Decentration and stress and strain state in hip joint of patients with dysplasia of the hip

Abstract

Abstract Purpose of the study: to find a correlation between radiological and biomechanical parameters of a hip joint (HJ) in case of dysplastic instability for dystrophic process in the joint study. Materials and methods. Clinical and radiological data of 154 patients aged from 2 to 13 years with dysplastic instability of a HJ the development of which was being observed from 2 to 5 years (189 joints) were studied. The following parameters of a HJ were determined: acetabular index, cervico-diaphyseal angle, centration of the femoral head. All the measurements were performed by plain pelvis radiographs without special positioning the patient. The stress and strain state in the acetabulum and proximal part of the femur was studied using the finite element method. The stress intensity (stress of von Mises, measured in MPa) was chosen as a basic characteristic, which is one of the most informative one for the stress and strain state evaluation. Results. The study has shown that of the three types of decentration such as coaxial, non-coaxial with downward and lateral acetabular center displacement from a HJ center, non-coaxial with downward and medial acetabular center displacement, the last-mentioned has turned out to be less favorable. Moreover, an increased angle of deviation corresponded to greater width and sclerosis intensity of a HJ subchondral surface and also fragmentation of the femoral head was observed in the majority of cases. Studied stress and strain state in a HJ has revealed that decentration in a joint is a key factor, which leads to osteartrosis development. The large decentration, the more severe stress intensity at the lower part of the femoral head and at the upper border of the acetabulum was. Thus, the value of stress intensity at the upper part of the femoral head decreased and was 2.3 MPa comparing with the normal one 2.5 MPa; at the lower part it increased to 2.3 MPa, that was twice higher than the normal one. On the front edge of the acetabulum the stress state level was about 2.3 MPa, which was almost four times more than the normal one (0,6 MPa) in this region. Increase in acetabulum curvature didn’t change the pattern of stress and strain state distribution, but it led to increase in strain level especially at the posterior superior part of the acetabulum and, respectively, at the contact region of the femoral head. Conclusions. The femoral head rotational center shift changes the pattern of stress and strain state distribution in a HJ in comparison with the norms, increases the stress at its lower part and also increases stress state at the upper acetabular edge. Increase in acetabulum curvature doesn’t change the pattern of stress and strain state distribution, but it leads to increase in strain level especially at the acetabular posterior superior part and, respectively, at the contact region of the femoral head. Decentration of HJ elements is the main etiological factor of its instability leading to the further dystrophic transformation of the joint. Biomechanic modeling of decentration in a HJ using the finite element method has proved the fact of complex changes in HJ structures at the early stages of the pathological process development, revealed understanding of the decentration process in a joint, contributed to working out the dystrophic process early prevention and treatment tactics.

Keywords

• hip dislocation
• decentration
• hip joint