Female Urethroplasty: A Brush with the Continence Mechanisms

The only approaches to female urethroplasty (Anterior/dorsal and Posterior/ventral) are by far the most utilized techniques for the reconstruction of a structured female urethra. They involve splitting the urethra across the stricture segment and placing a graft (buccal, vaginal, or inner labial skin) to interpose, which is then quilted to the underlying bed. Effective development of the plane between the urethra and sphincteric muscles seldom happens, and often, they get transected in the midline and are further kept apart by the graft, which is fixed to a bed. Strangely, continence remains preserved in most cases, and this makes us ponder how female urethral continence mechanisms work.

Functionally, the female resting urethral profiles show an increase in urethral pressures beginning from the bladder neck, but the maximum closure pressures are in the mid-urethra region. However, the anatomical basis of the resting continence mechanism is of utmost significance before we contemplate any tampering with these mechanisms during reconstruction.

The bladder neck has a minor but significant contribution to the resting continence. This continence is provided by the loops of detrusor fibers thrown as slings around the bladder neck from all sides, which tighten with bladder filling and slide away when the neck funnels for voiding. This relatively passive continence mechanism is further supported by an active component provided by a rich density (much less as compared to males) of alpha-adrenergic receptors.

The major contributor to continence is the striated (yet nonfatigable) rhabdosphincter. This muscle is distinct from the pelvic floor and has three components, namely the pars urethralis, the compressor urethrae, and the urethrovaginal sphincter. This sphincteric complex starts surrounding the urethra just distal to the bladder neck, but the maximum density is at around the mid-urethra level and then ends before the last quarter of the length of the urethra. The circular and longitudinal smooth muscles of the urethra add very little to the continence. The sphincteric muscles derive innervation from each side with the least nerve density in the midline. Hence, a midline urethrotomy preserves the functional muscle bundles on either side.

In addition to these sphincters, the extensively rich vasculature in the submucosal layers makes the mucosa thrown into interdigitating folds which provide the mucosal seal. This hormone-sensitive mucosal seal is a significant contributor to the resting continence mechanism.

The various ligaments that hold the pelvic structures (and their natural angles) and the pelvic floor muscles provide little contribution to resting continence but are significant adjuncts preventing stress-associated incontinence.