According to previous studies1, for Crowe type Ⅱ and type Ⅲ DDH patients, it is difficult to achieve satisfactory CC when reconstructing the acetabulum due to the shallow acetabulum and bone defects. Placing the cup in the bone stock above the true acetabulum with sufficient bone mass can increase the CC and ensure the initial stability of the acetabular cup13–16, 23. Here, we found that elevating the cups with different sizes to 23.76mm above the true acetabulum increased the CC by 19.28% during acetabular reconstruction for patients with Crowe type Ⅱ and type Ⅲ DDH, which is consistent with the findings of previous studies12, 14–16, 23. In addition, the mean elevated height identified in this study meets the safety range determined by Hirakawa et al.24, who recommended that the cup should be placed < 35 mm vertically from the interteardrop line.
Sufficient host bone coverage of the acetabular cup is required to achieve satisfactory long-term implant survival for DDH patients following THA. Many studies have demonstrated that HHC technique is a feasible method for reconstructing the acetabulum at the level of true acetabulum in DDH patients14–16. In this study, except for the 48-mm and 50-mm cups, other cups achieved the mean CC of greater than 90% after elevating from true acetabulum. Many studies have shown that HHC technique does not cause aseptic loosening of cups, and the outcomes after medium and long-term follow-ups are satisfactory23, 25–28. Therefore, we believe that the HHC technique is a valid method to reconstruct acetabulum for DDH patients.
Few studies reported the analysis of acetabular reconstruction for Crowe type Ⅱ and type Ⅲ DDH patients based on CT of pelvis and acetabular cup prosthesis model. Komiyama29 used a cup with diameter of 50mm, and positioned the cup with an inclination of 40° and an anteversion of 20° to simulate acetabular reconstruction in 32 patients with Crowe type Ⅱ and type Ⅲ DDH, the cup center-edge (Cup-CE) angle was used to evaluate CC. The result shows that, at the anatomical hip center, the mean Cup-CE was − 4.3 ± 11.8°, 13 hips (40.6%) satisfied Cup-CE ≥ 0°, and the Cup CE angle ≥ 0° was used as the cut off value for the required bone coverage in their study, which was approximately equal to bone coverage of acetabular cup ≥ 60%. Xu et al16. reported acetabular reconstruction for Crowe IV DDH patients with 44-mm cup at a position of 40° abduction and 20° anteversion, the simulated mean in situ 3D CC was 78.60% (67.67%-92.51%). In 2018, Liu et al15. evaluated CC at the true acetabulum for 20 Crowe type Ⅲ DDH hips with 44mm cup model, the inclination was set to 45° constantly, and three anteversion groups with 0°, 5°, and 10° were set. The CC was 65.87% ± 7.82%, 67.77% ± 8.02%, and 68.98% ± 6.97% respectively. To our knowledge, the present stud is the first one to evaluate CC at the true acetabulum for Crowe type Ⅱ and Ⅲ DDH patients based on 3D reconstruction images and the results are different from previous views and studies, that is, using cups with diameter of 38mm, 40mm, 42mm, 44mm, 46mm, 48mm, and 50mm to reconstruct acetabulum, the cups can obtain a mean 3D bone coverage of 74.69 % at the true acetabulum. According to the existing literatures30–32, no additional operation is required to ensure the initial stability of cups during acetabular reconstruction if CC is greater than 70%.
This study analyzed CC of 44mm cups at the true acetabulum and the distribution of uncovered portion in this study. Many existing studies2, 14, 15, 17, 33, 34 have described the bone defect of the true acetabulum in DDH patients from the perspective of pelvic morphology, but few studies analyzed the uncoverage from the perspective of acetabular cup. We all know that setting a proper position of the acetabular cup is the only thing we can do during the THA, so it is the most important to carry out relevant research from the perspective of the acetabular cup. The present study is the first one that analyzed the uncovered portion of cups in THA and drew a map of uncovered portion of cups that represents bone defects of true acetabulum. From this map, we can intuitively see the distribution and probability of the uncovered portion of the acetabular cup. The results show that 40% of the uncovered portion of cups are located inside the gravity line of hip, we believe that if the uncovered portion of the cup is located inside the gravity line of the hip joint, hearing forces on the acetabular cup may lead to early loosening4. So, we can predict that about 60% of type Ⅱ and type Ⅲ DDH patients can obtain satisfactory CC without additional operation when reconstructing acetabula at the true acetabulum with 44-mm cups. This proportion is not very large, but it is really a surprising finding. That is, during acetabular reconstruction for type Ⅱ and type Ⅲ DDH patients, if careful preoperative design is carried out, in many patients, placing acetabular cup at the true acetabulum can also obtain good CC and initial stability can be ensured.
On the other hand, from the results of this study, if the acetabular bone defect is too large and bone grafting is necessary, the bone defect distribution map can be used as a guide for orthopedic surgeons to determine the location of bone grafts during preoperative design.