A ganglion cyst is a cystic swelling containing myxoid matrix and lined by a pseudomembrane (1, 2). These cysts sometimes occur in the knee, mainly arising from the synovial membrane of ligaments. The prevalence of intra-articular ganglion cyst of the knee ranges from 0.2% to 1% as determined by magnetic resonance imaging (MRI) and 0.6% as determined by arthroscopy (3). A ganglion cyst arising from the infrapatellar fat pad is very rare ; only a few cases have been reported to date (4, 5) and age in these cases ranged from 22 to 77 years. Here, we describe the case of a pediatric patient with a ganglion cyst arising from the infrapatellar fat pad, which was treated by arthroscopic excision.
CASE PRESENTATIONA 10-year-old boy complained of right knee pain without any triggers. He had no traumatic or infectious history in the affected knee. He was physically active ; he practiced the martial art Shorinji Kempo and played golf. He consulted a family doctor when the pain did not improve for 1 month. MRI revealed an intra-articular mass in the right knee, and the patient was referred to our department for further examination.At presentation to us, alignment of the lower limbs was normal. Range of motion (ROM) of the right knee was slightly limited in extension compared with the opposite side. The patellofemoral joint line was tender, but there was no swelling or localized warmth on the knee. The McMurray, Lachman, and varus/valgus stress tests were all negative. Although plain radiographs were normal, MRI showed a multilobular cystic lesion between the infrapatellar fat pad and anterior cruciate ligament (ACL) (Figure 1). The mass measured approximately 30 mm! 15 mm! 20 mm and was well circumscribed, with low intensity on T1 -weighted images and high to iso-intensity on T2-weighted and fat-suppressed T2-weighted images. The origin of the mass was unclear. The ACL showed no abnormal intensity on MRI.We considered ganglion cyst, parameniscal cyst, lipoma, hemangioma, pigmented villonodular synovitis (PVNS), and synovial sarcoma as a differential diagnosis. We considered this lesion was unlikely to be synovial sarcoma because the multilobular cystic lesion existed intraarticularly and has no calcification in this case. For diagnostic treatment, arthroscopic excision of the mass was performed. Superolateral, anterolateral, and anteromedial portals were used. At the beginning of the operation, we inserted the arthroscope into the superolateral portal to observe the entire mass in one field with reference to the preoperative MR images. Arthroscopy revealed a multilobular cystic mass covered with a thin capsule. It arose not from the ACL but from the infrapatellar fat pad. The mass was filled with myxoid matrix with a jelly-like consistency. No injuries to the cartilage, ligaments, or meniscuses were evident. The synovial tissue was hyperplastic around the anterior and lateral spaces of the groove. We also confirmed the impingement of the mass between the femur and infrapatellar fat pad during the knee extension. The cystic...