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Joint aspiration: The dry tap

Joint aspiration: The dry tap
Literature review current through: Jan 2024.
This topic last updated: Aug 04, 2022.

INTRODUCTION — In occasional patients, a joint that has suspected fluid on physical or radiographic examination yields a "dry tap" during diagnostic arthrocentesis, even after the needle has been redirected and is believed to be in the joint space. This problem comes up with surprising frequency with respect to the knee and also occurs with joints that are more difficult to tap, such as the ankle and shoulder. The proper approach to a dry tap in an individual patient has to take into account, above all, the degree of suspicion of a septic joint. When joint fluid is necessary for diagnosis, consideration should be given to the potential causes of a dry tap as well as to the need for advanced imaging.

The causes of and approach to the patient with a dry arthrocentesis will be reviewed here. Separate topic reviews related to joint aspiration in adults include the following:

(See "Joint aspiration or injection in adults: Technique and indications".)

(See "Joint aspiration or injection in adults: Complications".)

(See "Synovial fluid analysis".)

CAUSES OF A DRY TAP — Tapping a joint with an obvious large effusion in which the synovium bulges out in a mass-like fashion is generally not problematic. However, a person with obesity and poorly defined bony landmarks, altered anatomy, and cases of small/trace effusions may present technical problems. During arthrocentesis, a clinician experienced in the procedure may be certain of intraarticular needle placement when the needle is felt to smoothly glide deep to the superficial surface of the joint, between the cartilaginous surfaces. Technical failure in arthrocentesis may also occur as a result of altered joint anatomy due to prior trauma, surgery, or in cases of chronic deforming arthritis.

After excluding technical failure to enter the joint, there are three major explanations for a dry tap (table 1):

Mistaken physical diagnosis (ie, no effusion present)

Blockage of the bevel of the needle by plica, fat, or debris

Very high viscosity fluid or true lipoma arborescens

Dry tap in the knee — The relative frequency of the different possible explanations of failed arthrocentesis remains conjectural. Ultrasonography (US) can often solve the practical problem of a dry tap if multiple unguided attempts fail. In addition, the advent of more sensitive imaging techniques, especially magnetic resonance imaging (MRI) of the knee, has permitted many of the explanations for failed arthrocentesis to be vividly illustrated. A dry tap may occur in 10 percent of attempts in osteoarthritis, with the rate rising in more inflammatory conditions that have become chronic [1]. These dry taps can often be recouped with ultrasound guidance on a second attempt.

Probably the most common cause of failed arthrocentesis occurs when the examiner is mistaken and no joint effusion is present (image 1). Even the experienced clinician can make this mistake, particularly in a person with obesity. However, a similar problem can occur in patients of average weight. The accuracy of needle placement can be improved with ultrasound guidance, particularly of the more difficult to aspirate medium-sized joints (ie, wrist, ankle, or elbow). A small trial comparing the use of ultrasound guidance with landmark-guided aspirations of suspected medium-sized joint effusions found that the use of an ultrasound-guided approach for all patients would have avoided unnecessary aspirations in approximately 30 percent of patients who ultimately did not have an effusion [2]. (See "Musculoskeletal ultrasonography: Guided injection and aspiration of joints and related structures", section on 'Accuracy of needle placement'.)

Nearly all subjects have a variably sized triangular fat pad at the medial aspect of the patellofemoral compartment, which is the most common site to attempt arthrocentesis of the knee. Placing the needle in this location may yield no fluid if the fat pad is thick (image 2). The medial fat pad is the strongest individual reason for a switch from the medial to the lateral approach for a knee tap. It should be noted that the lateral approach, while favored by many clinicians, may be technically more challenging in the setting of significant patellofemoral osteoarthritis. Other approaches, such as medial or lateral infrapatellar, may also result in failure due to needle placement in Hoffa's infrapatellar fat pad. The approach least likely to yield fluid is the anterior approach on a flexed knee at 90 degrees, as is sometimes used for a patient who is a wheelchair user [1,3]. The yield in this situation can be improved by having a second operator simultaneously compress the effusion [4].

Chronically inflamed synovium may undergo subsynovial fat proliferation over a period of time and may become markedly thickened, much more so than is indicated by examination of the surface anatomy (image 3). This condition, when severe, is referred to as lipoma arborescens [5-7]. Placing the needle tip into this thickened synovium is another cause of dry tap.

A commonly invoked, but rarely present, explanation for failed arthrocentesis in the knee is a thickened medial plica (picture 1), which behaves as a dam or valve, obstructing the lumen of the needle (image 4) [8]. The medial patella plica is very common, representing a failure of complete involution of embryologic tissue separation of the three knee joint compartments [9] (medial, lateral, and patellofemoral). Symptomatic inflammation of the plica is rare, as is "dry tap" due to a medial plica. Lateral plicae are very rare.

Fluid can also become inaccessible by one or more routes when joints have only a low volume of fluid, or when the bevel becomes blocked by debris such as rice bodies or thick fibrin [10].

Highly viscous synovial fluid can be difficult or impossible to remove by smaller gauge needles, and increasing to a 20 or 18 gauge needle may be required. In some cases, the distended joint consists of areas of free fluid along with other areas occupied by semisolid gelatinous material too viscous to be withdrawn (image 5) [5]. Failure to aspirate fluid from a popliteal (Baker's) cyst, for example, has sometimes been attributed to inspissation of joint fluid into a gelatinous mass that has fluid-like characteristics on physical examination but that is too viscous to be withdrawn even through a large gauge needle. Such inspissated "pseudoeffusions" are sometimes confirmed at surgery [11].

Prior trauma or surgery may result in altered joint anatomy or abnormal compartmentalization of joint fluid, rendering standard arthrocentesis difficult. Similarly, anatomic changes due to a chronic deforming arthritis may make arthrocentesis technically challenging.

Some patients have more than one cause for a dry tap, as with lipoma arborescens and a gelatinous effusion (image 6).

Dry tap in other joints — Other joints not as well studied by MRI retain their complexity in the "dry tap" situation. In particular, the several normal anatomic compartments of the shoulder often require more than one attempt by different routes, US, or even arthroscopy.

A dry tap in a patient with apparent bilateral ankle effusions may be due to a periarthritis without true effusion. This can occur in sarcoidosis and gout. In the latter disorder, a single drop of interstitial fluid expressed from needle puncture wound of an unsuccessful arthrocentesis can be applied to a slide for examination under polarized light.

Unilateral ankle effusions can be crystal-induced "cluster attacks" in which several anatomically adjacent joints are involved. These latter attacks can sometimes be made to yield fluid by switching from the true ankle to the subtalar joint, one finger's breadth below the prominence of the lateral malleolus (figure 1).

APPROACH TO THE PATIENT WITH A DRY TAP — After a failed arthrocentesis, we generally proceed with some form of diagnostic imaging to confirm the presence of intraarticular fluid. Depending on the imaging modality, it can also be used to guide the joint aspiration.

Radiography is useful in confirming a distended joint in the knee and elbow, in particular. However, simple radiographs cannot distinguish simple or complex fluid from thickened synovium.

The authors prefer ultrasonography (US), as this is an effective tool for both detecting a joint effusion and facilitating needle placement for arthrocentesis. While increasing, the availability of this technique for use in the clinical setting varies widely due to potential costs of equipment, regulatory and reimbursement policies, and differences in experience and training between musculoskeletal clinicians. US can be used for direct guidance of needle insertion, with concurrent ultrasound visualization of the area for injection, or for indirect guidance, in which the target region is visualized for the purpose of marking the needle insertion site and of estimating the depth and direction of needle placement.

The use of ultrasound guidance in rheumatology practice continues to evolve and is discussed in detail separately. (See "Musculoskeletal ultrasonography: Nomenclature, technical considerations, and basic principles of use" and "Musculoskeletal ultrasonography: Clinical applications" and "Musculoskeletal ultrasonography: Guided injection and aspiration of joints and related structures".)

Other imaging modalities, including fluoroscopy, single energy computed tomography (CT), and MRI, can also sometimes be used to confirm the presence of a joint effusion (see "Imaging techniques for evaluation of the painful joint"). Additional clinical considerations regarding specific joints are described below.

In terms of imaging guidance, the suspected acute septic prosthetic joint merits special attention. An acute septic prosthetic joint is an orthopedic emergency, requiring immediate accurate diagnostic tap. If standard non-imaging-guided attempts fail to yield fluid, the authors prefer fluoroscopic guidance as the technique of choice. Fluoroscopy allows direct visualization of key landmarks, and the concurrent injection of contrast material absolutely confirms successful intraarticular needle placement. Culture yield can potentially be improved by careful intraarticular injection of sterile, preservative free saline followed by reaspiration (lavage) of a suspect prosthetic joint that remains a dry tap even with fluoroscopic guidance [12]. The utility of joint lavage in the setting of suspected septic total joints is controversial, with possible increased sensitivity, but decreased specificity (higher false positives) [13]. The diagnosis of prosthetic joint infection is discussed in detail separately. (See "Prosthetic joint infection: Epidemiology, microbiology, clinical manifestations, and diagnosis", section on 'Diagnosis'.)

Knee effusions — There are four important teaching points for the approach to dry taps of knee effusions [5]:

Unsuccessful aspiration from the medial side should be followed by a lateral approach. Many prefer to attempt aspiration from the medial side first because the surface landmarks and shape of the patella are more obvious medially.

A helpful maneuver in second attempts is to have a second operator compress the fluid from three sides of the knee. US should be performed if these two modalities are ineffective and if a septic joint is suspected.

The physical examination can occasionally be falsely positive, even when performed by experienced examiners; US or MRI may noninvasively confirm the presence of absence of "tapable" fluid when this situation is suspected.

With chronic effusions, it may be impossible to obtain fluid from either the medial or the lateral approach due to a gelatinous mass obstructing the needle (image 5).

Hip effusions — Hip effusions are seen well on ultrasound in most adults, as demonstrated in a study promoting the bedside use of joint US in the rheumatologic setting [14]. In children, however, more mobile femoral heads are sometimes pushed anteriorly, obscuring the existence of hip effusion from the vantage point of the anteriorly placed ultrasound window. US is useful for average to thin adults and is considered by pediatric radiologists to be their imaging modality of choice (over fluoroscopy) for children suspected of hip joint effusion. For heavier patients, and in cases of prostheses, the standard imaging study for the hip requiring arthrocentesis remains arthrocentesis under fluoroscopy.

Shoulder effusions — As with other joints, US can be used to confirm a glenohumeral joint effusion and guide the aspiration. Ultrasound is particularly useful in distinguishing fluid collections that may be limited to the true glenohumeral joint compartment versus those isolated within the more superficial subacromial-subdeltoid bursa. US of the shoulder and an approach to an ultrasound-guided aspiration of the glenohumeral joint is presented elsewhere. (See "Musculoskeletal ultrasound of the shoulder", section on 'Sonographic appearance of shoulder pathology' and "Musculoskeletal ultrasonography: Guided injection and aspiration of joints and related structures", section on 'Shoulder joint and subacromial/subdeltoid bursa'.)

Wrist effusions — The wrist consists of three normally non-communicating compartments: the distal radioulnar joint (DRUJ), the radiocarpal joint, and the midcarpal joint. Additionally, multiple extensor and flexor tendons with their sheaths cross the wrist superficially. Infectious processes may only affect one of these separate compartments initially, eventually spreading to affect multiple. Due to this complexity, the authors prefer US in the wrist for detecting and aspirating fluid in the case of suspected infection (image 7 and image 8 and image 9). Approximately 5 percent of emergency department nontraumatic wrist presentations are due to septic joint [15].

SUMMARY AND RECOMMENDATIONS

A joint that appears to have an effusion on physical examination may occasionally yield a "dry tap" during diagnostic arthrocentesis, even after the needle has been redirected and is believed to be clearly in the joint space. Explanations for a dry tap that should be considered, after extraarticular placement of the needle tip is excluded, include mistaken physical diagnosis (ie, no effusion present); blockage of the bevel of the needle by plica, fat, or debris; and very high viscosity fluid or true lipoma arborescens (table 1). (See 'Introduction' above and 'Causes of a dry tap' above.)

The most common cause of failed arthrocentesis of the knee may be the absence of a joint effusion, despite an apparent effusion on examination; this may occur more frequently in a person with obesity. Other causes include insertion of the needle into the medial patellar fat pad or alternatively Hoffa's infrapatellar fat pad, fatty replacement of chronically inflamed synovium (lipoma arborescens), a low volume of fluid, blockage by debris, and use of a needle of insufficient gauge for a gelatinous effusion. The presence of multiple anatomic compartments in a joint, such as the shoulder and wrist, or periarthritis and swelling without true arthritis, as sometimes occurs in the ankle, are also potential causes. (See 'Dry tap in the knee' above and 'Dry tap in other joints' above.)

Technical failure may also occur as a result of altered joint anatomy due to prior trauma or surgery, or in cases of chronic deforming arthritis. In some instances, fluid may localize in an atypical location. Ultrasound guidance for aspiration in suspected cases is useful. (See 'Causes of a dry tap' above.)

Ultrasonography (US) is often helpful in patients with a dry tap, enabling precise visualization of fluid location and for guiding needle placement. (See 'Approach to the patient with a dry tap' above and "Musculoskeletal ultrasonography: Clinical applications" and "Musculoskeletal ultrasonography: Guided injection and aspiration of joints and related structures".)

Unsuccessful aspiration from the medial side of the knee should be followed by a lateral approach. It may also be helpful in second attempts to have a second operator compress the fluid from three sides of the knee. US should be performed if these two modalities are ineffective and if a septic joint is suspected. (See 'Knee effusions' above.)

The standard approach to the hip requiring arthrocentesis remains performance of arthrocentesis under fluoroscopy, although US may be helpful for visualization of hip effusions, especially in adults. As with other joints, US can be used to confirm a glenohumeral joint or wrist effusion and guide the aspiration. (See 'Hip effusions' above and 'Shoulder effusions' above and 'Wrist effusions' above.)

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