Comparative study of cylindrical vs circular ring magnets for colonic anastomosis in rats

Abstract

BACKGROUND

Magnetic compression anastomosis (MCA) offers a simple and reliable technique for inducing anastomoses at any point along the digestive tract. Evidence regarding whether the design of the MCA device influences the anastomosis effect is lacking.

AIM

To investigate any difference in the side-to-side colonic anastomosis effect achieved with cylindrical vs circular ring magnets.

METHODS

We designed cylindrical and circular ring magnets suitable for side-to-side colonic anastomosis in rats. Thirty Sprague-Dawley rats were randomly divided into a cylindrical group, circular ring group, and cylindrical–circular ring group (n = 10/group). Side-to-side colonic anastomosis was completed by transanal insertion of the magnets without incision of the colon. Operation time, perioperative complications, and magnet discharge time were recorded. Rats were euthanized 4 weeks post-operatively, and anastomotic specimens were obtained. The burst pressure and anastomotic diameter were measured sequentially, and anastomosis formation was observed by naked eye. Histological results were observed by light microscopy.

RESULTS

In all 30 rats, side-to-side colonic anastomosis was completed, for an operation success rate of 100%. No postoperative complications of bleeding and intestinal obstruction occurred, and the postoperative survival rate were 100%. The operation time, magnet discharge time, anastomotic bursting pressure, and anastomotic diameter did not differ significantly among the three designs (P > 0.05). Healing was similar across the groups, with gross specimens showing good anastomotic healing and good mucosal continuity observed on histological analysis.

CONCLUSION

This study found no significant difference in the establishment of rat side-to-side colonic anastomosis with the use of cylindrical vs circular ring magnets.

Key Words: Magnetosurgery; Magnetic compression anastomosis; Colonic anastomosis; Magnet; Rats

Core Tip: Magnetic compression anastomosis (MCA) is a new anastomosis approach that differs from suture and staple anastomosis. Multiple studies have demonstrated the feasibility of MCA in the digestive tract, but whether MCA establishment is affected by the shape of the magnets has not been determined. In this study, the effect of differently shaped magnets was examined in a rat model of side-to-side colonic anastomosis. The results showed no significant difference in the establishment of side-to-side colonic anastomosis with the use of circular vs cylindrical magnets. Moreover, the findings of this study support Yan-Zhang's Peripheral Decision Theory of digestive tract MCA.

INTRODUCTION

Magnetic compression anastomosis (MCA) is a new anastomosis mode, which differs from suture anastomosis and staple anastomosis in that the anastomosis is established by a magnetic compression technique with no foreign body retention. This leads to a better quality anastomosis effect, and current studies have shown that MCA can be effectively achieved at any point throughout the entire lumen of the digestive tract from the esophagus to rectum[1-5]. In addition, the need for anastomosis is not limited to the digestive tract, as some cases require vascular anastomosis[6,7] or ureteral anastomosis[8,9]. In some special cases, MCA combined with an endoscopic technique can be applied in place of open surgery or laparoscopic surgery for gastrointestinal anastomosis[10,11]. Therefore, to some degree, MCA can replace the traditional mode of digestive tract anastomosis. Accordingly, MCA has wide application potential due to the reliable anastomosis effect and potential for diverse clinical translations.

However, although MCA offers important advantages, it remains in the stage of technological innovation and clinical testing[12,13]. Researchers' interest continues to be focused on new technological innovations and case-by-case application of MCA for specific patients. Basic research studies in support of theoretical innovation of MCA are lacking. In the early stage, using the colonic magnetic anastomosis model in Sprague-Dawley rats, we established the Yan-Zhang's staging digestive tract MCA by analyzing anastomosis specimens obtained continuously and dynamically[14]. Such finding is valuable to the basic understanding of MCA and can provide key support for the construction of a magnetic surgical system.

In the existing literature regarding magnetic anastomosis, the shapes of magnets designed by researchers for digestive tract anastomosis include cylindrical[15,16], circular[17,18], oval shaped[19], and irregularly shaped[20-22]. The purpose for the design of diverse magnet shapes has been only for easy operative placement and easy discharge. However, whether the establishment of anastomosis between magnets is affected by differences in the designed shape of the magnets remains unknown. Moreover, evidence is lacking for regarding the type of magnet compression surface that will be most conducive to the establishment of anastomosis. Therefore, in the present study, we designed magnets of two different shapes, cylindrical and circular, and tested the influence of magnet shape on the anastomosis effect in a rat model of side-to-side colonic anastomosis.

MATERIALS AND METHODS

Experimental animals

A total of 30 Sprague-Dawley albino rats (n = 15 males; n = 15 females), weighing 200-250 g, were acquired from the Laboratory Animal Center of the Xi’an Jiaotong University (Xi’an, China) and used as experimental animals. The animals were acclimatized to laboratory conditions (23 °C, 50% humidity, 12-hour/12-hour light/dark cycle, and food and water provided ad libitum) for 1 week before use in experiments.

Ethical statement

The experimental protocol was approved by the Animal Ethics Committee of Xi’an Jiaotong University (license No. 2021-1534). The research protocol and all experimental procedures were conducted strictly in accordance with the Guidelines for the Care and Use of Experimental Animals issued by the Xi’an Jiaotong University Medical Center.

Study design

In this study, all rats were randomly divided into three groups: Cylindrical group, circular ring group, and cylindrical-circular ring group. The numbers of female and male rats in each group were ensured to be equal. In all rats, the colon was not cut open after the abdominal cavity was opened, and the daughter magnet (DM) and the parent magnet (PM) were placed into the proximal and distal ends of the colon successively through the anus. Once place, the DM and PM were attracted to each other. The operation time was recorded. After operation, the general state of rats, the time to magnet discharge, and the complications of intestinal obstruction were observed and recorded. The rats were euthanized 4 weeks after the operation, and the burst pressure and diameter of the anastomosis were measured. The formation of the anastomosis was observed by the naked eye and light microscopy.

Magnets

For use in this study, we designed a circular magnet with an outer diameter of 7 mm, an inner diameter of 4.2 mm, and a height of 2 mm. To simulate the bottom surface of a cylindrical magnet, the inside of a circular magnet was filled with epoxy resin glue and cured sufficiently. All magnets were made of N40 sintered-type neodymium-iron-boron (NdFeB), and the surface of the magnets was protected by nickel plating, with saturation in the height direction. The magnet designs are illustrated in Figure 1A and B. The surface magnetic flux density was 220 mT, and the magnetic force between the two magnets was 4.10 Newton at zero distance. The masses of the cylindrical magnet (the modified circular magnet) and the circular magnets were 0.36 g and 0.33 g, respectively. The magnetic field viewing film showed that the magnetic field distribution of the cylindrical magnets (modified circular magnets) was the same as that of the circular magnets (Figure 1C). The magnetic force curve for the magnets is shown in Figure 1D.

Figure 1 Magnets. A: Front views of the cylindrical magnet and circular ring magnet; B: Side view of the magnets once attracted; C: Magnetic field viewing film showing the magnetic field distributions for the cylindrical magnets and circular ring magnets; D: The magnetic force curve.

Surgical procedure

The Sprague-Dawley rats were fed a slag-free liquid diet 1 day before the surgery. They were fasted, and their water intake was restricted at 4 hours before the operation. The animals were anesthetized via intraperitoneal injection of pentobarbital sodium solution (3 mg/100 g). After confirming the loss of the paw withdrawal reflex, the animals were fixed to an operating table, and their hypogastrium was shaved. For all rats, the abdominal cavity was accessed via a 3 cm median incision in the skin of the lower abdomen and abdominal musculature. The DM was pushed through the anus into the distal colon. A cotton swab was used to push this magnet into the proximal colon under direct vision, at which time the PM was pushed through the anus into the distal colon. After adjusting both magnets in their appropriate positions, they were attracted near the anti-mesenteric portion of the colon. The colon was placed back in the abdominal cavity, which was closed with a silk suture. The surgical procedure is illustrated in Figure 2. For the cylindrical group, cylindrical magnets were used as both the DM and PM. For the circular group, circular ring magnets were used as both the DM and PM. For the cylindrical-circular ring group, a cylindrical magnet was used as the DM and a circular ring magnet as the PM.

Figure 2 Schematic of the surgical procedure. A: A magnet is inserted through the anus into the proximal portion of the colon; B: Another magnet is inserted through the anus to the distal end of the colon; C: The two magnets in the colon are attracted; D: The magnets are discharged after the side-to-side colonic anastomosis is established. DM: Daughter magnet; PM: Parent magnet.

Postoperative care

After recovery from anesthesia, the rats were fed normally. During the first 3 days after surgery, pethidine (0.1 mg/100 g) was injected intramuscularly every 12 hours. The mental state, feeding, and defecation of rats were closely monitored after the operation. The stool was checked daily until the magnets were expelled through the anus. The time to magnet discharge was recorded for each rat.

Tissue harvesting and burst pressure measurement

The rats were euthanized via intraperitoneal injection of pentobarbital sodium solution (6 mg/100 g) at 4 weeks after the operation, and a gross specimen of the anastomosis was obtained from each rat (the proximal end was the colon bypass anastomosis with 3-5 cm of the proximal colon, and the distal end was the anus). A catheter was introduced through the other end and ligated using a single silk suture. The entire anastomosis segment was immersed in 0.9% saline. Subsequently, the intraluminal pressure was increased gradually, and the value was recorded when the first air bubble leaked from the colon and rose to the surface of the liquid. This pressure was considered the burst pressure.

Anastomotic gross observation and anastomotic diameter measurement

The colon specimens were dissected longitudinally from both sides of the anastomosis, and the formation of the anastomosis was observed by naked eye. The tip of the conical aperture scale was inserted into the colon anastomosis, and the intestinal anastomosis specimen was slid toward the end of the handle with appropriate force. When the resistance increased, the movement was stopped. At this time, the scale corresponding to the conical aperture scale indicated the diameter of anastomosis.

Histological analyses

Each anastomotic specimen was soaked overnight in 10% formalin for fixation and embedded in paraffin before the preparation of 4 μm-thick sections. The sections were stained with hematoxylin and eosin (H&E) and Masson trichrome stain and examined under a bright-field microscope.

Finite element simulation analysis

To model the compression stress on the intestinal wall generated by the attraction of the magnets of different shapes, we used ANSYS software (ANSYS Corporation, United States, ANSYS 2021R1) for analysis. The stress distribution on the intestinal wall between the DM and PM was calculated, and the deformation of the intestinal wall was simulated.

Statistical analysis

IBM SPSS statistical software version 20.0 was used for data analyses. Normally distributed quantitative data were described using mean and standard deviation, whereas non-normally distributed data were represented as median. Differences between the groups were compared using analysis of variance or nonparametric test. Values of P < 0.05 indicated a significant difference.

RESULTS

Procedural parameters

The designed magnets were inserted through the anus, and side-to-side colonic anastomosis was successfully achieved in all animals. Magnet insertion went smoothly, and attraction between the magnets was achieved once the second magnet reached the target position without perforation or bleeding. The operation times for the cylindrical group, circular ring group, and cylindrical-circular ring group were statistically similar at 12.00 ± 2.29 minutes, 11.05 ± 2.28 minutes, and 11.25 ± 2.30 minutes, respectively (P = 0.624). The intraoperative procedures for the three designs are illustrated in Figure 3.

Figure 3 Surgical procedure. A: For the cylindrical group, a cylindrical daughter magnet (DM) and cylindrical parent magnet (PM) were placed in the proximal and the distal end of the colon (black arrows); B: The two cylindrical magnets attracted together (yellow circle); C: For the circular group, a circular ring-DM and circular ring-PM were placed in the proximal and distal ends of the colon (black arrows); D: The two circular ring magnets attracted together (yellow circle); E: For the cylindrical–circular group, a cylindrical-DM and a circular ring-PM were placed in the proximal and distal ends of the colon, respectively (black arrows); F: The cylindrical-DM and circular ring-PM attracted together (yellow circle). DM: Daughter magnet; PM: Parent magnet.

Survival rate and postoperative adverse events

All rats survived for 4 weeks after the surgery until the end of the experiment, for a survival rate of 100%. The animals exhibited a normal mental state and food intake postoperatively, and none of them suffered complications, such as bleeding or intestinal obstruction.

Expulsion time of the magnets

After the formation of anastomosis, the magnets were discharged without complication after similar durations among the three designs. Magnet discharge occurred after 4.60 ± 0.97 days, 4.50 ± 0.97 days, and 4.8 ± 1.14 days in the cylindrical group, circular ring group, and cylindrical-circular ring group, respectively (P = 0.760).

Tissue harvesting, burst pressure, and anastomotic measurements

Four weeks after the operation, the rats in all groups, and colon samples covering at least 3 cm in either direction from the anastomosis were obtained. The water pumping method was used to measure the burst pressure of the anastomosis in each sample, and the burst pressure did not differ significantly among the cylindrical group, circular ring group, and cylindrical-circular ring group (212.80 ± 16.28 mmHg, 220.20 ± 14.98 mmHg, and 217.60 ± 15.51 mmHg, P = 0.567). The anastomotic diameters in the three groups were 6.94 ± 0.30 mm, 6.87 ± 0.34 mm, and 6.97 ± 0.37 mm, respectively, with no statistical difference among them (P = 0.809). Figure 4 shows the measurement process of anastomotic burst pressure and anastomotic diameter. Sample data and observation indicators are presented in Table 1.

Figure 4 Measurement of anastomotic parameters. A: Measurement of the burst pressure of the anastomosis; B: Measurement of the anastomotic diameter.

Table 1 Sample data and observation index data, n (%).

	Cylindrical group	Circular ring group	Cylindrical-circular ring group	P value
Rats, n	10	10	10	1.00
Gender
Male	5 (50)	5 (50)	5 (50)	1.00
Female	5 (50)	5 (50)	5 (50)
Operative time (minute)	12.00 ± 2.29	11.05 ± 2.28	11.25 ± 2.30	0.624
Time to magnet discharge (day)	4.60 ± 0.97	4.50 ± 0.97	4.80 ± 1.14	0.760
Bursting pressure (mmHg)	212.80 ± 16.28	220.20 ± 14.98	217.60 ± 15.51	0.567
Anastomotic diameter (mm)	6.94 ± 0.30	6.97 ± 0.37	6.87 ± 0.34	0.809

Gross appearance and histological characteristics of the anastomosis

The colon portions on either side of the anastomosis were dissected longitudinally, and the mucosa at the site of the anastomosis appeared smooth and free of any foreign material (Figure 5). H&E and Masson trichrome staining showed good continuity of the mucosal layer, basal layer, and serosal layer at the anastomosis in specimens from all three groups (Figure 6). No significant difference in the appearance of the anastomosis was observed among the three groups. Necrotic tissue staining of the intestinal wall showed that in the cylindrical group, the two intestinal walls were tightly attached and fused (Figure 6C). In the circular group, the two sides of the intestinal wall tissue that had been pressed together by the magnetic ring were fused, and the two sides of the intestinal wall were separated from the area in the middle of the magnetic ring (Figure 6F). In the cylindric-circular magnet group, the uncompressed intestinal wall tissue in the middle was close, but the intestinal wall tissue on both sides could still be clearly distinguished (Figure 6I).

Figure 5 Anastomotic gross specimens. A-D: Gross specimen of colon anastomosis from the cylindrical group; E-H: Gross specimen of colon anastomosis from the circular ring group; I-L: Gross specimen of colon anastomosis from the cylindrical–circular ring group.

Figure 6 Representative images of histological staining of the colonic anastomosis. A and B: Hematoxylin & eosin (H&E) and Masson trichrome staining of the anastomosis in the cylindrical group; C: H&E and Masson trichrome staining of the necrotic tissue between the daughter magnet (DM) and parent magnet (PM) in the cylindrical group; D and E: H&E and Masson trichrome staining of the anastomosis in the circular ring group; F: H&E and Masson trichrome staining of the necrotic tissue between the DM and PM in the circular ring group; G and H: H&E and Masson trichrome staining of the anastomosis in the cylindrical–circular ring group; I: H&E and Masson trichrome staining of the necrotic tissue between the DM and PM in the cylindrical–circular ring group.

Modeling of compression stress on the intestinal wall

We used finite element simulation analysis, and the results showed that the compression stress on the intestinal wall generated by the surrounding area of the magnet during the establishment of magnetic anastomosis was similar in the three groups (Figure 7).

Figure 7 Finite element simulation analysis. A: Stress distribution after intestinal wall compression in the cylindrical group; B: Stress distribution after intestinal wall compression in the circular ring group; C: Stress distribution after intestinal wall compression in the cylindrical–circular ring group.

DISCUSSION

In previous basic studies on MCA, researchers have discussed the influence of magnetic force on the establishment of digestive tract anastomosis and proposed the most suitable magnetic forces for bilioenteric anastomosis and enteroenteric anastomosis[23,24]. However, few investigators have examined the influence of the structure of the magnets’ compression surface on anastomosis. Jamshidi et al[25] designed a gradient magnet for digestive tract anastomosis in the early years of MCA research, and their results showed that the gradient magnet could shorten the time to anastomosis establishment time with the use of a uniform magnet but without significantly altering the anastomotic burst pressure[25]. In clinical applications, cylindrical and circular magnets are more commonly used. In some reports, to facilitate magnet placement, cylindrical magnets were designed to have small holes in the center[26], and we also ascribed this design to the cylindrical magnet in the present study. Although magnets of both cylindrical and circular shapes both established a good anastomosis, no study had examined whether the anastomosis effect was influenced by the two different compression surface designs.

In the present study, the influence of the difference in the compression surface structures of cylindrical and circular ring magnets used for anastomosis on the effect of magnetic anastomosis of the digestive tract was investigated for the first time. The results showed no significant differences in the time to magnet discharge, anastomotic burst pressure, and anastomotic diameter among the three groups. In all groups, the mucosal layer was smooth and healed well. Histological staining of necrotic intestinal wall tissue from the three groups of magnets showed no significant difference in the intestinal walls of the fully compressed areas. Necrotic changes occurred in the circular group and the uncompressed area of the cylindric-circular group due to ischemia, and no obvious fusion was observed between the two intestinal walls in these areas. However, H&E and Masson trichrome staining showed good continuity of the mucosal layer of the anastomosis in all three groups. The results of this study showed that there was no significant difference in the establishment of side-to-side colonic anastomosis in rats with the use of cylindrical vs circular ring magnets.

Further analysis showed that although the true compressed area of the circular ring magnet to the intestinal wall tissue was smaller than that of the cylindrical magnet, the uncompressed intestinal wall tissue in the center of the circular ring magnet also experienced ischemia and necrosis. Because the circular ring magnet creates a circular ischemic area, in this case, the intestinal wall tissue in the middle of the circular ischemic area will undergo tissue ischemia even if not compressed. Therefore, under the premise that the magnetic force meets the anastomosis demand, the peripheral area of the magnet plays a decisive role in the establishment of the magnetic anastomosis, which is consistent with the Yan-Zhang’s Peripheral Decision Theory of digestive tract MCA. The results of finite element simulation analysis showed that the compression stress on the intestinal wall generated by the area of the magnets during the establishment of magnetic anastomosis was the same in all three groups, further verifying the rationality of Yan-Zhang’s Peripheral Decision Theory.

In the present study, no significant difference in the anastomosis effect within the digestive tract was observed with or without holes in the press surface of the magnet. Therefore, in the design of a digestive tract magnetic anastomosis device, the decision to use either a cylindrical or circular magnet depends on the convenience of magnet placement and magnetic force. This finding has important clinical implications.

This study has two major advantages. First, we construct cylindrical magnets by cementing on the basis of the structure of circular magnets, the purpose of which was to eliminate the influence of magnetic differences between different groups on the establishment of anastomosing. This method was simple and effective. Secondly, the magnets were inserted through the anus with the assistance of laparotomy, which can avoid colon incision, simplify the operation, and eliminate the influence of colon incision and suturing on the observation indicators. A disadvantage of the study is that rats were used as the model animals, and the conclusions would be more convincing conclusions if the results were obtained in larger animal models (dogs or pigs), which have a gut anatomy more similar to that of humans.

CONCLUSION

With application of the same magnetic force, cylindrical magnets and circular ring magnets showed no significant differences in the establishment of side-to-side colonic anastomosis in rats. Based on the results of this study, the Yan-Zhang’s Peripheral Decision Theory is reasonable.