ISSN: 2643-4016 Fukuoka and Todo. Int Arch Orthop Surg 2018, 1:003 Volume 1 | Issue 1 Open Access International Archives of Orthopaedic Surgery ORIGINAL RESEARCH Analysis of Principal Stress Projection in Femur with Total Hip Ar- throplasty using CT-image Based Finite Element Method Kosei Fukuoka1 and Mitsugu Todo2* 1Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan Check for updates 2Research Institute for Applied Mechanics, Kyushu University, Japan *Corresponding author: Mitsugu Todo, Associate Professor, Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan, Tel: +81-92-583-7762, Fax: +81-92-583-7762 lum and the femoral head are replaced, and three com- Abstract ponents are required: A stem and a spherical head that The research demonstrates a novel three-dimensional finite replaces the femoral head, and a hemispherical cup with element analysis to analyze biomechanical changes in a femur with implant. The method uses principal stress pro- an inner liner to replace the acetabulum. While funda- jections to determine trabeculae trajectories in the femur. mental concept of hip arthroplasty has changed little Visual comparisons of the projections of the femurs show since its appearance in medicine, the implants have ex- areas of cortical and cancellous changes due to changes in perienced numerous changes in terms of material and tensile and compressive stresses, revealing areas of stress shielding. The method also includes a bone remodeling al- design [1]. During the early stages of development, the gorithm to simulate bone adaptation. In the experiment, a THA implant had a metal-on-metal (MOM) bearing com- patient-specific femur model with inhomogeneous material bination. All components were made cobalt-chrome al- properties was created from CT scans. Models of femurs loy and bone cement was used to fixate the implant to with implants varying in type and size were also created. the bone. The implant head was also similar in size as Maximum load during walking was simulated with realistic muscle forces, and stress projections of the femur model the anatomical head, giving the hip a natural physiolog- were compared to trabecular trajectories of a real femur for ical range of motion (ROM). When it was revealed that validation. Bone remodeling was also simulated to inves- MOM produced high frictional torque, metal-on-poly- tigate changes in projections over time. High correlation mer became the popular bearing combination choice. It was found between the principal stress projections of the computational model and trabeculae trajectories of a real fe- soon became evident that debris from the plastic liner mur. Changes in projections were evident for implant mod- due to volumetric wear led to osteolysis and causing the els, suggesting stress shielding and bone remodeling. The implant to loosen. Thus, the implant head size was re- analysis provides an effective method for planning implants duced to decrease the wear, which also decreased the that are ideal for patients and for designing future implants ROM of the hip, limiting the patient’s mobility. The next that preserves the biomechanics of the femur to maintain its physiology. fifty years saw the introduction of two more bearing combinations (ceramic-on-polymer and ceramic-on-ce- Keywords ramic), though much of the implant designs and the small Biomechanics, Finite element analysis, Total hip arthroplas- head size remained unchanged. Presently, hip implants ty, Bone remodeling are available in all bearing combinations and in various shapes and sizes. THA implants with large head size are Introduction also becoming popular, as researches have shown that with larger head sizes, lubrication regime changes from Total hip arthroplasty (THA) is a type of joint replace- boundary lubrication to fluid-film lubrication [2]. New ment surgery that has been the standard treatment for researches in materials have also led to bearings having patients with osteoarthritis, avascular necrosis, and/or very low frictional coefficients. In addition, porous coat- fracture within the hip joint. In THA, both the acetabu- Citation: Fukuoka K, Todo M (2018) Analysis of Principal Stress Projection in Femur with Total Hip Arthroplasty using CT-image Based Finite Element Method. Int Arch Orthop Surg 1:003. Accepted: September 18, 2018; Published: September 20, 2018 Copyright: © 2018 Fukuoka K, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Fukuoka and Todo. Int Arch Orthop Surg 2018, 1:003 • Page 1 of 10 • ISSN: 2643-4016 ings around the stem have led to the development of has introduced the use of telemetry devices to deter- cement less implants, providing stronger bone fixation mine hip contact force in real time. Combining the data than using bone cement. with musculoskeletal modeling has allowed research- ers to mimic realistic physiological loading conditions However, there are still risks to hip arthroplasty and [22-25]. Numerical and computational bone adaptation thousands of revision surgery is conducted each year. theory has also been suggested, following three basic The main indications for surgery that concerns the im- concepts: Alteration in bone architecture due to me- plant are aseptic loosening, dislocation, and the wear of chanical stimulus, calculation of updated material prop- acetabular component [3]. The main risk factors are can erty, and calculation of new stress and strain fields from be categorized as being mechanical, biological, or bio- updated material properties [26-30]. Computationally mechanical. Mechanical risk factors include incorrect derived principal stress projections have also been eval- determination of implant size, position, and orienta- uated with trabeculae structure of the femur with suc- tion. As mentioned earlier, using a smaller implant head cess [31,32]. size decreases the ROM of the hip and increases the risk of the implant impinging with the cup, leading to Despite of the advancements in materials and de- dislocation [4,5]. Biological risk factors are derived from signs, hip implants continue to cause complications. improper choice of implant type. With implants using The National Joint Registy (NJR) for England and Wales, bearing combinations of a polymer, the wear particles which has a large population and high rates of coverage, are associated with osteolysis, causing aseptic loosen- have recently revealed that the revision rate for MOM ing of the implant. With MOM implants, the release of implants, specifically HRA implants, was much higher metal ions has been linked to metallosis. In addition, than other bearing combinations [33,34]. The findings the size of head has been known to coincide with volu- have led to government health agencies in England and metric wear, with increasing head size leading to great- the United States to release alerts on the use of particu- er wear [6]. Both mechanical and biological risks have lar hip implants. Furthermore, within the past 10 years, been heavily discussed in literature, and with proper major implant companies have recalled their implants. clinical assessment and pre-surgical planning, the oc- While researches concerning implant design have been currence of these risks could be minimized. However, conducted, most have been based on minimizing me- biomechanical risk factors are based on the remodeling chanical and biological risks. Patient-specific implants of the bone, or the biological response due to mechan- have also been simulated; however, the advantages ical change. Stress shielding is one factor in which the have been minor compared to the cost and time to man- body load is shifted from the femur to the implant, caus- ufacture such implants [35-39]. Furthermore, long-term ing osteopenia [7-11]. Such condition can cause aseptic safety has not been addressed, which is a key factor for loosening due to instability in fixation, or cause sudden femur and implant survival, as well as the quality of life fatal damage to the femur, such as fracture. In order to for the patient. Ideally, there should be minimal differ- clinically assess biomechanical risks from an engineer- ences between the biomechanics of an intact femur and ing perspective, researchers have focused on the bone that of a femur with implant. Any alterations in loading adaptation theory [12-18]. will affect the stress and strain distribution, which will cause respective changes in the trabecular structure The first bone adaptation theory, known as “Wolff’s and henceforth the bone density. Thus, the primary de- Law” suggests that the bone adapts to changes in stress sign criteria of an implant must be based on minimizing and tries to minimize load. Wolff also illustrated his ver- biomechanical risks, followed by the minimization of sion of principal stress projections and postulated the mechanical and biological risks. In order to assess the “Trajectorial Theory”, in which he noted that cancellous effectiveness and safety of the implant design, it is nec- bone architecture is formed of a network of perpendic- essary to first replicate the biomechanics of the femur, ular intersections [19]. While Wolff’s Law has remained test under realistic physiological conditions, followed by as the philosophy of bone remodeling, the Trajecto- bone remodeling simulation to predict long-term out- ry Theory has been dismissed by modern research, as come. To the best of our knowledge, the combination of it has been found that the cancellous architecture is the three works has not yet been reported in literature. non-orthogonal, and its orientation is governed clearly by loading [20]. The objectives of this study were to realistically sim- ulate the physiological condition of the femur, to in- Finite element analysis (FEA) has frequently been vestigate the effects of an implant in the femur, and to used to simulate physiology of the femur, using pa- utilize the obtained results for optimization of implant tient-specific three-dimensional (3D) models made from designs.