The patient presents with left knee pain for approximately 3 weeks. There is no known trauma. A complete ESSR / SonoSkills ultrasound knee protocol was performed.
Ultrasound examination of the left knee demonstrates findings consistent with synovitis of the left knee joint.
There is intra-articular swelling at multiple locations, with capsular thickening and synovial proliferation. These changes are visible in several compartments, including the suprapatellar recess, the region of the medial patellofemoral retinaculum, the subpatellar region, and posteromedially at the semimembranosus-gastrocnemius bursa.
The posteromedial fluid collection at the semimembranosus-gastrocnemius bursa is consistent with a Baker cyst. Mild neovascularity is present within the synovial tissue, supporting active synovial inflammation.
The ultrasound findings indicate an intra-articular knee process. The exact underlying cause cannot be reliably determined by ultrasound alone, as intra-articular structures such as menisci, cartilage, cruciate ligaments, and bone marrow are not sufficiently assessed with this modality.
1. Final diagnosis
Extensive synovitis of the left knee joint with intra-articular effusion, capsular thickening, synovial proliferation, mild neovascularity, and an associated Baker cyst.
2. Differential diagnosis
The most relevant differential diagnosis includes a chronic reactive synovial process secondary to intra-articular pathology, such as meniscal pathology, chondral injury, or degenerative joint disease. Inflammatory arthritis or another synovial disorder should also be considered, particularly in the presence of clear synovial proliferation. Infectious arthritis or crystal arthropathy is less likely based on the provided information, but should be considered depending on clinical findings and laboratory results. A primary anterior cruciate ligament injury appears less likely due to the absence of trauma, although intra-articular injuries cannot be reliably excluded by ultrasound.
3. Teaching points
Ultrasound is well suited as a first-line imaging modality for detecting knee effusion, synovitis, capsular thickening, synovial proliferation, hyperemia, and Baker cysts. However, ultrasound is limited for evaluating deeper intra-articular causes of synovitis, including meniscal tears, cartilage defects, cruciate ligament injury, bone marrow pathology, and the full extent of synovial disease. MRI is therefore recommended when the cause of synovitis remains unclear or when intra-articular pathology is suspected.
4. Injury/disease information
Knee synovitis represents inflammation and proliferation of the synovial lining. It may occur as a reactive response to mechanical intra-articular pathology, such as meniscal or chondral disease, or as part of an inflammatory, infectious, or crystal-induced arthropathy. A Baker cyst often reflects increased intra-articular fluid production and communication with the semimembranosus-gastrocnemius bursa. Further evaluation with MRI may help identify the underlying cause and guide treatment. Echoguided intra-articular injection may be considered when not contraindicated and when clinically appropriate, to reduce inflammatory activity and pain and to facilitate restoration of normal movement and targeted exercise.
Marc is founder of, and trainer at, SonoSkills. SonoSkills is an organization dedicated to MSK ultrasound education. He's also an MSK Sonographer at EchoConsult in Roermond, the Netherlands.
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