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CT and especially MRI of primary pelvic hydatidosis, particularly if T2-weighted images are included, allow for easier recognition of cystic changes in bone, and their relationship to skeletal structures and soft tissues (Figs. 3.136 and 3.137).

The entire process may be quiescent for a year or longer, but as the hydatid is reactivated, interval radiographs show a progressive creeping type of bone destruction leading to complete osteolysis. The sacrum, ilium, ischium, and/or pubis may be completely destroyed, leaving only a shell of bone containing poorly defined trabecular strands and irregular calcifications (Figs. 3.135 and 3.136). Pathological fractures often occur at this stage, allowing daughter cysts to escape into the soft tissues with hydatid abscess formation. Multiple dense, rounded masses become visible in the surrounding soft tissues, and occasionally calcification of the ectocyst is noted, implying demise of the parasite (Fig. 3.126).

The widespread destruction of bone may extend across the sacroiliac joint, hip joint, and pubic synchondrosis (Figs. 3.136, 3.137, 3.138). The end result may be a complete deterioration of the pelvis on one or occasionally both sides, with crumbling and collapse of the innominate bone and dislocation of the femur, so that the femoral head migrates inwards to approximate itself against the sacrum (Figs. 3.137 and 3.138). The head, neck, and trochanteric portions of the femur may be destroyed, and rarely the entire femur may show diffuse multicystic disease (Fig. 3.138C). Despite the vast loss of bone structure in some patients, the complicating osteitis with dense bone formation tends to stabilize the process, although the end result is a crippling deformity (Fig. 3.135).

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Fig. 3.136 A 72-year-old Saudi woman with primary osseous pelvic hydatid disease. (A) AP view of the pelvis reveals extensive destruction of the right ilium and sacrum (curved white arrows) with some extension into the adjacent soft tissues (black arrows). (B, C, D) Contrast-enhanced CT scans at different levels show destruction of the right ilium and sacrum by an expansile cystic process, and extensive soft tissue abscess formation (small white arrows) with multiple small gas pockets (curved large white arrows) in (B). The sacrum, including its left wing and adjacent soft tissues, is also involved (large white arrow). The extent of soft tissue involvemnt is best seen in (D) (arrows). (E) MRI T2-weighted (spin echo, 2000/100) image shows cystic masses of high signal intensity replacing bone trabeculae and extending into the pelvis and subcutaneous tissues. (F) Coronal MRI reconstruction shows the destructive process in the right ilium, sacrum, and adjacent tissues (arrows). (Courtesy of Dr. von Sinner and Seminars in Roentgenology, 1997).

Fig. 3.137 A 50-year-old Saudi woman with left pelvic pain of over 3 years' duration. A diagnosis of osteomyelitis was made elsewhere after purulent material was recovered from the left pubic bone, and she was treated with ampicillin. On admission, a painful left groin swelling was noted. Later, it was found the lesion was expanding despite antibiotics, and conventional radiography (A) showed extensive destructive lesions involving the left pubis, ischium and ilium, as well as the left acetabulum, femoral head and neck. The hip joint cartilage is narrowed and the femoral head protrudes through the acetabulum medially. (B) A radionuclide bone scan (20 mCi Tc99m MDP) shows increased uptake of isotope in the left innominate bone, especially in the ischium and acetabulum. (C-G) Six months later, noncontrast CT scans at different levels show further extension of the destructive lesions in the area of the left pubis (C), left hip joint, femoral head and neck (D), trochanteric area and proximal femur (D,E), ilium (F), and sacrum (G). In addition, a cystic soft tissue mass adjacent to bone protrudes into the pelvis (C-F-arrows) and impinges on the bladder (E- arrowhead). Ultrasound of the abdomen showed no other lesions. Exploration of the left pelvis with curettage was performed. (H) Small irregular fragments of cystic, mucoid, glistening pieces of tissue from the iliac crest, some of which contained clear fluid as well as laminated membranes on microscopy suggestive of hydatid cysts. (I) The femoral head was loose and was extracted, with areas of necrotic material noted. The 3.5 cm specimen of the femoral head shows an irregular glistening 2 cm cyst on an uneven articular surface with fibrin deposits and loss of articular cartilage. (J) Microscopic examination shows several fragments of hyaline or laminated parasitic membranes but no germinal epithelium or scolices. (Courtesy of Dr. von Sinner and Skeletal Radiology, 1990).

Fig. 3.138 Extensive hytadid disease involving the pelvis and hips in three different patients. (A) There is extensive bubbly cystic destruction of most of the left hemipelvis with a moderate degree of expansion. There has been collapse and crumbling of the innominate bone in the region of the acetabulum with inward dislocation of the femur so that the femoral head is driven deep into the pelvis in the direction of the sacrum. This extensive bubbly cystic pattern must be differentiated radiographically from other entities such as myeloma, metastatic thyroid or kidney carcinoma, angioma, giant cell tumor, fibrous dysplasia, unicameral bone cyst or aneurysmal bone cyst. (Courtesy of Dr. Emanuel Levin, Brooklyn, New York.) (B) Extensive hydatid disease of the left hemipelvis and femur in a 50-year-old Iranian man with swelling and pain in his left thigh of four months' duration which began as a small, slightly painful mass and got progressively larger. The mass was nontender, mobile, and extended from the iliac crest to the gluteal fold. The AP radiograph of the pelvis and hips shows extensive bubbly cystic destruction of the left hemipelvis with moderate bone expansion. The process has extended across the pubic symphysis to involve the right pubis as well. The acetabulum is no longer recognizable and the hydatid infection has involved the entire proximal femur and shaft. The femoral head and neck are partially destroyed by the multilocular, expansile, destructive process and the femur is rotated and dislocated out of the acetabulum and lies superior and lateral to its normal position. There are huge soft tissue masses representing extension of the hydatid infection beyond the bones into the soft tissues of the left pelvis and thigh, with hydatid cysts and abscesses present there. (C) Hydatid involvement of the proximal right femur and shaft in a 45-year-old Iranian woman. The patient had swelling of her right thigh, hip and buttock as high up as the iliac crest of 10 months' duration. Hip movements were painful and limited. There is a pathological comminuted intertrochanteric fracture of the right hip with marked varus deformity and superolateral displacement of the femoral shaft with relation to the neck. There is a moderate degree of callus formation about the lower portion of the fracture site, but bony union is incomplete and a crippling deformity will persist. Note the honeycombed, lytic destruction of the femoral shaft and neck due to the underlying hydatid process. (B and C courtesy of Dr. Massoud Majd, Washington, D.C.).

In the pelvis, the bubbly cystic, expansile pattern seen in many patients with osseous hydatid disease must be differentiated from myeloma, metastatic renal or thyroid carcinoma, angioma, giant cell tumor, fibrous dysplasia, bone cyst, aneurysmal bone cyst, and other cystic lesions. If the destructive process is extensive and is associated with a large soft tissue mass and calcification, it must be differentiated from a primary malignancy of bone such as chondrosarcoma. If there are coexisting round masses in the lungs due to pulmonary hydatid disease, the possibility of metastases from a bone sarcoma may be erroneously entertained. However, the chronicity of the process and the formation of sclerotic bone in the later stages will exclude malignancy, and the pulmonary masses will grow very slowly.

Occasionally, there may be a soft tissue hydatid in the pelvis which does not originate in or destroy bone. It may be indistinguishable from other causes of pelvic soft tissue masses, even if it has calcium in its wall (Fig. 3.13).