Biomechanical analysis of the treatment of intertrochanteric hip fracture with different lengths of dynamic hip screw side plates

Abstract

BACKGROUND:

Dynamic hip screw (DHS) is a common implant used to treat stable-type intertrochanteric hip fractures. There are many factors that can affect the success rate of the surgery, including the length of side plates. It is therefore important to investigate the biomechanical effect of different DHS side plates on bones.

OBJECTIVE:

In order to reduce the likelihood of an implant failure, the aim of this study was to use finite element analysis (FEA) to investigate and understand the effect of side plates with different lengths in DHS.

METHODS:

In this FEA study, a 3D model with cortical bone, cancellous bone, side plate, lag screw, and cortical screws to simulate the implantation of DHS with different lengths of side plate (2-hole, 4-hole, and 6-hole) for intertrochanteric hip fractures was constructed. The loading condition was used to simulate the force (400 N) on the femoral head and the stress distribution on the lag screw, side plate, cortical screws, and femur was measured.

RESULTS:

The highest stress points occured around the region of contact between the screw and the cortical bones. The stress on the femur at the most distal cortical screw was the greatest. The shorter the length of the side plate, the greater the stress on the cortical screws, resulting in an increased stress on the femur surrounding the cortical screws.

CONCLUSIONS:

The use of DHS with 2-hole side plate may increase the risk of side plate pull-out. The results of this study provide a biomechanical analysis for selection of DHS implant lengths that can be useful for orthopaedic surgeons.

Keywords

Biomechanics finite element analysis dynamic hip screw intertrochanteric hip fracture

1. Introduction

Dynamic hip screw (DHS) is the implant of choice for stable-type intertrochanteric hip fractures [1]. DHS provides compression at the fracture site and is comparatively easy to operate [2, 3]. Lag screw cut-out is one of the most common complications encountered with DHS. Appropriate tip apex distance and fracture reduction are believed to decrease the risk of lag screw cut-out [4, 5, 6]. Side plate pull-out from the femur shaft is also a complication of DHS [7], but discussion on this issue is scant. A longer side plate can provide more screws to grasp the bones, but this will result in a more invasive operation resulting in a larger wound and more soft tissue destruction. Although many studies have investigated the effects of different lengths of side plates through clinical observations [7, 8, 9, 10, 11], few studies used biomechanics as a basis for investigation [12, 13]. Therefore, orthopaedic surgeons need a biomechanical analysis to select side plates with suitable lengths for their patients in order to reduce postoperative complications.

Previous studies indicated that 4-hole side plates are common choices for treatment, but some scholars argued that 2-hole side plates provided sufficient stability in biomechanical analysis and clinical practice [7, 8, 9, 10, 11, 12, 13]. McLoughlin et al. conducted a cadaveric experimental study to investigate the biomechanical status of 2-hole and 4-hole side plates after implantation, and the results showed no substantial difference in terms of stability [12]. In fact, the 2-hole DHS is biomechanically as stable as the 4-hole DHS in cyclic and failure loads. Some studies found that using 2-hole side plates may result in the pull-out of the side plate. Laohapoonrungsee et al. reviewed 83 patients who received DHS implantation with 2-hole side plates and found that two patients experienced pull-out of the side plate [7]. Ríha et al. reported that out of 32 patients who received DHS implantation with 2-hole side plates, one experienced side plate pull-out [8]. These studies found that using DHS with a two-hole side plate produces satisfactory results. However, there are still a few cases where the screws were pulled out or a fracture collapsed. Yian et al. investigated the optimal number of cortical screws needed for stable side plate fixation, and found that three cortical screws provide optimal tension distribution [14]. Although many studies have examined the lengths of side plates and the number of cortical screws [7, 8, 9, 10, 11, 12, 13, 14, 15], few studies have conducted a complete biomechanical analysis for different lengths of side plates [13].

The applications of finite element analysis (FEA) in medical research are mainly in orthopaedic or dental biomechanical analysis [16, 17, 18, 19]. The advantage of FEA is that it can be used for observation of stress distribution, displacement, and strain, providing the mechanical status of the overall structure that cannot be obtained from experimental observations. Some researchers have used FEA to investigate the effects of different lengths of lag screws and barrels [19]. Rooppakhun et al. used FEA to investigate the mechanical performance of DHS with different lengths of side plates, and the results showed that there are no significant differences with the increasing plate length in implant stress and fracture stability [13]. However, their study did not include an investigation of the effects of stress distribution on the femur and cortical screws.

On the basis of the descriptions in the above-mentioned studies and to understand the effects of different side plate lengths on patients, the aim of this study was to use FEA to conduct a biomechanical analysis of three different side plates with varying lengths (2-hole, 4-hole, and 6-hole) used in DHS. The study mainly investigated the stress distribution on the lag screw, side plate, cortical screws, and femur. The results of this study provide a biomechanical analysis that can be useful for orthopaedic surgeons to obtain ideal surgical outcomes and provides a biomechanical basis for future design and development of implants.

2. Materials and methods

2.1 Building a simulation geometry model

This study mainly examined the three-dimensional (3D) FEA computer simulation of side plates of different lengths in DHS; therefore, a FEA model of three DHS groups with varying lengths (2-hole, 4-hole, and 6-hole) that are commonly encountered in clinical practice was established (Fig. 1a). The models used in this study are mainly classified into five components, namely cortical bone, cancellous bone, side plate, lag screw, and cortical screws. A femur model was established using computed tomography images (Visible Human Project) provided by the National Institutes of Health in the United States (the bone model used in this study is the male’s right femur). This model was divided into two components, namely cortical and cancellous bones, and a fracture site with an interval of 1 mm was established at the greater trochanter. With regards to DHS and cortical screws, we used 3D computer-assisted design (CAD) software (Solidworks, Dassault Systems SolidWorks Corp, Waltham, MA, USA) for drawing. In addition, Solidworks was used to combine the femur, DHS, lag screw, and cortical screws (4.5 mm in diameter, the screws used to have threads). The lag screw was implanted in the middle of the femoral head. A 10 mm distance from the tip of the lag screw to the apex of the femoral head was used as a basis for implantation. In addition, the tip-apex distance (TAD) value was 22.5 mm (TADAP $=$ 10 mm, TADLAT $=$ 12.5 mm) (TAD $=$ tip-apex distance measured on anteroposterior radiograph (TADAP) $+$ tip-apex distance measured on lateral radiograph (TADLAT)). This study divided DHSs into three groups according to their lengths: 2-hole DHS (first group), 4-hole DHS (second group), and 6-hole DHS groups (third group). After the 3D computer model was completed, the constructed model was imported into the FEA software (ANSYS Workbench 18, ANSYS, Inc., Canonsburg, PA, USA) for analysis.

2.2 Loading and boundary conditions

This study mainly simulated the force on the femoral head in a scenario where a man stands upright. This study provided one load condition and one boundary condition (Fig. 1b). The loading condition was used to simulate the force on the femoral head when a subject stands upright, so a downward force of 400 N (Z-axis direction) was applied to the femoral head to simulate the force on both legs [19, 20]. In addition, for the boundary conditions used in this study, a fixed support was provided at the distal end of the femur so that the displacement at the X-axis, Y-axis, and Z-axis were set at 0. The contact between the lag screw and the barrel of the side plate was set as a no separation type, and contact between other parts was set as a bonded type. In ANSYS Workbench, the no separation option allows some degree of slipping while in contact. On the other hand, the bonded option couples the two faces (or edges) together, leaving no gap [21].

2.3 Material properties of the model

The model used in this study was composed of five parts, namely the femoral cortical bone, femoral cancellous bone, lag screw, side plate, and cortical screws. The material properties set up in this study were obtained from previous studies [22, 23, 24, 25]. All materials were assumed to be homogeneous, isotropic, and linear elastic. Therefore, two independent parameters (Young’s modulus and Poisson’s ratio) were used to express the material properties. Table 1 shows the material properties used in this simulation.

Table 1
Material properties used in this simulation [22, 23, 24, 25]

Material	Young’s modulus (MPa)	Poisson’s ratio
Cortical bone	17000	0.3
Cancellous bone	1000	0.3
Side plate	200000	0.3
Lag screw	200000	0.3
Cortical screw	118000	0.3

After FEA analysis, we mainly used the distribution of von Mises stress and principal stress as observation indicators. The von Mises stress is suitable for the observation of strength for metallic materials. With regard to bone structure, the principal stress provides a suitable scalar measure of stress intensity. The von Mises stress is defined $\sigma_{\textit{von}}=\sqrt{\frac{1}{2}[(\sigma_{1}-\sigma_{2})^{2}+(\sigma_{1% }-\sigma_{3})^{2}+(\sigma_{2}-\sigma_{3})^{2}]}$ , where $\sigma_{1}$ , $\sigma_{2}$ and $\sigma_{3}$ represent the principal stress along the three axes.) The von Mises stress regions observed were those on the lag screw, side plate, and cortical screws. The principal stress distribution status on the lateral and medial sides of the femur was determined to examine the differences in DHS when side plates of different lengths were used.

2.4 Convergence test

Before performing the FEA, a convergence test was carried out on the constructed model to obtain more accurate results with the FEA model when performing a simulation analysis. With regard to the model for convergence test, we mainly used control of mesh size to achieve the result of testing convergence. The dimensions of the mesh were 5, 4, 3, and 2 mm, and quadratic tetrahedral elements were mainly used for the mesh in the ANSYS Workbench software. Although this study provides the size of the mesh, the software automatically refines the mesh in a place with a large curvature in the function of the mesh, such as the threads on the screw. A downward force of 400 N (Z-axis) was exerted on the femoral head as the load condition. A fixed support was provided at the distal end of the femur as its boundary condition (Fig. 1b). The different mesh sizes were used for convergence testing. We observed the maximal value of von Mises stress on the lesser trochanter of cortical bone as a marker for the convergence values, which are listed in Table 2. When we observed the convergence values of the von Mises stress on the lesser trochanter of cortical bone in three different models, we found differences in convergence of 2.45%, 0.97%, and 0.36% (level of convergence, 97.55%, 99.03%, and 99.64%, respectively). From previous studies, we found that this convergence result is acceptable for this study and that a convergence of 5% is the stop criterion for the convergence test [19, 26]. These results show that the model used in this study converges. Therefore, the use of these finite element mesh models to examine DHSs of different lengths is reasonable. After the convergence test, the three FEA models used in the study all used a 3 mm mesh for quadratic tetrahedral elements as a standard for mesh segmentation (Fig. 1c) and as an analysis simulation for mechanics in the ANSYS Workbench FEA software. Figure 1c shows the number of nodes and elements used in the three models.

Table 2
Results of the convergence test

Mesh size

Peak von Mises

stress on the lesser

trochanter in 2-hole

DHS model (MPa)

Levels of convergence (%)

Peak von Mises

stress on the lesser

trochanter in 4-hole

DHS model (MPa)

Levels of convergence (%)

Peak von Mises

stress on the lesser

trochanter in 6-hole

DHS model (MPa)

Levels of convergence (%)

5 mm

5.092

95.46

4.576

98.26

4.608

99.25

4 mm

5.094

95.97

4.600

98.78

4.623

99.56

3 mm

5.203

97.56

4.612

99.03

4.626

99.64

2 mm

5.334

–

4.657

–

4.643

–

Figure 1.

(a) The 3D computer model used in this study. (b) The loading and boundary conditions. (c) The mesh status of the 3D computer models and the number of nodes and elements used in the three mesh models.

Figure 2.

(a) Stress distribution on the lag screws. (b) Stress distribution on the cortical screws.

3. Results

In this finite element analysis study, the analyzed results can be represented by a color graph. Figure 2a shows the von Mises stress distribution on the lag screws. Among the three groups, the largest stress values on each lag screw were: 2-hole DHS, 281.18 MPa; 4-hole DHS, 280.15 MPa; and 6-hole DHS, 280.4 MPa. The largest stress values were all found on the fracture site. The differences in the largest stress values between the lag screws in the three groups were not remarkable.

Figure 2b shows the von Mises stress distribution on the cortical screws. The largest values on the cortical screws in each group were: 2-hole DHS, 71.934 MPa; 4-hole DHS, 34.684 MPa; and 6-hole DHS, 29.606 MPa. The largest stress values all occurred around the points where the screws were in contact with the cortical bones. In the 2-hole DHS group, the stress on the cortical screws was greatest.

Figure 3.

(a) Maximum principal stress distribution on the lateral side of the femur. (b) Minimum principal stress distribution on the medial side of the femur. (c) Cross-section maximum principal stress distribution on the femur.

Figure 3a shows the maximum principal stress (because the lateral side is subjected to tensile stress) distribution on the femur when observed laterally after implant placement. We found that when the length of the side plate was longer, there would be a wider range of areas with low stress. Figure 3b shows the minimum principal stress (because the medial side is subjected to compressive stress) distribution when the femur is observed medially. It shows that greater stress is produced around the contact point between the cortical screws and the medial cortex of the femur. The results show that the bone around the most distal screw will experience greater stress. In the 2-hole DHS group, this stress value was greatest. Figure 3c shows the cross-section maximum principal stress distribution on the femur.

4. Discussion

DHS is a common implant used to treat stable-type intertrochanteric hip fractures. The factors that determine the success of DHS in fixation of intertrochanteric fractures include fracture pattern, quality of fracture reduction, and position of the lag screw [2, 3, 4, 5, 6, 22]. Lag screw cut-out and side plate pull-out are common complications of DHS failure. It is generally believed that a longer side plate can provide greater stability and minimize the risk of side plate pull-out. However, a longer side plate will result in a longer wound and greater soft tissue damage. Some studies have noted that implantation of a 2-hole side plate with a smaller incision can provide sufficient fixation but that side plate pull-out may still occur [7, 8, 9, 10]. Some studies also reported failure due to cortical screw breakage [13]. In this study we successfully used FEA to investigate the biomechanical effects of DHS side plates of different lengths after implantation. We showed that the shorter the length of the side plate, the greater the stress on the cortical bone of femur bone that may be induced. The results of this study can provide a mechanical basis for the selection of side plate lengths by orthopaedic surgeons.

The results of this study showed no substantial differences in stress values on the lag screw in each group (Fig. 2a). The main reason for this could be that the lengths of the lag screw and barrel used in this study were fixed. Previous studies pointed out that when a longer lag screw was used, the stress on the lag screw was greater than when a shorter lag screw was used [19]. The use of a side plate with a shorter barrel will result in greater stress on the lag screw and barrel. This study used lag screws and barrels of the same length; therefore, the differences in stress on the lag screw and barrel in each group were not remarkable.

In addition, we examined the stress distribution on the cortical screws. The largest stress occurred around the points where the screws were in contact with the cortical bones (Fig. 2b). This could be because, when the cortical screws passed through the cortical bone and cancellous bone, as the Young’s modulus of the cortical bone was larger (the stress on the cortical bone was larger than that on the cancellous bone), the cortical screw was bound by the cortical bone; therefore, the cortical screw sustained greater stress at both sides where it was in contact with the cortical bone. Although the peak stress on cortical screws of this study was lower than the ultimate strength (the ultimate strength of stainless steel is approximately 850 MPa) [23], cortical screws breakage may be induced by fatigue failure [24]. In some clinical cases of DHS failure, the location of the break of cortical screws was consistent with the results of this study [7]. In addition, the longer the length of the side plate, the more cortical screws are used, and the lower the stress on the cortical screws. The main reason is that the more cortical screws are used, the larger the contact surface area between the cortical screws and the femur, which can decrease the stress on the cortical screws.

Through observations of the stress values on the femur, we found that at the side of contact with the side plate, due to the support of the implant, the stress on the femur was relatively lower. This can be explained by the stress shielding effect. The stress shielding effect becomes apparent when the difference in elastic modulus between the implanted material and the bone is large. The stress originally sustained by the bone becomes sustained by the implant, thus reducing the stress in the bone. In addition, observations of the medial side of the femur revealed that the bone surrounding the cortical screws was subjected to greater stress, particularly the most distal cortical screw, where the stress on the bone was greatest, and the stress in the 2-hole DHS group was larger than that in the other two groups (4-hole and 6-hole DHS groups). Previous studies found that on normal bone, the ultimate strength value is approximately 105 to 120 MPa [23, 25]. However, age and disease also affect the ultimate strength value of cortical bone. Thus, screw pull-out may occur with greater stress values. Therefore, the use of a 2-hole DHS in patients may increase the risk of side plate pull-out, and a longer side plate can be chosen to avoid screw pull-out. In addition, clinically, the thread on the cortical screw may also cause higher stress on the cortical bone due to the stress concentration.

The FEA used in this study has some limitations. The material properties were assumed to be homogeneous, isotropic, and linearly elastic to simplify the simulation in this study and were set by referencing previous studies [17, 19, 26, 27]. Although the mechanical response of the cortical and cancellous bone is visco-elasto-plastic, the effects of loading rate on strength of the proximal femur is important to evaluate. With the homogeneous, isotropic, and linearly elastic model, the effect of time dependent behavior is omitted, leading to a limitation for this work. Although this study underwent some simplification and used conditions that might had differences with actual situations, it showed a clear trend for the topic being investigated.

Based on the observations of FEA in this study, the biomechanical status of DHS side plates of different lengths was explored. Although the results obtained from this analysis may have some differences with actual situations, the data provide reference for orthopaedic surgeons when selecting side plates of different lengths. This can also decrease implant failures, enabling patients to attain a better prognosis. In the future, based on the results of this study, clinical researchers can choose a suitable DHS side plate for gait analysis or related research.

5. Conclusions

This study investigated the effects of side plates of different lengths in DHS. The results showed no substantial differences in the stress on the lag screw and barrel when different side plates were implanted. This study also showed that the shorter the length of side plate (2-hole), the greater the stress on the cortical screws. This will increase the stress on the femur surrounding the cortical screws. The use of DHS with a 2-hole side plate may increase the risk of side plate pull-out. The results of this study provide a biomechanical analysis for the selection of DHS implant lengths that can be useful for orthopaedic surgeons.

Footnotes

Acknowledgments

The authors acknowledge the United States National Library of Medicine (NLM) and the Visible Human Project as the image source to build the FEA model in this study. They would like to thank Taichung Veterans General Hospital (TCVGH-HK1088001, TCVGH-1085105C and TCVGH-1087320C) in Taiwan for providing the funding for this research. In addition, they would also like to thank the 3D Printing Research and Development Group, Taichung Veterans General Hospital for helping them build the simulation computer model of this study.

Conflict of interest

The authors declare that they have no conflicts of interest.

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Biomechanical analysis of the treatment of intertrochanteric hip fracture with different lengths of dynamic hip screw side plates

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Materials and methods

2.1 Building a simulation geometry model

2.2 Loading and boundary conditions

2.3 Material properties of the model

Table 1 Material properties used in this simulation [22, 23, 24, 25]

Table 2 Results of the convergence test

5. Conclusions

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Material properties used in this simulation [22, 23, 24, 25]

Table 2
Results of the convergence test