Malignant Fibrohisticytoma of the Knee in a Patient with HIV.

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.

INTRODUCTION

This case report describes a patient infected with HIV presenting with anterior knee pain. The final diagnosis was malignant fibrohistiocytoma, a rare condition among knee pathologies. 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.

CASE REPORT

The patient WPO, from the city of São Paulo (Brazil), a 75-year-old caucasian male, had been receiving antiretroviral treatment in Hospital das Clínicas of São 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). He also had lipodystrophy syndrome diagnosed five years ago. 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.

The initial suspicion was inflammatory abnormalities of Hoffa's fat pad. 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 2.0×1.6×2.0 cm, with low signal and no impregnation with contrast. This was interpreted as a solid fibrous mass. The patellar tendon was found thickened, with foci of partial ruptures and with peritendinous edema.

Figure 1

Sagital MRI image, showing thickening of Hoffa's fat pad and adjacent synovial membrane, with foci of partial rupture of the patellar tendon

Figure 2

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.

Figure 3

Intraoperative aspect of the lesion in Hoffa's fat pad, with an intact patellar tendon

None of the cultures was positive. On the 14th 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 pathological analysis confirmed free resection margins.

The patient has a good functional outcome, walking with the aid of one crutch and without any recidive after 18 months of the surgery.

DISCUSSION

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,[1] osteonecrosis,[2] carpal tunnel syndrome,[3] adhesive capsulitis of the shoulders,[4] modifications of body shape, and changes in lipid and glucose metabolism.[5]

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. Another complaint frequently observed in this group is patellofemoral pain.[6] 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.[7]

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.[8] It mainly affects the appendicular skeleton. This tumor originates especially in the deep fascia and muscles.

Malignant fibrohistiocytoma usually presents as a deep painless mass, without other local or systemic symptoms. It is diagnosed by means of needle or incisional biopsy. The staging depends on the histological grade, size, and presence of metastasis, which is the most important prognostic factor regarding survival. The overall five-year survival ranges from 36 to 52%.[9]

The treatment is based on surgical resection with wide margins. 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.[10] 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. Conditions that are unknown or not understood cannot be treated adequately.