen in the current case. The treatment of aneurysmal bone cyst usually is surgical. 4,12,16 Aneurysmal bone cysts in expandable bones can be treated with resection, with an extremely low rate of recurrence. 1,4,9,10,16 In the other locations, intralesional excision with curettage and bone grafting has been the treatment of choice. 1,4,12,23 Curettage must be done by opening a large window in the most thinned area of the cortical shell. 5,21 Use of a high-speed burr to extend the margin beyond the reactive zone of the lesion is advised. 11 Phenol is effective as a local adjuvant in reaching the crevices in the cortical bone to which the lesion has permeated, and killing any surface residue within the lining bed. 6,20 Phenol should be applied carefully, because it may damage the soft tissue around the lesion and skin. 19 Large aneurysmal bone cysts in the proximal femur frequently require surgical intervention. Surgical treatment should eradicate the lesion sufficiently to preclude recurrence while restoring bone integrity to prevent a subsequent pathologic fracture or deformity. 14,21 It also should provide good stabilization when a pathologic fracture has occurred, as in the current patient. The authors classify the treatment of pathologic fractures of the proximal femur associated with bone cysts in children into three types based on skeletal maturity and the amount of bone loss in the femoral neck and lateral proximal femur (Fig 8).Fig 8A–F.: The authors’ classification system for the treatment of pathologic fractures of the proximal femur associated with bone cysts in children is shown. (A) In Type IA, a moderately-sized cyst is present in the middle of the femoral neck. There is enough bone in the femoral neck and lateral proximal femur (lateral buttress) to allow fixation with cannulated screws after curettage and bone grafting. (B) In Type IB, a large cyst is present at the base of the femoral neck. Although there is enough bone in the femoral neck, there is loss of lateral buttress, so a pediatric hip screw and a side plate should be considered rather than cannulated screws, after curettage and bone grafting. (C) In Type IIA and (D) Type IIB, a large lesion is present in the femoral neck, so there is not enough bone beneath the physis to accept screws. There are two options for treatment of these bone cysts: (1) After curettage and bone grafting, parallel pins across the physis can be used in combination with a spica cast. (2) The patient can be treated in traction until the fracture heals (with subsequent spica cast) followed by curettage and bone grafting. In (E) Type IIIA and (F) Type IIIB, the physis is closing or closed. The lateral buttress is present in Type IIIA hips, so cannulated screws can be used to stabilize the fracture after curettage and bone grafting. In hips with Type IIIB fractures, the loss of lateral buttress makes it necessary to use a pediatric hip screw and a side plate after curettage and bone grafting. In all types, the current authors recommend spica cast immobilization after surgery. lat = lateral.Reconstruction using polymethylmethacrylate after curettage was reported as an alternative adjunctive therapy for large metaphyseal lesions. 19 However, bone cement does not resorb, and it is difficult to predict whether and how long the plugged bone can tolerate full weightbearing without mechanical failure, especially around the femoral neck area. Irradiation also has been used for those lesions that are surgically inaccessible, but it is associated with many adverse effects, especially in children, including growth arrest, injury to the gonads, and secondary sarcoma. 4,7,8,23 Reported local recurrence rates range from 0% in patients in whom total wide excision was achieved, 10% to 59% recurrence in patients who had intralesional excision, 6% recurrence in patients who had intralesional excision followed by radiotherapy, and 11% recurrence in patients treated with radiation alone. 1,4,9,11,12,16,27 Excision with intralesional cryosurgery was reported to show an 18% recurrence rate with increased morbidity. 17 Aneurysmal bone cyst is a benign lesion, but can be locally aggressive and therefore increases the risk of pathologic fracture. Magnetic resonance imaging is useful in differentiation from other lesions, such as unicameral bone cyst. Extended curettage with use of a high-speed burr and bone grafting is the treatment of choice when en bloc resection is not feasible. Phenol may be used as a local adjuvant. The surgical treatment of an aneurysmal bone cyst associated with a pathologic fracture should include eradication of the lesion to decrease local recurrence and stable internal fixation to allow the fragments for healing in satisfactory alignment.