BACKGROUND: To compare levator ani defects and pelvic floor function among women with prolapse and controls. METHODS: Levator ani structure and function were measured in a case–control study with group matching for age, race, and hysterectomy status among 151 women with prolapse (cases) and 135 controls with normal support (controls) determined by pelvic organ prolapse quantification examination. Magnetic resonance imaging was used to determine whether there were “major” (more than half missing), “minor” (less than half of the muscle missing), or no defects in the levator ani muscles. Vaginal closure force at rest and during maximal pelvic muscle contraction was measured with an instrumented vaginal speculum. RESULTS: Cases were more likely to have major levator ani defects than controls (55% compared with 16%), with an adjusted odds ratio of 7.3 (95% confidence interval 3.9–13.6, P<.001) but equally likely to have minor defects (16% compared with 22%). Of women who reported delivery by forceps, 53% had major defects compared with 28% for the nonforceps women, adjusted odds ratio 3.4 (95% confidence interval 1.95–5.78). Women with prolapse generated less vaginal closure force during pelvic muscle contraction than controls (2.0 Newtons compared with 3.2 Newtons P<.001), whereas those with defects generated less force than women without defects (2.0 Newtons compared with 3.1 Newtons, P<.001). The genital hiatus was 50% longer in cases than controls (4.7±1.4 cm compared with 3.1±1.0 cm, P<.001). CONCLUSION: Women with prolapse more often have defects in the levator ani and generate less vaginal closure force during a maximal contraction than controls. LEVEL OF EVIDENCE: II

Approximately 11% of U.S. women undergo surgery for pelvic floor dysfunction, including genital organ prolapse and urinary and fecal incontinence. The major risk factor for developing these conditions is giving vaginal birth. Vaginal birth is a remarkable event about which little is known from a biomechanical perspective. We first review the functional anatomy of the female pelvic floor, the normal loads acting on the pelvic floor in activities of daily living, and the functional capacity of the pelvic floor muscles. Computer models show that the stretch ratio in the pelvic floor muscles can reach an extraordinary 3.26 by the end of the second stage of labor. Magnetic resonance images provide evidence that show that the pelvic floor regions experiencing the most stretch are at the greatest risk for injury, especially in forceps deliveries. A conceptual model suggests how these injuries may lead to the most common form of pelvic organ prolapse, a cystocele.

Treatment strategies for stress incontinence are based on the concept that urethral mobility is the predominant causal factor with sphincter function a secondary contributor. To our knowledge the relative importance of these 2 factors has not been assessed in properly controlled studies. The Research on Stress Incontinence Etiology project is a case-control study that compared 103 women with stress incontinence and 108 asymptomatic controls in groups matched for age, race, parity and hysterectomy. Urethral closure pressure, urethral and pelvic organ support, levator ani muscle function and intravesical pressure were measured and analyzed using logistic regression and multivariable modeling. Mean +/- SD maximal urethral closure pressure was 42% lower in cases (40.8 +/- 17.1 vs 70.2 +/- 22.4 cm H(2)O, d = 1.47). Lesser effect sizes were seen for support parameters, including resting urethral axis and urethrovaginal support (d = 0.41 and 0.50, respectively). Other pelvic floor parameters, including genital hiatus size and urethral axis during muscle contraction (d = 0.60 and 0.58, respectively), differed but levator strength and levator defect status did not. Maximum cough pressure, which is an assessment of stress on the continence mechanism, was also different (d = 0.43). After adjusting for body mass index the maximal urethral closure pressure alone correctly classified 50% of cases. Adding the best predictors for urethrovaginal support and cough strength to the model added 11% of predictive ability. The finding that maximal urethral closure pressure and not urethral support is the factor most strongly associated with stress incontinence implies that improving urethral function may have therapeutic promise.