A 63-year-old man presented with a 2-years history of progressively increasing left-sided knee pain. He reported no trauma. Physical examination of the lower extremity revealed no skin abnormalities, but painless swelling. Based on a suspicion of meniscus pathology, radiographs and magnetic resonance imaging (MRI) were ordered.
Imaging studies included radiographs and MRI.
Frontal and lateral radiographs show findings of Osgood-Schlatter disease, osteoarthritis, and a soft tissue swelling in the suprapatellar pouch.
MRI imaging shows intraarticular mass-like synovial proliferation with numerous frond-like projections occupying the suprapatellar pouch, with signal corresponding to fat on T1 and fat-sat proton-density fast spin-echo (PD FSE). Large osteophytes, cartilage loss, meniscal tear and synovitis consistent with osteoarthritis were also seen.
Lipoma arborescens is a rare, benign intraarticular lesion in which there is a lipomatous tissue proliferation characterized by replacement of the subsynovial tissue by mature adipocytes, giving rise to prominent intra-articular villous proliferation of the synovium [1]. It may result from a chronic inflammatory synovitis with resultant hyperplasia of the fatty subsynovial tissue, but often there is no recognized history of arthropathy. In fact, the exact aetiology of lipoma arborescens remains unclear and most of the cases arise de novo [1].
The knee joint is most commonly affected and especially the suprapatellar pouch [2], however, other joints can be involved too [1]. It commonly affects a single joint but some cases with polyarticular involvement were also reported [1, 2].
Most of the cases affect people between 50 and 70 years [1].
Clinically, presentation is with painless joint swelling, frequently with an associated effusion.
Laboratory test (ESR, rheumatoid factor and uric levels) are not altered, joint aspirate is usually negative for crystals and cells and culture for joint fluid is also negative [3].
Plain radiograph usually demonstrates chronic changes due to ostheoartritis. We can also see swelling and opacification of the suprapatellar pouch, data that are suggestive of effusion, although occasionally it is able to detect fatty lucencies. These findings are more evident on lateral plain film.
Ultrasound imaging findings can be joint effusion with echogenic frond-like excrescences into the effusion.
CT of a lipoma arborescens will show fatty infiltration of the joint space.
MRI is the diagnostic modality of choice to identify frond-like fatty projections [1, 3]. The lesion follows the signal intensity of fat on all sequences (T1 and T2: high signal; will saturate on fat-suppressed sequences). Where effusions coexist, visualisation of the fronds is improved. Even though intravenous contrast MRI is not necessary to identify the characteristic frond-like fatty projections of this tumour, it is useful to show sinovial proliferation. Bony erosions are uncommon [4]. There are some MRI findings that are pathognomonic for lipoma arborescens (Take home message), like the presence of a synovial mass with a frond-like architecture, which exhibits fat signal intensity on all pulse sequences. It also shows suppression of signal with fat-selective presaturation and associated joint effusion. It is also important to note the absence of magnetic susceptibility effects from haemosiderin, a typical characteristic of pigmented villonodular synovitis [3].
Open or arthroscopic synovectomy with excision of the lesion is the treatment of choice. Recurrence of the lesion is uncommon [5].
Lipoma arborescens
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1. X-ray Findings:
On the anteroposterior and lateral knee X-ray, mild degenerative changes of the articular surface are observed, with no significant bony destruction or obvious signs of fracture. Soft tissue swelling can be seen around the knee joint capsule; in some patients, localized lucent areas may be present but are not consistent. Overall, these findings suggest possible effusion or synovial proliferation.
2. MRI Findings:
- A large amount of lobulated, villous proliferative tissue is seen in the knee joint, especially around the suprapatellar bursa.
- These villous structures show high signal intensity similar to fat on T1- and T2-weighted sequences, and the signals are suppressed on fat-suppression (Fat-Sat) sequences.
- Meanwhile, a joint effusion is visible, making the boundary between these proliferative structures and the surrounding soft tissues clearer.
- There is no obvious presence of hypointense hemosiderin deposition signal (commonly observed in pigmented villonodular synovitis, PVNS) and no marked erosion or destruction of the subchondral bone trabeculae.
Considering the patient’s age (63 years), chronic painless knee swelling, and the above imaging findings, the following differential diagnoses are suggested:
1. Lipoma arborescens: Typically characterized by fatty, villous synovial proliferation within the joint. On MRI, the signal intensity matches that of fat, and signal suppression is observed on fat-suppression sequences.
2. Pigmented villonodular synovitis (PVNS): Usually presents with extensive synovial proliferation, low signal foci due to hemosiderin, and possible erosive bone changes. Such changes are not evident in this case.
3. Synovial chondromatosis: Commonly shows scattered cartilaginous or calcified nodules within the joint. On MRI or X-ray, calcifications or cartilaginous signals can be seen, which do not match the primarily fatty composition in this case.
4. Articular lymphangioma or other rare synovial lesions: Relatively uncommon, and typically do not demonstrate purely fatty signals.
Based on the patient’s clinical presentation (chronic, painless joint swelling) and imaging features (fatty, villous proliferations on MRI suppressed in fat-suppression sequences), while excluding pigmented villonodular synovitis and synovial chondromatosis, the most likely diagnosis is:
Lipoma arborescens.
1. Treatment Strategy:
- Surgical Intervention: For patients with significant symptoms or major joint dysfunction, arthroscopic or open synovectomy with excision of the involved tissue may be considered. Most cases have a low recurrence rate postoperatively.
- Conservative Treatment: If symptoms are mild, initial management consisting of joint aspiration, anti-inflammatory and analgesic therapy, alongside regular follow-up, can be adopted. Surgery may be deferred if no significant functional impairment is observed.
2. Rehabilitation / Exercise Prescription (FITT-VP Principle):
- Type of Exercise: Early stages should focus on range-of-motion exercises and low-impact activities (such as cycling, swimming, or water therapy) to reduce stress on the knee.
- Frequency (F): 3–5 times per week.
- Intensity (I): Initiate with low to moderate intensity, guided by perceived exertion and heart rate, avoiding excessive weight-bearing or sudden twisting of the knee.
- Time (T): Each session should last 20–30 minutes initially, gradually increasing to around 45 minutes depending on tolerance.
- Type (T): Include range-of-motion training, mild strength training, and moderate aerobic exercises. After arthroscopic surgery, follow postoperative rehabilitation protocols, gradually introducing flexion and extension exercises.
- Progression (P): As pain and swelling subside and joint function improves, strengthen the thigh muscles using exercises like straight leg raises and partial squats. Monitor closely for worsening pain or swelling, and adjust the plan as needed.
3. Precautions:
- In older adults with osteoporosis or limited cardiopulmonary reserve, exercise must be initiated gradually under professional supervision.
- If surgical treatment is performed, follow medical advice for immobilization or restricted weight-bearing. Begin functional exercises at the appropriate time to aid joint function recovery and reduce the risk of adhesions.
- Regular follow-up is advised to monitor for recurrence or any decline in joint function.
Disclaimer: This report is a reference analysis based on the currently provided images and clinical data. It does not substitute for an in-person consultation or professional medical advice. If you have any questions or if your condition changes, seek medical attention promptly.
Lipoma arborescens
