Anatomy, Abdomen and Pelvis, Bones (Ilium, Ischium, and Pubis)

Article Author:
Anna Wobser
Article Editor:
Randy Wobser
Updated:
10/27/2018 12:32:03 PM
PubMed Link:
Anatomy, Abdomen and Pelvis, Bones (Ilium, Ischium, and Pubis)

Introduction

The human pelvis is a group of bones fused together and functions to join the axial skeleton (bones of the head, neck, and vertebrae) to the lower appendicular skeleton. The axial skeleton joining the pelvis is the spinal column, specifically, the lumbar spine and the bones of the coccyx. The appendicular skeletal bones the pelvis joins are the two femurs. The main function of the pelvis is support and locomotion for the human. It provides attachment points for muscles, tendons, and ligaments that allow for support and changes in orientation of the axial and appendicular skeleton that allows for changes in orientation between the two, which is best described as movement. While relatively fixed joints bind the axial skeleton, the appendicular skeleton (the femurs) are joined to the pelvis via a relatively free-floating ball and socket joint.  This allows maximal mobility of the joint. The pelvis may be considered the first step in the "linkage" of the axial skeleton to the lower appendages that allow locomotion of the human. [1]

In discussing the pelvis, a distinction needs to be made between the "pelvis" and the "pelvic girdle," as the 2 are often used interchangeably. The pelvic girdle, or OS coxae, consists of the fused bones identified as the ilium, ischium, and pubis. Two of these girdles are fused together in the anterior by the pubic symphysis, and in the posterior at the sacrum and it's attached coccyx. This entire group of 2 pelvic girdles and the sacrum is known as the pelvis.

Structure and Function

Clinicians must understand the pelvis structure to understand its function. The pelvic girdle, or "hip bone" as it is commonly referred to, is comprised of 3 bones on each side. These bones are the ilium, ischium, and pubis. These hip bones are attached to the sacrum in the in the most dorsal area by ligaments. These are known as sacroiliac ligaments. In addition to the ligaments of this joint, the interposing areas of the sacrum and the ilium have matching, irregular contours to increase the joint strength. At the most ventral area of each pubis, a there is a fibrocartilaginous joint. It is amphiarthrotic, meaning that is somewhat flexible. The appendicular skeletal "hip bones" and the axial skeleton sacrum form the entire bony pelvis.

The most important purpose of the pelvic girdle is as a mechanical structure associated with locomotion. The relatively fixed joint with the lumbar spine and the stable structure provided is ideal for the load bearing of the upper body. The hip joint, or acetabulum, is at the central location on each side of the pelvis. The girdle provides origin points for various muscles of the legs that allow for movement of the femoral head in the acetabulum. The distance from the acetabulum to the crest of the iliac allows for greater leverage of the muscles attached than if those muscles originated more closely to the joint. This would give an evolutionary advantage that allows for more rapid movement than other forms of locomotion at a lower energy cost. The pelvis is also connected by muscles to the axial skeleton to allow orientation and stabilization.

The pelvis has a "floor" that has several functions. The first function is to contain the visceral organs. This provides structural integrity to the floor (caudal region) of the pelvis and prevents prolapse through the pelvic outlet. The second function is to control the opening of the anus, vagina in females, and the urinary outlet. This is accomplished through several layers of musculature and ligaments. This floor of the pelvis will be described in a later section.

Embryology

The earliest appendicular skeletal elements appear at approximately 28 days, which coincides with the appearance of limb buds. Condensation of mesenchymal tissue in the fifth week form in the limb buds to form bone models. These models undergo chondrification to form hyaline cartilage bone models beginning in the sixth week. At this point, radiologic studies can discern a basic skeletal system. By the time the fetus reaches the gestational age of 9 weeks (7 weeks after fertilization), chondrification centers in the ilium, ischium, and pubis first appear and grow rapidly. The following week the first ossification center is present in the iliac crest. At the same time, fusion if the sacrum and ilium occur. After the tenth week of gestation, the fusion of all the bones of the girdle and pelvis has occurred. Primary ossification centers are present. Secondary ossification continues post-birth. Complete ossification is not effected until adulthood. [2]

Blood Supply and Lymphatics

The aorta bifurcates into the common iliac arteries at the level of the umbilicus. At the pelvic brim, the iliac arteries bifurcate into the internal and external iliac arteries. The external iliac exits the pelvis through the inguinal ligament and supplies the ipsilateral lower extremity with blood. The internal iliac artery supplies the pelvic organs, perineal floor, and gluteal muscles with blood. It has 2 major divisions, known as the anterior and posterior. The anterior division supplies the upper part of the bladder via the umbilical and superior vesicular arteries. The pelvic floor, head of the femur, and ilium are supplied via the obturator artery. The trigone of the bladder, prostate, and seminal glands are supplied by the inferior vesical artery. The middle rectal artery supplied the rectum with blood. The internal pudendal artery supplies the perineal muscles and skin, as well as the as the penis and clitoris. In women, the uterine artery supplies blood the uterus, fallopian tube, ovary, cervix, and vagina. The posterior division includes several branches. The iliolumbar artery supplies blood to the psoas major, iliacus, and quadratus lumborum muscles. It also supplies the cauda equina. The lateral sacral arteries supply blood to the sacral plexus and vertebral canal. Other minor arteries supply blood to the pelvis and do not originate from the iliac. The superior rectal artery supplies blood to the rectum and anus. The median sacral artery anastomoses with the with the lateral sacral arteries. It is a vestigial, direct continuation of the aorta. Finally, the ovarian artery supplies blood to the ovaries and anastomoses with the uterine artery.

There are several groups of lymph nodes in the pelvis. They include the external, internal, and common iliac nodes. They also include the sacral, pararectal, lumber, and inguinal nodes. There is extensive communication for these node groups, and it facilitates spreading of cancer to other pelvic or abdominal organs.

Nerves

There are two plexuses of nerves that provide motor and sensory innervation to the pelvis. They are the sacral plexus and the coccygeal plexus. [3]

The sacral plexus arises from L4-S4 spinal nerves. It includes the sciatic nerve L4 through S3. This is the longest nerve in the body. It innervates almost the entire skin of the leg as well the muscles of the leg and foot. The S2-S4 pudendal nerve innervates both the skin and muscles of the perineum. The superior gluteal nerve L4 through S1 and the inferior gluteal nerve L5 through S2 innervate the gluteal muscles. The nerve to the quadratus femoris L4 through S1 innervates the quadratus femoris and gemellus inferior muscles of the hip. The nerve to the obturator internus muscle L5 through S2 innervates said muscle. The nerve to the S1-S2 piriformis muscle innervates said muscle. The S2-S3 perforating cutaneous nerve innervates the medial and lower part of the buttock. The S2-S3 posterior femoral cutaneous nerve innervates the skin on the posterior thigh/leg and the perineum. The S2-S4 pelvic splanchnic nerves provide parasympathetic innervation of all pelvic organs.

The coccygeal plexus S4 through S5 and coccygeal nerves innervate the coccygeus and levator ani muscles, and well as the skin immediately dorsal to the anal vent.

The obturator nerve arises from the lumbar plexus L2 through L4 and innervates the skin of the medial aspect of the thigh.

Parasympathetic and sympathetic innervation is also present. The superior hypogastric plexus provides sympathetic innervation to the pelvis. The inferior hypogastric plexuses contain both parasympathetic and sympathetic fibers. The S2-S4 pelvic splanchnic nerves S2-S4 provide parasympathetic innervation to pelvic organs.

In general, parasympathetic and sympathetic innervation works as expected in the "fight or flight" response. Parasympathetic output causes gastrointestinal (GI) peristalsis, and contracts muscles for defecation and urination. It is also involved in engorgement of erectile tissue. Sympathetic output acts antagonistically to the parasympathetic output and is also involved in muscle contraction during orgasm. Remember the medical school phrase that compares the male sexual function to firing a pistol: "point (parasympathetic) and shoot (sympathetic)."

Muscles

The muscles of the pelvis are divided into 3 groups. Some muscles attach to the trunk and provide movent and stabilization. Other muscles attach to the hip and thigh and provide stabilization and locomotion. Finally, the muscles of the pelvic floor and perineum are located wholly within the pelvis. These are involved in support of the pelvic floor as well as in urogenital/Gi functions. [4][5]

There are 3 layers of the pelvic floor musculature. From caudal (superficial) to cephalad (deep), they are the urogenital triangle, the urogenital diaphragm, and the pelvic diaphragm.

The urogenital triangle consists of several muscles. The superficial transverse perineal muscle originates at the ischial tuberosity and extends to the central, tendinous perineal body, which lies immediately ventral to the anus. It is involved primarily with the support of the pelvic floor. The second muscle is the ischiocavernosus. It extends from the ischial tuberosity to the ischiopubic rami and is involved in supporting the erect male penis, the female vagina, as well as flexing the anus. The third muscle is the bulbocavernosus (bulbospongiosus in men). It is located in the midline of the pelvis and describes a dorsal to a ventral path starting from the tendinous body and coursing to the ischiocavernosus. In both males and females, it contributes to erection. In the female, the fibers are divided and course laterally around the vaginal introitus. The final muscle in the urogenital triangle is the anal sphincter. This is attached to the dorsal end of the perineal body or central tendinous portion, and some fibers spread to the levator ani posteriorly. The function of this muscle is defecatory regulation.

Deeper (more cephalad) into the pelvis is the urogenital diaphragm. It is a triangular ligament with a string muscular component that stretches from the pubic symphysis to the to the ischial tuberosities. It occupies the ventral portion of the pelvic outlet. It is composed of several muscles. The urethral sphincter is an autonomically-controlled internal sphincter located at the urethro-vesicular junction. In males, the external sphincter is located just caudal to the prostate, and in females, it is located just caudal to the internal sphincter. The external sphincter is skeletal muscle and is under voluntary control. Both the internal and external sphincter are involved in regulating urination. The deep transverse perineal muscle extends from the inferior rami of the ischium and inserts into the tendinous median plane. It aids in support of the pelvis. The caudal and cephalad borders of this muscular-tendinous plane are embedded in a layer of fascia. The caudal layer of fascia is known as the perineal membrane.

The third, most cephalad (deep) layer is the pelvic diaphragm. It is a thin and muscular layer that extends from the pubic ramus to the coccyx and insert laterally into arcus tendinous fascia. It is a funnel-shaped sling and represents the inferior (caudal) border of the internal pelvic cavity.  It is composed of several muscles and endopelvic fascia. The largest muscle is the levator ani. This muscle is composed of 3 muscles: the puborectalis, pubococcygeus, and the iliococcygeus. The levator ani's ventral border is the posterior surface of the superior pubic rami. It courses bilaterally around the pelvis and inserts at the arcus tendinous. This is a condensation of the membrane of the obturator internus muscle. Between the left and right levator pass the urethra, anal canal, and vagina (in females).

Physiologic Variants

The most significant physiologic variant of the pelvis relates to child-bearing. There are 4 recognized variants of the pelvis as classified by the "Caldwell-Moloy" system. [6]

Gynecoid

This is the classic female pelvis and is most suited for childbirth and is found in roughly half of all females. The inlet (cephalad border) is ovoid to almost circular. The pubic arch is wide as is the pelvic outlet. The sacrum is deeply curved, and the ischial spines are relatively blunt. Overall, the internal dimensions of this pelvic type are the largest and most capable of childbirth.

Android

This is the typical male pelvis. It is present in approximately 20% of women. The inlet is heart-shaped, and the sidewalls are more convergent. The sacral curve is more shallow. Overall, internal dimensions are smaller than those of the gynecoid pelvis and childbirth is relatively difficult due to this.

Anthropoid

Common in men and present in approximately 25% of women. It has an ovoid inlet and is greater in dimension from front to rear than side to side. The pubic angle is somewhat less than the gynecoid but greater than the android pelvis. This pelvis is generally considered suitable for childbirth.

Platypelloid

A very wide and shallow inlet is the hallmark of this pelvis. It is present in approximately 5% of women. This pelvis is commonly associated with a high transverse arrest of labor due to the inability of the fetal head to navigate the inlet due to insufficient space from the anterior to posterior of the inlet. Normal vaginal birth is very uncommon with this pelvis.

To assist childbirth with any type pelvis, the ligaments binding the pubic bones become more pliable due to the hormone "relaxin" made by the placenta. It allows the pelvis to enlarge by facilitating limited motion of the pubic symphysis. This is also a common source of pain in later pregnancy.

Surgical Considerations

The surgeon must be aware of several considerations to navigate the pelvis successfully. The most common will be discussed here. The ureters enter the pelvic brim at the bifurcation of the common iliac and can be found lying over this bifurcation. In a pelvis with anatomy distorted by the disease process, location of the ureter may be known by identifying it at the entrance and tracked by palpation or dissection to the lower pelvis. This way, a surgeon may ensure that he or she is working safely away from them during pelvic surgery. Additionally, the ureters course approximately 2 cm from the cervix at the level of the uterine artery. One study showed that in 15% of women, that distance is less than 5 mm. This may explain the observed incidences of ureteral ligation during hysterectomy. [7]

Clinical Significance

The pelvis has several disease processes that are of clinical significance, and they are predominately in the female pelvis. Scar tissue or adhesive tissue bridges from ovulation or several disease processes can be a source of both pain and infertility. When a woman ovulates, the follicular liquor is released, and the follicle becomes hemorrhagic. This can cause abnormal tissue bridges to form through activation of natural healing mechanisms. This may cause pain when tension is placed on these adhesions as well as infertility from physically interfering with the transport of the egg to the fallopian tube. [8]

  • Endometriosis is a disease in which the uterine lining (endometrium) is found outside of the uterus. This causes bleeding in the pelvis during every menses. The blood irritates the peritoneum acutely and causes pain.  In addition, the activation of natural repair mechanisms promotes the formation of adhesions, or scarring, that are involved in chronic pain.  Interestingly, the amount of endometriosis correlates with the degree of infertility but does not correlate with the level of pain a woman may feel. Normal treatment for these women is hormonal to stop menstruation and progression of the disease. Laparoscopic surgery may be used to lyse scar tissue bridges and ablate areas of endometriosis. In recalcitrant cases, a hysterectomy with removal of fallopian tubes and ovaries is required to provide relief.
  • Pelvic inflammatory disease causes pain and infertility by promoting the formation of scar tissue and interfering with fertilization. This is an ascending infection from the uterus that causes a massive inflammatory response as it travels through the cervix, uterus, tubes, and into the pelvis.  Often scar tissue is found between the liver and anterior abdominal wall as the infection travels throughout the abdomen.  An abscess can form in either or both fallopian tubes.  These usually respond to antibiotics but may need to be treated surgically.
  • The uterus may be a source of pain symptoms in the pelvis. Uterine fibroids, which are benign tumors of unicellular origin, may cause pain when they become hypoxic. This is not uncommon during pregnancy. In other cases, they may cause symptoms such as urinary frequency, constipation, or bloating merely due to occupying space. They may be treated with a GHRH agonist, or surgically. If they are not symptomatic, they are generally not treated.

Pelvic floor laxity may be of clinical significance in the female pelvis. Damage to the endopelvic fascia for any reason can lead to weakened support of the vagina. By far the greatest risk factor for this condition is natural childbirth. A weakened anterior vagina can cause incontinence of urine and prolapsing of the bladder (cystocele) into the vagina to varying degrees. Weakened apical support can allow the cervix and uterus to prolapse into, and in some cases, out of the vagina. A weakening of the posterior wall of the vagina can allow the small bowel (enterocele) or rectum (rectocele) to prolapse into the vagina. In severe rectoceles, a woman must insert fingers into the vagina to facilitate defecation by supporting the rectum. In almost all cases of prolapse, a woman will feel the pressure that progresses throughout the day.   A wide range of treatments is available. If symptoms are minimal, the physical therapy is usually offered. If they are more severe, then surgical correction of the defect is usually done. In women who are not sexually active or are not surgical candidates, a pessary may be placed into the vagina to provide support.

Pain from the bladder may be of clinical significance. This may be from a urinary tract infection that is easily treated with antibiotics, to more chronic diseases of the bladder such as interstitial cystitis.

Other disease processes such as fibromyalgia and muscle spasm may present in the pelvis. With fibromyalgia, the symptoms will usually not be isolated to the pelvis. With muscle spasms, there is frequently a history of sexual trauma and pain with initial penetration during coitus due to the spasming of the pelvic floor musculature. Treatment is usually physical therapy.

Genetic or acquired deformities/injuries to the bony structure of the pelvis may be of clinical significance to the patient. Standard radiologic procedures may be used to diagnose these.