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1Vol.:(0123456789)Scientific Reports | (2021) 11:18294 | accuracy of gap and step‑off measurements in acetabular fracture treatmentA. M. L. Meesters1*, K. ten Duis1, J. Kraeima2, H. Banierink1, V. M. A. Stirler1, P. C. R. Wouters1, J. P. P. M. de Vries1, M. J. H. Witjes2 & F. F. A. IJpma1*The assessment of gaps and steps in acetabular fractures is challenging. Data from various imaging techniques to enable accurate quantification of acetabular fracture displacement are limited. The aim of this study was to assess the accuracy of pelvic radiographs, intraoperative fluoroscopy, and computed tomography (CT) in detecting gaps and step‑offs in acetabular fractures. Sixty patients, surgically treated for acetabular fractures, were included. Five observers (5400 measurements) measured the gaps and step‑offs on radiographs and CT scans. Intraoperative fluoroscopy images were reassessed for the presence of gaps and/or step‑offs. Preoperatively, 25% of the gaps and 40% of the step‑offs were undetected on radiographs compared to CT. Postoperatively, 52% of the gaps and 80% of the step‑offs were missed on radiographs compared to CT. Radiograph analysis led to a significantly smaller gap and step‑off compared to the CT measurements, an underestimation by a factor of two. Approximately 70% of the residual gaps and step‑offs was not detected using intraoperative fluoroscopy. Gaps and step‑offs that exceed the critical cut‑off indicating worse prognosis often remained undetected on radiographs compared to CT scans. Less‑experienced observers tend to overestimate gaps and step‑offs compared to the more‑experienced observers. In acetabular fracture treatment, gaps and step‑offs were often undetected and underestimated on radiographs and intraoperative fluoroscopy in comparison with CT scans. This means that CT is superior to radiographs in detecting acetabular fracture displacement, which is clinically relevant for patient counselling regarding treatment decisions and prognosis.Pelvic radiographs and computed tomography (CT) scans are used to assess the fracture pattern and to determine the amount of displacement in acetabular fractures. Fracture gap and step-off measurements aid in the decision-making process regarding treatment strategy and preoperative planning. Limited data is available from direct comparisons of radiographs and CT scans and their ability in detecting fracture displacements15. Intraopera-tive fluoroscopy is used to evaluate whether the fracture fragments have been adequately reduced, however it is unknown how accurate this is and how much of the gap and step-off can be detected compared to radiographs and CT scans.Traditionally, residual displacements are graded by Matta’s criteria6. The largest gap or step-off on post-operative radiographs determines the quality of the fracture reduction. Studies reporting acetabular fracture treatments routinely correlate the clinical outcome to the amount of residual displacement24,69. These studies either use radiographs or CT scans to detect gaps and step-offs. Controversy exists about using CT scans for the postoperative evaluation of acetabular fractures, due to higher radiation exposure and higher costs4,10. Never-theless, postoperative CT scans are increasingly being performed to assess the residual displacement and screw positions13,11,12. Therefore, this study uses CT scans as a reference. Also, a standardised CT-based measurement method was recently introduced enabling consistent determination of residual displacement13. Verbeek et al.13 concluded that their standardised method is reliable for the assessment of reductions and that CT scans revealed worse reduction compared to radiographs.Understanding the accuracy and limitations of the imaging modalities assists in the interpretation of studies reporting functional outcomes after acetabular fracture surgery: Can we accurately predict hip survivorship and the patient’s rehabilitation process when different imaging modalities are still being used? We hypothesized that radiographs and intraoperative fluoroscopy imaging underestimate the extent of the fracture displacement, OPEN1Department of Surgery, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands. 23D Lab/Department of Oral and Maxillofacial Surgery, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands. *email:;
2Vol:.(1234567890)Scientific Reports | (2021) 11:18294 | the degree of underestimation was still unknown. Thus, the aim of this study was to assess the accuracy of radiographs and intraoperative fluoroscopy compared to CT scans in detecting gaps and step-offs in acetabular fractures.Materials and methodsPatients. All the patients in the pelvic registry (N = 256) who had suffered an acetabular fracture between 2007 and 2018, and were operated on using open reduction and internal fixation in our academic level one trauma centre (N = 138), were reviewed. Cases were included if a complete data set was available with pre- and postoperative radiographs and CT scans as well as intraoperative fluoroscopy images. Bilateral acetabular frac-tures with concomitant pelvic ring injury (N = 2) and patients with skeletal immaturity (N = 16) were excluded. The baseline characteristics were retrieved from the electronic patient files and all fractures were re-classified according to the AO/OTA and Letournel & Judet classification systems1416. All patients were approached by posted mail or telephone and asked if they had a conversion to total hip arthroplasty (THA) at follow-up. Our study was performed in line with the principles of the Declaration of Helsinki. It was reviewed and a waiver (no. 2016.385) was provided by the institutional Medical Ethics Review Committee of the University Medical Center Groningen. Exemption was provided by the Ethics Committee regarding obtaining informed consent, in accord-ance with the Dutch law that this research does not qualify as medical research with humans.Imaging assessment. All the measurements were performed by two trauma surgeons with > 5 years of experience in pelvic surgery, two trauma surgeons with < 5 years of experience in pelvic surgery and one PhD candidate in pelvic surgery. First, the maximum gap and step-off were measured on the radiographs (the anter-oposterior, obturator oblique or iliac view; standard Judet views). Second, the final intraoperative fluoroscopy images were assessed for the presence of a gap and/or step-off in any of the standard Judet views. Finally, the maximum fracture gap and step-off were measured on axial, coronal and sagittal CT slices, similar to the method introduced by Verbeek et al.13. Each median gap or step-off measurement, determined by all five observers, was used to compare the measurements between the different imaging modalities. All CT scans had a maximum slice thickness of 2 mm. The residual displacement on the postoperative images was graded according to Matta’s criteria6. Particular emphasis was placed on the evaluation of the imaging techniques and not on the results of the surgical treatment.Statistical analysis. The radiographs’ measurements were compared to CT measurements with the Wil-coxon signed rank test (in SPSS version 23, IBM, Chicago, IL, US). The sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) of detecting gaps and step-offs using radiographs and intra-operative fluoroscopy was calculated using the CT measurements as a reference. Furthermore, the radiological findings (median postoperative gap and step-off sizes) were correlated with the clinical outcomes (conversion to THA) by using a Wilcoxon signed rank test. For patients with or without a THA, the quality of the fracture reduction was assessed on both postoperative radiographs and CT scans. A postoperative gap of ≥ 5 mm and/or a step-off of ≥ 1 mm was considered an inadequate reduction according to the criteria of Verbeek et al.17 For both radiographs and CT scans, the ability to correlate inadequate fracture reduction to conversion to THA was assessed by using descriptive statistics. Finally, gap and step-off measurements of more- compared to less-experienced observers were assessed by using a Wilcoxon signed rank test. A p-value of ≤ 0.05 was considered significant.ResultsPatients. Sixty patients with acetabular fractures, treated with open reduction and internal fixation, were included. The patient characteristics are presented in Table 1. Twelve out of 60 patients (20%) received a THA after a mean follow-up of 25 ± 22 months. One patient died within a month after the accident.Preoperative imaging. A gap was observed on 45 patients’ preoperative radiographs, while a gap was present on the preoperative CT scan in all 60 patients. Radiograph analysis led to a significantly smaller gap com-pared to the CT measurements (P < 0.001), an underestimation of the gap by a factor of two (Fig. 1). The median size of the undetected gaps was 15 mm (IQR 10–23 mm) (Table 2). The sensitivity, specificity, PPV and NPV are presented in Table 2. Figure 2 shows a case example where no displacement was observed on the radiographs and intraoperative fluoroscopy whereas the CT scan revealed that displacement was indeed present.A step-off was observed in 34 patients’ radiographs, while the CT scan revealed a step-off in 57 patients. The radiographs demonstrated a smaller median step-off (4 mm) than the CT scans (9 mm), meaning there was a tendency to underestimate the step-off by a factor of two (Fig. 1). The median size of the undetected steps was 12 mm (IQR 5–15 mm) (Table 2). In one case, a step-off was observed on the radiograph while the CT revealed a medially displaced quadrilateral plate instead of a step-off.Intraoperative assessment. A gap was observed in 18 patients using fluoroscopy, whereas the postop-erative CT demonstrated a gap in all 60 patients. The median size of the gaps not detected by fluoroscopy was 5 mm (IQR 4–6 mm) on the corresponding CT images (Table 2). A step-off was observed in 11 patients using fluoroscopy, in contrast to 43 patients on the postoperative CT. The median size of the undetected steps using fluoroscopy was 3 mm (IQR 2–4 mm) (Table 2).