Finite Element Analysis Of The Stress Distribution In The Dome Region Of Acetabulum With Medial Migration Of Femur Head After Acetabular Fracture Operation

Objective:(1) The definition and measurement method of femoral head medial displacement index after acetabular fracture operation.(2) Through the inner wall acetabular osteotomy to establish the finite element models of the different hip joint under the femoral head position.(3) To study the influence of magnitude and distribution of contact stress on acetabular weight-bearing with the finite element analysis after the femoral head medial displacement,which provides reference for clinicians study of hip biomechanical changes after postoperation of acetabular fractures, and predict the correlation between the changes and hip traumatic osteoarthritis.Methods:(1) To follow-up in patients after acetabular fracture surgery and collect medical records, and screen out of 8 cases of different degree of femoral head displacement of patients. 8 patients underwent anteroposterior pelvic X-ray and CT scan examination,all patients with pelvic CT scan data in DICOM format and stored into the CD, and finish reconstruction of the 3-D models of the pelvis in the Mimics software. The femoral head medial displacement index was measured on plain films and three-dimensional models respectively.The data was statistically analysed and compared the differences between the two sets of data are statistically significant.(2) Choose a healthy adult males(33 years, 70kg), extract pelvic CT scan DICOM data, building a three-dimensional simulation model of the left hip by Mimics17.0software, use the software sector in surgical simulation tools within the acetabular osteotomy sidewall osteotomy, femoral head and the cut in the inner sidewall of the bone synchronous shift. The model number of the column processing, such as cartilage after reconstruction, gridding, assignments and other operations, the model into finite element analysis software ANSYS, then the relevant modifications, set after hip surface contact between the property to 4mm for the mobile unit to different shift position within the femoral head of the hip model group, including normal hip joint total of six groups, and establish a finite element model of the hip mechanics analysis under different shift positions within the femoral head.(3) Applying the same load and setting the boundary conditions to simulate standing with one foot stress distribution acetabular roof weight-bearing area, after theadoption of the same restrictions within relatively shift between the femoral head of the hip size and distribution of contact stress on all models changes, combined with shifts in the clinical analysis of the femoral head and acetabular fracture surgery traumatic arthritis correlations provide a theoretical basis.Results:(1)Based on multi-slice CT and DICOM technology, the Mimics software rebuild pelvis and unilateral hip three-dimensional simulation model.The geometry was consistent with the real anatomy, and can be used for further study.(2)In 8 cases of acetabular fracture patients with postoperative pelvic radiograph and three-dimensional reconstruction model of femoral head displacement index were measured, The data of the two groups were t test of paired data. t = 2.522, P = 0.046,P< 0.05. The data of the two groups has statistically significant difference.(3) The Mimics and 3-matic software supply the model with cartilage reconstruction,surface mesh, volume mesh and so on, which forms a left femur with 117402 nodes and 83714 tetrahedral units, innominatum with 218557 nodes and 152197 tetrahedral units. The model is based on gray value assigned and stored as finite element analysis software can import models.(4) In finite element analysis software ANSYS, the corresponding contour plots can display seven groups of hip joint model of acetabular dome region stress distribution.When standing on one leg normal hip acetabular roof load distribution of stress, no stress concentration points, peak stress range 3.01 MPa. With the shift of the femoral head, hip and femoral acetabular bearing area top area of contact stress concentration,and the peak value of up to 16.47 MPa, especially when the femoral head is larger than 6mm shift the peak stress exceeds the normal tolerance range.Conclusion:(1) On standard AP pelvic X-ray, the measurement of the femoral head medial migration index has certain implications, but it is difficult to obtain absolute standard pelvic AP X-ray because of the influence factors such as the subject’s position and fracture.Three-dimension reconstruction model of the pelvis can be arbitrarily adjusted the pelvis, and the measured results have a higher reliability.(2) Based on the pelvic CT data, the 3-D reconstruction and finite element analysis software can accurately constructed normal and different femoral head displacement hip joint finite element analysis model.(3) With the femoral head medial displacement, the hip joint biomechanics environment changes, and the top acetabular weight-bearing area and stress peak bothincrease.This is probably one of the factors for the occurrence of traumatic arthritis after acetabular fracture surgery.(4) When acetabular fractures, the acetabular medial wall should be anatomic reduction and rigid internal fixation. It can help restore the position of the femoral head after operation.