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acute sacroiliac joint pyogenic infections are rare and account for 1 - 2% of orthopedic infections .
up to 60% of these patients have predisposing risk factors such as previous trauma , immune deficiency , intra - venous drug abuse , pelvic inflammatory diseases and pregnancy .
most of the reports however are of cases in children and adolescents with limited reports in adults .
the case outlined in this report adds to the body of knowledge by highlighting the difficulty in diagnosing the infection in patients with inflammatory arthropathy and by challenging the long - held belief that all such cases require surgical intervention .
in addition we believe the report demonstrates the efficacy of daptomycin in the treatment of orthopaedic infections related to mrsa .
we report the case of a 50-year - old gentleman who presented to the accident and emergency department at our institution with a 2 week history of malaise , fever and lower back pain .
his examination revealed a fever of 39c and he was tender on palpation over the left side of the lower back and left sacroiliac joint .
radiographs of the lumbosacral spine and sacroiliac joints demonstrated evidence of lumbar spondylosis and reduced disc space between l5 and s1 ( figure 1 ) .
laboratory investigations revealed raised inflammatory markers with a c - reactive protein of 324 mg / l ( normal 0 - 10 ) , white blood cell count of 18x 10 /l ( normal 4 - 11 ) and a neutrophilia of 14.9x10/l ( normal 2.0 - 7.5 ) .
blood cultures taken at the time were subsequently found to be positive for mrsa infection . despite the evidence pointing towards sepsis as a cause for the gentleman 's symptoms , preliminary examination and investigations failed to identify a clear focus for this .
an mri scan of the lumbosacral spine and pelvis was requested on the presumption that this was the most likely source of the infection .
figure 2 shows the mri scan of the lumbosacral spine demonstrating degenerative changes in the l5-s1 disc with reactive endplate changes .
mri images extended over the pelvis revealed a left iliopsoas collection measuring 7x4 cm with inflammatory changes and oedema involving the iliacus muscle and extending into the left groin .
a ct scan was also performed to further evaluate the sacroiliac joint and showed erosive changes within the left sacroiliac joint ( figure 4 ) .
this may have been secondary to the infective process but equally there was a suggestion that these changes may be secondary to sacroiliac joint arthritis secondary to psoriasis .
the patient had been commenced on intravenous vancomycin and linezolid on the basis of the blood cultures .
the basis for an aggressive first line combined treatment was the absence of a clear septic focus prior to the mri findings and the abnormally high inflammatory markers .
the treatment was converted to daptomycin on the advice of the microbiologists . following the findings of the mri
scan a surgical opinion was sought from the pelvic team who felt that access to the joint and the deep lying abscess was technically difficult .
this was further compounded by the presence of active psoriatic plaques across the patient 's abdomen and groin , impeding the surgical approach . in the context of the patient already having shown signs of improvement on the antibiotics ,
the patient did continue to improve and was treated with an 8 week course of daptomycin in all .
his inflammatory markers remained marginally high ( crp and esr between 40 - 50 ) for a period after his treatment but this coincided with a flare up of his psoriasis which was now being treated with ultraviolet light therapy .
the patient had serial liver function and renal function tests , performed throughout his treatment with daptomycin and these remained within normal range .
this was necessary whilst the patient was treated with daptomycin due to the reported side effects such as myositis , acute renal failure secondary to rhabdomyolysis and impairment of liver functions .
serial follow up mri scans demonstrated complete resolution of the iliopsoas abscess and iliacus muscle oedema .
he however developed claustrophobia over the course of his many scans and for this reason his final imaging was with a ct scan ( figure 6 ) .
septic arthritis of the sacroiliac joint is a rare disease entity with non- specific symptoms and signs contributing towards the difficulty in diagnosing this condition .
this can be further compounded by coexisting disease processes such as inflammatory arthritis and this has been demonstrated in the case reported above .
cases of sacroiliac joint infections in adults have been reported in the literature since the early nineties .
it has been postulated that infections in the pelvic area occur due to the sluggish circulation in the venous plexus of batson .
this coupled with the slow sub - chondral circulation initiates infections on the iliac side of the sacroiliac joint .
the common routes taken are along the iliopsoas tendon , into the hip joint or the lower spine .
staphylococcus aureus has been reported as the commonest causative organism in sacroiliac joint infections and this is in keeping with most other orthopedic infections .
methicillin is a semi - synthetic group of penicillin that was introduced in the 1960s to overcome the resistance that staphylococcus aureus had developed to the conventional penicillin .
it has an additional acyl group on the beta lactam ring that gives it resistance to the penicillinase enzyme produced by the organism .
resistance to methicillin is attributable to the mec a gene , which is a component of the staphylococcus cassette chromosome ( scc ) .
this gene encodes for the expression of an alternative penicillin binding protein pbp2a that renders methicillin ineffective .
the main challenge in the early diagnosis of our patient was to differentiate septic sacroilitis from psoriatic sacroilitis .
the original moll and wright criteria for diagnosis of psoriatic arthritis has been used prior to 2006 .
the criteria included : inflammatory arthritis ( peripheral , sacroilitis or spondylitis)presence of psoriasisnegative serological test for rheumatoid inflammatory arthritis ( peripheral , sacroilitis or spondylitis ) presence of psoriasis negative serological test for rheumatoid in 2006 the new classification of psoriatic arthritis ( caspar ) study group criteria were introduced .
this included additional features such as dactylitis , nail psoriasis and positive family history to increase the sensitivity of the diagnosis particularly in patients without cutaneous manifestations of psoriasis .  
the patient in this report presented with bleeding psoriatic patches and dactylitis in addition to the lower back pain .
the presentation could well have been attributed to a flare up of his psoriatic arthritis , however the presence of fever , elevated inflammatory markers together with leukocytosis and neutrophilia did not correlate with psoriatic sacroilitis alone .
the diagnosis of septic arthritis of the sacroiliac joint was supported by the mri findings and the improvement seen in the patient 's condition following antibiotic therapy .
surgical debridement with or without fusion of the sacroiliac joint is the main surgical option for this condition and the largest series reported in the literature consisted of 22 patients .
we however managed to treat the patient in this report non - operatively in the form of an 8 week course of daptomycin .
it contains a 10 carbon lipid side chain which is produced by addition of decanoic acid to the growth medium during fermentation ( figure 7 ) .
the first step is calcium - mediated binding of the daptomycin molecule to the cell membrane of the organism with conformational change and oligomerisation of daptomycin .
these side effects were not encountered during the 8 week course of treatment in our patient .
the case presented here contributes to the body of existing literature on the treatment of sacroiliac joint septic arthritis .
we believe this is the first case of an infection of this nature having been successfully treated with a course of daptomycin .
the case also highlights the difficulty in establishing the diagnosis in the presence of inflammatory arthropathy .
sacroiliac joint septic arthritis is a rare disease entity representing 1 - 2% of all cases of septic arthritis 
 . establishment of the diagnosis is often challenging given the non - specific presenting features and the potential cross - over with other pathologies . 
 we report the case of a 50 year old gentleman who suffers with psoriasis and presented with sacroiliac joint septic arthritis complicated by methicillin resistant staphylococcus aureus ( mrsa ) bacteraemia and an iliopsoas abscess . 
 this was successfully treated conservatively with a course of the novel antibiotic daptomycin .