Introduction {#S1}
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Hydatid cyst disease is a parasitic disease caused by a cestode known as echinococci. The genus Echinococcus includes three species; *Echinococcus multilocularis, Echinococcus vogeli* and *Echinococcus granulosus. Echinococcus granulosus* is the most common cause of hydatid disease in man.^[@R1]--[@R3]^ The definitive hosts are dogs, foxes, and other carnivores. The tapeworms live in the small bowel of these hosts and infected ova are shed in the feces. When ingested by intermediate hosts such as man, sheep, or cattle, the larvae enter the portal circulation. The larvae eventually reach the liver; where most of them are trapped. Sometimes, larvae reach the lungs and other areas of the body and form cysts. The life cycle is completed when the definitive hosts consume infested viscera of the intermediate host.

Bone lesions are reported in only 1--2% of cases.^[@R4]^ The strong structure of osseous tissue limits the growth of the hydatid cyst, which spreads along medullar and trabecular channels. The trabeculae are slowly resorbed due to pressure without any cortical extension.

The cysts extend to surrounding soft tissues if the bone cortex is eroded. The disease affects long bones, vertebral column, pelvis, and costae in order from least to most affected region.^[@R1],[@R5]^ ([Table 1](#T1){ref-type="table"}). We report a case of hydatid disease of the left pelvic and femoral bones with cartilage destruction of the ipsilateral hip joint revealing a retroperitoneal location of hydatid cyst.

Table 1Bone manifestation of Echinococcus.^[@R6]^LocationPercentageSpine30%Pelvis/hip20%Femur/tibia15%Humerus15%Ribs/scapula10%Phalanges5%Head5%

Case Report {#S2}
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A 35-year old woman presented with one year history of pain in her left hip. There was no history of prior trauma. On physical examination, there was pain and limitation on motion of the left hip. Treatment that included anti-inflammatory drugs and exercise did not alleviate the pain.

Laboratory studies revealed normal results. A simple radiograph of hip showed intraosseus cystic lesions in the left pelvic bone and femur with uniform space narrowing of the left hip ([Figure 1](#F1){ref-type="fig"}).

Figure 1A simple radiograph of hip showed intraosseus cystic lesions in the left pelvic bone and femur with uniform space narrowing of the left hip.

Our patient was admitted initially for a probable coxarthrosis. Because the radiological aspect was suspect, a femoral biopsy was performed confirming the presence of *Echinococcus granulosus* within the osseous and synovitis lesions. Magnetic resonance imaging (MRI) showed multiple cystic lesions were present in the left pelvic bone, femoral bone and pelvic cavity ([Figure 2](#F2){ref-type="fig"}).

Figure 2Magnetic resonance imaging (MRI) was performed, multiple cystic lesions were present in the left pelvic bone, femoral head and pelvic cavity. (A). Horizontal image. (B). Coronal image.

The retroperitoneal hydatid cyst was managed by aspiration. The patient received mebendazole 40 mg/kg/day which was well tolerated. She referred to only mild pain in her left thigh. A new simple radiograph after three months showed multiple cystic lesions involving the left pelvic bone and enlargement of lesions ([Figure 3](#F3){ref-type="fig"}). She has received albendazole 10 mg/kg/day with no changes in clinical symptoms and mild enlargement of the cysts on imaging evaluation. A Girdlestone arthroplasty was suggested but our patient doesn\'t believe in this option.

Figure 3Simple radiograph three months later, enlargement of the lesions with pelvic deformity.

Discussion {#S3}
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Echinococcosis is a parasitic disease produced by the larval stage of echinococcus granulosus. Hydatid disease of bone is rarely seen in humans and it has been reported in only 1--2% of cases of echinococcosis.^[@R4]^ The vertebral column, the pelvis, the long bones and the skull are most commonly involved.^[@R7],[@R8]^

Hydatid cysts of bone remain asymptomatic over a long period, and are usually detected after complication or at an advanced stage when lesions have become extensive.^[@R9]^ There are a variety of complications when osseous involvement is present; they include deformity, pathological fractures, secondary infection or neurological problems, such as paraparesis.

Hydatid disease joint involvement is usually due to secondary extension from the adjacent bone, although primary hydatid synovitis after hematogenous spread of the infection can be seen.^[@R10]^

Diagnosis of the hydatid bone disease is primarily based on findings of X-ray and CT scans.

The radiological signs include lucent osseous lesions associated with expansion of the bone and thinning of the cortex. In patients with these signs, soft tissue calcification is highly suggestive of hydatid disease. Although hip joint destruction is uncommon, it can be seen, as in our case when pelvic and femoral bones are involved. MRI and computed tomography are valuable in delineating the extent of bone and soft tissue abnormalities.^[@R11]^ The diagnosis is often made on the basis of the characteristic radiographic appearance of the lesions.

However, a definite pre-operative diagnosis without histological examination is often difficult as there are no pathognomonic signs, radiological findings may be confused with those of other tumoral lesions, and serological tests are of limited value.^[@R11]^

Hydatid bone must be present in the differential diagnosis of chronic monoarthritis.^[@R12]^ Infestation of hip prosthesis has been reported.^[@R13]^ Giant cell tumors, solitary bone cysts, aneurismal bone cysts, fibrous dysplasia, bone metastasis, neurofibromatosis, chronic osteomyelitis, tuberculosis of the bone, brown tumor (hyperparathyroidism), and various other neoplastic lesions should be considered in the differential diagnosis of osseous hydatid disease.^[@R9]^

Echinococcus joint disease is usually due to secondary extension from an adjacent bone. Transarticular extension from the pelvic bone to the femur or sacrum, similar to the present case, has been reported in the literature.^[@R14],[@R15]^ We present a patient who developed hydatid disease of the left pelvic and femoral bones with cartilage destruction of the ipsilateral hip joint revealing a retroperitoneal location of hydatid cyst.

In our case, hydatid bone disease was suspected from radiologic findings. Femoral and synovitis biopsy confirmed the presence of Echinococcus granulosus within the osseous and synovial lesions.

The literature offers various treatment approaches.^[@R2],[@R16]--[@R18]^ Until recently, the basic treatment has been surgical excision or resection ([Table 2](#T2){ref-type="table"}). Unfortunately, the results have been discouraging.^[@R9],[@R23]^ Complete surgical excision with a wide margin of healthy tissue and curettage is the basic approach for osseous hydatid disease. This surgery may be difficult, with a high risk of recurrence. Radical resection in the pelvis and hip is extremely challenging and total eradication of parasitic osteitis is almost impossible.^[@R17],[@R23]^

Table 2Cases of pelvic and hip hydatid disease, treatment methods and outcomes.AuthorsBone localizationNumber of casesTreatmentComplicationOutcomeSiwach 2009^[@R19]^Femur and pelvis1Amputation and chemotherapySepsis and death-Khan 2008^[@R20]^Pubis, femur and pelvis1Chemotherapy-UnsatisfactoryMasse 2004^[@R18]^Ischion, hip and pelvis1Surgery-SatisfactoryMartinez 2001^[@R21]^Pelvis and hip8Surgery and chemotherapyChronic lesions 37%Satisfactory 63%Bulzunegui 1997^[@R10]^Pelvis, femur and hip1Surgery and chemotherapy-SatisfactoryWirbel 1995^[@R9]^Pelvis, femur and hip1Total hip replacement and chemotherapyRecurrence and instability of hipUnsatisfactoryAgarwal 1992^[@R22]^Pelvis, femur and hip2Surgery and chemotherapy-Satisfactory

Reconstructive surgeries after radical excision are almost technically impossible in the pelvis and hip, although in the past, hip arthroplasty^[@R2]^ and custom-made prosthesis^[@R8]^ have been tried. Extensive surgical approaches are always accompanied by the dangers of recurrence and infections. Even patients in a good general condition may not tolerate such surgeries. Sepsis may be a cause of death.^[@R14],[@R24]^ Overall, a review of the literature reveals a poor prognosis if the disease is extensive in the pelvis and femur.^[@R19]--[@R22],[@R24],\ [@R25]^

Isolated medical therapy is not adequate for controlling the process, but it can be added to surgery or, used like isolated therapy when complete excision is not possible.^[@R2],[@R26]^ Mebendazole, albendazole, and antihelmintic drugs are used for chemotherapy. Albendazole has been found to be better absorbed than mebendazole and exhibits superior efficacy against helminths.^[@R2]^ The aim of this work is to alert orthopedic surgeons to this morbid condition and to emphasize the fact that this disease should be suspected in cystic osseous lesion of hip, especially in endemic areas of the world. Skeletal involvement is usually secondary to visceral hydatidosis that we must research. Early diagnosis allows eradication and salvage of the bone and the hip joint. Delayed diagnosis and misdiagnosis are always charged with the risk of handicap, recurrence, and sepsis.
