Chapter 40 MIDURETHRAL TO DISTAL URETHRAL SLINGS

MIDURETHRAL TO DISTAL URETHRAL SLINGS

The first part of this chapter addresses the anatomic basis for the midurethral sling operations, and the second part describes the surgical techniques themselves.

Short History of Midurethral Slings

The first prototype midurethral sling, the “intravaginal slingplasty” operation, was performed in 1986. This operation ^1,² was based on the anatomic work of Robert Zacharin³ and the observation that an implanted plastic fiber causes a controlled deposition of collagen. It was reasoned that a tape placed at the site of the pubourethral ligament, without tension, would create a scar tissue reaction and strengthen the weakened ligament. It was also reasoned that a midurethral tape placement without abdominal attachment would eliminate most of the postoperative pain and urinary retention associated with sling and colposuspension procedures. These hypotheses were proven in the prototype intravaginal slingplasty operations. The one remaining problem, a high rate of foreign body reaction to Mersilene, was resolved by using polypropylene.²

Structural Anatomy

Without ligamentous support, the bladder, urethra, and vagina have no shape, strength, or function. Fibroligamentous attachments connect urethra and bladder base to the vaginal membrane (Fig. 40-1), so any laxity of the underlying vagina may influence the urethra and bladder base. Stress urinary incontinence (SUI) is a mechanical process.¹ To better understand the pathogenesis and treatment of SUI, it is useful to examine how ligaments, fascia, and muscle forces all combine to give shape, strength, and function to the vagina. All the component structures in Figure 40-2 are interconnected, and all have a role in the functioning of the system. It follows that more than one damaged structure may need to be repaired for optimal restoration of a particular function.

Figure 40-1 Unsupported organs have no strength, structure, or function. X indicates fibroligamentous attachments of distal urethra and bladder base to the vagina. The proximal urethra is not attached. PUL, pubourethral ligament; USL, uterosacral ligament.

Figure 40-2 The suspension bridge analogy. The vagina is suspended anteriorly by the pubourethral ligament (PUL), posteriorly by the uterosacral ligament (USL), superiorly by arcus tendineus fascia pelvis (ATFP), and by attached fascia (f). Muscle forces (arrows) stretch the vaginal membrane against the suspensory ligaments to create shape and strength. PS, pubic symphysis; S, sacrum.

Anatomy of the Urethra’s Attachments

Figure 40-3 is a live anatomic study⁴ of the structures involved in urethral closure. They are, in order of importance, the pubourethral ligament (P), the suburethral vaginal hammock (H), and the external urethral ligaments (E). The external ligament E attaches the external urethral meatus to the anterior surface of the pubic bone. The pubourethral ligament P descends from the posterior surface of the pubic bone.³ Its medial part (m) attaches to the middle part of the urethra, and its lateral part (l) to the pubococcygeus muscle and, by extension, to the vagina.

Figure 40-3 A, Suspensory ligaments of the distal urethra: view into the vagina, left paraurethral incision. The labels m and l indicate the medial and lateral branches of the pubourethral ligament, P. E, external urethral ligament; f, Foley catheter; H, hammock. B, Schematic sagittal view. EUL, external urethral ligament; PUL, pubourethral ligament.

In 1963, Robert Zacharin demonstrated that the vaginal hammock is inserted into the lateral closure muscles (Fig. 40-4). This anatomy is consistent with dynamic video radiographic studies and ultrasound observations (Figs. 40-5 and 40-6).

Figure 40-4 Anatomy of the vaginal hammock. Coronal section, cadaveric study. Arrows show direction of muscle forces. H, suburethral vaginal hammock; LA, levator ani also known as; pubococcygeus muscle PCM; U, urethra; V, vagina.

Figure 40-5 A, Abdominal ultrasound image and schematic diagram of distal urethra at rest. E, external urethral ligament attaching external meatus to the anterior surface of pubic bone; H, suburethral vaginal hammock; PS, pubic symphysis; U, urethral cavity. B, Same patient, image while straining. Note how the hammock is closed from behind (arrows). The ligament E needs to be firmly anchored for optimal action of the closure forces.

(From Petros PE, Ulmsten U: An integral theory of female urinary incontinence. Acta Obstet Gynecol Scand 69[Suppl 153]:7-31, 1990.)

Figure 40-6 Role of the pubourethral ligament (PUL) during straining and micturition: radiographs taken at rest (A), on straining (B), and with micturition (C) in the same patient. The pubourethral ligament (PUL) is a critical anchoring point for the muscle forces (large arrows) in both straining and micturition. During straining, three muscle forces (arrows) act against the PUL. On micturition, the forward force relaxes; the backward/downward vectors (diagonal arrows) pull against PUL to open out the urethra and bladder neck. The white broken lines are drawn against fixed bony points. Note the downward angulation of the levator plate (LP) in B and C. B, bladder; CX, cervix; R, rectum; U, urethra; USL, uterosacral ligament; V, vagina.

Dynamic Anatomy of Urethral Closure

Abdominal ultrasound ⁵ (see Fig. 40-5B) demonstrates the contribution of the hammock and external ligament (E) to urethral closure. Closure occurs from behind, presumably by forward stretching of the hammock (H) against (E) by contraction of the pubococcygeus muscle (PCM).

Importance of Midurethral Anchoring for Closure and Micturition

The importance of a midurethral anchoring point is evident on the sequence of radiographs in Figure 40-6. During straining (see Fig. 40-6B, arrow), the distal urethra is pulled forward against the pubourethral ligament (PUL) for “urethral closure.”¹ The proximal urethra is stretched backward and downward against the PUL (arrows) for “bladder neck closure.”¹ During micturition (see Fig. 40-6C, arrows), the forward muscle force appears to relax, allowing the posterior urethral wall to be pulled open by the backward/downward muscle forces, right down to the PUL anchoring point. This opening vastly reduces the intraurethral resistance, facilitating the expulsion of urine. A surgical attachment at the bladder neck (Burch, fascial sling) may forcibly restrain the urethra from being stretched open during micturition, so that the detrusor has to expel urine through a narrower channel with a much higher resistance. The patient perceives this symptomatically as a slow flow, stopping and starting, incomplete emptying, and so on. From these radiographs it can be deduced that, whatever the operation performed for SUI, anchoring at or near the mid-urethra without excessive tension is an important factor in preventing postoperative urinary retention.

Clinical Anatomy: “Simulated Operations”

The individual contribution of the PUL, hammock, and the external urethral ligament (EUL) to urethral closure can be demonstrated directly by the office technique of “simulated operations.” This technique also directly tests the theory underlying this work ¹ for truth or falsity. A patient with known urine loss on coughing is tested with a full bladder in the supine position.⁷ Pressing upward on the anterior vaginal wall at the level of mid-urethra with a hemostat during coughing (Fig. 40-7) provides a temporary anchoring mechanism and mimics a midurethral sling. This maneuver immediately reduces urine loss in approximately 70% to 80% of patients.⁷ Taking a fold of the hammock with a second hemostat (“pinch test”)⁸ usually restores complete continence in the remaining 20% to 30% of patients.⁷ Anchoring the external urethral meatus may decrease urine loss in approximately 10% of patients. The synergistic actions of these structures can be demonstrated by anchoring one and then two structures simultaneously.