Stress urinary incontinence (SUI) is a fairly common medical problem affecting about 40% of women. The current treatment strategies include conservative and surgical options. Midurethral slings are the gold standard surgical option with a cure rate of 80-90% [1]. Despite the sling procedure offers high efficacy, mesh and biomaterial related drawbacks are not uncommon. Novel biological biomaterials might help especially after failed/complicated synthetic mesh surgeries. To test the novel biomaterials, rats and rabbits were effectively utilized for biocompatibility. However, the slings were undersized to fit in the small animals’ anatomical spaces, which might not reflect the properties of human-sized slings and devices. Sheep cadavers were carried out in literature for hands-on training of sling techniques. Our aim was to create a reliable larger animal model of human-sized transvaginal sling placement. We are the first to report survival surgery for transvaginal slings in sheep animal model.

We used eight Dorset retired breeder, female ewes (160-175 pounds). Under general anesthesia, ewes have been positioned on their backs (dorsal recumbency) to mimic lithotomy position. After skin and vaginal surgical preparations, relevant vaginal and abdominal dimensions were reported (Figure 2).  We did measure clitoral dimensions, labia majora length, vaginal length, perineum length, the width of the vagina when opened using self-retaining retractors and stays and length of symphysis pubis (Figure 2).  A disposable tape measure was used for all measurements for making the comparison.
We used human-sized slings and trocars for implantation (e.g. Lynx™ Suprapubic Mid-Urethral Sling System, Boston Scientific, MA, USA). Using the sterile instruments, surgery was mimicking human TVT procedure. Sterile Foley’s 12F urethral catheter was inserted (Figure 1a). We did a 3 cm incision in the anterior vaginal wall, 1 cm below the urethral meatus. Dissection was done bilaterally to reach out to the endopelvic fascia. Two suprapubic 1 cm-size skin incisions 5 cm-apart and located 7 cm above the pubic symphysis of the animal. The trocars traveled through the suprapubic incisions on each side in a downward motion with surgeon's forefinger acting as a guide (from the vagina) during insertion from the suprapubic region to the vagina (Figure 1d). The trocars passed through each incision, through the retropubic space, hugging the posterior aspect of the symphysis pubis, then perforating the endopelvic fascia and exiting through the already made vaginal incision. Cystoscopy was done to ensure that bladder and urethra are not injured (figure 1b and 1c). Ewes urethra accommodated up to 16Fr. cystoscopy sheath. The sling was hooked to the trocars. The trocars on both sides were pulled to the suprapubic skin incision, placing the sling under the urethra without tension (figure 1e). Closure of vaginal incisions was done using 2-0 absorbable sutures (figure 1f). Animals’ intraoperative and postoperative follow up charts were recorded.

The sheep vaginal anatomy was feasible to perform the placement of human-sized sling devices and surgical instruments. However, average vaginal dimensions were less than human. Mean vaginal length of sheep was 8.4 cm (figure 2b) compared to the reported 9.6 cm in human [2]. All average measurements are shown in figure 2d. The mean symphysis pubis’ length was 6.7 cm which is longer than reported human symphysis pubis (from 2.6 to 4.6 cm [3]). We had to make higher abdominal incisions (6 cm above the pubic bone) to accommodate the trocar curvature with the lengthy symphysis pubis without injuring the internal abdominal organs. Mean operative time was 44 ±12 minutes. Blood loss was less than 50 cc. All ewes survived the surgery and recovered smoothly. All animals urinated normally without haematuria or obstruction since the first postoperative day. After two weeks, Incisions were healed without signs of erosions, infection or rejection.

We are the first to report vaginal sling survival surgery in the sheep model. The sheep vaginal dimensions are suitable for vaginal surgery experiments. Spontaneous prolapse has been reported in elder ewes suggesting the similarity of both physiological mechanisms and anatomical support of the pelvic floor in human and sheep. The novel biomaterials had been tested in small sizes suitable for the proof of biocompatibility. Mechanical testing of the small-sized biomaterials might not reflect the same properties when translated to human-sized slings. Suburethral implantation of slings into female sheep model might be ideal for translation of the biomaterial slings before clinical trials. Our focus of future research will be to induce incontinence in the sheep model.

We were able to implant human-sized slings into female ewes using the human instruments and devices. The vaginal anatomy of the female sheep might suggest a reliable model for pre-clinical testing of novel biomaterials and sling devices.

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