A 45-year-old woman working in education and participating in recreational, low-intensity horse riding presented with recurrent swelling of the right knee for more than one year. Episodes of knee effusion typically persisted for approximately 1–2 weeks and subsequently resolved spontaneously.
There was no clear relationship with physical activity. Even considerable loading, including approximately 15,000 steps during a school trip, did not result in a definite exacerbation. Pain was mild (VAS approximately 3/10) and was described predominantly as tenderness rather than distinct pain, particularly when rising from a seated position or applying force through the knee. There was no reported erythema or local warmth. During episodes of marked effusion, knee flexion and movement were restricted because of intra-articular pressure.
The patient also reported symptoms involving the fingers and shoulders, raising the clinical possibility of a systemic or inflammatory musculoskeletal disorder. More recently, occasional mild stabbing symptoms had also developed in the left knee, without swelling.
During childhood she had experienced bilateral knee symptoms with occasional locking episodes, at that time suspected to be related to cartilage pathology. No recent radiographs were available.
Ultrasound examination of the right knee was performed according to the ESSR knee scanning protocol.
The suprapatellar recess contained an anechoic intra-articular effusion. Associated hyperechoic synovial thickening and proliferation were present, consistent with synovial hypertrophy. These findings indicate an active or chronic synovial response, although the underlying etiology cannot be determined by ultrasound alone.
The quadriceps and patellar tendons demonstrated a normal fibrillar echotexture without evidence of tendinopathy or tear.
Medially, the superficial and deep components of the medial collateral ligament were intact and normal in appearance. The accessible peripheral portion of the medial meniscus showed no abnormality or extrusion. The pes anserinus and associated bursa were unremarkable. No significant sonographic features of medial compartment osteoarthritis were identified.
Laterally, the lateral collateral ligament and iliotibial tract were normal. The accessible peripheral portion of the lateral meniscus demonstrated no sonographically visible abnormality or extrusion.
The trochlear cartilage showed preserved coverage and a regular appearance. No significant osteophyte formation was identified, and the examined patellofemoral structures were unremarkable.
Posteriorly, there was no Baker cyst. The semimembranosus and semitendinosus tendons demonstrated a normal appearance. No abnormalities were identified in the examined superficial bursae.
Overall, ultrasound demonstrated recurrent intra-articular effusion associated with distinct synovial hypertrophy, without a convincing local mechanical or structural abnormality to explain the recurrent hydrops.
1. Final diagnosis
Recurrent atraumatic right knee effusion with suprapatellar synovial hypertrophy, sonographically consistent with synovitis. No significant abnormality was identified in the examined tendons, ligaments, peripheral meniscal portions, bursae, cartilage, or accessible bony margins.
2. Differential diagnosis
Given the recurrent atraumatic monoarthritis, synovial hypertrophy, and additional symptoms involving the fingers and shoulders, an inflammatory or systemic arthropathy is an important diagnostic consideration, including rheumatoid arthritis or another peripheral inflammatory arthritis. Depending on the clinical and epidemiological context, Lyme arthritis, crystal-associated arthritis, and reactive arthritis may also be considered. In persistent unexplained monoarthritis, less common proliferative synovial disorders such as lipoma arborescens, tenosynovial giant cell tumor/PVNS, or synovial chondromatosis remain differential considerations.
3. Teaching points
Recurrent knee effusion without significant degenerative, tendinous, ligamentous, or peripheral meniscal abnormality should prompt careful assessment of the synovium. Ultrasound can readily demonstrate joint fluid and synovial hypertrophy but cannot reliably determine the etiology of synovitis. Clinical distribution of symptoms in other joints is therefore important when distinguishing a localized knee disorder from a systemic inflammatory process.
4. Injury/disease information
Synovitis represents thickening and inflammation of the synovial lining and may result in recurrent or persistent joint effusion. It can occur in inflammatory arthropathies, infection, crystal deposition disease, reactive arthritis, mechanical joint disorders, and proliferative synovial conditions. When recurrent monoarthritis remains unexplained, correlation with clinical findings, laboratory investigations, synovial fluid analysis during an active effusion, and, where indicated, MRI can provide additional diagnostic information.
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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