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A study on the use of artificial networks for prediction of Tensile strength of Silk

Braided surgical sutures used in medical application

By

Gurumurthy.B.Ramaiah

1

, Radhalakshmi Y. CHENNAIAH

1

, Gurumurthy K.Satyanarayana rao2

1.Central Silk Technological Research Institute (CSTRI), Central Silk Board, Ministry of

Textiles, Madivala, Bangalore-560 068. INDIA,2.

Department of Electronics andCommunication University Visveshvaraya College of Engineering (UVCE), Bangalore

UniversityK. R. Circle, Bangalore-560 001. INDIA ; E-mail:[email protected]

Medical textiles are one of the fast growing areas of technical textiles today. The

nonabsorbable sutures made from natural fibers like silk sutures finds wide applicationused due to its characteristic properties. Silk sutures are nonabsorbable, sterile, non-

mutagenic surgical sutures composed of natural proteinaceous silk fibers called fibroin.Artificial Neural networks are used extensively to predict material properties in various

sections of Textiles. This paper focuses on the study of tensile strength of silk braidedsurgical sutures for medical application using artificial neural networks. Silk braided

surgical sutures confirming to USP (United States pharmacopeias) is used for

experimental data generation along with standard ASTM methods for determination ofinput parameters to the model. Constant rate elongation testers confirming to USP have

been used for experimental data generation and Neural network modeling is carried out

using Matlab.7.1 software. In the present research comparisons were made with the

Experimental and Network predicted tensile strength values which fall within theobjectives of the study.

Keywords: Linear tensile strength, Knot-breaking strength, Knot-holding capacity

1. Introduction

Figure 1: Use of Textiles in Medical Products


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Textile Medical products (Figure 1) typically employed in cords and sutures are braided

structures. Braided structures can be designed using several different patterns, either withor without a core. Because the yarns criss-cross each other, braided materials are usually

porous and may imbibe fluids within the interstitial spaces between yarns or filaments.

To reduce their capillarity, braided materials are often treated with a biodegradable

(polylactic acid) or nonbiodegradable (Teflon) coating. Such coatings also serve toreduce chatter or noise during body movement, improve hand or feel, and help position

suture knots that must be transported by pressure from a surgeon's finger from outside thebody to the wound itself.

A suture is a medical device that doctors, embalmers, and surgeons use to hold skin,internal organs, blood vessels and all other tissues of the human body together after they

have been severed by injury, incision or surgery. They must be strong so that they do not

break, non-toxic, hypoallergenic (to avoid adverse reactions in the body), and flexible (sothey can be tied and knotted easily). In addition, they must lack the so called "wick

effect", which means that sutures must not allow fluids to penetrate the body through

them from outside, which could easily cause infections.

For nonabsorbable surgical sutures, the predominant areas of concern are strength,

capillarity, sliding and positioning of knots, knot security, and handling characteristics.The recent focus of suture research has been on improving the structure of the braids.The

present study is limited to use of physical properties of fibres and yarns with braiding

parameters, particularly its resistance to tensile strength of the knotted thread andpredicting their properties using back-propagation neural networks.

The need to test the tensile strength of the knotted thread is based on the fact that, as thesurgeon tightens the suture's knot, it is expected that the stitch will not loosen until the

proliferative phase of the tissular healing is reached. The analysis of the mechanical testresults underlines such importance, since the strength data of the knotted threadinvestigated in this study is used for building a prediction model using neural networks.

The tensile strength characteristics of suture threads has been studied using artificialneural networks viewing to guide surgeons in the selection of the optimal thread by

comparing several suture materials .In the present study comparisons were made with the

Experimental and Network predicted tensile strength values which fall within the

objectives of the study.

The analysis of the results of network predicted tensile strength of the knotted thread withthe experimental samples of this study for different silk braid threads showed that such

results are similar to the ones reported by experimental trial ones.

2. Theoretical Modeling for circular braidsResearch in the past has presented mechanical models based on the concept of a fabric

structure of repeated unit cells (e.g. Ko et al. 1989, Phoenix 1978). Goff introduced

parameters such as crimp into the equations of braid geometry (1976) and more recently


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differential geometry has been used by Du and Popper (1990) to describe the braid

structure. One missing element from the literature is a study of the changes in yarn cross-sectional geometry as a function of braid structure. Most models assume that the yarn in

the braid is either round or race-track shaped, and that this shape is a known parameter

prior to modeling. The work of Gowayed (1992) for three dimensional braided fabrics

shows that the yarn cross-section of the yarns in such a fabric go through substantialchanges in cross-sectional shape depending on location within the braided unit cell.

Kawabata (1973) has presented models of yarn deformation in woven fabrics as afunction of axial applied tension and transverse compression, but this has not been linked

to braided fabrics. Pastore (1993) presented a model of yarn cross-sectional changes in a

woven fabric as a result of transverse pressure applied to the yarns, but this has not beenextended to braided fabrics.

The purpose of this study is to present a neural network model which would overcome

the non-linear constraints which are present in theoretical modeling and experimental

results of tensile knot strength is predicted and correlated with Network predicted ones.

Traditionally, braid diameter and cover factor were configured through manual

adjustment of machine settings until a satisfactory result was achieved. More recently,scientists have derived equations that make it easier to predict such values. A brief review

of the literature relevant to both diameter and cover factor is presented below.

Figure 2: Illustration of effect of braid angle on the apparent yarn width.

Braid DiameterKo and Pastore (1989) assume circular yarns will be inserted into the braid and

approximate the yarn diameter with the equation

(1)


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A = D2 /4

Braid Cover Factor

Zhang et al. (1997) also explored braids and developed equations designed to determinethe cover factor of the braid based on three variables: braid angle, helical length, andbraid diameter. Zhang defined cover factor as the ratio of yarn-occupied area within a

unit cell to the area of the unit cell. His research proved that if any two variables are

specified, the third could be determined. The first case is when braid diameter is held

constant. The cover factor of a braided structure is determined as:

(4)

where:

K = cover factor

wy = width of yarn

Nc = number of carriers

db = braid diameter (constant)

/2 = braid angle

3. Silk Braids for suture threadsSilk was first widely used as a suture material in the 1890s. It is a braided material

formed from the protein fibers produced by silk worm larvae. Although silk is considered

a nonabsorbable material it is eventually degraded in tissue within 2 years. Silk hasexcellent handling and knot-tying properties and is the standard to which all other suture

materials are compared. Its knot security is high, its tensile strength is good, and its tissue

reactivity is high. Silk suture threads are produced from filaments of natural silk of 20-22

deniers. Degummed, scientifically twisted, treated, colored & compactly braided to giveexcellent strength & handling properties. Flexible, elongates to support for optimum

knotting. Wax coated for smooth tissue passage & does not soak up in body fluids.

Braided silk surgical sutures having increased tensile strength and knot strength are

prepared by (1) presoaking conventionally-prepared unstretched braided silk sutures in a

non-corrosive liquid (preferably water) for at least about 30 minutes to wet the silk fibers,(2) stretching the presoaked sutures while the same are immersed in a non-corrosive

liquid (preferably water), and then (3) drying the stretched silk sutures while maintaining


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the stretched length and not allowing shrinkage to occur during drying. The wet-

stretching method of the invention makes possible the stretching of braided silk sutures toa greater extent than that obtainable by conventional hot-stretching techniques, up to

about 21/2 times greater, without damage to the sutures. The wet-stretched sutures of the

invention have greater tensile strength and knot strength than the prior art hot-stretched

sutures of comparable diameter.

Figure 4 : Silk suture thread manufacturing

The United States Pharmacopoeia(USP) is one of the official compendium for the suture

industry. It sets standards and guidelines for suture manufacture. Suture sizes are given

by a number representing diameter ranging in descending order from 10 to 1 and then 1-0to 12-0, 10 being the largest and 12-0 being the smallest at a diameter smaller than a

human hair.

The ideal suture would be totally biologically inert and cause no tissue reaction. It would

be very strong but simply dissolve in body fluids and lose strength at the same rate that

the tissue gains strength. It would be easy for the surgeon to handle and knot reliably. Itwould neither cause nor promote complications.

Currently, sutures are designed to result in the most desirable effect for any givensituation as determined by those administering the sutures. Taken into consideration in

the manufacture and use of sutures are properties such as stress-strain relationship, tensile

strength (and rate of retention), flexibility, intrinsic viscosity, wettability, surfacemorphology, degradation, thermal properties, contact angle of knots, and elasticity. In an

attempt to achieve this "ideal" suture the textile industry has come up with many options.Properties such as stress-strain relationship and tensile strength have a direct effect onhow much force at a given rate the closure will be able to withstand. For example, a

cough would impose a fast rate of elongation whereas edema or hemorrage would impose

a slow rate of elongation.


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4. Materials and Methods

As technology advances, testing techniques improve and become more specific for the

application of sutures. The greatest percentage of testing that is done on sutures is doneon those suture materials already existing in practice. This is due to the virtual newness of

the application of testing techniques to the suture product although, a fairly small, yetincreasingly important number of tests are done on possible new suture materials. In thetesting of sutures, conclusions such as the fact that it is possible to predict long-term

tensile properties of material under dynamic load from their dynamic loss factor and loss

modulus attract significant importance. Some of the research test areas for sutures are:

breaking strength, elongation-to-break, Young's modulus, knot security, viscoelasticproperties, tissue reaction, cellular response, cellular enzyme activity, suture

metabolism chronic toxicity, teratologics, mutagenicity, carcinogenicity, suture

allergenicity, immunigenicity, cell cultures etc.

Physical tests for sutures are either standard methods used to demonstrate agreement or

compliance with compendum requirements or research tests measuring fundamentalproperties simulating performance under use conditions. Some of the most common

methods by which tests are conducted are also methods widely used in other areas of

medicine or industry. A few examples of these methods or instruments are as follows:infrared spectrophotometers, nuclear magnetic resonance spectrophotometers, projection

microscopes, tensile testers, light microscopy, and degradation studies in vivo.

The knotted specimens were prepared using surgeons knot as shown in figure 5 and 6

respectively.

Figure 5 : Surgeons knot stage I Figure 6: Surgeons knot stage II

The tensile strength test specimens are prepared as shown in Figure 7 and Tensile tests recarried out shown in Figure 8. The tensile strength testing machine consists of suitable

clamps for holding the specimen firmly and using either the principle of constant rate of

load on specimen or the principle of constant rate of elongation of specimen, as describedin the USP standards for surgical sutures[16][17].


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Figure 7: Rubber tube with specimen Figure 8: Specimen placed between clamps

Sutures are tested immediately after removal from their sterile packages without drying

or conditioning as per United States Pharmacopoeia, except when testing for compliancewith BP (British Pharmacopoeia) or EP (European Pharmacopoeia) requirements which

specify prior conditioning as described in the monographs for various suture types.

Diameters of sutures are measured using a gauge of the dead-weight type with a presser-

foot 12.7 +/- 0.02mm in diameter. The diameter of each strand is measured at three pointscorresponding roughly to one-fourth, one-half, and three-fourths of the strand length.

For knot pull breaking strength, the suture is tied with a surgeon's knot with one turn

around a flexible rubber tubing of 6.5 mm inside diameter and 1.6 mm wall thickness.

The suture is then attached to a suitable testing machine and tested at a rate such that thespecimen breaks in less than twenty seconds.

In all strength tests, it is important to keep in mind that the breaking strength retention ofabsorbable and nonabsorbable sutures should be considered separately because the

strength retention of the absorbable sutures will be quite different than that of the

nonabsorbable suture.

The apparatus has two clamps for holding the strand. One of these clamps is mobile. The

clamps are designed so that the strand being tested can be attached without anypossibility of slipping. Gauge length is defined as the interior distance between the two

clamps. For gauge lengths of 125 to 200mm, the mobile clamp is driven at a constant rate

of elongation of 30 +/- 5 cm per minute. For gauge lengths of less than 125mm, the rate

of elongation per minute is adjusted to equal 2 times the gauge length per minute. Forexample. A 5cm gauge length has a rate of elongation of 10cm per minute. Tensile test

results for different surgical silk braid sutures are shown in Table 1.


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Table 1: Braided surgical suture material description and their tensile test results (Tests carrie


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4. Neural Network ModelingNeural networks provide significant benefits in medical research. They are actively being

used for such applications as locating previously undetected patterns in mountains ofresearch data, controlling medical devices based on biofeedback, and detecting

characteristics in medical imagery, Here we propose a Back-propagation neural networkto predict the tensile knot strength of surgical silk braided sutures. The Model is shown inFigure 9. with Inputs to the model being Average Diameter, Linear density of the yarn,

Fiber density, Apparent yarn width, braid angle and Number of carriages. The output of

the neural network model is Tensile strength and the data for targets was experimentally

generated.

Figure 9: A silk braid surgical suture neural network model

4.1 Silk braid yarn structural parameters used for Neural network model Inputs

a) Average Braid diameter of the yarn: The average braid diameter (mm) is calculated

based on the 10 strands of suture as directed under sutures- Diameter (861). The average

diameter of the strands being measured is within the tolerances prescribed in theaccompanying table for the size stated on the labels. IN the case of braided or twisted

suture, none of the observed diameters is less than the midpoint of the range for the next

smaller size or is greater than the midpoint of the range for the next larger size.


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b) Linear density of yarn (denier): Denier is a unit of measure for the linear mass

density of fibers and yarns. It is defined as the Mass in grams per 9,000 meters

c)Fiber density (g/cm3) - Fiber density is the ratio of mass to volume of the fiber. Fiberdensity is determined by using a Psychometer, density balance (Archimedes principle) or

Density gradient method. ASTMD3800 [5] gives Archimedes and sink/float method ofdetermining fiber density where as ASTM D1505 [6] gives the determination of fiberdensity using density gradient column methods.

d) Apparent yarn width This gives the measurement of the width of the yarn which iscalculated from Diameter of the braid and cosine of its braid angle. This is a process

variable and varies with braid angle. Apparent yarn width is given by the equation

Wy = Dy / cos ( /2) where Wy = apparent width of yarn

Dy = diameter of yarn

= angle between the two yarn systems (twice the angle between the braid axis and oneyarn).

e) Braid angle() - A type of finger weaving, braiding is a process of interlacing lengths

of hair or of intertwining strands of yarn or other material to form a fabric. Although theterms braiding and plaiting are often used to mean the same thing, there is a difference in

method. In plaiting, the strands being braided are linked with adjoining ones; in braiding,

the strands simply cross over or under oneanother Braid angles for surgical sutures rangefrom approximately 15 to 75 (/2).

f) No of carriages - A braiding machine carrier is fitted to a frame; a fiber spool mountedis attached to the frame; A carriage is a fiber take-up assembly including a spiral spring;gear train on the frame for mechanically connecting the take-up spring assembly to the

fiber spool mount for winding the spiral spring as a spool on the mount rotates; and amagnetic clutch, coupled to the take-up spring assembly, for preventing over winding of

the spiral spring. No of carriages determine the thickness of the braid.

4.2 Back-propagation neural networksBack propagation neural networks (Algorithm) (figure 10- Back Propagation algorithm)are loosely based on the neuronal structure of the brain and provide a powerful statistical

approach for exploring solutions of non-linear systems (Rumelhart 1986). This studyemploys a back propagation neural network which was used to correlate inputinformation with matched output values. The solution space is the set of all combinations

of coefficients for the equations that are being used for modeling a system of interest.

Table 2 gives the Neural Network model parameters and training graphs are given in

Figure 11 and 12 respectively.


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Figure 10: Back-propagation neural network algorithm

Table 2: Neural Network Model parameters for silk braid surgical suture prediction

model

Model parameter Value/Description

1.No. of Input Nodes 6

2.No of output nodes 1

3. No of Hidden layers 2

4. Activation function log sigmoid

5. Scaling of inputs

and outputs

0 to 1

6. Network learning

algorithm

Back-propagation

gradient descent

method

0 1 2 3 4 5 6 7 8 9

x 104

10-6

10

-5

10-4

10-3

10-2

10-1

100

101

90000 Epochs

Training-Blue

Goal-Black

Performance is 0.00351063, Goal is 1e-005

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

x 104

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

53889 Epochs

Training-Blue

Goal-Black

Performance is 9.96309e-006, Goal is 1e-005

Figure 11 and 12 : Training graphs for Tensile knot strength prediction tests using

Matlab 7.1 software Neural Network model


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Table 3: Selected Input parameters for silk braid surgical suture used for Ne


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Table 4 : Outputs for silk braid surgical suture used for Neural Network Mode

5. Results and Discussions

Figure 13: Best linear fits between Experimental tensile strength and Networkpredicted tensile strength for silk braid surgical suture material Training set


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Figure 14: Best linear fits between Experimental tensile strength and Network

predicted tensile strength for silk braid surgical suture material Testing set

The analysis of the results of network predicted tensile strength of the knotted thread with

the experimental samples of this study for different silk braid threads showed that such

results are similar to the ones reported by experimental trial ones. Their best linear fits areshown in figure 13 and 14 respectively.

Conclusion

The tensile strength characteristics of suture threads has been studied using artificial

neural networks viewing to guide surgeons in the selection of the optimal thread by

comparing several suture materials .In the present study comparisons were made with the

Experimental and Network predicted tensile strength values which fall within theobjectives of the study.

Acknowledgements

The authors wish to thank Mr.Shrish Nene, Lifeline sutures, Bangalore, Bangalore TestHouse, Vasanth Kumar, Sutures India for their help in conducting this study and

providing necessary information during analysis and investigation of surgical suture

materials.


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16. United states Pharmacopeias, Official compendia of standards, volume 1, USP3

NF25, Pg.376

17 United States Pharmacopeias, Official compendia of standards, volume 3, USP3

NF25, Pg.3262-3265.

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