Comparison between tungsten and steel polypectomy snares: evaluation of depth of colonic thermal wall injury in a pig model

• Introduction
• Materials and methods
• Animal model and study protocol
• Assessment of the thermal properties of the steel and tungsten snares
• Statistical method
• Results
• Test I
• Test II
• Discussion
• Power output of generator
• Current mode
• Electrosurgical device
• References

Background and study aims: Thermal injury of the colonic wall during polypectomy may induce complications such as bleeding and perforation. To date, the role of the snare material in these injuries has not been examined. The aim of this study was to evaluate the depth of colonic post-polypectomy thermal wall injury induced by tungsten and steel endoscopic snares, in an effort to reduce electrosurgery-related complications.

Materials and methods: This was a single tertiary center experimental study in a porcine model. A total of 90 polypectomies where performed in three live pigs under general anesthesia, using both steel and tungsten snares by cut, coagulation, and blend current modes. The pigs were then euthanized and their colons examined histologically.

Results: Steel snares induced significantly deeper tissue injury than tungsten snares in the pure cut mode (Pearson χ ² = 6.136, P = 0.013). The ordinal logistic regression analysis showed that the current mode and snare material were significantly associated with the ordinal score for the depth of injury. Thus, cut mode was positively associated with a lower score and coagulation mode with a higher score. In addition, tungsten was significantly associated with a lower depth of tissue injury.

Conclusions: Due to its inherent electrical properties, tungsten is very well suited for manufacture into electrosurgical endoscopic devices. Thus, tungsten snares may be advantageous for routine use in endoscopic polypectomy, although further studies are needed to confirm these promising findings in human patients.

Introduction

Endoscopic polypectomy is a common interventional procedure performed using endoscopic snares and electrosurgical current. However, the current applied by an endoscopic electrosurgical generator to the intestinal wall causes thermal injury, with the potential complications of bleeding, post-polypectomy syndrome (transmural burn), and colonic perforation [1] [2] [3] [4].

To reduce the rate of electrosurgery-related injury, the effects of several variables have been investigated, such as the size and shape of the snare and the type of current applied (pure cut, coagulation, blend). However, very few studies have examined the role played by the different snare materials.

Recently, a new tungsten endoscopic snare became commercially available and, to our knowledge, it is the first of its kind available in Europe. The physical and electrical properties of tungsten make it very well suited for manufacture into electrosurgical devices. Therefore, the aim of this study was to compare tungsten and steel endoscopic snares with respect to the depth of the colonic wall injury resulting from electrosurgical endoscopic polypectomy. In a porcine model, the two snare types were tested with the three standard current modes – pure cut, coagulation, and blend.

Materials and methods

Animal model and study protocol

The study was conducted on three 6-month-old female pigs (Large White), each weighing approximately 40 kg. The animals were placed under general anesthesia and then underwent colonoscopy, performed using a double-channel colonoscope (EC 450 D5; Fujinon, Saitama, Japan).

Current was supplied by a single electrosurgical generator (MSC 601; Söring, Quickborn, Germany) operating in three different modes – pure cut, coagulation, and blend. Electrical power for all modes was 60 J at a setting of 30 W, and was automatically stopped after 2 seconds by a timer-switch (XTM 11 – 3; Tectronix, Richardson, Texas, USA). To simulate the stalk of a polyp, the colonic mucosa was gathered into the snare by a grasping forceps, after which the snare was closed. The snare diameter was adjusted to 5.0 mm by means of a stopper on the snare handle.

To standardize the polypectomy, electrosurgical current was allowed to flow for 2 seconds through the snare, followed by release of the tissue without removal of the mucosa.

For each of the three different current modes, 15 polypectomies were randomly performed with either a steel or a tungsten endoscopic snare (Innoflex 36 LODM and IAP 24 LODMT, respectively; Innovamedica, Milan, Italy), both of which are commercially available. All snare devices were carefully chosen to ensure that their size, shape, and profile were the same regardless of the current mode selected for each procedure.

Electrosurgical current was applied to a total of 90 separate sites, and each polypectomy site was identified by hemoclips color-coded for snare type and current mode. Each experimental site was sufficiently distanced from the others to avoid overlap between adjacent sites.

After all electrosurgical applications had been completed, the anesthetized animals were euthanized by an intravenous injection of embutramide, mebezonium iodide, and tetracaine hydrochloride (Tanax; Intervet Italia, Segrate, Italy). The colon was then removed and underwent histological examination by a single pathologist who was blinded to the details of the procedures.

The results of the histological examination were converted into an ordinal score, ranging from 1 to 5, according to a system for scoring the depth of tissue injury [5]. This system has greater clinical relevance than simple measurements of depth injury in millimeters. The five levels of injury were defined as follows.

1. Injury involves the submucosa but does not reach the muscularis propria.

2. Injury involves the muscularis propria but is limited to within the internal circular muscle layer.

3. Injury involves the muscularis propria and extends to the external longitudinal muscle layer.

4. Injury involves the muscularis propria, including heat-induced degeneration of the entire thickness of the external longitudinal muscle layer.

5. Perforation of the serosa.

The experimental protocol was approved by the Ethical Animal Use and Care Committee of the University of Naples “Federico II.”

Assessment of the thermal properties of the steel and tungsten snares

To better understand the different behaviors of the snare materials, an ex vivo model was designed using porcine colon. Thermal measurements were made under the same conditions as described above and supervised by the Department of Aerospace Engineering of the University of Naples “Federico II.”

For these thermal measurements, infra-red thermography was carried out according to a validated method using a thermocamera (ThermaCam SC6000; Flir Systems, Wilsonville, Oregon, USA) working in the infra-red range of 8 – 9.2 µm [6] [7]. The sensitivity was 20 mK at ambient temperature, thus enabling the detection of small temperature variations.

To characterize the heating of the steel and tungsten snares, two tests were carried out for each one:

• Test I, with the open snares laid on the colonic mucosa

• Test II, with the partially closed snare catching the polyp stalk. This test was chosen because the snare wire is very thin (0.5 mm), such that it disappears between the tissue folds when the snares are totally closed. Therefore, the tests were performed with the snare simply positioned, rather than completely closed, ensuring that good contact was established with the tissue while the infrared camera recorded the initial heating of the snare.

For all tests, thermal images were acquired over time at 60 Hz.

Statistical method

For the statistical analysis, all observations were assumed to be independent as each polypectomy site was well separated geographically from the others.

A total of 90 histological assessments were scored (15 for each of the three different current modes with the steel and tungsten endoscopic snares, respectively).

Statistical comparisons among the three current modes for the same snare were performed using the Pearson chi-squared test. For the between-group analysis (steel vs. tungsten snares), statistical comparisons were performed separately for each current mode using the Pearson chi-squared test. Ordinal logistic regressions analysis was applied to estimate the relationships between an ordinal dependent variable (the five-point depth of tissue injury) and the current modes and snare material as primary predictor variables. Parameter estimates as well as Wald and confidence intervals (CIs) were also calculated.

Significance was defined as P < 0.05. The SPSS software package for Windows (release 12.0; SPSS Inc., Chicago, Illinois, USA) and GraphPad Prism version 5.01 for Windows (GraphPad Software, San Diego, California, USA) were used for the statistical analyses.

Results

[Fig. 1] is a graphical representation of the frequency of each score, expressed as a percentage for the different materials (i. e. steel and tungsten) for all current modes. In the within-group comparisons, the percentage of the depth of tissue injury significantly differed among the three different current modes (pure cut, blend, and coagulation) for both the steel snare (χ ² = 15.20, P  = 0.019) and the tungsten snare (χ ² = 16.24, P = 0.003). Between-group comparisons showed that the steel snare induced significantly deeper tissue injury than the tungsten snare in procedures performed with the pure cut mode (χ ² = 6.136, P  = 0.013). Using blend current mode the steel snare induced deeper tissue injury than the tungsten snare, but it just failed to reach statistical significance (χ ² = 5.040, P = 0.08). No difference was found between the two materials using the coagulation current mode (χ ² = 2.707, P = 0.258). In a single case, the steel snare resulted in a level 4 injury, using coagulation current mode.

Ordinal logistic regression analysis showed that current mode and snare material were significantly associated with the ordinal score for the depth of injury ([Table 1]). Thus, cut mode was positively associated with a lower score and coagulation mode with a higher score. In addition, tungsten was significantly associated with a lower depth of tissue injury.

In thermal determinations of the two materials with the above-described tests I and II, there was a quicker and more sudden rise in temperature, a better uniform standard temperature distribution, and a quicker drop in temperature with the tungsten snare compared with the steel snare.

Test I

[Fig. 2] shows the thermal images of the steel and tungsten snares, obtained in the unpowered and powered states. As the snare wires undergo non-uniform heating due to imperfect contact with the colonic mucosa, the evaluations focused only on zones of perfect contact ([Fig. 2 c, d]), for steel and tungsten, respectively.

To evaluate the temperature variation over time, the step-wise rise in the temperature of each snare was calculated. A comparison of the plots obtained for steel and tungsten ([Fig. 2 e]) shows that both snare types finally reached a temperature that was about 50 °C higher than the ambient value. However, tungsten (red plot) heated up faster than steel and cooled down faster as well.

In addition, a three-dimensional representation of the temperature distribution over an entire snare at several time points was plotted for each material ([Fig. 3 a, b]). The two plots clearly show that the temperature distribution was more homogeneous for the tungsten snare.

Test II

Thermal images taken at different time points during electrical power-on are shown in [Fig. 4]. The two temperature maps again clearly show that tungsten heating was faster, steadier, and more homogeneous than steel heating. The evolution of the temperature increase was also evaluated, with the variations in temperature plotted over time for each material. A comparison of the two plots ([Fig. 5]) revealed a well differentiated temperature increase for both snares; however, tungsten ([Fig. 5 b]) heating followed a more regular step-like pattern.

Discussion

In digestive endoscopic surgery, a specific and desired therapeutic effect is achieved by the balanced control of the interactions between three variables: the power output of the generator, the current mode, and the choice of device. A disturbance in this balanced relationship may result in surgical complications (i. e. hemorrhage or the so-called post-polypectomy syndrome) [8] [9]. The latter is marked by serosal inflammation and perforation, which occur in 0.1 % and 0.8 %, respectively, of patients undergoing colonoscopic polypectomy [2] [3] [8] [9].

Power output of generator

The power output of the electrosurgical generator is usually chosen on the basis of several factors (i. e. the endoscopist’s clinical and personal experience, the anatomical site of the intervention, the shape and size of the lesion to be treated, and the presence of co-morbidity), and it remains unchanged throughout the intervention.

Current mode

Several studies [3] [5] [9] have demonstrated a descending order of tissue injury using coagulation, blend, and pure cut current modes. In a Japanese study [5], tissue damage was five times deeper with the coagulation mode than with the blend mode and ten times deeper than with the pure cut mode.

Electrosurgical device

In polypectomy, the choice of the snare is determined by the shape, size, and thickness of the targeted lesion. The focus of most studies has been the manufacturing variables of the various snare types with respect to thermal wall injury (monowire or braided multiwire, insulated or non-insulated tip, flat, circular or rectangular section of the cutting wire, shape and size of the device). By contrast, very few studies have addressed the physical and electrical properties of the different snare materials. In terms of avoiding thermal injury to the intestinal wall, the most important of these properties are thermal conductivity, specific heat, electrical resistivity, and thermal volumetric capacity, as shown in [Table 2] for steel and tungsten.

Electrical resistivity measures how strongly a material opposes the flow of electric current. A low electrical resistivity and, complementarily, a high thermal conductivity are properties of a material that readily allows the movement of an electrical charge.

When a high-frequency current is passed from an electrosurgical generator through a polypectomy snare grasping a polyp, electrical resistance in the tissue generates heat. The high density of the current at the point of contact between the snare and the polyp tissue results in a steep rise in tissue temperature. When the temperature reaches 100 °C, the water in the cells vaporizes, the cells explode, and the tissue is cut. A snare with high electrical resistivity and low thermal conductivity hampers current flow during a polypectomy. Consequently, the temperature rise is slower, causing heat spread to the deep wall tissues and thereby their thermal injury.

As is evident in [Table 2], tungsten has a much higher thermal conductivity (more than ten times) and a lower thermal capacity than steel. This means that, compared with steel, tungsten heats up faster after the electrical power generator is switched on and cools down faster after it is switched off. Moreover, the temperature across the wire section of a tungsten snare is more uniform under heating and cooling conditions. Our study therefore demonstrates that due to its electrical properties, tungsten is very well suited to the manufacture of electrosurgical endoscopic devices.

In conclusion, a comparison of tungsten and steel snares under all three current modes used in polypectomy demonstrated that in tungsten snares the generated heat is not only homogeneously distributed, it is also well concentrated, allowing a more selective cut than that achieved with steel snares. This advantage, which is strictly a function of the inherent thermal and electrical characteristics of tungsten, results in a low rate of thermal-related complications when used in conjunction with either the cut or the blend current mode. These findings support the broad use of tungsten snares in endoscopic polypectomy, although further studies in human patients are needed to confirm these promising findings.

Competing interests: None.

Acknowledgments

The engineers Nicola Migliaccio and Bruno Cicatiello are gratefully acknowledged for their excellent technical support.

• References

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Corresponding author

• References

• 1 Macrae FA, Tan KG, Williams CB. Towards safer colonoscopy: a report on the complications of 5000 diagnostic or therapeutic colonoscopies. Gut 1983; 24: 376-383
  CrossrefPubMedSearch in Google Scholar
• 2 Waye JD, Lewis BS, Yessayan S. Colonoscopy: a prospective report of complications. J Clin Gastroenterol 1997; 15: 347-351
  PubMedSearch in Google Scholar
• 3 Nivatvongs S. Complications in colonoscopic polypectomy: lessons to learn from and experience of 1576 polyps. Am Surg 1988; 54: 61-63
  PubMedSearch in Google Scholar
• 4 Parra-Blanco A, Kaminaga N, Kojima T et al. Hemoclipping for postpolypectomy and postbiopsy colonic bleeding. Gastrointest Endosc 2000; 51: 37-41
  CrossrefPubMedSearch in Google Scholar
• 5 Chino A, Karasawa T, Uragami N et al. A comparison of depth of tissue injury caused by different modes of electrosurgical current in a pig colon model. Gastrointest Endosc 2004; 59: 374-379
  CrossrefPubMedSearch in Google Scholar
• 6 Meola C, Carlomagno GM. Recent advances in the use of infrared thermography. Meas Sci Technol 2004; 15: 27-58
  CrossrefPubMedSearch in Google Scholar
• 7 Meola C, Carlomagno GM. Application of infrared thermography to adhesion science. J Adhes Sci Technol 2006; 20: 589-632
  CrossrefPubMedSearch in Google Scholar
• 8 Parra-Blanco A, Kaminaga N, Kojima T et al. Colonoscopic polypectomy with cutting current: is it safe?. Gastrointest Endosc 2000; 51: 676-681
  CrossrefPubMedSearch in Google Scholar
• 9 Van Gossum A, Cozzoli A, Adler M et al. Colonoscopic snare polypectomy: analysis of 1485 resections comparing two types of current. Gastrointest Endosc 1992; 38: 472-475
  CrossrefPubMedSearch in Google Scholar