Analysis of the Biomechanical Behavior of Osteosynthesis Based on Intramedullary Nails in Femur Fractures

C H A P T E R

11 Analysis of the Biomechanical Behavior of Osteosynthesis Based on Intramedullary Nails in Femur Fractures Sergio Gabarre*, Jorge Albareda†,‡,§, Luis Gracia¶,k, Sergio Pu
ertolas¶,k, Elena Ibarz¶,k, Antonio Herrera‡,§,k *Vlaams Instituut voor Biotechnologie, Leuven, Belgium †Department of Orthopaedic Surgery and Traumatology, Lozano Blesa University Hospital, Zaragoza, Spain ‡Arago´n Health Research Institute, Zaragoza, Spain §Department of Surgery, University of Zaragoza, Zaragoza, Spain ¶Department of Mechanical Engineering, University of Zaragoza, Zaragoza, Spain k Arago´n Institute for Engineering Research, Zaragoza, Spain

11.1 INTRODUCTION Femoral shaft fractures are among the most severe injuries of the skeleton. In particular, these fractures are the most serious of the long bones of the body, characterized by high morbidity and mortality [1, 2], and are frequently associated with significant complications and sequelae. They represent around 13% of total skeleton fractures [3]. For this reason, it is necessary that they are treated because of their complexity, seeking the most appropriate method depending on the characteristics and location of the fracture (Fig. 11.1), as well as the patient. Although there is no universally accepted classification, these fractures have been classified according to their location by Wiss et al. [4]

FIG. 11.1

Femoral zones.

Advances in Biomechanics and Tissue Regeneration https://doi.org/10.1016/B978-0-12-816390-0.00011-X

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FIG. 11.2 Location of femoral fractures according to Wiss’ classification.

(Fig. 11.2). In addition, diaphyseal fractures, by their degree of comminution, have been classified by Winquist and Hansen [5] (Fig. 11.3), type IV being the most difficult to treat and with increased complications and sequelae because of their instability. The treatment of these fractures, always surgical, since the 1980s has been done by using intramedullary nails [6], which have many designs. These nails have revolutionized the treatment of diaphyseal femoral fractures, increasing their indications to the totality of fractures between zones 2 and 5 of Wiss et al. [4], regardless of the type of fracture, and presenting high values of consolidation with complications and sequelae [7, 8]. Since then in a bid to improve results, changes in nail design, morphology, material manufacture, screw configuration, and surgical approach have been introduced. Currently, there are stainless steel or titanium nails; slotted instead of grooved nails; hollow or solid nails to give greater rigidity; and oblique, transverse, spiral blade screws, etc. These developments have increased the stability and degree of fixation of the screws in the osteoporotic bone, and with anterograde or retrograde surgical approaches have increased their indications to more distal fractures. There are also reamed or unreamed nails to minimize vascular injury. Reamed nails decrease the risk of pulmonary embolism caused by the increase in intramedullary pressure produced during reaming [9], although this point continues to be controversial because some authors do not find a significant difference in pulmonary embolism between reamed and unreamed nails [10]. That is to say, there are multiple therapeutic possibilities but there is no consensus on the indication of each type of nail or surgical approach to the different types of fractures. Regarding the use of clinical findings as an aid in decision making, most are satisfactory, but a definitive conclusion has not been reached as to therapeutic indication. The use of reamed or unreamed nails is a persistent discussion [7]. In a meta-analysis performed in [11], there is scientific evidence of the best results of reamed nails against unreamed nails in terms of resurgeries, consolidation delays, and pseudarthrosis. Similar results with both types of nails and techniques as to breakage of the implant and the production of distress and respiratory failure [11] have been found. However, complications have been found in the use of reamed nails, especially in polytraumatized patients with lung injury [7], and because of this unreamed nails have been designed in an attempt to decrease the respiratory impact of reamed nails. However, this theoretical beneficial effect of unreamed nails in polytraumatized patients with respiratory involvement is not clinically proven [11]. There is some controversy because a number of authors have achieved excellent results with few complications using unreamed nails [12]. With respect to retrograde nails, Papadokostakis et al. [13], in a meta-analysis that studied the results of treatment with a retrograde nail in distal and diaphyseal fractures, found that this type of nail is a treatment option for distal fractures, but not for diaphyseal fractures because it produces high rates of pain in knees and a greater number of pseudarthroses and resurgeries than when using anterograde nails. These higher rates of failure in retrograde nails are due to the use of unreamed nails of small diameter, smaller than the diameter of the femoral medullary canal. At present, comparative results with anterograde nails are being discussed. Thus retrograde nails are preferable in patients with difficult access to the greater trochanter, such as obese and pregnant patients, and in patients with ipsilateral fracture of the tibia, which is tactically advisable when treating both fractures with a unique approach [7].

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11.1 INTRODUCTION

FIG. 11.3

217

Femoral fractures according to Winquist and Hansen’s classification.

Despite the high number of designs, techniques, and materials, the intramedullary anterograde reamed nail with a static combination of screws continues to be the reference treatment of fractures of femurs located between the 2 and 5 Wiss zones [4, 14, 15], depending on the success of the treatment, the characteristics of the fracture, the body habits of the patient, the associated lesions, and the experience of the surgeon using this technique [7]. Experimentation with artificial bones or corpses in the lab, or with experimental animals, trying to study the biomechanical behavior of various types of osteosyntheses in different fractures, is an important aid to clinical practice for determining the therapeutic indication appropriate for every type of fracture. In this field, there are many studies of all kinds of variables in terms of models of nails, techniques, and types of fracture, but the results are inconclusive and sometimes divergent, particularly regarding their application to human clinical practice, which must be corroborated with clinical studies. Research in experimental animals presents application difficulties due to anatomical differences and load conditions, including complex application studies on cadaver bone or plastic anatomical models [16]. Due to differences between experimentation in vivo and in vitro, finite element (FE) models have emerged as a powerful tool that simulates different biological systems in both physiological and pathological conditions, although there are few articles studying the behavior of intramedullary nails in femoral bone. With regard to fractures of the femur and biomechanical behavior of the different osteosynthesis techniques, experimental works studying multiple variables have been developed [17]. The location and type of fracture is a factor of utmost importance. The most studied fractures have been supracondylar or distal femoral fractures because of their greater complexity, greater number of complications, and multiple treatment options. Traditionally, their treatment has been based on plates associated with dynamic screws, screws, or monoblock sheets; however, due to the lateral location of the plate, frequent medial collapses in unstable fractures are produced [18]. For this reason, retrograde intramedullary nails have been designed specifically to treat this type of fracture in an attempt to improve the biomechanical behavior of the fracture and implant, and minimize surgical aggression; however, this type of fracture is still subject to breakage of the implants at the level of the holes of the unplaced screws [19]. Nevertheless, in biomechanical experimental studies using artificial I. BIOMECHANICS

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bones, the classic and traditional anterograde nails introduced in a retrograde manner have shown better results in terms of stability of this distal fracture of the femur compared to those specifically designed as retrograde nails for this type of fracture [20]. Despite the new designs, anterograde nails continue to be used with good clinical results in these complex fractures, although some nails have increased the number of distal screws and the possibility of placing them on different planes of space to increase the stability of the fracture [21]. With respect to its diaphyseal location, Montanini et al. [22] on an FE model of the diaphyseal fracture treated with intramedullary nails found that immediately after the fracture, the loading should not exceed the critical tensions of breakage of the implant into the hole in the proximal screw. These stresses in the nail disappear once the fracture is consolidated, which has been clinically confirmed [23–25]. Comparing only the biomechanical behavior of the intramedullary nails and osteosynthesis plates in fractures at the diaphyseal location using FE models, plates can allow greater stability to the focus of the fracture, while nails suffer increased deformities in the monopodal support. The increase in the diameter of the nail is critical because an increase of 2 mm decreases the deformation of the nails by 40% [26] and therefore increases the chances of failure. This work confirms the results published by Heiney et al. [27] in the case of unstable distal fractures of the femur, where comparing nails with plates showed that there was a greater chance of failure of the implant in nails than in plates. A debated and unsolved point is nail locking. On some models, the proximal screw is only one oblique from the greater to the lesser trochanter, while in other models there are two proximal transverse screws. Placement depends on the type of fracture, the type of screw, its diameter, stresses caused by screws and brittle points in the nail holes of unplaced screws, its proximity to the fracture focus, placement plans, etc.; these points remain clinically and biomechanically unclarified. In terms of the diameter of the screws, there must be a compromise between a maximum value, which does not exceed 50% of the diameter of the nail to ensure its resistance [28], and a minimum value, which ensures their resistance to the loads and stresses to which they are subjected. The smaller is the diameter of the nail, smaller should be the diameter of the screws. This is why some models of unreamed nails have increased their diameter to allow larger diameter screws. The proximity of the distal screws to the fracture focus is an important point. The closer the screws are to fracture focus increases the stresses and forces supported by screws, while if they are further away from the focus of fracture, rotational stability improves and their chances of failure are lower [29]. They must always be positioned perpendicular to the axis of the nail with at least two screws, except in transverse fractures without comminution in which a unique screw may be sufficient. Wahnert et al. [30] compared the stability achieved with different types of distal screws (screws and coiled sheets) and with a nail plate using an artificial model of osteoporotic femoral distal fracture subjected to rotational and axial loads. The conclusion is that in these fractures and against rotational stresses, distal locking of the nail with four screws with different angles is greater regarding the stability granted to the fracture than other types of locking systems with two screws lateral to medial or using a coiled sheet, and is similar to the stability granted with the nail plate. Locking with four distal screws obtained the best biomechanical results in terms of joint stability against rotational and axial stresses, and results on distal bolts have been confirmed in clinical studies [21]. Chen et al. [19], in a biomechanical study using FEs and artificial bones, explored the stresses in the screws and the rigidity of osteosynthesis in a retrograde nail in the treatment of distal femoral fractures. They came to the conclusion that distal screws are more important with respect to the stability of the fracture than proximal screws. A screw placed next to a fracture increases the rigidity of the mounting in oblique fractures, although this increase is not transcendent in transverse fractures and an unplaced screw determines an increase in stresses in the whole nail by 70%, facilitating their failure by breakage of the implant. Nail material has also been studied. P
erez et al. [31] examined the biomechanical behavior of nails of stainless steel and titanium using an FE model in femoral fractures in children. The model is not applicable to fractures in adults or to the behavior of the intramedullary nails, but the conclusion is that titanium behaves best, since stainless steel creates stress-shielding areas in the bone that increase the risk of refracture once implants are removed. However, Kaiser et al. [32] obtained different conclusions in terms of nail material. In this case, they compared intramedullary nails of steel or titanium in the stabilization of diaphyseal femoral fractures using artificial bones, and concluded that steel allows greater stiffness to the mounting and that titanium must only be used in cases of allergy to metals or in cases where future scans by magnetic resonance imaging are needed. The behavior of different materials for intramedullary nails in the treatment of diaphyseal fractures of the distal femur in adult has not yet been studied. In conclusion, primitive stainless steel anterograde intramedullary nails with a static combination of screws are nowadays the reference treatment for femoral fractures from zones 2 to 5 of Wiss; however, there has been no clear demonstration of the superiority of other nails and techniques specifically designed for the treatment of certain fractures. Broad discussions on the material to be used (stainless steel or titanium), on the route of entry of the nails (anterograde or retrograde), reamed or unreamed, and the placement and types of distal screws depending on the type of fracture to treat are still ongoing. I. BIOMECHANICS

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11.2 METHODOLOGY OF SIMULATION As already indicated, treatment of fractures of the femur between the 2 and 5 zones of Wiss is performed with the intramedullary nail with different combinations of screws. There are multiple designs of nail, steel or titanium, reamed and unreamed, solid or hollow, anterograde or retrograde, and various types of locking system. The use of one or other depends on the type and location of the fracture, the characteristics of the patient, and the experience of the surgeon. There is no scientific, clinical, or experimental evidence that demonstrates the best results and indications for each type of nail in each type of fracture. In view of the difficulties experienced with in vitro testing or in experiments with living subjects, FE simulation models have been developed to carry out research on biomechanical systems with high reproducibility, versatility, and limited cost. These models allow the study to be repeated as many times as desired, being a nonaggressive investigation of modified starting conditions (loads, material properties, etc.). However, work continues on the achievement of increasingly realistic models that allow placement of the generated results and predictions into a clinical setting. To that end, it is necessary to use meshes suitable for every particular problem, regarding both type of element and size. This is necessary to perform a sensitivity analysis to determine the optimal features or, alternatively, the minimum mesh necessary to achieve the required accuracy [33]. Thus through the development of a computational model based on the FE method, it is possible to study the biomechanical behavior of the same nail made of two materials (steel or titanium), introduced with or without reaming, anterograde or retrograde, with different types of locking systems in different types of stable and unstable fractures of the femur. This is done to find the best indication and therapeutic technique for each type of femoral fracture from the subtrochanteric to the supracondylar region. For this purpose, the methodology consists of the development of an FE model of a femur, on which will be simulated various types of fractures in the subtrochanteric, diaphyseal, and supracondylar areas, stabilized through various assemblies and materials of intramedullary nails. The mechanical strength of the nail against bending and compressive loads is also studied to determine its maximum strength. Subsequently, a comparative analysis of the different types of fixation in fractures is developed to verify what is the optimal solution in each of the analyzed cases. The biomechanical results are evaluated in correlation with observed clinical results. Thus it is possible to observe biomechanical needs for each type of fracture to find the optimum combination of variables (type of nail, locking system, material, surgical approach, etc.) ideal for their treatment. To develop an FE model, the first phase is to generate the geometry of the different parts that define the model. One of the most significant aspects of biomechanical systems is their geometric complexity, which greatly complicates the generation of accurate simulation models. Thus the use of scanners together with three-dimensional (3D) images obtained by computed tomography (CT) generate geometric models that combine high accuracy in the external form with an excellent definition of internal interfaces. The method requires not only appropriate software tools, capable of processing images, but also compatibility with the programs used later to generate the FE model [33]. Development of the model of a healthy femur is crucial to perfect the whole process of simulation, and to obtain reliable results. A 3D FE model of the femur from a 55-year-old male donor was developed. To obtain a faithful geometry, the bone was scanned using a 3D Roland Picza laser scanner (Fig. 11.4). This device offers two sweep modes: a plane-based and a rotary sweep with a scanning resolution of 0.2 mm. The scanner provides a first approximation of the outer geometry represented by a cloud of points. By means of its own scanning software (Roland Dr. Picza 3) [34], general and rough cleaning operations were performed and eventually treated afterwards in Pixform software [35]. The scanned file was subjected to a specific cleaning protocol to pull the 3D image out of scan noise (deleting spikes, cleaning abnormal and nonmanifold surfaces, fixing bad normals, etc.). Those initial rough geometries with noisy faces and screws attached to a support to fix firmly the bone while scanning are shown in Fig. 11.5. Local operations could also be carried out as smoothing and bridging gaps, because a fully closed geometry was required to continue with the process. Fig. 11.6 displays the final geometry of the femur after the cleaning and geometry treatment process was accomplished. Eventually, when a closed geometry was obtained, it was finally wrapped by an ensemble of B
ezier parametric surfaces as is exhibited in Fig. 11.7. The order and number of these surfaces were varied until a proper fidelity was reached. Analysis tools were provided in this software to check deviation from the original geometry of the surfaces generated. Afterward, they were exported to I-DEAS 11 NX Series software [36]. First, each volume was checked for suitability for meshing. If not, problematic surfaces were identified and the possible source problem was referred back to Pixform and Rhinoceros software [37]. I. BIOMECHANICS

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FIG. 11.4 Roland LPX-250 3D laser scanner.

FIG. 11.5 Initial geometries directly obtained from scanning.

Because the 3D scanner provided only the outer geometry, CT images of the femur were needed to quantify mineral bone density and eventually assign its corresponding Young’s modulus. A CT image treatment was performed with Mimics software (Belgium) [38]. A CT scan (512  512 acquisition matrix, field of view ¼ 240 mm, slice thickness ¼ 0.5 mm in plane resolution) was obtained using a Toshiba Aquilion 64 scanner (Toshiba Medical Systems Zoetermeer, Netherlands). Stacks of images from each bone were processed using Mimics. A threshold of 700 Hounsfield units was chosen to start cleaning the stack of images for the bone. This threshold served to establish an approximate border between cortical and cancellous bone. I. BIOMECHANICS

11.2 METHODOLOGY OF SIMULATION

FIG. 11.6

Smoothed and treated geometry of the femur.

FIG. 11.7

Surface ensemble wrapping previously treated scanned geometry for the proximal femur.

221

Masks, 3D objects, and 3D polylines were generated during an iterative process until a smoothed and properly cleaned geometry was obtained without any artifacts, spikes, or geometric irregularities. After this, the whole set of polylines generated was exported to I-DEAS. The initial set of polylines was reduced to smaller equally spaced groups depending on its distance: 4 mm, 3 mm, 2 mm, and 1 mm. The minimal distance was placed at the femoral head and distal femur part. These areas are of major interest because they are the region where boundary conditions are imposed. Fig. 11.8 shows a superposition of the 3D volumes of cortical bone with the corresponding CT image at a certain level (available with clipping in Mimics). The next step of the process was the proper alignment and orientation of the initial 3D geometry with the one exported from Mimics. Homolog polylines from the scanned anatomical model were generated (see red lines in Fig. 11.9). These polylines were obtained by cutting the bony geometry by auxiliary planes at the same previous Z levels. The aim was to create a connection between both geometries. Each geometry level or bone cut was treated by means of a novel algorithm to assign the corresponding density values between both geometries. The connection between corresponding levels was made to determine the start/end homolog points for each level represented in Fig. 11.9 by the pairs of black arrows on the right side of the image. The intramedullary nail Stryker S2 model (Stryker, Mahwah, NJ, USA) was used for the study, with a length of 380 mm, a wall thickness of 2 mm, and an outer diameter of 13 mm. The corresponding locking screws have an outer diameter of 5 mm. The geometrical model for the nail and the screws was generated by means of the NX I-DEAS program (Figs. 11.10–11.12). I. BIOMECHANICS

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FIG. 11.8 Cross-section of femur volume belonging to cortical bone with the CT image corresponding to this level.

FIG. 11.9 Assignment scheme between Mimics (left) polylines and bone cut (right) polylines.

On the geometrical model of the healthy femur, it is necessary to make the appropriate modifications to simulate different fractures, according to the classification of comminution of Winquist and Hansen, considering different locations (subtrochanteric, diaphyseal, and supracondylar fractures). For this purpose, pairs of outdated uneven surfaces around the desired fracture gap were generated (Fig. 11.13). This process was carried out in NX I-DEAS. After obtaining the geometry of the fractured femoral bone, the intramedullary nail, and the screws, the intramedullary nail with the corresponding screws was positioned in the femur using NX I-DEAS software in the same way as one would carry out a real surgery. This assembly of the computer-aided design model was performed under the supervision of a surgeon. After defining the geometry, the mesh can be generated. Bone, nail, and screws were modeled with linear tetrahedra with a reference size of 1.5 mm, using NX I-DEAS. Two details of the final mesh of bone are shown in Fig. 11.14. Afterwards, an interpolation technique was adopted to assign the property to every mesh element of each bone located in between consecutive splines. Each tetrahedron was reduced to its barycenter, projecting it to each plane between where it was situated. Once all these projections were done, density assignment to every projection was calculated by means of the developed algorithm in FORTRAN [39]. The stiffness was assigned to every bone element depending on the previous density assignment.

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FIG. 11.10

223

Geometrical model of the intramedullary nail: (A) front view of antegrade nail; (B) sagittal view of antegrade nail; (C) detail in perspective of the head of the nail; (D) detail in perspective of the tip of the nail.

FIG. 11.11

Geometrical model of the set nail and screws: (A) front view; (B) sagittal view.

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FIG. 11.12

Detail of screws in the intramedullary nail: (A) head; (B) tip of the nail.

FIG. 11.13

Detail of surfaces around the fracture gap.

FIG. 11.14

Final mesh of the femur. I. BIOMECHANICS

11.2 METHODOLOGY OF SIMULATION

FIG. 11.15

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Perspective details of the fracture generated along with the homologous points to measure the micromovement (marked with dots).

Subsequently, in the fracture site, pairs of homologous points were determined (Fig. 11.15). These points, selected from the mesh nodes located opposite to each other, will be used to measure the micromovements and identifying trends in forming bone callus to the imposed loads. The model of the fractured femur was meshed according to the previously described conditions. In the same way, the intramedullary nail and screws were meshed again using NX I-DEAS software. The FE model of the intramedullary nail is shown in Fig. 11.16. A linear tetrahedral was used to mesh the complete osteosynthesis model. The final FE model is shown in Fig. 11.17. To guarantee the accuracy of the FE results, a sensitivity analysis was performed to determine the minimal mesh size required for an accurate simulation. For this purpose, a mesh refinement was performed to achieve a convergence toward a minimum of the potential energy, with a tolerance of 1% between consecutive meshes. As an example, the statistics corresponding to one of the FE models are presented in Table 11.1. In the FE simulation, the appropriate characterization of the mechanical behavior of the different materials, which is usually very complex, is essential. Once the inner interface between the cortical and trabecular bone was determined in the way explained before, material properties were assigned to the FE model in NX I-DEAS. They were assumed linear elastic isotropic properties for the bone, with variable values related to the processed CT images [40]. The metallic nail was made of 316 LVM steel or Ti-6L-4V and the metallic screws were made of 316 LVM steel, both assumed to be linear elastic isotropic. Table 11.2 summarizes the mechanical properties values used in different materials. Concerning the load conditions, a load case associated with an accidental support of the leg at early postoperative stage has been considered. This load was quantified to be about 25% of the maximum gait load. According to Orthoload’s database (Fig. 11.18), the hip reaction force and abductor force, referring to 45% of the gait, corresponded to the maximum and most representative load [42]. Forces generated by the abductor muscles were applied to the proximal area of the greater trochanter, in agreement with most classic authors’ opinions [43, 44] (Fig. 11.19). Fully constrained boundary conditions (Fig. 11.20) were applied at the distal part of each femur (at the condyles).

FIG. 11.16 Finite element model of the intramedullary nail: (A) front view of anterograde nail; (B) sagittal view of anterograde nail; (C) detail in perspective of the head of the nail; (D) detail in perspective of the tip of the nail. I. BIOMECHANICS

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FIG. 11.17

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Cross-section of the femur joint, intramedullary nail, and screws.

TABLE 11.1 Mesh Statistics of the Finite Element Models Part of model

Element type

Number of elements

Femoral bone

4-node linear tetrahedron

427,939

Cortical bone

4-node linear tetrahedron

216,169

Trabecular bone

4-node linear tetrahedron

211,770

Intramedullary nail

4-node linear tetrahedron

574,547

Screw #1

4-node linear tetrahedron

2417

Screw #2

4-node linear tetrahedron

1454

Screw #3

4-node linear tetrahedron

1111

Screw #4

4-node linear tetrahedron

2818

Total

1,007,869

TABLE 11.2 Material Properties Elastic isotropic a

Cortical bone [41] a

Trabecular bone [41] b

316 LVM steel b

Ti-6L-4V a b

Young’s modulus (MPa)

Poisson coefficient

20,000

0.3

959

0.12

192,360

0.3

113,760

0.34

Values supplied by the manufacturer. Value corresponding to the maximum bone density. Elemental values were assigned according to the explained algorithm.

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FIG. 11.18 Simulated gait cycle introducing hip reaction forces and abductor (abd) muscle forces. The maximal hip reaction force was 2.54% body weight (BW). The red dotted line is the Fz hip reaction force for the scaled 3.75% BW gait cycle.

FIG. 11.19

Scheme of the load conditions applied to the model. Abd, abductor.

FIG. 11.20

Boundary conditions applied: femoral condyles constrained.

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FIG. 11.21

Illustration of workflow followed through the acquisition of the geometry, finite element model generation, material assignment, and eventually simulations developed.

A key issue in the FE models is the interaction between the different constitutive elements of the biomechanical system, especially when it results in essential conditions affecting the behavior to be analyzed. In this way, the biomechanical behavior of this kind of osteosynthesis depends basically on the conditions of contact between the intramedullary nail and bone, so that the correct simulation of the interaction conditions determines the reliability of the model. The study was focused on the immediately postoperative stage. Thus the interaction at the fracture site did not take into account any biological healing process. Contact interaction was assumed between the outer surface of the nail and the inner cortex of the medullary canal of the femur. Tied interaction between screws and cortical bone was considered, whereas contact between screws and femoral nail was simulated. The selected friction values of bone/nail and nail/screws were 0.1 and 0.15, respectively, in accordance with the literature [45–47]. Of interest, other similar studies modeled bone/nail interaction as frictionless [22, 48]. The Abaqus 6.11 program [49] was employed for the calculations and postprocessing the results of the previously generated models in NX I-DEAS software. To summarize, a schematic workflow is depicted in Fig. 11.21, exhibiting the overall software used to generate FE models and perform different simulations.

11.3 TYPES OF FRACTURES AND OSTEOSYNTHESIS For the treatment of fractures to femurs between the 2 and 5 Wiss zones, an intramedullary nail with corresponding screws is used. However, the characteristics of this indication are not unique, since there are multiple combinations according to the geometric design of the nail, its material, the type of surgical approach, locking system, etc. Thus the use of one or another system depends on the type and location of the fracture, the characteristics of the patient, and the experience of the surgeon. To help surgeons choose the best osteosynthesis in each case, two different studies were performed (A and B). The “A” study consisted of an analysis of the biomechanical behavior of a single system of osteosynthesis for different types of fracture (in terms of type and position along the femur). On the other hand, the “B” study corresponded

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to an analysis of the biomechanical behavior of different systems of osteosynthesis (with different locking systems) for different types of fracture with the same location in the femur (distal). All the considered fractures were modeled as transverse by means of an irregular surface developed to represent a closer geometry to the actual fracture. The effect of the gap size was unclear in the literature. So, the majority of the reviewed in vivo studies referred to a gap size ranging from 0.6 to 6 mm [40, 50], whereas in FE simulation articles it ranged from 0.7 to 10 mm [47, 51]. Thus for the “A” study, three different fracture gaps were studied: 0.5 mm (considered as a noncomminuted fracture), 3 mm (the most referenced value found in the literature, representing a mid-value), and 20 mm as an example of a comminuted fracture. In addition to this, three localizations of the fracture were studied: proximal, medial, and distal for each gap size. Only one combination of screws was studied: one oblique placed proximally and two transversely at the distal part. On the other hand, the purpose of the “B” study was to investigate the optimal screw combination and gap size for a single distal fracture location, considering the same three gap sizes: 0.5, 3, and 20 mm, respectively. Thus four combinations of locking screws were considered: one oblique proximal screw combined with four configurations of the three distal ones, two lateral-medial and one anteroposterior. Table 11.3 summarizes the list of FE models simulated for the “A” and “B” studies (9 and 12 FE models, respectively). These models will be duplicated, since each one of these cases is carried out considering the two studied materials (stainless steel and titanium) of the nail. Finally, to validate the conclusions obtained from simulations, a clinical follow-up was carried out for both studies, approved by the Ethics Committee of the Institute of Health Sciences of Aragón (protocol number CI PI15/0214). Thus for the “A” study, a sample of 55 patients, 24 males and 31 females, with a mean age of 52.5 years was obtained, all of them treated with femoral nail Stryker S2. Localizations of fractures were 32 in the right femur and 33 in the left femur. On the other hand, for the “B” study, a sample of 15 patients, 6 males and 9 females, with a mean age of 53.2 years was obtained, all of them treated with the same nail as in the “A” study. Localizations of fractures were 10 in the right femur and 5 in the left femur. The grade of comminution was measured in both cases according to the scale of Winquist and Hansen [5]. The distribution of cases corresponding to fracture localization and fracture grade are included, for the “A” and “B” studies, in Table 11.4.

TABLE 11.3

Different Configurations Considered in the Finite Element Simulation

FE model

Proximal screws

Distal screws

Fracture location

Gap size

Screw configuration

A-01

Oblique (#1)

2 L/M (#2, #3)

Proximal

0.5 mm

A-02

Oblique (#1)

2 L/M (#2, #3)

Proximal

3 mm

A-03

Oblique (#1)

2 L/M (#2, #3)

Proximal

20 mm

A-04

Oblique (#1)

2 L/M (#2, #3)

Medial

0.5 mm

A-05

Oblique (#1)

2 L/M (#2, #3)

Medial

3 mm

A-06

Oblique (#1)

2 L/M (#2, #3)

Medial

20 mm

A-07

Oblique (#1)

2 L/M (#2, #3)

Distal

0.5 mm

A-08

Oblique (#1)

2 L/M (#2, #3)

Distal

3 mm

Continued

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Different Configurations Considered in the Finite Element Simulation—cont’d

FE model

Proximal screws

Distal screws

Fracture location

Gap size

Screw configuration

A-09

Oblique (#1)

2 L/M (#2, #3)

Distal

20 mm

B-01

Oblique (#1)

2 L/M + 1 A/P screws (#2, #3, #4)

Distal

0.5 mm

B-02

Oblique (#1)

2 L/M + 1 A/P screws (#2, #3, #4)

Distal

3 mm

B-03

Oblique (#1)

2 L/M + 1 A/P screws (#2, #3, #4)

Distal

20 mm

B-04

Oblique (#1)

1 L/M + 1 A/P screws (#2, #3)

Distal

0.5 mm

B-05

Oblique (#1)

1 L/M + 1 A/P screws (#2, #3)

Distal

3 mm

B-06

Oblique (#1)

1 L/M + 1 A/P screws (#2, #3)

Distal

20 mm

B-07

Oblique (#1)

1 L/M + 1 A/P screws (#3, #4)

Distal

0.5 mm

B-08

Oblique (#1)

1 L/M + 1 A/P screws (#3, #4)

Distal

3 mm

B-09

Oblique (#1)

1 L/M + 1 A/P screws (#3, #4)

Distal

20 mm

B-10

Oblique (#1)

2 L/M screws (#2, #4)

Distal

0.5 mm

B-11

Oblique (#1)

2 L/M screws (#2, #4)

Distal

3 mm

B-12

Oblique (#1)

2 L/M screws (#2, #4)

Distal

20 mm

A/P, anteroposterior; L/M, lateral-medial.

11.4 RESULTS The FE simulations allowed the mobility results for the different cases analyzed to be obtained. Fig. 11.22 shows, for the “A” study, the deformed shape amplified (25) and the vertical displacement maps corresponding to noncomminuted fractures (gap size 0.5 mm), mid-value gap (gap size 3 mm), and comminuted (gap size 20 mm). In Fig. 11.23, the same results can be observed for all four combinations of screws and steel nail corresponding to the “B” study.

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231

11.4 RESULTS

TABLE 11.4 Statistics for the Clinical Follow-Up Study

Wiss zone

Cases

Comminution grade

Cases

A

2

7

None

29

3

11

1

9

4

22

2

9

5

15

3

1

4

7

B

Total

55

Total

55

5

9

None

9

5

5

2

5

5

1

4

1

Total

15

Total

15

U. U3 0.18 0.13 0.09 0.04 –0.00 –0.05 –0.10 –0.14 –0.19 –0.23 –0.28 –0.33 –0.37

(A)

(B)

(C)

FIG. 11.22 Deformed shape (25) and vertical displacement maps, for the “A” study, corresponding to distal fractures: (A) noncomminuted (gap size 0.5 mm); (B) mid-value gap (gap size 3 mm); (C) comminuted (gap size 20 mm).

The study of micromotions at the fracture site was measured as the relative motion between pairs of homologous points defined from opposed nodes depicted in Fig. 11.15. The maximum amplitude of micromotion between homologous points at the fracture site for steel and titanium nails is reported in Fig. 11.24 for the “A” study and in Fig. 11.25 for the “B” study, respectively. Thus Fig. 11.24A shows that the most rigid behavior belongs to the distal fracture (40.69–66.43 μm), followed by the medial one (51.96–73.39 μm), and proximal one (60.29–90.29 μm). Micromotion amplitude follows the same growing tendency with the increase in gap size for all three fracture locations. For the titanium nail, Fig. 11.24B shows the same tendency at the three fracture locations observed previously: micromotions at distal fracture ranges from 62.02 to 123.71 μm, followed by the medial one (ranging from 75.88 to 139.80 μm), and finally the proximal one (varying from 93.07 to 140.83 μm). If the ratio of the amplitudes between both materials is calculated, a pitchfork of 1.46–2.00 is obtained, which is located within the range of Young’s modulus ratio for both materials (1.69). On the other hand, in Fig. 11.25 it can be observed that the most rigid behavior of both nail materials corresponds to the fourth interlocking system: 40.69 μm (gap size of 0.5 mm) and 48.33 μm (gap size of 3 mm), whereas the first one (three distal screws) shows the best stability in terms of micromotions for the biggest gap size of 20 mm: 63.50 μm. The second and third screw combinations exhibit a similar behavior when the nail material is changed to titanium among the three gap sizes.

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11. ANALYSIS OF THE BIOMECHANICAL BEHAVIOR OF INTRAMEDULLARY NAILING

U. U3

U. U3 0.19 0.14 0.10 0.05 0.00 –0.05 –0.09 –0.14 –0.19 –0.24 –0.28 –0.33 –0.38

(A)

0.22 0.17 0.11 0.06 0.01 –0.04 –0.10 –0.15 –0.20 –0.26 –0.31 –0.36 –0.41

(B)

U. U3

U. U3 0.23 0.17 0.12 0.06 0.01 –0.04 –0.10 –0.15 –0.21 –0.26 –0.31 –0.37 –0.42

(C)

0.19 0.14 0.10 0.05 0.00 –0.05 –0.09 –0.14 –0.19 –0.24 –0.28 –0.33 –0.38

(D)

Deformed shape (25) and vertical displacement maps, for the “B” study, corresponding to a distal fracture: (A) 1st interlocking system; (B) 2nd interlocking system; (C) 3rd interlocking system; (D) 4th interlocking system.

FIG. 11.23

With respect to the evaluation of global stability by measuring the displacement at the head of the nail (insertion point at the trochanter), Figs. 11.26 and 11.27 show the results obtained for the “A” and “B” studies for both intramedullary materials. In this way, when evaluating global stability, in the “A” study, the trend is reversed with respect to the amplitude of axial micromotion. In this case, the proximal fracture is the most rigid, followed by medial fracture and distal fracture. This result is obtained because when the physiological loads at the head of the femur are applied, the intramedullary nail blocks the global movement of the femoral head “sooner” for the proximal fracture than for the distal one. According to gap size influence, there is a marked increase in the interfragmentary movement as well as global stability when the gap increases. Thus for the steel nail, values range from 1.33 mm (proximal fracture, 0.5 mm

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11.4 RESULTS

233

FIG. 11.24 Amplitude of axial micromotion (μm), for the “A” study, corresponding to different nail materials: (A) steel intramedullary nail; (B) titanium intramedullary nail.

gap) to 2.01 mm (distal fracture, 20 mm gap), whereas the titanium nail yields a higher rate of global movement: 1.62 mm (proximal fracture, 0.5 mm gap) to 3.14 mm (distal fracture, 20 mm gap). By calculating the ratio of the global movement between both materials a pitchfork of 1.22–1.56 is obtained. However, in the “B” study, global stability of each fixation system follows similar tendencies as the aforementioned amplitude of micromotion for the steel nail and titanium nail. The global movement at the top of the nail was measured yielding the most rigid behavior for the fourth interlocking system: 1.75–2.01 mm for the steel nail, whereas for the titanium nail, the first screw combination showed the smallest motion for the first interlocking system: 2.81 and 2.80 mm (3 mm and 20 mm gap size, respectively). For the smallest gap size, the fourth interlocking system was again the most stable in terms of global movement (2.36 mm). Analogously to the analyzed micromotions, the second and third fixation systems yield similar results for both materials in the two gaps associated with comminuted fractures.

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11. ANALYSIS OF THE BIOMECHANICAL BEHAVIOR OF INTRAMEDULLARY NAILING

Amplitude of axial micromotion (μm), for the “B” study, corresponding to different nail materials: (A) steel intramedullary nail; (B) titanium intramedullary nail.

FIG. 11.25

Concerning intramedullary nails, Figs. 11.28 and 11.29 show the von Mises stress maps in the nail corresponding to proximal and distal fractures, respectively, for a gap of 3.0 mm. As can be seen, the maximum stress values in the nail are located at the position corresponding to the site of fracture (i.e., near the top of the nail for proximal fracture and near the bottom of the nail for distal fracture), due to the bending effect produced on the nail connecting the two parts of the fractured femur. Moreover, a high stress concentration appears in the screw hole nearest to the fracture site. In any case, due to the low level of load applied, the values do not affect the material yielding stress, so the nail strength is not compromised. Finally, for the screws, Fig. 11.30 shows the von Mises stress maps for a proximal fracture (gap size 3 mm). The figure shows a high stress concentration in the upper screw, very near to the fracture site, while the lower screws are quite discharged. In the same way, Fig. 11.31 shows the von Mises stress maps for a distal fracture (gap size 3 mm). In this case, the biomechanical behavior is completely different, showing similar stress values in both upper and lower screws. Although the stresses tend to concentrate in the lower screws, the presence of two screws allows for a better

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11.4 RESULTS

235

Global movement of the top of the nail (mm), for the “A” study, corresponding to different nail materials: (A) steel intramedullary nail; (B) titanium intramedullary nail.

FIG. 11.26

load transmission between nail and bone, generating a moment that balances the bending effect appearing in that zone. With respect to the clinical follow-up, Table 11.5 shows the mean time of the fracture consolidation, except for Grade 3 in the “A” study, since it was not finally considered because only one case was assessed, and Grades 2 and 3 in the “B” study, since there are no cases. Thus, for both studies, it can be observed that the healing time increases with higher comminution grade. In view of Table 11.5, the clinical results are in accordance with the FE simulations results, obtaining a longer healing period for fractures with worse stability.

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236

11. ANALYSIS OF THE BIOMECHANICAL BEHAVIOR OF INTRAMEDULLARY NAILING

Global movement of the top of the nail (mm), for the “B” study, corresponding to different nail materials: (A) steel intramedullary nail; (B) titanium intramedullary nail.

FIG. 11.27

11.5 CONCLUSIONS Different FE models have been developed, on the one hand, to analyze various types of fractures in the subtrochanteric and diaphyseal supracondylar area with several gap sizes, stabilized with a single combination of screws for the intramedullary nail, and, on the other hand, to characterize the stability of different interlocking systems and identify the optimal one for every type of fracture in the distal location. In addition, the mechanical strength of the nail against bending and compression efforts was studied comparing two nail materials: stainless steel and titanium alloy. The results of the FE simulations were compared with a set of clinical cases included in the clinical follow-up. In this way, the following conclusions were obtained: • A good agreement between clinical results and the simulated fractures in terms of gap size was found. Noncomminuted fractures have a minimum mean consolidation time (4.1 months), which coincides with appropriate mobility at the fracture site obtained in the FE simulations, whereas comminuted fractures have a higher mean consolidation period (7.1 months), corresponding to excessive mobility at the fracture site obtained by means of FE simulations. The healing time rises as the comminution grade increases. I. BIOMECHANICS

237

11.5 CONCLUSIONS

S, Mises (Avg: 75%) 173.79 159.33 144.86 130.39 115.92 101.46 86.99 72.52 58.06 43.59 29.12 14.66 0.19

(A) FIG. 11.28

(B)

Von Mises stress maps in the nail for proximal fracture (gap size 3 mm): (A) whole nail; (B) detail corresponding to the fracture site.

S, Mises (Avg: 75%) 147.0 134.7 122.5 110.2 98.0 85.8 73.5 61.3 49.0 36.8 24.5 12.3 0.1

(A) FIG. 11.29

(B)

von Mises stress maps in the nail for distal fracture (gap size 3 mm): (A) whole nail; (B) detail corresponding to the fracture site.

• Regarding the best nail material, the mobility rate with the titanium nail was higher than with the steel nail. So, the steel nail confers a stiffer fixation system, which is better for osteosynthesis. In particular, the obtained results between both nail materials (stainless steel and titanium alloy) show a higher mobility when using titanium nails, which produce a higher rate of strains at the fracture site, amplitude of micromotions, and bigger global movements compared to stainless steel nails. I. BIOMECHANICS

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11. ANALYSIS OF THE BIOMECHANICAL BEHAVIOR OF INTRAMEDULLARY NAILING

S, Mises (Avg: 75%) 217.07 199.00 180.94 162.87 144.80 126.74 108.67 90.60 72.54 54.47 36.40 18.34 0.27

S, Mises (Avg: 75%) 23.23 21.32 19.40 17.49 15.58 13.66 11.75 9.84 7.92 6.01 4.10 2.18 0.27

(A) FIG. 11.30

(B)

Von Mises stress maps in the screws, proximal fracture (gap size 3 mm): (A) upper screw; (B) lower screws.

S, Mises (Avg: 75%) 57.19 52.53 47.87 43.21 38.56 33.90 29.24 24.58 19.92 15.26 10.60 5.94 1.28

S, Mises (Avg: 75%) 55.10 50.54 45.98 41.42 36.86 32.30 27.74 23.18 18.62 14.05 9.49 4.93 0.37

(A) FIG. 11.31

(B)

Von Mises stress maps in the screws, distal fracture (gap size 3 mm): (A) upper screw; (B) lower screws.

TABLE 11.5

Time Consolidation for the Clinical Follow-Up (Months)

Study

Comminution grade

Mean time of the consolidation

A

Noncomminutes

4.1

1

4.9

2

6.2

4

7.1

Noncomminutes

4.8

1

5.2

2

5.2

B

• Among the studied combinations of distal screws, the one with two distal screws medial-lateral provided the best results in terms of stability at the fracture site and global movement at the top of the nail along the three fracture gap sizes. This tendency is because the locking effect is maximized when the distance between the distal screws is increased. This parameter is limited by the proximity to the fracture site and the distance to the femoral condyles. In conclusion, an anterograde locked nail is particularly useful in the treatment of a wide range of supracondylar fractures with proximal extension into the femoral diaphysis, which confirms that this technique is nowadays the reference surgical treatment for this kind of fracture.

Acknowledgments This research has been partially financed by the Fundacion Mutua Madrileña (Research Projects: AP162632016) and by the Government of Spain: Ministry of Economy and Competitiveness (Research Project: DPI2016-77745-R).

I. BIOMECHANICS

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I. BIOMECHANICS