A 35 years old female with no comorbidities and an unremarkable medical history came to us complaining of a 3 years old mild progressive mechanical knee pain, it was fairly relieved on analgesics, physical exam revealed mild local tenderness along the distal bony aspect of the distal femur, full range of motion, neurovascular examination was normal, and no other abnormalities in the musculoskeletal system or any signs on knee derangements, x-rays AP and lateral views were obtained showing an ill-defined non-destructive lytic lesion in the distal femur (Fig. 1).
Differential diagnosis was simple cyst, aneurismal bone cyst or chondromyxoid fibroma so MRI was ordered and it showed a well-defined heterogenies non-aggressive lesion in the distal femur with low signal on t1 weighted images and mixed signal on t2 and PD images measuring 7 x 4 x 3 cm (Fig. 2). while a simple bone cyst doesn't correlate with the age of the patient and ABCs usually have a characteristic MRI appearance -multiple fluid lines- and tend to be more expansile and aggressive, chondromyxoid fibroma might be the most probable diagnosis but it is usually more demarcated from the adjacent bone with a scalloped and a sclerotic rim. Malignant bone lesions don't have that subtle hideous presentation, and of course infection the great mimicker of lesions is a possibility, however there is no local nor systemic symptoms of infection and the labs were within normal limits.
Due to the untypical radiological and clinical appearance the decision of biopsy was made to optimize the planning of management which came in as benign bone tissue with intervening fibrous tissue and addition four giant cells suggesting aneurismal bone cyst after correlation with the radiological examination (Fig. 3).
So we went for surgery with extensive curettage and reconstruction of the gap left behind, however the visual look of the lesion intra-operatively was not correlating with ABC so the decision of marginal resection was made and the gap was filled with bone cement and the bone was prophylactically fixed with an anatomical distal femur plate (Fig. 4). The resected tissue was send again for histological study and came in as fragments of bony and inflamed fibrotic tissue showing granulation tissue formation and fragments of pink acellular laminated material suggestive of germinal layer of the hydatid cyst (Fig. 5).
The drain was withdrawn the next day of surgery and the patient was discharged into physiotherapy program with partial weightbearing and prophylactic anticoagulant medicine based on her weight and it was referred to a general surgeon consultant where full body CT scan was made and it came negative with no other lesions identified. Infections control consultant was made and the patient was given a course of Albendazole for 3 months.