a Departamento de Ortopedia, Santa Casa de São Paulo, São Paulo, SP, Brasil
b Serviço de Ortopedia e Traumatologia, Hospital Israelita Albert Einstein, Universidade Federal de São Paulo (Unifesp), São Paulo, SP,Brasil
c Instituto de Ortopedia, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo (USP), São Paulo, SP, Brasil
d Escola Paulista de Medicina, Universidade Federal de São Paulo (Unifesp), São Paulo, SP, Brasil
Introduction
The infrapatellar fat pad, also known as Hoffa's fat, is an intra-articular and extrasynovial structure that forms part of theanterior compartment of the knee.
The main ailments that affect Hoffa's fat are degenera-tive joint disease, inflammatory pathological conditions andsequelae of trauma. Tumors or pseudotumoral lesions mayalso affect this fat pad, although these are rare.
Because of abundant innervation, these alterations fre-quently present significant symptoms, although the presenceof calcifications has only been reported in a small number ofcases.
Here, we report on a case of a single intra-articular lesionin the anterior compartment that was associated with severearthrosis.
Case report
The patient was a 78-year-old black woman with hypertensionand diabetes who complained of progressive pain in her leftknee that had been evolving over a period of more than 6 years,in association with functional limitation. On physical exami-nation, her axis was seen to present valgus of 15?and she hadmuscle atrophy of ++/3+. She had a painful range of motionfrom 10?to 130?, which was associated with crepitation inthree compartments. Palpation showed that the volume in theinfrapatellar region was greater than normal, with hardenedconsistency and limited mobility.
Radiographic examinations showed a calcified oval-shapedinfrapatellar image with well-delimited outlines, and alsohypodense areas associated with degenerative alterations andjoint pinching in the lateral femorotibial space. Examinationson the patient from 3 years earlier documented the samelesion and showed that there had not been any radiographicalterations over the past 36 months (Fig. 1).
Because the lesion presented benign characteristics, wechose to resect it en-bloc and perform total knee arthroplastywithin the same operation. A medial parapatellar access tothe left knee was opened and this showed a mass withhardened characteristics, visible laminas in the cartilage, well-defined outlines and lack of adherence to deep planes. Wedid not identify any communication with the femur or tibia(Fig. 2).
After tumor resection and implantation of the prostheticcomponent, the patient evolved without complications (Fig. 3).
Anatomopathological examination revealed a nodulewith a knobbly surface of whitish gray color, measuring3.3 cm × 2.5 cm. After sectioning, it was seen to have astratified appearance of hardened consistency and therewas also a central cavity measuring 1.6 cm × 0.5 cm (Fig. 4).Under a microscope, the nodule was seen to be formedby typical chondrocytes that were immersed in a chon-droid matrix and distributed in a stratified manner, withareas of calcification, ossification and degeneration, andwith formation of pseudocysts. The lesion was covered by asynovial membrane and it displayed flattened coating cells.The diagnosis was concluded to be synovial chondromatosis(Fig. 5).
Discussion
Osteochondromas are the commonest benign bone tumorsand are often located in the metaphyseal region of the longbones. However, extraskeletal presentations of these tumorsare rare.1There are three variants of extraskeletal osteochon-droma: synovial chondromatosis, para-articular chondromaand soft-tissue chondroma.
Synovial chondromatosis of the knee is an uncommonentity. Knowledge on this subject is based on reports on singlecases or small series of cases. It is defined as benign prolif-eration of multiple cartilaginous bodies in the synovial tissuethat is present in joints, bursas or tendinous sheaths.3It isbelieved that free cartilaginous fragments in joints undergocalcification and/or ossification through the process of meta-plasia formation.
The process of metaplasia formation can be divided intothree phases: confined to the synovial membrane; activationof the synovial membrane; progression to free bodies; and alate stage with an inactive synovial membrane and residualfree bodies.
Milgram divided synovial chondromatosis into threecategories: (a) free bodies originating from osteochondral frac-tures; (b) degenerative arthritis or avascular necrosis that
leads to fragmentation of the cartilage; (c) primary syno-vial chondromatosis.4In cases of chondromatosis secondaryto osteoarthrosis, as presented here, metaplasia is formedin fragments coming from subchondral fractures or in frag-mented cartilage.
The clinical condition generally affects a single joint. Theknees are the most frequent location, followed by the hips,shoulders and elbows.5This condition affects individualsbetween their third and fifth decades of life, and especiallymales.5,6The symptoms are pain, edema and limitationson the range of movement.5,6The results from the physicalexamination range from normal to a finding of a palpablemass.
The radiographic findings comprise multiple lesions thatare rounded or oval-shaped, of regular outline and with a cal-cified appearance. The main differential diagnosis for synovialchondromatosis is synovial sarcoma, in which the calcifica-tion is irregular, coarse and generally extra-articular.
The treatment is surgical, with excision of the lesions,preferably combined with total synovectomy.6Partial syn-ovectomy has been correlated with greater recurrence of thelesions.
Para-articular osteochondromas are often located in theknees, but cases in the elbows, hips and ankles have also beenreported.8Around 50 cases of para-articular osteochondromashave already been described.8Hoffa's fat is the most prevalent
location in the knees.8The presence of residual synovial tissuepossibly demonstrates that the primary neoplastic conditionsoriginated in and were confined to this structure.
Reith et al.2defined three criteria for diagnosing para-articular osteochondroma: a single lesion, as seen radiolo-gically and clinically; histologically composed of bone andcartilaginous tissue; and presentation as an extrasynoviallesion.
The nomenclature used to describe this condition is con-fusing. Similar lesions have previously been reported as capsu-lar osteochondroma, extraskeletal osteochondroma, ossifyingchondroma, para-articular chondroma, giant extrasynovialintra-articular osteochondroma and Hoffa's disease.
Complete resolution of the symptoms occurs in the major-ity of the cases, after complete excision of the lesion. Lesionrecurrence is a rare event.
It is believed that because of the clinical, radiologi-cal and histopathological similarity between Hoffa's diseaseand intra-articular chondromas, these conditions are closelyrelated. Mechanical alterations such as valgus or recurvatum,and rotational instability, contribute toward increasing theimpact between these structures.
The radiological diagnosis is based on viewing the osteo-chondroma in its corresponding setting, but it only becomesvisible after the process of ossification.10Histologically, trabec-ulated bone that is characteristic of endochondral ossificationis presented, with a covering of hyaline cartilage.
We therefore conclude that, in the case presented here,the radiographic findings of a single lesion and its locationinside Hoffa's fat favor a diagnosis of para-articular osteochon-droma secondary to Hoffa's disease. However, histologically,the lesion was compatible with synovial chondromatosis. Thisdiagnosis was probably related to the associated condition ofsevere osteoarthrosis of the knee.
Conflicts of interest
The authors declare no conflicts of interest.
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