Intra-articular synovial lipoma of the knee joint (located in the lateral recess): A case report and review of the literature

1. Introduction 

Lipomas are one of the most common of all benign neoplasms of soft tissues, and although they may arise almost anywhere in the body [1], intra-articular synovial lipomas are rarely seen. As far as we know there have been only twelve case reports of intra-articular synovial lipoma in English language journals [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Intra-articular synovial lipomas should be differentiated from other similar lipomatoid conditions such as villous lipomatous proliferation of the synovial membrane (lipoma arborescens) [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26] or Hoffa disease [27], [28], [29], [30], [31]. Intra-articular synovial lipomas mainly develop in the knee joint [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. They typically arise from fat pad area [6], [8], [10], or from the suprapatellar pouch [3], [4], [5], [7]. One from the medial meniscus [2] and two located in the femoral intercondylar notch [9], [11] were also reported. However, there has been one reported case of an intra-articular synovial lipoma in the hip [12] and one in a facet joint of the lumbar spine [13]. This article reports on a case of a catching knee caused by an intra-articular synovial lipoma with a stalk located in the lateral recess of the knee joint arising from the suprapatellar pouch. As far as we know, no intra-articular synovial lipoma located in the lateral recess has been reported, and two similar cases of locked knee caused by the impingement of the lipoma within the patellofemoral joint have been reported. And this article also describes how it can be distinguished from other lipomatoid conditions such as villous lipomatous proliferation of the synovial membrane (lipoma arborescens) or Hoffa disease. We also emphasize the importance of considering an intra-articular synovial lipoma in the differential diagnoses when examining a patient with a catching or locking knee. The patient was informed that data concerning the case would be submitted for publication.

2. Case report 

A 66-year-old female presented with a mass involving the left knee joint. The patient suffered from sudden knee pain and a catching or slight interruption of normal motion of the left knee three times in the previous three months. The sudden knee pain and catching of the left knee was relieved by flexing and extending her knee. The patient had never noticed the mass before the symptoms appeared. The patient's local orthopaedist referred the patient to our hospital in July 2005. There was no history of trauma to the left knee, and the patient was systemically well with no underlying joint disorder or general disease.

Physical examination revealed a globular soft tissue mass measuring about 2 by 3 cm on the anterolateral side of the left knee joint. The palpable mass was soft, mobile, well defined, and non-tender. Full extension and 140° of flexion (full flexion) was possible during examination. Results of laboratory tests, including a white blood cell count, erythrocyte sedimentation rate, C-reactive protein level and rheumatoid factor, showed no abnormalities.

Plain radiographs of the left knee joint showed no remarkable change. Magnetic resonance imaging (MRI) was performed on 0.5-T scanners (Signa; General Electric Medical Systems) with QD extremity coils. Spin echo (SE) T1-weighted images (TR 550, TE 15) on axial, sagittal and coronal planes and Gradient echo (GE) T2⁎-weighted images (TR 600, TE 24) on axial and sagittal planes, an 18 cm field of view, 5.0 mm thick sections with a 1.0 mm interslice gap, and a 288×192 matrix were obtained. MRI showed an intra-articular tumor between the lateral condyle of the femur and the patella (Fig. 1A,B). The tumor exhibited high signal intensity on T1-weighted images (Fig. 1A) and a signal intensity similar to subcutaneous fat on T2⁎-weighted images (Fig. 1B). A small amount of joint effusion, but no signs of injury to the menisci or ligaments were demonstrated on MRI.

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Fig. 1. Magnetic resonance imaging of the lesion. A: Spin echo (SE) T1-weighted axial MR image (TR 550, TE 15) showing an intra-articular tumor in the lateral patellofemoral joint with high signal intensity (arrow). B: Gradient echo (GE) T2⁎-weighted axial MR image (TR 600, TE 24) showing an intra-articular tumor in the lateral patellofemoral joint with a signal intensity similar to subcutaneous fat (arrow).

The patient subsequently underwent an operative arthroscopy, which revealed a smooth, globular, yellowish, encapsulated tumor extending into the lateral recess from the surface of the lateral condyle (Fig. 2). The tumor had a stalk on the anterior aspect of the lateral condyle and small blood vessels were observed on the surface of the tumor (Fig. 2). No synovitis or villous synovial proliferation was observed. No associated injury of the joint structures, including menisci or cruciate ligaments, was observed. The tumor was totally excised under arthroscopic guidance using a shaver and a grasper. After the excision, slight bleeding was observed from the base of the stalk. Macroscopic analysis revealed a pedunculated tumor 2.1×2.0×0.8 cm in size. The outer surface of the tumor had a smooth white capsule, and the cut surface of the tumor revealed yellowish adipose tissue with a 2.0 cm stalk (Fig. 3).

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Fig. 2. Photograph showing the arthroscopic appearance of the lesion revealing an ovoid, yellowish, encapsulated tumor extending into the lateral recess from the surface of the lateral condyle. The tumor has a short stalk (arrow) on the anterior aspect of the lateral condyle and small vessels are observed on the surface of the tumor. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3. Photograph showing the gross appearance of the lesion: a pedunculated, ovoid tumor, 2.1×2.0×0.8 cm in size. The outer surface of the tumor has a smooth white capsule and the cut surface of the tumor reveals yellowish adipose tissue. The length of the stalk is about 2.0 cm long (arrow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Histologic examination of the specimen revealed a tumor composed of mature adipose cells covered by a thin fibrous layer of varying thickness and normal synovial lining cells. Macrophage and small blood vessels were present among the lipocytes. No lipoblasts or atypical cells were found. The histopathological diagnosis was an intra-articular synovial lipoma (Figs. 4, 5A,B). The postoperative course was uneventful. The patient was asymptomatic with full range of motion of the knee and no recurrence 6 months after the surgery.

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Fig. 4. Microphotograph showing the histologic appearance of the lesion in low magnification. Histologically, the tumor is composed of mature adipose cells covered by a thin fibrous layer of varying thickness and normal synovial-lining cells (hematoxylin and eosin, ×2).

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Fig. 5. Microphotograph showing the histologic appearance of the lesion in high magnification. A: The tumor is mainly composed of mature adipose cells covered by a fibrous layer and normal synovial-lining cells. There are no lipoblasts or atypical cells (hematoxylin and eosin, ×20). B: Macrophage and small blood vessels are present among lipocytes. There are no lipoblasts or atypical cells (hematoxylin and eosin, ×20). The histopathological diagnosis was intra-articular synovial lipoma.

3. Discussion 

Lipomas, one of the most common of all benign neoplasms of soft tissues, are composed of mature adipocytes and [1] can arise as solitary masses in any part of the human body where there is fatty tissue [3], [32]. They are subclassified according to the site of origin as 1) subcutaneous (most frequent site), 2) intermuscular and intramuscular, 3) intrathoracic, 4) intraperitoneal and retroperitoneal, 5) intraoral, 6) arising in various organs, 7) arising in the central or peripheral nervous systems, and 8) synovial and bone lipomas [32]. Among these lipomas, intra-articular synovial lipomas are exceedingly rare [33]. To our knowledge, there are only a few case reports in the English language literature of intra-articular lipomas [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Most intra-articular synovial lipomas occur in the knee joint [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], with one reported in the hip joint [12], and one in a facet joint of the lumbar spine [13]. In the present case, the lipoma was located in the lateral recess of the knee joint. This localization has not been reported in other case reports. In examining a patient with a symptom in the knee joint, careful physical examination of lateral or medial recess as well as tibiofemoral joint space, patellofemoral joint space and suprapatellar pouch is essential.

Intra-articular synovial lipomas arise within the joint either by penetrating the synovial membrane or as a result of fat overgrowth from within the intra-articular synovial tissue [32]. Jaffe [2] suggested that lipomas are likely to arise from the subsynovial fat on either side of the patellar ligament or on the anterior surface of the femur. In our patient, the lipoma had a thin synovial layer and a stalk which originated from the anterior aspect of the lateral condyle of the femur. These facts suggested that the lipoma in our patient developed from the posterior suprapatellar fat pads and overgrew within the synovial tissue.

The size of the lipoma in our patient was 2.1×2.0×0.8 cm, relatively small in size. Because of its relatively small size, the lipoma could move beneath the patella (patellofemoral joint) during the movement of the knee joint and become caught between the patella and the anterolateral aspect of the femur. There are some reports of locked knee caused by the impingement of the lipoma within the patellofemoral joint [3], [5], in the intercondyler notch [7], [10], or in the anterior horn of the meniscus [2]. In our patient, the lipoma was relatively small, which did not lead to a locked knee but only to a catching or momentary hindrance to movement. When a lipoma is caught or locked between the articular surfaces, within the intercondyler notch or in the meniscus, it could be strangulated if the stalk becomes twisted. The strangulation causes pain in the knee, as in our patient. There are some reports of strangulated lipomas secondary to volvulus about their stalks [2], [5], [9], [10].

Intra-articular synovial lipomas should be distinguished from other similar lipomatoid conditions such as Hoffa disease [27], [28], [29], [30], [31] and villous lipomatous proliferation of the synovial membrane (lipoma arborescens) [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. In the past, some authors confused intra-articular synovial lipoma with these other conditions. Hoffa disease is a syndrome of infrapatellar fat pad impingement described by Hoffa in 1904 [27]. The pathogenesis of Hoffa disease is a recurrent impingement of the infrapatellar fat pad between the articular surfaces in the knee [27], [28], [29], [30], [31]. Initially, a minor trauma to the fat pad alae results in an acute inflammatory response. The swollen and inflamed alae are more susceptible to repetitive impingement between the articular surfaces of the tibia and femur or between the femur and patella. With persistent trauma, the infrapatellar fat pad enlarges with hemorrhage, fat necrosis, fat replacement, and fibrous tissue resulting in a tumor-like lesion [29], [30], [31]. Since the lipoma in our patient originated from the anterolateral aspect of the femur, it was not difficult to distinguish from Hoffa disease; however, physicians should consider the possibility of Hoffa disease (a syndrome of infrapatellar fat pad impingement) when a lipoma developing from the infrapatellar fat pad is detected.

Villous lipomatous proliferation of the synovial membrane is the term proposed by Hallel et al. [16] instead of the term lipoma arborescens. The condition is characterized by replacement of subsynovial tissue by mature adipose cells and is a relatively rare, benign intra-articular lesion, which mainly occurs in the suprapatellar pouch of the knee joint [16], [19], [21], [22], [25]. The exact etiology is unknown, and most cases appear to arise de novo, although some reported cases were associated with osteoarthritis [16], [17], [20], [21], rheumatoid arthritis [15], diabetes mellitus [17], [20], gout [22], psoriatic arthritis [24], or joint trauma [14], [18], [22]. Macroscopically, it has a frond-like appearance with numerous broad-based polypoid or thin papillary villi composed of fatty yellow tissue [16], [23], [24]. Histopathologically, it is characterized by marked villous proliferation of the synovial membrane and hyperplasia of the subsynovial fat [16], [22], [24], [25]. It is usually difficult to discriminate between intra-articular synovial lipoma and villous lipomatous proliferation of the synovial membrane with microscopy of high magnification because of their similar appearance: a collection of mature fat cells covered by fibrous tissues or synovial membrane. The most remarkable distinction between these two lesions is seen grossly. Intra-articular synovial lipoma usually appears as a small, yellowish, solitary polyp-like mass, round to oval in shape, with a short stalk, while villous lipomatous proliferation of the synovial membrane usually appears as a large, frond-like mass.

With the widespread use of MRI, this diagnostic modality plays an important role in diagnosing intra-articular lesions such as intra-articular synovial lipoma, villous lipomatous proliferation of the synovial membrane (lipoma arborescens) and Hoffa disease. Typically, images of intra-articular synovial lipoma and fatty tissue appear the same with high signal intensity on both T1-weighted and T2-weighted images [7], [9]. Marui et al. [8] reported atypical MR images with low signal intensity on T1-weighted images and high signal intensity on T2-weighted images. They explained the difference in the signal intensity on T1-weighted images by the histologic extent of mucoid degeneration in the tumor they imaged. In Hoffa disease, low signal intensity areas on both T1-weighted and T2-weighted images are observed because of the hemosiderin and fibrin present [31]. In villous lipomatous proliferation of the synovial membrane (lipoma arborescens), a villous synovial mass or mass-like subsynovial deposits with a signal intensity similar to that of fat on all pulse sequences are observed [19], [21], [22], [23], [24], [25], [26]. Villous lipomatous proliferation of the synovial membrane (lipoma arborescens) has a characteristic morphological appearance helpful in diagnosing the condition.

Resection of the lesions under arthroscopic guidance or by an open surgical method is curative for intra-articular synovial lipoma and Hoffa disease [27], [28], [29], [30], [31]. The treatment for lipomatous proliferation of the synovial membrane (lipoma arborescens) is open [16], [24], [26] or arthroscopic synovectomy [23], and recurrence rarely occurs.

In summary, we have described an intra-articular synovial lipoma in the knee of a middle-aged female who suffered from sudden knee pain and a catching of the knee. The symptoms were relieved after the arthroscopic resection. Intra-articular synovial lipoma should be considered in the differential diagnoses when examining a patient with sudden knee pain, and a catching or locking knee.