1 Introduction¹

Acetabular dysplasia is the main cause of secondary osteoarthritis of the hip in the Asian population.¹ The ilium, the bone located above the acetabulum, is considered to be most affected in acetabular dysplasia. Regarding the pelvic anatomy of patients with developmental dysplasia of the hip (DDH), Kumeta et al.² reported that dysplasia was present not only in the acetabular region but also in the entire pelvis. It was reported that these affected pelvises take the form of an “inward wing.“^2–4 That is, in patients with DDH, the distance between the bilateral superior anterior iliac spines is short and the anterior iliac opening angle is large compared to those in the normal group. Fujii et al.³ also found differences between these distances in the sagittal plane, and reported differences in iliac rotation and extension of the innominate bone in patients with DDH. Bones other than the ilium are also known to have abnormal rotation. Suzuki⁵ reported that the rotation of the pubic sciatic bone was greater than that of the iliac bone. Fujii et al. divided acetabular dysplasia into four patterns: anterior deficiency, posterior deficiency, global deficiency, and mild deficiency, with anterior deficiency and global deficiency accounting for more than two-thirds of all patterns.^3,6–8 Abnormal acetabular direction has also been reported. Fujii et al. reported that 18% of patients with dysplasia had acetabular retroversion.^9–11 The relationship between the femur and the angle and anatomical differences of the sacrum and the vertebral bodies have also been reported, and as a result, the hip-spine syndrome has been proposed.^12–15

As DDH predominantly affects women, most of these past studies have focused on women in the study cohort. The study by Kobayashi et al. is one of a few studies to examine differences in sex among patients with DDH. They measured the pelvis of 17 males and 50 females with DDH using the same method as that reported by Fujii et al., and reported that male pelvises with DDH were narrower than those of females.¹⁶ The number of males in the study was also small,¹⁷ indicating that it was difficult to collect a similar number of male patients with DDH. This study for DDH based on women alone is reliable for analyzing accurate anatomical characteristics rather than for drawing comparisons using measurements that would have resulted from a study that included a small number of men with differences in the pelvis size and anatomical characteristics of the pelvis. Since these anatomical studies often measure the pelvis as a whole or with at least two or more elemental bones such as the iliac bone, pubic bone, ischial bone, and sacrum, it has not been possible to elicit in detail which part is actually dysplastic in DDH. Moreover, when the distance or angle between two points is measured on the pelvis as a whole, the values are related to the abnormal size, shape, and rotation of the bones. To identify the detailed anatomical features of acetabular dysplasia, it is necessary to determine whether there are any anatomical abnormalities as well as their locations. However, currently, no studies have measured the pelvic bones (iliac bone, pubic bone, and ischial bone) in isolation. Therefore, we aimed to clarify the characteristics of the pelvic anatomy related to the ilium in DDH using three-dimensional computed tomography (3D-CT).

2 Materials and methods

We compared the iliac bones, which are responsible for the coverage of the femoral head in three dimensions, between an acetabular dysplasia group (DDH group) and a control group, and then we clarified the differences between the two groups. This retrospective study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments. It was approved by the institutional review board of our university (Approval No. XX), and all the study participants provided verbal informed consent at the beginning of the study. All 3D-CT images obtained at the hospital were constructed and created by a 3D model and measured using 3D preoperative planning software ZedHip (LEXI, Tokyo, Japan). All 3D-CT images were acquired using a helical CT scanner (Aquilion CX; Toshiba Medical Systems Corporation, Tokyo, Japan) with 1-mm slice thickness.

2.2

2.2 Image analysis

We recorded the locations of the anterior superior iliac spine (ASIS), anterior inferior iliac spine (AIIS), posterior superior iliac spine (PSIS), posterior inferior iliac spine (PIIS), iliac crest (IC), and the acetabular center (AC), and the distances between each point were measured. The points of ASIS, AIIS, PSIS, and PIIS were determined from ridges on the 3D model. The IC was defined as the outermost point on the iliac crest. The AC was defined as the center of the circle fitted to the acetabular edge in the acetabular model (Fig. 1) using the method described by Köhnlein et al.¹⁸ We measured nine parameters: five parameters without the AC (ASIS-AIIS, ASIS-PSIS, ASIS-IC, and AIIS-PIIS) and the distances between these five points and the AC (AC-ASIS, AC-AIIS, AC-PSIS, AC-PIIS, and AC-IC) (Fig. 2). In order to investigate whether these parameters were related to the acetabular coverage of the femoral head, we determined whether the parameters correlated with the CE angle, which is usually used as a reference for acetabular coverage in clinical practice.

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Fig. 1 Acetabular center (AC) was defined as the intersection of the perpendicular line from the center of the circle passing through the acetabular edge and the acetabulum.

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Fig. 2 The points ASIS, AIIS, PSIS, and PIIS were determined from the ridges on the 3D model. IC was defined as the outermost point on the iliac crest. We measured the distances between each ridge (ASIS-AIIS, ASIS-PSIS, ASIS-IC, and AIIS-PIIS) and the distances from the AC to the ridge (AC-ASIS, AC-AIIS, AC-PSIS, AC-PIIS, and AC-IC). ASIS, anterior superior iliac spine; AIIS, anterior inferior iliac spine; PSIS, posterior superior iliac spine; PIIS, posterior inferior iliac spine; IC, Iliac crest; AC, the acetabulum center.

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3 Results

Table 2 shows the respective values of 112 hips of 112 patients (55 cases in the DDH group and 57 cases in the control group) measured using the 3D model, as well as a comparison of the results between the groups. There were no significant differences between the groups with regard to distances between sets of two points that did not involve the AC (ASIS-AIIS, ASIS-PSIS, ASIS-IC, and AIIS-PIIS). However, regarding distances between two points that involved the AC (AC-ASIS, AC-AIIS, AC-PSIS, AC-PIIS, AC-IC), AC-ASIS, AC-AIIS, and AC-PIIS were significantly shorter in the DDH group than in the control group (P < 0.05). There was a correlation between AC-AIIS and a weak correction between AC-ASIS and AC-IC when compared to the CE angle (Table 3). Intraobserver and interobserver reliabilities were excellent, with ICCs of 0.96–0.99 and 0.82–0.97, respectively.

4 Discussion

To the best of our knowledge, there have been no pelvic anatomical studies confined to the ilium. We did not find any significant difference between the DDH and control groups with regard to the distance between iliac points excluding the AC. This suggests that there is no significant difference in the size of the bone itself in the so-called iliac wing above the acetabulum and that there is no bone dysplasia in the iliac wing in patients with DDH.

In the evaluation of the installation angle of the cup, the anterior pelvic plane, which consists of three points among the bilateral superior anterior iliac spines and the pubic symphysis, is frequently used as the pelvic axis in modern total hip arthroplasty. Therefore, a detailed examination of iliac anatomical abnormalities affecting the location of the ASIS is an essential requirement for determining the true pelvic axis in DDH. Patients with DDH have internal recessed forms such as the so-called “inward wing”; however, if an inward-wing actually exists in the ilium of patients with DDH, it is necessary to clarify whether it is due to abnormal rotation of the entire ilium or an anatomical deformation of the ilium itself. In measuring the distances between the ASISs across the entire pelvis, it was unclear if the measurements accurately represented the actual anatomy because they included elements of both bone rotation and dysplasia.

We believe that the internal recess formed in patients with DDH is caused only by abnormal rotation of the iliac wings, not by dysplasia. Nevertheless, because we measured straight line distances, it is not possible to compare detailed differences in the shape of the ilium, such as the curvature of the iliac wings. Demir et al.²⁰ measured straight distances using Turkish dry hip bones, using the same measurements we used in our study. Their values for ASIS-AIIS, ASIS-IC, and AIIS-PIIS measured 41.46 ± 5.02 mm, 60.33 ± 10.15 mm, and 117.51 ± 7.70 mm, respectively. In this study, these values were 40.0 ± 4.6 mm, 61.2 ± 9.0 mm, and 123.3 ± 9.0 mm, respectively. There were some discrepancies in the measured values, possibly owing to the differences in pelvic anatomy between different ethnicities.^21,22

There was a significant difference between the groups in terms of distances between the two points including the AC, suggesting that there is dysplasia of the iliac bone including the acetabulum. In addition, as the AC-AIIS, AC-ASIS, and AC-IC correlated with the CE angle in the present study, the results suggest that the dysplasia of the iliac bone including the acetabulum is related to the acetabular coverage of the femoral head. Nevertheless, the method for identifying the AC in our study may be affected by the deficiency and direction of the acetabulum. This will be discussed in terms of the following three effects: (1) effect of the roof, (2) effect of deficiency, and (3) effect of retroversion. Regarding the effect of the roof, the angle between the sagittal axis and the circle at the acetabular margin in the coronal plane should be more acute in the DDH group than in the control group. The AC in the DDH group is identified below that of the control group. We expected that the distance between all sets of points would be longer in the DDH group; however, the opposite was the case. Regarding the effect of deficiency, as described above, acetabular deficiency and global deficiency were common; therefore, if an error occurs in the center position, the AC in the DDH group would be identified posteriorly and inferiorly. We expected that AC-ASIS and AC-AIIS would become longer and that AC-PSIS and AC-PIIS would become shorter in the DDH group; however, the opposite was the case. Regarding the effect of retroversion, the AC in the DDH group should be identified anteriorly. We expected that AC-ASIS and AC-AIIS would become shorter, and AC-PSIS and AC-PIIS would become longer; however, there were no significant differences in AC-PSIS and AC-PIIS. These findings suggest that the method for identifying the AC in this study resulted in no substantial differences between the DDH and control groups. By measuring the pubis and the ischium at the same points in the future, it will be possible to validate the accuracy of our method for identifying the AC.

The iliac proximal growth line is the iliac wings, and the distal point is the Y cartilage. We found that there was dysplasia, especially in the antero-superior part of the acetabulum. In other words, in patients with acetabular dysplasia, iliac dysplasia occurs only in the Y cartilage during the growth stage, and the anterior side of the Y cartilage tends to be dysplastic. The fact that there was a significant difference in the anterior superior direction when the acetabulum part was included and that there was no significant difference with regard to the shape of the iliac wing suggests that the acetabulum is shifted antero-superiorly (toward the ASIS).

There are several limitations in this study. First, the study cohort was limited to Asian women. DDH is common among women and rare among men; therefore, it is difficult to draw comparisons between the sexes. Furthermore, pelvic anatomy and pelvic size differ between the sexes, which has been reported even in patients with DDH.¹⁶ As the measured values may vary significantly, only women were surveyed in this study. Therefore, comparisons including men will be necessary in the future. Second, the coverage was used to evaluated only the correlation between the CE angle and the parameters in the present study. Though CE angle has been used as a value of acetabular coverage for the femoral head in clinical practice, it is a two-dimensional evaluation on the aspect passing through the center of femoral head. However, we found a tendency toward dysplasia in the anterior superior region, which appears to reflect the results of anterior and global deficiency. Therefore, it is important to evaluate the relationship between the three-dimensional parameters for the acetabular coverage and anatomical abnormalities in more detail in future studies. Third, the AC should not be the center of the Y cartilage. In this study, only the iliac bone was measured; therefore, it is desirable to measure the acetabulum only with the iliac component, that is, from the center of the Y cartilage. However, because it was difficult to identify the center of the Y cartilage on CT, the AC was treated as the center of the Y cartilage and was used as the iliac margin. Identification of the true Y cartilage center is desirable for more accurate comparisons. In the future, it will be necessary to compare the shapes of the ilium; to evaluate the pubis, ischium, and ilium; and to compare the measured values with the coverage.

5 Conclusions

In conclusion, our study showed that there were no significant differences in size between the DDH and control groups in the upper part that did not include the acetabulum; however, there was a significant difference in the acetabular part. These findings suggest that, in patients with DDH, bone dysplasia occurs only the iliac bone near the acetabulum and not in the iliac wings. The acetabulum in patients with DDH has to shift antero-superiorly (toward the ASIS) secondary to osteogenesis in the acetabulum. The dysplasia of the iliac bone including the acetabulum may be related to the acetabular coverage of the femoral head. Identifying how anatomical features of patients with DDH differ from those of the normal pelvis will assist in setting the true pelvic axis and facilitate the planning for the placement of the cup at the appropriate angle. Therefore, future studies are needed to clarify the detailed anatomical features of DDH such as anatomical differences between sexes.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Author contributions

Noriaki Sako: Analysis and Writing the original draft. Nobuhiro Kaku: Conceptualization, Methodology, and Review & editing. Yuta Kubota: Validation. Yoshiki Kitahara: Data curation. Hiroaki Tagomori: Project administration. Hiroshi Tsumura: Supervision. All authors have read and approved the final version to be published.

Unblinded ethics statement

This retrospective study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments. It was approved by the institutional review board of our university (Approval No. 1052, June 17, 2016).