Emergency and acute care physicians are commonly called upon to evaluate patients with a painful or swollen joint where definitive diagnosis will depend upon joint fluid being obtained for analysis or therapeutic reasons. While many physicians feel comfortable tapping a knee effusion, the comfort level for performing arthrocentesis in other joints is often much lower. On occasion, the provider's discomfort with the procedure may stem from the complete absence of recognizable landmarks due to the patient's morbid obesity. Whether because of lack of familiarity with the regional anatomy or unfamiliarity with the landmarks or techniques traditionally relied upon for the procedure, arthrocentesis of less commonly tapped joints such as the elbow, ankle, shoulder, hip, acromioclavicular, or metatarsophalangeal joints are often referred to rheumatologists, interventional radiologists, or orthopedists. The growing use of ultrasound for procedural applications is likely to change this historical pattern. Arthrocentesis of all these joints can easily be added to the armamentarium of procedures performed by the emergency physician if certain basic procedural principles are applied and a few basic anatomic and sonographic pointers for each area are noted.
Joints in the human body, while built on similar principles, vary in their individual design and size. Sampling fluid from each requires knowledge of the particular anatomy of the specific joint. In the following section, each joint will have a brief discussion of anatomy specific to the individual application.
Technique and Ultrasound Findings
Most ultrasound machines now purchased for use in acute, emergency, and critical care settings will be capable of supporting diagnostic presets and the transducers required for arthrocentesis. A 7.5–10 MHz linear array transducer will be used for most joints except the hip, where a curvilinear 5 MHz or lower frequency transducer may be utilized. Doppler evaluation should be performed to identify any vascular structures surrounding the effusion or along the planned aspiration approach in order that they may be avoided. Image depth should be adjusted appropriately for the joint in question (the 80/20 rule applies: 80% of the image should focus on the anatomic area of interest, with 20% left over in the far field so as to see adjacent anatomy) and image depth should be set to the level of the bony cortex of the relevant joints being imaged.
For precise localization, the joint fluid collection should be mapped in two orthogonal planes and the skin marked with indelible ink on either side of the transducer. Connecting the lines will create a target with the aspiration site in the center of the “+”; note should be taken of the optimal aspiration angle. The depth from the skin surface to the deepest portion of the effusion should be noted so that an appropriate length needle can be chosen for the procedure. Most effusions will be mapped and marked and then aspirated using a freehand technique. If real-time guidance is used, such marking will not be required, but the site should be prepared in a sterile manner and ultrasound transducer properly draped. Guiding the needle to a joint effusion in real time is little different from vascular access or abscess drainage techniques.
Graded compression and power Doppler can help distinguish synovial fluid from synovial proliferation. A color flow signal may be noted at the border between synovium and where joint fluid is moving because of compression caused by the transducer. Freely mobile loose bodies will occasionally be seen within a joint effusion and will typically reveal themselves by virtue of their hyperechoic acoustic profile and accompanying posterior acoustic shadowing. They are typically found in the suprapatellar bursa of the knee or in the elbow joint and can be seen to move with either gentle palpation of the effusion or pressure on the transducer.
The suprapatellar bursa is a large synovium lined pouch that is really an extension of the joint space of the knee. It is located at the anterior distal femur extending about a hand breadth above the adult knee joint, bounded superiorly by skin, subcutaneous tissue, and quadriceps tendon and inferiorly by prefemoral fat and the femur. When the knee joint is distended with fluid, the deepest collections will be found where the suprapatellar bursa bulges out on either side of the quadriceps tendon, particularly at the lateral suprapatellar recess. The suprapatellar bursa is the largest bursa in the body and can distend to accommodate a large volume of fluid. Numerous other bursae also surround the knee joint. Two that are of clinical relevance are the gastocnemiosemimembranosis bursa (found in the medial popliteal fossa), and the subcutaneous prepatellar bursa. The former often communicates with the knee joint, and when distended can cause pain and swelling in the popliteal fossa and is known as a Baker's cyst. The subcutaneous prepatellar bursa lies immediately below the skin over the lower patella, does not communicate with the knee joint, and may become swollen and infected due to local trauma.
The normal knee joint will have little or no fluid visible within the joint space. The highest yield sites for detection of an effusion will be in the lateral and medial recesses of the suprapatellar bursa. A simple effusion appears as a hypoechoic fluid collection separated from the brightly echogenic femoral cortex by a thin layer of hyperechoic prefemoral fat (Figure 20-16). With a more chronic process, inflammatory synovial changes (pannus) may be appreciated and will appear as a thickening of the synovium or lobulations within the joint space (Figure 20-17). An effusion due to intraarticular hemorrhage may initially appear echo-free but later exhibit a homogenous mid-level gray echotexture consistent with clotted or partially clotted blood (Figure 20-18). A knee joint effusion should not be confused with other fluid collections that can be found around this joint. A prepatellar bursitis appears as a hypoechoic fluid collection that is seen in the subcutaneous tissue anterior to the patella and proximal patellar ligament and is often surrounded by hyperechoic edematous soft tissue typical of cellulitis (Figure 20-19). A Baker's cyst communicates with the knee joint and classically appears as a hypoechoic stomach-shaped fluid collection in the medial posterior fossa of the knee (Figure 20-20).
Transverse sonogram of a knee effusion at the lateral suprapatellar recess. Hyperechoic prefemoral fat is seen just below the hypoechoic effusion.
Transverse sonogram of a more chronic knee effusion with thickened, lobulated appearing synovium.
Midline sagittal sonogram of a knee hemarthrosis. The curved shadow of the patella is seen to the right of the image and the bright echo of the anterior femoral cortex below. Just above the prefemoral fat there is a homogenous mid-grey echotexture layer (arrow) consistent with hematoma.
Sagittal midline sonogram of a prepatellar bursitis. Note the thickened hyperechoic skin, some debris within the fluid-filled prepatellar bursa, the shadow from the echogenic patella on the left inferior border of the image, and a portion of the fibrillar patellar ligament.
Transverse sonogram at the medial popliteal fossa demonstrating the characteristic stomach-shaped fluid collection of a Baker's cyst.
The knee is optimally scanned with the patient supine and the slightly flexed knee supported from behind with a sheet or towel for patient comfort. Scanning is most easily performed in a paramedian longitudinal plane just above the patella. Small volumes of fluid will first be seen in the lateral or medial recesses of the suprapatellar bursa. Longitudinal and transverse scanning of the fluid collection is followed by marking of the skin with indelible ink to form a “+” designating the site for aspiration (Figure 20-21). Depth from the skin surface to the effusion and the optimal aspiration angle should be noted. From this point the procedure does not differ from blind aspiration unless real-time guidance is chosen by the sonologist. In this case, the transducer should be dressed in a sterile sheath, and sterile coupling gel should be utilized. An approach allowing for long-axis visualization of the needle is best and is identical to other needle guidance techniques.
Knee aspiration technique at the lateral suprapatellar recess: two-person technique where pressure is applied to the contralateral recess for maximal joint cavity distension. Sterile drape omitted for purposes of illustration.
Sonographic evaluation of the hip joint can be successfully accomplished with a variety of transducers, ranging from a 3–5 MHz curved array or sector transducer to a 7.5–10.0 MHz linear array transducer. With the transducer aligned along the long axis of the femoral neck (orientation marker pointing toward the umbilicus), the normal hip will appear as a brightly curved line about 3–6 cm below the skin surface, convex along the surface of the femoral head on the left side of the image, then gently concave along the femoral neck to the right. The area just anterior to the femoral neck is termed the anterior synovial recess and represents the potential space between the femoral neck and the joint capsule where a hip effusion will preferentially collect (Figure 20-22). A thin line of hypoechogenicity may be noted adjacent to the cortex of the femoral head; this line corresponds to the articular cartilage. The acetabular labrum will often be seen as an echogenic area to the immediate left of and slightly superior to the femoral head. The joint capsule is of variable echogenicity, sometimes difficult to identify, other times clearly visible as an echogenic layer 3–8 mm in thickness extending from the acetabular labrum to the base of the femoral neck (Figure 20-23). The joint capsule is usually readily identified when an effusion is present. The commonly accepted sonographic criteria for defining a hip effusion in a native hip include (1) a convex bulging joint capsule with a fluid stripe greater than 5–6 mm; or (2) when compared to the asymptomatic joint, a greater than 2 mm increase in the distance from the cortical echo to the joint capsule. A perpendicular measurement of the effusion is taken at its widest anteroposterior dimension between the surface of the femoral neck and the inner surface of the joint capsule Comparison with the contralateral hip should be routine (Figure 20-24).
Ventral oblique sonogram of the normal hip. The prominent curve of the femoral head is seen on the left of the image and leads to a concave region along the femoral neck where an effusion will preferentially collect.
Ventral oblique sonogram of the hip. The joint capsule is seen as a hyperechoic, horizontally oriented layer extending from the acetabular labrum to the femoral neck. A small amount of joint fluid is noted below the joint capsule.
Ventral oblique sonogram of a hip effusion in a patient with reactive arthritis. The anterior synovial recess is distended with fluid and the capsule is seen to bulge anteriorly. The hyperechoic echo from the aspirating needle is seen in the upper right of the image.
In the prosthetic hip the sonographic landmarks will obviously be different. Transducer alignment should still be along the long axis of the now prosthetic femoral neck aiming toward the umbilicus. A series of four horizontally oriented hyperechoic lines will be noted. First, a short segment to the left of the image may be seen that corresponds to the acetabular component of the prosthesis. Adjacent to this line is a somewhat wider, more superficially located horizontal line that represents the head of the prosthesis. A prominent metallic reverberation or ring-down artifact will be seen behind the prosthesis during real-time scanning. Next, and to the immediate right, a longer and somewhat deeper echogenic horizontal line will be seen. This line corresponds to the neck of the hip prosthesis and a metallic ring-down artifact will be noted here as well. Finally, a bright and somewhat thicker echo will be noted to the right of the image. This echogenic line is located a few millimeters more superficially than the echo from the prosthetic femoral neck and represents the anterior surface of the most proximal portion of the remaining native femur where the prosthesis has been inserted. It is typical to see a small amount of hypoechoic fluid surrounding the neck of the prosthesis. As noted above, no joint capsule will be seen since it will have been removed during hip replacement surgery (Figure 20-25). A perpendicular measurement is taken of the width of the fluid collection located between the superior surface of the most proximal edge of the remaining native femoral cortex and the edge of the pseudocapsule above. A fluid collection with a width of greater than 3.2 mm at this location is considered to be abnormal (Figure 20-26).
Ventral oblique sonogram of a normal prosthetic hip. Four echogenic lines are noted. From left to right: a short segment that corresponds to the acetabular component of the prosthesis; next, a wider more superficially located line that corresponds to the femoral head (a prominent metallic reverberation is noted below it); next, the long prosthetic femoral neck; and finally, a somewhat more anterior and more echogenic line that corresponds to the most proximal portion of the remaining native femur. A small amount of fluid is normally seen anterior to the prosthetic femoral neck (arrow).
Ventral oblique sonogram of a septic prosthetic hip. A reverberation artifact is seen emanating from the prosthetic femoral head. A large fluid collection is seen anterior to the prosthetic femoral neck. A 5-mm fluid collection was measured between the most proximal native femur and the pseudocapsule above (arrow) (>3.2 mm is considered abnormal).
In children, the sonogram of the hip appears somewhat different from that of the adult. The growth plate of the femoral capital epiphysis produces a curved notch in the convexity of the femoral head and, depending on the degree of ossification, a linear lucency in the anterior head of the femur. The hypoechoic region anterior to the notch represents the cartilaginous acetabulum and should not be mistaken for an effusion (Figure 20-27).
Ventral oblique sonogram of a pediatric hip. A notch is seen in the femoral head that corresponds to the growth plate of the femoral capital epiphysis. The hypoechoic area adjacent to this notch corresponds with the still cartilaginous acetabulum and should not be mistaken for an effusion.
If a decision is made to aspirate the collection, either a mapping technique and a subsequent freehand aspiration or an aspiration under real-time guidance may be employed. With the ultrasound marking approach a line over the femur and best point of entry are marked and from there the procedure is a blind one like a typical hip aspiration. If a real-time aspiration technique is employed, a sterile sheath over the probe and sterile gel will be utilized. The transducer should be aligned along the long axis of the femoral neck as described previously and note should again be made of the location of the femoral vessels in order that they may be avoided by the aspirating needle. Once the effusion is identified and its most bulging portion centered in the monitor image the needle is inserted at one end of the probe and guided in long axis to the fluid collection just as in other aspiration and vascular access procedures (Figure 20-28).
Ultrasound-guided hip aspiration technique. The needle is advanced in line within the long-axis scan plane of the transducer and its characteristic reverberation artifact is used to guide the needle tip into the effusion. For purposes of illustration, the sterile drape and probe cover are not shown.
Scanning of the ankle is usually performed with a 5–10 MHz linear array transducer. In a sagittal scan plane over the distal tibia, a brightly echogenic horizontal line that corresponds to the anterior tibial cortex will be noted about 1 cm below the skin surface. As the transducer is moved further distally, a V-shaped recess will appear to the right of the image, formed by the distal tibia on the left and the dome of the talus on the right. This location is the region where the anterior synovial recess of the ankle joint is found and is normally filled by an anterior intracapsular fat pad (Figure 20-29). A small amount of echo-free fluid may be seen at the base of this recess and a collection of <3 mm in anteroposterior height is considered normal. In sagittal midline orientation, an ankle effusion will typically appear as a prominent triangular area of sonolucency that fills this V-shaped recess. The fibrous joint capsule is sometimes seen as a distinct and somewhat echogenic structure lying horizontally just anterior to the upper border of the effusion (Figure 20-30). More medially, the effusion will often take on a more rectangular configuration (Figure 20-31). On occasion, there may be significant associated soft tissue swelling of the overlying skin accompanying the inflammatory process within the joint. On a transverse image through the joint line, the anterior tibial/dorsalis pedis artery will be located somewhat medial to the midline and appear as a hypoechoic circle just above the hypoechoic effusion (Figure 20-32).
Sagittal midline sonogram of a normal ankle joint. The V-shaped recess is formed by the distal tibia on left and the talar dome on the right, and is filled by the anterior intracapsular fat pad. No fluid is seen in this example.
Sagittal midline sonogram of an ankle effusion. The joint capsule is seen as somewhat echogenic structure just above the hypoechoic effusion. The cortical echoes from the distal tibia and talar dome outline the posterior surface of the triangular effusion.
Sagittal medial paramedian sonogram of an ankle effusion. The ankle effusion in this location appears more rectangular in configuration.
Transverse sonogram of an ankle effusion. The location of the hypoechoic anterior tibial/dorsalis pedis artery (arrow) should be marked on skin with a “Ø” so that it may by avoided during the aspiration. The deep peroneal nerve is located just medial to the artery.
Once a fluid collection has been identified in long-axis orientation, the transducer should be positioned so that the deepest portion of this V-shaped recess is located in the exact center of the image. A horizontal line can then be drawn on the skin with an indelible marker on either side of the transducer at the site that corresponds to the deepest portion of the effusion. Next, a transverse view should be obtained and the location of the anterior tibial/dorsalis pedis artery should be marked on the skin with an “Ø” so that it may be avoided during the aspiration. Note should be taken of the angulation of the transducer, if any, as well as the degree of ankle plantar flexion and this information used to aim the needle in the appropriate direction during the aspiration. The patient is then sterilely prepped and draped, anesthetized, and the aspiration performed with a freehand technique (Figure 20-33). If a real-time technique is desired, the largest subjective image of the effusion is obtained in a transverse orientation with simultaneous visualization of the artery. The needle can be inserted under direct visualization somewhat lateral to the artery using a superior or inferior approach. This technique requires additional preparation and a sterile probe cover, however, and is usually not required.
Freehand ankle aspiration technique. The location of both the deepest portion of the anterior recess and the anterior tibial/dorsalis pedis artery has been marked on the skin. Needle entry is laterally to the artery. When fluid is obtained, the needle can be held fixed in place with one hand while the syringe aspirates the effusion with the other. Sterile drape omitted for purposes of illustration.
Ideally, the patient should be seated on a stretcher with the elbow held in 90 degrees of flexion and the forearm resting in neutral position a folded towel. In transverse orientation at the level of the medial and lateral epicondyles, the posterior echogenic surface of the humerus flattens (Figure 20-34) and then, somewhat more distally, forms a centrally located echogenic “U”-shaped depression that corresponds to the olecranon fossa (Figure 20-35). This space is normally filled with a posterior fat pad that exhibits a mid-level gray echotexture with some areas of increased internal echogenicity. In a longitudinal midline orientation over the distal elbow, the echogenic posterior surface of the humerus will be seen to the left of the image, the olecranon fossa and the posterior fat pad will appear in a “V”-shaped recess in the center, and the echogenic posterior surface of the olecranon will be located more superficially on the right side of the image. The triceps tendon may be apparent just below the skin as a horizontal, somewhat hyperechoic structure with a fibrillar echotexture. The distal portion of the triceps muscle appears as a hypoechoic structure just beneath the tendon, superficial to the posterior fat pad (Figure 20-36A). In a longitudinal posterior midline orientation, an elbow effusion will appear as an anechoic fluid collection that pushes the posterior fat pad superiorly (to the left of the image) and distends the joint capsule posteriorly (Figure 20-36B). An effusion may appear hypoechoic or exhibit complex echogenicity depending on etiology (Figure 20-37).
Transverse sonogram of posterior humerus at the level of the medial and lateral epicondyles. The brightly echogenic posterior humerus transitions from its rounded profile above to the flat profile seen here. As the transducer is moved a bit further distally, a U-shaped depression will be seen in the humerus that corresponds with the olecranon fossa.
Transverse sonogram of the normal posterior elbow at the level of the olecranon fossa (also known as the posterior recess). The U-shaped depression in the humerus corresponds with the olecranon fossa and is filled by the posterior fat pad.
Sagittal sonogram of the normal posterior elbow (A). Humerus to the left, olecranon process to the right. The olecranon fossa is V-shaped in this orientation and the somewhat echogenic posterior fat pad is seen filling the recess (arrow). The fibrillar triceps tendon is seen just below the skin and it inserts on the echogenic olecranon on the right of the image. The hypoechoic region below the tendon is a portion of the triceps muscle. Sagittal sonogram of the posterior elbow in a patient with an anechoic effusion (B). The echogenic fat pad has been pushed superiorly and the joint capsule bulges posteriorly.
Transverse sonogram of the posterior elbow of a patient on warfarin with atraumatic elbow pain and an elevated INR. The olecranon fossa is filled with clotted blood. The bony outline of the fossa is brighter than usual because of posterior acoustic enhancement.
If fluid is present, the horizontally aligned transducer should be positioned such that the largest subjective image of the fluid collection is centered on the monitor image. The skin should then be marked with indelible ink at both ends of the transducer and the ends connected with a horizontal line. Alternatively, the line may be constructed by scanning in a midline sagittal position (orientation marker facing up) and the deepest portion of the recess marked on either side of the transducer. This line will determine the optimum vertical location for the aspiration. Needle insertion should always be 1–2 cm lateral to the midline in order to remain remote from the medially located ulnar nerve and avoid the centrally located triceps tendon. The aspirating needle should be aimed toward the midline, however, in order to access the deepest portion of the always centrally located posterior recess. The skin is prepped, draped, and anesthetized as usual and the aspiration performed freehand (Figure 20-38). If a real-time technique is preferred, the needle tip should be inserted in an out-of-plane approach from above the transversely oriented transducer.
Elbow arthrocentesis technique—posterior approach. The effusion has been mapped and marked. Needle insertion is lateral to the midline to avoid the triceps tendon and to stay well remote from the ulnar nerve. The needle should be medially angulated so that it will reach the deepest portion of the centrally located recess.
On the transverse sonogram of the anterior shoulder at the level of the coracoid process, an echogenic layer of skin and subcutaneous tissue will be seen overlying a thicker, hypoechoic, and horizontally striated layer that corresponds to the anterior portion of the deltoid muscle. Deep to the deltoid muscle lie two distinct, brightly echogenic lines that represent the medial humeral head (seen as a large, smoothly curved echo in the lateral portion of the image), and the anterior surface of the coracoid process (seen as a somewhat flatter, anterior echo medially). The hypoechoic rim surrounding the humeral head represents cartilage and should not be mistaken for a layer of fluid. The coracoid process is distinctive by virtue of the dense posterior acoustic shadow that it casts (Figure 20-39). If the profile of the medial humerus appears flat (Figure 20-40), the arm should be slightly externally rotated to obtain the more desirable rounded profile. The middle of the space between these two bony echoes will identify the vertical line where the effusion will be tapped, usually several centimeters inferior to the level of the coracoid. An effusion will appear as a hypoechoic collection that extends past the midline into the axillary recess of the joint capsule.
Transverse sonogram of the right anterior shoulder. The deltoid muscle is seen as a thin hypoechoic layer just below the skin. The curved medial humeral head appears on the left and the coracoid process with its pronounced posterior acoustic shadow on the right. The midline between these two structures represents the sagittal plane where the aspiration should occur (several centimeters below the level of the coracoid process, however).
Transverse sonogram of the right anterior shoulder. The deltoid muscle appears more prominent in this example, and the coracoid process on the right is brightly echogenic. The profile of the medial humerus appears flat because the arm is too internally rotated. With slight external rotation, the more desirable curved profile of the humeral head will be obtained.
On the transverse sonogram of the posterior shoulder just below the posterior angle of the acromion, the echogenic layer of skin and subcutaneous tissue will be seen overlying a substantially thicker hypoechoic layer that represents the posterior portion of the deltoid muscle. Deep to the deltoid, the triangular or beak-shaped infraspinatus muscle (hypoechoic) and tendon (hyper- or hypoechoic, depending on the angle of insonation) will be seen pointing laterally over the curved echogenic line that corresponds to the humeral head (Figure 20-41). A thin hypoechoic rim that corresponds to articular cartilage may be noted adjacent to the hyperechoic head of the humerus. Medial to the humeral head, two additional lines will be noted: a slightly more superficial echogenic line corresponding to the dorsal glenoid rim, and a somewhat deeper horizontal echogenic line corresponding to the posterior surface of the scapula (Figure 20-42). An effusion will appear as an anechoic region in the groove between the humeral head and the dorsal glenoid rim. Finally, a subacromial (or subdeltoid) effusion or bursitis will appear as a hypoechoic collection immediately beneath the deltoid muscle but superficial to the supraspinatus tendon (Figure 20-43). The effusion will best be seen from an anterior transverse approach at the level of the coracoid process (similar to the scan technique employed when assessing the biceps tendon for bursitis). The fluid collection seen with a subacromial (or subdeltoid) bursitis will not extend significantly inferior to the coracoid process; this characteristic can help distinguish it from an intra-articular effusion on an anterior view of the shoulder.
Transverse sonogram of the right posterior shoulder. The deltoid muscle is seen below the skin in the near field as a thick hypoechoic layer. The triangular or beak-shaped infraspinatus muscle (hypoechoic) and tendon (hyperechoic) are seen pointing to the right over the curved echogenic humeral head. The glenoid rim is seen as an indistinct echogenic line medial to the humeral head.
Transverse sonogram of the right posterior shoulder. In this example, the infraspinatus tendon is noted to be brightly echogenic on the left of the image but hypoechoic on the right; this is due to tendon anisotropy. With slight movement of the transducer, the entire tendon could be visualized. The glenoid rim is seen as a more distinct curved structure just medial to the larger curve of the head of the humerus. The site for the aspiration will be between the glenoid rim and the medial humeral head (arrow). The flatter echo to the left of the glenoid emanates from the surface of the scapula.
Transverse sonogram of the right anterior shoulder in a patient with a chronic subacromial (subdeltoid) bursitis. A large, complex hypoechoic collection is seen immediately beneath the deltoid muscle. The bursal fluid contains some echogenic debris within it and the synovium appears thickened and lobulated below. The echogenic anterior surface of the proximal humerus is seen beneath the effusion with the hypoechoic rounded biceps tendon resting within the bicipital groove (arrow). The bursal sac is seen to extend somewhat medial to the proximal humerus on the right side of the image.
Shoulder arthrocentesis is typically performed from either an anterior or a posterior approach and can be ultrasound assisted or ultrasound guided. For the seated or supine patient, an anterior approach may be used (Figure 20-44). The arm is ideally extended in slight abduction with the palm facing up. A 7.5 MHz linear array transducer is placed in transverse orientation at the level of the coracoid process. The “V”-shaped recess seen between the medially located coracoid process and the laterally located medial head of the humerus is aligned in the center of the image. The skin on both sides of the transducer should be marked with an indelible marker at the precise location of the base of this recess and a vertical line drawn along this axis. The optimal site for aspiration will be on this line, but several centimeters inferior to the level of the coracoid process. If performed blindly, the needle should be advanced perpendicular to the skin surface.
Shoulder arthrocentesis technique—anterior approach. The space midway between with coracoid process and the medial humeral head has been mapped and marked with a vertical line. The aspiration should occur perpendicular to the skin several centimeters below the level of the coracoid (at the horizontal line) and the needle should always remain lateral to the coracoid process.
For the posterior approach, the patient should be sitting with the elbow flexed and with the forearm resting at the side in neutral position (Figure 20-45). The transducer is placed in transverse orientation approximately 2–3 cm inferior and 1–2 cm medial to the bulge of the posterior acromion. The optimal image of the joint space can usually be obtained by tilting the lateral edge of the transducer slightly inferiorly from the horizontal plane. Beneath skin, subcutaneous tissue and deltoid muscle, the triangular or beak-shaped infraspinatus muscle and tendon will be seen overlying the echogenic curve of the medial humeral head. Slightly deeper and just medial to the humerus, a less distinct echo will be seen that corresponds to the dorsal glenoid labrum. An effusion will appear as an anechoic or hypoechoic collection adjacent to the curved head of the humerus, filling the groove between it and the medially located glenoid labrum.
Shoulder arthrocentesis technique—ultrasound-guided posterior approach. The effusion is mapped and marked several centimeters inferior and medial to the bulge of the posterior angle of the acromion. The aspiration needle is guided to the space between the glenoid rim and the medial humeral head under ultrasound guidance. Sterile drape and probe cover are omitted for purposes of illustration.
The needle entry site should be from the lateral edge of the transducer in the plane of the ultrasound beam. The transducer is covered with a sterile sheath, and after a sterile prep, drape, and local anesthesia, the needle should be aimed toward the fluid collection in the groove between the medial humeral head and the glenoid rim. The aspiration path is fairly horizontal; this will enhance visualization of the metallic needle and help guide precise placement of the needle tip. The joint capsule should be punctured along the medial border of the humeral head, slightly lateral to the glenohumeral joint so as to avoid contact with the circumflex scapular vessels and the suprascapular nerve that are located medial to the glenoid rim.
Patients will occasionally present with joint pain that is ultimately discovered to be due to a pathologic process affecting the acromioclavicular, sternoclavicular, or first metatarsophalangeal joints. Ultrasound allows for precise mapping of these superficially located joints. The acromioclavicular and sternoclavicular joints are located on the lateral and medial end of the clavicle, respectively, and contain an articular disc that separates and cushions the clavicle from the abutting acromion or sternal articular surfaces. The fibrous capsule of the joint inserts on the bone immediately adjacent to the articular surfaces. The capsule of the first metatarsophalangeal joint surrounds the joint, extending from the nonarticular bony surfaces of the distal metatarsal bone to the proximal portion of the proximal phalanx.
The acromioclavicular and sternoclavicular joints are most easily scanned by first placing the transducer sagittally over the upper chest in order to identify the bright superficial echo from the clavicle (Figure 20-46). The transducer is then slid either medially or laterally, and at the joint of interest, is rotated along the long axis of the clavicle spanning the relevant joint space. The hypoechoic V-shaped recess that corresponds to the joint space is marked on either side of the transducer, and then marked again with the transducer turned orthogonally. The two lines are connected and the center of the “+” marks the location for the aspiration. The first metatarsophalangeal joint is best scanned axially over the joint space and a similar mapping technique is utilized. Alternatively, the acoustic shadow of a paper clip can be used here to map the desired puncture site. After the usual sterile prep, drape, and local anesthesia, the mapped areas are vertically punctured at the mapped sites and aspirated.
Longitudinal sonogram across the right acromioclavicular joint in a patient with gout. The hypoechoic V-shaped recess between the acromion on the left and the clavicle on the right represents the site where an aspiration or steroid injection would be directed.