Comparative Study of Fixation Devices for Intertrochanteric Fractures

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1 Comparative Study of Fixation Devices for Intertrochanteric Fractures C. Sticlaru * A. Davidescu Politehnica University of Timişoara Politehnica University of Timişoara Timişoara, România Timişoara, România Abstract The paper presents a comparison of some fixation devices for intertrochanteric fractures. The devices studied are: the fixed plate, the sliding screw-plate device, the intramedullary hip nail and Ender wire. The devices are designed in ProEngineer (ProE). The 3D models were analyzed in Ansys. Different materials are investigated and aspects concerning the stress and strain state, the loading of the contact area between bone fragment and bone device. Keywords: intertrochanteric fracture, fixation devices, finite element method I. Introduction Hip fractures, particularly in older persons, result in problems that extend far beyond the orthopedic injury, with repercussions in the areas of medicine, rehabilitation, psychiatry, social work, and health care economics. A hip fracture is generally a fracture of the proximal femur. Such injuries are divided into three categories, according to the anatomical area in which they occur (Fig. ). Femoral-neck fractures are located in the area distal to the femoral head but proximal to the greater and lesser trochanters and are considered intracapsular fractures, because they are located within the capsule of the hip joint. This anatomical characteristic has important implications for healing. Fractures in this area, particularly those involving marked displacement, can disrupt the blood supply to the femoral head and are therefore associated with an increased incidence of healing complications (nonunion and osteonecrosis of the femoral head). Intertrochanteric fractures, which occur in the wellvascularized metaphyseal region between the greater and lesser trochanters, are extracapsular fractures that do not interfere with the blood supply to the proximal femur. These fractures are thus not associated with the healing complications characteristic of femoral-neck fractures. The most common complications of intertrochanteric fractures are mal union and shortening as a result of the deforming forces and compromised bone quality in this area of the proximal femur. Subtrochanteric fractures are those that occur just below the lesser trochanter. Femoral-neck and intertrochanteric fractures account for over 90 percent of hip fractures, * carmen.sticlaru@mec.upt.ro arjana.davidescu@mec.upt.ro occurring in approximately equal proportions and subtrochanteric fractures account for the remaining 5 to 0 percent []. The estimated incidence of hip fractures in the United States is 80 per 00,000 populations []. The incidence increases with age, doubling for each decade after 50 years, and is two to three times higher in women than in men. The incidence is two to three times higher in white women than in nonwhite women. Other risk factors for hip fracture include a maternal history of hip fracture, excessive consumption of alcohol and caffeine, physical inactivity, low body weight, tall stature, previous hip fracture, use of certain psychotropic medications, residence in an institution, visual impairment, and dementia []. Osteoporosis is an important contributing factor, because it decreases the skeleton s resistance to injury, and approximately 90 percent of hip fractures in the elderly result from a simple fall. The characteristics of a fall (the direction, site of impact, and protective response) and the habitus are recognized as important factors influencing the risk of hip fracture from a fall.

2 This paper deals with two of the most frequently of hip fracture types [2, 3 and 4]: the pertrochanteric and intertrochanteric fracture (Fig. 2) Fig. 2. The intertrochanteric and pertrochanteric fractures Some studied references (e.g. [7]) deal with analysis of the relevant mechanical load system, such as the singleleg stance phase of normal gait, showing that the forces acting on the femur are essentially in the frontal plane. For this reason the analysis were made using a twodimensional FE model, [7], and the mechanical responses of the bone-implant system are dominated by deformation and stress acting in the frontal plane. In this paper, the model was studied using a 3D model; the applied environment in FEM represents muscle and reaction forces which have effect on the femur in a typical loading condition loading during fast walking [7]. The main purpose of this study was to investigate some fixation devices from mechanical point of view, taking into account the stress and strain state, the total deformation and the processes in the contact area between the bone fragments, respectively bone - fixation devices for two representative fractures. II. The FEM Models An anatomical model of the femur (the inner and the outer surfaces) was digitized using Microscribe G2 equipment. The anatomical model was delivered by University of Medicine and Pharmacy from Timişoara. A section of the obtained 3D model is presented in Figure 3. Bone material shows an anisotropic material behavior. In order to consider the non-homogeneity of the cancellous bone, the corresponding finite element model was split into subzones, each of which had different material properties [7]. The partitions are presented in Figure 3 and the material characteristics in Table [7]. The Young s modulus for the cortex was considered 4500 MPa and the Poisson s ratio of 0.3 was used for all materials. The intact and fractured femurs were developed in ProE using the points delivered by Microscribe equipment (Fig. 4a, b, b2). Starting from points, spline curves were created, then surfaces and finally the solid model. Fig. 3.The bone model Zone E cortex E cancellous TABLE I. The material characteristics for the bone Four fixation devices were investigated (Fig. 5): - the fixed plate (a); - the sliding screw - plate device (b); - the intramedullary hip nail device (c); - the Ender wires device (d). a. Fig. 4 The investigated models: a. intact femur; b. intertrochanteric fracture b2. pertrochanteric fracture Three different biocompatible materials for implants were analyzed: - Titanium (Ti-6Al-4V) having Young modulus Pa, Poisson coefficient 0.36 and density 4620 kg/m³, - Stainless Steel l36l (Young modulus.96 0 Pa, Poisson coefficient 0.3 and density 7900 kg/m 3 ); - Co-Cr alloy (Young modulus.93 0 Pa, Poisson coefficient 0.3 and density 7750 kg/m 3 ).

3 The intact femur and the assemblies were imported in Ansys. The results for the intact femur (in the frontal plane) (fig. 6) are similar to the results obtained for a planar model studied in the frontal plane [7]. a a2. Fig. 5. The fracture s assemblies for the studied devices: a. The fixed plate b. The sliding screw-plate device c. The intramedullary hip nail device d. Ender wires device a. the environment b. von Mises stress c. von Mises strain Fig. 6 The results obtained for the intact femur in frontal plane The environment of the model consists of (fig. 6a): - the hip joint reaction force; the angle Φ of the joint reaction force to the vertical is 6. The magnitude of the reaction force takes into account the human body weight and the action of the three major muscle groups, the abductors, the vastus lateralis and the iliopsoas. - the distal end of the femur is rigidly fixed. III. Results and Conclusions The magnitude of the reaction force depends on the activity of the human subject. Parameterized analyzes (four scenarios in which the input parameter is reaction force) were run in Ansys. These scenarios simulate different activities: standing, standing on one limb, fast walking and walking upstairs. Considering that the most unfavorable case is fast walking, next pictures represent the results for this situation (the environment presented in fig. 6 is applied to all models). The next figures are presented in the same order: - the fixed plate, - the sliding screw-plate device, - the intramedullary hip nail, - Ender wires. The same bone model was used in the simulation and the environment is equivalent, so that the results can be used to compare the fixation devices and different fractures. It can be seen that the device s materials has no significant influence upon stress and strain state, respectively upon the frictional stress, pressure, penetration and sliding distance from fractured region. The stresses permissible for the implants were not exceeded in any of the variants considered. That means there can not appear cases of implants failure due to breakage of their components.

4 The results are in the same range of values for all above mentioned aspects. The stress and strain state reaches small values: 3 5 MPa, mm/mm. For the cortical bone, the maximum values appear in the contact region bone-implant, corresponding to the transmission of forces between the implant and the femur in this zone (fig. 8). Fig. 7. The equivalent von Mises strain for studied fractures with fixed plate, the sliding screw-plate device, the intramedullary hip nail The results for the studied implants and fractures show no significant differences. The stress range of approx. 7 MPa, which according to Hassler (named in [7]) is relevant for the destruction of bone cells due to compression, was not exceed for any of the variants considered. The maximum values appear in different regions of the cancellous bone in case of the studied fractures, because of the fracture position relative to the regions with different bone properties. Fig. 8 The pressure between implant and cortical bone for studied fractures fixed with all studied devices The total deformations (Fig. 9) have values in range of 2 2 mm for all studied cases. In case of an exterior device

5 component (fixed plate - fig. 9 a, sliding-screw plate fig. 9 b), the values for total deformations are greater, this means that the devices are unfavorable versus the others. The devices with intramedullary component cause smaller values for total deformations due to increased stiffness of the femur (fig. 9 b, d). The smallest value for total deformation appears for the Ender wires (fig. 9 d), aspect that could be predicted considering the length of the device. status, gap, penetration, distance, and reactions) can be investigated at once for a given scoping. From medical point of view, one of the most important contact results during the healing process is the sliding tendency between the bone fragments. The sliding distance from the contact area can be seen in figure 0. It is obvious that the sliding distance must have insignificant values. Analyzing the contact results it can be mentioned: - for all studied devices, the values are small, in range of mm; - the minimum value occurs for the fixed plate device, an expected result because the device consists of a single part; - the maximum value appears for the Ender wires, also, an expected result because the device has no fixation (the wires can relatively slide to each other and to the bone); a. b. c. d. Fig. 9 The total deformation for the studied devices: a. the fixed plate b. the sliding screw-plate device c. the intramedullary hip nail device d. Ender wires device Another aspect that should be mentioned is related to the stability of the devices. The three Ender wires can rotate relative to each other and relative to the bone. Also, the wires can slide along the femur creating discomfort and harm the patients. Many subsequent studies have investigated the stress distribution within the human femur [-4] but most of these have concentrated on stresses developed in the femoral shaft and the effects of implanted devices. However, a small number of references have drawn attention to the interaction between the bone fragments during the healing. Ansys allows verifying the transfer of loads (forces and moments) across the various contact regions. In this way, all possible contact results (pressure, frictional stress, Fig. 0 The sliding distance for the fracture area for the studied fractures: a. the fixed plate b. the sliding screw-plate device c. the intramedullary hip nail device d. Ender wires device - for the screw-plate device and the intramedullary hip nail device the values are acceptable from medical point of view; - the distribution of the sliding distance is favorable in case of the intramedullary hip nail due to the proportion of sticking contact from the contact area.

6 - the sticking contact occurs in the regions where no sliding distance appears (dark blue in fig. 0). In figure an interesting aspect is presented the sliding distance between the three Ender wires. Due to its design, this device allows relative motion between its parts. This phenomenon represents a major inconvenience because the wires may provoke injuries to the patients (the wires can penetrate the patients skin). This fact was also, revealed by the physicians and patients. The value for the sliding distance is significant in lower half bone region 0.6 mm. For all the others devices this problem is eliminated due to the configuration of component parts. Fig. The sliding distance between the Ender wires for the studied fractures The gap values for the small fragment of the cancellous bone are very small and all negative, which means that the contact between the bone and the device is correct. The healing process develops in good conditions (fig. 2). From all aspects and devices analyzed by FEM, some general conclusions can be depicted: - from medical point of view the best device is the intramedullary hip nail; in this case occurs the minimum values for relative motions between the bone fragments; the stress and strain state for the femur are favorable; - the most unfavorable case is the Ender wires due to the relative displacements in fracture area; also, the patient may be hurt by the wires because they can penetrate the skin, shown by the sliding distance between the wires during loading; - the intramedullary hip nail device and the Ender wires increase the stiffness of the bone due to the inner position in transversal section of the femur; - the devices that have components on the outside of the bone provide an asymmetrical loading of the bone section; - the devices that have components in the intramedullary canal generate greater values for penetration in the contact area. Fig. 2. The gap between the cancellous bone fragments and the device It is important to study the reconstruction of the bone because some contradictory results occur and the physician must take the right choice for the fixation device. By FEM analyses one can develop a realistic model with fewer simplified hypotheses; this fact allows acquiring accurate results and the right selection of the fixation device. References [] Steinberg E., Blumberg N., Dekel S., The fixion proximal femur nailing system: biomechanical properties of the nail and a cadaveric study, Journal of Biomechanics 38 (2005) [2] Sadowski C. and others, Treatment of Reverse Oblique and Transverse Intertrochanteric Fractures with Use of an Intramedullary Nail or a 95 Screw-Plate. Journal of Bone & Joint Surgery jbjs.org Vol 84-A(3), March [3] Cornell C., Internal Fracture Fixation in Patients with Osteoporosis Journal of American Academy of Orthopaedic Surgeons, (2):09-9, [4] [5] [6] [7] Gahr R.H., Leung K.S., Rosenwasser M.P., Roth W. The Gamma Locking Nail, Einhorn-Press Verlag GmbH Reinbek, ISBN , 999.