ISSN 0972-978X 

 
 
 
 
 
 
 
 
 
 
 
 
  About COAA
 

 

 

 

 

 

 

ORIGINAL ARTICLE

Investigation Of A Trochanteric Fi-Nail Intramedullary Implant Fixation Using The Finite Element Method

Evangelos N. Kaselouris(1), Demetrios T. Venetsanos(1), Christopher G. Provatidis(1), Fragiskos N. Xypnitos(2), Vassilios Nikolaou(2), John Lazarettos(2), Nicolas E. Efstathopoulos(2)

(1) Laboratory of Dynamics and Structures, Mechanical Design and Control Systems Section, School of Mechanical Engineering, National Technical University of Athens.

(2) B’ Orthopaedic Department, Medical School, University of Athens.

Address for Correspondence:

Fragiskos N. Xypnitos,

B’ Orthopaedic Department, Medical School, University of Athens

3-5 Agias Olgas Str

142 33 Nea Ionia, Athens, Greece

Phone:  0030-210-6611808  / 0030-6944-443316
Fax    :  0030-210-6611808
E-mail:
frank_xypnitos@yahoo.gr

Abstract:

The efficiency of an intramedullary nail fixation device, used in cases of trochanteric and subtrochanteric fractures, is defined by several parameters, two of which are the location and the number of distal screws that are used. Towards this direction, the present paper investigated the effect of the two aforementioned characteristics implementing the finite element method (FEM). The left proximal femur of a 93-year old man was scanned and two series of full 3D models, introducing an intramedullary Fi-nail, were developed. The first series, consisting of five models, concerned the use of a single distal screw inserted in five different distal locations. The second series, consisting of four models, concerned the use of four different pairs of distal screws. Each model was analyzed with the FEM twice, first considering that the femur is fractured and then considering that the fracture is healed. The main conclusion derived from this investigation was that, for Fi-nails with a single distal screw, stresses around the nail hole were reduced with proximal placement of the distal screw but the area around the nail hole where the lag screw is inserted becomes more stressed. Furthermore, for Fi-nails with a pair of distal screws, placing the pair of distal screws at a specific location is most beneficial for the mechanical behavior of the femur/Fi-nail assembly.

J.Orthopaedics 2009;6(4)e1

Keywords:

Intramedullary nailing; proximal femoral fractures; finite element method (FEM); contact analysis

 

Introduction:

Intramedullary nailing is an established surgical technique for the treatment of proximal femoral fractures. Intramedullary nails are load-sharing devices, allowing the bone to transmit compressive forces while maintaining axial alignment [1]. The Fi-nail (Sanatmetal Ltd. – Hungary) is an implant designed for the treatment of trochanteric and subtrochanteric fractures of the proximal femur. Implant failures have been reported in the literature mainly due to lag screw cut-out [2-4] or fractures of the femoral shaft due to excessive local stress loading around the distal locking screws or near the nail tip [4].

As far as finite element investigations are concerned, Wang et al. [5] investigated the effects of nail length, nail distal stiffness and material stiffness on the structural behavior of the system while Sitthiseripratip et al. [6] investigated the developed stresses in the trochanteric gamma nail (TGN) throughout the healing process of the bone in the fracture zone. On top of that, biomechanical studies of intramedullary nails have also been reported [7, 8]. Furthermore, with respect to the implanted nails, it has been reported that titanium nails (Ti) had increased biomechanical stability compared to stainless steel nails during tests of torsion and compression for both transverse and comminuted fractures [9].

The present paper, using the FEM, investigated the effects of the location of the distal screw, as well as the number of the implanted distal screws in the stress fields developed on the femur/ nail assembly.

Materials and Methods:

Finite element models

The left proximal femur of a 93-year old man, who had underwent an intramedullary nailing for a trochanteric fracture of the right femur (fig.1a), was scanned with computer tomography (CT) using 1mm slice thickness. The CT dataset was imported into Mimics (medical image processing software by Materialise N.V., Belgium) where a 3D graphic model was created. The 3D model was then imported into Ansys ver.10 (Finite Element Analysis software).

A commercially Fi-nail was used (nail: 219mm long, proximal diameter 15mm, distal diameter 10mm, 5º valgus curvature). The lag screw was 90mm long, 10mm in diameter and placed at 125º with respect to the nail. The distal screw(s) had a 10mm diameter. Based on this geometrical information, a 3D model of the Fi-nail was first created in Ansys and then virtually inserted into and aligned with the intramedullary canal of the femur model. The lag screw was inserted below the femoral head center and 10mm away from the outer boundary of the femoral head, in accordance with Parker MJ [10]. In addition, the distal screw(s) and the femoral fracture were added (fig.1b). The fracture under consideration was type 31-A1.3 according to AO-ASIF and it was introduced as an idealized plane gap of 2mm thickness in the trochanteric region. The single distal screws were inserted as indicated in table 1. The location L1 (fig.1a) is placed 162.5mm from the proximal nail head, the other locations being 11.3mm away from each other (five single-distal-screw configurations). These positions of the distal screw are possible with the current implant configuration. With respect to the pairs of distal screws, one screw was considered to be fixed at the location L1 and the other was placed at one of the other Li,i=2,3,4,5 locations, its distance from the proximal head being denoted as zp. (four pairs-of-distal-screws configuration: ([L1, L2], [L1, L3], [L1, L4], [L1, L5)]). In total, nine models were developed.

The femur and the implant were meshed using eight-node brick elements (SOLID185), and ten-node tetrahedral elements (SOLID92), respectively. The nail-endosteum, the nail-lag screw interactions and the fracture interfaces were modeled using contact surfaces (“TARGEI70” and “CONTA174”). The use of dissimilar element types for the different components of the construct is due to the different geometry complexity of the bone and the implant. A 0.1mm gap between the nail and the lag screw was applied. In total, each model had approximately 420000 elements and 240000 nodes.

Location

Distance  

162,5

173,8

185,1

196,4

207,7

 

Table 1: Locations of single distal screws

Material properties

Linear elastic properties were attributed to all of the materials involved, as shown in table 2, even though the distribution of the elastic moduli of the cortical and cancellous bone is slightly arbitrary [5, 11, 12].

 

 

Young modulus

Poisson’s ratio

Cortical bone

17

0.30

Cancellous bone (intertrochanteric region)

0.32

0.30

Cancellous bone (femoral head)

1.3

0.30

Fi-nail (titanium)

110

0.30

 

Table 2: Muscles and joint reaction forces for the one-legged stance phase configuration

 

 

Applied forces (N)

 

A-P

M-L

S-I

Joint reaction force

130

1062

-2800

Abductor muscle force

 

-430

1160

Ilio-tibia tract

 

 

-1200

Iliopsoas

-560

-78

525

 

 

Loading, boundary conditions and analysis

A one-legged stance-phase load configuration was applied. This load case is analytically described in table 3, while the distal end was fixed. All modes were analyzed using a non-linear contact analysis approach.

Investigation strategy

A diagrammatic presentation of the present investigation is shown in fig.3. In total, 18 different analyses were carried out (nine models, each one analyzed twice), divided as follows:

·          Case 1:       fractured femur with a single distal screw (5 models)

·          Case 2:       healed femur with a single distal screw (5 models)

·          Case 3:       fractured femur with a pair of distal screws (4 models)

·          Case 4:       healed femur with a pair of distal screws (4 models)

For the examined models, the stress fields were recorded for:

·          the distal screw (the screw itself and the area around the corresponding nail hole)

·          the lag screw (the screw itself and the area around the corresponding nail hole) 

·          the proximal femoral head

 

Evaluation of results

The quantities explicitly recorded were:

·          the von Mises equivalent stress,

·          the nodal displacement of the femoral head, denoting, in the case of the fractured femur, the dislocation of the fractured femoral parts and in the case of the intact femur, the deformation of the femur.

The quantities expressed in a normalized form aimed at revealing how much stressed a screw is with respect to its corresponding nail hole (relative stress state) were defined as:

 

     (1)

 

where  stands for ‘Normalized Index’,  is the maximum von Mises equivalent stress, while  and  are defined in section 2.1. The stresses, the displacements and the normalized indices were first recorded for the analyses carried out (section 2.3) and then plotted versus distance  (for models with a single distal screw,  for models with a pair of distal screws), as follows:

·          Distribution of the maximum von Mises equivalent stress versus ,

·          Distribution of the maximum proximal femoral head displacement versus ,

·          Distribution of the Normalized Indices (Eq.(1)) describing the change in the mechanical behaviour, versus .

Overall, the evaluation procedure was carried out in four steps:

Step E1:Separate evaluation of each case using the aforementioned plots

Step E2: Comparison between fractured and healed femur, for a single and a pair of distal screws
Step E3:
Evaluation of each case using the Normalized Indices  

Step E4:            Comparison between a single distal screw and a pair of distal screws, for fractured and healed femur

The aforementioned steps are denoted in fig.3, with the code names ‘EXyz’, where X stands for a numerical index (‘1’ for Step E1, ‘2’ for Step E2, etc), the subscript y refers to the number of distal screws (‘s’ and ‘p’ denote ‘a single distal screw’ and ‘a pair of distal screws’, respectively), while the subscript z refers to the type of femur ( ‘f’ and ‘i’ denote ‘fractured’ and ‘healed’ femur, respectively).

Results :

Indicative stress and displacement fields are illustrated in fig.2. When the fracture is taken into consideration, the discontinuity of the displacement field at the area of fracture is clearly shown in fig.2a. On the contrary, when the fracture is considered to be healed, thus material continuity at the area of the fracture is ensured, the corresponding displacement field is continuous, as fig.2b illustrates. Finally, in case where a pair of distal screws is used, the area around the proximal distal screw, the screw itself and the corresponding nail hole, is more stressed, as fig.2c shows.

Figure 1: (a) X-ray with the examined locations  of distal screws marked as dotted lines and (b) full 3-D model of the fractured proximal femur carrying a Fi-nail

 

Figure 2: (a) Discontinuous displacement field for a fractured femur, (b) continuous displacement field for a healed femur and (c) stress field for a Fi-nail with a pair of distal screws

 

 

Figure 3: Diagrammatic presentation of the investigation carried out

                               (a)

                                 (b)

Figure 4: Maximum von Mises equivalent stress versus distance  for (a) the distal screw and (b) the lag screw (case: fractured femur, single distal screw)

                                (a)

                                 (b)

Figure 5: (a) Maximum displacement of the femoral head and (b) Normalized Indices versus distance  (case: fractured femur, single distal screw)

 

                               (a)

                                (b)

Figure 6: Maximum von Mises equivalent stress versus distance  for (a) the distal screw and (b) the lag screw (case: healed femur, single distal screw)

                                            (a)

                                               (b)

Figure 7: (a) Maximum displacement of the femoral head and (b) Normalized Indices versus distance  (case: healed femur, single distal screw)

                                            (a)

                                             (b)

Figure 8: Maximum von Mises equivalent stress versus distance  for (a) the distal screw located at  and (b) the distal screw located at  (case: fractured femur, pair of distal screws)

                                             (a)

                                            (b)

Figure 9: (a) Maximum von Mises equivalent stress for the lag screw and (b) maximum displacement of the femoral head versus distance  (case: fractured femur, pair of distal screws)

                                (a)

                                   (b)

Figure 10: Maximum von Mises equivalent stress versus distance  for (a) the distal screw located at  and (b) the distal screw located at  (case: healed femur, pair of distal screws)

                                  (a)

                                    (b)

Figure 11: (a) Maximum von Mises equivalent stress for the lag screw and (b) maximum displacement of the femoral head versus distance  (case: healed femur, pair of distal screws)

                                   (a)

                                            (b)

Figure 12: Normalized Indices versus distance  for (a) a single distal screw and (b) a pair of distal screws

 

Evaluation of each examined case separately

Case 1: Fractured femur with a single distal screw

The plots in fig.4a illustrate that the maximum equivalent von Mises stress increases linearly with respect to the distance  of the distal screw from the proximal end of the nail. As the distance increases, so does the maximum equivalent von Mises stress (fig.4a). Fig.4b shows that the maximum von Mises stress developed on the lag screw is almost insensitive to the distance , while the maximum von Mises stress, developed on the nail and around the hole where the lag screw is inserted, decreases with respect to the distance . From fig.5a, it yields that the maximum displacement of the femoral head increases almost linearly. Finally, the maximum von Mises stress at the fracture zone was recorded to be .

 

Case 2: Healed femur with a single distal screw

The plots in fig.6 are similar to those in fig.4. From fig.6a, it is clear that the maximum equivalent von Mises stress is linearly correlated to the distance . From fig.6b, it yields that the maximum von Mises stress developed on the lag screw is almost insensitive to the distance . It is clear that the distally placement of the single distal screw is more beneficial to the stress field of the nail hole for the lag screw, but it has the opposite effect to the stress field of the nail hole for the distal screw. From fig.7a, it yields that the maximum displacement of the femoral head increases linearly with respect to the distance .

Case 3: Fractured femur with a pair of distal screws

From fig.8a it is evident that the plotted maximum equivalent von Mises stresses are practically constant. Fig.8b shows that there is strong linear correlation between the illustrated maximum equivalent von Mises and the distance  and fig.9a suggests that the plotted maximum equivalent von Mises stress are practically constant. Finally, the maximum von Mises stress developed along the fracture zone was .

 

Case 4: Healed femur with a pair of distal screws

The plots in fig.10 and fig.11a illustrate changes of the maximum equivalent von Mises stress with respect to the distance . Fig.10 shows that there is a strong linear correlation between the plotted maximum equivalent von Mises stress and the distance . and fig.11a suggests a linear correlation of the plotted quantities but for a more narrow range. From fig.11b, it yields that the displacement of the proximal femoral head decreases as the distance  increases. Finally, it was found that the strains on the part of the bone where the second distal screw is inserted are always higher than the corresponding strains on the part of the bone where the first distal screw is inserted.

 

Comparison between fractured and healed femur

Comparison between Case 1 and Case 2

The results [figs.(4,5) and figs.(6,7)] indicate that the stresses in the Fi-nail are gradually reduced during the healing process of the bone in the fracture zone. More particularly, the maximum von Mises stress for the fractured femur in correlation to the healed femur is:

·          higher on the distal screw ( - )

·          higher on the corresponding nail hole ( - ),

·          higher on the lag screw ( - ), and

·          higher on the corresponding nail hole ( - ).

Furthermore, the maximum displacement of the femoral head is also higher ( - ).

 

Comparison between Case 3 and Case 4

A comparison between figs.(8,9) and figs.(10,11), using the healed femur as reference, shows that the maximum von Mises stress of the fractured femur is higher:

·          on the distal screw at  (-),

·          on the corresponding nail hole ( - ),

·          on the distal screw at  (-),

·          on the corresponding nail hole ( - ),

·          on the lag screw ( - ), and

·          on the corresponding nail hole ( - ).

The maximum displacement of the femoral head is also higher ( - ).

 

Evaluation of each case using the Normalized Indices

At the fractured femur with a single distal screw, the relative stress state is almost constant at a value of , while, for the lag screw, it increases linearly with respect to the distance  (fig.5b).

At the healed femur with a single distal screw, the relative stress state presents a linear decrease for the distal screw,, while for the lag screw, the relative stress state increases linearly with respect to the distance  (fig.7b).

At the fractured femur with a pair of distal screws (fig.12a), both the distal screw at  and the distal screw at  present a correlation of 2nd degree polynomial (relative coefficient  in both cases). The maximum value for normalized index concerning the distal screw at  appears for the pair of screws . However, the maximum value for normalized index concerning the distal screw at  appears for the pair of screws . The normalized index concerning the lag screw decreases slightly as the distance  increases.

At the healed femur with a pair of distal screws, for the distal screw at  a decrease up to  appears (fig. 12b) as the distance  increases, while for the distal screw at  a decrease up to  appears. For the lag screw, it appears a constant normalized index.

 

Comparison between a single distal screw and a pair of distal screws

For the fractured femur and when a single distal screw, rather than a pair, is used, the developed maximum von Mises stress is higher:

·          on the distal screw at location  at least by ,

·          on the corresponding nail hole at least by , and

·          on the lag screw.

The opposite holds for the corresponding nail hole. Furthermore, the proximal femoral head displacement is approximately the same in both cases.

For the healed femur and when a single distal screw, rather than a pair, is used, the developed maximum von Mises stress is higher both on the distal screw at location , and on the corresponding nail hole. Furthermore, the maximum von Mises stress developed on the lag screw is practically the same in both cases, while for the corresponding nail hole, it is most beneficial to use a single distal screw. Finally, the proximal femoral head displacement is lower when a pair of distal screws is used, with the screws being far away from each other.

Discussion :

For the models carrying a single distal screw, the basic remark was that placing the distal screw distally is beneficial to the proximal part but not to the distal part of the nail. The more distally the distal screw is placed the larger the cantilever, with respect to the distal screw, of certain imposed force components becomes, thus resulting in larger moments and in a generally more stressed state.

With respect to the models carrying a pair of distal screws, the basic remark was that placing the distal screws far away from each other is beneficial to both the proximal femoral head displacement and the stress state of the distal Fi-nail part. A distal screw is nothing else than the foundation of the Fi-nail in the intramedullary canal, thus as the distance between two distal screws becomes larger, the foundation of the implant becomes more rigid, causing lower femoral head displacements.

The loads that are applied at the femoral head and transmitted to the femoral shaft through the distal screw(s). If two distal screws participate in this load transmission, then the resulting stress state is lower than that caused when only one distal screw is used because while the load is the same there are two paths towards the femoral shaft instead of one. Therefore, in general, the use of two distal screws results in a less stressed nail near its distal end.

Conclusions:

The main conclusions of the present investigation are the following:

· The more distally a single distal screw is placed the more stressed the screw itself and its corresponding hole on the Fi-nail get, relieving at the same time the area around the nail hole of the lag screw. This holds for both the fractured and the healed femur.

· When a pair of distal screws is introduced then, the distal area of the nail generally gets less stressed while the opposite holds for the area around the Fi-nail/lag screw connection. In addition, the presence of two distal screws far away from each other results in lower proximal femoral head displacements and lower stressed distal part of the Fi-nail.

·  The stress field at the area of fracture is not significantly influenced by the presence of a single distal screw or a pair of distal screws.

Reference :

  1. Knothe U, Knothe Tate ML, Klaue K, Perren SM. Development and testing of a new self-locking intramedullary nail system: testing of handling aspects and mechanical properties. Injury, 2000, 31:617–626.

  2. Haynes RC, Poll RG, Miles AW, Weston RB. Failure of femoral head fixation: a cadaveric analysis of lag screw cut-out with the Gamma locking nail and AO dynamic hip screw. Injury, 1997, 29:337–41.

  3. Kawaguchi S, Sawada K, Nabeta Y. Cutting-out of the lag screw after internal fixation with the Asiatic Gamma nail. Injury, 1998, 29:47–53.

  4. Vicario C, Marco F, Ortega L, Alcobendas M, Dominguez I, Lopez- Duran L. Necrosis of the femoral head after fixation of trochanteric fractures with Gamma locking nail. A cause of late mechanical failure. Injury, 2003, 34:129–34.

  5. Wang CJ, Brown CJ,Yettram AL, Procter P. Intramedullary nails: some design features of the distal end. Medical Engineering & Physics, 2003, 25:789–94.

  6. Sitthiseripratip K, Van Oosterwyck H, Vander Sloten J, Mahaisavariya B, Bohez ELJ, Suwanprateeb J, et al. Finite element study of trochanteric Gamma nail for trochanteric fracture. Medical Engineering & Physics, 2003, 25:99–106.

  7. Seral B, Garcia JM, Cegonino J, Doblaré M, Seral F. Finite element study of intramedullary osteosynthesis in the treatment of trochanteric fractures of the hip: gamma and PFN. Injury, 2004, 35:130-5.

  8. Filardi V, Montanini R. Measurement of local strains induced into the femur by trochanteric Gamma nail implants with one or two distal screws. Medical Engineering & Physics, 2007, 29;38-47.

  9. Mahar AT, Lee SS, Lalonde FD, Impelluso T, Newton PO. Biomechanical comparison of stainless steel and titanium nails for fixation of simulated femoral fractures. Journal of Pediatric Orthopaedics, 2004, 24(6): 638–41.

  10. Parker MJ. Cutting-out of the dynamic hip screw related to its position. Journal of Bone and Joint Surgery  [BR], 1992, 74-B:625.

  11. Wang CJ, Yettram AL, Yao MS, Procter P. Finite element analysis  of a  Gamma nail    within a fractured femur. Medical Engineering & Physics, 1998, 20:677-683.

  12. Brown CJ, Wang CJ, Yettram Al, Procter P. Intramedullary nails with two lag screws. Clinical biomechanics, 2004, 19:519-525.           

This is a peer reviewed paper 

Please cite as: Fragiskos N. Xypnitos: Investigation Of A Trochanteric Fi-Nail Intramedullary Implant Fixation Using The Finite Element Method.

J.Orthopaedics 2009;6(4)e1

URL: http://www.jortho.org/2009/6/4/e1

ANNOUNCEMENTS

 


 

Arthrocon 2011


Refresher Course in Hip Arthroplasty

13th March,  2011

At Malabar Palace,
Calicut, Kerala, India

Download Registration Form

For Details
Dr Anwar Marthya,
Ph:+91 9961303044

E-Mail:
anwarmh@gmail.com

 

Powered by
VirtualMedOnline

 

 

   
© Copyright of articles belongs to the respective authors unless otherwise specified.Verbatim copying, redistribution and storage of this article permitted provided no restrictions are imposed on the access and a hyperlink to the original article in Journal of Orthopaedics maintained. All opinion stated are exclusively that of the author(s).
Journal of Orthopaedics upholds the policy of Open Access to Scientific literature.