Abstract
Background
Pelvic inlet and outlet fluoroscopy views are routinely used in operative treatment of posterior pelvic ring injuries. In this study, we aimed to evaluate the angles of pelvic inlet and outlet fluoroscopic view, their differences with hip flexion and the correlation of these differences with sacral slope changes.
Materials and methods
Sagittal reconstructions of 100 lumbopelvic CT were used to measure sacral slope, pelvic inlet and outlet view angles. The range of pelvic inlet–outlet view angles and their relation with age, sex and sacral slope were analyzed. In ten of these 100 patients, who were undergone a second CT imaging, hips were passively flexed to 60° to change pelvic tilt. The difference in sacral slope and pelvic inlet–outlet view angles in different positions were compared.
Results
Mean angles for inlet view, outlet view and sacral slope were 28.9, 41.4 and 37.0, respectively. There was no difference between males and females (p > 0.05). Pelvic outlet angles had a negative correlation with age (p < 0.05). Sacral slope changes with hip flexion showed a negative correlation with inlet angles and positive correlation with outlet angles (p < 0.05). The differences in sacral slope, pelvic inlet and outlet view angles between two measurements were equal.
Conclusions
The pelvic inlet and outlet view angles shows a wide range without a standard so we suggest preoperative CT scan to plan the optimal angles before pelvic ring surgery. The difference in these angles due to pelvic tilt during the surgery may be corrected by measuring the sacral slope difference.
Keywords: Pelvis, Inlet view, Outlet view, Sacral slope, Fluoroscopy
Introduction
Pelvic inlet–outlet views are used in the diagnosis and treatment of posterior pelvic ring injuries [1]. Pelvic inlet and outlet views combined with lateral fluoroscopic images are used peroperatively for percutaneous fixation of iliosacral injuries [2]. Sacral morphology and optimal angles to obtain proper images show great differences among population [3–6]. Preoperative computerised tomography (CT) imaging is necessary to understand the structure of sacrum, plan the peroperative fluoroscopy angles for pelvic inlet and outlet views, and define the pathways for fixation screws [3, 7]. Peroperative definition of optimal fluoroscopy angles without preoperative planning may cause prolonged surgical time and increased radiation exposure. Patient positioning without preoperative planning may also limit free movement of fluoroscopy arms to provide optimal views [3].
Sacrum has an oblique position in sagittal plane. Fluoroscopy arm should be tilted to provide the proper visualization. The angle for optimal pelvic inlet and outlet view could be measured by sagittal midline CT images [1, 5]. Pelvic inlet and outlet image provides the axial and coronal view of the S1 vertebra, respectively. Coronal view is used to avoid penetration of inferior neural foramen and superior L5-S1 disc. Axial view shows anteroposterior safe trajectory of iliosacral screw to prevent anterior cortex and posterior spinal canal penetration. Another essential point of preoperative planning is the evaluation of sacral morphology. Sacral dysmorphism was reported in nearly half of the adult population [8]. It was proposed that, safe trajectory is not existed in the first sacral segment for a transsacral screw placement in case of sacral dysmorphism, however second sacral segment is available for it [9].
Pelvic tilt defines the peroperative fluoroscopy angles for pelvic inlet–outlet views. The difference in patient position would lead to a difference in the pelvic tilt [10]. It has been previously shown that the pelvis can move from a position of 5° (± 9.5°) extension tilt when supine to a 3° (± 11.9°) extension position when standing and to a 29° (± 12.2°) flexion position when sitting [11]. Pelvic tilt and sacral slope are related and the degree of pelvic position changes are equal to sacral slope differences. Pelvic tilt and sacral position may change peroperatively under several circumstances including: (1) Different hip position (flexion-neutral-extension); (2) relaxation of lumbar muscle spasm under anesthesia; (3) inclination of the operating table. Preoperatively planned angles with CT for optimal peroperative inlet–outlet views may alter cause of the mentioned reasons. However, the relation between preoperatively measured sacral slope and inlet–outlet angles does not change. Lateral fluoroscopy is suboptimal for measuring pelvic inlet and outlet angles during surgery as the anatomic landmarks are difficult to be viewed in a single image. But, sacral slope could be measured more readily with lateral fluoroscopic images and it could be used to define optimal pelvic inlet–outlet angles peroperatively; if the relation between sacral slope and these angles was measured preoperatively.
In this study, we aimed to measure the range of angles for pelvic inlet and outlet views in consecutive patients without lumbar or pelvic pathology and sacral dysmorphism. We hypothesized that the optimal angles for pelvic inlet and outlet view may change with hip flexion position and these changes may correlate with the differences in sacral slope. Another aim was to prove that differences in sacral slope measurements may be used for the correction of differences in pelvic inlet and outlet measurements in different hip positions. We also aimed to define a novel method to get proper intraoperative inlet and outlet views by using sacral slope differences between the preoperative CT images and peroperative lateral fluoroscopic views.
Materials and Methods
This study was approved by Institutional Review Board. A hundred consecutive lumbopelvic CT image series for urinary stone evaluation were obtained from our institution’s radiology database. All CT images were obtained from a single device (Somatom Definition Flash-Siemens) and scans were reviewed by using the picture archiving system (Synapse Pacs). Skeletally mature patients which are older than 18 years with no lumbar or pelvic pathology and sacral dysmorphism were included in the study. CT images with obvious pathology or severe degenerative findings were excluded. CT imaging were performed from L1 upper endplate to the femoral heads in all included patients. Routinely, all CT images were obtained in supine position with both hips in neutral. Three measurements were made in all CT images. Measurements were performed by three orthopaedic surgeons until the consensus was reached. Pelvic inlet–outlet view fluoroscopy angles and sacral slope were measured. In ten of the patients who had a second CT imaging (necessary for re-assessing urinary stones), both hips were passively flexed to 60° to alter pelvic tilt (Fig. 1).
Fig. 1.
60° of hip flexion position
Midline sagittal reconstruction view were used for measurements. Reference line was provided in all CT images as a parallel line to the CT table and lying surface. Another line vertical to this reference line was drawn. Anterior edge of S1 superior endplate and anterior edge of S2 inferior endplate were used for drawing a line simulating pelvic inlet view. The degree between this line and the vertical line was pelvic inlet fluoroscopy view angle (Fig. 2). Secondly, a line formed by connecting upper border of pubic bone with the centre of S2 corpus. The measurement between vertical line and this line was pelvic outlet fluoroscopy view angle (Fig. 3). The angle between the tangential line to the superior end plate of S1 and the vertical line was sacral slope (Fig. 4). In ten patients, mid-sagittal imaging in two different positions were used for calculations. Lumbar lordosis was also measured in these patients. Cobb’s method was used for the measurement of lumbar lordosis (Fig. 5). The difference in lumbar lordosis, sacral slope and pelvic inlet–outlet view angles in different positions were noted for further analysis.
Fig. 2.
Pelvic inlet view angle. Blue line is parallel to the computerised tomography (CT) table and red line is vertical to it
Fig. 3.
Pelvic outlet view angle. Blue line is parallel to the CT table and red line is vertical to it
Fig. 4.
Sacral slope angle. Blue line is parallel to the CT table and red line is vertical to it
Fig. 5.
Cobb angle
Our hypothesis was that the optimal angles for pelvic inlet and outlet view may change under certain circumstances and sacral slope differences may be used for the correction of the optimal angles. Clinical correlation of our hypothesis was made by five volunteer patients who were previously operated for iliosacral screw fixation. Postoperative CT was used for measurements of sacral slope, pelvic inlet and outlet angles. The relation between sacral slope and pelvic inlet–outlet angles were simply measured by calculating their differences. Patients were taken to the operation room for fluoroscopic examination. Lateral fluoroscopic views of first sacral vertebra were obtained in both hips in extension and 60° flexion position. Sacral slope measured for both positions. The difference between CT measured and lateral fluoroscopy measured sacral slopes was used to calculate the angles of optimal pelvic inlet and outlet views. Later, fluoroscopy arm was adjusted to anteroposterior position and moved to the calculated inlet and outlet view angles while hips in extension and 60° flexion position. Three surgeons agreed with consensus whether the views obtained were optimal and similar in both positions. A smartphone with a goniometer application was placed over the fluoroscopy arm to measure the degree of it during inlet and outlet views. Sacral slope measurements and fluoroscopy arm angles during pelvic inlet and outlet views were noted.
Mean, standard deviation, median, lowest, highest, frequency and ratio values were used in descriptive statistics of data. Distribution of variables were measured by Kolmogorov Smirnov test. Mann–Whitney u test was used for analysis of independent quantitative data. Wilcoxon test was used for evaluating dependent quantitative data. Spearman analysis was used for evaluating the correlation between variables.
Results
Consecutive CT imaging of a hundred patients were evaluated. 60 of the patients were male and the other 40 were female. Mean age of the patients were 39.1 years (min 19–max 76). Mean angles for inlet view-outlet view and sacral slope were 28.9, 41.4 and 37.0, respectively (Table 1). There is no significant difference between males and females for pelvic inlet, outlet and sacral slope angles (p > 0.05) (Table 2). Pelvic inlet and sacral slope angles have no correlation with age (p > 0.05). However pelvic outlet angles have a negative correlation with age (p < 0.05) (Table 3).
Table 1.
Demographic features and average angles
| Min–max | Median | Avr. ± s.d./n–% | ||
|---|---|---|---|---|
| Age | 19.0–76.0 | 36.0 | 39.1 ± 15.6 | |
| Age | ||||
| < 50 | 75 | 75.0% | ||
| ≥ 50 | 25 | 25.0% | ||
| Sex | ||||
| Male | 40 | 40.0% | ||
| Female | 60 | 60.0% | ||
| Inlet angle | 10.0–74.0 | 27.0 | 28.9 ± 8.9 | |
| Outlet angle | 23.0–58.0 | 41.0 | 41.4 ± 6.2 | |
| Sacral slope | 20.0–60.0 | 35.5 | 37.0 ± 7.8 | |
Table 2.
Comparison of pelvic angles between males and females
| Female | Male | p | |||
|---|---|---|---|---|---|
| Avr. ± s.d | Median | Avr. ± s.d | Median | ||
| Inlet angle | 27.8 ± 9.4 | 27.0 | 29.6 ± 8.6 | 30.0 | 0.207m |
| Outlet angle | 41.7 ± 7.9 | 42.5 | 41.2 ± 4.9 | 41.0 | 0.341m |
| Sacral slope | 38.5 ± 8.3 | 38.0 | 36.1 ± 7.4 | 35.0 | 0.230m |
mMann–Whitney u test
Table 3.
Correlation of pelvic angles with age
| Inlet angle | Outlet angle | Sacral slope | |
|---|---|---|---|
| Age | |||
| r | − 0.143 | − 0.272 | 0.048 |
| p | 0.157 | 0.006 | 0.635 |
Spearman correlation
Statistically significant correlation is given in bolditalic ( p < 0.05)
In ten of the cases pelvic inlet–outlet angles, sacral slope and lumbar lordosis were measured in two different hip positions; in extension and 60° of passive hip flexion (Table 4). The difference between two measurements for inlet, outlet, sacral slope and lumbar lordosis angles in different hip positions were not significant (Table 5). Sacral slope changes between two measurements in different hip positions showed a negative correlation with pelvic inlet angles and positive correlation with pelvic outlet angles (p < 0.05). Lumbar lordosis differences have a positive correlation with sacral slope and pelvic outlet angles, whilst a negative correlation with pelvic inlet angles (Table 6). The differences in sacral slope between two measurements were equal to the differences in pelvic inlet and outlet angles, but not with lumbar lordosis (Table 7).
Table 4.
Demographic features of patients with two measurements in different hip positions
| Min–max | Median | Avr. ± s.d./n–% | ||
|---|---|---|---|---|
| Age | 20.0–63.0 | 33.5 | 34.9 ± 11.6 | |
| Sex | ||||
| Female | 4 | 40.0% | ||
| Male | 6 | 60.0% | ||
Table 5.
Comparison of measured angles between two different hip positions, extended and 60° flexed
| Min–Max | Median | Avr. ± s.d | p | |
|---|---|---|---|---|
| Inlet angle | ||||
| I. measurement (hip extended) | 13.0–38.0 | 29.5 | 27.8 ± 7.7 | 0.765w |
| II. measurement (hip flexed) | 8.0–39.0 | 30.0 | 27.5 ± 10.6 | |
| Outlet angle | ||||
| I. measurement (hip extended) | 34.0–50.0 | 42.0 | 41.6 ± 5.2 | 0.858w |
| II. measurement (hip flexed) | 35.0–52.0 | 40.0 | 41.8 ± 6.0 | |
| Sacral slope | ||||
| I. measurement (hip extended) | 27.0–60.0 | 32.5 | 37.3 ± 11.4 | 0.858w |
| II. measurement (hip flexed) | 24.0–70.0 | 32.0 | 37.5 ± 15.0 | |
| Lumbar lordosis | ||||
| I. measurement (hip extended) | 10.0–61.0 | 23.5 | 30.7 ± 17.4 | 0.052w |
| II. measurement (hip flexed) | 17.0–69.0 | 32.0 | 35.0 ± 18.6 | |
wWilcoxon test
Table 6.
Correlation of measured angles with different hip positions
| Outlet angle | Sacral slope | Lumbar lordosis | |
|---|---|---|---|
| Inlet angle | |||
| r | − 0.991 | − 0.991 | − 0.718 |
| p | 0.000 | 0.000 | 0.019 |
| Outlet angle | |||
| r | 1.000 | 0.690 | |
| p | 0.000 | 0.027 | |
| Sacral slope | |||
| r | 0.690 | ||
| p | 0.027 | ||
Spearman correlation
Statistically significant correlations are given in bolditalic ( p < 0.05)
Table 7.
Differences of angles between two measurements
| Case | Age | Sex | Inlet angle differences | Outlet angle differences | Sacral slope differences | Lumbar lordosis differences |
|---|---|---|---|---|---|---|
| 1 | 30 | E | − 4 | + 4 | + 4 | + 3 |
| 2 | 27 | K | − 1 | + 1 | + 1 | + 3 |
| 3 | 27 | E | 0 | 0 | 0 | + 1 |
| 4 | 41 | K | − 3 | + 3 | + 3 | + 2 |
| 5 | 36 | E | + 15 | − 15 | − 15 | − 4 |
| 6 | 63 | K | − 5 | + 5 | + 5 | + 10 |
| 7 | 33 | E | − 5 | + 5 | + 5 | + 7 |
| 8 | 34 | E | + 5 | − 5 | − 5 | − 2 |
| 9 | 38 | E | + 5 | − 5 | − 5 | + 10 |
| 10 | 20 | K | − 10 | + 10 | + 10 | + 15 |
Five patients, who were previously operated for iliosacral screw fixation, were analyzed with postoperative follow-up CT and fluoroscopic examinations while hips in extension and 60° flexion position. Three of the patients were male and two were female. Mean age of the patients were 31 years (min 24–max 37 years). Mean postoperative period was 10 months (min 6–max 15 months). Sacral slope, pelvic inlet and outlet view angle were different in all three measurements for each patient. The mean difference of sacral slope, between CT measurement—hips in extended position and CT measurement—hips in 60° flexed position were 6.8° (min 3–max 10) and 6.2° (min 2–max12), respectively. It was observed that the corrected inlet and outlet angles provide the optimum and similar views in both positions. The measurements of sacral slope, pelvic inlet and outlet angles were given in Table 8.
Table 8.
Sacral slope, pelvic inlet and outlet angles of five clinical cases
| Computerized tomography | Hips in extension | Hips in 60° flexion | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Sacral slope | Pelvic inlet | Pelvic outlet | Sacral slope | Pelvic inlet | Pelvic outlet | Sacral slope | Pelvic inlet | Pelvic outlet | |
| Case 1 | 38 | 19 | 39 | 46 | 11 | 47 | 50 | 7 | 51 |
| Case 2 | 35 | 28 | 50 | 45 | 18 | 60 | 39 | 24 | 54 |
| Case 3 | 45 | 18 | 44 | 54 | 9 | 53 | 43 | 20 | 42 |
| Case 4 | 51 | 22 | 43 | 54 | 19 | 46 | 55 | 18 | 47 |
| Case 5 | 42 | 21 | 44 | 46 | 17 | 48 | 51 | 12 | 53 |
Discussion
In this study, it was showed that pelvic inlet, outlet and sacral slope angles have a wide range of variation among population. Although mean values of these angles were provided in this study and in literature, their standard use in clinical practice is not optimal cause of the wide range of variation. Preoperative CT scan to plan the optimal angles before pelvic ring surgery is proposed in literature [3]. However, the angles that were measured in preoperative CT images may vary in operation room due to several circumstances [2]. The change in sacral slope remains constant. Sacral slope can also be measured in the lateral fluoroscopic images just prior to surgery. The difference of sacral slope between preoperative CT and fluoroscopic lateral image can be used to determine optimal angles to obtain intraoperative inlet and outlet views. The main suggestion of this study is that the ‘sacral slope’ can be used as a corrective parameter to get appropriate intraoperative inlet and outlet views.
During the preoperative planning phase, angles to get optimal inlet and outlet views should be measured. Additionally, we suggest to measure the sacral slope in preoperative CT images. We also suggest to get a proper lateral fluoroscopic image to measure sacral slope prior to surgery after the anaesthetics administration. Perfect superimposition of iliac wings is necessary for a proper lateral view. The difference of the sacral slope between the preoperative CT and lateral fluoroscopy images should be noted. In patients with posterior pelvic ring injuries, peroperative reduction attempts with hip flexion and relieved lumbar muscle spasm with anesthesia may cause significant differences in pelvic tilt, so the pelvic inlet and outlet angles. The noted sacral slope difference and preoperative measurement angles could be useful to calculate optimal peroperative pelvic inlet and outlet angles. The fluoroscopy technician should be informed to obtain a proper lateral view, measurement of sacral slope and calculation of peroperative optimal angles. Also, we recommend to use a goniometer (smartphone applications are useful) placed over the fluoroscopy arm for exact positioning of it during inlet and outlet views.
Traditionally, pelvic inlet and outlet views are obtained in 45° [5]. A standard angle for these imaging may cause inappropriate radiographies [1]. Sagittal midline CT images are used to measure angles for pelvic inlet and outlet views [1, 5, 12]. In a study, Ricci et al. defined the reference lines on computerised tomography imaging to reflect the optimal pelvic inlet and outlet views [5]. They suggested to obtain inlet and outlet images at 25° and 60°, respectively [5]. Pekmezci et al. reported similar values of optimal pelvic inlet and outlet angles with Ricci et al. by the same measurement methods [12]. However, they also used 3-dimensional CT analysis to evaluate optimal angles and conclude that 25° and 45° angles should be used for pelvic inlet and outlet views, respectively [12]. In another study, computed tomography scans of 110 Indian people were evaluated by the same methods with the Ricci et al.’s study. They reported mean inlet view angle was 33° and mean outlet view angle was 56° [1].
Appropriate pelvic inlet and outlet fluoroscopic views are necessary for iliosacral screw fixation of posterior pelvic ring injuries [13]. Outlet view is used for avoiding inferior neural foramen and superior L5-S1 disc penetration. Inlet view shows anteroposterior safe trajectory of iliosacral screw to prevent anterior cortex and posterior spinal canal penetration. Inappropriate views with incorrect angles are misleading in iliosacral screw fixation of posterior pelvic ring injuries. In a study, Gusic et al. measured average inlet and outlet view angles of 30 patients; 22° and 42°, respectively [3]. They used a parallel line to the anterior cortex of S1 and S2 body for inlet view and a line connecting upper part of symphysis and centre of S2 body for outlet view. They reported that the free space under the operating table should be at least 145 cm for adequate manoeuvre of fluoroscopy [3]. In the present study, we used the same measurement method and we found average inlet and outlet view angles 29° and 41°, respectively.
Preoperative CT measurements and peroperative fluoroscopy angles for optimal pelvic inlet and outlet views may show some differences. Eastman et al. reported that the difference of preoperatively measured angles with CT images and intraoperative fluoroscopy angles for inlet and outlet views are within 5° [2]. Positioning of the patient, relaxation of muscular spasm under anesthesia and inclination of the operating table are possible causes of these variations. In this study, we used passive hip flexion in ten patients to provide pelvic tilt change simulating possible peroperative changes. We showed that the differences in sacral slope, pelvic inlet and outlet view angles between two measurements were equal.
The clinical implication does not change between normal subjects and the patients previously operated. Clinical correlation of our findings was made by five volunteer patients who were operated previously for iliosacral screw fixation. It was observed that sacral slope, pelvic inlet and outlet view angles were different between CT measurements and fluoroscopic examinations on operation table. The mean sacral slope difference was found to be 6°–7° between CT measurements and fluoroscopic examinations. Correction of pelvic inlet and outlet angles by using sacral slope difference in different circumstances has been shown to be an effective method.
We suggest preoperative CT scan planning of pelvic inlet–outlet angles. However, surgeons should be aware of the conditions that the preoperatively measured angles may change with hip flexion or other circumstances, intraoperatively. We present the method of preoperative and peroperative sacral slope difference measurement for correction of preoperative planning to provide optimal imaging and avoid prolonged surgery time or increased radiation exposure.
In clinical practice, a simple algorithm could be given to the fluoroscopy technician to correctly calculate peroperative optimal pelvic inlet and outlet angles based on preoperative CT scan measurements and peroperative sacral slope measurement. Peroperative pelvic inlet and outlet angles can be calculated in three steps.
Preoperative sacral slope angle (CT scan) − Peroperative sacral slope angle = Sacral slope difference
Preoperative pelvic inlet angle (CT scan) + Sacral slope difference = Peroperative pelvic inlet angle
Preoperative pelvic outlet angle (CT scan) − Sacral slope difference = Peroperative pelvic outlet angle
The weak point of this study is the low number of cases with second CT scan in hip flexion position. However, the correlation of our results with clinical cases has been showed. Another limitation is that the clinical cases were previously operated patients. We recommend further studies with patients evaluated during surgery to analyze the relation of preoperatively planned and peroperatively obtained pelvic inlet–outlet angles.
Conclusion
The pelvic inlet and outlet view angles shows a wide range without a standard so we suggest preoperative CT scan to plan the optimal angles before posterior pelvic ring surgery. The difference in pelvic inlet and outlet view angles due to pelvic tilt during the surgery may be corrected by measuring the sacral slope difference.
Funding
There is no funding source.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
Ethical Standard
This study was approved by Institutional Ethical Review Board.
Informed Consent
Informed consent was obtained from all individual participants included in the study.
Footnotes
Publisher's Note
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Contributor Information
Deniz Aydın, Email: deniz.aydin@neu.edu.tr, Email: denizayd@yahoo.com.
Enes Sarı, Email: enes.sari@neu.edu.tr.
Kaan Erler, Email: kaan.erler@med.neu.edu.tr.
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