Background and Indications for Examination
Joint pain and swelling are common complaints in the acute care setting. Diagnostic arthrocentesis may be required to rule out an infectious process or hemarthrosis. Joint aspiration may also relieve joint pain and allow for infusion of medications when appropriate. Much of the technique and equipment for ultrasound-guided arthrocentesis is the same as for a landmark-based approach. The use of ultrasound can determine the presence of a joint effusion and increase the success rate of arthrocentesis while decreasing complications. Ultrasound may also assist in directing a needle into a joint space even when no effusion exists (ie, for injection of dye to determine joint disruption in trauma or for injection of medications).
Probe Selection and Technical Considerations for Arthrocentesis
A linear-array probe is best suited for arthrocentesis because of its high resolution and linear configuration. A curvilinear abdominal probe with a 3.0–5.0 MHz frequency can also be used for deeper joints such as the hip or in obese patients. A “hockey stick” or pediatric linear probe may be used for smaller joints.
If available, a superficial or bone preset should be utilized on the machine in order to optimize the image obtained.
The focus should be adjusted so that it is at the level of the joint of interest. This will help to improve the lateral resolution of the image.
The sonographer should initially start off deep and survey the whole joint. The depth should then be reduced so that the area where the needle will be introduced takes up most of the screen.
The overall gain should be adjusted in order to enhance the brightness of the image as needed. It is often better for the sonographer to increase the far gain of the image only, using time-gain compensation (TGC), in order to improve the quality of the deeper structures. It is important to not use too much gain, which can result in obscuring other important structures within the joint, such as anechoic blood vessels.
Arthrocentesis is an invasive procedure, and therefore, sterile precautions with a sterile probe cover and gel should always be used.
Like many procedures, ultrasound guidance for arthrocentesis can be performed by either a static or dynamic technique. Static ultrasound guidance involves using the image to identify the synovial space, depth, angle of entry, and vessels/nerves or other obstacles. The sonographer then marks a spot on the skin where success is likely. The procedure is then carried out in a fashion similar to landmark-based arthrocentesis. The main advantage of this technique is that it leaves the clinician with two free hands to complete the procedure.
Dynamic ultrasound guidance uses real-time imaging to perform the arthrocentesis. It has the advantage of visualizing the needle entering the synovial space and the ability to redirect the needle as required. Needles with special echogenic tips may be used to help visualization and guidance into the joint. This technique may initially be more challenging for the novice, but ultimately it is the preferred method. When using dynamic guidance, sterile precautions with probe covers and sterile gel must be used, as in central venous access.
An intermediate technique is a two-person dynamic guidance procedure, where the probe is held by one person and the procedure performed by the other. Either technique may be used depending on operator preference, resources, and time availability, although mastery of the single-operator dynamic approach will result in the most consistent success.
Performing Ultrasound-Guided Arthrocentesis
The knee is the one of most commonly tapped joints in the body. It is often performed successfully using only anatomic landmarks. However, ultrasound may be useful in cases where the anatomy is distorted by soft tissue swelling or bony displacement. The suprapatellar bursa is contiguous with the knee joint; therefore, an effusion of the suprapatellar bursa signifies a knee joint effusion and vice versa (Fig. 21-1). Most often, the medial or lateral recess of this bursa is the site for joint aspiration. The patient should be supine and the knee flexed to 15°–45°. The knee should be scanned on the medial and lateral sides just posterior to the patella in both the transverse and coronal planes (Fig. 21-2). An effusion will appear as an anechoic or hypoechoic area abutting the distal femur (see Fig. 21-1). It may have internal echoes in the case of clotted blood, a septic joint, or a chronic inflammatory process (Fig. 21-3). The needle should be directed toward the largest pocket of fluid. It is important to differentiate the suprapatellar bursa from other bursa and cysts in the region, as these may not communicate directly with the joint space.