Department of Biomedical Engineering

VanderbiltUniversity

ThermalFuse Suture

Team Members:
Katie Good

Emily Schaefer

Advisor:

Bruce Beyer, M.D.

Paul King, Ph.D.

Date of Submission:

April 24, 2007

Good, Schaefer 1

Table of Contents

Abstract2

Introduction3

Problem Statement4

Literature Review4

Patent Search5

Methodology5

Results and Discussion11

I. Data Analysis11

II. Safety, Health, and Risk Analysis12

III. Economic and Market Consideration14

Conclusion15

Recommendations16

I. Future Direction16

II. Ethical Issues and Regulations17

References17

Appendices20

Abstract

Our advisor, Dr. Bruce Beyer, a surgeon of Obstetrics and Gynecology at VanderbiltUniversityMedicalCenter, desired an alternative suturing method which only requires movement orthogonal to the wound. He believes the new method would be especially beneficial in the training environment and would result in reduced patient rehospitalization because the sutures would be more secure. The newly developed suturing method would affect many women, 1.2 million women give birth by cesarean section per year1 and 600,000 women have hysterectomies each year2. Several closure procedures were identified and eliminated. The twist tie method, however, fit all of the criteria and allowed us to use suture material already developed and commonly used in the operating room. The twist tie method closes by thermally fusingthe suture material to itself with a hair straightener. The Vicryl sutures fused together and held strong, however, we were unable to fuse the Dexon sutures. Both the 0-gauge and 2-0 gauge Vicryl sutures fused at 92.22°C had a tensile stress (241.811 N, 58.908 N, respectively) comparable to the tied sutures (207.744 N, 63.375 N, respectively) of the same gauge. Both the 0-gauge and 2-0 gauge Vicryl sutures fused at 73.89°C had a lower tensile stress (186.255 N, 19.945 N, respectively) than the sutures fused at 92.22°C or the tied sutures. These results show that applying heat to the Vicryl sutures did not compromise the integrity of the material. The results also indicate that the strength of the fused suture at the 92.22oC temperature is comparable to the tied suture of the same gauge. Since the suture strengths are comparable, there is promise for development of this closure method for sealing pedicles and closing tissue in cesarean sections and hysterectomies. The next step was to develop a heating element that is more conducive within an operating room. The device needs to be sterilizable and waterproof. Several design ideas were considered when developing the heating element. Our prototype heating element, which employed the same principle as the hair straightener, was developed using coiled stainless-steel wire as a thermally conductive heat source. Gold plated aluminum heating surfaces were attached to a modified hemostat and contained the coiled stainless-steel wires. Future work still needs to be done on the heating element design for the safety.

Introduction

It takes time and practice for surgeons to become proficient at what they do. This is especially true when tying sutures to seal pedicles and close wounds in deep cavities. Our advisor, Dr. Bruce Beyer, a surgeon and assistant professor of Obstetrics and Gynecology at VanderbiltUniversityMedicalCenter, noticed that less experienced surgeons have greater difficulty in properly tying sutures in confined areas common to gynecological surgeries.

Deep cavity medical procedures, specifically cesarean sections and hysterectomies, restrict a surgeon's ability to properly maneuver when performing tasks such as suturing. Traditional suture tying requires lateral movement in order to suitably secure the knot and ensure the suture will not loosen. Dr. Beyer desired an alternative suturing method which only requires movement orthogonal to the wound. He believes the new method would be especially beneficial in the training environment and would result in reduced patient rehospitalization because the sutures would be more secure. Dr. Beyer does not want to replace all types of sutures, but believes an alternative design has specific applications for surgeries which involve joining together thick tissues or tying off pedicles, which can occur during a cesarean section or in a hysterectomy. The newly developed suturing method would affect many women, 1.2 million women give birth by cesarean section per year1 and 600,000 women have hysterectomies each year2. Women who receive cesarean sections are 2.3 times more likely to result in rehospitalization due to wound complications and infection3 than those who have natural births. In a study comparing abdominal and vaginal hysterectomies, 14.9% of a group of 82 women, who had abdominal hysterectomies, were rehospitalized. The number of rehospitalization cases due to these procedures could be greatly reduced with a new suturing technique providing a more secure and reliable closure method, which would reduce the incidence of wound opening, infection, and internal hemorrhaging. The average hospitalization costs for a cesarean section is approximately $7,1864 and $3,0815 for a hysterectomy. Additional costs for rehospitalization are burdensome for patients.

Problem Statement

Our advisor asked us to design a suturing method which would be less cumbersome for deep cavity procedures such as hysterectomies and cesarean sections. An alternative suturing method which only requires movement orthogonal to the wound or pedicle is desirable. Dr. Beyer believes a new method would be especially beneficial in the training environment and would result in reduced patient rehospitalization because the sutures would be more secure. Our aim is not to replace all types of sutures or suturing methods, but believe an alternative design has specific applications for surgeries which involve joining together thick tissues or tying off pedicles in deep cavities.

Literature Review

There exists a wide variety of methods for closing tissue, most of which have been adapted from everyday closure methods. Some examples of surgical closure methods adapted from commonplace closure methods include tied sutures, staples, tissue adhesives6,7 and adhesive paper-tape. These methods have been previously studied,8,9,10 but are not all appropriate for all applications. For example, tissue adhesives are only used for small low-tension skin wounds and would not be appropriate for sealing a pedicle or sub-dermal wounds.11 A broad review of surgical procedures and tools has been done for obstetric and gynecological applications,11 which provided insight into the current surgical methods. By evaluating the current methods, we were able to identify more specific design criteria for an improved suturing technique.

Patent Search

In conducting a patent search we found apatent for a device which fuses the tail ends of the suture knot to ensure that the knot does not slip12. However, this device does not have a pinching mechanism and is only proposed for sealing the ends of a traditionally tied suture together. The method proposed in the patent still entails tying traditional sutures and does not solve the current problem of not having enough room to tie a secure suture. This device may, however, be useful in procedures where there is adequate room to tie proper sutures and where fusing the ends would act as an added measure to ensure the suture did not loosen.

A device which actually operates on a similar principle to a zip-tie is the Wiseband, produced by Wisebands Ltd.13 However, this device is not intended for internal suturing and is only approved for use prior to surgery for closure of tissue-deficit skin wounds. It operates by applying unidirectional tension to a flat plastic band (5mm wide and 50cm long) attached to a metal surgical needle.14 The plastic band is only temporarily left in the skin and is removed upon completion of surgery. While the mechanical operation of this device might prove useful for a future design of a suturing system, this device itself has an entirely different intended purpose.

Methodology

Dr. Beyer proposed the idea of a suturing technique with the same mechanics as a cable zip-tie. He favored the clicking mechanism, which prevents a cable tie from loosening and avoided any lateral movement when closing. From his initial idea, several design goals and criteria were established. The suture needed to be bioabsorbable, have an tensile strength (80- 100N)and degradation rate (3-4 weeks) comparable to suture materials currently used forthese specific gynecological procedures, utilize unidirectional tension, and maintain suture integrity while providing a simple closure method for the surgeon. Several closure procedures were identified and eliminated. The "Problem Formulation and Brainstorming" section of Innovation Workbench (Appendix A) aided in establishing design constraints and modifications to the traditional suture method. Cable zip-tie and other similar closure methods such a trash bag tie (Figure 1) were explored initially. However, feasibility issues arose when attempting to develop a suture which would use "teeth" as a closure method. To meet the absorption criteria we needed to use an absorbable material, but the materials that fit our design criteria do not have the structural properties necessary to maintain such a mechanical closure. These designs would need to be formed by injection molding and were out of the price range provided. The approach altered to incorporate similar characteristics using products already available. The final two ideas considered were a beaded model (Figure 2, a) and a twist tie model (Figure 2, b), per Dr. King's suggestion. The beaded model maintains the clicking mechanism of the zip-tie, but with beads instead of teeth. The beads would actually be a more favorabledesign for a suture because it would not have sharp edges. In order to keep the loop that slides over the beads in place, the loop would have to consist of a shrinkable material and treated to lock it in place. Once again the manufacturing method prevented further development, as it was established that either 3-D printing or injection molding would be needed for this process. The twist tie method, however, fit all of the criteria and allowed us to use suture material already developed and commonly used in the operating room. The twist tie method closes by thermally fusing the suture material together after twisting the two ends of the suture.

In order to fuse the suture, a material witha relatively low glass transition temperature was necessary. Materials considered included common suture materials. Dexon, a homopolymer of glycolic acid15, and Vicryl, copolymer made from 90% glycolide and 10% L-lactide with a polyglactin 370 and calcium stearate16coating were eventually tested. These two suture materials were chosen initially for their tensile strength of 80 – 100N and their degradation rate of 3 – 4 weeks (Figure 3). These materials were considered optimal materials for our design because their glass transition temperature was low enough to fuse together. Table 1 shows the various melting and glass transition temperatures for the polymers used in manufacturing the suture materials used in this experiment. To ensure these assumptions were accurate, suture material was acquired from the Vanderbilt University Medical Center Hospital Supply and tested. Zero and 2-0 gauge braided and coated Vicryl along with 3-0 gauge, braided, uncoated Dexon were purchased. These two suture materials were chosen for their desirable properties; Vicryl and Dexon both degrade rapidly and havea relatively low glass transition and melting temperature.

To test the suture material for its ability to thermally fuse, the suture was wrapped once around a white cylindrical tube, simulating a pedicle or portion of thick tissue, the two ends of the suture material were then clamped together by hemostats, and the suture material was twisted together by twisting the hemostats. Once the twisted part of the suture was flush and secure against the cylinder, heat was applied to the twisted section of the suture as close to the cylinder as possible (Figure 4). For our initial tests the heat supply was provided by a hair straightener. Two different temperature settings were used to find the optimal fusing temperature. These two settings were 7.5, which equates to 73.89°C and 9, which equates to 92.22°C. Higher temperatures were tested, but melted the suture material too quickly.

Experimental trials using the two different suture gaugeswere conducted. The twisted sutures were clamped in the hair straightener for 40 seconds and then allowed to cool. The Vicryl sutures fused together and held strong, however, we were unable to fuse the Dexon sutures. A possibility for the Dexon not fusing is the absence of an external coating on the suture.

Once we established that the Vicryl sutures could be fused, we needed to test their tensile strength. The fused suture tensile strength needed to be comparable to the tied sutures tensile strength for use in these surgical procedures. If heating the sutures compromised the integrity of the suture material, the fused Vicryl could not be used. To test the tensile strengthof the sutures an Instron Tensometer in the Skin Disease Research Core Center (SDRCC) was used. To ensure the strength of the fused sutures was comparable to traditionally tied sutures, Dr. Beyer, an experienced surgeon, tied three Vicryl sutures of each gauge. Each suture was placed between the vacuum activated clamps on the tensometer. The sutures were positioned so that the fused or tied junction was not in either clamp. The Blue Hill software captured and calculated the tensile stress of each suture as the tensometer pulled on opposite ends. The true tensile strength was then calculated according to the cross-sectional area of the suture. Three tests were run for each of the suture gauges, temperatures, and closing methods.

Next we needed to develop a heating element which was more appropriatefor use in an operating room. The device needs to be sterilizable and waterproof. Several design ideas were considered when developing the heating element. Initially, heated tweezers were desired because they would be ideal for clamping the suture material and use thermal conduction to produce heat. However, we could not purchasethe device because it did not have a variable temperature setting and the price was beyond our means. Other current surgical techniques were also explored. Dr. Robert Galloway, a professor of Biomedical Engineering at VanderbiltUniversity,allowed us to use an electrocautery unit in his lab to test if we could fuse the sutures with direct current. While this method was unsuccessful for fusing the sutures, it was established that the appropriate heating element would need heat the suture through thermal conduction, not through direct current. We realized that we would need to develop our own heating element and could not use an instrument already present in an operating room. Initially, a hair straightener was taken apart and modified by reducing the size of the resistance coils. This technique, however, did not work because shortening the coils reduced the resistance of the device. By reducing the resistance a greater amount of current traveled through the coiled wires,destroying them. Using the same principle as the hair straightener, a new heating element was designed using coiled stainless-steel wire as a thermally conductive heat source. Gold plated aluminum heating surfaces were attached to a modified hemostat and contained the coiled stainless-steel wires (Figure 5). This heating element had a smaller heated surface area than the hair straightener. To provide power to these coiled wires, the two ends of the wires were attached toDC+ and DC- ports of a capacitor. This specific capacitor was a Nippon Chemi – con 50V, 68,000μF capacitor. The DC+ and DC- ports of the capacitor were connected to a bridge rectifier which was then connected to a toroidal transformer. The ACin of the toroidal transformer was plugged into a voltage-variable transformer. More specifically, a Superior Powerstat ® with an input of 120V and a variable output of 0-140V. A full circuit diagram can be seen in Figure 6. The sutures were then fused with the new heating element to determine the correct voltage setting on the variable transformer. The voltage was increased at increments of 5V until a secure suture closure was established. The best voltage for fusing the sutures is approximately 40V. Although our prototype was able to fuse the sutures, it is still not an appropriate device to be used within the operating room. The large capacitor and primitive insulation on the heating pads would not be very safe for a patient or surgeon.

Results and Discussion

I. Data Analysis

As previously described the Dexon sutures did not successfully fuse when exposed to the heating element. The Vicryl sutures, however, did fuse together creating a reliable suture closure method. Since the Dexon sutures did not fuse, further testing and research was conducted using only the Vicryl sutures.

Both the 0-gauge and 2-0 gauge Vicryl sutures fused at 92.22°C had a tensile stress comparable to the tied sutures of the same gauge. Both the 0-gauge and 2-0 gauge Vicryl sutures fused at 73.89°C had a lower tensile stress than the sutures fused at 92.22°C or the tied sutures. The results from the tensometer are summarized in Figure 7. Missing data was a result of the tensometer not functioning properly or the suture slipping out of the clamps. The average tensile strength across all three trials was also calculated and is listed in Table 2.