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1 Department of Radiology, University of Michigan Medical Center, 1500 E.
Medical Center Dr., Ann Arbor, MI 48109-0326.
2 Present address: Department of Radiology, St. Joseph Mercy Health System, 5301
Huron River Dr., Ann Arbor, MI 48197.
3 Present address: Valley Radiologists, Ltd., 5322 W. Northern Ave., Glendale,
AZ 85301.
4 Present address: Akron Radiology Inc., 525 E. Market St., Akron, OH
44304.
5 Center for Health Outcomes, Innovation, and Cost Effectiveness Studies, 3A-14,
300 N. Ingalls Bldg., Ann Arbor, MI 48109.
Received September 25, 2002;
accepted after revision December 31, 2002.
Address correspondence to J. A. Jacobson.
Abstract
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MATERIALS AND METHODS. Twenty-one consecutive patients with clinical suspicion of hip joint effusion were examined on sonography by one of five musculoskeletal radiologists with experience in musculoskeletal sonography. All 21 patients underwent diagnostic arthrocentesis (fluoroscopic in 16, sonographic in five) to confirm the presence or absence of joint effusion. A retrospective analysis of the sonograms was made to assess the size of the distention of the anterior joint recess (anteroposterior dimension) and the echogenicity (anechoic or other relative to muscle), and correlation was made to the presence or absence of joint effusion.
RESULTS. Joint effusion was seen on diagnostic arthrocentesis in 10 (48%) of the 21 patients. Seven of the 21 patients had native hips and 14 had prior hip surgery. Retrospectively, no significant difference was found with regard to the size of the anterior recess distention (p = 0.34) or echogenicity (p = 0.2) when comparing the patients with and without joint effusion.
CONCLUSION. Anterior recess distention and echogenicity could not reliably be used as an indicator of adult hip joint effusion, either in native or postoperative hips. Diagnostic arthrocentesis was necessary to establish or exclude the presence of hip joint effusion.
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Several authors have used sonography to detect joint effusion in the native adult hip [2], in particular for the evaluation of a septic joint [3, 4]. In the native adult hip, anterior capsule distention of 7 mm or more and asymmetric distention of 1 mm or more suggest hip effusion [2]. Sonography has also been used to assess joint effusion after hip arthroplasty [5–7]. In this situation, anterior pseudocapsule distention of 3.2 mm or more at the proximal anterior femoral cortex indicates an infected hip prosthesis [7]. The importance of sonography in detection of soft-tissue extension from a loose and infected hip arthroplasty has also been emphasized [7]. In our clinical practice at the University of Michigan, we have noted difficulty in confidently identifying a hip joint effusion on sonography in the adult population, particularly when an effusion is small in a patient with a large body habitus and in the presence of a hip arthroplasty. The objective of this study was to evaluate anterior recess distention and echogenicity as a determinant of hip joint effusion in the adult native and postsurgical hip, using diagnostic arthrocentesis as the gold standard.
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Standardized sonography included anterior imaging in the sagittal and axial planes over the femoral neck and evaluation of the adjacent soft tissues, including posterolateral imaging over the region of the greater trochanter. The native femur was identified by its characteristic reflective surface contours. The metal surface of a hip prosthesis was reflective with posterior acoustic reverberation. The region over the femoral neck was assessed for joint effusion, identified by anechoic or hypoechoic distention of the anterior recess. Extraarticular extension of a joint process was noted, as well as the presence of hypoechoic or anechoic fluid posterolateral to the greater trochanter, the area of the trochanteric bursa. The presence of a joint effusion and extraarticular extension of joint fluid were imaged on sonography. The patients were 10 men and 11 women, with a mean age of 44 years (range, 18–78 years).
All patients underwent sonographically guided or fluoroscopically guided arthrocentesis, depending on the referring clinician's request, with percutaneous aspiration performed with an 18- or 20-gauge spinal needle with stylet via an anterior approach. On sonography, intraarticular needle placement was confirmed with visualization of the needle entering the anterior joint recess and fluid return. On fluoroscopy, if no fluid was aspirated, intraarticular needle placement at the femoral neck medially was confirmed with injection of 1- to 5-mL of iodinated contrast material into the hip joint. An aspiration was regarded as positive if 1 mL or more of fluid was aspirated.
A blinded retrospective analysis was made by one of the authors to measure the anteroposterior dimension (in millimeters) of the anterior recess over the femoral neck for distention. In the native hip, the anterior recess was measured from the anterior femoral cortex to the outer joint capsule. In the hip that had undergone arthroplasty, pseudocapsule distention was measured over the femoral neck and at the most proximal aspect of the native femur. In addition, anterior recess distention echogenicity was characterized as anechoic or other (relative to muscle). The nonparametric Mann-Whitney test was used to determine if a correlation existed between the retrospectively measured anteroposterior recess distention and the presence or absence of effusion at arthrocentesis. Using the Fisher's exact test, the retrospective evaluation of the characteristic of the fluid, anechoic or other, was correlated with this same presence or absence of effusion at arthrocentesis.
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Of the 21 patients, 10 (48%) had joint effusion at diagnostic arthrocentesis (one native hip and nine arthroplasty hips). Five of these patients had infected joint fluid at aspiration (one native hip and four arthroplasty hips). There was no effusion at arthrocentesis in 11 patients (52%), including six with native hips and five with arthroplasty hips. All seven patients with extraarticular extension of a joint process had hip arthroplasties, six of the seven had effusions, and two of the six were infected. One of the patients with a native hip had infected trochanteric bursal fluid proven by sonographically guided aspiration.
The retrospective measurements of anterior recess distention are as follows: Of the seven native hips, the patient with effusion at arthrocentesis had an anterior recess distance of 5 mm (Fig. 1). The other six patients with native hips without effusion at arthrocentesis had a mean distance of 10.5 mm (range, 4–25 mm) (Figs. 2 and 3). With regard to the 14 patients with hip arthroplasties, the five with no effusion at arthrocentesis had a mean distance over the prosthesis neck of 7.8 mm (range, 0–15 mm) (Fig. 4), whereas the mean distance in the remaining nine arthroplasties with effusion at arthrocentesis was 13.4 mm (range, 5–25 mm) (Figs. 5 and 6A, 6B). There was no statistically significant difference in the anterior recess distance between patients with and those without an effusion in both native hip and arthroplasty groups (p = 0.34). Last, measurements obtained at the junction of the native femoral neck and prosthesis in the 14 patients who underwent hip arthroplasty showed a mean of 3.8 mm (range, 0–6 mm) in hips without effusion (Fig. 4) and 8 mm (range, 4–15 mm) in hips with effusion (Figs. 5 and 6A, 6B).
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The following represents the retrospective determination of echogenicity for each joint distention: Of the 10 joints with positive fluid found at arthrocentesis, five were anechoic and five were categorized as other (not anechoic). Of the 11 joints with no fluid seen at arthrocentesis, two were anechoic, whereas nine were categorized as "other." No statistical difference was found in the anterior recess echogenicity between hips with and without joint effusion (p = 0.2).
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The normal anterior joint capsule of the hip consists of two fibrous layers covered with a synovial membrane [1] (Fig. 1). The anterior layer extends inferiorly from the labrum adjacent to the iliopsoas muscle, and after it inserts in the intertrochanteric region of the femur, the capsule is then reflected superiorly along the femoral neck as the posterior layer. Each layer measures 2–4 mm and is isoechoic to the psoas muscle in 48%, hyperechoic in 31%, and hypoechoic in 2% of hips [1]. The thin synovial membrane which covers the fibrous capsule is not normally visible on sonography. A joint effusion is visible between these two fibrous layers of the joint capsule over the femoral neck (Fig. 1).
The diagnosis of a joint effusion in the native hip has been based on the distance of joint capsule distention; joint effusion is suggested when there is 7 mm or greater of anterior recess distention or asymmetry between hip joints of 1 mm or more [2]. However, in our seven native hips, the hip with joint effusion had an anterior distention of 5 mm (Fig. 1), whereas the six hips without effusion had a mean distention of 10.5 mm (Figs. 2 and 3). One explanation for this finding is that hypoechoic synovium may also abnormally distend the anterior hip joint recess in the absence of joint effusion (Fig. 3). In patients with a large body habitus, internal echoes in the synovium may be difficult to identify, and the synovium may appear similar to fluid in echogenicity. The presence of hyperemia on color or power Doppler imaging will assist in differentiating synovium and effusion, although synovitis does not always show hyperemia. In addition, accurate measurement of anterior recess distention may be difficult in the patient with a large body habitus.
With regard to patients who had undergone the hip arthroplasty, we found an anterior pseudocapsule distention over the prosthesis neck of 13.4 mm (range, 5–25 mm) in the nine patients with effusion (Figs. 5 and 6A, 6B), and distention of 7.8 mm (range, 0–15 mm) in the five patients without effusion (Fig. 4). The degree of overlap in these two groups did not allow accurate prediction of effusion based on pseudocapsule distention measurements. The periarticular soft-tissue echogenicity may be altered after hip surgery; the anterior soft tissues may appear hypoechoic and thickened without joint effusion (Fig. 4). With regard to pseudocapsule distention at the level of the native femur, we found a mean of 8 mm (range, 4–15 mm) in those with effusion (Figs. 5 and 6A, 6B) and a mean of 3.8 mm (range, 0–6 mm) in those without effusion (Fig. 4), but overlap of these measurements did exist. Although van Holsbeeck et al. [7] found that pseudocapsule distention of 3.2 mm or greater at the native femur indicated an infected loose hip arthroplasty, three of our five patients without joint effusion had pseudocapsule distention equal to or greater than 5 mm (Fig. 4).
Retrospective review of the sonographic images also showed that the echogenicity of the distended anterior hip recess did not correlate with the presence or absence of joint effusion. The increased thickness of the overlying soft tissues in the adult population with a large body habitus may cause anechoic joint fluid to be poorly visualized and appear hypoechoic. Complicated hypoechoic joint fluid may also appear similar to hypoechoic synovitis (Fig. 3). Although hyperemia is not uniformly present, its identification on color or power Doppler imaging would suggest synovitis and not complex fluid.
One advantage of sonography in the evaluation of hip pain is the ability to identify extraarticular soft-tissue fluid collections that do not communicate with the hip joint. Although seven of our eight patients with soft-tissue abnormalities did have hip joint communication (Figs. 6A, 6B) (all had hip arthroplasties), one patient with a native hip had isolated fluid in the greater trochanteric bursa, which was percutaneously aspirated with sonographic guidance and proven to be infected. This patient's greater trochanteric bursitis would not have been diagnosed if evaluation relied solely upon fluoroscopically guided hip aspiration. Additionally, there is a theoretic risk of seeding a sterile joint with fluoroscopic hip joint aspiration if the needle was passed through an unrecognized overlying soft-tissue abscess. We therefore advocate the use of sonography in screening the soft tissues for noncommunicating fluid collection, especially over symptomatic areas and incision sites.
Possible limitations to this research include the small number of patients and small number of infected hip joints in our patient population. Color Doppler and power Doppler imaging, which would assist in differentiating synovitis and joint fluid, were not uniformly used. In addition, tissue harmonic imaging was not routinely used in the evaluation of hip joint effusion and therefore the utility of this technique was not assessed.
We conclude that anterior hip recess distention and echogenicity as seen on sonography are unreliable in the diagnosis of hip joint effusion in the adult population and that arthrocentesis should be used to exclude or confirm hip joint effusion. Our results suggest that sonography is important in diagnosing extraarticular noncommunicating soft-tissue fluid collections and that sonography should be considered before fluoroscopic hip joint aspiration.
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