Septic arthritis (SA) of the native adult hip is a rare orthopaedic emergency that requires prompt diagnosis and treatment (Balato et al., 2021; Lum et al., 2018; Carpenter et al., 2011; Clerc et al., 2011; Rutz and Brunner, 2009). The reported incidence of acute bacterial arthritis of a single native joint in developed countries fluctuates around 10 per 100 000 patients per year in the general adult population (Arieli et al., 2021; Kennedy et al., 2015; Clerc et al., 2011; Mathews et al., 2010; Margeretten et al., 2007; Kaandorp et al., 1995). The hip joint is involved in only a small fraction of these cases (Ross, 2017; Kennedy et al., 2015; Carpenter et al., 2011). Given this low incidence, data on the topic mainly originate from retrospective cohort studies (Mathews et al., 2010). As a result, there are no generally accepted diagnostic and therapeutic algorithms for SA of the native hip joint (Balato et al., 2021). Treatment of acute bacterial arthritis consists of surgical intervention and antibiotic therapy (Goldberg and Sexton, 2022; Balato et al., 2021; Lum et al., 2018; Ross, 2017; Mathews et al., 2010; Rutz and Brunner, 2009). Surgical modalities range from arthroscopic or open debridement to (staged) total hip arthroplasty (THA) and (modified) Girdlestone procedures, using a variety of anatomical approaches (Balato et al., 2021; D'Angelo et al., 2021; Russo et al., 2021; Davis and Zamora, 2020; Lum et al., 2018; Rutz and Brunner, 2009).

Clinical presentation and laboratory findings are frequently atypical and imprecise (Ross, 2017; Carpenter et al., 2011; Clerc et al., 2011; Margaretten et al., 2007). Fever at presentation is, for example, only present in around 60 % of cases (Ross, 2017; Clerc et al., 2011; Mathews et al., 2010; Margaretten et al., 2007), and joint pain might be confounded by pre-existing osteoarthritis (Clerc et al., 2011). In addition, serum markers, more than synovial fluid markers, suffer from low sensitivity and specificity (Varady et al., 2022; Ross, 2017; Carpenter et al., 2011).

It is apparent that both over- and underdiagnosing SA of the native hip joint will have severe consequences. Advanced imaging, for example, fluorodeoxyglucose positron emission tomography (FDG PET), magnetic resonance imaging (MRI), and computed tomography (CT), is therefore often requested in the diagnostic work-up of patients in whom clinical presentation and/or biochemical findings are ambiguous (Goldenberg and Sexton, 2022; Casali et al., 2021; Lum et al., 2018; Ross, 2017; Simpfendorfer, 2017; Mathews et al., 2010; Karchevsky et al., 2004; Weishaupt and Schweitzer, 2004; Learch and Farooki, 2000). It can demonstrate inflammation in and around the hip joint, although it is important to note that it is not always possible to differentiate between other non-infectious types of inflammation (Ravn et al., 2023; Casali et al., 2021; Mathews et al., 2010; Weishaupt and Schweitzer, 2004). It can also provide information on pre-existent osteoarthritis, avascular necrosis, and other joint pathologies that might indicate that joint sacrifice or replacement should be preferred over attempts at femoral head preservation (Davis and Zamora, 2020; Ross, 2017; Mathews et al., 2010). Additionally, FDG PET-CT can identify distant infectious foci (Casali et al., 2021).

The exact role of advanced imaging in the diagnostic work-up of patients with potential SA of the native hip joint remains unclear. Its use, to date, is mostly restricted to supporting or opposing diagnosis in patients with an ambiguous presentation (Goldenberg and Sexton, 2022; Casali et al., 2021; Davis and Zamora, 2020; Elsissy et al., 2020; Ross, 2017; Simpfendorfer, 2017). We hypothesised that the application might be broader since the presence of adjacent infections could directly influence surgical decision-making and outcomes (Rosenfeld et al., 2016). For example, Shoji et al. (2021) reported concomitant musculoskeletal infection in patients with an iliopsoas abscess as a risk factor for failure of conservative treatment.

Our primary aim was to determine the prevalence of extra-articular infectious manifestations on pre-operative advanced imaging in patients with proven SA of the native hip joint. Secondary aims were to determine whether a certain pattern of infectious manifestations could be identified, whether advanced imaging could guide surgical approach, and whether prognostic factors for prolonged hospital stay, number of surgeries, and femoral head preservation could be found.

For this study, a comprehensive analysis of advanced imaging in 25 patients with SA of the native hip joint was performed. It was found that one-third of patients had an abscess in one anatomical region, one-third of patients had abscesses in multiple anatomical regions, and only one-third of patients had no substantial abscess. Gluteal abscesses were especially common in patients with SA due to contiguous spread, while abscesses in the iliopsoas region were more common in patients with hematogenous infections. Different surgical approaches or a combination of approaches were deemed necessary to adequately deal with these various presentations of SA of the native hip joint. No significant prognostic factors could be identified.

The finding of extra-articular abscesses in 64 % of patients has several implications. Firstly, it shows that SA of the native hip might behave differently from other forms of septic arthritis, for example, the knee joint. Literature regarding the prevalence of soft-tissue abscesses in SA of native joints is scarce; however, we suspect that the frequent presence of extra-articular collections is a characteristic unique to SA of the native hip. This could be the result of the delay in presentation/diagnosis that is frequently present due to the deep location of the hip joint, the fact that it often concerns patients with underlying comorbidities (including immune deficiencies and paraplegia) (Ruythooren et al., 2023; Huang et al., 2020), and the unfamiliarity with the pathology (Ross, 2017; Kennedy et al., 2015; Carpenter et al., 2011). These factors could, in turn, explain the observation by Huang et al. (2020) that SA of the hip joint is associated with the highest risk for mortality compared to SA of other (more superficial) joints (Kao et al., 2019).

Secondly, it questions the suitability of arthroscopic debridement as a stand-alone intervention, as current guidelines on pyomyositis recommend drainage if an abscess is present (pyomyositis stage two or three) (Baddour, 2022). However, to date, direct comparison between open and arthroscopic debridement has not shown a significant difference in outcome between the two treatment modalities (Khazi et al., 2020; de Sa et al., 2015). We do note that those findings are based on retrospective studies where the initial incentive of surgeons choosing arthroscopic lavage remains unclear. It is possible that arthroscopy was mostly performed in patients with less extensive infections.

Thirdly, it poses the question of whether advanced imaging should be performed on a regular basis prior to initial debridement. After all, it is needed to determine the most efficient surgical approach since our findings showed that one standard approach does not suffice to access all the observed extra-articular lesions. In addition, it would allow for a more patient-tailored treatment protocol and could lower the number of surgical interventions needed. Elsissy et al. (2020), in contrast, suggest that advanced imaging should be used very insightfully when clinical suspicion of SA is high as it might unnecessarily delay surgical intervention; however, SA of the hip might be an exception to this rule. A large retrospective review on septic arthritis of the native joints performed by Lauper et al. (2018) did not find a significant difference in the risk of sequelae when surgical intervention is delayed in patients without sepsis. Furthermore, as noted above, a significant delay is often already present in patients with SA of the native hip. In our sample, the median delay until initial debridement was 12 d, with an interquartile range of 15. The question remains of which type of imaging would be most suited.

CT is the most readily available and widespread type of investigation in the acute setting (Eurostat, 2021). This corresponds with our findings, where two out of three patients receiving advanced imaging underwent a CT investigation. Yet, receiving pre-operative CT imaging was significantly correlated with further investigation via MRI or FDG PET-CT, independent of the presence of an abscess. This questions the value of CT imaging in the surgical management of SA of the hip, as based on our sample, it seems surgeons are not satisfied with the information provided by the investigation. There was, however, no significant difference in the outcome. It is important to note that due to the retrospective nature of this study, we cannot make any assumptions on the value of CT imaging in diagnosing patients with potential SA since our sample was restricted to patients in whom bacterial infection of the joint was confirmed.

MRI is the imaging modality of choice in musculoskeletal infections because of its superior representation of soft tissues and sensitivity for pathological oedema (Simpfendorfer, 2017). As expected, we found that the sensitivity of MRI was significantly higher for peri-articular soft-tissue abnormalities (p=.002). This was, however, not the case for the detection of abscesses. We therefore suspect that the added soft-tissue resolution of MRI might not be indispensable in the management of SA of the hip. In addition, MRI is less widely available and cannot be performed in some patients (e.g. claustrophobia, implanted devices) (Simpfendorfer, 2017).

Whole-body FDG PET-CT provides additional functional information and can detect distant foci of infection (Casali et al., 2021). This is particularly helpful in patients presenting with less specific clinical findings or when multi-joint involvement is suspected. In our series, six out of seven patients who underwent FDG PET-CT had an infection due to hematogenous seeding with a less specific clinical appearance (e.g. fever of unknown origin) or pain in multiple joint regions. We found no significant difference in outcome regarding the different types of advanced imaging. We therefore conclude that any kind of pre-operative advanced imaging could suffice, and choices should be based on availability and clinical presentation. If directly available, MRI is the investigation of choice in patients with overt and exclusive involvement of the hip joint and FDG PET-CT in patients where the clinical focus is less uniform or hematogenous spread is expected. These examinations provide more satisfactory information for surgical planning and thus may avoid the need for a second investigation.

This study has several limitations, the most important being its small sample size. Despite including all patients treated during a 16-year period at a large university hospital (±2000 beds), only 41 patients with SA of the native hip joint were eligible for inclusion, and only 25 received some form of advanced imaging pre-operatively. This could be a rationale for not finding significant prognostic factors associated with a prolonged hospital stay, infection control within two interventions or fewer, and preservation of the femoral head. Therefore, we believe that – at present – treatment decisions should be mainly based on the route of infection and the state of the joint, as previously suggested (Ruythooren et al., 2023). The extent of extra-articular abscesses should guide the surgical approach but not the decision on whether to preserve, sacrifice, or replace the hip joint. A second limitation is that imaging did not happen using a single standardised protocol and was performed in multiple different centres utilising diverse equipment. Furthermore, selection bias is present since not every adult patient with expected SA of the native hip received advanced imaging, and no standardised protocol was in place to determine who did. Lastly, our sample might not be directly extrapolatable to other centres or countries.

Based on the results of this study, we recommend performing advanced imaging in patients with suspected or proven septic arthritis of the native hip joint, as abscesses are present in 64 % and might require varying anatomical approaches.