CC BY-NC-ND
0Asian Journal of Surgery (2017) xx, 1-9
Asian Journal of Surgery
ORIGINAL ARTICLE
Intertrochanteric fracture: Association between the coronal position of the lag screw and stress distribution
Changxiang Liang a,b, Ruiping Peng c, Nan Jiang a, Guoping Xie a, LeiWang a, Bin Yu a *
a Department of Orthopaedics and Traumatology, Nanfang Hospital of Southern Medical University, Guangzhou 510515, China
b Department of Orthopaedics, Guangdong General Hospital and Guangdong Academy of Medical Sciences, Guangzhou 510080, China
c Department of Ophtalmology, Third Affiliated Hospital of Sun Yat-sen University, Guangzhou 510630, China
Received 24 October 2016; received in revised form 14 January 2017; accepted 7 February 2017
KEYWORDS
cut-out; finite element
analysis; intertrochanteric
fracture; lag screw position; PFN-A
Summary Background/Purpose: The best position of the lag screw in the femoral head for the fixation of intertrochanteric fracture is controversial. Traditional view suggests that it should be positioned in the central axis of the femoral neck with a tip—apex distance (TAD) of <25 mm, but the mechanical properties have not been reported yet. Herein, we aimed to investigate internal fixation with the lag screw placed in different positions on the femoral coronal plane by performing a finite element analysis and to identify a reasonable lag screw position after the internal stress distributions at the femoral head.
Methods: A three-dimensional finite element model of a healthy male's femur was set up, on which the intertrochanteric fracture model with proximal femoral nail antirotation (PFN-A) was based. Nine modalities of the model were established in accordance with different lag screw positions in the femoral head. Three-dimensional finite element calculations were conducted, and the distribution trends of characteristic high-stress concentration points were observed.
Results: The area of high-stress concentration was distributed from the top of the femoral head to the medial cortex of the trochanteric region. Four characteristic high-stress concentration points were observed, and the following trends indicated that the lower the position of the lag screw, the greater its length.
* Corresponding author. Department of Orthopaedics and Traumatology, Nanfang Hospital of Southern Medical University, No. 1838, Guangzhou Avenue North, Baiyun District, Guangzhou 510515, China. E-mail address: binyudoc@163.com (B. Yu).
http://dx.doi.org/10.10167j.asjsur.2017.02.003
1015-9584/© 2017 Asian Surgical Association and Taiwan Robotic Surgical Association. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Conclusions: A longer and lower lag screw may make the fixation sustain greater stress, reduce bone tissue stress correspondingly in intertrochanteric fractures fixated with PFN-A, and sustain greater stress and more cyclic load at the same bone density.
© 2017 Asian Surgical Association and Taiwan Robotic Surgical Association. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Internal fixation is the preferred treatment for intertrochanteric fracture, a typical osteoporotic fracture.1 However, as one of the most important mechanical complications, cut-out failure occurs after intramedullary and extramedullary treatment,2-5 with an incidence between 1.4% and 19%, depending on the fracture type and implant used.6-8 This may result in fracture displacement again, pain, joint dysfunction, lower limb shortening, and limping.9
The concept of the tip-apex distance (TAD) was introduced in 1995 by Baumgaertner, who supported the center-to-center position for the placement of a lag screw with a TAD of <25 mm.10 A TAD of <25 mm and even 20 mm has been proven to reduce the incidence of lag screw cut-out effectively. Alternatively, a lag screw should be placed as deeply as possible in the axis along the head and neck. Currently, TADs of <25 mm have been widely accepted by most scholars to guide fixation of intertrochanteric fracture.11
Although positioning a lag screw in the central axis of the femoral head and neck on the lateral view is generally accepted, the best position of a lag screw in the femoral head on the coronal plane remains controversial. Recently, some scholars have questioned the TAD concept and the traditional view that a lag screw should be placed in the central axis of the femoral neck. Kane believed that when a lag screw is placed lower, its biomechanical stability may not be less than that when it is placed in the central axis; even the TAD was >25 mm.12 Kuzyk's study showed that a lower placed lag screw could produce the highest axial and torsional stiffness. He suggested that a lag screw should be placed lower in the femoral head and neck on the ante-roposterior view and in the center on the lateral view.13 Kashigar retrospectively analyzed 170 cases with intramedullary fixation for intertrochanteric fractures. He believed that a lag screw placed lower in the femoral head was conducive to reduce the incidence of cutting off and put forward the concept of calcar-referenced TAD (Cal-TAD).14
So far, to our knowledge, no study has been conducted on the mechanical properties of the lag screw position on the coronal plane, probably because of limitations of clinical trials and specimens' biomechanical study. The purpose of this study was to investigate the internal stress distributions on the femoral head and the internal fixation when a lag screw is placed at different positions in the femoral coronal plane using different modalities of the three-dimensional (3-D) finite element (FE) model. We hypothesized that our findings will help determine a reasonable lag screw position.
2. Materials and methods
2.1. Research equipment and design principles
Before computed tomography (CT) scanning, a geometrically accurate FE model of the entire femur was obtained using 3-D reconstruction of femoral CT images obtained in the neutral unloaded position from a healthy male volunteer (age, 32 years; height, 175 cm; weight, 78 kg) who was free from any disease or injury of the hip joint.
Using data on a proximal femoral nail antirotation (PFN-A) implant (Synthesis, USA) and ProE4.0 (PTC, USA), we performed a 3-D virtual reproduction. The length and diameter of the PFN-A intramedullary nail were 200 and 11 mm, respectively. The screw blade was of standard diameter, and its length varied for different models (for model set-up, see the text below). The distal locking screw was simplified into a cylindrical shape.
Continuous CT scanning was performed for the full-length femur, and the scan data were stored in the Digital Imaging and Communications in Medicine (DICOM) format. A total of 489 images were obtained. The two-dimensional data in DICOM format were introduced into Mimics 13.0 (Materialise, Belgium), and a proper grayscale threshold was set up. Image segmentation was performed. Inter-trochanteric osteotomy was simulated for A1 bone fracture according to AO classification.15 A single fracture line extended from the medial greater trochanter to the lesser trochanter (Figure 1). The geometric model of the femur with internal fixation was assembled and introduced into Geomagic 10.0 (Geomagic, USA) for the smoothing treatment to obtain the precise finite element and final analytical models. The models were input in the ANSYS 14.5 software to assign material attributes and constraints to the models. The loading process was simulated, and the stress nephrograms were output.
2.2. Material properties
In this study, the implants and bone were set to be homogeneous and isotropic.16-18 The proximal femur model was set up with reference to Wang's method.19 The material-related properties were as follows: the proximal femur cancellous bone elastic modulus was 620 MPa with Poisson's ratio of 0.29; the femoral cortical elastic modulus was 16,800 MPa with Poisson's ratio of 0.30; and the PFN-A titanium alloy (Ti-6Al-7Nb) material elastic modulus was 110,000 MPa with Poisson's ratio of 0.33.18
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Figure 1 Dimensional finite element model of intertrochanteric fracture with PFN-A fixation. A1 type intertrochanteric osteotomy fractures of AO classification were simulated, and the single fracture line extending from the greater trochanter to the lesser trochanter. Three stresses were applied to the model. (1) Femoral head: F1 = 2460 N at a 23° angle with the frontal plane and 68° with sagittal plane. (2) Greater trochanter: F2 = 1700 N a 24° angle with the frontal plane and 15° with the sagittal plane. (3) Small trochanter: F3 = 771 N at a 41° angle with the frontal plane and 26° with the sagittal plane.
2.3. Boundary conditions and stress load
As the forces acting on the head of the femur vary significantly, we used the femoral coordinate system defined by Bergmann et al.20,21 We set the posterior condylar connection on the coronal plane as the x-axis, the sagittal axis as the y-axis, and the femoral shaft axis as the z-axis. A one-legged stance load configuration was simulated in this study.
We used the commonly used simplified model that considered only three forces as follows19,21—23: the acetabular fossa force applied to the femoral head, the abductor force applied to the greater trochanter, and the iliopsoas force applied to the femoral lesser trochanter. According to the biomechanical model defined by Oken et al, a load of 2460 N was applied on the femoral head at a 23° angle with the frontal plane and 68° angle with sagittal plane, and in the same direction with calcar and trabecular bones.23,24 The force abductor acting on the greater trochanter was 1700 N at a 24° angle with the frontal plane and 15° angle with the sagittal plane. The iliopsoas force acting on the femoral lesser trochanter was 771 N at a 41° angle with the frontal plane and 26° angle with the sagittal plane (Figure 1).
We set the degrees of freedom constraints of all the nodes of the lower edge of the condyle in the 3-D FE model of the femur to zero as the boundary condition. This means that the displacement of each remote node in the x-, y-, and z-axes was zero.20,25 We assumed that the fracture surface was completely broken, and the friction coefficient was 0.2.26 A PFN-A nail was inserted in the medullary cavity through the greater trochanter apex, and we assumed that there was no direct contact between the intramedullary nail and the surrounding bone structure. A lag screw and nail were inserted at 130° in the femoral fracture fragment from the lateral femoral view. We assumed that the front end of the lag screw and the femoral head cancellous bone were in close contact without sliding and that the trailing end of the lag screw was not in contact with the surrounding cancellous bone. The lag screw and nail were in sliding contact. The lock screw was closely connected with the bone cortex and intramedullary nail.
2.4. Internal fixation model group
The femoral head was considered to be round on the coronal plane. Figure 2 shows that the femoral head was divided by six lines, of which three were parallel to the lag screw and the other three were perpendicular to the lag screw. Each line was divided into four equal parts by three perpendicular lines, and we obtained nine sections. We set the end of the lag screw to be in these sections and obtained the following nine modalities of the FE model: high—short, high—mid, high—long, axis—short, axis—mid, axis—long, low—short, low—mid, and low—long (Figure 2).
Figure 2 Model group. According to lag screw vertex position in the femoral head, nine models were set up. A = high—short; B = high—mid; C = high—long; D = axis—short; E axis—mid; F = axis—long; G = low—short; H = low—mid; I = low—long.
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Figure 3 Von Mises stress distribution in the coronal plane of the nine models. A = high—short; B = high—mid; C = high—long; D = axis—short; E = axis—mid; F = axis—long; G = low—short; H = low—mid; I = low—long.
2.5. Evaluation
FE analysis was implemented after loading stress on the nine modalities of the FE model. The distribution trends of the characteristic high stress concentration points in these modalities were observed. A characteristic high stress was found in the following areas: (1) the calcar; (2) the cancellous bone in front of the lag screw tip; (3) the junction of the lag screw and nail; and (4) the distal fracture surface. Then, the maximum stress in these high-stress areas was compared for the nine modalities.
3. Results
The von Mises stress in the nine modalities was distributed in accordance with certain rules (Figure 3). Owing to the stress-shielding effect of the lag screw, the area of low stress concentration was above the lag screw, while the high-stress area was below the lag screw. The area of high-stress concentration was distributed from the top of the femoral head to the medial cortex of the trochanteric
region. The highest point of the stress proximal fracture fragment was at the medial femoral cortex/calcar (Figure 4A, yellow ellipse). The maximum stress value at this point in these modalities is shown in Figure 5. A stress concentration area was also found in the cancellous bone on the top of the lag screw (Figure 4A, red circle), and the maximum stress value at this point is shown in Figure 6.
The outer part of the nail and lag screw sustained tensile stress, while the medial part sustained pressure stress. An evident stress concentration area was found at the junction of the nail and lag screw, and a certain degree of stress concentration was observed at the junction of the spiral and rod portions of the lag screw and mid medial portion of the nail. The highest stress point was at the inferomedial junction of the nail and lag screw (Figure 4B, red circle), and the highest stress values in the nine modalities are shown in Figure 7.
Certain rules dictate the distribution of von Mises stress on the fracture surface. The pressure stress on the fracture surface above the lag screw was low, while that below the lag screw was high. The highest stress point was at an anterior orientation. A stress concentration area was also
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Intertrochanteric fracture
Figure 4 (A) Stress on the proximal fracture fragment in the coronal plane. The yellow oval shows the maximum stress area of the proximal fracture fragment, and the red circle shows the high-stress area above the lag screw. (B) Stress on the lag screw and nail. The red circle shows the maximum stress area of the lag screw, i.e., at the junction of the lag screw and nail.
0 5 10 15 MPa
Figure 5 The maximum stress value of the calcar.
Figure 6 The maximum stress value of the bone at the lag screw tip.
observed above the small trochanter. The highest stress values on the fracture surface are shown in Figure 8.
4. Discussion
From the perspective of FE analysis, the occurrence of a microfracture is decided by the internal stress and fatigue strength of a unit of cancellous bone. Continual microfracture of the bone contacting the screw tip may lead to
Figure 7 The maximum stress value of the lag screw and nail junction.
displacement and cut-out of the lag screw if the internal stress of the cancellous bone is higher than its fatigue strength. Fatigue strength of a unit of cancellous bone relates to bone mineral density, and internal stress of a unit of cancellous bone relates to various factors, such as fracture type, reset stability, fixation type, and the lag screw location, in the femoral head. The individual influence of these factors is unclear, except for the lag screw
Figure 8 The maximum stress value of the distal fracture surface.
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location in the femoral head, which is extremely important.13'27
Retrospective clinical and biomechanical specimen studies can be performed to address the impact of coronal plane location of the lag screw on the mechanical structure. However, since deep and long lag screws are used in most clinical cases, collecting enough samples to compare the mechanical properties of the lag screw tip at different locations on the femoral coronal plane would be difficult. In biomechanical specimen studies, the lag screw location can be controlled, but researchers may not be able to collect enough homogeneous femoral head specimens to exclude the effects of other confounding factors, such as age, sex, bone mineral density, and the femoral head shape. In a 3-D FE analysis, we can set the lag screw at different locations in the same femur model and have only one variable, the lag screw location, in different models. Without interference of other factors, we can effectively study the impacts of coronal plane locations of the lag screw on the mechanics (stress distributions).
Herein, we observed certain regular stress distributions in nine models. The area of high-stress concentration distributed along the compression trabeculae, indicating cancellous bone in this region, is prone to microfracture. If the proximal fracture fragments tend to get varus, the lag screw is likely to pierce the top of the femoral head due to the action of microfracture and varus trend, which is called cut-out. If the proximal fracture fragments tend to slide outward (e.g., lack of lateral support), the lag screw is likely to pierce the inside of the femoral head, which is called cut-through.
In this research, we found the highest stress point at the inner side of the junction of the femoral neck and femoral shaft after internal fixation of intertrochanteric fractures. The reasons for the stress concentration at this point may be as follows:
(1) Usually, stress focuses at the corner, and this point is the top of the inner cortex arc of the femur.
(2) This point is a gathering place of pressure trabeculae, through which human body gravity is transmitted to the lower limb.
(3) An offset is observed between the limb alignment and the femoral shaft axis, and the varus stress of the offset is concentrated at this point.
After analysis of the maximum stress at this point in the nine models, we observed that the longer and lower the lag screw, the less the maximum stress, indicating that a lower and longer lag screw can sustain more stress and offset the varus trend of the proximal fracture fragments.
The second highest stress point on the proximal fracture fragment was found inside the lag screw peak. The huge elastic modulus difference between internal fixation and cancellous bone may be the most important reason for this stress concentration. In addition, as the angle stabilization of the lag screw and nail causes the lag screw to bear an anti-varus-rotation effect, the lag screw peak becomes the fulcrum of the varus stress. In the nine models, the stress at this point in short screw models was significantly greater than that in middle and long screw models, with no
significant difference in stress between middle and long screw models. Thus, we believe that a short lag screw may not be strong enough to offset the varus trend of the proximal fracture fragment because the varus trend is concentrated at this point.
The highest stress point on the lag screw was its junction with the nail. Owing to the angle stabilization and dynamic sliding function of the PFN-A lag screw, the von Mises stress concentration at this point suggests the ability of the femoral head and neck to bear the varus stress. The longer and lower the lag screw, the higher the von Mises stress, indicating that a longer and lower lag screw can sustain more varus stress.
In a normal femur, the upper half of the cross-section between the greater trochanter and the lesser trochanter sustains tensile stress, and the lower half sustains oppression stress. After internal fixation, the stress on this cross-section changes, and the master oppression stress distributes below the lag screw, particularly under the front section. In the nine models, we found that the highest stress on the longer and lower lag screws was approximately half of that on the shorter and upper lag screws. This means that lower and longer lag screws may play two roles. First, implant fixation sustains more stress and reduces stress on the bone tissue. Second, longer and lower lag screws evenly distribute the stress on the fracture surface.
Based on the analysis, when PFN-A fixation is used for an intertrochanteric fracture, a longer and lower lag screw may allow for the fixation to sustain higher stress, thus reducing pressure on bone tissue. Furthermore, we believe that a longer and lower lag screw may bear greater stress and more cyclic load when the bone density is not changed.
We observed on radiography of the proximal femur that the calcar femorale distributes from the top of the femoral head to the medial cortex of the proximal femur. According to the Wolff Theorem, this is in agreement with the distribution of proximal femur stress and that the direction of proximal femur stress should be consistent with the grain of calcar trabeculae. When the screws in the femoral head and neck were aligned with the central axis of the femoral neck and the TAD was 25 mm, the terminal of the screw blade was roughly located along the central axis of the trabecula (Figure 9). According to the TAD theory, a TAD of <25 mm indicates that the screw tip is about to cross the talar trabecular line of the femur.
Geometrically, femoral eccentricity would lead to the inversion of the proximal fracture block in the inter-trochanteric fracture. The medial fracture end is the pivot point of inversion (Figure 10A, point i). After internal fixation, the angle between the screws for the femoral head and neck and intramedullary nail effectively reduces femoral eccentricity. Therefore, the principal compressive stress acts on the screws for the femoral head and neck. If the screw tip does not cross the stress line medially, femoral eccentricity would still exist and inversion of the proximal fracture block may occur. The screw tip acts as the pivot point of the inversion (Figure 10B, point ii), where the stress is highly concentrated. If the screw crosses the stress line, this means the internal fixation completely counteracts the femoral eccentricity. As the screw tip is no
Figure 9 The blue line indicates the central axis of the femoral head; the yellow line indicates the center of the compression trabecula; the red double-headed arrow indicates the distance from the intersection of the yellow and blue lines to the subchondral bone of the femoral head, which is approximately 15 mm. When the screws of the femoral head and neck are inserted along the blue line and TAD is <25 mm, the screws of the femoral head and neck can completely cross the stress line of the trabecula.
longer the pivot point, the inversion of the proximal fracture block becomes unlikely (Figure 10C).
Based on geometrical analyses, the high-stress region in the cancellous bone of osteoporotic patients is usually associated with a higher risk of microfracture of the cancellous bone. The inversion trend of the proximal femur may lead to screw cut-out and inversion of the proximal fracture block. If the length of the screws for the femoral head and neck is sufficient to counteract the femoral eccentricity, a trend of inversion would not exist. Therefore, screw cut-out is unlikely even for patients with osteoporosis.
In this study, we made the following hypothesis: The femoral head is divided into the lateral superior and medial inferior regions by taking the central axis of the talar trabecular line as the boundary on the coronary plane (Figure 11). The screws are inserted in the right position, with the tip of the screws for the femoral head and neck positioned in the medial inferior region (optimally in region A of Figure 11 and less optimally in region B). This is important for counteracting the femoral eccentricity and inversion trend, thus reducing the risk of screw cut-out.
Our hypothesis explains why a TAD of <25 mm can prevent cut-out. When the screw is placed on the central axis of the femoral neck on the coronary plane, a TAD of <25 mm is equivalent to the situation where the screw tip crosses the central axis of the talar trabecular line (stress line P). Moreover, it also explains why the mechanical structure of the screws placed in the inferior position is better. As the stress increases from the medial superior region to the lateral inferior region, the inferiorly placed screws for the femoral neck and head are more likely to cross the stress line. Therefore, they more effectively bear the compressive stress and the mechanical structure is better as well.
We propose from a biomechanical perspective that the placement of the screws for the femoral head and neck can be guided by the talar trabecular line. Screw cut-out can be prevented by properly counteracting the femoral eccentricity. To validate our hypothesis, a grouping design was used for the experiment. However, the experiment result failed to validate our hypothesis. It only demonstrated that longer screws placed in an inferior position sustained a larger proportion of stress.
Screw cut-out may be caused by various reasons, such as improper placement of the screws for the femoral head and neck. Owing to the limitations of the 3-D FEM method, the impact of compression and tension trabeculae inside the femoral head and neck on the general mechanical structure cannot be studied with our model. Another limitation of our study is that the distribution of talar trabeculae was only considered on the coronary plane and that the distribution was not symmetrical on the sagittal plane. The stress lines
Figure 10 A schematic of the relationship between the screws of the femoral head and neck and the trabecular line. (A) After intertrochanteric fracture, the proximal bone fragment will undergo inversion and dislocation with respect to point i. (B) The screws of the femoral head and neck do not cross the trabecular line, and points i and ii jointly serve as the pivot points of inversion for the proximal femur. (C) The screws of the femoral head and neck cross the trabecular line, and the inversion trend of proximal femur is counteracted by the screws of femoral head and neck. The pivot point of inversion shifts to point ii.
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Figure 11 The medial-inferior region of the femoral head. Region A indicates that it is completely inferior to the compression trabecula. Region B indicates the inferior half of the trabecula.
on the sagittal plane are not distributed along the midline of the femoral head and neck. Therefore, whether placing the screws along the central axis of the femoral head and neck on the sagittal plane is reasonable remains unclear. Further mechanical investigations are needed with cadaverous specimens and follow-up of clinical cases.
5. Conclusions
Longer and lower lag screws may allow fixation to sustain greater stress, consequently reducing bone tissue stress in the intertrochanteric fracture fixated with PFN-A, and sustain greater stress and more cyclic load at the same bone density. The results of this study support the principle of the TAD theory that a lag screw should be placed as deep as possible and also suggest that better mechanical structure can be achieved when a lag screw is located lower.
Conflicts of interest
All of the authors declare that they have no conflicts of interest regarding this paper.
Acknowledgments
This work was supported by Natural Science Foundation of China (Grant No. 81572165) and Guangdong Provincial Medical Scientific Research Foundation, China (Grant No. A2015053).
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