A 30-year-old man was investigated for a painful right swollen knee. Imaging investigations included ultrasound and MRI of the knee. Diagnosis was confirmed at arthroscopy of the knee. The differential diagnoses for unusual swellings of the knee are explored along with some imaging features.
A 30-year-old man presented with a history of right knee pain and swelling. There was no history of trauma or connective tissue disease.
Initial assessment by ultrasound showed a diffuse solid echogenic mass in the supra-patella pouch, which extended into both sides of the joint with a small effusion. (Fig. 1) The mass had a normal vascular appearance and did not compress. No villous frondlike projections were seen. The patella and quadriceps tendon were normal.
Further investigation by MRI confirmed a diffuse fatty mass in the supra-patella pouch (Figs. 2 & 3). The joint cavity was expanded by the mass with a small effusion and minimal synovial thickening demonstrated. No joint erosions, marrow oedema, haemosiderin or calcification was evident on MRI. (Figs. 4, 5, 6, 7)
The lipoma arborescens was confirmed at arthroscopy (Fig. 8 & 9) and the patient subsequently underwent a synovectomy.
Lipoma arborescens is a rare condition that forms part of the differential diagnosis for non-infectious knee swelling. This includes synovial chondromatosis, pigmented villonodular synovitis (PVNS), synovial haemangioma and rheumatoid arthritis. The above were excluded because the patient had none of the typical features of these conditions mentioned below.
Synovial chondromatosis can show low / intermediate signal on T1 images in the synovium and joint space. T2 signal characteristics vary depending on the amount of calcification or ossification. PVNS shows low signal synovium associated with haemosiderin deposition. Synovial haemangiomas appear as intra/extra-articular lesions which are intermediate signal on T1 & T2 with fluid voids in abnormal vessels or calcified phleboliths. Rheumatoid arthritis shows low signal synovial tissue on T2 which can indicate fibrous pannus formation.
Lipoma arborescens has two different forms; the villous frondlike synovial mass has been described on ultrasound and MRI. The other form is a mass of subsynovial fat deposition that has been shown on MRI as a well marginated single nodule on the supra-patella bursa, without irregularity or synovial hyperplasia [1, 2]. The usual ultrasound appearances of the villous form are of a hyperechoic vascular frondlike mass. The projections have been shown to move around inside an effusion during real time imaging with knee manipulation. [3, 4] The solid form of lipoma arborescens has rarely been shown using ultrasound before.
Lipoma arborescens has to be differentiated from synovial lipoma. The arborescens form often has a joint effusion, synovial cyst and joint erosions along with the subsynovial fat deposition. The synovial lipoma is a localized rounded / oval fatty mass without synovial changes. [1]
The original ultrasound suggested synovial lipoma because of the solid echogenic mass. The MRI confirmed a diffuse mass filling the joint cavity as shown on ultrasound. The arthroscopy and subsequent histology showed a solid mass fatty synovial proliferation, the solid form of lipoma arborescens. We feel that our readers should be aware of this unusual form.
Solid form of lipoma arborescens.
1. Ultrasound findings: Significant solid, relatively high-echo soft tissue proliferation can be observed within the right knee joint cavity, appearing as lobulated or lobular protrusions with clear margins. With joint movement, a small amount of fatty protrusions in the fluid-dark area can be seen floating or shifting.
2. MRI findings:
• Multiple lobulated fatty signal shadows can be seen in the joint cavity and subsynovial area. They appear hyperintense on T1WI, and predominantly hyperintense on T2WI, with mild synovial thickening in certain areas;
• The lesion is mainly located in the knee joint cavity, showing lobulated or villous fatty proliferation on the synovial surface, possibly involving the suprapatellar bursa and the joint space;
• No clear bone destruction or prominent cartilage erosion is noted, and the articular surfaces remain relatively intact;
• Moderate joint effusion is present around the joint cavity; no abnormal involvement is seen in the adjacent soft tissue.
Based on the patient’s clinical presentation at age 30 (right knee swelling and pain) and the above imaging characteristics, the following primary differential diagnoses should be considered:
1. Synovial Chondromatosis: Common findings include synovial cartilage nodules and intra-articular loose bodies. On MRI, T1 signals are usually low to intermediate, with calcified or ossified areas appearing as low signal on T2;
2. Pigmented Villonodular Synovitis (PVNS): The synovium typically exhibits low signal (especially in gradient-echo sequences) with hemosiderin deposition, often accompanied by joint destruction;
3. Synovial Haemangioma: Often appears as intermediate or slightly high signal on MRI. After contrast enhancement, vascular tumors show enhancement, and vascular flow voids or phleboliths may be present;
4. Rheumatoid Arthritis: Marked synovial thickening, with possible articular surface erosion and cartilage destruction. Fibrotic thickening may appear hypointense on T2WI;
5. Synovial Lipoma: Typically presents as a single round or oval fatty mass with clear boundaries; the synovium itself shows relatively minimal change and does not exhibit diffuse proliferation.
Considering the patient’s clinical symptoms (persistent knee swelling and pain), the MRI findings of diffuse subsynovial fat accumulation, and the correlation with the degree of discomfort and physical exam, a rare fatty lesion should be prioritized in the differential.
Combining the arthroscopic findings and corresponding pathological examination results, which indicate solid fatty synovial proliferation, the diagnosis is Lipoma Arborescens with a solid morphology. This condition commonly affects the knee and is frequently associated with joint effusion and synovial thickening. Arthroscopy and pathology are essential for a definitive diagnosis.
1. Treatment Strategy:
• Arthroscopic Surgery: For patients with significant symptoms or impaired joint function, partial synovectomy or surgical removal of the lipomatous lesion under arthroscopy can effectively alleviate symptoms and prevent disease progression;
• Conservative Management: For mild symptoms that do not affect daily life, short-term observation and symptomatic treatment may be considered, including anti-inflammatory analgesics and avoiding excessive weight bearing.
2. Rehabilitation and Exercise Prescription:
• Early Stage (1–2 weeks post-surgery): Focus on reducing joint pain and swelling. Begin with passive and assisted active range-of-motion exercises (e.g., seated or supine knee flexion and extension). Avoid excessive loads and high-impact maneuvers;
• Intermediate Stage (2–6 weeks post-surgery): Gradually increase lower-limb muscle strengthening, such as isometric quadriceps contractions and light resistance exercises. Within pain tolerance, engage in low-impact aerobic activities like cycling or swimming for about 15–20 minutes each session, 3–4 times a week, progressing gradually in duration and intensity;
• Late Stage (6 weeks post-surgery and beyond): Under medical guidance, progressively restore joint function. Activities may include slow jogging with short strides, balance training, and coordination exercises. Adjust exercise loads based on the patient’s physical condition and pain level.
Throughout the rehabilitation process, applying the FITT-VP principle (Frequency, Intensity, Time, Type, Volume, Progression) is recommended. Adjust exercise frequency, intensity, duration, and type according to individual circumstances. Monitor changes in pain levels, joint function, and swelling. Seek medical attention promptly if any abnormalities arise.
Disclaimer: This report is a reference-based medical analysis derived from the available data and does not replace an in-person consultation or professional medical advice. For diagnosis and treatment, please consult relevant specialists in orthopedics, radiology, or rheumatology.
Solid form of lipoma arborescens.
