A Novel Surgical Technique in a Sheep Model for Suburethral Graft Implantation

Summary

This article explains a new animal model designed for testing suburethral graft materials. In this model, a mesh tape is implanted through a vaginal incision in ewes, and the animals are observed over a three-month period. This model is essential as it lays the foundation for experimental research on grafts.

Abstract

Suburethral polypropylene mesh tapes for women were initially introduced into clinical practice for the treatment of stress urinary incontinence without being tested in an appropriate animal model. The goal of this work has been to establish and evaluate a suitable animal model for the assessment of suburethral tape grafts. Allowing for the anatomical similarities between ewes and women and recognizing the inevitable differences in posture, but recognizing prior work, this report presents a step-by-step description of the surgery involved in an animal model that simulates the implantation of support materials for treating stress urinary incontinence in women. Seven ewes with a mean weight of 27.7 SD 5.1 kg underwent surgery. In the lithotomy position, a ventral vaginal wall incision was made. In five animals, using an introducer, a polyurethane implant was placed underneath the urethra and fixed with absorbable sutures. The incision was closed, and a betadine vaginal pack was placed after 24 h. Two animals received transvaginal polypropylene mesh using similar surgical steps but no fixation sutures. After surgery, all animals urinated freely and had regular bowel movements. No animals showed signs of vaginal erosion or graft infection during the three-month follow-up. This large animal model accurately reproduces the surgical technique used in women, thereby providing a valuable platform for evaluating new materials for stress urinary incontinence treatment.

Introduction

Stress urinary incontinence (SUI) affects 26% of women in a USA prevalence study; in addition, 16% more women have both stress and urgency incontinence¹. Tension-free vaginal tape evolved as a less morbid surgical treatment for SUI associated with a comparable success rate and faster recovery than the gold standard colposuspension^2,3. However, suburethral polypropylene (PP) mesh tapes were introduced without testing in a relevant animal model, leading to adverse responses^4,5. In recent years there have been reports of significant complications following the use of polypropylene not relatable to surgical technique and thought to be possibly related to tissue reaction to the material itself including mesh erosion through the vagina, chronic pain and infection⁶. The most commonly reported surgical complications included bladder perforation, urinary retention, and bleeding^6,7.

Nearly 100 products have been developed for genito-urinary surgical implantation application⁸. Major concerns have been identified regarding the implantation of mesh for the treatment of pelvic organ prolapse^6,7. There is a high incidence of spontaneous pelvic floor prolapse in ewes and, on this basis, a sheep model for pelvic organ prolapse in women has been advocated and has demonstrated graft erosion^9,10,11. Recent basic science work has added support to the suggestion that tissue reaction to polypropylene may play an important role in the development of untoward complications with this material¹². Generic animal studies have investigated tissue reaction in the abdominal wall of small animals to PP mesh after implantation^13,14. Because these models do not reproduce the anatomical environment of mesh implantation in human surgery, a sheep model -- featuring anatomical parallels to human surgery -- was evaluated. This model has proven effective in evaluating new implant materials for pelvic organ prolapse surgery, placing the graft between the rectum and vagina^9,10,11.

More recently, contemporaneous work replicating the transvaginal retropubic technique used in human SUI surgery reported a top-down approach for tape insertion in three ewes¹⁵. A recent report from the present authors highlighted the versatility of this model, demonstrating its suitability for both a retrograde (bottom-up) insertion approach using the TVT device and a top-down approach for the insertion of polypropylene with the TVT device and a novel material manufactured from polyurethane¹⁶. This video-augmented article describes in detail the preparation and use of this sheep model for the insertion of both polypropylene using the TVT device and the novel new material using the bottom-up approach. The TVT insertion technique is as previously reported³. The polyurethane is inserted mounted on a needle introducer and passed from an anterior vaginal wall incision upward into the retropubic space, replicating the bottom-to-top path used for insertion of TVT.

To validate the model, the feasibility of creating a surgical plane between the urethra and vagina was assessed, with careful attention to avoid injury to either structure. Potential pelvic and intra-abdominal complications associated with needle-guided tape passage were also evaluated. Additionally, this study examines a novel electrospun polyurethane graft, previously untested for suburethral implantation, and compares its tissue response and biocompatibility to that of conventional polypropylene mesh three months postoperatively.

Protocol

The hospital's Institutional Review Board (IRB) and the Animal Care and Use Committee reviewed and granted approval for the protocol (ethics approval number: 2230017). This approval adheres to internationally accepted principles of humane experimental techniques. All procedures and postoperative care were conducted in a facility accredited by the Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC).

Ensure that qualified veterinarians always conduct medical management and anesthesia.

1. Animal preparation and anesthesia

1. In the preoperative room, give the sheep atropine sulfate 0.02-0.05 mg/kg I.M. once for the induction of anesthesia.
2. Administer the analgesic carprofen at a dosage of 2-5 mg/kg SC.
3. Shave the skin around the lower abdomen and inner thighs using a hair clipper.
4. Induce anesthesia using propofol 2-4 mg/kg IV as needed and ketamine 10-12 mg/kg I.M. once.
5. Insert a 7 mm diameter endotracheal tube under direct vision using a laryngoscope and connect the tube to a mechanical ventilator. Use oxygen and isoflurane inhalation anesthesia, 5% for induction, and 2-3% for maintenance throughout the procedure. Check the endotracheal tube's proper placement, auscultate the chest and abdomen during inhalation, and observe the chest wall movement.
6. Place a pulse oximeter under the tongue's surface and fix it with adhesive tape.
7. Observe the jaw tone for flaccidity to monitor the depth of anesthesia. Complete flaccidity means deep anesthesia, and jaw spasticity means the animal is waking up. Monitor the anesthesia machine for pulse rate and oxygen saturation changes from baseline.
   NOTE: A 10% increased pulse rate or a drop in oxygen saturation suggests the animal is reviving from deep anesthesia, requiring an adjustment in anesthetic dosage. Check deep reflexes every 15 min with ear pinching. Increase the anesthetic dose upon recovery from deep anesthesia.

2. Skin preparation and draping

1. Carry out all surgical procedures under sterile conditions.
2. Place the animal supine. Scrub the skin of the lower abdominal wall, the perineum, and the vagina with a 7.5% solution of povidone-iodine. Then, dry the skin with a sterile absorbent towel.
3. Administer Cefazolin sodium 25 mg/kg IV at the induction of anesthesia. Next, drape the animal with sterile surgical drapes, exposing the perineum.
4. Insert a 12-French urethral Foley catheter for bladder drainage. (Figure 1A).

3. Electrospun polyurethane mesh implantation

1. Incise the ventral vaginal wall longitudinally (Figure 1B). Develop the space between the vaginal wall and the urethra with lateral dissection using Metzenbaum scissors¹⁶.
   1. Through the anterior vaginal wall incision, insert the scissor blades closed, direct them to the pelvic side wall on one side, open the blades, and then withdraw them out.
   2. Repeat the procedure several times till the created space can admit the index finger into the retropubic space (Figure 1C,D). Repeat the dissection on the opposite side¹⁶.
2. Use the electrospun polyurethane implant in five healthy adult multiparous ewes. (Table of Materials).
   1. Thread a polypropylene one monofilament suture on a 36 mm taper point rounded needle through the lateral suture hole at both ends of the implant.
   2. Pass the sutures cranially via the retropubic tissues using a needle introducer (Figure 1E)¹⁶.
   3. Identify the tip of the introducer at the anterior abdominal wall, bring it out through a five mm transverse anterior abdominal wall incision, and bring the suture out.
3. Place two additional absorbable sutures through the implant's two side holes to fix it in position (Figure 1F)¹⁶. Pull the implant cranially with the polypropylene sutures, seat it underneath the urethra, and tie the sutures (Figure 1G)¹⁶. Tie the abdominal ends of the polypropylene sutures together and close the skin over the ends using 3-0 absorbable sutures.
4. Close the anterior vaginal wall with 2-0 absorbable sutures (Figure 1H)¹⁶. Utilize interrupted sutures and the surgical knot technique.
5. Remove the catheter. Place a betadine vaginal pack and remove it after 24 h.

4. Polypropylene tape implantation

1. Use a polypropylene transvaginal tape implant in the second group of animals, consisting of two healthy, adult, multiparous ewes (Table of Materials).
2. Incise the ventral vaginal wall longitudinally. Develop the space between the vaginal wall and the urethra with lateral dissection retropubically.
3. Attach the tape and its protective sheath to the product-dedicated introducer. Pass the introducer cranially via the retropubic tissues (Figure 2A). Identify the tip of the introducer at the anterior abdominal wall and bring it out through a five mm transverse anterior abdominal wall incision. Deliver the polypropylene tape.
4. Pull the tape cranially and let it sit underneath the urethra (Figure 2B). Cut the central marker of the tape to release the protective sheaths. Pull the protective sheaths from the abdominal end and discard them. Do not fix the tape with sutures, as it is self-fixing. Trim the abdominal ends of the tape and cover them with skin closure.
5. Close the anterior vaginal wall with absorbable 2-0 sutures. Utilize interrupted sutures and the surgical knot technique.

5. Postoperative protocol

1. Postoperatively, observe the animals for pain and complications and allow them access to water and food ad libitum.
2. Administer analgesics starting eight hours after the preoperative dose: carprofen 2-5 mg/kg SC every 8-12 h as needed for three days postoperatively.
3. After full recovery, house the sheep together and allow them to move freely. Observe the animals daily, treat any signs of pain or distress, or carry out euthanasia.

6. Endpoint

1. Follow the animals for three months and perform tissue harvesting under general anesthesia for histopathological analysis.
2. Examine the vagina under general anesthesia using a self-retaining retractor to look for evidence of erosion or gross anatomical deformity.
3. Identify and excise the graft with the surrounding tissues and preserve it in 10% formalin for histopathological examination.
4. While under general anesthesia, administer a sufficient dose of IV anesthetic for euthanasia or follow the euthanasia method approved in the animal protocol by the institution.

Results

Seven ewes with a mean weight of 27.7 SD 5.1 Kg underwent surgery (Table 1). Figures 1-2 show the surgical procedure. There were no complications or morbidity directly attributable to the surgical procedure. All animals tolerated the surgery and exhibited normal eating behavior, bowel movement, and micturition in the next postoperative days. There was no vaginal bleeding after removal of the pack. All the animals survived to the endpoint. During the follow-up, none of the animals exhibit...

Discussion

The critical step in the current study is to dissect a plane between the vagina and urethra, develop a retropubic passage, guide a trocar-mounted graft cranially through this plane, and lay the graft in a suburethral position. This animal model effectively simulates the implantation of suburethral mesh in women, making it a unique tool for surgical experimentation^2,16. Although there are previous experiments evaluating the PP mesh as a possible synthetic graft fo...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Advantage Fit Blue System (polypropylene tape)	Boston Scientific Corporation 300 Boston Scientific Way Marlborough, MA 01752	M0068502120	Needle diameter: 2.7mm, Mesh thickness: 0.66 mm, Pore size: 1182 μm, Fiber size (diameter): 0.15 mm
COVIDIEN Nellcor MAXA	Covidien Medtronic, Operational Headquarters 710 Medtronic Parkway, Minneapolis, MN 55432-5640 USA		Pulse oxymeter
COVIDIEN Shiley Hi-Lo Oral/Nasal Tracheal Tube	Covidien Medtronic, Operational Headquarters 710 Medtronic Parkway, Minneapolis, MN 55432-5640 USA		Endotriacheal tube 7 mm ID
Drager Fabius	Draeger Arabia Co.Ltd. P.O.Box 365642. Riyadh 11393 Kingdom of Saudi Arabia.		Anesthesia machine and monitors, Siemens SC 7000 monitor, vaporizers , flowmeters for oxygen, air and nitrous oxide.
Electrospun polyurethane fascio-mimetic implant	University of Sheffield, Sheffield, United Kingdom.	NA	Graft material in a tape shape.
Polypropylene suture	Johnson & Johnson Medical N.V., Belgium	V8015	2.0 suture on rounded needle
Sterile procedure pack	Arabian Medical Manufacturing Company , P.O. Box 355033, Riyadh, 11383, Saudi Arabia	CPT-01000-022	Povidone iodine tray, includes Povidone iodine 7.5% 120 ml, Povidone iodine 10% 120 ml, applicator sponges, sterile towels.