Interpretation
Several reference lines have been proposed to stage POP on MRI, but to the best of our knowledge, no reference lines represent the axis of the normal vagina9,10. In this study, we compared the PS3L with the pubococcygeal line (PCL). The PCL is the most widely used and recommended reference line for POP staging with MRI3,9. Traditionally, in situations in which the bladder neck and vaginal vault or distal edge of the cervix descend below the PCL on MRI, the diagnosis of prolapse is established3. As shown in this study, the PCL was under the vaginal axis, which is consistent with previous studies3,9. In fact, the PCL was thought to approximate the axis of the levator plate18. Other lines, such as the midpubic line and the perineal line, were introduced and were expected to correspond to the level of the hymen19,20. The hymen is the fixed reference point recommended by the International Continence Society and is used by urogynaecologists to stage POP11. Clinically, successful surgical treatment for prolapse from an anatomical perspective has been defined as no apical descent greater than one-third into the vaginal canal or anterior or posterior vaginal wall beyond the hymen21. However, the plane of the hymen is anterior to the pubic bone and crosses the urethral meatus4. Evaluations of POP based on these lines are not in situ assessments and could result in underestimation, further leading to incomplete or incorrect surgery. Nearly one-third of patients undergoing surgery for POP repair were estimated to require reoperation within 4 years after the initial surgery22. After prolapse surgery, new pelvic floor symptoms may develop, while preexisting pelvic floor symptoms may improve, worsen, or remain unchanged21.
In this study, the measurements based on the PS3L showed superiority over those based on the PCL because they were more concentrated, that mainly because relative to the PCL, the PS3L has an orientation that mostly conforms to the normal vaginal anatomy, and in situ evaluation may reduce deviations to a certain degree. Therefore, quantifying POP based on the PS3L may be more likely to allow quantification and grading of the extent of POP.
However, the measurements still showed variation in these parameters, even in the young women, possibly because the uterosacral ligament is attached anteriorly mostly but not exclusively on the posterior aspect of the cervicovaginal junction23, posteriorly broadly to the first three sacral vertebrae and variably to the fourth sacral vertebra24. In addition, the definitive role of paravaginal support in the middle third of the vagina and its contribution to the development of prolapse are still unknown25,26. In this study, the vaginal points in the elderly women were slightly lower than those in the younger women. Ageing, multiparity, and especially previous vaginal delivery are high risk factors for POP27, and a moderate degree of prolapse in continent women based on MRI has been reported due to its ability to measure actual pelvic organ descent19,28. However, in most cases, overall, as shown in this study, the PS3L may represent the axis of the normal upper two-thirds of the vagina