JIMT

Fecal incontinence is a disease in which feces in the rectum suddenly leaks out of the anus due to lack of control bowel movements. It generally occurs in older people or people in nursing homes or shelters¹. Because the leak is without warning, the fecal incontinence has a significant negative impact on the quality of patients’ life. It causes mental distress to the patients and adverse effect on their social life.

When a large area of the anal sphincter is destroyed and then fecal incontinence occurs, standard sphincter repair surgery or anoplasty are performed². If these attempts fail, a medical device that reinforces the sphincter can be implanted into the anus through a transplanting procedure. Artificial bowel sphincters have been used for this purpose, but recently, a magnetic anal sphincter was introduced as a less invasive method as shown in Fig. 1^3-5.

Figure 1. FENIX^® Continence Restoration System (Torax Medical Inc., St. Paul, MN, USA). (A) Working principle. (B) Closed position with a knot.

This device still has room for improvement. The final step in the above-mentioned fecal incontinence prevention surgery is to secure the device around the upper anal canal by tying the knot connected to both ends’ suture line of the device together. The incised wound is then closed with absorbable sutures to prevent drainage⁶. Although it is known to be less invasive and have a better prognosis than surgery using the conventional artificial bowel sphincters, the surgery is still inconvenient and time-consuming for colorectal surgeons due to the multiple suturing processes.

Among the multiple sutures, what this study focused on is the final knotting process that connects the two ends’ suture line of the device. This knot should be easy to tie but secure the device firmly. For the procedure using the Lehur’s conventional device, however, the colorectal surgeon directly knots the suture line ends attached to the eyelets. This is a difficult or time-consuming for novices, and there is a risk that it will come loose even if they are tied well.

To make this final knotting step easier and faster, this study proposes using a snap-fit mechanism instead of the knot. In Fig. 1B, the yellow arrows indicate the colorectal surgeon’s knot, which ties the sutures attached to the eyelets at each end of the sphincter device. In contrast, Fig. 2 shows the device just before connecting with a proposed snap-fit instead of the knot. To prevent tissue surrounding the anal canal from penetrating into this area, this junction is covered with a cap (20 mm in diameter).

Figure 2. Magnetic anal sphincter with proposed snap-fit knotting.

Before joining, the two main parts of this connection look like Fig. 3. The snap has four elastic cantilevers (18 mm long), each of which has a wedge-shaped tip. There is a step from this tip to the cantilever. To connect, the snap is pushed into the fit, which connects as shown in Fig. 4. At this time, the cantilever is elastically deformed, and the snap is combined with the fit to maintain the connection⁷. These connection forms a firm fastening that is not reversed. If the colorectal surgeon wants the remove the device, the wire in Fig. 2 should be cut during the following reoperation.

Figure 3. Proposed cantilever snap-fit.

Figure 4. Cantilever’s behavior during connection.

To make sure, we firstly fabricate several designs with 3D printing. The result is shown in Fig. 5. It was manufactured using Objet30 Pro 3D printer (Strasys Ltd., Eden Prairie, MN, USA). Its material is acrylate-based plastic. Among the various designs, we tried to make the snap-fit small but enough to fit together well and not to be broken. In particular, the cantilever length was carefully determined, and the wedge surface was also created with a smooth curve. This design was then utilized to make a metal snap-fit from stainless steel (SUS 316L) using a metal 3D printing process. A new magnetic anal sphincter with the snap-fit, magnets coated with silicone, and an elastic rubber band wire, was fabricated as shown in Fig. 6. Stainless steel, silicone coating, and rubber bands were selected for biocompatibility.

Figure 5. Snap and fit for connection. (A) Snap with 4 cantilever & wedges. (B) Fit with hollow inner space. L: cantilever length.

Figure 6. Fabricated magnetic anal sphincter with biocompatible metallic snap-fit. (A) Snap-fit made of SUS 316L. (B) Magnetic anal sphincter with the proposed connection.

To check whether the magnetic anal sphincter with the proposed connection worked well, a phantom mimicking a rectum and anus was devised as shown in Fig. 7A. This phantom has a vertical pipe, like a rectum, and a flexible tube that mimics the anal canal. Water corresponding to feces is supplied into the pipe then inner pressure is developed. Meanwhile, some water branch out of the pipe goes to the pressure sensor, which measures the pressure inside the canal.

Figure 7. Experimental phantom and anal sphincter’s squeeze pressure. (A) Closed state when P<2 kPa. (B) Open state when P>2 kPa. P: pipe-rectum, tube–anus canal, water–feces.

According to reports in the literature, the maximum pressure exerted when a patient pushes stool out of the rectum is approximately 3–40 kPa (20–300 mmHg)⁸. Assuming that defecation begins when a pressure of 2–3 kPa is applied to a patient’s sphincter, we checked whether the magnetic anal sphincter with the connection opens well at the pressure range and discharges water corresponding to feces. As a result, it was confirmed that defecation occurred properly as shown in Fig. 7B.

It is anticipated that the connection time will be reduced by using the proposed snap-fit compared to the time required to connect the conventional sphincter device. In the conventional device, a knot is needed to connect both ends. According to discussion with a colorectal surgeon, it takes about 10–30 seconds to tie one knot. Assuming that he/she ties two knots to securely tie the sutures at both ends, it is expected to take 20–60 seconds. If the proposed snap fit method is used, this time will take approximately 13 seconds, and it is expected to take less time since the connection is completed with a single push.

During more than 100 experiments on the phantom, the metal snap-fit has never separated. Typically, surgical knots should be tied more than twice to ensure adequate tension and prevent from coming undone⁹. Therefore, it can be said that the connection method proposed here is safe and guarantees a certain degree of tension.

This magnetic anal sphincter uses a magnet. Since the magnet is made of heavy metals such as neodymium or samarium, which are harmful to the human body, biocompatibility may be problematic if it is not properly coated. Additionally, even with coated magnets, there is a risk of inflammation and necrosis if tissue gets caught between the magnets in the closed position. It seems a good idea to wrap the outside of the magnetic anal sphincter with a donut-shaped silicone tube to prevent tissue pinching.

Since the snap-fit type connection is a permanent connection, it has the disadvantage of not being able to be removed and corrected during the surgical procedure. Therefore, the appropriate position and length must be secured during the surgical process and then the snap-fit connection must be completed at the end. If the proper connection is not made, the surgeon must cut the rubber band wire and use another new device. Some snap-fit allow modification, so it is worth trying to apply them to the device.

If the outer surface of the cantilever snap-fit’s wedge is not designed to be smooth and curved, it may not be easy to join or it may be broken. If the length of the cantilever is not appropriate, it may be too difficult to fit or may be too loosely coupled. In this study, the first attempts to fabricate several snap fits were performed with plastic materials because the 3D printing process using metal is very expensive. The metal can be titanium and stainless steel. The former is more expensive, and the latter is cheaper. However, the titanium is recently more widely used in medical implant.

The range of pressure that opens the anus in the phantom used in this study was set to over 2 kPa, but other literature reports that the anus opens at pressures greater than this pressure¹⁰. Therefore, it is necessary to find the appropriate pressure, conduct more new experiments, and perform statistical analysis.

The time it takes for a surgeon to tie a surgical knot was estimated to be 10–30 seconds through discussion with the surgeons, but this is not based on appropriate statistics. Therefore, more data on this is required. It was thought that the connection of this device has been robust after more than 100 experiments. However, if the actual patient defecated daily and his expected survival was 5 years, more than 1,800 experiments would be needed. Therefore, this also needs to be analyzed by conducting more experiments and obtaining more data.

In this study, the previously proposed magnetic anal sphincter device was improved to enable easy, fast and robust connection. For this connection, a snap-fit mechanism made of metal was designed and manufactured, and its usefulness was tested on a phantom. As a result, it was possible to develop a magnetic anal sphincter device with a connector that allows for a faster, easier, but more robust connection. The differences between the conventional and proposed connections are listed in Table 1.

Table 1 . Differences between the conventional and the proposed connections.

	Conventional way	Proposed way
Connection method	Tying square or surgeon’s knot	Pushing snap into fit in cap
Required instruments	Finger, needle, and scissors	Fingers
Advantages	Familiar to surgeons	Fast, easy, and robust

However, this study still has risks to the human body, irreversibility of the surgery, high cost of the device, and limitations of the experiment. Therefore, additional animal or human test is necessary.

No potential conflict of interest relevant to this article was reported.

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