Background: Septic arthritis in children is a joint threatening condition with potentially severe consequences; however, long-term outcome data is lacking. This study aims to determine 1-20-year outcomes following septic arthritis of hip and knee joints in children in an Australian population. Methods: All paediatric patients with septic arthritis of the hip or knee from 1995 to 2015 treated at our Australian institution were retrospectively assessed. Clinical features, treatment and investigation results were recorded. Long-term functional and radiological outcomes, infection recurrence and reoperation rate at final follow-up (mean 8.5 years, range 1.0-20.3; hip versus mean 7.7 years, range 1.1-20.3; knee) were recorded. Results: Sixty-four patients (37 hip, 27 knee) met inclusion criteria. Fifty-two patients (81.3%) attended follow-up. No mortalities or late infection recurrence occurred. Three patients (1; hip versus 2; knee) had a later operation. Median Oxford scores were excellent (48; hip versus 48; knee); however, a significant proportion had a degree of impaired function (31.3%; hip versus 42.1%; knee). Radiological outcomes were excellent in knees more commonly than hips (81.3%; hip versus 100%; knee). Conclusions: Outcomes at 1-20 years for the majority of patients following septic arthritis of the hip and knee are excellent with early joint irrigation and intravenous antibiotics. Our results demonstrate a significant proportion of patients following septic hip arthritis have mild to moderately poor functional and radiological outcomes. Those with septic knee arthritis demonstrated universally excellent radiological outcomes and mild functional impairment in approximately one-third of cases.

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have assumed that the analyses were by per-protocol because in table 2 of our manuscript, the sample sizes for the analyses at 3-4 month were less than those for the analyses at 1 week. The former comprised subjects for whom outcome data were available, not subjects who adhered to the intent of treatment. Indeed, there was no 'cross-over' of subjects in our study, which means the results of analyses by intention-to-treat would have been the same as by results per-protocol anyway.An important point was made regarding vitamin D levels increasing in the daily group at 4 months despite a poor adherence rate of 33%. There are plausible explanations for this. Following production in skin from ultraviolet B light exposure, vitamin D undergoes hydroxylation, to form 25OHD (the form measured in routine vitamin D assays to assess vitamin D status) and then to the biologically active form of 1,25(OH) 2 D. Both 25OHD and 1,25(OH) 2 D are then inactivated by 24 hydroxylase to form 24,25(OH) 2 D and 1,24,25(OH) 3 D, respectively. We hypothesise that when this system is exposed to a high bolus dose of vitamin D, metabolism to its inactive form is upregulated, thereby resulting in a substantial decline and lower mean 25OHD at 4 months.In addition, the vitamin D supplement is unhydroxylated and, hence, is not measured in the assay used. It is unlikely that taking vitamin D supplements just prior to the 4-month blood test would explain why vitamin D levels in the daily group are increasing despite a 33% non-adherence rate. However, most poorly adherent infants (9/12) were on mixed feeding regimens and consequently received some vitamin D in their formula feeds. We also reiterate that while there was statistical significance between mean 25OHD levels between both groups at 4 months, this was not clinically significant as these values were greater than 50 nmol/L and therefore considered replete for clinical purposes.