Aims of study:
Although pubovaginal sling surgery is believed to exhibit its effect partly by correcting urethral hypermobility, there is scarce data to support this hypothesis. We compare female bladder neck mobility, before and after pubovaginal sling surgery, and correlate it with surgical outcome.

Methods:
Before and after the surgery, patients received complete urodynamic studies including videourodynamics. Maximal bladder neck decent distance below lower margin of symphysis pubis (MBND) by abdominal straining and functional bladder neck mobility (MBND/increased intravesical pressure) were determined. We identified intrinsic sphincter deficiency (ISD) as valsalva leak point pressure lower than 60cmH2O. Surgical outcome was judged by questionnaire survey and follow-up videourodynamics. We defined "cure" as no objective stress incontinence.

Results:
30 female patients, including 12 with isolated intrinsic sphincter deficiency (ISD) and 18 having bladder neck hypermobility with or without ISD, received pubovaginal sling surgery using either autologous rectus fascia or freeze-dried allograft fascia lata. They had a mean follow-up of 16 months. All of isolated ISD patients were cured. Their MBND (0.48+0.12cm), maximal urethral closure pressure(MUCP), cystometric capacity, voiding detrusor pressure at maximal flow rate did not change following the surgery. However, their maximal uroflow rates were significantly decreased by the surgery, from 38 down to 25ml/sec. 14 of 18 hypermobility cases were cured. Pubovaginal sling surgery significantly reduced their bladder neck mobility with a decreased MBND from 2.9+0.4cm to 0.03+0.13cm and a reduced functional bladder neck mobility from 0.50 + 0.07 to 0.01+ 0.01 cm/10cmH2O. Most patients experienced a slightly reduced urinary flow rate with a decreased maximal uroflow rate from 32.5+2.5 to 21.2+2.3 ml/sec, although there was no changes in cystometric capacity and voiding detrusor pressure at maximal flow rate. Four hypermobility patients had early failure with recurrent stress incontinence within 3 months. In contrast to cured cases, these four failed cases still had their bladder neck below symphysis pubis at resting and retain similar bladder neck hypermobility after the surgery (MBND 3.1 vs. 3.0 cm). Their maximal uroflow rate also remained unchanged.

Conclusions:
Our findings show a close correlation between decreased bladder neck mobility and surgical success following pubovaginal sling surgery, particularly for patients with urethral hypermobility. It seems that the sling limits mobility of the bladder neck and proximal urethra, forming a "hammock area" to prevent stress urinary leakage. Failure to properly limit bladder neck mobility leads to unsatisfactory surgical outcome. We suspect that the sling also has a restrict effect on bladder outlet opening, which prevents inadvertent bladder outlet opening during abdominal stress and slightly reduces urinary flow rate during voiding, as observed in most successfully treated cases.