Osama Ahmed

Polytetrafluoroethylene Suture Performance under Uniaxial Tension: A Comparison of Two Stitch Techniques

25 April 2007

BACKGROUND:

There are numerous suturing techniques currently being used in the surgical world. Therefore it is important to compare these techniques in order to determine which will deform least with an increase in load. Two of these stitching styles, Running Locked (RL) and Pulley, were previously tested for this under uniaxial tension by use of predetermined weights (200g increments); still-photos were taken with a CCD Camera (640x480 pixels) after each applied load and deformation was determined by measuring pixel distances from the top and bottom of the stitch. The slopes of the formulated Displacement vs. Load curves for each technique were compared with an unpaired t-test of unequal variance. The results showed that there was no difference between the slopes (one-tail p-value =0.0518). This p-value is only slightly larger than the 0.05 cut-off. Also, the average slope of the RL appeared higher than that of the Pulley stitch (Figure 1 in Appendix) which suggests that the two techniques could possibly differ.

Aside from these points, the data collection methods used in the suture lab (Experiment 2) had many limitations. For one, the suture lab involves taking a still photo after every 200 g load addition. The restraints of such a technique can easily be overcome by applying the uniaxial tension by Instron machinery[1]. Some advantages of using the Instron include: (1) bypassing camera pixel-to-mm calibration, (2) steady load rate which will give a continuous data set, contrasted to incremental loading of Experiment 2 and, (3) less room for human error (camera movement, rough handling of weights, etc).

This proposal hopes to expand on Experiment 2 by optimizing the protocol. Also, polytetrafluoroethylene (PTFE) thread will replace the nylon thread used to suture the surrogates together. Recent reports have advocated the use of PTFE sutures for replacement or reinforcement of ruptured or elongated mitral valve chordae tendineae (Cochran and Kunzelman 1991). For this reason, the biomedical relevance of using PTFE instead of nylon can be attested for by comparing the two stitch techniques.

AIMS HYPOTHESIS:

Results from Experiment 2 suggest that the RL technique might be stronger than the Pulley stitch because the average slope was higher for RL than for Pulley (Figure 1 in Appendix). However, this was not statistically backed (p = 0.0518). The many restraints contained in the suture lab procedure may have introduced errors that contributed to the above mentioned p-value. This proposal hopes to identify these limitations and optimize the protocol so that more accurate results may be obtained.

This proposed experiment will determine the force-deformation plots from the output values of the Instron 4444. The slopes of the linear portions of each technique will be compared. As such, it is hypothesized that the RL technique will have a significantly higher slope(i.e. be able to withstand more force with less deformation) than the Pulley stitch and is therefore expected to be a stronger stitch.

EQUIPMENT:

Major Equipment:

  1. Instron Model 4444 with LabView (for direct visualization of live data collection)

The Instron provides continuous data points that will allow for more accurate measurement of the needed fracture properties. It also minimizes the amount of influence the student may have on the set up. In Experiment 2, possible errors include moving the camera which invalidates the predetermined calibration, rough placement of the weights and incorrect no-load positioning. Use of the Instron eliminates all problems specifically associated with using the predetermined weights and image analysis technique of Experiment 2.

Lab Equipment:

  1. Cutting equipment: scalpel, razor blade, and scissors
  2. Length measuring instruments: ruler and calipers
  3. Weight set (500 g, 1 kg, 2 kg)

Scissors are recommended but if the surrogate material is too thick a razor blade or scalpel may be used depending on the student’s comfort level with such equipment. It will be necessary to measure the dimensions of the surrogate material needed for suturing and the amount of material clamped in the instron should remain constant. The weight set will be used for Instron calibration[2].

Supplies:

  1. Needle set
  2. PTFE thread
  3. PTFE-Filled Delrin Sheet 1/8" Thick, 12" X 12"

Refer to Experiment 2 (or other sources) for directions on the RL and Pulley techniques. At least two needles are needed so that one student may work on suturing 4 samples with one technique while another student uses the other technique.

PTFE sutures have become widely used in chord replacement because of its“availability, theoretical simplicity, and demonstrated long-term durability” (Duran and Pekar 2003). Therefore, PTFE thread will be used to mimic current surgically established suturing materials. The Delrin Sheets will be used as the “skin” surrogates. They have a tensile strength much higher than the PTFE thread (7500 psi compared with a maximum of 4000 psi) which will increase the chances that failure will occur at the stitch zone instead of on the surrogate material.

Newly Purchased Equipment:

  1. Jaw Faces for Flats (Serrated, 50 X 30 mm)

The serrated jaw faces will be used to reduce the chances of the Delrin material slipping during testing. This is important because the focus of this experiment is to reduce chances of error and promote more accurate data collection.

PROPOSED METHODS & ANALYSIS:

Sample Preparation:

  • Cut sixteen 2 cm x 5 cm “skin” surrogates from the 4”x12” Delrin sheet. You may want to prepare more samples so as to practice using the Instron.
  • Using the PTFE thread, suture eight of the sixteen pieces with the running lock stitch (n=4). Connect the other eight pieces of fabric using the Pulley stitch (n=4). One student should stitch with the RL while another student uses the Pulley technique. One person working on a specific technique reduces the chances of having drastically different stitches.
  • Use four equally spaced stitches per sample.

Instron Calibration:

  • Verify load cell transducer settings (refer to Experiment 3 in lab manual).
  • Make sure that the serrated jaw faces have been installed.

Instron Tensile Testing:

  • Ensure that the no-load position is the same for all samples. Remember that the focus is on the suture area and you can predetermine the distance between the stitch area and the clamp and keep this distance constant between all samples.
  • To reduce the amount of “slack” that the sample has you should (1) fasten the sample at the top clamp first then tighten it at the bottom clamp and then (2) “jog” the sample until it is taut (refer to Experiment 3 in lab manual).
  • Test all 8 samples with a cross-head speed of 10 mm/min (Cochran and Kunzelman 1991). This is the rate tested for PTFE sutures; however, the group may decide that 10 mm/min is too time consuming. If so, the number of stitches may be reduced to three or two stitches so as to hasten sample failure.
  • Note down the rupture position, shape, and any other visible rupture properties.

Data Collection and Analysis:

  • Save data files in a safe place before moving on to next sample.
  • Plot Force-Deformation data and determine the slope of the initial linear region.
  • Compare the slopes (n=4 per suture technique) with an unpaired t-test of unequal variance.
  • Use the one-tailed p-value as a measure of significance.

Time Allocation:

12noon------1------2------3------4------5------6pm


POTENTIAL PITFALLS & ALTERNATIVE METHODS:

The general purpose of this proposal is to identify the limitations of the testing techniques used in the Suture Lab and to optimize the protocol so as to reduce chances of inaccuracy due to procedural errors. However, even with this new protocol there are some drawbacks that can still hamper data collection. These issues are addressed below with possible resolutions.

1)The PTFE thread may damage the surrogate by severing it at the stitch points. Such damage will cause early failure and will yield inaccurate fracture point results. A combination of the proposed alternatives should be used.

Possible resolution #1: Further the distance of the stitch points from the edge of the surrogate. This will reduce the chances of premature failure because the thread will have a greater distance to “cut through” in order to reach the edge of the surrogate.

Possible resolution #2:Choose a different surrogate material that specifically resists tearing.

Possible resolution #3: Increase the thickness (1/8” to ¼”) of the surrogate material to reduce chances of tearing. An added benefit of increasing surrogate thickness is that it will reduce slippage at the clamp points.

2)Deformation of the Delrin surrogates will contribute to the total deformation of the sample. Such results will reduce the slope of the force-deformation plots by increasing deformation faster than necessary.

Possible resolution #1: Reduce the length of the Delrin surrogate so as to maximize the stress on the stitch area and minimize the stress on the surrogate.

Possible resolution #2: Instead of actual surrogate length reduction, it is possible to simply reduce the distance between the clamp and stitch area. Such a change is justified by the same reasoning in the above resolution.

3)Needle work discrepancies may arise such as: (1) the spacing between stitches is irregular and (2) the lengths of the thread used may differ from surrogate to surrogate. Such variations may increase or decrease force-deformation values between samples of the same group. Note that even though it is not expected that these differences will account for large divergences (> 5%), this experiment does hope to teach the student about possible errors and show ways to increase accuracy.

Possible resolution for (1): Mark the points of needle insertion with a pen beforehand and keep the spacing between insertions constant. Measure space distances and quantified resolution.

Possible resolution for(2): Measure the amount of thread needed beforehand to promote regularity between samples. Practice on a couple of samples to estimate how much thread is needed then measure that with a ruler to X length ± 0.5 mm.

BUDGET:

Product / Cost (each) / Amount Purchased / Amount needed
PTFE Thread / $13.97 / 2 / 1 per group
PTFE-Filled Delrin Sheet / $48.33 / 8 / 4”x12” piece per group
Jaw Faces for Flats / $300.00 / 2 / 1 per Instron in lab

Details:

PTFE Thread (online catalog number 84935K36 @ [3]): Made of Virgin Electrical Grade PTFE (Fluorpolymer). Length 100’. Diameter (0.035±0.002)". Tensile strength of 2500 to 4000 psi. One spool of thread supplied to each group per week. There is more than enough thread for the 20 groups.

PTFE-Filled Delrin sheets (online catalog number 8678K311 @ Each order will be 12”x12” (±0.125" each) and (1/8±0.025)” thick. Tensile strength of 7500 psi. Each group however will only be provided with 4”x12” cuts, which is more than enough for the sixteen 2 cm x 5 cm pieces needed for this lab. 8 pieces will be purchased for sake of leniency.

Jaw Faces for Flats(online catalog number 2702-141 @ These serrated jaw faces are 50 mm wide x 30 mm high (2.0 in x 1.2 in) and allow for specimens up to 15 mm (0.6”) in thickness. These jaw faces will drastically reduce any chances of surrogate slippage from the clamps.

The total price will be $1014.58 which leaves more than $985 for shipping, handling, and any other needed purchases.

REFERENCES:

1)Cochran RP and KunzelmanKS. Comparison of viscoelastic properties of suture versus porcine mitral valve chordate tendineae.J Card Surg. 1991 Dec;6(4):508-13.

2)Duran CM and Pekar F. Techniques for ensuring the correct length of new mitral chords.J Heart Valve Dis. 2003 Mar;12(2):156-61.

3)Winkelstein B. Experiment 2: Imaging Techniques for Measuring Suture Displacement During Uniaxial Tension. BE210 Spring 2007 Lab Manual.

4)Winkelstein B. Experiment 3: Instron Tensile Testing. Structural & Material Properties of Chicken Skin. BE210 Spring 2007 Lab Manual.

APPENDIX:

Figure 1: Average Displacement vs. Load (200 g increments) comparison of RL and Pulley stitch methods.

[1] Instron 4444 is available in lab.

[2] Refer to “Experiment 3” in 2007 BE210 lab manual for details on calibration.

[3]McMaster-Carr is a supplier to industrial and large commercial facilities worldwide (blackboard).