After approval by the Institutional Review Board (University Hospital of Geneva 09-029R, Mat-Ped 09-008R), medical charts of all children admitted to our tertiary care hospital from January 2010 to December 2013 for suspicion of osteoarticular infection of the hip were retrospectively reviewed. Analyses were based on clinical records, demographics (age and gender), body temperature, and laboratory data including bacterial cultures (from blood and bone samples), white blood cell count (WBC), platelet count, erythrocyte sedimentation rate (ESR) and serum C-reactive protein (CRP). Since 2007, all samples are also analyzed using either a broad range PCR assay or a new, real-time PCR assay specific to K. kingae. Radiological studies of the hip were available for all patients, and included plain radiographs, and MRIs.
For children less than 5 years, MRI was performed under general anesthesia whereas the examination was realized without sedation in older children. MRI studies were performed on a 1.5-T Avanto (Siemens) machine and the protocol included coronal turbo spin echo T1-weighted sequences (TR/TE = 500/ 12), axial turbo spin echo T2-weighted images (TR/ TE =6420/99), 3 days T2 STIR SPACE images (TR/ TE =2000/201, thickness = 1,3–1,5 mm, isometric), and coronal (TR/TE = 639/13) and axial (TR/TE = 350/8,8) fat-suppressed spin-echo T1-weighted images after intravenous administration of gadopentetate dimeglumine (Dotarem) at a dose of 0.1 mmol/kg body weight. Contrast enhanced images were obtained directly after contrast injection and no dynamic sequences were obtained. Then children were transferred under general anesthesia to the operating room where joint fluid or bone aspirate specimen were taken and sent to the laboratory for Gram staining and immediate inoculation onto Columbia blood agar (incubated under anaerobic conditions), CDC anaerobe 5 % sheep blood agar (incubated under anaerobic conditions), chocolate agar (incubated in a CO2-enriched atmosphere), and brain-heart broth. Incubation time was 10 days. All samples were also analyzed using either a broad range PCR assay or a new, real-time PCR assay specific to K. kingae.
MRI studies were anonymized, coded and transferred to a dedicated computer station and stored in random order. Two senior pediatric radiologists (LM and MA) independently reviewed MR images. Only patients with MR signs of isolated SA were included in this study, whereas children with concomitant SA and AHO at initial investigation were excluded from the study. According to the literature, we analyzed the images as follows:
-
1.
AHO was diagnosed when there was focally decreased marrow signal intensity on T1-weighted images and focally increased marrow signal intensity on fluid-sensitive images (fat-suppressed T2-Weighted and STIR sequences). On enhanced, fat- suppressed T1-weighted images, osteomyelitis usually presents as focal abnormal bone marrow enhancement [17, 19] and sometimes, in children, as focal enhancement defects of the epiphyseal cartilage [20].
-
2.
We considered isolated SA when there was joint effusion with or without soft tissue involvement. Joint effusion was quantified as mild if there was only a small hyperintense line on T2-Weighted sequences, moderate if there was a more significant effusion and important if the femoral head was eccentric.
-
3.
The enhancement pattern of the femoral epiphyseal head cartilage and bone nucleus was qualitatively compared to the contralateral side on substraction images, and on slices with the most intense enhancement.
On the Fat SAT T1 enhanced sequence, the normal cartilaginous femoral epiphysis appears uniformly hyperintense (Fig. 1a arrow) before the appearance of femoral head ossification centers. With the appearance of ossification centers, normal cartilaginous femoral epiphysis presents a « striate » aspect due to enhanced vessels (Fig. 2a arrows). Ossification centers are uniformly enhanced in comparison with the pre-contrast appearance and surrounded by a thin hyperintense line (Fig. 2a arrowhead). The cartilaginous part reduces with age and this « striate » aspect usually disappears by the second year of life.
In our study, decreased perfusion was considered when the pathological femoral epiphysis was uniformly hypointense in comparison to the contralateral side (Fig. 1a arrowhead) if the ossification centers were not present. If present, when they were hypointense, the surrounding line did not enhance and the “striate” aspect of the cartilaginous part was absent (Fig. 3a arrows). We did not measure enhancement ratios.
Conventional X ray studies of the hip were performed in all cases one month after the MRI and images were reviewed by the same two radiologists: signs of AHO sequelae (lytic lesions, sclerosis, or disappearing of ossification centers) were reported if present.