Effects of rotational acetabular osteotomy on the mechanical stress within the hip joint in patients with developmental dysplasia of the hip

Abstract

In this study we used subject-specific finite element analysis to investigate the mechanical effects of rotational acetabular osteotomy (RAO) on the hip joint and analysed the correlation between various radiological measurements and mechanical stress in the hip joint. We evaluated 13 hips in 12 patients (two men and ten women, mean age at surgery 32.0 years; 19 to 46) with developmental dysplasia of the hip (DDH) who were treated by RAO. Subject-specific finite element models were constructed from CT data. The centre-edge (CE) angle, acetabular head index (AHI), acetabular angle and acetabular roof angle (ARA) were measured on anteroposterior pelvic radiographs taken before and after RAO. The relationship between equivalent stress in the hip joint and radiological measurements was analysed. The equivalent stress in the acetabulum decreased from 4.1 MPa (2.7 to 6.5) pre-operatively to 2.8 MPa (1.8 to 3.6) post-operatively (p < 0.01). There was a moderate correlation between equivalent stress in the acetabulum and the radiological measurements: CE angle (R = -0.645, p < 0.01); AHI (R = -0.603, p < 0.01); acetabular angle (R = 0.484, p = 0.02); and ARA (R = 0.572, p < 0.01). The equivalent stress in the acetabulum of patients with DDH decreased after RAO. Correction of the CE angle, AHI and ARA was considered to be important in reducing the mechanical stress in the hip joint.

Effects of sclerotic changes on stress concentration in early‐stage osteonecrosis: A patient‐specific, 3D finite element analysis

“…After downloading CT data in the digital imaging and communication in medicine format (DICOM: National Electrical Manufactures Association [NEMA], Rosslyn, VA) onto a personal computer, a FEM in each patient was generated using the Mechanical Finder software program, version 7.0 (Research Center for Computational Mechanics, Tokyo, Japan). This program includes a three‐dimensional (3D) modeler as well as a meshing component and FE solver, and creates a non‐linear FEM that shows the shape of the individual bone and the distribution of its density …”

“…The model with a 1‐mm mesh size could not be resolved due to the limited capacity of our computer. For each of the models, the total strain energy was calculated at a load of 900 N under the same loading and boundary conditions . The percentage differences in total strain energy between the 2‐ and 3‐mm models, the 2‐ and 4‐mm models, the 2‐ and 5‐mm models, and the 2‐ and 6‐mm models were 1.05%, 4.56%, 8.94%, and 18.2%, respectively.…”

“…The pelvis, the articular cartilage, and the femur were assumed to be completely bonded. We artificially modeled the articular cartilage as a homogeneous isotropic material . For each FEM, the mean number of elements was 831,633 (range, 557,894–1,025,080) and the mean number of nodes was 152191 (range, 103568–186518).…”

“…The elastic modulus of the bone was determined from CT density values, as previously reported . First, the apparent bone mineral density (p in g/cm 3 ) of each element was determined using Hounsfield units.…”

“…Using a finite element model (FEM) analysis, it is possible to predict stress distribution in the hip joint in vivo . Recent FEM studies have adopted the concept of Drucker–Prager equivalent stress, which treats bone as a brittle material . Shear stress was found to be concentrated at the sclerotic boundary in a previous FEM study that used samples from surgically resected femoral heads with advanced‐stage osteonecrosis .…”