Working Diagnosis:
Left SI joint sacroiliitis, likely infectious secondary to Propionibacterium acnes

Treatment:
Oral clindamycin for 6 months

Outcome:
Successful treatment with oral clindamycin 6 months with full return to activity and sport. Occasional back pain after strenuous activity but very active and doing well.

Repeat MRI pelvis w/wo contrast (7/2017) = Left SI joint still widened. Interval decreased T2 enhancement - improved left sacrum osteomyelitis/septic arthritis, left SI joint abscess. Case Photo #6

NM bone spec (8/2017) - slightly increased uptake in left SI joint due to healing of known infection. No other abnormal uptake. Case Photo #7

Author's Comments:
Persistent pain in children is always a concerning feature that warrants continued care. In our case, as his symptoms changed, an initial diagnosis of contusion and hematoma changed to SI joint osteomyelitis - which led to turnover of care from sports to pediatric infectious disease- that was then successfully treated with 6 months of antibiotics. As with our patient, leukocytosis is variable/nonspecific, seen in only 36% at time of presentation. ESR/CRP are also often elevated at presentation but normalize by a few weeks. The diagnosis of osteomyelitis, especially of the SI joint in children, is uncommon, unclear at presentation, and often diagnosed late. Thus, it is important to keep this diagnosis in mind. XRs are usually normal early on but should be done initially to rule out other causes of pain. But MRI, having greater detail, sensitivity, and less radiation, is the imaging of choice (especially in pelvis osteomyelitis). Diagnosis is confirmed by inflammatory changes on pathology. Pathogen identification then guides appropriate antibiotic treatment.

Editor's Comments:
Overall infectious sacroiliitis is rare, most cases reported in the medical literature are in children, and most of the reported pathogens are Staph species. In most cases there are no identified sources. The index of suspicion must be high and there must be lower threshold to image if young children present with hip or pelvic pain without a clear source.

References:
1. Dartnell J, Ramachandran M, Katchburian M. Hematogenous acute and subacute paediatric osteomyelitis: a systematic review of the literature. J Bone Joint Surg Br. 2012;94(5):584.

2. Guillerman RP. Osteomyelitis and beyond. Pediatr Radiol. 2013;43 Suppl 1:S193. Epub 2013 Mar 12.

3. Rolandsen Riise O, Kirkhus E, Samson Handeland K, et al. Childhood osteomyelitis - incidence and differentiation from other acute onset musculoskeletal features in a population-based study. BMC Pediatr. 2008;8:45.

4. Saavedra-Lozano J, Falup-Pecurariu O, Faust SN, et al. Bone and Joint Infections. Pediatr Infect Dis J. 2017;36(8):788.

5. Ford LS, Ellis AM, Allen HW, Campbell DE. Osteomyelitis and pyogenic sacroiliitis: a difficult diagnosis. J Paediatr Child Health. 2004;40(5-6):317-9.

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