This project demonstrates that advanced MR imaging and 3-D modeling can provide a detailed anatomic depiction of the normal posterior compartment structures in asymptomatic, nulliparous, living women. It also illustrates clear posterior compartment boundaries provided by muscular and connective tissue structural components that can be reliably visualized in nulliparas. Our approach of using characteristic features in the various scan planes provides an inventory of anatomic findings and the frequency with which these features can be seen. It extends previous cadaver-based anatomical work by demonstrating a technique that can be used in living women.
There are several clinical implications of being able to directly image elements of the posterior compartment. Improved surgical outcomes have been achieved in other surgical disciplines through accurate anatomical identification of structural abnormalities as has been possible with lumbar disk disease or complex shoulder injury. Several steps are needed to accomplish this goal. First, it is necessary to identify the appearance of normal structures in imaging studies as we have done. Second, the appearance of different structural failures must be established in symptomatic women by comparing changes seen in their anatomy to normal. Third, the accuracy of the imaging modality to diagnose injuries is established through surgical exploration. Knowing normal MR cross-sectional and 3-D anatomy provides a guide to basic features of anatomy undisturbed by changes that occur during vaginal delivery; deviations and differences in women with defects of the posterior compartment can then be easily compared to normal anatomy. The list of normal features provided here can now be used to target areas for investigation that seeks to identify specific areas of structural failure.
The future application of these techniques to study symptomatic women with posterior wall failure can be expected to provide important information to guide surgical therapy and help understand operative failure. Directed repair of identified defects holds the theoretical promise of avoiding unnecessary surgical disruptions of normal areas and hopefully de novo complications such as the occurrence of postoperative dyspareunia6
. MR studies also have the advantage of providing objective depictions of living women’s anatomy. Clinically, this may help in studying operative failure by providing a permanent record of anatomy that can be examined postoperatively and thus help identify preoperative anatomical factors that may be associated with operative failure. In addition, several different observers, blinded to subject complaint and clinical findings, can independently evaluate anatomy to test differing hypotheses using the same dataset. This may help improve on the poor inter-examiner and intra-examiner agreement that investigators have found with clinical exams for prolapse.7
Previous studies using surgical and cadaveric dissections have investigated the structures of the posterior compartment such as the rectovaginal septum,8
as well as its attachments to the pelvic sidewall5
. Although our group has significant experience with cadaveric anatomical studies, we have found that MR analysis and 3-D models have added value; these analyses provide an inventory of normal characteristic cross-sectional and 3-D features in living women, in whom clinical symptoms and physical findings are known9
. MR imaging has been used to study normal anatomy of the levator ani muscles and the anal sphincter10,11,12,13
but, not to our knowledge, the overall structural organization of the posterior compartment. While previous studies have used 3-D modeling techniques for studying the levator ani 14,15,16,17,18
and anal sphincter,19
this is also the first study, to our knowledge, to have built a model of the posterior compartment in its entirety. Our 3-D model helps to elucidate the complex interaction of the anatomy as a whole and thus, is valuable in the ultimate understanding of the posterior compartment structure and function. 3-D modeling demonstrated the boundaries of the posterior compartment as well as how specific MR cross-sectional anatomy related to each other in 3-D space.
In interpreting the results of our study, several limitations must be kept in mind. Our MR images are acquired with a 0.5 cm interval between images. This may account for why thin structures oblique to the scan plane, i.e. the uterosacral ligaments, were not seen in 100% of the images examined. While we do not think that this is a significant limitation for analysis of general aspects of normal anatomy, it may become more important in studying disruptions or defects in determining if a structure is actually missing or not imaged. The perineal body region, where localized defects in the fibromuscular layer of the posterior vagina occur as described by Richardson in 1995,20
and which resulted in changes in surgical treatment,21,22
are not well visualized at present. The newer, 3 Tesla magnets will allow good images with 0.2 cm slice intervals which should improve this situation in the near future. For our current projects, we are investigating the 3-T magnet as well as new protocols to improve our visualization of structures and overcome some current limits on visibility. In addition, at our current sample size we are not able to comment on racial differences that may exist in the posterior compartment anatomy due to the high proportion of Caucasians in our study sample.
While we are not currently advocating the routine use of MRI as a pre-operative imaging tool, based on our demonstrated ability to image normal features in this area, it seems likely in the near future that routine imaging may be warranted. This imaging may serve to objectively identify specific abnormalities in each of the regions of posterior vaginal wall support prior to surgical intervention. At present, defect identification depends on surgical dissection which does not allow for tailored treatment or surgical plan pre-operatively and requires dissection of areas that may not need repair. Pre-operative imaging studies may benefit patients in the same way that detailed MR imaging currently help orthopedic surgeons manage a complex knee injury23
The normal posterior compartment has characteristic anatomic features that can be elucidated using MR imaging and 3-D modeling.