Influence of the fixation region of a press–fit hip endoprosthesis on the stress–strain state of the “bone–implant” system
Introduction
Numerous causes of the hip joint impairment can be listed. The most common disorder of the hip joint is a pathological destruction of a joint cartilage, called osteoarthritis. Congenital deformation of the hip joint, accompanied by damage, contributes to the destruction of both the bone and the cartilage. This leads to pain and immobilisation of the hip joint. Nowadays, hip replacement is one of the most effective methods of treatment of hip joint diseases, which enables to both save the patient from pain and, simultaneously, restore full static–dynamic functions of the joint [1], [2], [3]. According to the USA analytical services, an increase of 180% in the number of total hip replacements will be observed by 2030 [4].
However, hip replacement is also accompanied with negative consequences which may occur about 10 years [5] after installation of the hip joint endoprosthesis (Fig. 1). In the case of excessive concentration of stresses on small areas of the femur, an increase in the density and volume of the bone tissue is observed, i.e. the cortical hypertrophy arises [6], [7]. Factors which increase the risk of femur fractures are patient's reduced bone density, defects and cracks of the femur, and also the peculiarities of installing the endoprosthesis in the bone [8], [9]. In the case of exclusion of any volumes of bone structures from the process of transferring the loads, atrophy and lysis of the bone tissue is possible [10]. Moreover, in patients who have osteolysis of the bone surrounding the implant, osteoporosis, or allergic reactions to metal, loosening and instability of endoprosthesis components may occur [11]. If the limit of elasticity or durability of materials used to manufacture implants is exceeded, plastic deformation or destruction of the prosthesis is likely to happen [12].
In the process of designing and constructing implants, efforts should be aimed at solving the afore-mentioned problems and developing fundamentally new designs of implants. These new designs of structures must be based on the data of biomechanical research, application of new materials and modern manufacturing methods [13], [14], [15]. Taking these elements into consideration ensures creation of high-quality implants being able to maintain their functional properties for a long time. One of the most important stages in the development of endoprosthesis is biomechanical rationale for performance and reliability of implants [16], [17], [18], [19].
The efficiency of the “bone–implant” biomechanical system is defined by the condition of the stress–strain state and mechanical behaviour of each element of the system under the action of functional loads. If in certain areas of the bone or the implant the functional load causes stress which exceeds some value (e.g., tensile strength, fatigue limit, etc.), then a destruction or plastic deformation of one or more components occurs, which leads to partial or complete loss of functionality of the whole system. It usually happens among patients who have an overly active lifestyle or suffer from osteoporosis and/or overweight. It may also happen in the case of subsurface voids or inclusions (or both), which could increase the stress two or even three times. Moreover, such condition may be caused by local weakening, incorrect attachment of the stem during surgery, or incorrect choice of the size of the prosthesis, etc. [20].
Currently, one of the most effective and informative methods of research of problems related to biomechanics is the method of numerical simulation, i.e. the finite element method (FEM). Thanks to the FEM, it is possible to avoid difficulties associated with the use of analytical methods for calculation of the stress–strain state of biomechanical systems and, most importantly, to get results with high accuracy [21], [22], [23]. In general, numerous approaches simulating bone behaviour and the process of adaptation to the applied loads have been already developed and some authors proposed techniques to reduce stress shielding effect [24], [25], [26], [27]. However, in the literature, there have been no studies combining the FEA, used to analyze the influence of the region of fixation of a press–fit hip endoprosthesis on the state of the “bone–implant” system, with a bone remodelling model. The following research provides a numerical analysis of the stress-strain state of the femur and a tapered stem, considering different types of stem fixation in the medullary canal of the femur under the action of functional loads. The analysis is used to determine the best conditions for long-term functioning of the "bone–implant" system, what will lead to the improvement of results of the surgery.
Section snippets
Methods
For the analysis, a three-dimensional model of the femur has been developed based on the results of a computed tomography (CT) (Fig. 2).
For this study, frozen cadaver bones were used. Bones were scanned at the Dnipropetrovsk State Medical Academy. Scanning was conducted using an AQUILION RXL 16 (Toshiba Medical Systems) 16-slice CT scanner. DICOM images were obtained with a 0.5–mm slice thickness. In the first step, CT images of the femur have been acquired for subsequent segmentation of the
Results
For all considered types of fixation of the prosthesis being under the action of functional loads, intensity and distribution of stresses and strains were obtained for both medial and lateral sides of the stem and the femur.
In the case of diaphyseal fixation (A, AB, ABC), the stress–strain state of the stem was determined by combining the bending moment acting in the frontal plane and compression forces acting in the axial direction. Longitudinal tensile stress arises on the lateral side and
Discussion
Prosthesis materials and designs (type of fixation) are the factors that most greatly influence the stress distribution in the bone around implants [48]. In the considered implants, the maximum stress is found on the medial side, in the contact zone of the prosthesis stem and the cortical layer of the femur. The region of stress concentration for such fixation is determined by the distance between the region of application of the load and the fixed part of the stem as well as the dimensions of
Conclusions
The paper presents the results of application of the numerical analysis which enabled to determine the stress-strain state of the femur and mechanical behaviour of a tapered stem endoprosthesis of the hip joint for different types of prosthesis fixation. The calculations have shown that distal fixation may lead to stress concentration in the distal femur. In such a situation, the risk of the stress shielding effect or the bone fracture should be taken into account. Also, the values of stresses
Conflict of interest statement
None of the authors has a conflict of interest in relation to this study.
Acknowledgment
The work has been supported by the National Science Centre of Poland under the grant OPUS 9 No. 2015/17/B/ST8/01700 for years 2016–2018.
Ievgen Levadnyi received the Master of Engineering degree from Dnepropetrovsk National University in 2015. Currently doing Ph.D. course in Biomechanics Engineering at Lodz University of Technology, Lodz, Poland.
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Ievgen Levadnyi received the Master of Engineering degree from Dnepropetrovsk National University in 2015. Currently doing Ph.D. course in Biomechanics Engineering at Lodz University of Technology, Lodz, Poland.
Jan Awrejcewicz currently Head of the Department of Automation, Biomechanics and Mechatronics, Ph.D. School on 'Mechanics' and graduate/postgraduate programs on Mechatronics associated with LUT.
Marcio Fagundes Goethel Ph.D. in Biomechanics. He has experience in Biomechanics, with emphasis on signal processing, spine implant.
Loskutov Alexander is full professor of Dnipropetrovsk State Medical Academy. Since 1991 Head of the Department of Orthopedics and Traumatology. He specializes in adult hip replacement and also has a special interest in the development of techniques for efficient, less invasive, high quality joint replacement.