Kategoria: MSK Ultrasound and Ultrasound-Guided Procedures (EN)

Joint aspiration, or arthrocentesis, is performed mainly to collect synovial fluid when the cause of an effusion needs to be clarified — particularly when infection, gout or another inflammatory process is suspected. Aspiration can also reduce pressure from a large effusion, but not every swollen joint needs to be drained. Ultrasound can confirm that fluid is present, identify the best pocket and help guide the needle safely, although it is not essential for every straightforward aspiration.

Polish version: Punkcja stawu — po co pobiera się płyn?

When is joint aspiration particularly important?

The most urgent indication is suspected septic arthritis. A joint that becomes acutely very painful, swollen and hot requires prompt assessment, and infection remains possible even without a high fever. The SANJO guideline recommends aspiration as soon as septic arthritis is suspected because synovial-fluid analysis is central to confirming or excluding the diagnosis.

Synovial fluid may also be collected when gout or calcium pyrophosphate crystal arthritis is suspected, when a large or atypical effusion appears after trauma, when the cause of an inflammatory effusion is unclear, or when fluid rapidly returns after previous aspiration.

What is tested in synovial fluid?

Depending on the clinical question, the sample may be assessed for the white-cell count and differential, crystals, Gram stain and bacterial culture. Additional microbiological or molecular testing can be selected when indicated.

No single synovial-fluid white-cell threshold independently proves or excludes infection in every patient. Results must be interpreted together with symptoms, examination, blood tests, imaging when appropriate and the patient’s risk factors.

Why is aspiration important when infection is suspected?

Blood tests alone are not enough. A normal C-reactive protein concentration does not reliably exclude septic arthritis. In a clinically stable patient without sepsis, synovial fluid should generally be obtained for microbiology before antibiotics are started, because prior antibiotic treatment can reduce culture yield. This principle does not mean delaying urgent antimicrobial treatment in an unstable or septic patient.

The presence of crystals does not automatically rule out a simultaneous bacterial infection. Gout or calcium pyrophosphate deposition and septic arthritis can coexist, so a crystal-positive sample still has to be interpreted in the full clinical context.

Can the appearance of the fluid give the diagnosis?

No. Cloudy, bloody or unusually coloured fluid may provide a clue, but visual appearance is not sufficiently reliable to establish the cause of an effusion. Laboratory analysis is needed when infection, crystal disease or another inflammatory process is a realistic possibility.

What does ultrasound add?

Ultrasound can confirm whether a collection is present and show where it is largest. It can help the operator avoid vessels, tendons and other sensitive structures, visualise the needle as it advances, and improve access when the effusion is small, deep or anatomically difficult. It can also be useful after a previous unsuccessful landmark-guided attempt.

A systematic review of knee arthrocentesis found ultrasound guidance to be associated with greater procedural accuracy, less procedural pain and a larger volume of aspirated fluid compared with landmark-guided aspiration. A more recent meta-analysis in knee osteoarthritis also reported better accuracy. These findings do not mean that every uncomplicated knee aspiration requires ultrasound; the value of guidance depends on the joint, the amount and location of fluid, anatomy and operator experience.

How is the procedure performed?

The skin is prepared using strict aseptic technique and a needle is introduced into the joint through an appropriate route. When infection is suspected, meticulous asepsis is especially important. The needle should not be passed through an abscess and, where possible, should avoid inflamed or infected skin.

Aspiration and injection are separate decisions. If joint infection is a realistic possibility, corticosteroid should not be injected before the situation has been clarified. Removing fluid may relieve pressure, but aspiration itself does not treat the underlying cause of the effusion.

What about a painful joint replacement?

A painful or swollen prosthetic joint requires a separate diagnostic pathway. Aspiration can be an important part of investigating periprosthetic joint infection, but interpretation follows dedicated criteria and should not be reduced to simply “draining fluid” from the joint.

When does a swollen joint need urgent assessment?

• acute severe joint pain with rapidly increasing swelling,
• a red or hot joint, with or without fever or chills,
• inability to bear weight or move the joint because of pain,
• immunosuppression, diabetes, a recent procedure or another active infection together with a new effusion,
• a prosthetic joint with sudden pain, fever, wound drainage or a marked loss of function.

Septic arthritis can damage articular cartilage rapidly, so suspected infection should not be managed by simply waiting to see whether the swelling settles.

Questions to discuss with an orthopaedic surgeon

Does every knee effusion need aspiration?

No. Aspiration is most useful when the result can clarify the diagnosis, when infection or crystal disease is suspected, or when a large effusion is causing significant symptoms. A small, clinically explainable effusion may not require aspiration.

Does every aspiration need ultrasound?

No. Ultrasound is especially useful when there is little fluid, anatomy is atypical, the joint is deep or access is difficult, or a previous attempt has failed. A straightforward aspiration of an obvious superficial effusion may be performed accurately without imaging by an experienced operator.

Can the fluid be judged by colour alone?

No. Appearance can suggest possibilities but cannot reliably distinguish infection, crystal disease and other inflammatory causes. Laboratory testing remains necessary when the diagnosis matters.

References

1. Ravn C, Neyt J, Benito N, et al.; SANJO guideline group. Guideline for management of septic arthritis in native joints (SANJO). J Bone Joint Infect. 2023;8:29–37. DOI: 10.5194/jbji-8-29-2023.
2. Uson J, Rodriguez-García SC, Castellanos-Moreira R, et al. EULAR recommendations for intra-articular therapies. Ann Rheum Dis. 2021;80(10):1299–1305. DOI: 10.1136/annrheumdis-2021-220266.
3. Wu T, Dong Y, Song HX, Fu Y, Li JH. Ultrasound-guided versus landmark in knee arthrocentesis: A systematic review. Semin Arthritis Rheum. 2016;45(5):627–632. DOI: 10.1016/j.semarthrit.2015.10.011.
4. Deng X, Li D, Li Y. Ultrasound-guided versus blind arthrocentesis in knee osteoarthritis: A systematic review and meta-analysis. Medicine (Baltimore). 2025. DOI: 10.1097/MD.0000000000041389.

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Content prepared by: Tomasz Poboży - orthopaedic surgeon.

Polska wersja

Comparing the painful side with the asymptomatic side can help distinguish an individual anatomical feature from a true abnormality. It is not, however, an automatic deciding test. The opposite limb may contain asymptomatic changes, and some disorders are bilateral.

Each patient provides their own reference point

Reference ranges for tendon thickness or nerve size are useful, but they do not capture the full range of biological variation. Comparing both sides can show whether a difference is pronounced in that particular patient. This is especially useful when the finding is subtle or lies close to the boundary of what is considered normal.

What can an orthopaedic surgeon compare during ultrasound?

• tendon thickness and internal structure,
• nerve cross-sectional area and echogenicity,
• the amount of fluid in a bursa or tendon sheath,
• how a structure behaves during movement,
• vascularity on Doppler imaging,
• bony contours and anatomical relationships.

Side-to-side comparison is most useful when the images are obtained in the same plane, with a similar limb position and identical machine settings. Otherwise, an apparent asymmetry may reflect technique rather than disease.

Ultrasound is not only a static image

One of the advantages of musculoskeletal ultrasound is the ability to observe tissues in real time. A tendon may snap only during a particular movement, a nerve may change position as a joint bends, and tissue impingement may appear only in a specific position. Comparing both sides during the same manoeuvre can therefore be more informative than comparing two isolated still images.

Why is the other side not always a “healthy control”?

Imaging abnormalities can occur without pain. This applies, among other things, to degenerative changes in tendons, entheses and menisci. Conversely, a disorder may affect both sides even when symptoms are present only on one side. A symmetrical appearance therefore does not exclude disease, and asymmetry by itself does not prove that the source of pain has been identified.

The image must be matched to the exact site of symptoms

The greatest value comes from combining three elements: the history, physical examination and ultrasound findings. If an abnormality is located where the patient hurts, changes during the movement that provokes symptoms and fits the clinical mechanism, it becomes more likely to be relevant. If it is incidental and does not correspond to the symptoms, it should not determine treatment on its own.

This approach was also important in a study of 430 patients evaluating meniscal abnormalities with ultrasound, co-authored by Tomasz Poboży. Ultrasound was treated as one component of the clinical assessment, with attention to the type of symptoms and the limitations of the method.

When can side-to-side comparison be misleading?

• after a previous injury or operation on the currently asymptomatic side,
• in bilateral or systemic disorders,
• when the limb or transducer position is not equivalent on both sides,
• when the difference lies within normal biological variation,
• when the visible abnormality does not match the location of symptoms.

Practical takeaway

The opposite side is a valuable reference, but it does not replace knowledge of anatomy, diagnostic criteria or examination of the patient. Good ultrasound practice is not about finding any asymmetry. The question is whether the observed difference actually explains the symptoms and whether it should change further management.

Back to: Diagnostic Tests

More about: MSK Ultrasound and Ultrasound-Guided Procedures

References

• Poboży T, Konarski W, Janowski K, Michalak K, Poboży K, Domańska-Poboża J. Ultrasonographic Assessment of Meniscus Damage in the Context of Clinical Manifestations. Medicina. 2025;61(8):1339. DOI: 10.3390/medicina61081339.
• European Society of Musculoskeletal Radiology. Musculoskeletal ultrasound technical guidelines. Current technical recommendations and regional scanning protocols.

This material is educational and does not replace an individual medical consultation.

Content prepared by: Tomasz Poboży - orthopaedic surgeon.

One of the major advantages of musculoskeletal ultrasound is the ability to observe tissues while they move. A tendon, nerve or part of a joint may look normal at rest but move abnormally, impinge on a neighbouring structure or reproduce a characteristic snap only in a specific position.

Polish version · For Patients

A static image answers only part of the question

MRI and CT provide extensive anatomical information, but they usually show the body in a fixed position. Ultrasound can be performed while the patient or examiner moves the joint. This means the examination can assess not only how a structure looks, but also how it behaves.

The movement should not be arbitrary. It should be chosen from the history and clinical examination to reproduce the position in which pain, clicking, locking or snapping occurs. Dynamic imaging is most useful when it answers a specific clinical question.

What can dynamic ultrasound reveal?

Tendon movement

During movement, ultrasound can assess structures such as the long head of the biceps tendon in the bicipital groove and the peroneal tendons behind the lateral malleolus. A brief subluxation may not be visible while the joint is held still.

A snapping nerve

The ulnar nerve may appear stable with the elbow extended but move over the medial epicondyle during flexion. Ultrasound can show the exact moment of displacement and help determine whether it corresponds to the patient’s snapping sensation. Movement alone is not necessarily abnormal; it has to be interpreted together with symptoms and the condition of the nerve.

Impingement during a specific movement

In selected situations, ultrasound can show the behaviour of tissues beneath the acromion, hand tendons, the iliotibial band or structures responsible for snapping around the hip. The image may help explain a mechanical phenomenon, but it should not be interpreted without considering pain and function.

What does the dynamic part of the examination look like?

The anatomy is first assessed at rest. The probe is then kept over the structure of interest while the patient performs a defined movement, sometimes slowly and sometimes against gentle resistance. The manoeuvre can be repeated from another angle or compared with the other side, remembering that asymmetry does not automatically mean pathology.

Why is “it snapped on ultrasound” not enough?

Some tendons and nerves move or sublux even in people without pain. Conversely, significant symptoms can arise from structures that ultrasound cannot assess sufficiently well. The finding therefore has to be integrated with the exact location of symptoms, injury mechanism, clinical tests and, when appropriate, X-ray, MRI or CT.

Probe position, movement speed, muscle tension and examiner experience also affect the assessment. An overly forceful manoeuvre can unnecessarily increase pain, while the wrong movement may fail to reproduce the problem.

When is dynamic assessment particularly useful?

• when clicking or snapping occurs only in a specific position;
• when intermittent tendon or nerve subluxation is suspected;
• when symptoms change with muscle contraction;
• when the aim is to distinguish smooth tissue gliding from mechanical conflict;
• when a static image does not explain a reproducible symptom.

Key point for an orthopaedic surgeon

Dynamic ultrasound is not a separate “better test”. It is a way of using ultrasound to answer a question about tissue behaviour. Its greatest value comes when the movement is selected to reproduce the symptom and the observed mechanism fits the clinical examination.

References

1. American College of Radiology, American Institute of Ultrasound in Medicine and collaborating societies. Practice Parameter for the Performance of a Musculoskeletal Ultrasound Examination. ACR–AIUM practice parameter.
2. Fodor D, et al. The EFSUMB Guidelines and Recommendations for Musculoskeletal Ultrasound – Part I: Extraarticular Pathologies. Ultraschall in der Medizin. 2022.
3. Pan G, et al. Current status of dynamic musculoskeletal ultrasound for evaluation of extra-articular and intra-articular pathologies. 2024. Full text.

Content prepared by: Tomasz Poboży - orthopaedic surgeon.

After a puncture, cut or stepping on a sharp object, ultrasound can help locate retained glass, wood, metal or plastic in the hand or foot—especially when the fragment is not visible on an X-ray. A normal ultrasound does not exclude every retained foreign body, and the decision to remove a fragment depends on its material, depth, symptoms and proximity to nerves, vessels, tendons or a joint.

When should a retained foreign body be suspected?

Common examples include a splinter, thorn, piece of glass, metal or plastic after an injury to the hand or foot. Suspicion is higher when the patient remembers the object entering the skin, the wound was deep, or part of the object broke during an attempt to remove it.

Possible clues include focal tenderness, a sharp sensation during movement or weight bearing, a palpable lump, recurrent redness, drainage from the wound or difficulty fully bending a finger. Symptoms may develop only after several days or weeks. Pain alone, however, does not prove that a foreign body remains: a haematoma, tendon or nerve injury, fracture and infection can produce similar symptoms.

X-ray or ultrasound—which is more useful?

X-rays show some materials well

Plain radiographs are usually the first test when glass, metal or another radiopaque material is possible. They can also reveal a fracture. Wood, thorns and some plastics are much less visible. A very small fragment, or one projected over bone, may also be difficult to identify.

Ultrasound shows soft tissues and anatomical relationships

Ultrasound can show the foreign body itself and the tissue reaction around it. It can help determine the fragment’s depth and its relationship to a tendon, tendon sheath, nerve, blood vessel or joint. For superficial foreign bodies, the skin can be marked before a procedure and, in selected cases, removal can be guided in real time.

Ultrasound also has limitations. Detection depends on the fragment’s size, material and location, the time since injury and the examiner’s experience. Air in a fresh wound, proximity to bone and deep location can make assessment more difficult. In a meta-analysis of 17 studies, pooled ultrasound sensitivity was 72% and specificity 92%. A positive result can therefore be very helpful, but a normal result does not completely exclude a retained foreign body when clinical suspicion remains high.

When are CT or MRI needed?

CT may be useful for deeply located foreign bodies, complex anatomy or an equivocal radiograph. MRI is used selectively, particularly when soft-tissue complications need to be assessed. If a metallic fragment is possible, MRI should not be ordered blindly; the type and location of the material should first be established.

Does every foreign body need to be removed?

No. Removal is more often indicated when the fragment causes pain, infection, restricted movement or compression of a nerve, vessel or tendon. Organic material such as wood or a thorn is more likely to provoke inflammation and infection. Foreign bodies close to a joint, tendon sheath or neurovascular structures deserve prompt assessment.

A small, deeply located, inert and asymptomatic fragment can sometimes be left in place when retrieval would cause more tissue damage than benefit. The decision should balance the expected benefit of removal against the procedural risk. Tetanus protection should also be reviewed according to the wound and vaccination history.

What does ultrasound-guided removal involve?

The fragment is first located and the safest access route is planned. In an appropriate superficial location, a small incision can be made and the instrument followed under real-time ultrasound, potentially reducing unnecessary tissue dissection. Not every case is suitable for this approach.

Operating-room conditions may be required when the object is deep, brittle, large, fragmented, close to a nerve or vessel, has penetrated a tendon, or lies within a joint. Blind attempts to dig out an unseen fragment with a needle or tweezers can push it deeper, break it into smaller pieces and increase the risk of infection.

What should you do after a fresh injury?

• Do not probe a deep wound with household instruments.
• If an object remains deeply embedded, do not force it out.
• Cover the wound with a clean dressing and reduce loading of the painful hand or foot.
• Note what material caused the injury and when it occurred.
• For a deep, contaminated or puncture wound, seek advice about cleaning, antibiotics when indicated and tetanus prophylaxis.

Warning signs

Urgent assessment is needed for increasing pain and swelling, rapidly spreading redness, pus, fever, red streaking along the limb, numbness or weakness of finger or toe movement, a pale or cold hand or foot, severe pain when moving a finger, or a wound that may have penetrated a joint. A high-pressure injection injury requires immediate medical attention even when the skin opening looks minor.

Key points for an orthopaedic surgeon

Ultrasound is especially valuable when a retained fragment is not visible on X-ray or when its relationship to soft tissues needs to be mapped. It is not a perfect test. When the history and symptoms still strongly suggest a retained foreign body, one normal imaging result should not automatically end the diagnostic process.

Sources

1. Davis J, et al. Diagnostic Accuracy of Ultrasonography in Retained Soft Tissue Foreign Bodies: A Systematic Review and Meta-analysis. Acad Emerg Med. 2015;22(7):777–787. DOI: 10.1111/acem.12714. PubMed.
2. Rupert J, Honeycutt JD, Odom MR. Foreign Bodies in the Skin: Evaluation and Management. Am Fam Physician. 2020;101(12):740–747. Full text.

Polish version · Diagnostic Tests · For Patients

Content prepared by: Tomasz Poboży - orthopaedic surgeon.