Arab J Sci Eng (2014) 39:2205–2214 DOI 10.1007/s13369-013-0777-9
RESEARCH ARTICLE - MECHANICAL ENGINEERING
Dynamic Fatigue of Braided Textile Ligaments S. Marzougui · L. Kilani · S. Ben Abdessalem · F. Sakli
Received: 16 February 2012 / Accepted: 13 July 2013 / Published online: 6 September 2013 © King Fahd University of Petroleum and Minerals 2013
Abstract A new fatigue tester simulating the movement of the knee is designed and developed in this paper. This fatigue tester allows combining simultaneously cyclic solicitations: bending–traction, twist and abrasion on prostheses ligament. The fatigue test of braided ligaments made of polypropylene was performed in a liquid environment at 37 ◦ C. The combined cyclic solicitations generated an increase of breaking load and stiffness and a decrease of ultimate strain of the braided structure. The new behavior of tested ligaments and the effect of fatigue test parameters on braided ligaments mechanical performances were discussed. Keywords Fatigue tester · ACL · Braided prosthesis · Knee motion
S. Marzougui (B) · S. Ben Abdessalem · F. Sakli Textile Engineering Laboratory, High Institute of Technological Studies of Ksar Hellal, Avenue Hadj Ali Soua 5070, Ksar Hellal, Tunisia e-mail:
[email protected] L. Kilani High Institute of Technological Studies of Kairouan, Kairouan, Tunisia
1 Introduction The anterior cruciate ligament (ACL) is the main ligamentous stabilizer of the femur on the tibia. The ACL shearing at the knee articulation is the most frequent injury of ligaments with more than 100,000 ACL reconstructions per year in the USA [1]. The artificial ligament can be used in replacement or reinforcement of a broken natural ligament. These prostheses can be manufactured from various materials. Biocompatible materials currently used as ACL implants include polyethylene terephthalate (PET) polyester fiber, PTFE fiber, polypropylene and polyacrylonitrile fiber [2,3]. An artificial ACL can be braided, knitted or weaved. After cleaning, drying and sterilization by β-irradiation, the ACL is carried out [4]. The braided prostheses are commonly used for ACL reconstruction. The ligaments Gor-Tex® , which provide very satisfactory results for prostheses ligament, were braided structures [5,6]. The braiding technique was also employed to prepare the tendon grafts used for ligament prosthesis [7]. The ideal ACL replacement should be biocompatible and have sufficient mechanical properties. There are several commercially available synthetic grafts. They exhibit excellent short-term results but too many long-term clinical outcomes such as lack of stiffness or elasticity, poor resistance to abrasion with bones and high incidence of fatigue failure. The most difficult problem of the in vitro fatigue test of ligament prosthesis is the lack of knowledge regarding the biomechanics, that is, the in vivo load conditions in the knee articulation. According to Weiss and Gardiner [8], the knee movement has six freedom degrees: three rotations and three translations. The description of knee motion can be accomplished by relating movement to three principle axes: the tibia–shaft axis, the medial–lateral axis and the anterior– posterior axis (Fig. 1).
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Arab J Sci Eng (2014) 39:2205–2214 ACL Flexion-extension
Femur Joint distraction Varus-valgus rotation
Medial-lateral translation Anterior-posterior translation
Tibia
mitted to submit artificial ligaments to combined solicitations, simultaneously, of traction–bending, twist and abrasion, was designed and developed. This fatigue tester, simulating the knee movement, enabled us to study the mechanical behavior of ligament prosthesis and therefore lifetime.
2 Materials and Methods 2.1 Conception and Development of the Fatigue Tester
Internal-external rotation
Fig. 1 Schematic diagram illustrating the six degrees of motion of the human knee joint
The knee articulation therefore cannot be considered as a simple pivot connection. Indeed, the flexion–extension corresponds to a rotation and a sliding of the femoral condyles on the tibia. This involves cyclic solicitations of the ACL: traction–bending and rotation of the ACL. The lesion of the ACL causes the instability of the knee articulation. It is, therefore, necessary to substitute the ACL with synthetic ligaments, which have to satisfy complex mechanical needs (bending, traction, twist and abrasion of the ACL prosthesis on the tibial and femoral tunnel). The prediction of the fatigue behavior of artificial ACL under its real functioning conditions has a great interest for the prosthesis manufacturers who model the architecture of these structures in function of their mechanical requirements. In the literature, the mechanical characterization of artificial ligament was performed with conventional tests such as tensile strength and stiffness measurement [2]. The literature also refers to the combination of different solicitations tests to evaluate the ability of the ligament to withstand cyclic stresses. These stresses, which are not applied simultaneously, permit to evaluate the intrinsic properties to the textile structure, but they do not reflect the real mechanical properties applied in vivo to the ligament. The most of fatigue testers presented in the literature generate only a limited number of simultaneous cyclic stresses. For example, the ligament ligastic® has been tested with a sinusoidal force and a bending angle of 60◦ . To study the fatigue of prostheses ligament, several researchers have used fatigue testers that permit to combine, simultaneously, solicitations of traction and twist [9,10]. While the study of the knee joint describes the coexistence of at least four simultaneous solicitations at the ACL, namely, tensile, bending, twist and friction [11]. We propose to study the influence of cyclic solicitations on braided ligament prosthesis, made of polypropylene yarns, using a new fatigue device and an experimental design approach. The new experimental fatigue device, which per-
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The interest of the fatigue tester is to combine an iterative and cyclic force of traction, bending, twist and abrasion on prostheses ligament. The repetitive deformations of the braided ACL prostheses occur in a saline medium that simulates the action of the synovial liquid in the knee articulation. The developed fatigue tester (Fig. 2) is principally composed of the mechanisms of bending–traction, twist, abrasion and a stainless bath for the saline solution. 2.1.1 Bending–Traction Mechanism Traction and bending are ensured by the same mechanism. A sheep femur exerts a pressure in the middle of the prosthesis sample gripped between two clamps. Under this pressure, the sample undergoes traction and bending at the same time (Fig. 3). Figure 4 shows the bending–traction mechanism. The connecting rod (1) is connected to an eccentric disk with a ball-and-socket joint. The eccentric (eccentricity e = 20 mm) is driven by an electric motor (13) which turns with a constant speed of 100 rounds/min. The connecting rod (1) have cyclic movement with a frequency of 1.66 Hz and an amplitude c = 40 mm in the (ox) direction. The other end of the connecting rod is connected to the piece elbow (2) through a ball-and-socket joint. A distance L 1 separates rod (1) and shaft (3). The piece elbow (2), fixed on the shaft (3), can turn around (oy) with a β angle. The shaft (3) has a direction of (oy). The rolling bearing (5) is fixed on the piece elbow (2). A distance L 2 separates shaft (3) and rolling bearing (5). The rolling bearing (5) can slide on the smooth piece (4), which generates a vertical cyclic displacement of the parts (4) and (7) along (oz) axis. This shifting, ensured by a guiding bar (6), generates a deflection ( f ) of the sample (Fig. 3). The bending rod (7) generates a bending of the tested sample with deflection f that can vary between 9.4 and 60 mm (Table 1). We can vary the deflection f value by varying the two adjustable distances L 1 and L 2 (Eq. 1). The bending angle of the sample (θ ) (Fig. 3) is given according to Eq. 2. Table 1 gives the bending parameters of adjustment. L 1 and L 2 values were chosen in such a way that we apply twist and bending levels cited in the literature [9,12,13].
Arab J Sci Eng (2014) 39:2205–2214
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Fig. 2 The developed fatigue tester
z y
2
x
1
9
12 L1 11
10
7
8 L2
5
14
15 13
Testing zone
4 6
3
1: Connecting rod 2: Piece elbow 3: Shaft 4: Smooth piece of aluminium 5: Rolling bearing 6: Guiding bar 7: Bending rod 8: Abrasion rod 9: Twist rod 10: Flat bar 11: Mobile clamp 12: Eccentric disk 13: Electric motor 14: Sheep femur 15: Sample
Fig. 3 Combined cyclic solicitations of the sample
Fig. 4 Bending–traction mechanism
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Parameters
Min value
Max value
13.6◦ and 53◦ (Table 1) and can have zero value when twist rod (9) is disassembled from the mechanism.
L 1 (mm)
50
170
2.1.3 Abrasion Mechanism
L 2 (mm)
40
240
f (mm)
9.4
192
θ (◦ )
10.81
124
β (◦ )
13.60
53
Table 1 Summary of the fatigue tester adjustment parameters
f = L 2 sin(β) = c θ = 2ar ctg
Le 2f
L2 L1
(1)
Figure 6 shows abrasion mechanism. The abrasion rod (8) is fixed by its two ends via two rolling bearing. One end is fixed to the tester machine frame and the other end is fixed to the bending rod (7). The bending rod (7) applies abrasion effort through a sheep femur. However, the anatomy of the sheep stifle joint is similar to that of the human knee joint [14]. The combination of rotation and translation movement of the bending rod (7) generates simultaneously traction, bending and friction on the sample.
(2) 2.2 Experimental Fatigue Study
where L e = 200 mm is the length of the tested sample. 2.1.2 Twist Mechanism The twist mechanism is shown in Fig. 5. The twisting constraint is applied with a rotating clamp. The rotation of this clamp, fixed on the flat bar (10), is ensured by the cyclic movement of the twist rod (9). This rod is driven by the motor through the connecting rod (1). This twist mechanism enables clamps to have cyclic twist of the tested prosthesis with a β angle (Fig. 3). The β angle can be adjusted between Fig. 5 Twist mechanism
Fig. 6 Abrasion Mechanism
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Our experimental study was based on the investigation of the influence of the prosthesis surrounding medium, the number of solicitations cycles, the bending and twist angles on the breaking load, the stiffness and the ultimate strain of braided prostheses. This testing procedure consists of submitting the sample to cyclic solicitations at various levels of bending, twist angles and number of cycles, using the developed fatigue tester. For the bending and twist angles, we considered those cited in the literature. Drouin et al. [12] carried out the mechanical tests by combining the bending
Arab J Sci Eng (2014) 39:2205–2214 Table 2 Studied fatigue test factors and their levels
2209 Bending angle (◦ )
Twist angle (◦ )
Level
Factors medium
I
Dry
30
0
6,000
II
Saline water (0.9%)
45
15
30,000
60
30
42,000
III
Number of cycles
Fig. 7 Determination of braid angle
and twist solicitations on the prostheses ligament. Bending angle was varied between 0◦ and 30◦ and the twist angle was varied between −15◦ and +15◦ . For the study of the mechanical properties of the ligament after ACL reconstruction, Matthias et al. [13] used a bending angle of 45◦ . This bending angle of the ACL corresponds to a knee flexion of an angle 90◦ . In the literature, it has been reported that flexion tests were carried out on artificial ligament with a deviation angle of 60◦ , corresponding to the inclination angle of the tunnel from the tibial plateau [9]. We used an experimental design method to study the effect of the solicitation parameters (called factors) on the mechanical properties of the ligaments. For the combination of the various levels of factors, we used a fractional design of Tagauchi. A Taguchi design, or an orthogonal array, is an experimental design method that usually requires only a fraction of the full factorial combinations. The term “orthogonal array” means that the design is balanced so that factor levels are weighted equally. Therefore, each factor can be evaluated independently of all other factors, so the effect of one factor does not influence the estimation of another factor. A Taguchi design requires an optimal number of tests by keeping an acceptable quality of results. This experimental design method has several advantages compared to the traditional methods of experimentation. It allows decreasing the number of tests and to study consequently an important number of factors [15]. About the medium influence, we studied two cases: the case of the dry cyclic solicitations and the case of the cyclic solicitations in a solution of 0.9 % saline water. In the literature, it was reported that this solution simulates the effect of the synovial liquid [8,12,16,17]. We kept the temperature of the solution equal to 37±2 ◦ C, the knee internal temperature [18] using the immersion heater. For the study of
the other factors, we have chosen to vary each factor on three levels. The creation and analysis design of Tagauchi were generated by Minitab Software. The factors and their levels are illustrated in Table 2. The corresponding experimental design is given by columns 1–5 of Table 4. The braid angle is an intrinsic characteristic of the braided structure. Its variation leads to important changes of the braided ligament aspect [19]. The braid angle α (Fig. 7) is the half of the angle made by crossing filaments in the braided ligament. It is related to the braiding machine take-up speed, which is proportional to a cogwheel ratio. We developed a new method for the measurement of braid angle based on image analysis. Digital photographs were taken by a digital microscope to determine the braid angle. These images were treated by Microsoft Visual Basic 6.0 software to determine the braid angle. The used image processing consists of two lines (l1 and l2 ) drawn parallel to the axis of two interlaced yarns by clicking on two points belonging to each of the same filament. For each line, an equation is calculated allowing the determination of the intersection point coordinates. The braid angle is defined as being half of the angle made by the two lines (Fig. 7). For the purpose of testing, we fabricated a triple braided ACL of 6 mm diameter using a circular braiding machine “LESMO” with a 16-carrier arrangement. A polypropylene yarn having a count of 210 g/km (tex). The circular braiding machine has a regular sequential motion (1/1) of the carrier to interlace the 16 yarns and a simple circular braid is then obtained (Fig. 8). In the same circular braiding machine, a central core can be added inside the simple braid. We braided a double braid composed of the 16 yarns and having a simple braid as a central core. Then a triple braid composed of 16 yarns and the double braid as a core was fabricated (Fig. 9).
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ament with those of the calculated averages of the 18 experimental values presented in Table 4. Then, to simplify the analysis of the test factors’ effects on the mechanical properties of the tested braided ligament, we used the statistic delta defined as the difference between the high and the low effect of each factor. Fig. 8 Circular braided structure
3 Results and Discussions 3.1 Effect of Fatigue Test on Braided Ligament Aspect
Fig. 9 Core-braided structure
The tested sample is 200 mm long. After fatigue test, the mechanical properties of the ACL sample are measured using a constant speed gradient dynamometer “LRX2.5K Lloyd, England” (10 mm/mn) normally used for classic fabrics. The ligament is grabbed by the dynamometer clamps and undergoes a longitudinal traction until rupture. At the breaking instant, the exact breaking strength and the ultimate strain are registered. For each combination of factor levels, five samples were tested. Mean breaking load, mean ultimate strain and mean stiffness were calculated. The virgin braided ACL prosthesis has the following properties: • • • •
Breaking strength: 1,673.80 N Stiffness: 73.35 N/mm Ultimate strain: 107.87 % Braid angle : 43.6◦
We compared the mechanical properties of non-tested braided ligaments with the properties of tested ones. For this purpose, we compared the values of the breaking load, the stiffness and the ultimate strain of the non-tested braided lig-
Fig. 10 Braided ligament images after fatigue test
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The fatigue test, equipped with the friction mechanism, had important effects on the braided ligament aspect. Indeed, according to visual evaluation, the tested samples show the presence of fluff due to the abrasion of the structure with the sheep femur (Fig. 10). This abrasion is as significant as the number of solicitation cycles is important and the bending angle is low. The low bending angle permits a permanent contact between the braided ligament and the sheep femur and entails the friction damage. 3.2 Effect of Fatigue Test on Braided Ligament Angle In order to investigate the effect of the fatigue test on the braiding angle, we measured this angle for the different tested samples with the developed method of the measurement of braid angle. Table 3 shows that the fatigue test generates a deformation of the braided ligament which appeared by a reduction of the braid angle of the tested sample compared to virgin braided ligament. 3.3 Effect of Fatigue Test on Mechanical Properties of Braided Ligaments The results of the experiments are illustrated in Table 4. Figure 11 shows the effects of the cyclic loading on the mechanical properties of a polypropylene braided ligament.
Arab J Sci Eng (2014) 39:2205–2214 Table 3 Variation of the braid angle with fatigue test conditions
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Fatigue test parameters
Bending angle (◦ )
60
30
45
30
Twist angle (◦ )
30
30
0
0
30,000
30,000
42,000
6,000
38.60
39.60
39.70
40.60
Number of cycles Braid angle (◦ )
Virgin braided ligament
43.60
Table 4 Mechanical properties of fatigue-tested braided ligaments in the different test conditions Test
Fatigue testing parameters
Tested braided ligament
Medium
Number of cycles
Twist angle
Bending angle
Breaking load (N )
1
I
I
I
I
1,674.60
76.50
100.23
2
I
I
II
II
1,675.60
91.78
87.55
3
I
I
III
III
1,658.20
65.39
125.97
4
I
II
I
I
1,699.00
71.53
103.20
5
I
II
II
II
1,699.60
105.48
97.99
6
I
II
III
III
1,972.20
97.53
96.45
7
I
III
I
II
1,628.00
94.88
98.35
8
I
III
II
III
1,775.50
95.23
75.76
9
I
III
III
I
1,670.80
44.16
97.71
10
II
I
I
III
1,810.00
94.39
104.00
11
II
I
II
I
1,820.40
87.44
110.00
12
II
I
III
II
1,820.20
80.58
115.50
13
II
II
I
II
1,803.50
90.65
103.79
14
II
II
II
III
1,839.00
106.97
102.66
15
II
II
III
I
1,734.70
76.93
100.28
16
II
III
I
III
1,809.80
108.74
95.68
17
II
III
II
I
1,823.10
81.49
110.24
18
II
III
III
II
1,824.20
87.33
98.91
1740 1720 1700 1680 1660 1640
90
110
85
108
Stiffness (N/mm)
1760
Ultimate strain (%)
Breaking load (N)
1780
80 75 70 65
1620
Non-tested braid
Tested braid
Stiffness (N/mm)
Ultimate strain
106 104 102 100 98
Non-tested braid
Tested braid
Non-tested braid
Tested braid
Fig. 11 Effects of fatigue test on the mechanical properties of the polypropylene braided ligament
Compared to non-tested braided ligament, the breaking load increases 89.44 N, the stiffness increases 13.15 N/mm and the ultimate strain decreases 6.25 %.
These tables also show the average effect of each factor on the studied property when it passes from one level to another.
3.4 Effect of Fatigue Test Factors on the Mechanical Properties
3.4.1 Effect on Breaking Load
In order to determine the most influential factor, a classification can be performed (Tables 5, 6, 7) based on statistic delta.
According to the results illustrated in Table 5, the dominant parameters that influence the breaking load are first the medium (L = 92 N), then the bending angle (L = 74 N),
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Table 5 Analysis of effects on the breaking load (N)
Level
Medium
Bending angle
Twist angle
Number of fatigue cycles
I
1,717
1,737
1,737
1,743
II
1,809
1,742
1,772
1,791
1,811
1,780
1,755
92
74
43
48
1
2
4
3
III Delta: L Rank Table 6 Analysis of principal effects on stiffness (N/mm)
Level
Medium
Twist angle
Number of solicitation cycles
I
82.50
73.01
89.45
82.68
II
90.50
91.78
94.73
91.51 85.31
III
Table 7 Analysis of principal effects on ultimate strain
Bending angle
94.71
75.32
Delta: S
8
21.70
19.41
8.83
Rank
4
1
2
3
Level
Medium
Bending angle
Twist angle
Number of solicitation cycles
I
98.14
103.61
100.88
107.21
II
104.56
100.35
97.37
100.73
III
100.09
105.80
96.11
Delta: US
6.43
3.52
8.43
11.10
Rank
3
4
2
1
then the number of solicitation cycles (L = 48 N) and finally the twist angle (L = 43 N). The breaking load of a solicited braided ligament in saline medium is higher than that in the dry state (L = 92 N). The variation of the breaking load depends on bending and twist angle. In fact, with the increase in bending angle of 45◦ – 60◦ , the breaking load increases with L = 69 N, whereas L= 5 N when the bending angle passes from 30◦ to 45◦ . When the considered factor is the number of fatigue cycles (n), the breaking load increases (L = 48 N) when n passes from 0 to 30,000. However, when n passes from 30,000 to 42,000, the breaking load decreases with L = 36 N.
twist angle has an effect on the stiffness, which depends on the twist angle. Indeed, when the twist angle passes from 0◦ to 15◦ , the stiffness increases 5.28 N/mm. But when the twist angle passes from 15◦ to 30◦ , the stiffness decreases with S = 19.41 N/mm. The observations of the effect of the number of fatigue cycles (n) on stiffness are similar to those concerning twist angles. The stiffness increases 8.83 N/mm. However, n passes from 0 to 30,000. When n passes from 30,000 to 42,000, stiffness decreases with S = 6.2 N/mm.
3.4.2 Effect on Stiffness
The examination of the statistic delta values (Table 7) shows that the four factors can be classified in a decreasing order of importance as follows: the number of solicitation cycles, the twist angle, the medium and finally the bending angle. Table 7 shows that the number of fatigue cycles is the factor which has the most significant effect on the ultimate strain corresponding to a reduction of US =11.1 %. The influence of the twist angle on the ultimate strain depends on the angle value. Indeed, for a twist angle lower than 15◦ , the ultimate strain decreases with US = 3.51 %. When increasing the twist angle from 15◦ to 30◦ , the ultimate strain increases with US = 8.43 %. We also notice that the ultimate strain increases in the saline medium with US = 6.43 % compared to fatigue test without saline medium. When the bending
Based on the statistic delta, Table 6 shows that factors which present the main effect on the braided ligament stiffness are first the bending angle and then the twist angle. The number of cycle and the medium have lesser effect on the braided ligament stiffness. When the bending angle varies from 30◦ to 60◦ , stiffness increases with S = 21.70 N/mm. The variation of stiffness is more significant for a bending angle lower than 45◦ and the variation decreases for higher angles. The medium has also an effect on stiffness. Indeed, the solicitation in a saline medium increases the stiffness with S = 8 N/mm compared to a solicitation in a dry medium. It is also clear that the
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3.4.3 Effect on Ultimate Strain
Arab J Sci Eng (2014) 39:2205–2214
angle value increases with 15◦ , the ultimate strain decreases with US = 3.52 %. The application of the low number of fatigue cycles on a braided ligament involved an increase of the breaking load and stiffness and a decrease of the ultimate strain due to braided ligament structural changes. This can be explained by the fact that dynamic test increases the cohesion between yarns composing the braided ligament due to the rearrangement of these yarns under repeated constraints. This cohesion between yarns increases breaking load and stiffness and reduces ultimate strain. Due to cyclic loading, the braided structure in which yarns have a helical shape tend to reach a jammed state; yarns are more parallel and braiding angle is reduced. These results correspond with results reported by [19] who tested the mechanical behavior of circular hybrid braids made of polypropylene and PFT. Pastore and Amstrong [20] proved that braided fabrics go through a geometric transition under low loads until the fabric reaches the stage of jamming where yarns start to press against each other. The experiments performed by [2] on the optimization of the mechanical properties of artificial ligaments showed that when the braid angle decreases, the breaking load and the stiffness increase and the ultimate strain decreases. The rate of increase of the stiffness, the ultimate strain and the breaking load decrease when the number of fatigue cycles increases (Tables 5, 6, 7). The variation of these parameters for low number of fatigue cycles corresponds to a transitory phase called preconditioning of the sample. The importance of preconditioning was demonstrated by Graf et al. [10]. They noticed that after several cycles of load and unload on the ligament, load–strain curve was stabilized and breaking load and stiffness increased. This study indicates the importance of preconditioning before experimental testing or reconstruction ACL. During biological ACL reconstruction, the initial force applied during implantation to cause tension in the graft decreases over time as a result of stress relaxation. This effect has also been demonstrated by [21] with primate patellar tendon where the stress in the tendon was reduced to 69.8 % of the initial stress within 30 min. However, preconditioning the graft can reduce the amount of stress relaxation by approximately 50 % when compared with non-preconditioned [22]. Studies by Elmarzougui et al. [23] show that the preconditioning of braided prostheses ligament before fatigue test reduces the energy dissipated and the residual deformation of these structures under cyclic loading.
4 Conclusions In the present work, we developed a new fatigue tester for artificial ligament that simulates the efforts applied on the ACL during the knee movement. The design of this device
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was based on the knowledge of biological data concerning the knee articulation. The fatigue tests performed with the developed device on braided ACL samples showed that the repeated traction–bending–twist involved an increase of breaking load and stiffness and a decrease of ultimate strain due to structural rearrangement of braided prosthesis. The abrasion had an effect on the braided ligament aspect that appears by the apparition of the fluff. The experimental design method permitted to determine the effect of each parameter of the fatigue test on the mechanical behavior of the braided ligament. The change of properties of braided ligament for low number of stress cycles reveals the importance of preconditioning of prostheses ligament premarket which reduces the probability of failure of these prostheses ligament for a short period of use. The preconditioning of braided ligament prostheses before implantation avoids the laxity of the knee that can occur in the early days of reconstruction because of the permanent deformations due to rearrangement of the filaments of the prosthesis under the effect of repetitive deformation.
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