The acetabular reconstruction for Crowe III hips is a complex procedure and brings challenge to orthopedic surgeons because of the deformed acetabulum [16]. We have observed distinct acetabular morphologic variations based on the relationship between the false and the true acetabulum. In addition, we speculated different morphologies may influence the bone mass around the acetabulum, which further affected the cup position. Therefore, in this study, we aimed to 1) assess the acetabular morphologic variations of Crowe III hips by measurement; 2) study the influence of different morphologies on the cup position in THA.
Our measurement on the AP radiographs revealed that the morphology of the false acetabulum was significant different between the two groups. The larger Tonnis angle and the smaller CE angle indicated that the false acetabulum of IIIB hips has larger inclination, which is conducive to accurately classify the deformity. In addition, the superior wall of the true acetabulum in the IIIB group was significant thicker than the IIIA group (Fig. 1). One potential explanation was that the dislocated femoral head of IIIB hips migrated more laterally, resulting in more host bone above the true acetabulum to be preserved. However, not all the native bone in the true acetabulum can be effectively used in THA [17]. It is necessary to assess the position of acetabular component in the two group, to determine the influence of the acetabular morphologies.
Without structural bone grafts and any additional augmentation, acetabular reconstruction at the level of the true acetabulum was achieved in 11 (18.6%) hips of the IIIB group but no one in the IIIA group. This result demonstrated more bone mass in the true acetabular roof can provide a better containment for the placement of the cup to facilitate the anatomical reconstruction. Nevertheless, the majority of IIIB hips remained to be reconstructed in an elevated position. From the anatomical perspective, some authors have analyzed the bone stock around the acetabulum and supported the use of HHC technique in Crowe III hips [18, 19, 20]. Liu et al.[21] examined the cup coverage (CC) and rim contact (RC) in patients with Crowe III DDH, and they concluded that CC and RC increased with the peak value at 25mm and 22.5mm above the native rotation center. Previously, some studies suggested that the HHC leads to high rates of postoperative complications, including aseptic loosening, dislocation, leg-length discrepancy and limp [22, 23, 24, 25]. However, most recent clinical studies demonstrated efficacy of the HHC technique with the use of cementless acetabular cup and ceramic-on-ceramic coupling, which were also utilized in all the hips of this study[26, 27, 28]. In our study, the HHC was used in 51 (100%) IIIA hips and 48 IIIB (81.4%) hips. After eliminating the 11 IIIB hips which were reconstructed at the level of the true acetabulum, the height of dislocation was greater in the IIIB group, but the vertical distance of the rotation center was significantly lower than the IIIA group (31.2 ± 6.3mm vs 33.5 ± 4.5mm, p=0.040), which suggested the acetabulum of IIIB hips can accommodate a well-covered acetabular cup in a relatively lower position. Hence, we believed the extra bone stock in the superior wall of the true acetabulum plays a vital role in lowering the center of rotation.
To eliminate the influence of pelvic size, we performed the further evaluation using the four-zone system which was developed on the basis of AFHC. According to the four-zone system, 15 (31.3%) IIIB hips were located within the inferomedial quadrant which was the region proximal to the AFHC, while in the IIIA group, there were only 5 (9.8%) hips in this quadrant. When using HHC in the IIIA hips, we had no alternative but to implant the cup in the position of the false acetabulum. But for some IIIB hips, more preserved bone stock located at the junction of true and false acetabulum provided a “middle position” for acetabular component, which can not only permit adequate bone-implant contact, but reduce the height of the center of rotation (Fig. 6). However, the middle position was not always available, and when the bone stock at the junction was insufficient, the cup would only be placed in the position of false acetabulum using HHC. As shown in this study, 11 (22.9%) IIIB hips were located in the superolateral zone. Nevertheless, compared with 22 (43.1%) IIIA hips in the superolateral quadrant, it was significantly much less (p=0.033).
The height of dislocation is also an essential factor that influences the placement of acetabular cup. In our study, a significant difference was observed regarding the height of dislocation between the two groups. Through the correlation analysis, the vertical distance of the acetabular cup center was positively correlated with the height of dislocation in the IIIA group. As we described previously, when reconstructing the IIIA hip with HHC technique, the only location for acetabular component was in the false acetabulum. As a result, the higher the height of dislocation, the higher the acetabular component was placed. On the contrary, there was no significant correlation in the IIIB group, because implanting the cup at the junction of the true and the false acetabulum helps lower the cup position.
This study has several limitations. First, the observes found it sometimes difficult to distinguish the relationship between the false and the true acetabulum from the standard pelvic X-ray. Second, we used HHC technique, which still remained controversial, to reconstruct the majority of hips. However, the HHC have been demonstrated to provide a satisfactory result in many medium- to long-term follow-up studies [11, 26]. In addition, due to the characteristic of simplicity and effectiveness, many orthopedic surgeons prefer to use the HHC technique. Third, it was a retrospective study, raising the possibility of selection bias. Fourth, our 2D measurement of the morphology of the acetabulum on plain AP radiographs might not reflect the complex three-dimensional geometry around the acetabulum. Based on the CT data, 3D reconstruction and simulated implantation may be a more effective method.