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AJR 2004; 182:119-122
© American Roentgen Ray Society

MRI Findings of Septic Arthritis and Associated Osteomyelitis in Adults

Michael Karchevsky1, Mark E. Schweitzer2, William B. Morrison1 and J. Antoni Parellada1

1 Department of Radiology, Thomas Jefferson University Hospital, 111 S 11th St., Ste. 3390, Philadelphia, PA 19107.
2 Department of Radiology, Hospital for Joint Diseases, Bernard Aronson Plaza, 301 E 17th St., New York, NY 10003.

Received January 22, 2003; accepted after revision July 8, 2003.

 
Address correspondence to M. E. Schweitzer.


Abstract
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 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
OBJECTIVE. The purpose of this study was to describe the soft-tissue, synovial, and osseous MRI findings of septic arthritis.

MATERIALS AND METHODS. At 1.5 T (T1-weighted, T2-weighted or STIR, and contrast-enhanced images), 50 consecutive cases of septic arthritis were evaluated by two observers for synovial enhancement, perisynovial edema, joint effusion, fluid outpouching, fluid enhancement, and synovial thickening. The marrow was assessed for abnormal signal on T1- and T2-weighted images or after contrast enhancement. We noted whether the marrow signal was diffuse or abnormal in bare areas. MRI findings were compared with microbiologic, clinical, and surgical data and diagnoses.

RESULTS. The frequency of MRI findings in septic joints was as follows: synovial enhancement (98%), perisynovial edema (84%), joint effusions (70%), fluid outpouching (53%), fluid enhancement (30%), and synovial thickening (22%). The marrow showed bare area changes (86%), abnormal T2 signal (84%), abnormal gadolinium enhancement (81%), and abnormal T1 signal (66%). Associated osteomyelitis more often showed T1 signal abnormalities and was diffuse.

CONCLUSION. Synovial enhancement, perisynovial edema, and joint effusion had the highest correlation with the clinical diagnosis of a septic joint. However, almost a third of patients with septic arthritis lacked an effusion. Abnormal marrow signal—particularly if it was diffuse and seen on T1-weighted images—had the highest association with concomitant osteomyelitis.


Introduction
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 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Septic arthritis is a common, often disabling disease that requires early diagnosis for optimal outcome [1]. Diagnosing septic arthritis early in its course is important because delayed diagnosis may result in cartilage and joint destruction arising from the action of enzymes released from neutrophils, synovial cells, and bacteria [2]. Septic arthritis is traditionally a clinical diagnosis based on physical examination and prompt arthrocentesis [3]. The paucity of specific clinical findings at the time of presentation has been noted [4]. Moreover, clinical evaluation of joint effusion in deep joints, such as the shoulder or the hip, can be difficult [5], thus making the diagnosis even more challenging. Historically, before the era of MRI, imaging findings of septic arthritis were considered nonspecific [6]. There is a lack of detailed literature on the MRI findings of this disorder [7]. Consequently, we sought to systematically evaluate the soft-tissue, synovial, and osseous findings of septic arthritis on MRI.


Materials and Methods
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 Abstract
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 Materials and Methods
Results
 Discussion
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Fifty consecutive cases of septic arthritis were studied in 38 patients (12 women and 26 men; age range, 24–83 years; mean age, 54 years). Septic arthritis was confirmed in all 50 cases by positive results from a culture after joint aspiration or biopsy (n = 46) or by follow-up of response to antibiotic treatment (n = 4). Twenty-two contrast-enhanced control cases without infection were evaluated in 21 consecutive patients without septic arthritis (11 women and 10 men; age range, 24–83 years; mean age, 40 years). The presence of associated osteomyelitis was determined by bone biopsy. Imaging was always performed before joint aspiration or biopsy and before bone biopsy; in most cases (n = 43), imaging was performed within 3 days of hospital admission.

Imaging was performed on a 1.5-T unit (Signa, General Electric Medical Systems, Milwaukee, WI). Pulse sequences included sagittal T1-weighted (TR/TE, 500/10; early and late after contrast administration) and STIR (3,500/48; inversion time, 150 msec); coronal T2-weighted (5,000/500); and axial T2-weighted (4,000/60 fat-suppressed) and T1-weighted (500/12) sequences before and after the IV administration of gadolinium (n = 47). Studies after contrast enhancement were fat suppressed, with early images being acquired immediately and late images being obtained after a 15-min delay. Imaging planes and field of view varied by body part.

Images were retrospectively reviewed by two experienced musculoskeletal radiologists, working in consensus, for the presence of joint effusion [8], synovial thickening (intermediate signal intensity compared with the lower signal intensity of joint effusion on T1-weighted images) [9], perisynovial edema, synovial enhancement, fluid outpouching, and fluid enhancement. The bone marrow was assessed for abnormal signal on T1-weighted images, T2-weighted images, or contrast-enhanced images [10], as well as for the specific location and extent of marrow edema, in an attempt to distinguish reactive processes from infections [11]. Control cases were evaluated only for the presence of synovial enhancement. The frequency of each of these findings was calculated.


Results
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Discussion
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The metatarsophalangeal joint was the joint most commonly involved (n = 20), followed by the interphalangeal joint of the foot (n = 7), the hip (n = 7), the knee (n = 5), the sacroiliac joint (n = 4), the shoulder (n = 3), the sternoclavicular joint (n = 2), and the ankle and metacarpophalangeal joints (n = 1 each).

Staphylococcus aureus was the most commonly isolated organism (64%, n = 32), being present in the metatarsophalangeal joint (n = 13), the interphalangeal joint of the foot (n = 7), the hip (n = 3), the knee (n = 3), the sacroiliac joint (n = 2), the shoulder (n = 2), the ankle (n = 1), and the metacarpophalangeal joint (n = 1). Group A streptococcus was present in 20% of joints (n = 10): the metatarsophalangeal joint (n = 3), the hip (n = 3), the sternoclavicular joint (n = 2), the knee (n = 1), and the shoulder (n = 1). Escherichia coli was present in 10% of joints (n = 5): the metatarsophalangeal joint (n = 3), the hip (n = 1), and the sacroiliac joint (n = 1). Pseudomonas aeruginosa was isolated in 4% of all joints (n = 2): the metatarsophalangeal joint (n = 1) and the sacroiliac joint (n = 1). Neisseria gonorrhoeae was present in only one knee.

Synovial enhancement was nearly invariable, present in 98% (n = 47) of cases, whereas it was only present in 23% (n = 5) of control cases. This enhancement was usually associated with perisynovial edema (84%, n = 42) (Fig. 1A, 1B, 1C). Less frequently seen was synovial thickening (22%, n = 11).



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  		Fig. 1A. —35-year-old man with septic arthritis of knee. T1-weighted image shows enhanced thickened synovium (arrows) in suprapatellar bursa.

 


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  		Fig. 1B. —35-year-old man with septic arthritis of knee. On fat-suppressed T1-weighted image (TR/TE, 500/12), this synovium enhances after contrast administration (arrowheads); additionally, subcortical edema (arrow) is present in bare area of tibia.

 


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  		Fig. 1C. —35-year-old man with septic arthritis of knee. Perisynovial edema (arrows) is seen on fat-suppressed T1-weighted image (500/12).

 

Joint effusions were common, but not invariable (70%, n = 35) (Fig. 2A, 2B). They were present in 54% (n = 15) of small joints and in 91% (n = 20) of large joints. Surprisingly, fluid outpouching was seen in only half of the cases (53%, n = 26). Rarely was there fluid enhancement, even on delayed images (30%, n = 14).



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  		Fig. 2A. —52-year-old man with septic first metatarsophalangeal joint. Axial T2-weighted image shows no noticeable joint effusion (asterisk). Note enhancing edema in metatarsal head (arrows) that was shown on biopsy to be osteomyelitis.

 


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  		Fig. 2B. —52-year-old man with septic first metatarsophalangeal joint. Contrast-enhanced T1-weighted image shows synovial enhancement (arrowheads) and marrow enhancement.

 

There were 33 biopsy-proven cases of associated osteomyelitis. The marrow changes were diffuse in 86% (n = 43) and focal in 14% (n = 7). Frequently, this edema was visible on both T1-weighted (66%, n = 33) and T2-weighted (84%, n = 42) images. After IV administration of gadolinium, enhancement was seen in 81% (n = 38) of cases. Interestingly, in a third of cases (33%, n = 14) with the diffuse pattern of edema, osteomyelitis was not present (Fig. 3A, 3B), and in 29% (n = 5) of cases with normal T1 signal, osteomyelitis was seen at biopsy.



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  		Fig. 3A. —50-year-old woman with septic arthritis of left sacroiliac joint and negative findings for osteomyelitis on biopsy. On axial T2-weighted fat-suppressed image, reactive marrow edema in sacrum (asterisk) is greater than edema in ilium, consistent with reactive changes. Also noted is effusion posteriorly, anteriorly, and medially (curved arrows). What is striking is synovial outpouching (thick straight arrow), and its dissection away from joint (thin straight arrows) to form microabscess.

 


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  		Fig. 3B. —50-year-old woman with septic arthritis of left sacroiliac joint and negative findings for osteomyelitis on biopsy. Reactive edema enhances after IV administration of contrast material (arrow) on T1-weighted image. Note rim enhancement of fluid outpouching (arrowheads).

 


Discussion
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Septic arthritis is a common disease [12]. Any delay in diagnosis of septic arthritis may increase morbidity and lead to complications such as bone and cartilage destruction, osteonecrosis, secondary osteoarthritis, osteomyelitis, and eventually ankylosis [13]. Some patients may present with subtle findings and have inconclusive findings of laboratory studies [3]. Moreover, no single test has shown itself to be of sufficient diagnostic accuracy to serve as a definitive test for musculoskeletal infection [14].

MRI has been increasingly used to evaluate musculoskeletal infections [15] because it is useful for evaluating bone marrow, soft tissues, and joints. MRI findings in patients with septic joints have been described as abnormal as early as 24 hr after the onset of infection [6]. The sensitivity and specificity of gadopentetate dimeglumine–enhanced MRI with fat suppression were found to be 100% and 77%, respectively, for the detection of septic arthritis [16]. However, there is little detailed data on the specific MRI findings in patients with septic arthritis.

Effusions have been considered a sine qua non of septic arthritis [17]. In fact, in 75% of cases they lead to joint aspiration and culture [12]. We found effusions to be present in 70% of our cases. This frequency may be lower than expected because a disproportionate number of small joints were included in our study and increased intraarticular fluid may not be evident in small interphalangeal joints, especially if the joint capsule is eroded [18]. On MRI, we found a similar frequency of effusion in large joints, as has been reported in prior studies of sonography [17].

Although the synovium is barely perceptible on MRI [9], we found that it becomes thickened and therefore readily visible in septic arthritis, but only in a small percentage of cases (22%). This increase in synovial thickness is consistent with the histologically noted permeation of synovial stroma by a dense perivascular inflammatory infiltrate [19]. Synovial thickening has been seen consistently on sonography in infectious and inflammatory arthropathies [20]. In this situation, the superior resolution of sonography most likely explains the more accurate appraisal of the synovial membrane thickness.

The soft tissues of a healthy synovial joint show only minor contrast enhancement [21]. Our finding of fairly intense synovial enhancement (98%) is likely to indicate increased vascularity owing to vasodilation and angiogenesis [19], which are known manifestations of septic arthritis. Only 23% of healthy knees show contrast enhancement of the synovium. The enhancement in control cases in our study was also usually minimal. This contrast enhancement of hypervascular, inflamed synovium is in agreement with a prior MRI study [22]. Although we found occasional cases of synovial enhancement without joint effusion, enhancement was seen with joint effusion in most cases.

The enhancement of proliferating synovium may be difficult to distinguish from enhancement of joint fluid [23]. The rate of enhancement between synovial membrane and joint fluid differs, and dynamic contrast-enhanced MRI may separate the fast-enhancing synovium from the slower enhancing fluid [22]. We acquired early and late contrast-enhanced images but found fluid enhancement in only 30% of joints. Because 86% of small joints showed fluid enhancement, we believe that our delay was not adequate to allow significant diffusion of contrast material into the large joints, leading to a number of false-negative results for joint fluid enhancement.

Blood flow, as well as capillary permeability, is increased during inflammation [6]. Perisynovial edema, which we believe is due to increased permeability of inflamed blood vessels, also seems to correlate with the diagnosis of a septic joint, being present in 84% of cases. To our knowledge, this finding has not been previously described; however, it may be a nonspecific finding, as we noted, in inflammatory arthropathies.

Classically, synovial outpouching has been associated with effusions under pressure. In the setting of septic arthritis, continued synovial proliferation may lead to an effusion under pressure. However, we found joint outpouching in approximately half our cases (53%). The fact that fluid outpouching is more often seen in patients with recurrent synovitis may explain why we found it to be relatively uncommon in patients with acute septic arthritis. The differentiation of outpouching from abscess is both difficult and, to some degree, arbitrary. As synovial fluid dissects away from the joint, its connection may become tenuous, and the fluid may be considered an abscess [18].

MRI has been suggested to detect coexistent osteomyelitis in patients with septic arthritis [12]. Reactive bone marrow edema of septic arthritis may be easily confused with associated osteomyelitis [24]. It has been stated that in a true bone infection the marrow edema is larger and more multifocal than in reactive edema [25]. In our study, abnormal marrow signal, particularly if it was diffuse and seen on T1-weighted images, had the highest association with concomitant osteomyelitis. However, a third of patients with the diffuse pattern of edema did not have osteomyelitis, and 29% of cases with normal T1 signal did. This lack of specificity is disturbing but not surprising considering the difficulties in distinguishing reactive from infectious radiopharmaceutical uptake. This difficulty has also been suggested by prior authors to exist on MRI [26]. An important caveat is the possibility of false-negative results, particularly culture results, in osteomyelitis [18].

This investigation had significant limitations, including the small sample size, variability in joint location studied, and lack of true MRI criteria for effusions and synovial thickening. There was also significant population bias as to which patients were imaged and at what time over the course of their disease.

Accepting these limitations, we conclude that synovial enhancement and joint effusions had the highest correlation with the clinical diagnosis of septic joint. However, because we imaged patients with septic arthritis only, we can draw no absolute conclusions about the importance of these signs in the general population. In fact, most effusions were noninfectious. We do believe and have shown, however, that significant synovial enhancement is unusual in patients without articular inflammation. In addition, not previously described is the frequency of perisynovial edema, which was quite common in this population. However, absence of fluid in the joint, especially in small joints, is not an absolute negative predictor of infection. Abnormal marrow signal, particularly if diffuse and seen on T1-weighted images, had the highest association with concomitant osteomyelitis, but this finding may lead to significant false-negative results.


References
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 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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