Evaluation of Sacrum Morphology in Patient with Developmental Dysplasia of the Hip for Iliosacral Screw Fixation

Background: The purpose of this study was to evaluate the patients with developmental dysplasia of the hip (DDH) in terms of sacroiliac anatomy to proper placement of iliosacral screws. Methods: We retrospectively reviewed computed tomography (CT) records of 96 patients who were referred to our clinic. We mainly divided the patients into 2 groups; the iliosacral joint on the same side with DDH evaluated in the DDH group and on the contralateral side with DDH evaluated in the control group. The presence of the ve qualitative characteristics of sacral dysplasia evaluated according to Route in both groups. The DDH group divided into three subgroups according to Hartolakidis and Rout classications. The cross-sectional area, length of the osseous corridor, coronal and vertical angulation evaluated in both groups. Results: Sacral dysplasia observed %87.5 in the DDH group,%83.3 in the control groups. The DDH group also exhibited a signicantly lower S1 cross-sectional area and S1 iliosacral screw length than the control group (p:0.018,p:0,027; respectively). No statistically signicant difference was observed according to Hartolakidis(p>0.05). According to Rout, the S1 iliosacral screw length of the normal and transient groups were found to be signicantly higher than those of the dysplastic groups (p: 0.004, p:0.0001; respectively). The transient group also exhibited a signicantly lower S1 iliosacral screw length than the normal group (p:0.001). There were no signicant differences in S1 and S2 axial and coronal angulation, S2 cross-sectional area, S2 iliosacral screw length in the DDH groups (p>0.05) Conclusion: When iliosacral screw is planned for patients with unilateral DDH, surgeons should consider that there are high rates of dysplastic sacral changes, differences in S1 cross sectional area and iliosacral screw length compared to the opposite side, and asymmetric sacral dysplastic changes in the upper sacrum.

characteristics. Routt et al described six qualitative characteristics for the dysmorphic sacrum [10].
Radiological studies have shown that patients with sacral dysplasia have a more narrow and angled osseous corridor in the rst sacral segment compared with the normal and transitional groups [11;12]. In the literature, up to 41% sacral dysplasia has been observed in the normal population but there are variations by ethnicity and gender [5;12]. Imai et al. found that the patients with DDH have different pelvic morphology such as pelvic incidence and anatomical pelvic tilt than the normal population [13]. In patients with preoperative sacral dysmorphism, neurovascular damage occurs at a rate of 0-1%. Patients with sacral dysplasia have small cross-sectional area for safe screws insertion because of angled osseous corridors [14;15]. The relationship between sacral dysplasia and DDH has not been discussed enough in the literature. De ning sacroiliac anatomy is important to proper placement of iliosacral screws in patients with DDH.
The purpose of this study was to evaluate the patients with DDH in terms of sacroiliac anatomy to proper placement of iliosacral screws. The relationship between the severity of dysplasia and the sacral morphology was also evaluated.

Materials And Methods
We retrospectively evaluated the radiography and CT records of patients who had DDH from January 2011 to September 2020. We found 187 patients with DDH, who had available radiography, and CT records. This retrospective study was approved by the Local Ethics Committee.

Patient selection
We evaluated the pelvic CT scans of patients aged between 20 and 86 with adequate quality images available on the Picture Archiving Communication system and imaging included the lowest rib-bearing vertebrae and the entire pelvis. We included the patients with unilateral DDH.
After exclusion criteria were applied 96 patients were selected for the study. We mainly divided the patients into 2 groups; the iliosacral joint on the same side with DDH evaluated in the DDH group and on the contralateral side with DDH evaluated in the control group.
The DDH group divided into three subgroups according to Harto lakidis Classi cation. Patients with a femoral head in the acetabulum despite some subluxation with segmental de ciency of the upper wall were named as type A group. Patients in which the femoral head formed a false acetabulum above the true acetabulum but the false acetabulum was not completely separated from the true acetabulum was named as type B group. The femoral head was not connected to the true acetabulum was named type C group [16]. The cross-sectional area, length of the osseous corridor, coronal and vertical angulation compared between the groups.
The DDH group and the control group were evaluated according to Rout et al. DDH group also divided into 3 subgroups. According to Rout et al. to compared the cross-sectional area, length of the osseous corridor, coronal and vertical angulation between the groups. The upper sacral segment morphology was evaluated as normal, transitional, and dysplastic as described. Images containing 5 qualitative characters were evaluated as sacral dysplasia which is an upper sacral segment not recessed in the pelvis, the presence of mammillary processes, an acute alar slope, a residual disc between the rst and second sacral segments and noncircular upper sacral neural foramina [10]. It was evaluated as normal group without dysplastic characteristics, dysplastic group with all of the dysplastic characteristics, and transient group with less than 5 dysplastic characteristics .
The patient's age, gender, height, weight, diagnosis for which the CT scan had been ordered were recorded (Table 1). To determine the intraobserver reproducibility, intraclass correlation coe cients (ICCs) were calculated by randomly choose 30 patients. These patients were evaluated twice, 10 days apart.
The CTs of all patients were taken in our hospital using high resolution CT with 16 sections (Somatom Emotion; Siemens Healthcare, Germany)For the CT protocol, a 0,625 mm slice intervals while the angle was at 0.1° with the precision of 0.1 mm in length was taken. The tomographic images were reconstructed via PACS (picture archiving and communication system) (In nitt, Korea). For analyzing appropriate the cross-sectional areas and short width of the safe zones, the oblique sagittal images on multiplanar reformation images was used in the same screen that was on contiguous slices perpendicular to the axis of the sacral osseous corridor on axial reformats (1). Coronal angulation was measured, using a line drawn perpendicular to the axis of the osseous corridor and a line connecting the top of the iliac crests. Axial angulation was measured, using a line drawn perpendicular to the axis of the osseous corridor and a line connecting the posterior iliac spines (5) (Fig. 1).
The CT scans were reviewed blindly by a radiologist with at least 5 years experience in musculoskeletal imaging.

Statistical analysis
Statistical analysis was performed using the SPSS version 25 software (IBM Corporation, Armonk, New York, United States). Data were analyzed using descriptive statistics (mean, standard deviation, median, frequency, percentage, minimum, and maximum). The normal distribution of the data was evaluated using the Shapiro-Wilk test. Variance homogeneity was assessed using the Levene test. The intraobserver reproducibility were determined using intraclass correlation coe cients (ICCs). One-way ANOVA post hoc Tukey test was performed to compare descriptive statistical data (mean, standard deviation, median, frequency, ratio, minimum, and maximum) and quantitative data among the three groups with a normal distribution. The Student's T-test was used for the comparison of the two groups that showed a normal distribution. Pearson correlation coe cients were calculated to analyze the relationship. They were categorized according to Dancey and Reidy grading system as follows: 0, none; 0.1 to 0.3, weak; 0.4 to 0.6, moderate; 0.7 to 0.9, strong; and 1, perfect. A p-value < 0.05 was considered statistically signi cant.

Results
No statistically signi cant difference was observed in S1 and S2 cross-sectional area, S1 and S2 maximum estimated iliosacral screw length, S1 and S2 axial and coronal angles assessment by gender, BMI, height, age in both groups (p > 0.05).
According to side, The DDH group also exhibited a signi cantly lower S1 cross-sectional area and S1 iliosacral screw length than the control group (p:0.018,p:0,027; respectively). There were no signi cant differences in S1 and S2 coronal and axial angulation, S2 cross-sectional area and S2 iliosacral screw length between groups (Table 2). Minimal cross-sectional area, coronal and axial angulation, and maximum estimated iliosacral screw length were signi cantly greater in the rst sacral segment than in the second sacral segment in both groups (p < 0.05). In the DDH group, we found no correlation between S1 maximum iliosacral screw length and S2 maximum iliosacral screw length (r:9, p:0.645). A moderate correlation was observed between S1 minimum cross-sectional area and S2 minimum cross-sectional area (r:50, p:0.012). A moderate correlation was observed between S1 axial angulation and S2 axial angulation (r:49, p:0.014). A moderate correlation was observed between S1 coronal angulation and S2 coronal angulation (r:59,p:0.002). In the control groups, we found no correlation between S1 maximum iliosacral screw length and S2 maximum iliosacral screw length, between S1 coronal angulation and S2 coronal angulation, between S1 axial angulation and S2 axial angulation (p:0.948, p:0.539, p:0.242 ; respectively). A moderate correlation was observed between S1 minimum cross-sectional area and S2 minimum crosssectional area (r:63, p:0.001) ( Table 3). comparisons; the S1 cross-sectional area of the normal and transient groups were found to be signi cantly higher than those of the dysplastic group (p: 0.004, p:0.0001; respectively). The transient group also exhibited a signi cantly lower S1 cross-sectional area than the normal group (p:0.045). A statistically signi cant difference was observed in terms of the S1 iliosacral screw length between groups (p:0.001). According to bilateral comparisons; the S1 iliosacral screw length of the normal and transient groups were found to be signi cantly higher than those of the displastic group (p: 0.004, p:0.0001; respectively). The transient group also exhibited a signi cantly lower S1 iliosacral screw length than the normal group (p:0.001). There was no signi cant differences in S1 and S2 axial and coronal angulation, S2 cross-sectional area, S2 iliosacral screw length (p > 0.05) ( Table 4). When DDH group of the patients were divided into groups according to Harto lakidis, No statistically signi cant difference was observed in S1 and S2 cross sectional area, S1 and S2 maximum estimated iliosacral screw length, S1 and S2 axial and coronal angles assessment (Table 5). An ICC value of 0.9 was considered excellent, and values between 0.8 and 0.9 were considered good [17;18]. The intraobserver ICCs were 0.82 for the S1 Minimal cross-sectional area, 0.84 for the S1 maximum iliosacral screw length, 0.86 for the S1 axial angulation, 0.83 for the S1 coronal angulation, 0.85 for the S2 Minimal cross-sectional area, 0.81 for the S2 maximum iliosacral screw length, 0.80 for the S2 axial angulation, 0.84 for the S2 coronal angulation.

Discussion
The most important nding in our study is that the patients with DDH have upper sacral dysplasia in %87.5 of the DDH group. S1 cross-sectional area and S1 iliosacral screw length on the DDH groups were signi cantly lower than the control groups. When patients with developmental hip dysplasia were divided into groups according to the Harto lakidis classi cation, no signi cant difference was observed between the groups in terms of all measurements that were evaluated. There was a moderate correlation between S1 and S2 in terms of cross sectional area, axial angulation and coronal angulation in the DDH group The upper sacral dysplasia was observed in 12.2-54% in the population according to literature [10;19]. There is a variation even according to ethnic groups. In the Asian population more sacral dysmorphism was observed than the in western population [1;20]. Mendel et al. showed that S1 cross-sectional area differs according to the side, However, no statistically signi cant difference was observed [21]. Similarly, other studies showed that the normal population with sacral dysmorphism was symmetric in the radiographs and CT scans [11]. In our study, we observed lower S1 cross-sectional area and iliosacral screw length in the DDH group compared with the control group. According to Rout classi cation, we observed asymmetrical upper sacral dysmorphism in some patients; sacral dysplasia was observed in 87.5% ( 29.1% in the transitional group and 58.3% in the dysplastic group ) of the DDH group and 83.3% ( 41.66 % in the transitional group and 41.73% in the dysplastic group ) of the control group. Especially in cases with asymmetrical dysplastic changes, if it is not planned with proper preoperative preparation, more neurovascular damage may be seen because it could change the intraoperative markers to guide iliosacral screw.
In patients with DDH, the proximal femur has increased anteversion, shortened neck, decreased intramedullary canal size [22], and the acetabulum distorted into an oval shape, and in time, femoral head migrates to the anterosuperior of acetabulum [23]. In cases with dislocated hip, increased lumbar lordosis, scoliosis and valgus deformity of the knee joint may be observed clinically [8;24]. That causes changes in the coronal alignment of the hip and knee joints in patients with DDH [25]. The pelvic incidence and anatomical sacral slope changes were observed in patients with DDH [13]. Malalignment of the lower extremity may result from soft tissue or structural bone problems. On the other hand, since the grade of hip dislocation is proportional to the lower extremity malalignment [26]. We expected more dysplastic changes at higher grades of dislocations. In our study, no relation was found between grades and dysplastic changes. On the other hand, the higher rate of sacrum dysplasia in the normal side suggests that the DDH process creates changes on the opposite side even in unilateral DDH cases. This suggests that more detailed studies should be conducted to reveal the etiology of sacral dysplasia in patients with DDH.
In the literature, a connection was observed between sacral dysmorphic changes in S1 cross sectional area and S1 iliosacral screw length in the normal population. In the study by Kım et al., It was observed that the S1 cross-sectional area was higher in the normal group compared to the transient and dysplastic group according to the rout classi cation [1]. In Kaiser et al. study less cross-sectional area and iliosacral screw length were observed in people with dysplastic changes [5]. Similarly, in our study, less S1 crosssectional area and iliosacral screw length were observed in the dysplastic group compared to the transient and normal groups. However, no change was observed in S2 cross-sectional area and S2 iliosacral screw length according to the groups.
In the literature, iliosacral screw length, cross-sectional area, and axial and coronal angulations are controversial in terms of gender in the normal population. There are studies showing that women have less cross-sectional area, iliosacral screw length and less coronal and axial angulation than men [19;21]. On the other hand, there are studies showing that there is no relation with gender [21;27]. In our study, There was no relation between gender and all these measurements between the groups. In Balling et al.' study, it was observed that there were no gender-related changes in hips with upper sacral dysmorphism [27]. Since most of the patients in our study were upper sacral dysmorphism, it may be the reason why there were no gender-related changes.
Coronal and axial angulations were higher in the sacrums with dysplastic changes. In the dysmorphic sacrum, the upper sacral screw iliac cortical starting point is posterior and caudally located. Hasenboehler et al. previously reported the mean axial angulations was 19.27° for S1 [19]. Kaiser et al's study the mean axial angulations was 11 ± 10.5, the mean coronal angulation was 22.6 ± 11.1 for S1 [5].
In the study of Kaiser et al., It has been shown that the S1 cross sectional area, iliosacral screw length, and axial and coronal angles are greater than the S2 measurements [5]. On the other hand, Garden et al.
showed that in dysplastic sacrums, cross-sectional area, iliosacral screw length and axial and coronal angles are the same in the S1 and S2 [12]. In our study, although 87.5% of the patients had dysplastic changes, we found signi cantly higher results in all measurements between S1 and S2. This difference suggests that the dysplastic changes caused by DDH are different from normal populations but further studies are needed to determine this suggestion. In addition, it was observed that a positive correlation was found with the cross-sectional area, axial and coronal angulation between S1 and S2 in DDH group.
This suggests that in patients with DDH, changes due to DDH occur not only in the rst sacrum but also in the second sacrum at a similar rate.
The number of screws and screw diameter affect the iliosacral screw length. Long screws can be placed from the posterolateral to the anteromedial direction [20;28]. The screw diameters used in daily practice are 6.3, 7 and 8 mm, the screw diameters should be selected to leave a safe osseous distance around the screw. There are lots of studies investigating the measurements by different screw numbers and screw diameters [12;29;30]. In our study, we measured vertical and posterior locations which are available for 8 mm diameter iliosacral screws. The majority of patients in our study were dysplastic and had less crosssectional area, suggesting that multiple screws are less likely. We chose 2 mm higher diameter to have su cient bone stock. Therefore, a thickness of 10 mm area has been preferred to suit the single 8 mm screw.
In the Rout et al cadaver study, those whose hips did not show dysplastic changes were normal, those who showed all dysplastic changes were dysplastic, and those who did not show all dysplastic changes were included as transient. Six qualitative characteristics were associated with the dysmorphic sacrum [10]. On the other hand, in a study showed that the tongue-in-groove was a less reliable marker for dysmorphism [5] so in our study, when evaluating sacrum dysplasia, the tongue-ingroove was not accepted as one of the characteristics. Routt et al used the radiographic outlet images to de ne sacral dysmorphism [10]. In our study, computer tomography was chosen over conventional radiography because sacral dysmorphism could also result from differences in radiographic image used for evaluation of sacral dysplasia [5].
The most important limitation in our study was we included only patients with unilateral DDH because we could not nd enough patients with bilateral DDH. Second, all the patients were of Caucasian descent. It has been shown in the literature that ethnic differences have an impact on sacrum morphology. Further studies are needed for different ethnic groups. Another limitation is that it has been observed that 6 characteristic dysplastic characteristics have moderate interobserver reliability in order to classify according to rout classi cation [5]. In our study, although strong intraobserver reliability was observed in the evaluation made with a single observer, it is thought that the evaluations made with different observers may affect the results.

Conclusion
When iliosacral screw is planned for patients with unilateral DDH, surgeons should consider that there are high rates of dysplastic sacral changes, differences in S1 cross-sectional area and iliosacral screw length compared to the opposite side, and asymmetric sacral dysplastic changes in the upper sacrum.

Declarations
Ethics approval and consent to participate