Analysis of the Stress State of the «Hip — Implant» System Elements in Functional Loads on Hip Endoprosthesis

O.O. Loskutov, Ye.V. Levadnyi

Abstract


Despite the high performance of hip replacement, we should take into account the negative consequences associated with a violation of the biomechanical balance in «bone — endoprosthesis» system during its operation, and in particular of the femoral component. Objective of the study: analysis of the stress state of the femur and stem of prosthesis with metaphyseal-diaphyseal fixation type at different levels of femoral medullary canal. Materials and methods. Using finite element method, there has been carried out the numerical analysis of the stress state of the hip and stem of prosthesis with metaphyseal-diaphyseal fixation of ORTEN system for different types of its fixation in the medullary canal of the femur, under the influence of functional loads to predict the operational reliability of the endoprosthesis. When fixing the implant, dimensions of the canal in the place of fixation were taken 1 mm smaller than the size of stem of prosthesis. Results. Calculations have shown that stress state of stem of prosthesis is determined by a combination of bending moment in the frontal plane and the compression force in the axial direction. On the lateral side of stem and neck, there are formed tensile stresses, and on medial one — compressive, and the absolute value of the tensile stress at the same level of the stem is somewhat smaller than compressive. The va­lues of the stress on the lateral and medial sides differ on average by 8 %. It was found that for this type of stem of hip prosthesis, diaphyseal type of fixation is the most dangerous one, because it can lead to a stress fracture of the stem, especially small. In metaphyseal fixation type, as well as in support on calcar, stress values are significantly reduced. It was discovered that from the lateral side of the bone, tensile stresses occur, and from the medial one — compressive (tensile stress is 13 % less than compressive one). In diaphyseal, as well as in diaphyseal-distal-metaphyseal type of fixation, the most of the load on the stem bypasses the proximal femur and is transmitted directly to its diaphysis, thereby development of the proximal stress-shielding effect and bone hypertrophy become possible. When modeling the positioning of endoprosthesis in the femoral canal, the area of implant contact with the bone is strictly determined by the type of fixation of the prosthesis stem. Calculations have shown that an acceptable situation of the distribution of stresses in the femur is achieved when the area of the contact of endoprosthesis stem with the bone is ≥ 50 %.
Conclusions. Depending on the level of fixation in the femoral bone of the square section of endoprosthesis by ORTEN system with metaphyseal-diaphyseal fixation type, Mises equivalent stresses in the endoprosthesis vary widely from 30.6 to 195 MPa, and in the bone tissue their changes are minor — from 6.4 to 7.9 MPa. In case of diaphyseal fixation of stem endoprosthesis of this type, there is a possibility of stress concentration in the distal bone, which leads to its hypertrophy and the development of stress-shielding effect.


Keywords


arthroplasty; femoral component of the hip endoprosthesis; femoral bone; numerical simulation; stress state; finite element analysis

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DOI: https://doi.org/10.22141/1608-1706.6.16.2015.79694

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