Knowledge of urethral closure mechanics helps to optimize surgical methodology of the midurethral sling operation

Abstract

The mechanism for urinary continence is not obstructive. Ultrasound and video data indicate that 3 striated muscle forces contract in opposite directions around a competent pubourethral ligament (PUL) to close the distal urethra and bladder neck. If PUL is loose, both mechanisms are invalidated, because striated muscles need a firm insertion point to function efficiently. The patient now loses urine on effort. Referring back to original research, the various steps involved in a midurethral sling operation are analysed with a view to optimizing surgical results. These include an analysis of what causes postoperative urinary retention, why the components of the distal closure mechanism need repair and simple steps to avoid nerve and blood vessel injury.

Keywords: midurethral sling; pubourethral ligament; surgical methodology; urethral closure mechanism.

Figure 1

Transperineal ultrasound 'REST'. Patient in semirecumbent sitting position. S – symphysis; U – urethra; B – bladder; a – anterior vaginal wall; p – posterior vaginal wall

Figure 2

Transperineal ultrasound 'STRAIN'. Patient pushes downwards (Valsalva). The posterior pelvic muscles stretch the vaginal walls 'a' and 'p' backwards to pull open the posterior urethral wall. The urethra opens out (funnels) both proximally and distally.

Figure 3

Transperineal ultrasound 'MID/UR ANCHOR. A hemostat (arrow) is pressed gently upwards immediately behind the symphysis; this restores PUL length; 'a' & 'p' visibly tension, indicating restoration of the directional forces to enact bladder neck closure and distal urethral closure.

Figure 4

Urethral closure (straining) by oppositely acting striated muscle vectors (arrows). The pubococcygeus muscle (PCM) stretches the distal suburethral vagina forward against the pubourethral ligament (PUL) to close the distal urethra; m. levator plate (LP) stretches the distal vagina, bladder and proximal urethra backwards. This action tenses PUL and PVL; m. longitudinal muscle of the anus (LMA) pulls the proximal vagina and bladder base downwards against the uterosacral ligaments; this action rotates the bladder (broken circle) to close the urethral 'kink' at the bladder neck (1). Zone of critical elasticity (ZCE) signifies the significant elasticity in the vagina required to allow separate action of the opposite muscle vectors (arrows).

Figure 5

PUL is loose and has lengthened by ‘L’ (circle). LP and PCM weaken; LP/LMA contraction now funnels the bladder, as in Figure 3.