the difficulty in making the correct diagnosis because of the multitude of coinfections and comorbidities in the hiv population led the authors to a first erroneous diagnosis almost compromising the results of treatment . this experience is discussed further in the text . the patient wpo , from the city of so paulo ( brazil ) , a 75-year - old caucasian male , had been receiving antiretroviral treatment in hospital das clnicas of so paulo , since being diagnosed with hiv 12 years earlier ( now taking lamivudine , tenofovir , atazanavir , and ritonavir for the last three years ) . he was in a good general condition , with good viral and immunological control ( undetectable viral load and cd4 count of 653/ml ) . other laboratory tests were normal , and he did not have any other coinfections , had no drug addiction , and no other comorbidities . he sought medical assistance because of a complaint of patellofemoral pain in his right knee that had started one year earlier with increasing local volume and progressive inability to perform active extension . he had a knee extension lag of 30. he was able to walk only with a stick . the increased volume was concentrated below the patella and posterior to the patellar tendon , without local inflammatory signs , history of trauma , and infection . in addition , a tendinopathy with partial chronic lesion of the patellar tendon probably explained the clinical findings . it was decided first to perform a magnetic resonance imaging ( mri ) [ figures 1 and 2 ] . this revealed the presence of edema and thickening of the infrapatellar fat , with the formation of a nodule measuring 2.01.62.0 cm , with low signal and no impregnation with contrast . the patellar tendon was found thickened , with foci of partial ruptures and with peritendinous edema . sagital mri image , showing thickening of hoffa 's fat pad and adjacent synovial membrane , with foci of partial rupture of the patellar tendon axial mri image , showing a solid fibrous mass in hoffa 's fat pad the treatment for this situation is essentially nonsurgical . since our patient did not show any improvement through physiotherapy and drug treatment , surgical treatment was indicated . this was done with the aim of exploring the patellar tendon and reconstructing it if necessary , along with partial resection of hoffa 's fat and the nodule , with pathological analysis of the tissue . the intraoperative appearance of the knee showed an intact patellar tendon with an infiltrated tissue around hoffa 's fat , without any local inflammatory reaction [ figure 3 ] . it was decided only to biopsy this tissue and culture for bacteria , mycobacteria , and fungi . intraoperative aspect of the lesion in hoffa 's fat pad , with an intact patellar tendon none of the cultures was positive . on the 14 day after the operation , the results from the anatomopathological examination revealed that the nodule consisted of a high - grade malignant fibrohistiocytoma , without sufficient margins of resection . after ruling out metastasis [ by thoracic and abdomen computed tomography ( ct ) and bone scintigraphy ] , our patient underwent bloc resection of the entire joint , including the distal femur , proximal tibia , patella , and musculature of the distal quadriceps . the knee joint was reconstructed using an endoprosthesis , which was covered by rotating a flap from the medial gastrocnemius muscle and the extensor mechanism was reconstructed using the sartorius muscle . the patient has a good functional outcome , walking with the aid of one crutch and without any recidive after 18 months of the surgery . the most common causes for this pain are degenerative or traumatic modifications of the patellar and trochlear cartilage , tendonitis , inflammatory processes of hoffa 's fat , and patellar dysfunction caused by muscle weakness , shortening , or instability . the pain - specific localization was suggestive of an inflammatory process in hoffa 's fat or chronic partial lesion of the patellar tendon , which would explain the active extension deficit . within the context of considerable increase in life expectancy among patients infected with hiv achieved since the introduction of highly active antiretroviral therapy ( haart ) , certain osteoarticular consequences of prolonged duration of viral infection and its treatment have been observed , such as osteopenia / osteoporosis , osteonecrosis , carpal tunnel syndrome , adhesive capsulitis of the shoulders , modifications of body shape , and changes in lipid and glucose metabolism . a care and study group called the  bone - hiv  group was created in our hospital to assist patients with hiv / aids presenting any orthopedic complaint . the medical literature on this orthopedic abnormality is very sparse , and we have not found any reports or epidemiological characterization of patellofemoral pathological conditions among this group of patients . one of the few studies on this subject in the literature described abnormalities in hoffa 's fat in patients with hiv , which may have an etiological link to patellofemoral pain . so , the typical changes induced by long - term infection and therapy of hiv may have a negative impact on the extensor mechanism of the knee , which led us to an erroneous initial diagnosis , instead of a much more aggressive disease . malignant fibrohistiocytoma was first described in 1964 and is currently known as high - grade pleomorphic sarcoma in adult life . malignant fibrohistiocytoma usually presents as a deep painless mass , without other local or systemic symptoms . the staging depends on the histological grade , size , and presence of metastasis , which is the most important prognostic factor regarding survival . radiotherapy is administered in most cases , with good results relating to local control , either before or after the surgery . the role of chemotherapy is unclear , but it can be administered particularly in cases with a high risk of metastases . in this case , chemotherapy was not used because of unclear evidence of benefit and the risk of side effects . as the case evolved , it became clear that the abnormalities in the patient 's knee were unrelated to his chronic hiv infection . however , this case once again raises the issue of lack of knowledge of the long - term changes caused by this disease . since there have been sporadic reports of abnormalities of hoffa 's fat among such patients , along with the nonspecific presentation of the condition in our patient , we were led to believe that his condition could have been related to hiv . we again emphasize that studies on osteoarticular changes ( such as bone density , muscular mass , joint mechanics , and cartilage loading ) induced by chronic hiv infection need to be conducted , to epidemiologically define this population better and diminish the frequency of erroneous diagnoses among patients with hiv / aids .
this case report describes a patient presenting with anterior knee pain ( extensor mechanism pain ) , a poorly studied complaint in the hiv population . 
 the final diagnosis was malignant fibrohistiocytoma , a rare condition among knee pathologies , successfully treated with endoprosthesis after tumor resection . 
 this article focuses on what the authors learned after treating this patient , particularly on the difficulty in making a correct diagnosis of this group of patients due to lack of adequate epidemiological characterization . by assuming that the pathology was related to long - term infection and treatment of hiv ( knee hoffitis ) 
 , the authors underestimated the gravity of the case , almost compromising the result of treatment .