DiagnosisIntra-articular lipoma causing patellofemoral instability andsubluxation.DiscussionIntra-articular knee masses are rare, with an incidence ofonly 33 out of 2,200 knee arthroscopies described in theliterature [1]. Lipomas are common benign tumours, but anintra-articular localisation is exceptional. Two types oflipomatous entities can be found: solitary lipoma andlipoma arborescens. They differ in pathogenesis and grossappearance, although their microscopic features are similar.Lipoma arborescens, the more frequent of the two intra-articular tumours, is usually considered to be a secondarylesion, occurring in the context of either inflammatory ordegenerative joint disease, or after trauma [2–4]. Microscop-ically, lipoma arborescens is manifested as a diffuse hyper-plasic villous proliferation of the synovium in response tochronic irritation. Magnetic resonance imaging (MRI) exam-ination is pathognomonic, showing multinodular fatty tissueinfiltrating and protruding into the joint. The tissue showscontrast enhancement, and an effusion is often present.Solitary intra-articular lipoma is not associated with pre-existing pathology, and no causative factors can be identi-fied. Macroscopically, it is a soft yellow solitary mass, roundor ovoid in shape, uni- or poly-lobed, and finely encapsu-lated. It is usually small and often pediculated. It is thoughtto develop from the fat of the suprapatellar or retropatellarregions. On MRI the fatty signal shows increased intensity inT1 sequences and decreased intensity in all the sequenceswith saturation of the fat. There is no enhancement afterinjection of contrast medium.Seventeen cases of intra-articular lipoma of thekneewerefound in the international literature. This lesion is a rarecause of pain or internal articular dysfunction of the knee.Clinical presentation may vary from a simple palpable massto a more complex association of pain, swelling and internalderangement. There is a relationship between anatomicallocalisation and clinical presentation. Tumours of the supra-patellar region usually present as a simple palpable mass.However, localisation within Hoffa’s fat pad or between thefemur and tibia may have more varied manifestations, withpain, reduced range of motion and joint locking.Our case was an unusually large tumoral mass with apeculiar presentation. To our knowledge, it is the firstexample of patellofemoral instability with patellar sublux-ation due to intra-articular lipoma, although there has beenone previously reported case with perturbation of patellofe-moral tracking [5].