Hydatidosis is an endemic disease in different parts of the world. The most commonly affected organs in humans are liver and lung. However, bone hydatidosis is a very rare entity accounting for 0.5 to 2.5% of total cases, as most larvae are trapped by the liver and lung upon release of embryo in to the portal circulation [1–5]. It occurs most commonly in the spine (35%) and pelvis (21%), with long bone involvement of the femur (16%) and tibia (10%) seen less commonly [3, 4, 6].
Skeletal infestation of E. granulosus cyst occurs by hematogenous seeding . The initial location of the lesion in long bones is metaphysical or epiphyseal, later extending to the diaphysis . As no connective tissue barrier form in bone, daughter cysts extend into bone infiltrating and replacing medulla, leading to the constraints of this external layer, the cysts progressively enlarge, filling the medullary cavity to a variable extent [1, 4]. This can explain why bone hydatid disease is polycystic in contrast to other non-osseous locations . On the other hand, the rigid structure of the cortical bone does not allow the cyst to grow rapidly and lead to its erosion and pathological fracture and deformity, usually presents between the fourth and sixth decades of life [1, 2, 4, 7], as in the present case. This condition is rarely encountered in childhood [1, 4]. The growing cysts lead to bone destruction via three mechanisms: ischemic process through obstruction and compression of the nutrient vessels, mechanical process through compression of the surrounding tissues and a cellular process via osteoclast proliferation. There is no inflammatory reaction of bone except secondary infections which lead to sclerosis, abscess formation, and draining sinuses . The cysts occasionally lie dormant in the body for up to 20 years without producing clinical signs or symptoms. The lamellated structure of the osseous tissue prevents rapid growth of the cyst along medullary and trabecular channels [1, 2, 4].
Diagnosing bone hydatid disease is more challenging and often made at an advanced stage intraoperatively, as most cases do not have a specific clinical features and imaging characteristics, and are often asymptomatic [1, 2, 5, 6, 9, 10]. Preoperative diagnosis is primarily made by imaging studies like X-ray, Computed tomography (CT) scans and Magnetic Resonance Imaging (MRI) [3–5]. X-ray findings include monolocular, bilocular, or multilocular cysts. Monolocular cysts, as in this case, are rarely observed and are characterized 87 by their oval or polycyclic nonspecific lacunae of variable sizes. Progression of the disease takes place in 2 forms: formation of diverticuli and exogenous vesiculation . Although CT scan is valuable in depicting hydatid cysts, MRI is the most helpful technique, especially in the soft tissue involvement and spine [1, 6]. However, sometimes it is not possible to differentiate hydatid cyst appearance from malignancy . CT scan shows well-defined single or multiple cystic lesions that may cause cortical thinning without contrast enhancement. It may also show pathological fracture, cortical destruction and soft tissue extension with calcification . Accurate diagnosis may aided in some persons by eosinophilia (25 to 35% of all cases) and positive result of complement fixation tests, intradermal injection of hydatid fluid (casoni test), and indirect hemagglutination tests over a long period of time. Lesions are usually osteolytic and can involve cortical bone and extend to soft tissues . Therefore, hydatid bone disease should be considered in the differential diagnosis of osteolytic lesions, especially in endemic areas. These include chronic osteomyelitis, fibrous dysplasia of bone, osteosarcoma, and benign cystic lesions. The presence of a periosteal reaction, osteosclerosis, and calcification are not specific for hydatid bone disease . This case report of tibial hydatidosis is the first of its kind in Ethiopia and it was not suspected preoperatively and the immunological tests were not performed, although they are recommended.
The treatment of choice for osseous hydatid cyst is surgery with or without chemotherapy using Albendazole or Mebendazole [1, 5, 8, 11]. High rates of postoperative recurrence have highlighted the importance of adjuvant antihelmintic therapy which should be given preoperatively, if the diagnosis is known, and postoperatively in order to control the disease locally, avoid systemic spread and prevent recurrence [3, 8, 9]. Albendazole is the antiparasitic drug of choice because of its good intestinal absorption and effectiveness. It is given at a dose of 15 110 mg/kg daily in courses of 28 days. One course is given preoperatively and six or more courses postoperatively [1, 12]. Some authors advocate continuous treatment without a break, especially for complicated lesions such as hydatid disease of the bone is recommended [1, 13]. A treatment period of 2 years may be necessary although sometimes lifelong treatment is recommended. If albendazole alone is not effective, praziquantel can be added. This regimen can be used for patients with multiple recurrences and those unsuitable for surgery . Our patient was managed with initial chemotherapy followed by extended curettage. Functional outcome was excellent, and she was fully weight bearing with no pain or discomfort and regained motility of her right leg with no evidence of recurrence.