Large infrapatellar ganglionic cyst of the knee fat pad: a case report and review of the literature
© Nikolopoulos et al; licensee BioMed Central Ltd. 2011
Received: 14 February 2011
Accepted: 4 August 2011
Published: 4 August 2011
Large ganglionic cystic formations arising from the infrapatellar fat pad are quite uncommon and only a few are mentioned in the literature. An open excision in these cases is mandatory.
We report the case of a large infrapatellar fat pad ganglion in a 37-year-old Greek man with chronic knee discomfort. The ganglionic cyst originated from the infrapatellar fat pad and had no intrasynovial extension. The final diagnosis was determined with magnetic resonance imaging of the knee, and the lesion was treated with surgery.
These lesions are asymptomatic in most cases but often are misdiagnosed as meniscal or ligamentous lesions of the knee joint. Nowadays, the therapeutic trend for such lesions is arthroscopic excision, but when there is a large ganglion, as in this case report, the treatment should be an open and thorough resection. This report is intended mostly but not exclusively for clinical physicians and radiologists.
Cystic lesions around the knee are common. Of these, popliteal cysts are the most frequently encountered. Other cystic lesions, including meniscal or ganglion cysts, are less common [1, 2]. A ganglion, by definition, is a cystic swelling that is formed of myxoid matrix, which gives the ganglion a jelly-like consistency, and is lined with a pseudomembrane.
Ganglia about the knee are rare and usually are located within the joint, in juxtaposition to the joint, or in the soft tissues around the joint, within muscles, tendons, or nerves. Intra-articular small ganglia are often confused with meniscal cysts . Many of these lesions are incidental findings on magnetic resonance imaging (MRI) or arthroscopy, are of little clinical significance, and usually are asymptomatic.
Ganglion cysts do not have a fixed set of common symptoms and their symptoms may correlate with size and the location within the knee joint . Knee pain, clicks, stiffness, incomplete extension of the knee, and pain at the extremes of motion are common symptoms. Occasional findings include a palpable mass and bone erosion. Cysts anterior to the anterior cruciate ligament (ACL) tend to limit extension, and those posterior to the posterior cruciate ligament (PCL) to limit flexion.
The infrapatellar fat pad, known as Hoffa's fat pad, is located posterior to the patellar ligament and adjoining capsule separating them from the synovium. The differential diagnosis of swelling in the infrapatellar fat pad region, as we will show, includes lipoma, synovial cyst, meniscal cyst, and ganglion.
Our patient had no limitation of knee range of motion apart from a minor lack of flexion and no knee effusion, and he had tenderness over the swelling upon local palpation. The results of Lachman-Noulis, Apley, and McMurray tests were negative, and X-rays showed no bony abnormalities. Initially, our predominant diagnosis was medial meniscal cyst.
We decided to take our patient into surgery, but our main concern was the kind of surgical modality that was indicated for this case. Our dilemma was the choice between arthroscopic or open excision of the ganglion. After thoroughly researching the literature, we decided on an open procedure.
The postoperative period was uneventful, and our patient was able to return, with no complaints, to his job and previous activities within three weeks. When he was re-evaluated six months after the operation, his knee range of motion was normal and there was no palpable swelling. He had no complaints and no pain from the knee joint on gait or during sports.
Ganglion cysts within the knee cavity are rare and usually originate from the cruciate ligaments, the menisci, the alar folds that cover the patellar fat pad , and the popliteus tendon and from osteochondral fractures or subchondral bone cysts . The reported prevalences of intra-articular ganglia in the knee are from 0.2% to 1% on knee MRI and 0.6% on knee arthroscopy . Many cases of ganglia, ranging in size from 1.8 to 4.5 cm, have been reported , and occasionally they are bilateral. Most of them are incidental findings and of little clinical significance.
The first intra-articular knee ganglion was described by Caan  in 1924, and there have been several references to ganglia around the knee since then. Brown and Dandy  reported 38 intra-articular ganglia in 6500 knee arthroscopies and half of the patients had no other abnormality.
The differential diagnosis of knee cystic lesions must include ganglia, lipoma, synovial myxoma, meniscal or parameniscal cyst, synovial cysts, pigmented villonodular synovitis, synovial hemangioma, aneurysm, synovial sarcoma, and synovial chondromatosis . Symptomatic ganglia usually present with a history of or signs of mimicking an internal derangement of the knee. The differential diagnosis should include meniscal injury, loose body, chondral flaps, osteoarthritis, cyst of menisci, or discoid meniscus [4, 10].
Ganglion cysts do not have a fixed set of common symptoms, and their symptoms may correlate with size and the location within the knee joint . Knee pain, clicks, stiffness, incomplete extension of the knee, and pain at the extremes of motion are common symptoms. Occasional findings include a palpable mass and bone erosion.
Imaging studies include plain X-rays to exclude pathologies such as a loose body or other bone abnormalities. Ultrasound (U/S), computed tomography (CT) scan, and arthrography are not very helpful examinations, and MRI is the most sensitive, specific, accurate, and noninvasive method for depicting cystic masses, including their size and location. In addition, MRI helps to exclude neoplastic lesions and to detect other intra-articular pathologies . The characteristic findings of a ganglion cyst include a fluid-filled lesion with low T1-weighted and high T2-weighted signal intensities in MRI . In histological sections, ganglia show a dense connective-tissue capsule with a thick jelly-like content. Microscopy shows a pseudocystic space with small multifocal areas of mucoid degeneration.
A variety of treatment modalities have been employed to treat intra-articular ganglion cysts of the knee. Spontaneous size reduction has been reported . Excellent results with percutaneous aspiration using U/S and CT guidance have also been obtained . Recently, the trend is for arthroscopic excision of intrarticular cysts [12, 13]. However, the recurrence of ganglia after arthroscopic treatment has been reported with cyst reformation . In such cases, the recurrence risk is high; therefore, patients should be followed up more carefully .
We believe that puncturing the lesion in an attempt to reduce its content reduces its volume but does not alter its margins. On the contrary, when the lesion collapses, it is very difficult to pinpoint the margins of the pseudocapsule extension, especially when an arthroscopy is performed for a lesion within the fat pad. Therefore, we believe that puncturing the lesion poses a high potential risk of recurrence. Of course, open excision of an infrapatellar fat pad ganglionic cyst does not nullify the recurrence risk, but given the literature data mentioned above, arthroscopic treatment of such lesions has high recurrence rates .
On the other hand, when an open procedure for a large ganglionic cyst of the knee has been decided upon, the preservation of an intact synovium should be the main consideration of the surgeon. Unfortunately, in our patient, the preservation of an intact synovium was inevitable because of the lesion's firm attachment to the capsule (Figure 5C). Our decision to carry on with an open excision of the ganglion lesion was based more on our pursuit for a complete resection of the lesion in order to diminish the recurrent rates and less on avoiding synovium invasion. The substantial defect of the synovium that was left after the complete resection of the ganglion was repaired by approximating the defect margins with interrupted sutures. The latter would be quite difficult with an arthroscopic procedure that primarily invades the synovium in order to excise such lesions.
Our case regards a large intra-articular extrasynovial ganglion cyst and this is the reason we believe that arthroscopic intervention cannot provide a complete resection of the cyst. In such cases, the possibility of leaving even a small piece of wall lining poses a high potential risk of recurrence. Therefore, as mentioned above, an open surgical procedure is necessary.
A careful clinical assessment and an MRI study both contribute significantly to the determination of the nature, location, and size of ganglionic cyst. In addition, MRI helps in treatment decision making, as was demonstrated in our case, in which the ganglionic cyst was large and was located outside the synovium but within the fat pad.
We consider that open surgical excision should be reserved for cases of large ganglionic cysts because it can provide a complete resection of the lesion and minimize the risk of recurrence. On the other hand, arthroscopic treatment is more suitable for small lesions that lay strictly within the synovium.
Written informed consent was obtained from the patient for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
anterior cruciate ligament
magnetic resonance imaging
posterior cruciate ligament
The authors would like to acknowledge Graeme K Hesketh, a consultant radiologist from the Computed Tomography Department of General Hospital "Asclepeion Voulas", for his kind contribution to the English translation and writing of the manuscript.
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