Functional Anatomy of Urogenital Hiatus Closure: the Perineal Complex Triad Hypothesis

Abstract

Introduction: Urogenital hiatus enlargement is a critical factor associated with prolapse and operative failure. This study of the perineal complex was performed to understand how interactions among its three structures: the levator ani, perineal membrane, and perineal body-united by the vaginal fascia-work to maintain urogenital hiatus closure.

Methods: Magnetic resonance images from 30 healthy nulliparous women with 3D reconstruction of selected subjects were used to establish overall geometry. Connection points and lines of action were based on perineal dissection in 10 female cadavers (aged 22-86 years), cross sections of 4 female cadavers (aged 14-35 years), and histological sections (cadavers aged 16 and 21 years).

Results: The perineal membrane originates laterally from the ventral two thirds of the ischiopubic rami and attaches medially to the perineal body and vaginal wall. The levator ani attaches to the perineal membrane's cranial surface, vaginal fascia, and the perineal body. The levator line of action in 3D reconstruction is oriented so that the levator pulls the medial perineal membrane cranio-ventrally. In cadavers, simulated levator contraction and relaxation along this vector changes the length of the membrane and the antero-posterior diameter of the urogenital hiatus. Loss of the connection of the left and right perineal membranes through the perineal body results in diastasis of the levator and a widened hiatus, as well as a downward rotation of the perineal membrane.

Conclusion: Interconnections involving the levator ani muscles, perineal membrane, perineal body, and vaginal fascia form the perineal complex surrounding the urogenital hiatus in an arrangement that maintains hiatal closure.

Keywords: Levator ani; Pathophysiology of pelvic organ prolapse; Pelvic floor birth injury; Perineal body; Perineal membrane; Urogenital and levator hiatus.

Figure 1.. 3D reconstruction of the perineal complex made from an MRI of a vaginally nulliparous woman with normal support

a) Seen obliquely from below in the standing posture including the external anal sphincter (EAS); perineal body, (PB), urethra (U), and vagina, (V). b) Interconnections of the perineal complex are shown with the semitransparent pubic bones and perineal body, using the same orientation as panel a. The three transverse blue bars on each side indicate the lines-of-action for the fibers of the perineal membrane (PM), the red fibers represent the fibers of the medial levator ani (LA), and the white bars show connections through the perineal body (semitransparent white). The grey hoop represents the connective tissue surrounding the urogenital hiatus.

Figure 2.. Cadaver dissection of a specimen with normal support viewed from the left side

a) after removal of the lateral portions of the pubic bones. b) the pubic bones have been removed with their outline represented. The perineal membrane is held in position with forceps. c) the perineal membrane is reflected caudally to reveal the connections between the medial levator ani and perineal membrane (black arrow). The dotted line indicates the plane of histological section shown in Figure 3. CC, clitoral crus; CG, clitoral glans; PB, perineal body; PM, perineal membrane; VB, vestibular bulb; LA, levator ani.

Figure 3.. Whole pelvis cross sections in axial (a), coronal (b), and parasagittal (c) planes

Dense connections (arrowheads) between the levator ani (LA), perineal body (PB), perineal membrane (PM), and vaginal wall (V) are shown. Approximate location of sections in other panels are designated by dotted lines and letter indicating panel. Dotted rectangle denotes approximate location of Figure 4a. Cx, cervix, IT, ischial tuberosity; P, pubis; STP, superficial transverse perineal muscle; VB, vestibular bulb.

Figure 4.. Histological sections using Masson’s Trichrome stain

a) Parasagittal trichrome histological section generally oriented as shown in Figure 3c (different specimen) showing the insertion of the levator ani muscles into the dorsal surface of the perineal membrane (arrows) and an area with a lack of insertion (arrowhead). b) Coronal histological section from a different cadaver. The left side has been reflected across the midline to allow for labeling on one side. Note the connection between the levator ani muscle and dense endopelvic fascia (white arrowhead) and between the fascia and perineal membrane (black arrowheads). BSM, bulbospongiosus muscle; CC, clitoral crus; CG, clitoral glans; CU, compressor urethra; ICM, ischiocavernosus muscle; PB, perineal body; PM, perineal membrane; PR, periosteum of pubic ramus; VB, vestibular bulb; VG, vestibular gland.

Figure 5.. Demonstration of perineal complex kinematics

Panel a simulates the normal tone of the levator ani that holds the perineal body up in its normal location at the external urethral meatus. In Panel b, relaxation of the upward force simulates loss of levator ani muscle tone causing the perineal membrane to spread in the anterior-posterior direction to the extent allowed by its connective tissue that acts as a “catch chain” once extended. The prolapsed anterior wall can be seen in the enlarged hiatus. Panel c shows attempts to move the medial margin of the perineal membrane in the transverse direction that are restricted by the membrane which resists displacement more in this direction compared to the anterior posterior direction.

Figure 6.. Anatomical consequences of failures in the perineal complex

a) Dissection of a cadaver with normal anatomy showing the close connection of the perineal membranes connected through the perineal body (black dots) and normal hiatus (dashed line). b) dissection of a cadaver with prolapse revealing separation of perineal membranes (black dots), widened levator hiatus (horizontal arrow), and lengthening of the anterior-posterior diameter due to loss of levator action and spreading of the perineal membranes (vertical arrows). For comparison, the dashed line represents size of normal hiatus. (From Halban and Tandler 1907 [22]).