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Clin Orthop Relat Res. 2009 July; 467(7): 1688–1692.
Published online 2009 March 3. doi:  10.1007/s11999-009-0763-4
PMCID: PMC2690752

Magnetic Resonance Imaging Findings in Hematogenous Osteomyelitis of the Hip in Adults

Abstract

Hematogenous hip infections are rare in adults and the extent of infection into the bone or adjacent soft tissues may be underestimated, leading to inadequate surgical débridement. Using MRI, we sought to determine the extent of bone involvement and the presence of adjacent soft tissue abscesses in adults with hip osteomyelitis. We reviewed the records and MRIs in 11 adult patients (12 hips) with hematogenous osteomyelitis of the femoral head in 12 hips. Ten of 11 patients had one or more comorbidities. All patients underwent surgical débridement and received antibiotic therapy for 6 weeks. MRI revealed osteomyelitis distal to the femoral head in seven of 12 hips with extension into the medullary canal in three of these seven. Femoral head erosions were present in 10 hips, acetabulum osteomyelitis in 11, and acetabular erosions in six hips. Infection extended into adjacent soft tissues in eight of 12 hips. MRI demonstrated that the infection may extend distal to the femoral head or into the adjacent soft tissues. MRI may be useful for preoperative planning so that all regions affected by the infection can be treated.

Level of Evidence: Level III, diagnostic study. See Guidelines for Authors for a complete description of levels of evidence.

Introduction

Osteomyelitis may result from direct inoculation of pathogens after trauma or surgery, contiguous spread of an infectious process, or hematogenous seeding of the bone during bacteremia [16].

Hematogenous osteomyelitis of the proximal femur and hematogenous septic arthritis of the hip are not infrequent in pediatric patents [8, 13, 22]. Perlman et al. [22] reported the proximal femur was involved in 10 of 66 children (15%) with hematogenous osteomyelitis and the hip was also involved in five of these 10 infections. On the contrary, hematogenous hip infections are uncommon in adult patients [1]. Hip infections in adults usually develop as a complication of surgical procedures, especially hip arthroplasty [2, 9, 20].

Regardless of etiology, surgical débridement is the cornerstone of treatment of the infected hip in adults and excision arthroplasty may be required if osteomyelitis of the femoral head is present [1, 9, 20]. However, the rare occurrence of hematogenous hip infections in adults makes diagnosis and management problematic; the diagnosis may not be considered and may be missed initially, thereby leading to delayed treatment and allowing the infectious process to spread locally [1]. In addition, the extent of bone involvement and the potential extension of infection into the adjacent soft tissues are not apparent by conventional radiographs and may be underestimated, thereby leading to inadequate surgical débridement and persistence or recurrence of infection.

Magnetic resonance imaging (MRI) has high sensitivity for the diagnosis of musculoskeletal infection and also delineates with detail the extent of the osseous and soft tissue involvement [7, 11, 15, 17]. Studies on pediatric pelvic osteomyelitis using MRI demonstrate adjacent muscle abscesses in 20% to 55% of patients [4, 5, 14, 15, 17]. An MRI study on 44 paralyzed adult patients with osteomyelitis of the pelvis or hip demonstrated adjacent abscesses in 15 patients [10]. Knowledge of the extent of bone and soft tissue involvement will be useful for patient management and preoperative planning so that all locations of the infectious process are addressed [6].

Our purposes were to determine (1) the extent of bone involvement, and (2) the presence of adjacent soft tissue abscesses in adult patients with hip osteomyelitis of hematogenous origin.

Materials and Methods

We retrospectively reviewed the medical records of 11 adult patients (12 involved hips) treated for hematogenous hip infections with osteomyelitis of the femoral head from 2003 to 2006. There were nine male and two female patients with a mean age of 47 years (range, 33–59 years). The inclusion criteria were: (1) adult age (18 years or older); (2) hip infection, diagnosed based on clinical picture, laboratory exams, or positive cultures; (3) hematogenous origin of the infection, which was determined by excluding any patients with prior surgery or injury to the hip or pelvis (fractures, penetrating trauma or soft tissue loss over the hip or pelvis) who may have had direct inoculation of bacteria at the area of the hip or at the adjacent soft tissues; and (4) MRI evaluation prior to surgical management demonstrating femoral head edema indicative of osteomyelitis as reflected by decreased signal in T-1 weighted images and increased signal in STIR (short tau inversion recovery) T-2 weighted images (Fig. 1). We obtained prior ethical board approval.

Fig. 1
Coronal STIR image demonstrates hip effusion with femoral head and acetabular bone marrow edema, loss of articular cartilage and erosions.

Ten of 11 patients were classified as B hosts according to the Cierny-Mader classification [3] because of one or multiple comorbidities, the most common being intravenous drug abuse and smoking in six patients each (Table 1). Patients presented to the musculoskeletal infection ward of our institution with a considerable delay since the onset of symptoms of the involved hip(s); the mean delay was 42 days (range, 7–89 days).

Table 1
Patient comorbidities

All MRI exams were reviewed at the time of patient care by a single experienced musculoskeletal radiologist (TL) to determine the extent of involvement of the proximal femur and acetabulum, as well as the presence of adjacent soft tissue abscesses. A combination of T1-weighted sequences and STIR sequences in three cardinal planes was used. T1-weighted sequences were usually obtained in the axial and coronal plane and STIR sequences in the axial, coronal, and sagittal plane. No intravenous contrast material was used. All patients underwent surgical débridement, which included excision of the femoral head in 10 of 12 hips. Adjacent soft tissue abscesses were managed surgically or with CT-guided aspiration. Intraoperative culture results revealed a monomicrobial infection in nine hips (Staphylococcus aureus in five, Peptostreptococcus anaerobius in two, Streptococcus pyogenes in one, and Prevotella intermedia in one hip) and a polymicrobial infection in one hip (Staphylococcus aureus and Klebsiella pneumoniae). In two hips no organism grew in culture. Both patients were already on antibiotics at the time of surgery, had elevated C-reactive protein and erythrocyte sedimentation rate, and had previous blood cultures positive for Staphylococcus aureus. Staphylococcus aureus was the most common organism, present in six hips (three pathogens were oxacillin-sensitive and three were oxacillin-resistant). Antibiotic therapy was administered for 6 weeks postoperatively.

Results

Osseous involvement extended distal to the femoral head in seven of 12 hips; there were MRI changes consistent with osteomyelitis in the intertrochanteric area in seven hips (Fig. 2), and in three of these the infection extended into the medullary canal up to 10 cm distal to the lesser trochanter (Fig. 3). Other imaging findings included femoral head erosions in 10 hips (Figs. 1, ,2),2), osteomyelitis of the acetabulum in 11 hips with acetabular erosions in six hips (Figs. 1, ,2,2, ,3),3), and involvement of the sacroiliac joint and symphysis pubis in one patient each. This information was useful in guiding surgical débridement. When osteomyelitis extended to the intertrochanteric area, the cancellous bone of that area was débrided following excision of the femoral head and neck. When the MRI demonstrated medullary canal involvement, the canal was opened and reamed or curetted. Otherwise, if there was no evidence of intramedullary extension of infection, the medullary canal was not opened to avoid introducing bacteria into this area. When the acetabulum was involved, reaming of the acetabulum was performed.

Fig. 2A B
Coronal STIR (A) and T1-weighted (B) images demonstrate femoral head erosions and collapse. There is extensive bone marrow edema signal change present in the acetabulum and also in the femoral head extending distally into the intertrochanteric area.
Fig. 3
Coronal STIR image demonstrates extensive bone marrow edema of the left hip extending distally into the femoral shaft. There is also extensive bone marrow edema signal change of the acetabulum extending proximally towards the sacroiliac joint. Note marked ...

The infection extended into the adjacent soft tissues in the form of fluid collections (abscesses) in eight of 12 hips (Table 2). In six hips there were multiple soft tissue abscesses present (Fig. 3) and in two hips there was a single abscess (Fig. 4). Iliopsoas and subiliacus abscesses were the most common, present in four hips each.

Table 2
MRI findings in hematogenous osteomyelitis of 12 hips
Fig. 4
Coronal STIR image demonstrates an abscess (white arrow) medial to the iliopsoas tendon (black arrow) as it inserts onto the lesser trochanter. Also note the presence of fluid in the hip and the involvement of the acetabulum.

Discussion

Information on the extent of bone and soft tissue involvement in patients with osteomyelitis may be helpful to guide patient management and surgical débridement so that all locations of the infectious process are addressed. We aimed to determine the extent of bone involvement and the presence of adjacent soft tissue abscesses in adults patients with hematogenous hip osteomyelitis.

Our findings are limited to a specific patient group of compromised hosts with delayed presentation for treatment; the impaired host response to infection and the prolonged time interval from onset of symptoms to treatment may have contributed to the observed frequent spread of the infection beyond the hip. These findings may not be applicable to healthy patients who present for treatment without delay after developing a hematogenous hip infection. Nevertheless, the findings underscore the potential for underestimating the true extent of infection in cases of osteomyelitis involving the hip without the additional information obtained by MRI. Our MRI exams were not independently reviewed by at least two radiologists to reach a consensus on the diagnostic findings and there might be some variability in identifying the extent of the infections.

Intravenous contrast material, such as gadolinium, facilitates differentiating fluid collections/abscesses from edema of the surrounding soft tissues [5, 6]. However, intravenous contrast is not routinely used at our institution for the work-up of musculoskeletal infections. This is similar to suggestions by other authors [18, 19], who reported that bone inflammatory changes were similar in extent on contrast-enhanced T1-weighted sequences and on fast fat-suppressed T2-weighted sequences, such as STIR. In our experience, abscesses are also clearly demonstrated on STIR sequences, but if there is difficulty distinguishing phlegmon from abscess, then contrast is used. We prefer to use contrast only in select cases because of the added cost, the increased time necessary to inject and obtain additional sequences, and because of concerns for nephrotoxicity and nephrogenic systemic fibrosis in patients with renal insufficiency [21, 23].

Studies on pediatric patients with osteomyelitis of the pelvis have demonstrated MRI often detects associated soft tissue abscesses [4, 5, 14, 15, 17]. Muscle abscesses were present in four of 20 children with osteomyelitis of the pelvis in the series by McPhee et al. [17], and in 55% (21/38) of children in the series by Connolly et al. [5]. Using MRI, Huang et al. evaluated 44 adult patients with osteomyelitis of the pelvis or hip and found adjacent abscesses in 15 patients [10]. However, all patients in that study were paralyzed and many had decubitus ulcers so the origin of osteomyelitis is uncertain; moreover the authors did not provide data on the number of patients with hip osteomyelitis and the presence of associated abscesses in this subgroup [10].

In contrast, we evaluated patients with hip osteomyelitis comprising a homogeneous group regarding the etiology of infection. The absence of prior trauma, surgery, or soft tissue loss at the hip or pelvis assured the hematogenous origin of the infections and also prevented the confounding effects of postoperative or posttraumatic soft tissue changes on MRI imaging.

MRI has high sensitivity and specificity for the diagnosis of both osteomyelitis and soft tissue infections [7, 11]. MRI accurately depicts edematous, diseased bone and differentiates diseased bone from normal bone. However, differentiating reactive bone marrow edema from osteomyelitis is difficult as they have similar signal characteristics. Therefore, it should be noted that the extent of osteomyelitis could be overestimated because of reactive marrow edema and hyperemia adjacent to osteomyelitis.

Other imaging modalities may also be helpful in the evaluation of osteomyelitis. Bone scintigraphy can evaluate the whole skeleton for other areas of osteomyelitis, which can be useful in young children with diffuse symptoms, but does not provide anatomic detail and exact localization of the infectious process [4, 24]. Ultrasound can diagnose fluid collections but cannot provide data on bone involvement [19]. Computerized tomography can delineate sequestra and diagnose fluid collections but does not provide the detail that MRI does on early marrow changes [12, 17].

MRI can delineate the extent of involvement of the bone marrow and the surrounding soft tissues and has been described as imaging modality of choice for the evaluation of osteomyelitis of the pelvis in children [5, 15, 17]. We believe that MRI is a valuable diagnostic tool in osteomyelitis of the hip as well. The information provided may be useful for patient management and preoperative planning so that all locations of the infectious process are addressed.

MRI in adult patients with osteomyelitis of the hip demonstrates the infection may extend distal to the femoral head into the intertrochanteric area and medullary canal of the femur or into adjacent soft tissues.

Footnotes

Each author certifies that he or she has no commercial associations (eg, consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article. The authors have full control of all primary data and they agree to allow the journal to review their data if requested.

Each author certifies that his or her institution has approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

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Articles from Clinical Orthopaedics and Related Research are provided here courtesy of The Association of Bone and Joint Surgeons