How is the pelvic bone, the main support of a person, arranged?

In men and women, the pelvis forms a bony ring through which body weight is distributed to the lower limbs, but in women the pelvis has some features that are adapted for childbirth. The pelvis consists of four bones: the sacrum, the coccyx and two pelvic or innominate bones. The pelvic bones are attached to the sacrum via the sacroiliac synchondrosis and to each other via the pubic symphysis.

• Hip bone
• Urogenital diaphragm
• Female pelvis in obstetrics
• Classification of pelvis types
• Clinical determination of pelvic capacity

Hip bone

Each pelvic bone is formed by the fusion of the ilium, ischium and pubis. Connecting with each other, these bones form the acetabulum.

The ilium has an upper section, the wing, and a lower section, the body. The place where they connect has an arcuate shape - an arcuate line. There are several projections on the wing of the ilium: in front - the anterior superior iliac spine, slightly below it - the anterior inferior iliac spine; behind - the posterior superior iliac spine and the posterior inferior iliac spine.

The ischium makes up the lower and posterior third of the pelvic bone. It has a body, which participates in the formation of the acetabulum, and branches. The body and the branch form an angle between themselves, at the top of which there is a thickening - the ischial tuberosity. The ischial ramus joins the inferior ramus of the pubis. On the posterior surface, the branch of the ischium has a protrusion - the ischial spine. The ischium is involved in the formation of the lesser sciatic notch.

The pubic bone forms the anterior wall of the pelvis and consists of a body and two branches: the superior, horizontal and inferior, descending. The lower branches of the pubic bones form an angle - the pubic arch. The body of the pubis is involved in the formation of the acetabulum. At the junction of the ilium and pubic bones there is an iliopubic elevation. A bony ridge runs along the upper edge of the superior ramus of the pubis, ending in the pubic tubercle. Both pubic bones are attached to each other via the pubic symphysis. The pubic symphysis has a cavity inside that is filled with fluid and increases during pregnancy. Relaxation of the symphysis begins in the first half of pregnancy and is especially pronounced during the last 3 months. Regression of such relaxation begins immediately after childbirth and is completely completed after 3-5 months.

The sacrum consists of 5-6 vertebrae that are motionlessly connected to each other and has a uniformly concave anterior surface. The first vertebra of the sacrum is connected by cartilage to the fifth lumbar vertebra, forming a promontory. The sacrum is connected to each of the pelvic bones by flat cartilaginous sacroiliac joints, which have some mobility, and two ligaments: the sacroosteal and sacrohumpy.

The sacrospinal ligament runs from the posterior surface of the sacrum to the ischial spine, the sacrohumpy ligament runs from the posterior surface of the sacrum to the ischial tuberosity. These ligaments bend around the lesser and greater sacrosciatic notches and form the greater and lesser sciatic foramina. The coccygeal bone is usually formed by 4-5 fused vertebrae, and is attached to the distal end of the sacrum via a movable coccygeal joint. During childbirth, thanks to this joint, the tailbone can deviate by 1-1.5 cm.

The pelvic floor (perineum) is a group of fascia and muscles that supports the pelvic organs and is located in the area between the thighs from the tailbone to the pubic bone. The perineum is limited in front by the pubic symphysis, on the sides by the ischial tuberosities, and behind by the coccyx. The inferior surface of the levator anus muscle forms the superior border of the perineum. The floor of the perineum consists of skin and two layers of superficial fascia - the superficial subcutaneous fat layer (Camper's fascia) and the deep membranous layer (Collis' fascia). A transverse line drawn through the center of the perineum divides it into the anterior and posterior parts, or triangles - the urogenital (genitourinary diaphragm) and anal triangles (pelvic diaphragm).

The pelvic diaphragm (anal triangle) is a wide but thin layer of muscle that forms the lower border of the abdominal (and pelvic) cavity and consists of a wide funnel-shaped belt of fascia and muscle, extending from the symphysis to the coccyx between the walls of the pelvis. The pelvic diaphragm consists of 3 groups of muscles and fascia that cover:

• Elevator anus muscles;
• Coccygeus muscle;
• External sphincter of the anus.

These structures are evolved remnants of the tail muscles of lower animals. The levator ani muscle is the longest and strongest of all the muscles and forms a wide muscular belt extending from the posterior surface of the superior ramus of the pubis, the inner surface of the ischium, and between these two formations from the obturator fascia. Muscle fibers are distributed in several directions: in the urethra, vagina and rectum, forming functional sphincters around them. The levator anus muscle is divided into three paired components, which are named according to their anatomical location: the pubococcygeus, ischiorectalis and iliococcygeus muscles.

An important space of the pelvic diaphragm is the ischiorectal fossa - the space between the skin and the levator anus muscle on both sides of the anal canal, containing adipose tissue bounded by Collis's fascia. The ischiorectal fossa at the back is combined with the same one on the opposite side, forming a “horseshoe”.

Pathological anatomy

There are quite a lot of bone anomalies and they depend on a variety of factors, ranging from intrauterine bone underdevelopment (most often found in premature babies) and ending with injuries (dislocations, fractures), which subsequently led to pathology of the pelvic bones.

The most common anomalies are a wide, narrow or deformed pelvis.

1. Wide. Today, a clinically and anatomically wide pelvis is distinguished. This pathology is most likely in tall, overweight people.
2. Narrow. Just like wide, they are divided into clinically and anatomically narrow. The causes of a narrow pelvis may be impaired development inside the womb, insufficient nutrition, or some serious diseases, for example, rickets.
3. Deformation (displacement of bones). In 99% of cases, displacement occurs in the baby’s body at birth (if the child’s mother has deformed pelvic bones, then the child’s bones, as they pass through the birth canal, become bent and displaced, not only in the pelvis, but also in the entire skeleton). This pathology is transmitted from mother to child. And in only 1% of patients, pelvic deformation occurred as a result of injury.
4. Aplasia or hypoplasia - this disease, transmitted by inheritance, is quite rare, characterized by the absence or underdevelopment of one of the pelvic bones.
5. Deep acetabulum - the head of the femur is located more deeply. The pathology can be either unilateral or bilateral (most common).
6. Divergence of the pubic symphysis is most often observed in patients with disorders of the central nervous system, bladder or spinal exstrophy.

A clearer idea of ​​the extent of the anomaly is provided by X-ray data.

Urogenital diaphragm

The urogenital diaphragm (urogenital triangle) is a strong muscular sheath that occupies the area between the symphysis and the ischial tuberosities and passes through the triangular anterior part of the pelvic outlet. The urogenital diaphragm is located outside and inferior to the pelvic diaphragm and is formed by two spaces, or layers: superficial and deep.

The superficial space of the perineum is limited by the deep fascia of the perineum and includes 3 pairs of muscles:

• ischiocavernosus muscle;
• Bulbocavernosus, or bulbospongiosus muscle;
• Superficial transverse perineal muscle.

In this space are the bulbs of the vestibule of the vagina and the large vestibule glands (Bartholin's glands). The ischiocavernosus muscle runs from the medial surface of the ischial tuberosities under the pubic arch to the crura of the clitoris.

The bulbocavernosus, or bulbospongiosus muscle, which is also called the sphincter of the vagina, begins behind the tendon center of the perineum, passes from both sides of the vestibule of the vagina to the dorsal surface of the clitoris into the lower fascia of the urogenital diaphragm and forms the medial border of the superficial space of the perineum. The superficial transverse perineal muscle runs transversely from the front of the ischial tuberosities to the tendinous center of the perineum.

The deep space of the perineum (triangular ligament) is a closed space between the upper and lower fascia of the genitourinary diaphragm, on the sides - the places of entry of this fascia into the ischiopubic branches, which includes the following muscle groups:

• Sphincter of the urethra;
• Deep transverse perineal muscle.

The sphincter of the urethra begins from the pubic-sciatic branches, goes medially to the urethra, covers its distal section, as well as the anterior and posterior walls of the vagina. In women, it is poorly developed due to the fact that it is perforated by two openings: the urethra and the vagina.

The deep transverse perineal muscle consists of transverse muscle fibers that run along the posterior aspect of the urethral sphincter and enter the central tendon center of the perineum. Unlike men, in women this muscle plays a very minor role in the mechanism of urinary continence.

The blood supply to the perineum is carried out by the internal pudendal artery and its branches: the inferior rectal and posterior labial arteries. Innervation of the perineum occurs due to the pudendal nerve (from the second, third and fourth sacral segments) and its branches.

Clinical correlations

The ischial spines are of great obstetric importance, since the distance between them is usually equal to the smallest diameter of the pelvic cavity. They are also a guide for the advancement of the presenting part of the fetus along the axis of the birth canal. When a woman is positioned in the dorsal lithotomy position during childbirth, due to the mobility of the sacroiliac joints, the diameter of the pelvic outlet can increase by 1.5-2 cm. This circumstance is the main argument for placing a woman in this position during childbirth.

During childbirth, all layers of the perineal muscles form a wide muscular canal, which is a continuation of the bone birth canal. The paired levator anus muscle is important for the maintenance of the abdominal and pelvic organs, the distribution of intra-abdominal pressure together with the diaphragm and the muscles of the abdominal wall (for example, when coughing), the control of urine and feces, as well as for the process of childbirth (significant stretching of the imbricate-composed muscles fibers during the advancement of the fetus with their subsequent contraction). When this muscle contracts, the genital opening, rectum and vagina are compressed.

The presence of adipose tissue in the ischiorectal fossa facilitates stretching of the anal canal during defecation and the vaginal canal during the second stage of labor. It can become a place of accumulation of blood during postpartum hemorrhage (hematoma) or pus during abscesses and can hold up to 1 liter of fluid. Such abscesses can spread to the opposite side of the pelvis.

Symptoms of a pelvic bone fracture

Symptoms of a pelvic fracture are divided into two main groups:

• local manifestations;
• general manifestations.

Local signs

These include the following symptoms:

• acute pain;
• deformation of the pelvic bones;
• hematoma;
• edema;
• bone crepitus (sound phenomenon);
• shortening of the limbs (when bone fragments are displaced).

Symptoms depend on which part of the pelvis is damaged.

General symptoms

These include:

• traumatic shock;
• massive bleeding;
• compression of nerve endings;
• tachycardia (rapid heart rate);
• drop in blood pressure (blood pressure);
• loss of consciousness.

As a result of severe blood loss, traumatic shock develops. Shock is accompanied by sticky sweat and pale skin. Sometimes a fracture of the pelvic bone is accompanied by damage to internal organs. A hematoma may form in the abdominal cavity. If the urethra (urethra) is damaged, bleeding from the canal and urinary retention occur. A bladder rupture is manifested by the presence of blood in the urine (hematuria). Pelvic injuries have the following classification:

1. Fractures of certain bones. Such fractures heal quickly and are quite stable. The recovery period is short, however, only if the patient remains in bed.
2. Unstable fractures, in which the pelvic bones are displaced horizontally.
3. Fracture of the acetabulum. Trauma occurs to the bottom or its edges.
4. Fractures accompanied by dislocations.
5. Bilateral and unilateral fractures.

Female pelvis in obstetrics

The bony pelvis creates a solid foundation for the soft tissues of the birth canal and determines its direction and size. The bones of the female pelvis are thinner, the plane of the entrance to the small pelvis usually has the shape of a transversely narrowed oval, while the plane of the entrance to the male pelvis is funnel-shaped. The female pelvis is lower, wider and more spacious compared to the male; the pubic symphysis is shorter. The cavity of the female pelvis towards the exit becomes wider due to the flatness of the iliac bones, a greater distance between the ischial tuberosities and a large subpubic angle (90-100 ° compared to 70-75 ° in men).

From an obstetric point of view, the female pelvis is divided into two parts. The boundary between them is the boundary line. It runs along the inner surface of each ilium from the sacroiliac joint at the iliopubic eminence and divides the pelvis into two parts: the upper (large pelvis) and the lower (lesser, or true pelvis).

The large pelvis cannot serve as a guide for the capacity of the small pelvis, but it is easily accessible for measurement, and therefore some of its dimensions are used to roughly estimate the size of the small pelvis:

• Interspinous distance - the distance between the anterior superior spines of the iliac bones (25-26 cm);
• Intercrest distance - the distance between the most distant points of the iliac crests (28-29 cm);
• Interacetabular distance - the distance between the most distant points of the hip joints (30-31 cm);
• External conjugate - Baudeloc's conjugate, external obstetric conjugate - the distance from the fossa between the spinous processes of the last lumbar and first sacral vertebrae to the most protruding point of the symphysis (20-21 cm).

Small (true) pelvis

It is of greatest importance for childbirth. It is bounded above by the promontory of the sacrum, the border line and the upper edge of the pubic bones, and below by the outlet of the pelvis. The anterior wall of the small pelvis in the symphysis area is about 5 cm long, the posterior wall (in the sacral area) is about 10-12 cm. The lateral walls of the small pelvis are represented by the inner surfaces of the ischial bones. When a woman is in an upright position, the upper part of the pelvic canal is directed down and back, and the lower part forms an arc and goes down and forward. The lateral walls of the pelvis in an adult woman have a somewhat converging direction. The descending branches of the pubic bones in the normal female pelvis form a circular arch (subpubic angle 90-100 °), which allows the passage of the fetal head.

In the pelvis there are 4 conventional planes that help to navigate in determining the location of the presenting part of the fetus during childbirth:

— The plane of entry into the pelvis;

— The plane of the wide part of the pelvic cavity (passes through the largest diameter of the pelvis);

— The plane of the narrow part of the pelvic cavity (passes through the small diameter of the pelvis);

— The plane of exit of the small pelvis.

The plane of entrance to the small pelvis is limited posteriorly by the promontory and wings of the sacrum; on the sides - by the border line, in front - by the symphysis and the upper (horizontal) branches of the pubic bones. The configuration of the entrance to the female pelvis in 50% of women is more round than oval (gynecoid type of pelvis). In the plane of the entrance to the pelvis, 4 diameters have obstetric significance: straight (antero-posterior, true conjugate), transverse and two oblique.

The straight diameter is the real conjugate (internal obstetric conjugate) - the most important anteroposterior diameter, which is the smallest distance between the promontory and the internal superior edge of the symphysis (10-11 cm). The distance between the promontory of the sacrum and the upper edge of the symphysis (the anteroposterior diameter of the anterior opening of the pelvis) is called the anatomical conjugate and is equal to 11.5 cm.

Transverse diameter - the distance between the most distant points of the intermediate line (13-13.5 cm).

Oblique diameter - the distance between the sacroiliac joint on one side and the iliopubic eminence on the opposite side (12-12.5 cm). The right diameter is measured from the right sacroiliac joint, the left - from the left.

The plane of the wide part of the pelvic cavity is limited in front by the middle of the inner surface of the symphysis, on the sides by the middle of the hip sockets, and behind by the communication of the II and III sacral vertebrae. In the wide part of the small pelvis, the straight (12.5 cm) and transverse (12.5 cm) diameters are determined.

The plane of the narrow part of the pelvic cavity is limited in front by the lower edge of the pubic symphysis, on the sides by the spines of the ischial bones, and behind by the sacrococcygeal joint. In this plane, straight (11.5 cm) and transverse (10.5 cm) diameters are also distinguished.

The plane of exit of the small pelvis is limited in front by the lower edge of the pubic arch, on the sides by the ischial tuberosities, and behind by the apex of the coccyx. It has a straight diameter of 9.5 cm, but when the coccyx deviates, it can increase by 1.5-2 cm and equals 11-11.5 cm; and transverse diameter (between the ischial tuberosities), which is 11 cm (at least 8 cm). In the plane of the pelvic outlet, the anal sagittal diameter is also distinguished (a segment of the direct diameter from the apex of the coccyx to the point of intersection with the transverse diameter), which in a normal pelvis should not be less than 7.5 cm. When the narrow part of the pelvic cavity or the pelvic outlet is narrowed, the prognosis of vaginal birth depends on the size of the anal sagittal diameter.

So, in the plane of the entrance to the pelvis, the largest is the transverse diameter; in the wide part of the pelvic cavity, the straight and transverse diameters are the same (this plane has no special obstetric significance); in the narrow part of the pelvic cavity and in the exit plane, the largest are straight diameters. These provisions are important for understanding the biomechanism of childbirth with a normal pelvis.

Pelvic axis, or leading line of the pelvis , connecting the midpoints of the straight diameters of all planes of the small pelvis and is directed down and back when entering the pelvis, and down and forward when exiting.

The angle of inclination of the pelvis is formed between the plane of the entrance to the pelvis and the horizontal line when the woman is in a vertical position and is 45-60 ° (for non-pregnant women - 45-46 °).

What is anterior pelvic tilt?

The pelvis is the structure that connects the torso and legs. The main movements of the pelvis are rotation and tilting. With an anterior tilt of the pelvis, lumbar lordosis will increase and the hip joints will begin to bend.

To determine if you have an anterior pelvic tilt, stand with your back close to a wall and measure the distance between your lower back and the wall. With a “normal curvature” of the lower back, the space between it and the wall should allow your hand to pass through. For men, the normal anterior tilt is 4-7 degrees, for women 7-10. If the distance between the wall and your hand is greater than the thickness of your palm, then your pelvis is probably in an anterior tilt.

Classification of pelvis types

A line drawn through the transverse diameter of the plane of the entrance to the pelvis divides it into anterior and posterior segments. The shape of these segments is taken into account when classifying pelvic types. Thus, the nature of the posterior segment determines the type of pelvis, the anterior segment determines the tendency, which helps to identify mixed types of pelvis.

Gynecoid pelvis . The posterior sagittal diameter is slightly smaller than the anterior sagittal diameter, the sides of the posterior segment are rounded and wide. Considering that the transverse diameter of the pelvic inlet is almost the same as the anteroposterior one, the pelvic inlet has an almost round or oval shape. The pelvic walls are straight, the ischial spines do not protrude and the distance between them exceeds 10 cm. The pubic arch is wide.

The sacrosciatic notch is round. The sacrum is not deviated either anteriorly or posteriorly. It occurs in 50% of women and has the best prognosis for vaginal birth.

The anthropoid pelvis is distinguished by the fact that the direct diameter of the entrance to the pelvis exceeds the transverse one, therefore the shape of the entrance to the pelvis has the appearance of an oval, narrowed in the anteroposterior direction. The anterior segment is narrow. The sacrosciatic notch is wide, the walls of the pelvis somewhat converge. The sacrum is usually straight and has 6 vertebrae, making the anthropoid pelvis the deepest of all pelvic types. The ischial spines protrude somewhat. The subpubic arch is well defined, but may be somewhat narrowed. This type of pelvis occurs in 25% of white women and about 50% in representatives of other races.

Android pelvis . The posterior sagittal diameter of the entrance is significantly shorter than the anterior sagittal diameter, which limits the space for the fetal head. The walls of the posterior segment are not round and approach wedge-shaped. The anterior segment is narrow and triangular. The lateral walls of the pelvis tend to move closer together, the ischial spines protrude, and the subpubic arch is narrowed. The sacrosciatic notch is narrow. The sacrum protrudes somewhat into the pelvis and is of course straight, with an unpronounced depression. The posterior sagittal diameter decreases from the inlet to the outlet of the pelvis due to protrusion of the sacrum. May occur in 30% of women. A narrowed android pelvis has a poor prognosis for vaginal delivery.

Platypeloid pelvis is a pelvis that has a flattened gynecoid shape, with a short anteroposterior (straight and wide transverse) diameter. The angle of the anterior segment is very wide, the arcs of the anterior and posterior segments are of regular shape. The sacrum is short, the sacrosciatic notches are wide. This type of pelvis is less common (in 3% of women).

Exercise 2: lower your legs straight without lifting your back from the floor

1. Start by lying on your back with your legs straight up. Press your lower back to the floor. 2. Lower both legs down, keeping your knees straight until your lower back begins to lift off the floor. 3. Return to the starting position and do 2 sets of 20 repetitions. Keeping your lower back pressed to the floor is very important. If your back lifts off the floor, it means your abdominal muscles stop working and you start overworking your already tight hip flexors instead. It may be helpful if you place your hand between your lower back and the floor to make sure you keep your back pressed to the floor. As the strength of your abdominal muscles increases, you will be able to lower your legs lower without your lower back touching the floor.

Clinical determination of pelvic capacity

Diagonal conjugate

In many narrowed pelvises, the straight (antero-posterior) diameter of the pelvic inlet is reduced. To predict childbirth, it is important to determine this size, but this is only possible with a special instrumental study (X-ray pelvimetry, nuclear magnetic resonance and computer pelvimetry, ultrasound pelvimetry). But the distance between the lower edge of the pubic symphysis and the promontory of the sacrum (diagonal conjugate) can be determined during a gynecological examination.

When determining the diagonal conjugate, the doctor inserts two fingers into the vagina, determines the mobility of the coccyx and the nature of the anterior surface of the sacrum (vertical and lateral arches). In a normal pelvis, only the last three sacral vertebrae can be palpated, whereas in a narrowed pelvis, the entire surface of the sacrum is palpable. If the size of the diagonal conjugate exceeds 11.5 cm, the pelvic capacity is considered sufficient for vaginal delivery, provided that the fetus is of normal size.

Transverse narrowing of the pelvis (this type of narrowing of the pelvis can be observed with a normal anteroposterior diameter) can only be detected with a special study (X-ray pelvimetry, nuclear magnetic resonance and computer pelvimetry, ultrasound pelvimetry). With ultrasound pelvimetry, it is possible to determine the real conjugate, the size of the pelvic planes, the biparietal size of the fetal head, its location and insertion, and the expected weight of the fetus.

MRI of the pelvis with contrast

MRI of the pelvic bones shows multiple foci, which indicates metastatic lesions

Contrast-enhanced magnetic resonance imaging improves imaging capabilities. An enhanced study is carried out to diagnose the tumor process, including detection of metastases in the pelvic bones, early detection of tumor recurrence after treatment, clarification of the severity of the inflammatory process, destruction, etc. Gadolinium-based contrast has a high safety profile and does not require a preliminary assessment of renal function. Side effects from the introduction of a paramagnetic agent are recorded extremely rarely. In 99.9% of cases, the study proceeds without complications. Contraindications to contrast:

• history of allergic reaction to gadolinium (during a previous MRI scan);
• receiving replacement cleansing therapy for end-stage renal failure;
• period of bearing a baby;
• early childhood.

General contraindications to MRI:

• implanted cardiac and neurostimulators, hearing aids, injectors for drug delivery, prosthetic joints, vascular clips, etc.;
• the volume of the hips is greater than the diameter of the drum;
• acute condition requiring resuscitation;
• mental and neurological diseases with uncontrolled motor activity.

Exercise 3: bridge with leg straightening.

1. Lie on your back. Bend both legs at the knee joints. 2. Lift your pelvis up as much as possible. In this case, the shoulders should remain on the floor. 3. While in this position, straighten one leg and hold for 5 seconds. 4. Return this leg to the starting position and do the same with the other leg. 5. After this, return to the starting position and do 2 sets of 10 repetitions. This exercise trains your glutes, glutes, and abdominal muscle control. During the exercise, there should be no rotation of the body and/or flexion in the hip joint or supporting leg.