43-year-old patient with progressive left knee symptoms for approximately 1 year, without a known traumatic event. The patient now also reports symptoms lower in the kinetic chain, possibly related to compensatory movement changes. Ultrasound examination was performed using the full ESSR / SonoSkills knee protocol.
Ultrasound examination of the left knee demonstrates an extensive synovial process, with intra-articular swelling visible at multiple locations. There is capsular thickening and synovial proliferation, consistent with synovitis of the left knee joint.
The synovial abnormalities are present in several compartments, including the suprapatellar recess, the medial patellofemoral retinaculum region, the trochlear region, the subpatellar region, and the posteromedial knee. At the posteromedial aspect, distension of the semimembranosus–gastrocnemius bursa is present, compatible with a Baker cyst.
In addition, there is focal swelling around the mid-tibial plateau, approximately near the insertional region of the anterior cruciate ligament and meniscal structures. This may be part of the more diffuse capsular and synovial swelling pattern.
A primary anterior cruciate ligament injury is clinically less likely because there is no history of trauma. However, ultrasound is inherently limited for reliable assessment of deep intra-articular structures, including the anterior cruciate ligament, menisci, cartilage, bone marrow, and central joint structures.
Overall, the findings indicate a chronic intra-articular knee process. The exact underlying cause cannot be determined reliably by ultrasound alone.
1. Final diagnosis
Findings are consistent with extensive chronic synovitis of the left knee, with multicompartment intra-articular swelling, capsular thickening, synovial proliferation, and an associated Baker cyst. The ultrasound pattern suggests a chronic intra-articular knee process, but the exact underlying cause cannot be established with sufficient reliability on ultrasound.
2. Differential diagnosis
The most relevant differential diagnosis includes chronic reactive synovitis secondary to intra-articular pathology, such as meniscal pathology, chondral injury, or degenerative joint disease. Given the clear synovial proliferation and the long duration of symptoms, inflammatory arthritis or another primary synovial disorder should also be considered. Less likely, but dependent on clinical signs and laboratory findings, are infectious arthritis and crystal arthropathy. A primary anterior cruciate ligament injury is less likely anamnestically due to the absence of trauma, but intra-articular lesions cannot be reliably excluded by ultrasound.
3. Teaching points
Ultrasound is well suited for detecting knee effusion, synovitis, capsular thickening, synovial proliferation, and Baker cyst formation. Multicompartment synovial swelling is an important sign of an active or chronic intra-articular process. However, ultrasound has important limitations for evaluating central intra-articular structures such as the cruciate ligaments, menisci, cartilage, bone marrow, and deeper joint surfaces. In chronic progressive knee symptoms with extensive synovitis, MRI is better suited to assess the underlying intra-articular cause.
4. Injury/disease information
Chronic knee synovitis reflects persistent irritation or inflammation of the synovial lining. It may occur secondary to mechanical intra-articular pathology, degenerative joint disease, inflammatory arthritis, crystal disease, infection, or other synovial disorders. Synovial proliferation and capsular thickening suggest a longer-standing process. A Baker cyst often develops when increased intra-articular fluid communicates with the semimembranosus–gastrocnemius bursa. In this case, the extensive synovitis and 1-year progressive course support further evaluation with MRI, particularly to assess the menisci, cartilage, cruciate ligaments, bone marrow, and the extent and character of the synovial abnormality.
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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