Profiling surgical staplers: effect of staple height, buttress, and overlap on staple line failure

Surgery for Obesity and Related Diseases 4 (2008) 416 422 Original article Profiling surgical staplers: effect of staple height, buttress, and overlap on staple line failure Carlos M. Mery, M.D., M.P.H. a, Bilal M. Shafi, M.D., M.S.E. a, Gary Binyamin, Ph.D. a, John M. Morton, M.D., M.P.H. b, Michael Gertner, M.D. a, * a Biodesign Surgical Innovation Program, Stanford University, Stanford, California b Department of Surgery, Stanford University, Stanford, California Received May 13, 2007; revised October 5, 2007; accepted November 15, 2007 Abstract Keywords: Background: Few studies have been designed to assess the performance of surgical staplers. In this study, we analyzed the effect of staple height, buttressing, and overlapping of staple lines on staple line failure. Methods: Staple lines created on fresh porcine small bowel segments ex vivo were tested for leak pressure by insufflating air into the bowel under water and recording pressure at failure. Three separate experiments were done and included staple height (white, 2.5 mm, n 16; blue, 3.5 mm, n 16; green, 4.1 mm, n 16; one half of them buttressed); the absence (n 12) or presence (n 12) of an overlap in 3.5-mm staple lines; and the absence (n 14) or presence (n 11) of buttresses in 3.5-mm overlapping staple lines. Data are reported in median values and ranges; nonparametric tests were used for data analysis. Results: In the porcine small bowel, leak pressure was related to staple height; green loads had the worst profile (35 mm Hg, range 19 105) compared with the blue (79 mm Hg, range 9 177), and white (108 mm Hg, range 28 280) loads (P.006). Buttressing uniformly improved leak pressure for all staple loads (P.0001). No significant difference was found between lines with overlapping (59 mm Hg, range 32 121) and those without (42 mm Hg, range 22 75; P.162). Buttressing also improved the leak pressure of overlapping staple lines from 65 mm Hg (range 47 121) to 93 mm Hg (range 75 187; P.0014). Conclusion: Great variability was found in the leak pressures among the different applications of the same stapler. Staple height is an important determinant of leak pressure. The presence of an overlap did not affect leak pressure; in fact, a trend toward improvement was seen with overlapping staple lines. Buttressing improved all types of staple lines. (Surg Obes Relat Dis 2008;4:416 422.) 2008 American Society for Metabolic and Bariatric Surgery. All rights reserved. Stapler; Staple height; Buttress; Overlap; Crotch; Staple line; Leak; Leak pressure The relentless desire to create secure and reliable gastrointestinal anastomoses in a simpler fashion has sparked much of the technology development in surgery. Stapling has become commonplace in today s surgical procedures after its introduction almost 1 century ago [1]. However, Presented at the 24th Annual Meeting of the American Society for Bariatric Surgery, June 11 16, 2007, San Diego, California *Reprint requests: Michael Gertner, M.D., Biodesign Surgical Innovation Program, James H. Clark Center, Stanford University, 318 Campus Drive, Room E100, Stanford, CA 94305. E-mail: mgertner@stanford.edu only a few studies have reported on the performance of surgical staplers and provide a roadmap of how to best use them. Surgical staplers are expected to provide tissue approximation and hemostasis without causing ischemia or tearing of the tissue. The adequate performance of these instruments relies on achieving an optimal compression force that allows for this approximation but at the same time does not produce excessive tissue shearing and injury. On firing of the stapler, individual straight metal staples travel through the target tissue until they hit a hard anvil on the opposite side that bends the staple into a B position. The degree of 1550-7289/08/$ see front matter 2008 American Society for Metabolic and Bariatric Surgery. All rights reserved. doi:10.1016/j.soard.2007.11.008

C. M. Mery et al. / Surgery for Obesity and Related Diseases 4 (2008) 416 422 417 bending depends on the relationship between the height of the staple and the thickness and mechanical properties of the tissue. A short staple placed on a thick and firm tissue might not reach the anvil and would not conform into an adequate B structure. This is analogous to using a paper stapler on more sheets of paper than can be handled by that particular stapler. In contrast, a long staple placed on a thin piece of tissue will not provide the optimal compression needed for tissue approximation and hemostasis. To overcome this issue, staple cartridges with different heights are available for surgical staplers. However, guidance has been lacking for the surgeon as to which particular cartridge should be used for different applications. It is common practice for the manufacturers of surgical staplers to specify in their package inserts that particular cartridges should be used only for tissues that can comfortably compress to a particular thickness [2,3]. However, surgeons are not equipped with knowing what the final compressible thickness of a tissue will be before applying the stapler. The use of surgical staplers is exponentially increasing beyond their original intended use despite the lack of thorough research. Staplers are being applied to large blood vessels, adipose tissue, and thick organs such as the stomach; long and multiple staple lines are common practice; and staplers are now used as much in laparoscopy as in open procedures. A method that is routinely practiced but has not been necessarily studied or sanctioned by stapler companies is the so-called crotch or overlapping staple lines, in which a second staple line is placed over the first to create a longer staple line. Concern has been raised that overlapping staple lines used for the creation of the gastric pouch in Roux-en-Y gastric bypass could lead to devascularization of the tissue, which is to be avoided. Furthermore, repeated firings through previous staple lines might lead to further dulling of the knife blade in the case of linear staplers that use the same blade between cartridges. However, the specific effect of overlapping on the performance of staple lines is still unknown. Recently, buttresses or thin sheets made of different materials have been placed on 1 or both sides of the tissue to be stapled in an attempt to make the staple lines more uniform and to provide additional support, apposition, and hemostasis. However, their specific effect in the short- and long-term performance of staple lines and stapled anastomoses is still unclear. Despite all these efforts, anastomotic leaks are one of the most feared complications in gastrointestinal surgery, affecting.3 4.2% of patients undergoing Roux-en-Y gastric bypass [4 6] and 2.5 19% of patients undergoing colorectal resection [7 11]. A basic understanding of how different factors within surgical stapling affect the likelihood of staple line failure is crucial to minimizing the risk of anastomotic leaks. The goal of this study was to assess the effect of the staple height, buttressing, and presence or absence of overlapping staple lines on staple line failure using an ex vivo small bowel porcine model. Methods The study included three separate experiments aimed at assessing the effect of different variables on the leak pressure of a staple line: (1) staple height and buttressing of the staple line, (2) the presence of overlapping staples in the staple line, and (3) buttressing of overlapping staple lines. Specimens Fresh porcine small bowel was used for all experiments. The specimens were obtained from animals that had been killed after approved nongastrointestinal research studies. The specimens were either used immediately after retrieval or within 24 hours of refrigeration at 4 C. Because of the potential for uncontrolled variability in the tissue thickness, an attempt was made to use the same segment of small bowel for each experiment, and the different interventions included in each experiment were alternated on the same piece of bowel. Measurement All experiments were performed in the same manner (Fig. 1). A segment of small bowel, approximately 40 50 cm in length, was attached on 1 side (proximal) to a piece of tubing. A staple line with the desired characteristics was created on the distal end of the bowel. The distal portion of the bowel was immersed in a container filled with water. Air was then slowly introduced into the proximal bowel with flexible tubing and a handheld inflation bulb. The pressure in the system was continuously monitored with a pressure gauge (Omega Engineering, Stamford, CT). The pressure reading was recorded when air bubbles were first identified on the immersed staple line (leak pressure) or on the surface of the bowel itself (bowel burst pressure), according to 2 observers. After measurement, the bowel was deflated, and the next staple line was created proximal to the previous staple Fig. 1. Experiment setup for measurement of leak pressure from porcine small bowel.

418 C. M. Mery et al. / Surgery for Obesity and Related Diseases 4 (2008) 416 422 line or site of bowel burst. A segment of bowel was used until the length of bowel reached approximately 10 cm. Experiment 1: staple height and buttressing of staple line The goal of experiment 1 was to assess the effect on leak pressure of different staple heights (2.5, 3.5, and 4.1 mm) and the application of a buttress to the staple line. All staple lines were created with the use of an Ethicon Endo-Surgery Endopath ETS-Flex 45-mm linear stapler (Ethicon, Cincinnati, OH) for laparoscopic procedures. Staple lines were created with 3 different staple loads: (1) white (2 triple rows of 2.5-mm staples; n 16); (2) blue (2 double rows of 3.5-mm staples; n 16); and (3) green (2 double rows of 4.1-mm staples; n 16). A Seamguard buttress (W.L. Gore and Associates, Flagstaff, AZ) was applied to one half of the staple lines created with each of the different staple loads. Experiment 2: presence of overlapping staples in staple line Experiment 2 was designed to compare the leak pressure of a continuous staple line across the bowel with that of a staple line created by two separate staple firings that overlapped with each other. All staple lines were created with the use of an Ethicon Endo-Surgery Endopath ETS-Flex 45-mm linear stapler for laparoscopic procedures with a blue load (2 double rows of 3.5-mm staples). In the experiment, simple staple lines (n 12) were alternated with overlapping staple lines (n 12). For staple lines that overlapped, the first staple line was created at a 45 angle from the longitudinal axis of the bowel. The second staple line was then created at an inverse angle with respect to the first, creating a V shape. The leak pressure was measured and recorded as specified for experiment 1. The site of the leak on the staple line was also recorded. Experiment 3: buttressing of staple lines in addition to overlap Experiment 3 was designed to evaluate the effect of buttressing on overlapping staple lines. For that purpose, overlapping staple lines were created as specified in experiment 2 with the use of an Ethicon Endo-Surgery Endopath ETS-Flex 45-mm linear stapler for laparoscopic procedures with a blue load (2 double rows of 3.5-mm staples). Staple lines without buttressing (n 14) were alternated with staple lines with a Seamguard buttress (n 11). The leak pressure was measured and recorded as specified above. Statistical analysis Leak pressures (in millimeters of mercury) are reported as the median values and ranges. For experiment 2, which included 2 variables, the groups were stratified and analyzed accordingly. Leak pressures for each group were compared using the nonparametric Mann-Whitney U and Kruskall- Wallis tests. In the few instances in which the bowel burst before a leak occurred, the burst pressure was used as the leak pressure (the number of data points affected has been reported with each analysis). To minimize the potential for bias from this phenomenon, a time-to-event analysis was also used. Kaplan-Meier curves and log rank tests were used to analyze the staple line failure rate using pressure as the time variable and therefore creating a pressure-tofailure analysis. When the bowel burst proximal to the staple line before leaking from the staple line, the data points were censored at the particular burst pressure. All analyses were performed with JMP, version 6.0, for Windows (SAS Institute, Cary, NC). P.05 was considered statistically significant. Results Experiment 1: staple height and buttressing of staple line The effect of the staple height and buttressing was assessed by creating 48 staple lines (leak pressure, median 69.1 mm Hg, range 1.9 280). Bursting of the bowel before the staple line leak occurred in 6 cases (2 blue loads with buttressing, 1 white load without buttressing, and 3 white loads with buttressing). The leak pressure was significantly related to the type of staple load (Table 1). Overall, green staple loads had the lowest leak pressure (median 35.5 mm Hg, range 1.9 104.6) compared with the blue staple loads (median 79.4 mm Hg, range 9.3 177.5) and white staple loads (median 108.4 mm Hg, range 28.0 280.2; P.006). The use of a buttress on the staple line increased the leak pressure of all staple loads from 35.5 mm Hg (range 1.9 145.7) for staple lines without buttressing to 106.5 mm Hg (range 28.0 280.2) for those with buttressing (P.0001; Table 2). Fig. 2 depicts the failure rates of each staple load, with and without buttressing. It is clear that the green staple loads had a lower failure pressure than the blue and white loads. The addition of a buttress to the staple line shifted all curves Table 1 Leak pressure by type of staple load, stratified by presence or absence of buttress on staple line Buttress Load Median leak pressure (mm Hg)* P value Yes.008 White 121.4 (71 280) Blue 111.2 (28 177) Green 63.5 (34 105) No.0311 White 50.4 (28 145) Blue 59.8 (9 127) Green 18.7 (2 79) Data in parentheses are ranges. * Pressure at bowel burst for 6 cases (3 white load with buttress, 1 white load without buttress, 2 blue load with buttress).

C. M. Mery et al. / Surgery for Obesity and Related Diseases 4 (2008) 416 422 419 Table 2 Leak pressure by presence or absence of buttress on staple line, stratified by staple load type Load Buttress Median leak pressure (mm Hg)* P value White.0157 Yes 121.4 (71 280) No 50.4 (28 145) Blue.0209 Yes 111.2 (28 177) No 59.8 (9 127) Green.0046 Yes 63.5 (34 105) No 18.7 (2 79) Data in parentheses are ranges. * Pressure at bowel burst for 6 cases (3 white load with buttress, 1 white load without buttress, 2 blue load with buttress). to the right (improved failure pressure) but the differences among the staple loads remained. Experiment 2: presence of crotch in staple line A total of 24 staple lines were created to assess the effect of an overlapped row of staples on leak pressure. The median leak pressure was 49.5 mm Hg (range 22.4 121.4). Fig. 3. Comparison of leak pressure between staple lines with overlap and those without. No statistically significant difference was found between them. In no experiments did the bowel burst before failure of the staple line. As seen in Fig. 3, no significant difference was found in leak pressure between staple lines with an overlap (median 58.8 mm Hg, range 31.8 121.4) and those without (median 42.0 mm Hg, range 22.4 74.7; P.162). For those staple lines with an overlap, the leak occurred at the first staple line in 7, the second staple line in 3, and at the point of intersection in 2. Experiment 3: buttressing of overlapping staple lines To assess the effect of buttressing on the staple lines with an overlap, 25 staple lines were created using 2 staple firings for each. The median leak pressure was 84.1 mm Hg (range 46.7 186.8). A bowel burst occurred in 3 cases that were buttressed, all of them at a pressure 127 mm Hg. Buttressing significantly improved the leak pressure of the staple lines with an overlap, from 65.4 mm Hg (range 46.7 121.4) without buttressing to 93.4 mm Hg (range 74.7 186.8) with buttressing (P.0014). Discussion Fig. 2. Kaplan-Meier curves depicting time to failure for each staple load (Top) without buttressing and (Bottom) with buttressing. White and blue loads had better profile than green loads, as shown by displacement of failure curves to right. Buttressing improved profile of all staple loads. Surgical stapling instruments are being increasingly used for gastrointestinal anastomoses. Although it is recognized that stapled anastomoses are faster to create and lead to outcomes similar to those with hand-sewn anastomoses [12 14], the best technique and stapler profile to use remains unclear. It is also difficult to generalize the finding of these studies to other clinical scenarios. For example, it might be difficult to apply the results from a study of intestinal anastomoses to gastric anastomoses. The present study evaluated the different profiles of surgical staplers by measuring the leak pressure detected on an ex vivo porcine small bowel model. Our observations indicated that a great amount of variability in the leak

420 C. M. Mery et al. / Surgery for Obesity and Related Diseases 4 (2008) 416 422 pressures is present among the different applications of the same stapler. Second, in this particular model, the white loads (2.5-mm staple height) had a better leak pressure profile than did the blue loads (3.5 mm), which in turn had a better profile than the green loads (4.1 mm). Third, having overlapping staple lines did not affect the leak pressure. Finally, buttressing improved the leak pressure for all staple loads and for staple lines with and without an overlap. The great variability in leak pressures obtained with the different stapler applications, even those with the same stapler profile, is an interesting observation that could potentially be explained by the subtle differences in the thickness of the various areas of the same bowel or differences in the amount of compression applied by the stapler on the tissue. The tissue thickness and elasticity affect the shear and stress properties of the tissue, leading to differences in the strength of the staple line [4]. Similarly, an ideal compression pressure seems to exist for tissues of different thicknesses. Studies conducted at the Scientific Institute for Surgical Devices and Instruments in Russia in the 1960s showed that the optimal compression pressure for surgical stapling was 8 g/m 2 for gastric tissue and 6 g/m 2 for the esophagus and intestines [4]. Greater pressures can lead to tissue disruption, and lower pressures can be insufficient to appose the tissues adequately. Another possibility to explain some of the variability in leak pressures could be the rate of inflation of the bowel during the experiment. It is known that the rate of inflation of the bowel can slightly affect the pressure at which a piece of bowel leaks [15]. To minimize this confounding factor, the inflation pressure in these experiments was kept relatively constant by slowly compressing the bulb once every second until the staple line failed. As expected, the staple height proved to be an important determinant of leak pressure. In the present study, a shorter staple height (2.5 mm) was associated with improved leak pressure. It is clear that the tissue thickness should be taken into account when selecting the appropriate staple height. If the staple is longer than ideal, the tissue might not appose adequately, leading to a greater likelihood of leakage. In contrast, shorter staples might not conform into a full B morphology, leading to inadequate bowel closure, and their overcompression could cause undue tissue trauma and ischemia. The instructions of use for the different surgical staplers specify the appropriate staple height to use for different tissue thickness. For example, blue staple loads (3.5 3.8-mm staple height) should only be used if the tissue can be comfortably compressed to a 1.5-mm thickness and does not compress to 1.5 mm [2,3]. Obviously, most surgeons do not have the means to measure the end thickness of the tissue to be compressed. Until a better system to measure the tissue thickness or to anastomose tissue with less dependency on tissue thickness is available, clinical judgment is needed to determine whether a tissue is thick enough to warrant a green staple load (4.1 mm) or thin enough to require a white staple load (2.5 mm). For example, some laparoscopic bariatric surgeons use white loads for the small bowel and blue loads for the stomach. Buttressing with expanded polytetrafluoroethylene (Seamguard) significantly improved the leak pressure of all staple loads. This observation is in agreement with several studies that have compared plain staple lines with reinforced ones. Animal studies have shown that reinforcement of the staple line with either bovine pericardium [16] or porcine small intestinal submucosa [17,18] increases the burst pressure of the staple lines in the bowel or stomach. However, buttressing can interfere with visualization and repeated staple fires. Although some clinical studies have been performed to assess the clinical value of staple reinforcement, they have been inadequately powered to assess the effectiveness in reducing the occurrence of anastomotic leaks because of the relatively rare occurrence of leaks. In a retrospective study comparing 250 patients who underwent laparoscopic gastric bypass with bovine pericardium-reinforced staple lines with 100 patients who underwent the same surgery without staple reinforcement, no patients with reinforced staple lines had a postoperative leak compared with 2 of the 100 patients without reinforcement [19]. All the patients without reinforced staple lines had undergone gastric bypass historically earlier than those with reinforced staple lines, making the learning curve a possible explanation. Buttressing has also been shown to be associated with less bleeding from the staple lines [20]. The observations from the present study and other studies have suggested that reinforcement of the staple line might increase the range of tissue thicknesses that can be adequately stapled with a particular staple height. For example, in the present study, buttressing brought the leak pressures of staple lines performed with a green staple load (4.1 mm) to a range that might be acceptable for the tissue used. These data could be potentially translated into the standardization of staple heights, using buttresses to compensate for the differences in the tissue thickness. The long-term effects of buttressing in terms of mechanical strength and leak rates are still unknown. Although buttresses might allow for improved intraoperative results, their effect on healing during the ensuing days is equally, if not more, important to patient outcomes. Healing is affected by pressure and the buttress material. In a recent study, Hope et al. [21] used a porcine model to compare the burst pressures with and without buttressing in explants of both freshly created and 1-week-old anastomoses. Even though the initial burst pressures were greater with the use of a buttress, the burst pressures at 1 week were significantly lower than those without buttressing. How these findings relate to the occurrence of mechanical or ischemic leak rates is still unclear. More animal and clinical studies are warranted to determine the real effects of buttressing on longterm mechanical strength, blood supply, and, ultimately, the incidence of leaks.

C. M. Mery et al. / Surgery for Obesity and Related Diseases 4 (2008) 416 422 421 A common belief is that the application of a second staple line with the consequent creation of an overlap might weaken the staple line [4]. However, the results of the present study showed that the leaking pressure of staple lines with an overlap was not significantly different than those without an overlap. The addition of a reinforcing buttress to staple lines with an overlap had an effect similar to that for staple lines without it, significantly improving the leaking pressure. What exactly constitutes an acceptable leak pressure range in the acute setting remains unknown. In a study in which Burkitt and Donovan [22] measured the intraluminal colonic pressure in various postoperative patients, the measured colonic pressures were as great as 100 mm Hg at 8 days postoperatively. However, only 8% of the recordings were 10 mm Hg on the first 3 days after surgery, the period in which the mechanical strength and leak pressure might be most relevant. A recent study measuring intraluminal pressure by way of a nasogastric tube in patients with bowel dilation after Roux-en-Y gastric bypass showed that pressures can reach 22 mm Hg in these patients and 37 mm Hg in a porcine experimental model [23]. Thus, to avoid a leak, a staple line must be able to mechanically withstand these pressures in the early postoperative period. In addition, these anastomoses could be further stressed by nausea and emesis, increasing the need for a safe and reliable staple line. The present study was designed to study the mechanical strength of different staple lines. Several models are available to test the mechanical strength of an anastomosis or staple line. The most common ones include measuring the burst pressure, determining the bursting wall tension, and measuring the breaking strength. The bursting wall tension model is similar to the burst pressure model (used in the present study) but calculates the specific wall tension, taking into account the radius of the bowel, according to Laplace s law. The breaking strength model uses a system in which the bowel is pulled apart from both sides of an anastomosis, measuring the force needed to disrupt it. The simple burst pressure model was chosen for this study because both the bursting wall tension and the breaking strength models do not appear to provide any additional information, especially during the first week postoperatively [24]. Furthermore, even though some studies have suggested that the breaking strength might provide different information about an anastomosis than bursting strength [25], the bursting method is a multiaxial test that better simulates the physiologic conditions [15]. Not all anastomotic leaks occur because of mechanical failure. Leaks due to mechanical failure tend to occur mainly in the first several days postoperatively [4]. Once the mechanical strength of the anastomosis increases after 3 4 days [24], ischemia and material biocompatibility might play a more significant role. The results of the present study are relevant for that initial period when mechanical strength is a determining factor in anastomotic failure. However, even though greater compression (and lower staple height) might improve the mechanical profile of the staple line, the effects of these variables on the blood supply to the anastomosis were not addressed by this study and represent 1 of its major limitations. Microangiographic studies of sutured bowel anastomoses have shown that sutures placed closely and tied with moderate tension provide adequate mechanical strength without compromising the blood flow [26]. Itis therefore possible that a mid-staple length, perhaps with buttressing, could provide an adequate compromise between mechanical strength and staple line blood flow. Another potential limitation of the present study was that, because dead porcine small bowel was used, the results might not be generalizable to live human tissue or to tissues with different thickness profiles such as the colon or stomach. Also, the model might have some degree of subjectivity owing to the use of an analog system to measure the leak pressure, even though 2 observers were used. The results of this study have provided an initial framework to analyze the effect of different staplers on the mechanical strength of a staple line. Future studies are warranted to assess their effect on the strength of tissues with different characteristics and their in vivo effect on the blood supply of the anastomosis. Ultimately, it is only with careful experimentation that the optimal stapling instruments for different clinical scenarios will be defined and the rate of anastomotic leaks decreased. Conclusion The variability in leak pressures among the different applications of the same stapler is great. Staple height is an important determinant of leak pressure. In our study, the presence of an overlap did not affect leak pressure. In fact, a trend toward improvement was seen with overlapping staple lines. Buttressing improves the leak pressure for all types of staple lines. Future studies are warranted to better define the ideal stapler to use for different clinical scenarios. Disclosures The authors have no commercial associations that might be a conflict of interest in relation to this article. 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