Continuing Education Activity

Ultrasound-guided arthrocentesis is an advanced procedural method used to enhance the precision and safety of joint aspiration. Monoarticular arthropathy frequently presents in emergency and primary care settings, where distinguishing benign causes from septic arthritis is critical. Septic arthritis, if left untreated, can lead to rapid joint destruction and systemic complications. Clinical evaluation, including history, physical examination, and laboratory testing, may raise suspicion but often lacks specificity. Ultrasound offers real-time visualization of joint structures and effusions, allowing clinicians to localize fluid collections and guide needle placement with greater accuracy. This technique improves diagnostic yield, reduces procedural complications, and increases patient comfort. Its application is particularly valuable in evaluating joint effusions of uncertain origin, inflammatory arthropathies, or suspected infection.

This educational activity provides healthcare professionals with foundational and advanced knowledge of ultrasound-guided arthrocentesis. Participants develop competence in interpreting sonographic anatomy, selecting appropriate probes, and applying needling techniques tailored to specific joints. The course also emphasizes interprofessional collaboration, highlighting the roles of physicians, nurses, and ultrasound technologists in optimizing procedural success. By fostering communication and shared expertise, interprofessional teams can improve the accuracy of diagnoses, expedite treatment, and reduce adverse outcomes in patients presenting with joint effusions. Integration of ultrasound guidance into routine practice strengthens procedural confidence and promotes safer, more efficient care delivery.

Objectives:

• Identify appropriate clinical indications for ultrasound-guided arthrocentesis in monoarticular joint presentations.

•  

  Differentiate between benign and urgent causes of joint effusion using clinical and imaging criteria.

• Screen patients appropriately for joint aspiration based on presenting symptoms, risk factors, and diagnostic uncertainty.

• Collaborate with interdisciplinary teams, including orthopedics and infectious disease, for comprehensive patient care.

Introduction

Ultrasound (US) guided arthrocentesis has become an increasingly valuable tool in the emergency department (ED) for evaluating acute monoarticular arthopathies, particularly septic arthritis, which occurs at an annual rate of 10 per 100,000 patients in the United States.[1] In the ED, septic arthritis is the most critical of arthropathies to confirm and treat. While lab values and vitals may support clinical suspicion, synovial fluid analysis via arthrocentesis remains the diagnostic gold standard. If findings suggest septic arthritis, intravenous antibiotics should be initiated promptly, and orthopedic surgery should be consulted.[2][3]

Arthrocentesis is a common ED procedure with both diagnostic and therapeutic utility. Traditionally performed using anatomical landmarks, this blind approach has variable success rates (61%-78%) depending on the joint anatomy and effusion characteristics.[4][5][6][7] US enhances the accuracy of arthrocentesis by providing direct visualization of the effusion, allowing for precise needle placement using in-plane or out-of-plane techniques and reducing the number of attempts.[6][8][9][10] US guidance also enables detection of small effusions, as little as 4 mL, that may be missed using landmark-based techniques.[11][12] Study results show that this procedure improves aspirated volume, reduces patient-reported pain, and accelerates the diagnosis and management of septic arthritis.[13][14]

US-guided arthrocentesis offers several advantages that are particularly relevant in the pediatric population. Children have smaller joints, smaller effusion volumes, and may have difficulty tolerating procedural discomfort. US has demonstrated a diagnostic sensitivity and specificity of 80% and 90% for effusion detection, though further studies are needed to define its accuracy across all pediatric joints. Notably, its use in assessing the pediatric hip has shown significant promise, and case reports support the safety and feasibility of US-guided arthrocentesis in pediatric hips and knees.[15][16] 

Although most evidence currently pertains to the adult population, US-guided arthrocentesis can be applied to most major joints of the upper and lower extremities. Variation in technique, such as in-plane versus out-of-plane needle guidance, exists across the literature, and different approaches may be used depending on operator preference and clinical context. These same US techniques may also be adapted for intra-articular joint injections.[17][18][19]

Indications

The indications for US-guided arthrocentesis mirror those of traditional arthrocentesis. These include diagnosing septic arthritis, evaluating inflammatory arthropathies, administering intra-articular medications, assessing hemarthrosis, and evaluating synovial integrity following penetrating injuries.[1][20]

Contraindications

An absolute contraindication to US-guided arthrocentesis is cellulitis at the intended needle entry site, due to the risk of introducing infection into the joint space. Relative contraindications include bacteremia, coagulopathy, and the presence of a joint prosthesis.

Equipment

US-guided arthrocentesis requires specific equipment to ensure procedural accuracy, safety, and sterility. Proper selection and preparation of US and procedural tools are essential for optimizing visualization and successful joint aspiration. Equipment necessary includes:

• Antiseptic solution (povidone-iodine or chlorhexidine)
• Sterile gloves
• Sterile drape
• Local anesthetics (eg, lidocaine)
• 27-gauge needle for anesthetic administration
• 18–20 gauge needle for joint aspiration
• Collection tubes for synovial fluid analysis
• US machine with sterile gel and sterile probe cover
• High-frequency linear transducer or lower frequency curvilinear transducer (depending on joint depth)

Personnel

While a single operator can perform this US-guided arthrocentesis, having an additional team member can be helpful for patient support, managing equipment issues, handling the US probe before sterile preparation, and assisting with documentation. An assistant may also help maintain sterility and improve procedural efficiency, particularly in complex or high-acuity settings. 

Preparation

Before performing US-guided arthrocentesis, ensure all necessary equipment is readily available at the bedside and that the US machine is equipped with the appropriate transducer for joint evaluation. A high-frequency linear probe is typically used due to its superior resolution for visualizing superficial structures and soft tissue detail. However, a low-frequency curvilinear probe may be more suitable in cases requiring deeper penetration or a broader field of view, such as larger joints or deeper effusions.

Patient positioning varies depending on the joint being aspirated and the technique used. Administering analgesics or anxiolytics beforehand can improve patient comfort. Once the patient is positioned and equipment is prepared, the target joint should be cleansed with an antiseptic solution, such as povidone-iodine (betadine) or chlorhexidine, and draped in a sterile manner. A sterile US probe cover and gel should be applied to maintain the aseptic technique throughout the procedure. 

Technique or Treatment

Several common techniques are used for US-guided arthrocentesis, including in-plane and out-of-plane approaches. In-plane technique involves advancing the needle parallel to the US probe, allowing continuous visualization of the needle shaft and tip throughout the procedure. In contrast, the out-of-plane technique positions the needle perpendicular to the probe, with only a cross-sectional view, ideally the needle tip, visible on the screen.

When using the out-of-plane method, the needle should be inserted at a steep angle with careful triangulation to align the tip with the effusion seen on the US. A key principle is never advancing the needle tip beyond the probe's edge. If the tip is lost from view, the operator should stop advancing, slide the probe away from the needle to lose the visible portion, then slowly move back toward the needle to relocate the tip. These adjustments must be made without altering the probe's orientation to maintain spatial consistency. Recommended techniques for US-guided arthrocentesis of the ankle, elbow, hip, knee, shoulder, and wrist are outlined, recognizing that variations exist in the literature. Clinicians should select the approach that best aligns with their experience, the joint being accessed, and the patient's specific anatomy and clinical context.

Ankle Arthrocentesis

Identify the tibialis anterior tendon and the medial malleolus as key anatomical landmarks (see Image. Normal Ankle Ultrasound). Position the US probe with the marker oriented cephalad, locate the tibia on the screen, and slide the probe distally until the tibia-talus joint space is visualized. Maintain this orientation and shift the probe slightly medial so it lies between the tibialis anterior tendon and the medial malleolus. Using an out-of-plane approach, insert the needle at a steep angle adjacent to the center of the probe (see Image. Ultrasound-Guided Arthrocentesis of the Ankle). Begin aspiration once the needle tip is visualized within the effusion (see Image. Ankle Effusion Ultrasound).

Elbow Arthrocentesis

Position the patient with the elbow flexed at 90º, resting on a table for stability. Place the US probe longitudinally over the distal humerus to visualize the triceps muscle and tendon, posterior fat pad, distal humerus, and proximal ulna within the olecranon fossa (see Image. Ultrasound of Normal Elbow). Assess for an effusion deep to the triceps tendon in this view. If an effusion is identified, rotate the probe 90º into a transverse (short-axis) orientation to better visualize the triceps tendon in cross-section with the effusion beneath it. 

To estimate needle depth, freeze the image and measure the distance from the probe surface to the effusion. Insert the needle lateral to medial using an in-plane technique, maintaining a shallow, flat trajectory toward the effusion (see Image. Ultrasound-Guided Arthrocentesis of the Elbow). The lateral-to-medial approach allows continuous visualization of the needle tip throughout the procedure as it advances into the olecranon fossa. [21]

Hip Arthrocentesis

Position the US machine at the head of the bed before setup. After preparing the procedure site, place the probe over the inguinal groove with the probe marker oriented to the patient's anatomic right. This initial positioning should display the femoral nerve, artery, and vein in the near field, and the femoral head and neck in the far field. Next, rotate the probe so the inferior or medial portion faces the umbilicus. For right hip arthrocentesis, the non–marker side of the probe should be oriented toward the umbilicus; for left hip arthrocentesis, the marker side should be directed toward the umbilicus. This probe rotation allows for a consistent in-plane lateral-to-medial needle approach, regardless of the side of the hip.

From this view, the femoral head, femoral neck, anterior synovial recess, and iliofemoral ligament should be clearly visualized on the screen. Once these landmarks are identified, insert the needle in a longitudinal in-plane approach directly beneath the transducer. The needle should remain in view throughout the procedure as it is advanced into the anterior synovial recess. Begin aspiration once the needle tip enters the effusion.[22][23]

Knee Arthrocentesis

Begin by placing the US probe above the patella with the marker oriented cephalad to identify a suprapatellar effusion. The US window should include the superior patella, quadriceps femoris tendon (long axis), fat pad, and femur. Effusion typically appear as hypoechoic fluid collections between the quadriceps tendone and the femur, though echogenicity may vary depending on the effusion type.

Once the effusion is visualized, rotate the probe 90º into a transverse view with the marker facing the patient's right side. To approximate needle depth, freeze the image and measure the distance from the probe to the effusion. Insert the needle from lateral to medial using a flat, in-plane approach at the measured depth. Advance the needle into the suprapatellar space, ensuring continuous visualization of the needle tip as it enters the effusion beneath the quadriceps tendon. Avoid aspiration from the fat pad, as this can increase patient discomfort. 

Shoulder Arthrocentesis

US-guided shoulder arthrocentesis is typically performed using the posterior glenohumeral approach, which provides reliable visualization of the joint space between the glenoid fossa and the humeral head. First, to obtain this view, palpate the lateral scapular spine where it meets the humerus. Position a linear or curvilinear transducer just below the scapular spine in a parallel orientation, with the probe marker typically facing the operator's left to maintain orientation on the US screen. The patient should be seated upright, with the US machine positioned adjacent to or in front of the affected shoulder and the screen facing posteriorly. The clinician should stand behind the patient, with the probe marker oriented to the left side of the screen for consistent spatial interpretation, regardless of laterality. 

Once the probe is in position, slide it laterally to visualize the scapular notch, followed by the glenoid and humeral head. This view allows assessment of the glenohumeral effusions as well as shoulder dislocations.[24][25] If an effusion is identified, center it in the US window while maintaining the posterior glenohumeral view. Using an out-of-plane approach, insert the needle at a steep angle adjacent to the center of the probe. Advance the needle slowly, triangulating to the depth of the effusion. With proper technique and alignment, the needle tip should appear within the effusion on the US screen. 

Wrist Arthrocentesis

Position the patient with the wrist resting on a towel, palm down, to maintain slight flexion. Place the transducer transversely over the dorsal wrist, with the marker oriented laterally toward the ulnar side. This initial view should include Lister's tubercle on the distal radius, the extensor carpi radialis brevis and longus tendons.

Next, rotate the probe approximately 90º to align it longitudinally along the radius. This should bring the scaphoid and distal radius into view, allowing visualization of the radioscaphoid joint. Using an out-of-plane technique, insert the needle at a steep angle between the Lister tubercle and the extensor carpi radialis brevis tendon. Advance the needle until the tip enters the effusion space, then proceed with aspiration. 

Complications

Although rare, US-guided arthrocentesis may be associated with certain complications.

• Infection: Strict aseptic technique is essential to prevent introducing bacteria into the sterile joint space. The procedure should include sterile draping, skin preparation with an antiseptic solution, sterile gel, and a sterile US probe cover. Needle insertion through infected skin must be avoided.
• Bleeding: Hemarthrosis is uncommon but may occur, particularly in patients with coagulopathies or those taking anticoagulants. The needle should be inserted carefully to minimize risk, avoiding lateral movement that may shear surrounding tissues or disrupt nearby blood vessels.

Clinical Significance

Arthrocentesis is a versatile procedure with both diagnostic and therapeutic value. US guidance enhances the procedure by providing visualization of relevant anatomy, pathology, and needle trajectory. Compared to landmark-based techniques, US-guided arthrocentesis offers several advantages: improved ability to detect and aspirate small effusions, increased yield for larger effusions, faster time to diagnosis, reduced patient discomfort, and greater safety by avoiding neurovascular, cartilaginous, and ligamentous structures. 

Additionally, US allows for real-time documentation when video clips are saved to the medical record, supporting clinical transparency and quality assurance. These benefits make US-guided arthrocentesis the preferred technique when US is available; it also improves clinician confidence and may increase the likelihood of performing the procedure when indicated.[6]

Enhancing Healthcare Team Outcomes

Following aspiration, clear communication with nursing and laboratory staff is essential to ensure timely synovial fluid analysis. Proactive coordination helps prevent medical and procedural errors and delays in diagnosis and treatment. Effective interprofessional communication among physicians, nurses, pharmacists, and other team members has reduced hospital length of stay and overall healthcare costs. In managing monoarticular arthropathies, timely consultation with orthopedic surgery, infectious disease, internal medicine, and pharmacy teams is critical to optimizing patient outcomes and streamlining care.[26]