The most important finding 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 significantly lower than the control groups. When patients with developmental hip dysplasia were divided into groups according to the Hartofilakidis classification, no significant 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 significant 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 classification, 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 classification [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 cross-sectional 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]. Gardner et al’s study the normal sacrum’s axial angulation was 4.2 ± 3.60 and the dysplastic sacrum was 14.9 ±,9.0, the normal sacrum’s coronal angulation was 20.5 ± 6.3 and the dysplastic sacrum was 30.3 ± 4.6 [12]. In our study, the mean axial angulations was 17.92 ± 6.95 the normal S1 axial angulations was 9.16 ± 1.3 and dysplastic sacrum’s axial angulation was 19.4 ± 7.05, the mean coronal angulation was 15.7 ± 6.83 the normal sacrum’s coronal angulation was 10.86 ± 2.76 and the dysplastic sacrum was 18.94 ± 6.35. These results are similar to literature.
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 significantly 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 first 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 cross-sectional area, suggesting that multiple screws are less likely. We chose 2 mm higher diameter to have sufficient 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 define 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 find 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 classification [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.