| Background:The best approach for correct placement of the acetabular component in total hip arthroplasty(THA)remains controversial for patients with developmental dysplasia of the hip(DDH),especially for those with Crowe type Ⅱ and type Ⅲ DDH.When the acetabular cup is placed at the level of the true acetabulum,the bone defect caused by acetabular dysplasia may result in less cup coverage(CC)in THA.thus leading to poor initial stability and a low long-term survival rate.To achieve adequate cup coverage,several studies have recommended structural bone grafting and the use of extra-small cups for placing the acetabular cup in THA.However,it remains disputable whether these techniques are helpful for reconstructing the acetabulum in DDH patients,because these techniques are associated with various complications and uncertain long-term survival rates.The high hip center(HHC)technique,which means placement of the acetabular cup above the true acetabulum during acetabular reconstruction,is suitable for DDH patients,because it can make full use of the bone above the true acetabulum to ensure adequate host bone coverage of the cup.In addition,the HHC technique is also associated with some advantages such as reducing additional surgery,shortening operation time,reducing intraoperative exposure,and reducing the incidence of postoperative infection.Acorrdingly,there will be no cup loosening caused by complications such as bone resorption,bone collapse and uneven stress distribution caused by bone grafting that may result in surgical failure,and the limitation of hip joint mobility caused by small cups.Objective:Based on the existing research,the purpose of this study is to explore whether the upward movement of the rotating center can effectively improve the coverage of the cup in DDH patients of Crowe Ⅱand type Ⅲ,and whether this result is affected by the size of the cup,If not,then sum up the rule of cup coverage changing with the height of the rotating center,that is,which size of acetabulum cup was used,the positive results are most significant.So as to provide direct theoretical guidance for acetabulum reconstruction in THA of Crowe Ⅱand type Ⅲ DDH patients.Methods:Patients A total of 16 patients with a total of 20 hips were included.There were 2 males and 14 females,13 hips Crowe type Ⅱ and 7 hips Crowe type Ⅲ.All imaging findings of acetabulum morphology abnormalities are considered to be pelvic development.Computed tomography(CT)scan and 3D pelvic reconstruction:All patients received the same CT scanner in our hospital to obtain complete pelvic CT data,all CT data were imported into Mimics 17.0(Materialise,Belgium)software in DICOM format and then reconstructed the three-dimensional(3D)model for each pelvis.Determination and standardization of pelvis plane:After 3D image reconstruction,the position of each pelvis was adjusted according to the anterior plane of the pelvis determined by the anterior superior iliac spine and pubic tubercle,and then place the three-dimensional image of pelvis in the system coordinate.Adjust the front plane of pelvis to overlap with the plane formed by the X and Y axes of system coordinates,and the lines of iliac ridge on both sides are parallel to the X axis,so as to standardize all three-dimensional reconstruction images of pelvis.Simulating the implantation of the cup and calculating the coverage of the Cup host bone:We developed a group of acetabular cup models using 3-matic 9.0 software(Materialise).The diameter of the cups ranged from 38-50 mm in 2-mm increments.All cups had a shell thickness of 0.1 mm.These models were imported into the Mimics software in the STL(stereolithography)format,and the total surface area(St)was available.The cups were implanted into the reconstructed 3D images of the pelvis using the Mimics software to simulate placement of the acetabular component in THA.The position of all the cups was set at 400 abduction and 150 anteversion constantly.After the cup model was placed in the 3D reconstruction image of the pelvis,the surface area of the uncovered portion of the cup(Su),that is the surface area of the exposed portion,was obtained by the Boolean operation function of Mimics.The CC was then calculated by the following formula:CC=(St-Su)/St x 100%Firstly,the cup was placed at the level of the natural acetabulum,and then was moved vertically upward along the Z axis,2 mm at a time until the height of the upward movement reached 50 mm,and each time the cup model was slightly adjusted on the X and Y axes to achieve maximum bone coverage.Results:Except for the 50-mm cups,the cup coverage in the other groups at the true acetabulum can meet 70%,the recognized cup coverage for initial stability of cups.As the acetabular cups moved upwards from the true acetabulum,the CC increased gradually,untill reached the maximum,and then decreased gradually to a value even below 70%.The average cup coverage of each group increased by 18.92%compared with that at the true acetabulum,and the average elevated height was 23.87mm when the maximum coverage rate was obtained.With the increase of the cup size,the coverage at the true acetabulum and the maximum coverage decreased gradually.In the process of getting the maximum coverage,the cup coverage of each group was positively correlated with the elevated height and nearly increased linearly(P<0.001).The Pearson value of 40 mm-cup group is the largest,that is,when the 40 mm cup is used for acetabular reconstruction,the upward movement of the cup will inevitably increase the bone coverage of the cup.Conclusion:when reconstructing acetabulum for Crowe type Ⅱ and type ⅢDDH patients,compared with the reconstruction at the true acetabulum,moving the cup vertically can increase the cup coverage of host bone,and the maximum coverage can be obtained when the cup moves up to a certain height.In this study,when the cups get the maximum coverage,the elevated height meets the safety range recommended by previous studies.In the process of the cup moving upward from the true acetabulum till get the maximum coverage,the cup coverage was positively correlated with the elevated height. |
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