The perineal body (PB), also referred to as the central tendon of the perineum, is a fibro-muscular structure located in the midline of the perineum. Its exact location is the midline of the perineum at the junction of the anus and urogenital triangle in both females and males. In males, it is located between the anus and bulb of the penis, whereas in females it is found between the anus and the posterior limit of the vulvar orifice. The perineal body helps strengthen the pelvic floor. It provides attachments to the following muscles:
Two fascial structures join to the perineal body:
The perineal body is critical for maintaining the integrity of the pelvic floor, especially in females.
The perineal body may rupture during vaginal delivery. Once this occurs, it leads to a widening of the gap between the free borders of the levator ani muscles on both sides. This widening gap predisposes women to prolapse of the rectum, uterus, and sometimes the urinary bladder. It is also involved in the surgical procedure for anorectal tumors to achieve a tumor-free circumferential resection margin for reducing the risk of local recurrence.
The perineal body (PB) is the central fibrous skeleton of the perineum and has a pyramid shape in the male and a wedge-shaped in the female; laterally is formed by the perineal smooth muscle, the anterior continuity of the longitudinal anal muscle. The perineum is the region below the pelvic diaphragm; it consists of muscle-fascial formations arranged to close the pelvis inferiorly. The perineal musculature would have a different embryological origin than the musculature of the pelvic diaphragm and receives innervation by a group of motoneurons, grouped in the nucleus of Onuf, through the pudendal nerve branches. It extends from the skin to the inferior fascial surface of the pelvic floor and occupies a median position between the buttocks and the medial surface of the thighs. The perineum is bounded anteriorly by the pubic arch, laterally, in a posterior direction from the lower branch of the pubis and the ischium, to the ischial tuberosity and the sacrotuberous ligament, and posteriorly ends at the apex of the coccyx. It has a rhomboid shape, with a major axis directed anteroposteriorly; a transverse line passing through the ischial tuberosities divides it into two triangular regions:
The anterior and posterior triangle do not lie on the same plane: the urogenital triangle is tilted down and back, the anal triangle down and forward. The line that divides the two triangles are externally formed by the transverse muscles of the perineum: it fits on the ischial tuberosities laterally and mixed the fibers in the middle contributing, to create the perineal body (the central tendon of the perineum). The posterior border of the superficial perineal fascia joins the PB. Superiorly it is attached the rectovaginal/rectoprostatic septum continuing up to the arcus tendineus of the endopelvic fascia.
Studies confirm the variability in describing the PB components and acknowledge the difficulties in dissecting. The studies utilized thin-slice MRI and reported that the PB consisted of the bulbospongiosus muscle, the superficial transverse perineal muscle, the internal and external anal sphincters, the puboperinealis, and puboanalis portions of the puborectalis muscle.
The perineal body is the space in which the connective tissue of muscles and septa join. It is the meeting point of the superficial and deep layers of the pelvis contributing the balance of biomechanical forces. It integrates the excretory functions of the urogenital and anorectal organs absorbing the posterior visceral movements. Continence is maintained by the integrity of this system related to the respiratory function of the diaphragm and the postural and locomotor functions of the trunk and the inferior limbs. In the female, hormones during pregnancy influence tissue density and regulate the elongation properties of the perineal body and the pelvic soft tissues, stretching during childbirth.
During the concomitant formation of the first anlage of the pelvic organs, two muscle groups arise the pubis-caudal group and the cloacal group, or Gegenbauer's muscle.
From the pubic-caudal group will derive the coccygeal muscle and the levator anis muscle with their fibrous differentiations, the pubic-sacral or pubic-urethra-bladder-recto-sacral ligaments in man (the pubic-urethra-bladder-uterus-recto-sacral in the woman), the sacrospinous and sacrotuberous ligaments.
From the cloacal group, after the descent of the genitourinary septum that will separate the rectum, posteriorly, from the bladder and the urethra (urethra and vagina in the woman) anteriorly, will derive the sphincteric muscles (of the anus and of the urethra) and the bulbocavernosus, ischiocavernosus and the superficial and profound transverse muscles. There are close relationships and sometimes a fusion of various anatomical components. These include the levator anis muscle of the rectum and the urethra, the puborectalis muscle with the sphincter of the urethra that surrounds the prostate, the external anal sphincter, and with the deep transverse muscle of the perineum. These relationships have become functionally fundamental given the new upright posture and the bipedal gait acquired in humans.
During the very early fetal period, the mesenchymal tissue separating the orifices of the anus and vagina and the muscular cells surrounding them are clearly recognizable in specimens.
The blood supply depends upon the perineal branches of the pudendal vessels that nourish the superficial and muscular tissue of the perineum and the tissues placed inferiorly to the plane of the levator anis muscle.
The perineal body is innervated mainly by the perineal nerve, the broader and inferior terminal branch of the pudendal nerve, directed forward under the internal pudendal artery. The perineal artery and nerve accompany each other, and the nerve divides into the posterior scrotal (or labial) branches and muscular branches:
• Posterior scrotal or labial branches: medial and lateral. These pierce the inferior sheet of the urogenital diaphragm, or they run juxtaposed to its lower surface, to move anteriorly through the lateral wall of the urogenital tract with the scrotal or labial branches of the perineal artery. Their nervous distribution is to the skin of the vulva or scrotum, anastomosing with the perineal branch of the posterior cutaneous nerve of the thigh and with the inferior rectal nerve.• The muscular branches innervate the superficial transverse of the perineum, the bulbocavernosus, the ischiocavernosus, the deep transverse muscle of the perineum, the sphincter of the urethra and the anterior portion of the external anal sphincter and the levator anis muscle. A branch called the nerve of the bulb of the urethra comes off from the nerve directed to bulbocavernosus, crosses this muscle and innervates the spongy body of the urethra, ending in the urethral mucosa.
Several muscles form and are attached to the perineal body:
The perineal body is essential in maintaining the continence in humans; it is the central focus of the level III of the continence system of DeLancey. Damage occurs predominantly during childbirth for spontaneous tears or episiotomy; overstretching can cause neural and muscular-fascial lesions. Rectocele is a consequence of the PB lesion, wherein it divides into two parts joined by a stretched central part.
The perineal body should be the first structure investigated when evaluating pelvic floor function. PB moves inferiorly during inhalation (relaxed), and superiorly during exhalation as the pelvic floor muscles contract. During squeezing the perineal body lifts anterosuperiorly; evaluation of the force of the pelvic floor muscles is by using manual vaginal palpation. Movement of the PB against the fingers, inserted into the vagina, is prevented during active muscular contraction; a six-point scale (the Modified Oxford Grading System: 0 = no contraction, 1 = flicker, 2 = weak, 3 = moderate, 4 = good (with lift) and 5 = strong) describes the function. We can perform the test palpating the PB externally with a sufficiently light touch to sense with proprioceptive, tactile perception, the balance of force of the tissues converging into the PB from different directions. It is instructive to simplify the image of the perineal body as a knot made up of many strings tied together: the knot follows the tension of the strings; if not balanced the knot will change its position, and its movement will be impaired.