Septic arthritis is an orthopedic condition involving the shoulder in 8 to 21% of cases [3]. It mostly affects immunocompromised patients with co-morbidities as demonstrated in literature [1]. However, several cases of septic shoulder have been described throughout history following a routine vaccination, and more recently, following the covid-19 vaccination [4-5], where only two reported cases were about septic joints [6-7], whereas the four other patients presented with either an inflammatory or an autoimmune reaction to the vaccine [3,5,8]. Severe complications can occur, such as osteomyelitis, adhesive capsulitis and osteonecrosis the mortality rate may reach 11.5% of patients with septic arthritis of the shoulder [9].

Anterior injection of the deltoid, especially in thinner patients, exposes to a high risk of inadvertent subacromial bursal and glenohumeral joint penetration. The synovium is a well-vascularized structure with no limiting basement plate, and this allows easy access by bacteria [10]. Once bacteria are inoculated in the joint space, the low fluid shear allows it to adhere to joint structures andproliferate [11], generating an acute cascade of inflammatory reaction and the pathogens are eliminated [12]. However, if the infection is not resolved due to a defected immune system, the high levels of cytokines produced by the immuno-regulatory cells may result in joint effusion that increases intra-articular pressure, which prevents blood and nutrients from reaching and supplying the joint, leading to synovium and cartilage destruction. Permeation of synovial stroma by a dense perivascular inflammatory infiltrate increases synovial thickness. The increased vascularity owing to vasodilation and angiogenesis, and increased permeability of inflamed blood vessels are responsible forsynovial enhancement due to synovial hyper vascularity and peri synovial edema seen in MRI.

Septic arthritis diagnosis consists of a series of physical and biological examination. However, when a deep joint is affected, shoulder joint in our case, the patient can be paucisymptomatic and clinical findings can be nonspecific making the diagnosis even more challenging. In that case, we resort to imaging.

Plain radiograph may be normal in the very early stage of the disease. Otherwise, it shows a narrowing of the joint space due to cartilage destruction in the acute phase, destruction of the subchondral bone on both sides of a joint. And, if left untreated, reactive juxta-articular sclerosis or even ankylosis.

Ultrasound shows joint effusion, a thickened and hyper vascularized synovium on Color Doppler. And sometimes, it can be used to guide aspiration. CT features are similar to those on radiographs with a better assessment of osseous lesions. MRI is sensitive and more specific for early changes as early as 24H after the onset of infection [13]. It helps as well rule out differential diagnosis and evaluating osseous, cartilaginous and soft tissue lesions. The major findings comprise joint effusion in 70% of cases, synovial thickening and synovial enhancement after IV administration of gadolinium, associated with peri synovial and soft tissue edema on T2W implying a high correlation with the clinical findings in patients with septic joint [14]. These findings can be present in all stages of septic shoulder.

Fluid out pouching is also seen and results from fluid dissection from the joint due to continuous pressure, presenting a similar appearance with abscess [15]. MRI demonstrated bone marrow reactive edema which is seen as hypointense in T1w and hyperintense in T2w images and help distinguish it from associated osteomyelitis, where edema is larger, confluent and more diffuse causing a blurring in bone marginson T1W images [16], which becomes clearer after contrast administration. This aspect is referred to as the “ghost sign” [17]. Diffuse marrow edema and cyst formation indicate and advanced stage of septic arthritis and predicts a bad prognosis even after arthroscopic debridement [18]. MRI has its limitation as some findings may lead to over diagnosis, such as synovial enhancement without joint effusion, which can be present in 23% of healthy joints [16]. The use of thin needles of 16mm is recommended in adults less than 60 kg receiving a vaccination in the deltoid and any abnormal sign within the first 24 hours, pain or shoulder joint dysfunction, should raise suspicions of septic arthritis [4]. Early initiation of intravenous therapy is highly recommended to control the infection. The choice of antibiotics depends on the most probable pathogen in each health institute. Recommended duration of intravenous (IV) therapy, varies from two to six or seven weeks [19-20]. However, literature gives evidence for short-course IV therapy efficiency (seven days) [21] and longer-term treatment for patients who failed to respond to the shorter regimen. Aspiration guided by ultrasound helps decompress and provide clinicians with culture sample to identify the pathogen(s).

Arthrotomy and arthroscopy are two procedures used for surgical debridement of osteochondral fragments and areas of osteomyelitis. They also provide specimens in case of non-conclusive joint fluid aspiration results. Moreover, arthrotomy showed better clinical and radiological improvement compared to aspiration [22]. Any delay in septic arthritis management can causean irreversible joint damage within 48 hours of the onset of infection and result in important sequela, especially osteonecrosisdue to increase in intra-articular pressure which compromises blood circulation.

Conclusion
A relatively high rate of reoperation, morbidity, and mortality has been associated with septic arthritis of the shoulder. Accurate diagnosis relies mainly on imaging findings, especially MRI that provides the surgeon with the best assessment of soft tissue, cartilage and bone, which allows a prompt and proper treatment, thereby, preventing complications like septic shock and osteomyelitis. Most importantly, a proper intramuscular injection technique is mandatory to avoid such severer infections.

Acknowledgement: I would like to express my gratitude to my professors and all the colleagues who participated in the completion of this work.

Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of Interest: The authors declare no conflict of interest.

Author’s Contributions:
Ibtissam EL OUALI: Conception of the work, design of the work and acquisition of data.Drafting the work.
Chirihane AYADI: Acquisition of data.
Sara HABIB CHORFA: Acquisition of data.
Laila JROUNDI: Revising the work critically for important intellectual content.
F.Z LAAMRANI: Revising the work critically for important intellectual content and final approval of the version to be published.