Abstract: The success of wound healing depends on suture performances, such as slippage ratio, deformation recovery, knot performance and tensile properties. The main objective of this study is to determine the effect of manufacturing parameters related to braiding and hot stretching steps on suture performances. Obtained results showed that hot stretching contributes to macromolecular chains arrangement as well as suture performances and structural properties improving. The percentage of crystallinity of fabricated suture was ameliorated by hot stretching suture and reaches 25.51% under temperature of 170°C and during 3 min. An increase of hot stretching load to 15 N reduces slippage ratio and improves percentage of deformation recovery as a consequence of the improving of macromolecular chains arrangement.
INTRODUCTION
Braided structures are commonly used in surgery as ligaments, stents, nerves and sutures (Rajendran and Anand, 2002; Marzougui et al., 2009). Braided multifilament sutures are commonly used in surgery because of their excellent flexibility and handling proprieties compared to monofilaments. They are generally obtained by using a circular braiding machine. Monofilament sutures have a smooth surface and knots can be easily undone. They suffer from relatively high stiffness which creates problems for surgeons during knotting. Similarly, braided sutures have a rough surface and a greater tendency to break, in spite of being more flexible (Hockenberger and Karaca, 2004).
The braided suture manufacturing process is partially available in literature but the available data describes only some steps of this process (Hutton and Dumican, 2001; D’aversa et al., 2004; Pokropinski et al., 2005). There are lacks of information about manufacturing conditions of braided suture and especially about hot stretching step. Washington and Entrekinr (2001) developed a new technique to hot stretch sutures by using a new device permitting to apply a progressive stretching temperature and rate. They suggested temperatures ranges that improve suture performances but they considered only tensile strength and elongation of sutures as performance criteria.
Sutures key properties include tensile strength during tightening, knot security, surface morphology, knot slippage and suture behavior during healing period (Carr et al., 2009). Many testing procedures have been described in literature (Tomia et al., 1993; Hong et al., 1998; Bayraktar and Hockenberger, 2001; Heward et al., 2004; Carr et al., 2009; Debbabi et al., 2011). But there are no any official standard available today for this purpose (Carr et al., 2009; Debbabi et al., 2011). In previous works, Debbabi et al. (2011) proposed some methods to determine knot slippage ratio, percentage of deformation recovery, knot pull and strength pull curves. They identified also three main forces developed inside the knot when the knotted suture was subjected to longitudinal traction: flexural, frictional and compressive forces (Abdessalem et al., 2009).
In this study, the effect of manufacturing parameters related to braiding and hot stretching steps on suture proprieties such as slippage ratio, knot efficiency, tensile strength, and percentage of deformation recovery is studied.
MATERIALS AND METHODS
Suture manufacturing: This project was performed in the Textile Research Unit of Ksar Hellal and National Engineering School of Monastir, in 2008. In this research, braided sutures made of 16 non-texturized polyamide 6-6 yarns with 78 dtex count and 23 filaments per yarn were made up by using a LESMO circular braiding machine (LESMO machine, Type 80, Italy) with 16-carrier arrangement and five different cogwheel ratios (0.195, 0.179, 0.163, 0.143 and 0.130). A 1/1 interlacing braid geometry obtained through 1/1 sequential carrier motion was implemented. Obtained braided structures are noted in this study by majuscule letter “S” followed by the cogwheel ratio (Table 1).
| Table 1: | Fabricated sutures |
According to USP32-NF27 S2 monograph for non absorbable sutures and USP32-NF27 S2 <861> method to measure suture diameters, the fabricated polyamide braided yarns had a USP number of 1.
The braided sutures were hot stretched under different conditions by using a laboratory heat setting machine to obtain compact and uniform structure and better handling proprieties. Hot stretched suture are noted by majuscule letters “SS” followed by hot stretching conditions (Table 1). Suryadevara and Nozad (1996) recommended annealing temperature of the polyamide monofilament suture between 140 and 185°C. Washington and Entrekinr (2001) suggested a temperature of suture hot stretching between 10 and 65.5°C below the melting point of suture material. So, three hot stretching temperatures of 160, 170 and 180°C which are lower than the polyamide degradation temperature and belonging to the intervals suggested in Suryadevara and Nozad works, were used.
According to Cook (1998), braided and twisted structure can be stretched under a draw ratio situated between 15 and 35%. Washington and Entrekinr (2001) showed that the draw ratio of suture stretching should be between 6 and 25%. During hot stretching, suture was submitted to different stretching loads generating draw ratios that belong to these intervals. Table 1 shows the experimental plan used in this study where cogwheel ratio of braiding machine, temperature, time and load of hot stretching were varied.
Determination of fabricated suture characteristics: An optical digital microscope (Microscope word, DC5-420TH, USA) and Motic Image Plus 2.0 processing software were used to obtain images of the fabricated sutures.
| Fig. 1(a-b): | Image proceeding methods used to determine (a) suture pick count and (b) suture braid angle |
The Autocad software allowed the determination of pick count and braid angle of fabricated suture (Fig. 1). The pick count refers to the number of crossovers of sheath yarns per linear inch of suture. The braid angle is half of the angle made by crossing filaments in the braid. It is related to the suture take-up speed in the braiding machine which is proportional to the cogwheel ratio.
The percentage of crystallinity of hot stretched suture was determined using differential scanning calorimetry technique (DSC Mettler Telédo 832e). Temperature rate, temperature range and enthalpy of the ideal crystal ΔHf° were respectively equal to 10°C/min, 30°C to 300°C and 196 J g-1. The obtained values are presented in Table 2.
Determination of mechanical proprieties: A traction machine (dynamometer LLOYD, England) with a constant rate of extension was used for tensile tests according to ASTM D76-99 standard test method. The load cell was chosen so that the tensile force of tested sutures was between 10 and 90% of the load cell’s capacity (ASTM, 1999). All tests were carried in a controlled environment of 21±1°C temperature and 65± 2% relative humidity per ASTM D1776-04.
| Table 2: | Fabricated sutures characteristics |
| Fig. 2(a-b): | Tensile tests from the doctor side. (a) Straight pull test and (b) Knot pull test |
Ten specimens were tested in each test type and average value is presented in this study.
Figure 2 shows the used experimental procedure. Tests adopted from Instron® were used for straight pull and knot pull tests (Instron, 2004). For each tied specimen, a square knot was made around rubber tubing. The knotted and non-knotted specimen underwent a longitudinal traction until rupture at a strain rate of 200 mm min-1 with a 100 N load cell and 150 mm initial gauge length (Instron, 2004). The knot efficiency is determined by the percentage of tensile strength loss after knot tying.
The used tests methods to determine tensile performances of knotted suture from patient side were illustrated in previous works (Debbabi et al., 2011).
| Fig. 3: | Tensile test of knotted suture from the patient side |
Sutures were tied around two cylindrical sponges under a tying force of T0 during 3 sec at 200 mm min-1 strain rate to simulate roughly the surgeon’s tying force. The cut loop ends were clamped in the upper grip and the knot was placed around the pin added to the lower grip as shown in Fig. 3. The suture was finally subjected to tensile test till failure. The distance between the bottom of the upper grip and the pin was set at 10 mm. The selected load cell capacity was 500 N and the strain rate was 12.7 mm min-1.
The slippage ratio parameter which expresses the amount of slipped materials from the doctor side was developed by Debbabi et al. (2011). The Amount of slipped length upon tying (AST) was obtained by the difference between the extension in mm of the knotted suture from the doctor side and the extension in mm of the non-knotted suture from the doctor side at an applied force of 10 N.
The slippage ratio, noted SRi, is given by Eq. 1 where EKP is the extension in mm of the knotted suture from the patient side; ENKP the extension in mm of non-knotted suture from the patient side; AST the amount in mm that slipped during the tying; L the specimen length equal to 145.7 mm; Fi applied force from patient side and Fp the maximal presented force set at 70 N.
| (1) |
In order to determine the percentage of deformation recovery (DR%), suture tied under 10 N of initial tension was clamped, as shown in Fig. 3 and then subjected to a creep test in the traction direction using 100 N load cell and a 12.7 mm min-1 strain rate. The hold time was set to 15 min and the load limit was set to 20 N. The percentage of deformation recovery (DR%) was calculated according to Eq. 2 where EM is the maximal extension in mm and EF the final extension in mm.
| (2) |
RESULTS AND DISCUSSION
Effect of braiding step parameters: The obtained results in Table 2 show that suture braid angle and pick count are inversely proportional to cogwheel ratio. The coefficient of variation is low and never exceeds 3.5%. The cogwheel ratio is proportional to the take-up speed of a braiding machine which corresponds to the braid running speed. It can be regulated independently of the sequential motion speed of the carriers. At low take-up speed, yarns are interlaced with high braid angle, since the braid was pulled slowly and high pick count is obtained. At high take-up speed, the opposite phenomenon occurred; since interlacing speed remained unchanged, the high braid running speed led to low braid angle and low pick count.
Extensions at break and breaking load obtained after straight pull test of fabricated sutures with different cogwheel ratios are presented in Fig. 4 and 5. The suture extension at break of non hot stretched suture decreases from 73.97±2.78 mm to 57.37±3.05 mm, when cogwheel ratio increases from 0.130 to 0.195. Similarly, extension of hot stretched sutures present the same behavior and decrease from 59.63±2.49 to 47.93±1.74 mm with the increase of the cogwheel ratio. This is due to the fact that suture elongation corresponds to filaments reorientation in the direction of braid axis. These results are similar to those reported by Hristov et al. (2004), who tested the mechanical behavior of circular hybrid braids made of polypropylene and PET. On the other hand, suture breaking happened with lower force for suture having high braid angle than for suture having low braid angle (Fig. 4). The some result was reported in Abdessalem et al. (2009) works. It can be explained by the fact that at high braid angles, bending, compression and friction forces between the sutures filaments are high and therefore reduce suture strength.
Curves showing the variation of average slippage ratio of untreated sutures can be divided into two main regions (Fig. 6). In the first zone, when the load increases, the slippage ratio decreases until a minimum value. However, in the second zone, the slippage ratio increases when the load increases. For example, slippage ratio curve of non hot stretched suture made up with a cogwheel ratio of 0.163 shows two regions; in first region, slippage started off at 1.79% and dropped to about 1.20%, then steadily increased to about 2.86% in the second region. Initially when the knot is tied under T0 = 10 N tensioning force, the sum of the frictional, flexural and compressive forces (noted FFC) in the knot are lower than the applied force from the patient side, leading thus to a high initial slippage ratio. The difference between the sum of FFC forces and the pulling force decreases progressively with the knot packing. Consequently, the slippage ratios decreases and reach a minimum value. In the right zone of the curves, the applied pulling loads are higher than FFC forces and this leads to an increase of the slippage ratio.
Figure 6 shows a decrease of slippage ratio, when the braid angle decreases. This is linked to the decrease of suture bending rigidity and resistance to the lateral compression. In fact, sutures having low braid angles, show low rigidity and low resistance to compression (Charlebois et al., 2005; Yuksekkaya and Adanur, 2009). So, knot of suture having low braid angles is further compacted and tightened under the applied force from doctor side, inhibiting consequently slippage from patient side.
Figure 7 shows an increase of percentage of recovery deformation from 54.33±2.24% to 72.41±2.14%, caused by the decrease of cogwheel ratio from 0.193 to 0.130. This proves that braid having low pick count (high cogwheel ratio) recover more easily their deformations than braid having high pick count (low cogwheel ratio).
| Fig. 4: | Breaking Load of fabricated sutures using different cogwheel ratio |
| Fig. 5: | Extension at break of fabricated sutures using different cogwheel ratio |
| Fig. 6: | Slippage ratio curves of hot stretched and non hot stretched suture fabricated using different cogwheel ratio |
In fact, when braid filaments are more sloping to braid axis, they are more subjected to bending and compression forces and have more difficulties to return to their original positions after stress interruption.
Effect of hot stretching step: After hot stretching, sutures shows a decrease of breaking load (Fig. 8). At a given temperature, suture breaking load decreases with the augmentation of hot stretching time. The same result was reported by Yuksekkaya and Adanur (2009) who studied the effect of heat treatment on braid resistance. The author associated this phenomenon to the degradation of suture material. In the case of temperatures of 170 and 180°C, the decrease of the polymer percentage of crystallinity after the increase of hot stretching time (Table 2), seems to cause the decrease of suture breaking load. However, under hot stretching temperature of 160°C the percentage of crystallinity increases with the increase of hot stretching time as shown in Table 2. In this case, polymer degradation concerned probably only amino groups and not chains arrangement.
From Fig. 9, it can be seen that hot stretching involved a decrease of suture knot efficiency. This phenomenon is caused by filament weakening after sutures hot stretching. It is accentuated when increasing temperature, time and load that involved a significant degradation of suture material and a decrease of filament resistance to lateral compression.
| Fig. 7: | Percentages of deformation recovery of hot stretched and non hot stretched suture fabricated using different cogwheel ratio |
| Fig. 8: | Breaking Load of hot stretched sutures fabricated with the same cogwheel ratio and hot stretched under different conditions |
| Fig. 9: | Knot efficiency of hot stretched sutures under different conditions |
After hot stretching step, slippage ratio curves no longer show two zones (Fig. 10) because hot stretched suture knot are not packed under low forces, applied from patient side and material slips steady when increasing applied force which is higher than FFC forces. This can be explained by the fact that hot stretching treatment makes suture more resistant to compression force as demonstrated by Yuksekkaya and Adanur (2009).
| Fig. 10: | Slippage ratio curves of hot stretched sutures under different temperature and time conditions |
| Fig. 11: | Slippage ratio curves of hot stretched sutures under different load |
Figure 10 shows also that the slippage ratio increases with time and temperature of hot stretching. This is due to the fact that increasing hot stretching temperature and time contributes to the decrease of braid surface irregularities (i.e., suture friction coefficient) and resistance to lateral compression that allows suture slippage without knot packing. Figure 11 shows slippage ratio curves of hot stretched suture under three load levels. It can be seen that slippage ratio increases after hot stretching suture under 5 N. However, it decreases after hot suture stretching under a higher load (10 N and 15) because of the decrease of bending rigidity which permits an easy knot locking and prevents knot material slippage.
Before hot stretching step, suture, made up with a cogwheel ratio of 0.163, presented a percentage of deformation recovery equal to 65.41±2.09%. However, after hot stretching step, this percentage decreases to 58.39±1.42% in the case of hot stretched suture under temperature and time equal, respectively to 160°C and 6 min.
| Fig. 12: | Percentage of recovery deformation of hot stretched sutures under different conditions |
This phenomenon is probably caused by polymer chains degradation after this treatment. An improvement of this percentage was obtained when increasing hot stretching temperature and time. In fact, at high temperatures, the macromolecular chains are set in motion that allows chains arrangement by forming a new inter-chain connection during stretching step. Moreover, treatment time increasing allows macromolecular chains to relax and have further new stables positions. Consequently, sutures are more able to recover their deformations when hot stretched under high temperature and during longer treatment time.
Hot stretched suture under a load of 5 N showed the best deformation recovery (Fig. 12), as a consequence of relaxation of residual stress in the braided structure. Thus, heat treatment contributes to the elimination of stress accumulated after braiding step as demonstrated by Liu et al. (2006). The decrease of deformation recovery percentage of hot stretched suture under a load of 10 N is due to the reduction of chains arrangement revealed by the diminution of crystalline zones percentage from 25.% to 19.83 (Table 2). The increase of this parameter when increasing hot stretching suture load up to 15 N is explained by the new orientation of macromolecular chains proved by the increase of the percentage of crystallinity that reaches 22.09% (Table 2).
CONCLUSION
The available literature suffers from a lack of papers concerning the effect of manufacturing conditions and structural parameters of braided sutures on their mechanical properties. In this study, tensile properties, slippage ratio, knot efficiency and deformation recovery of polyamide braided sutures made up under different manufacturing conditions were determined. The obtained results showed that hot stretching treatment decreased suture breaking load and knot efficiency as a consequence of suture degradation. The application of high stretching load involved an improvement of tensile strength and deformation recovery and a reduction of knot slippage ratio. The increase of bending rigidity and compression resistance after hot stretching suture inhibit to suture packing and slippage ratio curves no longer show two zones. Further works will focus on the development of new suture treatments which can improve long term sutures proprieties after implantation in human body.