Introduction
A surgical site infection (SSI) is defined as an infection that occurs at a site of a prior surgical procedure.1 Surgical site infections are an inherent risk of any surgical procedure due to a compromise of the host-protective barrier.2 Although the overall health and economic burden of SSI in dogs and cats have not yet been quantified, SSI can lead to increased patient morbidity and mortality, a need for revision surgery, prolonged hospitalization, and client frustration and can impart a significant economic burden.3,4
The risk of SSI can be summarized into 3 main components: bacterial virulence, resistance of host, and dose of bacterial contamination.1,5 The most important factor influencing infection risk is the amount of bacterial contamination, whereby > 105 microorganisms/g of tissue markedly increases the risk of an SSI.5,6
Certain measures can be implemented to reduce the risk and impact of SSI,3 particularly efforts to reduce intraoperative contamination. Methods to reduce surgical site contamination include presurgical patient assessment (eg, treatment and elimination of pyoderma or any remote sites of infection before surgery, if possible), presurgical skin preparation, proper surgical site preparation, and aseptic technique.1,3 In small animal surgery, fecal contamination risk within the aseptic field during procedures located in the vicinity of the anus is a major concern. Certain additional steps have been suggested to reduce fecal-origin bacterial load, including the placement of anal purse-string sutures.7 At our institution, anal purse-string sutures are routinely placed preoperatively to reduce fecal contamination of surgical sites, particularly for soft tissue surgeries that involve the perianal region, such as anal sacculectomy, perineal herniorrhaphy, perineal urethrostomy, and tail amputation. In addition, these sutures are utilized for select orthopedic procedures involving the pelvic limbs such as tibial plateau leveling osteotomy, total hip replacement, and pelvic limb amputation via coxofemoral disarticulation.8–10 In our practice, approximately 336 anal purse-string sutures were placed during approximately 3,000 surgeries performed in 2019. We have observed that the efficacy of anal purse-string sutures to prevent intraoperative gross fecal leakage appears to be operator or technique dependent. To our knowledge, a specific anal purse-string technique has not been described in the veterinary or human literature, nor has there been validation of a consistent purse-string method to prevent gross fecal leakage.
The aim of the canine cadaveric study reported here was to compare 3 promising purse-string techniques to determine which technique was most consistently leak-proof. An additional goal was to determine a technique that employs the least amount of tissue bites to create a consistent leak-proof anal orifice closure. We hypothesized that there would be no difference in leak pressures among the purse-string techniques tested. We also postulated that an anal purse-string technique that employs the fewest number of bites and has small space between bites would sustain the highest leak pressures.
Materials and Methods
Cadaver preparation
Cadavers of 18 large-breed research dogs that weighed 25 to 30 kg and were euthanized by means of an IV infusion of pentobarbital sodium and phenytoin sodium (Euthasol) for reasons unrelated to the study were obtained. Cadavers were either used immediately after euthanasia or stored at a temperature < 4 °C prior to utilization, and all cadavers were used within 24 hours after euthanasia.
For each cadaver, fecal material was digitally evacuated from the rectum. After clipping hair from the caudal aspect of the ventral abdomen, a caudal ventral midline abdominal approach was sharply made by use of a No. 10 scalpel blade through the skin, subcutaneous tissues, and linea alba. The descending colon and rectum were isolated, and all palpable fecal contents were milked proximally. The descending colon was then transected 10 cm orad to the level of the pubis. The descending colon and rectum were flushed free of residual fecal material using a 14F red rubber catheter (Medline Industries Inc) and saline (0.9% NaCl) solution. Irrigation with saline solution was continued until the fluid exiting the rectum was grossly clear of fecal debris.
Anal purse-string suture placement
Monofilament 2-0 nylon on a 3/8-circle reverse cutting needle (Dermalon; Covidien Animal Health) was used for all anal purse-string suture constructs. All purse-string sutures were placed just beneath the dermis in a circumferential pattern in the cutaneous region of the anus by a single operator (JAY). Six square throws secured each anal purse-string suture. Each throw was tightened to a standard 9.8 N of force as measured with a tensiometer (Recreational Equipment Inc) attached to one free end of the suture and needle holders secured to the other. Three anal purse-string placement techniques were tested: 3 bites with 0.5 cm between bites (technique A), 5 bites with 0.5 cm between bites (technique B), and 3 bites with 1.0 cm between bites (technique C). To standardize the size and placement of suture bites among cadavers, marks corresponding to the number of bites (3 vs 5) and space between bites (0.5 vs 1.0 cm) were made on a circular template with a diameter of 2.0 cm (Figure 1). The appropriate designated template was centered over the anal orifice of the cadaver, and markings were made on the cutaneous region to denote and standardize placement of the purse-string bites (Figure 2). Tissue bites were oriented so the anal sac–neck region was avoided. In cadavers where this could not be avoided, bites were taken superficially to avoid incorporation of the anal gland orifice and neck. Each cadaver was only used once to test an assigned technique (ie, 6 cadavers were assigned to each technique).
Photographs of the 2-cm-diameter circular templates used to standardize suture bite size and spacing for 3 purse-string techniques (3 bites with 0.5 cm between bites [technique A], 5 bites with 0.5 cm between bites [technique B], and 3 bites with 1.0 cm between bites [technique C]) evaluated in a canine cadaveric study conducted to compare resistance to anal leakage among the techniques and identify the technique that required the fewest tissue bites to achieve a consistent leak-proof orifice. The study involved the use of 18 cadavers of large-breed dogs weighing between 25 and 30 kg that were euthanized for reasons unrelated to the study. Thus, there were 6 replicates/technique. The black shaded areas represent the spaces between suture bites.
Citation: American Journal of Veterinary Research 83, 3; 10.2460/ajvr.21.03.0050
Photographs that depict placement of the technique A circular template over the anus of a canine cadaver (A) and the template-guided markings placed on the cutaneous tissue of the perianal region to ensure standardized suture placement for all technique A constructs (B). The template was centered over the anal orifice, and the arrows denote the orifices of the anal glands. The base of the tail is located dorsally in both photographs.
Citation: American Journal of Veterinary Research 83, 3; 10.2460/ajvr.21.03.0050
Pressure testing
A 9F red rubber catheter (Medline Industries Inc) was inserted aborad into the rectal stump to a depth of 4 cm. A microtip pressure transducer (Mikro-Tip catheter-transducer; Millar Instruments Inc) was introduced within the 9F catheter via a custom-made 3-way adapter (ie, a 3-way Y connector glued to a chamber with a screw-in attachment; Figure 3). This transducer was zeroed before each experiment and was connected to a data acquisition system (SonoLab; Sonometrics Corp). The open colonic stump was sealed with a Doyen clamp placed perpendicular to the long axis of the colon as close to the catheter as possible. A curved Rochester-Carmalt forceps was then placed perpendicular to the long axis of the colon, with the curved tips oriented parallel to the catheter to prevent leakage. An infusion line was connected to the 3-way adapter, and isotonic saline solution was delivered at a rate of 900 mL/h with an infusion pump (Harvard Apparatus). Testing was done with the cadaver positioned in right lateral recumbency with the tail positioned in line with the axis of the spinal column. Leakage pressure, defined as the pressure (in mm Hg) at which saline was first observed leaking from the anus, was recorded. Leakage was visible as a droplet or moisture at the exit site (ie, anus).
Photograph of the experimental testing setup used to measure leakage pressure for the 3 purse-string techniques assessed in the canine cadaveric study described in Figure 1.
Citation: American Journal of Veterinary Research 83, 3; 10.2460/ajvr.21.03.0050
Statistical analysis
Prior to study initiation, data from a pilot study in which leakage pressures were assessed for 3 purse-string constructs similar to the 3 techniques assessed in this study were used for a sample size–power calculation (G*Power Software version 3.1.9.2; Institute for Digital Research and Education). Results of that calculation indicated that evaluation of 6 dogs per group would be sufficient to detect a minimum difference of 30 mm Hg in leakage pressure between any 2 purse-string techniques with 80% power and 95% confidence (ie, α = 0.05).
For each technique (A, B, and C), the data distribution for leakage pressure was assessed for normality by means of the Shapiro-Wilk test. Although data were normally distributed for all 3 techniques, because of the small number of replicates (n = 6) for each technique, leakage pressure was compared among the 3 purse-string techniques by means of a nonparametric Kruskal-Wallis test followed by the Dunn multiple comparison test for post hoc pairwise comparisons. Results were reported as the median (interquartile [25th to 75th percentile] range [IQR]). Values of P < 0.05 were considered significant, and all analyses were performed with a commercially available statistical program (Prism version 8.1.0 for Windows; GraphPad Software).
Results
Results of the Kruskal-Wallis test revealed that the median leakage pressure differed significantly (P = 0.003) among the 3 purse-string techniques. The median leakage pressure for technique A (101 mm Hg; IQR, 35 to 131.3 mm Hg) was significantly (P = 0.007) greater than that for technique C (19 mm Hg; IQR, 14.3 to 25.3 mm Hg; Figure 4). The median leakage pressure did not differ significantly between techniques A and B (50 mm Hg; IQR, 32.5 to 65 mm Hg; P = 1.000) or between techniques B and C.
Dot plots of leakage pressures for the 3 purse-string techniques described in Figure 1. For each plot, the circles represent the leakage pressure observed for each of the 6 replicates within the technique, the middle horizontal line represents the median, and the lower and upper horizontal lines (whiskers) delimit the interquartile (25th to 75th percentile) range. See Figure 1 for remainder of key.
Citation: American Journal of Veterinary Research 83, 3; 10.2460/ajvr.21.03.0050
Discussion
Placement of an anal purse-string suture has been recommended in select small animal surgical procedures involving the perianal and pelvic limb regions to help reduce the risk of fecal contamination of the surgical site. It has been reported that 1 g of dry dog feces contains an average of 3.9 × 107 CFU of enterococci.11 Hence, fecal contamination of surgical sites close to the perianal and pelvic limb regions could significantly influence the degree of contamination and risk of an SSI. In human vaginal reconstructive procedures, gross contamination of the perineal and perianal area can lead to complications in healing, especially when grafts are used. One study7 showed anal purse-string sutures employed in vaginal reconstructive procedures was effective in significantly decreasing gross fecal contamination of the surgical site. In the same study,7 microbiological analysis revealed greater surgical site contamination by fecal flora in patients who did not receive an anal purse-string suture. We assumed, therefore, that a consistently secure anal purse-string technique could likewise limit contamination within surgical sites in the regions of the anus and pelvis in animals. As of yet, the use of purse-string sutures has not been demonstrated to directly reduce SSI risk in veterinary medicine. The purpose of the present study was to evaluate 3 anal purse-string techniques and to determine which technique resulted in the most consistent leak-proof closure.
The median leakage pressure for technique A (3 bites with 0.5 cm between bites) was significantly greater than that for technique C (3 bites with 1.0 cm between bites) but did not differ significantly from the median leakage pressure for technique B (5 bites with 0.5 cm between bites). The median leakage pressure did not differ between techniques B and C. These results lead to rejection of the null hypothesis that there would be no difference in leakage pressures among the 3 purse-string techniques. This study also aimed to assess a technique that utilized the fewest number of tissue bites in the anal orifice to produce the most consistently leak-proof anal closure. Results indicated that the leakage pressure did not differ between techniques with an interbite space of 0.5 cm regardless of whether the purse-string suture consisted of 3 or 5 bites. However, when the purse-string suture consisted of 3 bites, the leakage pressure was greater for constructs with an interbite space of 0.5 cm versus 1.0 cm.
In the present study, the median leakage pressure was 101 mm Hg (IQR, 35 to 131.3 mm Hg) for technique A, 50 mm Hg (IQR, 32.5 to 65 mm Hg) for technique B, and 19 mm Hg (IQR, 14.3 to 25.3 mm Hg) for technique C. To our knowledge, there are no other studies reporting leakage pressures of anal purse-string sutures in the human or veterinary literature. In dogs, the resting baseline intraluminal colonic and rectal pressures reportedly range from 2 to 8 mm Hg,12,13 which is less than the median leakage pressure observed for any of the 3 purse-string suture techniques assessed in the present study. Therefore, any of the 3 purse-string techniques tested in this study should be able to resist physiologic colonic and rectal pressures at rest. In a study14 of 4 dogs utilizing the dynamics of a biologic, continent colostomy, mean maximal pressures immediately before bowel emptying ranged from 21 to 64 mm Hg. The median leakage pressures observed for techniques B and C of the present study overlapped the mean maximal colonic pressures prior to bowel emptying reported in that other study14 (ie, the leakage pressure was within the range of colonic pressure prior to bowel emptying for 4/6 replicates for technique B and was below the mean minimum colonic pressure prior to bowel emptying for 4/6 replicates for technique C). Thus, there may be risk of gross fecal leakage with technique B or C. The leakage pressure for 4 of the 6 replicates for technique A was well above the reported maximal colonic pressure prior to bowel emptying in that other study,14 which suggested that it may be preferable over techniques B and C. The leakage pressure for one of the experimental constructs in technique A of the present study was below the mean minimal colonic pressure prior to bowel emptying in that other study.14 This may have been a technical error during placement of sutures or tensioning of the throws when the knot was tied. One operator performed all experimental constructs to minimize interconstruct variation, and that construct may have been an outlier. The presence of an outlier in this study demonstrated that this procedure should be performed with due attention and precaution. Further investigation to reduce variation in pressures when this technique is carried out should also be considered in future studies.
Prior to commencement of the study, pilot testing was conducted to develop our methods and refine the study design. Methods were chosen on the basis of results of the pilot testing. Differences in suture location (anocutaneous vs cutaneous region) were assessed to determine which technique resulted in higher leak pressures. Placement of anal purse-string sutures with variable size and spacing of the needle bites in the anocutaneous zone resulted in consistently lower leak pressures, so it was decided to employ sutures only in the cutaneous zone for the constructs of the present study. In addition, pilot testing determined that leak pressure was unaffected when multifilament (Polysorb; Covidien Animal Health) or monofilament nonabsorbable suture material was utilized for the purse string, so a smooth monofilament suture was chosen to help reduce trauma from suture chatter during tightening. The mean ± SD straight pull failure load is 39.96 ± 0.85 N for commercially available 2-0 monofilament nylon.15 During the pilot study, it was noted that suture failure occurred when > 11.8 N of tension was applied while the square throws were tightened. Monofilament 2-0 nylon on a 3/8-circle reverse cutting needle was used for all anal purse-string constructs of the present study because that is the type of suture commonly used for this purpose at our institution and it was the same size as the suture used in a study involving human patients.7
Limitations of this study included the use of cadavers, which may not represent physiologic conditions in vivo. Cadavers have no anal sphincter tone or gastrointestinal peristalsis. However, when anal purse-string sutures are placed in live dogs under general anesthesia, anal tone would also be expected to be minimal or absent. Because cadavers have no anal tone, they likely represent a model most likely to leak, compared with live dogs. An additional limitation to this study was that the techniques assigned to the canine cadavers were not randomized. This project was essentially designed as a pilot study, hence randomization was not performed; however, techniques were standardized using the preformed templates. Randomization is recommended in future studies.
Placement of anal purse-string sutures is technically simple with no documented additional risk to veterinary patients and has not been associated with adverse effects in human patients.7 It would have been desirable to measure the length of time required to complete each purse-string technique because, in a clinical setting, these sutures are usually placed when the patient is under general anesthesia. In addition, microscopic contamination was not assessed given that this was a cadaveric study. Hence, future clinical studies on live patients are warranted. A study design to consider would be obtaining a swab of the perineal region for culture after clipping and aseptic preparation and a postprocedural swab prior to undraping of the patient to determine the degree of microscopic contamination in live patients undergoing a particular procedure.
Results of the present study suggested that placement of anal purse-string sutures resisted leakage at physiologic colonic pressures, regardless of the technique used. Technique A (3 bites with 0.5 cm between bites) is recommended over the other 2 techniques because it used the fewest tissue bites and had the highest resistance to leakage.
Acknowledgments
The authors have no conflicts of interest to declare. The authors thank Dr. Sangeeta Rao for assistance with data and statistical analysis.
References
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