Reconstructing dysplastic acetabulum presents considerable technical challenges for orthopedic surgeons, especially for a patient with Crowe III DDH. The placement of acetabular cup for Crowe III hips has several alternative positions in the presence of a false acetabulum, including the position of true acetabulum, false acetabulum and the middle of them. This study aimed to evaluate the outcomes of three different cup positions.
In our clinical opinion, an appropriate cup position depends primarily on three elements: (1) Lower limitation on selection and orientation of the acetabular component; (2) Be beneficial to intraoperatively correct leg length discrepancy; (3) Low incidence rate of postoperative complications.
Selection And Placement Of The Cup
Compared with the F group and M group, the diameter of acetabular component was significantly smaller when performing anatomical reconstruction. That could be explained by dysplastic deficiency of the true acetabulum, making it insufficient to accommodate a large sized acetabular cup. In fact, a larger cup and femoral head tended to be a preference which were reported decreasing postoperative dislocation rate among DDH patients(13). Moreover, Murray et al.(14) have demonstrated that a small sized cup and femoral head were associated with polyethylene wear and periprosthetic osteolysis in patients with hip dysplasia. Thus, polyethylene liner was not suggested when putting the cup at the anatomic position. In short, the position of true acetabulum restricted the choice of acetabular cup and liner.
Our results showed that anteversion in the A group was comparatively larger than that in other two groups even there was no statistically difference in inclination and anteversion among three groups. The main explanation was relatively more bone mass of posterior wall of true acetabulum. Because the cup was intraoperatively adjusted to attain adequate coverage, the orientation of the cup was partly determined by surrounding bone mass, especially the bone stock of superolateral rim and anterior and posterior wall which were always deficient in false acetabular position and middle position. However, Zheng et al.(8) have reported that medial wall thickness in Crowe III hips increased with the distance from teardrop and the bone-cup coverage could achieve at least 70% by medialization when the vertical height of the cup center ranged from 21 mm to 36 mm. In this study, the vertical height of the cup in the F group and the M group were respectively 31.6 ± 5.9 mm and 31.3 ± 6.7 mm. Accordingly, medialization could meet the requirements of bone-cup coverage with no need for excessively abducing the cup. But for anatomical reconstruction, due to a relatively thin medial wall, augmentation by structural autograft to supplement superolateral insufficiency was commonly required. Of the 15 hips in the A group, 5 hips utilized the femoral head autograft during cementless THA. In spite of the complex and time-consuming procedure of autograft, it’s notable that a cementless socket rather than a cement socket used in conjunction with a femoral head autograft have been demonstrated could provide good long-term results by Spangehl et al.(15) and Abdel et al.(16). Therefore, with the assistance of structural autograft and medialization, different cup positions wouldn’t strongly influence the orientation of acetabular cup.
Leg Length Discrepancy
Correcting leg length discrepancy was an important issue of THA for Crowe III hips. Our postoperative measurement showed a favorable result in each group and no difference was shown among three groups. However, a modular S-ROM femoral stem was utilized in the A group more frequently to restore the leg length. Sometimes, a cone instead of sleeve was applied to deepen the position of the stem in the femoral canal. By contrast, a common femoral prothesis such as Corail stem utilized in 19 hips (59.4%) of the M group and 7 hips (50%) of the F group, approving that it was more convenient to correct LLD when fixing the cup in the high rotation center. In brief, the elevation of the stem could be easily achieved by a larger size common femoral stem while the subsidence of the stem always needed a modular S-ROM femoral stem.
As is known, the complication rate is higher in patients with hip dysplasia than it is in patients who have osteoarthritis(17). Especially, a high hip center which was utilized in all cases of the F group and the M group was always associated with polyethylene liner wear, cup loosening, dislocation and limp(18–20). Nawabi et al.(20) reported a higher polyethylene liner wear rate of lateralized high hip centers. While different, we used a COC interface in all cases, as we hypothesized that the favorable wear characteristics of COC bearing surfaces may counteract to the excessive joint reaction forces. In addition, because it was thought that the risk of dislocation which is greater with a high hip center might result from the impingement, some measures were taken to avoid that, including completely removing of osteophytes, a larger size head, an increased femoral offset and sometimes a modular femoral stem to adjust appropriate anteversion. As a result, no dislocation occurred in our study. Furthermore, limp after a high hip center THA was also a major concern. However, recent studies on HHC technique have reported numerous favorable results of postoperative gait(21, 22). In this study, there were only 2 patients (4.8%) with limp. But it’s worth noting that the cup position of the F group was significantly more lateral than that of the M group. Though no limp and positive Trendelenburg sign was observed in the F group, lateralization would biomechanically increase the burden of the gluteus medius and potentially raise the risk of limp.
For an anatomical reconstruction, the main advantage was restoration of near-normal biomechanics. In this study, anatomic placement was only performed when contralateral hip was normal (10 hips) or Crowe IV (5 hips). It’s helpful for an equilibrium with a normal or anatomical reconstructed Crowe IV contralateral hip. Moreover, a potential particular complication in the A group was the bone absorption of autograft. But in fact, autograft in the cementless THA have been demonstrated to be a reliable method(16). All autografts integrated and no bone absorption occurred in our study also proved it.
Our study has several limitations. First, this was a retrospective study and all operations were performed in single center. Second, this study included a relatively small sample size, maybe because the series of patients with this deformity were not common. Third, the follow-up time varied greatly, from 1.1 to 11.3 years. All of the above reasons may have affected the objectiveness of this study. However, as the first study focusing on the Crowe III hips with a false acetabulum, we think this study is of great value.