Ultrasound of joints: inflammation, effusion, damage

An ultrasound examination of a joint doesn't show the entire joint, but primarily the soft tissues and fluid around and within the joint: the synovial membrane, effusion, tendons, ligaments, joint capsule, bursae, as well as superficial areas of cartilage and bone. This makes the method particularly useful for quickly determining whether there is inflammation, fluid, tendon damage, or bursitis. [1]

Unlike X-rays, ultrasound does not use ionizing radiation and can typically be used repeatedly for dynamic monitoring. This is important in chronic inflammatory joint diseases, when assessing the activity of the process and the response to treatment. [2]

Ultrasound detects inflammation not only by the presence of fluid, but also by thickening of the synovial membrane and increased blood flow within it, which is assessed by power Doppler. In rheumatology, these signs are standardized in the EULAR and OMERACT systems and are used to monitor synovitis. [3]

Another practical application of ultrasound is navigation for procedures such as joint punctures, effusion aspiration, and intra-articular injections. Systematic reviews show that ultrasound navigation increases accuracy and can improve clinical outcomes compared to "anatomical landmarks" without visual guidance. [4]

What structures can be assessed and why is ultrasound particularly useful for pain and swelling?

Ultrasound has limited ability to evaluate articular bone surfaces: bone reflects ultrasound, making it visible as a bright line of the cortical layer, while deeper bone is "invisible." This means that ultrasound is effective at detecting superficial defects of the cortical layer and some erosions, but it does not replace methods that evaluate bone marrow and intraosseous changes. [5]

The synovial membrane, joint capsule, and effusion are among the most sensitive areas for ultrasound. This method allows one to see not only the presence of fluid but also its distribution within the joint spaces, as well as distinguish between "simple effusion" and thickened, inflamed synovial tissue in grayscale imaging. [6]

Tendons, ligaments, and muscles are well-suited for ultrasound due to the high spatial detail achieved with high-frequency linear transducers and the ability to perform motion tests. This is advantageous in a number of applications, as it allows for visualization of tendon subluxation, clicking, impingement, or pain with specific movements during the examination. [7]

Paraarticular structures, particularly bursae and tendon sheaths, are often the source of pain but are not visible on radiographs. Ultrasound helps differentiate intraarticular problems from bursitis or tenosynovitis, thereby narrowing the diagnosis and choosing treatment options. [8]

Table 1. What exactly is most often assessed during an ultrasound examination of a joint

Structure	What can you see?	Typical clinical questions
Joint cavity	effusion, its volume and location	Is there fluid? Is a puncture necessary?
synovial membrane	thickening, villosity, Doppler signal	Is there synovitis, how active is it?
Joint capsule	thickening, stretching, defects	capsulitis, consequences of trauma
Tendons and sheaths	tendinopathy, ruptures, tenosynovitis	source of pain when moving
Joint bags	bursitis, thickening of the walls, fluid	periarticular swelling
Nerves near the joint	thickening, compression, inflammation	tunnel syndromes, neuralgia
Cortical bone layer	surface defects, erosions	inflammatory destruction near the joint

[9]

When is an ultrasound examination prescribed: indications and clinical scenarios

The most common indication is joint pain and swelling, requiring a quick assessment of effusion and signs of inflammation. This is especially true for suspected arthritis, reactive inflammation following overuse, exacerbation of osteoarthritis, and other situations where management depends on the presence of fluid and synovitis activity. [10]

Ultrasound is often prescribed after an injury if there is pain, limited motion, suspected ligament or tendon damage, or hemarthrosis. In such cases, ultrasound helps evaluate soft tissues and, if necessary, refer for magnetic resonance imaging to clarify intra-articular structures. [11]

In rheumatology, ultrasound is used to clarify the activity of the inflammatory process, including situations of clinical remission, when symptoms suggest the inflammation has resolved, but a vascular signal persists on Doppler. The presence and persistence of a Doppler signal in synovitis is associated with the risk of an unfavorable course and is used as an additional marker. [12]

In children and adolescents, ultrasound is particularly important when hip effusion is suspected in the presence of claudication and pain because it allows for rapid detection of fluid. Studies of ultrasound in pediatric emergency settings have demonstrated high diagnostic accuracy for detecting effusion when protocol is followed. [13]

Table 2. Typical indications and what exactly is being looked for on ultrasound

Situation	The main goal	What is considered a key finding?
The joint is swollen and painful	confirm or exclude arthritis	effusion, synovial hypertrophy, Doppler signal
After injury	assess soft tissues	partial or complete rupture of the tendon, hematoma, effusion
Chronic pain with no clear cause	narrow down the range of diagnoses	bursitis, tenosynovitis, enthesopathy
Suspected infection	quickly identify the effusion and refer it for puncture	significant effusion, hyperemia, signs of inflammation
Monitoring therapy for inflammatory arthritis	assess the activity of synovitis	dynamics of grayscale changes and Doppler signal

[14]

Preparation and implementation: what does a standard study look like?

Special preparation is usually not required, making the method convenient for initial diagnosis. Clothing that allows exposure of the joint being examined, as well as information about complaints, injuries, surgeries, and previous examinations, are usually sufficient.

A standard examination is performed using a multi-access protocol to assess the joint and periarticular structures from all key angles. Practical parameters, including documentation requirements and minimum examination elements, are described in the professional parameters for performing musculoskeletal ultrasound. [16]

An important feature is comparison with the opposite side and the use of dynamic tests. When pain occurs "on movement," this often provides more information than a static image, as it can show tendon displacement, friction, pinching, or localized tenderness at the moment of transducer compression.

If inflammation is suspected, Doppler modes are added to assess the vascular signal in the synovial membrane and at tendon attachment sites. In rheumatology protocols, standardization of settings and assessment is recommended to allow for comparison of results over time and between specialists. [18]

Table 3. Mini checklist before ultrasound examination of the joint

What to prepare	Why is this necessary?
List of symptoms and their duration	helps select priority scanning areas
History of injuries and surgeries	improves the accuracy of interpretation of scars, reconstructions, and fixators
List of medications, including anticoagulants	important if a puncture or injection is planned under ultrasound guidance
Previous images and conclusions	helps to compare dynamics and avoid duplicating research
Comfortable clothing, access to the joint	speeds up the procedure and reduces discomfort

[19]

Inflammation and effusion: which signs are considered the most informative?

On ultrasound, effusion appears as fluid within the joint recesses, which shifts with compression and position changes. An important practical point: effusion alone does not explain the cause, so it is assessed in conjunction with the synovial membrane and the clinical picture. [20]

Gray-scale synovitis includes synovial hypertrophy, or thickened synovial tissue, which differs from "simple fluid" in that it does not disappear under compression and has a more tissue-like structure. To improve reproducibility, the international EULAR and OMERACT projects have proposed harmonized definitions and scales. [21]

Power Doppler imaging adds information about the vascular activity of inflammation. The presence of a Doppler signal in synovial tissue is considered a sign of active inflammation, and in some studies, its persistence during clinical remission is associated with the risk of an unfavorable prognosis or relapse. [22]

Tenosynovitis and bursitis are common sources of periarticular pain. Ultrasound assesses fluid in the tendon sheath, thickening of the synovial lining, and the Doppler signal. For bursae, it assesses the presence of fluid and wall thickening. These findings are important because treatment for tenosynovitis and bursitis may differ from that for intraarticular arthritis. [23]

If infectious arthritis is suspected, ultrasound is useful because it quickly confirms the presence of effusion and facilitates safer visualization of the puncture. Laboratory tests of the synovial fluid then become crucial, as ultrasound cannot reliably differentiate infection from non-infectious inflammation without analysis of the puncture. [24]

Table 4. Common ultrasound signs of inflammation and their interpretation

Sign	What does most often mean?	What is being clarified further?
Effusion	the presence of inflammation or injury, sometimes overload	volume, distribution, clinical picture
Synovial hypertrophy	chronic or active inflammation	Doppler activity
Doppler signal in synovium	active inflammation	standardization of settings, dynamics
Tenosynovitis	inflammation of the tendon sheath	provocation of pain during movement
Bursitis	inflammation of the joint capsule	connection with overuse or arthritis

[25]

Injuries, degeneration and ultrasound-guided procedures

For tendon and ligament injuries, ultrasound is often used as a first-line method because it clearly reveals tears, degenerative changes, thickening, and disruption of fiber structure, and also allows for dynamic testing. This is especially relevant for superficial tendons and the rotator cuff, where ultrasound is often used in the initial diagnosis. [26]

For large ruptures and when planning surgery, magnetic resonance imaging typically provides a more complete picture: the degree of tendon retraction, muscle atrophy, and associated intra-articular damage. Therefore, in practice, ultrasound often answers the question "is there a rupture and where it is," while magnetic resonance imaging clarifies "how large it is and what surgical procedure to perform." [27]

In degenerative changes such as osteoarthritis, ultrasound is effective in detecting effusions, synovitis, cysts, tendon lesions, and bursae, but assessment of bone changes, limb axis, and osteophyte severity is often better accomplished with radiographs. Therefore, the methods are usually complementary, and the choice depends on the clinical question. [28]

Ultrasound is widely used to guide punctures and injections, particularly in small or difficult-to-reach effusions. Systematic reviews demonstrate improved accuracy and outcomes with ultrasound guidance compared to a "blind" technique using anatomical landmarks, particularly for the knee joint. [29]

In certain scenarios, such as chronic ankle pain, clinical criteria for selecting an imaging method are reflected in guidelines for the appropriateness of examinations. There, ultrasound is often considered an appropriate method when tendon pathology is suspected and as an option when the cause of pain is unclear after a normal X-ray. [30]

Table 5. When ultrasound is preferred and when another method is more often needed

Clinical question	Often a rational first step	When another method is added more often
Is there effusion and synovitis?	ultrasound	in case of complex anatomy or controversial data, magnetic resonance imaging
Suspicion of tenosynovitis or bursitis	ultrasound	if the zone is deeply located, magnetic resonance imaging
Suspected tendon rupture	ultrasound	magnetic resonance imaging for preoperative evaluation
Evaluation of bone changes in arthrosis	radiography	if intraosseous changes are suspected, magnetic resonance imaging
Unclear chronic pain after normal x-ray	ultrasound or magnetic resonance imaging according to the scenario	the choice depends on the suspected cause and availability

[31]

Limitations of the method and how to read the report so that it helps treatment

The main limitation of ultrasound is that it relies on the experience of the specialist and the examination protocol, and it cannot detect structures hidden by bone or deep joint elements. Therefore, if clinical findings and ultrasound findings do not match, further imaging or re-evaluation at a specialized center is usually required. [32]

In the conclusion, it is useful to look not only for a "verbal diagnosis" but also for descriptive elements: the presence and volume of effusion, signs of synovial hypertrophy, the Doppler signal, the condition of the tendons and bursae, and an indication of the approaches used. Such details allow the physician to correlate the clinical picture with the symptoms and choose a treatment. [33]

If the report includes terms from standardized scales, such as the grading of synovitis using grayscale and Doppler imaging, this increases the study's value for follow-up. When repeating studies, it is important to use a comparable protocol and comparable Doppler settings; otherwise, the comparison may be inaccurate. [34]

After receiving the results, three questions are usually helpful for the doctor: which findings explain the symptoms, which findings are incidental and do not require treatment, and what will be the criteria for dynamic improvement. This approach helps avoid overtreatment based on "beautiful findings" and focus on clinically significant changes. [35]

Table 6. Commonly used protocol phrases and what they usually mean.

Formulation	What does this most often mean?	What needs to be clarified
Joint effusion	fluid in the joint cavity	volume, is a puncture necessary?
Synovial hypertrophy	thickening of the synovium, often inflammation	Doppler activity
Doppler signal in synovium	active inflammation	degree, dynamics during treatment
Tendinopathy	degenerative changes in the tendon	is there a partial rupture
Tenosynovitis	inflammation of the tendon sheath	association with pain during movement
Bursitis	inflammation of the bursa	congestion or systemic inflammation

[36]