Since 1895, when Wilhelm Röntgen produced his first x-ray (of his wife’s hand), physicians have used imaging to "see" beneath the skin, having progressed from these early modalities to advanced imaging such as magnetic resonance imaging (MRI) and ultrasonography. These nonionizing imaging modalities afford significant resolution, allowing clinicians to appreciate the structure and anatomic relationships between joints, tendons, ligaments, and muscles.
Despite significant technological advancements since Röntgen's discovery, patients who present with common musculoskeletal complaints are routinely still evaluated only by x-ray. Not surprisingly, these studies often produce negative results, since x-rays offer poor visualization of ligaments, muscles, and other soft tissues. MRI is the best imaging modality for anatomic detail, but expense, availability, and even claustrophobia preclude its widespread use. Additionally, MR images are limited by being static.
Ultrasonography is available in many clinical settings (outpatient clinics, intensive care units, and emergency departments) as an adjunct to procedural guidance and critical care evaluation. One of its main advantages is that it allows for dynamic soft tissue examination. When used to evaluate the musculoskeletal system, bedside ultrasonography can provide clinicians with valuable information that radiography cannot. 
Joint aspiration, or arthrocentesis, is one of the most commonly performed procedures in the evaluation and treatment of joint diseases. Joint aspiration may aid in diagnosis by assessing synovial fluid for the degree of inflammation and for the presence of pathologic agents such as crystals or microorganisms. Additionally, arthrocentesis may prove therapeutic in some cases by relieving intra-articular pressure and by injecting intra-articular drugs (eg, steroids).
Although arthrocentesis is generally safe and simple, limited physician experience or technical difficulties (eg, patient positioning, body habitus) can make some cases challenging. In these cases, procedural guidance using ultrasonography may prove beneficial. [2, 3, 4, 5]
Ultrasonography has many diagnostic musculoskeletal applications, as noted below. This article focuses primarily on the use of ultrasonography for joint evaluation and for procedural guidance of arthrocentesis.
Effusions - Diagnosis and aspiration of joint and tendon sheath effusions
Bone - Demonstration of joint erosion
Muscle - Diagnosis of muscle trauma, tumor, or abscess
With specific regard to joint evaluation and arthrocentesis, the indications can be broadly separated into diagnostic and therapeutic, as follows:
- Evaluation of a suspected septic joint
- Identification of crystal arthropathy
- Pain relief by aspiration of synovial fluid or blood
- Drainage of septic effusion
- Injection of intra-articular drugs such as corticosteroids and anesthetics
Unlike x-rays and computed tomography (CT) scans, ultrasonography does not use ionizing radiation. It is noninvasive and does not require the use of contrast medium (dye). No absolute contraindications exist to diagnostic ultrasonography. However, patient positioning and comfort could preclude the use of ultrasound in some circumstances.
While no absolute contraindications exist to arthrocentesis, relative contraindications do exist, and, as with all procedures, the risks and benefits should be considered and discussed with the patient before beginning.
Caution should be used in patients with coagulopathy. A coagulation panel should be checked prior to the procedure in patients taking warfarin or when a coagulation abnormality is clinically suspected. Coagulopathy should be treated only if reversal is clinically appropriate in the context of the patient's underlying medical conditions. Even without reversal, coagulopathy should not deter the physician from performing arthrocentesis, if indicated. One trial of patients on warfarin found no complications after arthrocentesis. 
Inserting a needle through an overlying skin infection could theoretically seed the joint with bacteria from the skin. Therefore, try to avoid areas of the skin suspicious for cellulitis.
In general, diagnostic ultrasonography does not require any anesthesia. If, however, arthrocentesis is also performed, then local anesthesia will be needed.
Patients who are particularly anxious or in severe pain may require procedural sedation.
After identification of the optimal needle entry site (please see Technique section below for specific sites based on joint of interest), clean, drape, and prepare the region in the usual sterile fashion.
Using a 25- or 27-gauge needle, begin by making a subcutaneous skin wheal using an anesthetic (eg, 1% lidocaine without epinephrine) and then inject an additional 1-3 mL of anesthetic into the subcutaneous tissue overlying the chosen needle insertion site.
Avoid injecting directly into the joint space, since most local anesthetics have bacteriostatic or bacteriocidal properties that may interfere with synovial fluid analysis results. 
Equipment includes the following:
Portable ultrasound machine
Acoustic coupling gel
Appropriate transducer (probe)
Sterile gloves and drapes
Skin-cleaning solution (eg, chlorhexidine or povidone-iodine solution)
1% lidocaine without epinephrine
Syringe, 5 mL (for lidocaine injection)
Needle, 25 or 27 gauge (for lidocaine injection)
Syringe, 30 mL (for synovial fluid aspiration)
Needle, 18 gauge (for synovial fluid aspiration)
The type of transducer is an important consideration. Some transducers are well-suited to musculoskeletal ultrasonography, while others are designed for visualization of deeper structures.
An ultrasound transducer is designed to operate by emitting sound waves within a specific frequency range. The higher the frequency, the better the image resolution. This improved resolution, however, comes at the cost of sound wave penetration into tissues and results in poor visualization of deeper structures. Conversely, a transducer with a lower frequency range permits deeper tissue penetration but results in lower resolution images.
For most musculoskeletal applications, the linear, or small-parts, transducer is most useful (see image below). This is a high-frequency probe that provides high-resolution images of more superficial structures. In the obese patient with a significant amount of soft tissue overlying the joint of interest, a curvilinear probe, which has a lower frequency and greater tissue penetration, may be helpful.
In general, position the patient at a comfortable height and location for the examiner. The clinician performing the examination should be able to easily reach both the patient and the ultrasound machine without having to stretch, stoop, lean, or bend excessively to examine the patient.
Specific patient positioning depends on the particular joint of interest; see Technique section below for details.
General ultrasonography techniques
As with positioning, specific technique depends on the particular joint being examined. However, the following general techniques are important to understand and are applicable regardless of the joint, musculoskeletal region, or even organ system that is being evaluated.
Every ultrasound probe has an indicator (usually a mark or palpable protuberance) that identifies the probe's leading edge. The opposite end of the probe (without the indicator) is called the receding edge. The leading edge of the probe corresponds to the left side of the screen, and the receding edge corresponds to the right side of the screen (the transducer:screen relationship can be remembered by Leading edge:Left side of screen and Receding edge:Right side of screen). One can apply gel to the transducer and touch one end to confirm which side is which.
To produce an ultrasound image, an ample amount of acoustic gel should be placed on the probe. The probe should be placed on the region of interest with the leading edge pointed either toward the patient's head (cephalad) or toward the right side of the patient's body.
Once the transducer is placed on the patient's skin, a rectangular image (if using the linear array probe) will be displayed on the screen. The top of the image is called the near field and represents the superficial tissues nearest the probe. The bottom of the screen is called the far field and represents the deeper tissues further from the probe. The left side of the screen represents the probe's leading edge (which should be pointed toward the patient's head or right side), and the right side of the screen represents the probe's receding edge (which should be pointed toward the patient's feet or the patient's left side).
In the images below, the practitioner is evaluating the patient's knee; the femur is cephalad and the tibia is caudad relative to the probe. The transducer's leading edge is aimed cephalad (toward the femur), which appears on the left side of the screen.
The ultrasound image shown is created by emitting sound waves from the transducer and then "listening" for the reflected sound waves that return. The ultrasound machine processes the intensity of the returning echoes to create an image on the screen according to a grayscale spectrum of black, white, and many shades of gray. Whether a particular structure appears black, white, or gray on the screen depends on the tissue's echogenicity, which, in turn, depends on the tissue's density, water content, and elasticity. Black (anechoic) shapes generally represent fluid or artifact. Gray, or hypoechoic, images represent "soft" tissues and white, or hyperechoic, images represent dense tissues. The table below lists the echogenicity of various tissues. Sonographic examples of bone, muscle, and tendon are also shown.
A glenohumeral joint effusion may be seen with septic arthritis of the shoulder. It is also associated with a rotator cuff tear and can be detected by ultrasonographic examination of the biceps brachii tendon. 
Because the tendon sheath of the long head of the biceps brachii (highlighted in blue in the image below) communicates with the glenohumeral joint, increased synovial fluid collects in this dependent extension of the joint. As with most fluids, the collection appears anechoic (black) on the ultrasound screen. Note that a tiny amount of physiologic fluid is often seen on one side of the biceps tendon under normal circumstances, but excess fluid is considered abnormal, especially if the fluid is circumferential. 
To begin, have the patient sit on a rotating stool, if possible. Place the patient's arm in a neutral position with the elbow at his or her side and the dorsum of the hand lying on the thigh.
Holding the linear array transducer with the leading edge aimed toward the patient's right side, visualize the tendon sheath of the long arm of the biceps brachii in the short axis (cross-sectional view). Proper technique and patient positioning are shown in the image below.
Under normal circumstances, only a minimal amount of fluid should be seen between the biceps brachii tendon and the tendon sheath. With the presence of a glenohumeral joint effusion, fluid dependently travels from the glenohumeral joint space inferiorly and collects between the biceps brachii tendon and the tendon sheath. An example of a normal-appearing biceps brachii tendon is shown in the image below.
To aspirate the glenohumeral joint, begin by preparing the skin with cleansing solution and draping the shoulder with a sterile cover. Insert an 18-gauge needle attached to a syringe midway between the coracoid and the anterolateral corner of the acromion. Direct the needle posteriorly and slightly inferiorly. For details, see Medscape Reference topic Shoulder Arthrocentesis.
Knee effusions have many causes, including trauma, infection, arthritis,  and crystal deposition. Increased joint fluid in the knee is characterized ultrasonographically by anechoic distention of the suprapatellar recess (shown in red in the image below). With slight flexion of the knee, excess synovial fluid extends superiorly from between the patella and femur and preferentially collects deep to the quadriceps tendon. 
The best way to visualize fluid in the suprapatellar recess is via the anterior approach. Begin by placing the patient in the supine position with the knee exposed and slightly flexed (placing a rolled towel behind the patient's knee may aid in positioning), as shown in the image below.
Place the linear array transducer parallel to the quadriceps femoris muscle tendon just superior to the patella. This provides a longitudinal, or long axis, view of the quadriceps femoris muscle and tendon as well as the suprapatellar recess.
Sweep the probe proximally and distally along the quadriceps muscle and tendon to visualize the entire suprapatellar space. Synovial fluid within the space appears anechoic (black).
Knee effusion and knee hemarthrosis are shown in the image below. The image was acquired using the anterior approach described and shown above. Note the fibrillar quadriceps tendon in the near field, the hyperechoic (white) femur in the far field, and the patella at the receding (right) edge. The anechoic fluid bounded by the green arrowheads indicates the presence of a joint effusion and is fluid within the suprapatellar space.
The most common approach to knee arthrocentesis is the lateral approach at the level of the superior pole of the patella. The skin is prepared in the usual manner, and the superolateral aspect of the patella is palpated. An 18-gauge needle is inserted 1 fingerbreadth superior and lateral to this point. The needle is angled at a 45-degree angle beneath the patella.  For details, see Medscape Reference article Knee Arthrocentesis.
Ankle effusions can be caused by local factors such as infection, gout, or occult fracture or by systemic processes such as hemophilia, sickle cell disease, or rheumatoid arthritis. While conventional x-rays require up to 5 mL of synovial fluid before an ankle effusion can be detected, ultrasonography requires as little as 2 mL of fluid. [18, 19] The image below depicts normal ankle anatomy for reference.
The most sensitive location to identify an ankle joint effusion is over the anterior recess of the tibiotalar joint. An anterior approach to evaluating the ankle should be used: the linear array probe is oriented longitudinally at the level of the tibiotalar joint with the foot in mild plantar flexion; as always, the probe's leading edge should be aimed cephalad.
Once properly positioned, the tibia is seen at the left side of the screen (leading edge) and the talus is seen at the right side of the screen (receding edge). An example of correct probe placement and normal anatomy is shown in the first image below; in the second image, note the hypoechoic anterior fat pad between the tibia and talus.
An ankle effusion appears as an anechoic region in the anterior recess between the tibia and talus (see image below).
The anterolateral approach to ankle arthrocentesis is preferred because it reduces potential injury to the dorsalis pedis artery and peroneal nerve. Following usual preparation, skin cleansing, and draping, a needle is inserted at the joint line midway between the base of the lateral malleolus and the lateral border of the extensor digitorum longus. Extension of the foot by the practitioner against the patient's resistance may help to identify this space. The needle should be advanced perpendicular to the fibular shaft. For details, see Medscape Reference article Ankle Arthrocentesis.
Hip effusions can be common in patients with inflammatory diseases such as rheumatoid arthritis, but they can also arise in normal joints affected by osteomyelitis, trauma, toxic synovitis, and Legg–Calve-Perthes disease in children. [20, 21, 22, 3, 23] The most serious cause of hip effusions in both the adult and pediatric populations is septic arthritis, as it can lead to rapid joint destruction and significant morbidity. Plain films are not sensitive for detecting effusions, and magnetic resonance imaging (MRI) is typically unavailable in the emergency department. See the images below.
Synovial fluid is often not visualized by ultrasonography in a normal joint. When a hip effusion is present, it appears as an anechoic or hypoechoic collection that elevates the joint capsule, displacing the iliofemoral ligament from the femoral neck. The most prominent area of fluid accumulation occurs in the anterior synovial recess, an area located anterior to the femoral neck and posterior to the joint capsule. 
The approach most often used to examine a hip effusion is the anterior approach with the patient in a supine position and the hip in slight flexion and internal rotation. This position allows the fluid collection to move anteriorly. Transducers used include the 3- to 5-MHz curved array or 7.5- to 10-MHz linear array, and the probe should be placed in a sagittal oblique plane parallel to the long axis of the femoral neck, with the marker pointing toward the umbilicus. In this view, landmarks visualized include the femoral head and neck, acetabulum, ileofemoral ligament, and the anterior synovial recess. It is important to locate femoral vessels and position the probe lateral to the femoral vessels.
The sonographic criterion for a hip effusion in the adult patient is an anechoic fluid stripe greater than 5 mm located under the hyperechoic iliofemoral ligament, extending along the entire length of the femoral neck, or greater than 2 mm of asymmetry compared with the contralateral hip. 
Arthrocentesis is a sterile procedure, and the hip should be prepared and draped for needle aspiration. Following the technique described above, the femoral head should be visualized on the ultrasound screen. A sterile 18- to 22-gauge spinal needle attached to a 20-mL syringe can be used to aspirate the effusion under direct visualization.
To achieve adequate ultrasound images, remember to apply plenty of acoustic gel and pressure with the transducer. The 2 most common reasons for obtaining poor-quality images are too little gel and too little pressure.
The gain adjusts the amplification of the returning signal. Adjust the gain as needed to make black (anechoic) objects appear black on the screen. For example, if a urine-filled bladder appears gray (hypoechoic), decrease the gain to make it black (anechoic). If bone appears gray (hypoechoic), increase the gain to make it white (hyperechoic). By adjusting gain consistently, all tissues are displayed and recognized according to their relative echogenicity.
Anisotropy is a well-described phenomenon in medical ultrasound imaging; it describes any property that is directionally-dependent. Many tissues, such as tendons, have a lot of anisotropy, which means that their echogenicity is dependent on the angle of the ultrasound probe. To achieve the best possible images, hold the probe at 90 degrees to the tissue or structure of interest. The angle of insonation should be 90 degrees for B-mode images.
Allergic reactions from hypersensitivity to local anesthetics can occur. Frequently, this is due to the preservative and not to the anesthetic itself. In such cases, using a preservative-free anesthetic prevents this problem. For more details, see Medscape Reference article Infiltrative Administration of Local Anesthetic Agents.
Local bruising at the injection or arthrocentesis site may occur.
Damage to cartilage, tendons, or blood vessels can occur with improper needle placement. This can be avoided by reviewing the anatomy prior to the procedure and by using ultrasonographic guidance to help identify and avoid these structures.
Generally, hemarthroses are small and require no specific treatment. However, a hemarthrosis in a coagulopathic patient requires correction of the coagulopathy. Hematology consultation should be considered. Hemarthrosis is pictured in the image and videos below.