A unified pelvic floor conceptual model for studying morphological changes with prolapse, age, and parity

Abstract

Several 2-dimensional and 3-dimensional measurements have been used to assess changes in pelvic floor structures and shape. These include assessment of urogenital and levator hiatus dimensions, levator injury grade, levator bowl volume, and levator plate shape. We argue that each assessment reflects underlying changes in an individual aspect of the overall changes in muscle and fascial structures. Vaginal delivery, aging, and interindividual variations in anatomy combine to affect pelvic floor structures and their connections in different ways. To date, there is no unifying conceptual model that permits the evaluation of how these many measures relate to one another or that reflects overall pelvic floor structure and function. Therefore, this study aimed to describe a unified pelvic floor conceptual model to better understand how the aforementioned changes to the pelvic floor structures and their biomechanical interactions affect pelvic organ support with vaginal birth, prolapse, and age. In this model, the pelvic floor is composed of 5 key anatomic structures: the (1) pubovisceral, (2) puborectal, and (3) iliococcygeal muscles with their superficial and inferior fascia; (4) the perineal membrane or body; and (5) the anal sphincter complex. Schematically, these structures are considered to originate from pelvic sidewall structures and meet medially at important connection points that include the anal sphincter complex, perineal body, and anococcygeal raphe. The pubovisceral muscle contributes primarily to urogenital hiatus closure, whereas the puborectal muscle is mainly related to levator hiatus closure, although each muscle contributes to the other. Dorsally and laterally, the iliococcygeal muscle forms a shelflike structure in women with normal support that spans the remaining area between these medial muscles and attachments to the pelvic sidewall. Other features include the levator plate, bowl volume, and anorectal angle. The pelvic floor conceptual model integrates existing observations and points out evident knowledge gaps in how parturition, injury, disease, and aging can contribute to changes associated with pelvic floor function caused by the detachment of one or more important connection points or pubovisceral muscle failure.

Keywords: levator ani avulsion; levator ani muscle; levator bowl volume; levator hiatus; pelvic floor conceptual model; pelvic floor muscle injury; pelvic floor shape; pelvic organ prolapse; urogenital hiatus.

FIGURE 1. Illustrations drawn from cadaver dissection of the female pelvic floor

A, Left lateral view of a normal pelvic floor after removal of the upper pelvic organs. B and C, Midsagittal view of 2 women with pelvic organ prolapse. Red lines represent the urogenital hiatus, yellow lines represent the levator hiatus, and the black dotted line represents the levator plate. The 3 different images were chosen to illustrate that the pelvic floor changes seen with prolapse are different in different individuals (note the levators sagging downward in B, whereas they are more horizontal in C). Adapted from Halban and Tandler. ACR, anococcygeal raphe; ATFP, arcus tendinous fascia pelvis; CM, coccygeus muscle; EAS, external anal sphincter; ICM, iliococcygeal muscle; LP, levator plate; OIM, obturator internus muscle; PVM, pubovisceral muscle; R, rectum; U, urethra; V, vagina. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.

FIGURE 2. The 3D levator bowl models

The top row shows sample 3D levator bowl models. A, Nulliparous young patient. B, Multiparous young patient. C, Nulliparous older patient. D, Older multiparous patient affected by pelvic organ prolapse. E, Superimposed conceptual model of the bowl volumes in A and D using isocurves. Isocurves are the 3D curves representing the contour on 3D surfaces that are widely used in computer graphics and 3D model rendering. 3D, 3-dimensional. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.

FIGURE 3. PFC model

A, Illustrations drawn from cadaver dissection of the female pelvic floor from above, after removal of the upper pelvic organs showing the structures (underlined), elements, and connection points (black dots) of the proposed PFC model. B, A schematic view of all the different connections among the different elements (in circles) and anatomic structures (in rectangles), shown as color-coded solid lines relating to the structures shown in A. Bold arrows indicate well-known relationships between 2 elements, whereas dashed lines show less studied or less known connections. PFC, pelvic floor conceptual. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024

FIGURE 4. Structural elements of the proposed conceptual model

Examples of the proposed conceptual model from 3D MRI reconstructions from specific subjects made on the 3D Slicer. The top row shows a woman with normal support; the bottom row shows a woman with pelvic organ prolapse. The left panels show an MRI midsagittal view with the 3D model superposed. The central panels show a 3D view of the model and its orientation compared with the pelvic bones. In the right panels, the key muscles are represented as bands. The anatomic structures (underlined) include the levator ani muscle subdivisions (PVM, PRM, and ICM), the perineal body and membrane, and the anal sphincter ring. The structural elements include the levator hiatus and urogenital hiatus, the levator plate, the transverse shelf, the bowl “rim,” and the bowl itself. 3D, 3-dimensional; ICM, iliococcygeus; MRI, magnetic resonance imaging; PRM, puborectalis; PVM, pubovisceral muscle. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.

FIGURE 5. Structures, elements, and connection points of the pelvic floor conceptual model

Connection points are enumerated I to V as shown in Figure 4. L, left; R, right. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.

FIGURE 6. Areas of possible structural deformationin the conceptual model

The dotted line represents the normal configuration. The solid red line indicates an example of a structural alteration from the normal configuration. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.

FIGURE 7. Levator plate shape variations

Variations in 16 women with postoperative failure (red) and 19 women with normal postoperative support are shown. On the right, the analysis identified 2 main modes of shape variations (PC1 and PC2). At rest, PC1 accounted for 61% of shape variation, and PC2 accounted for an additional 30% of variation. The PC1 scores differed significantly between the success and recurrence groups, whereas PC2 score distributions were similar between groups. Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.

FIGURE 8. Biomechanical consequences of pelvic floor changes

A, Midsagittal scans from 2 different women at rest and strain. The top row shows a magnetic resonance image of a woman with normal support at rest and maximal Valsalva maneuver (strain). The middle row shows a woman with uterine prolapse at rest and maximal Valsalva maneuver. The bottom row shows a schematic model simulating the changes in hiatus size and levator plate angle that happen with prolapse. Red circles simulate the hiatuses, and green bands simulate the levator plate. B, Coronal scans from 2 different women at rest and strain. The bottom row shows a model simulating the tension on the apical ligaments in the case of impaired support at rest and maximal Valsalva maneuver. Gray circles simulate intra-abdominal pressure effect on the muscles (red band). Delancey. A unified pelvic floor conceptual model. Am J Obstet Gynecol 2024.