Pelvis is the region of the trunk that lies below the abdomen. Bony pelvis is the bowl shaped bony structure that forms the skeleton of this region of body. Its major functions include transmitting the weight of the body from vertebral column to the lower limbs, and protecting and housing the terminal parts of the gastrointestinal tract, urinary system and reproductive system.
Pelvis can be regarded as a basin with holes in its walls. The structure of the basin is composed of four bones which are joined by four joints. The bones are:
- Two hip bones: These form the lateral and anterior walls of the bony pelvis.
- Sacrum: It forms most of the posterior wall.
- Coccyx: It is a small bone that forms the lower part of posterior wall.
And the joints are:
- Symphysis pubis: It is the anterior joint between the two hip bones.
- Two sacroiliac joints: These are formed between hip joint of each side with the sacrum.
- Sacrococcygeal joint: It is the joint between the sacrum and coccyx.
The bony pelvis thus forms a strong basin shaped structure. This basin is divided into two parts by the pelvic brim. The brim is formed posteriorly by the sacral promontory, laterally by the iliopectineal line and anteriorly by the symphysis pubis. Above the pelvic brim lies the false pelvis, which is not of much clinical importance. Below the brim is the true pelvis.
Foramina in bony pelvis:
The holes of the basin are called foramina. Important foramina in the bony pelvis include:
- Anterior sacral foramina: These are present on the anterior surface of the sacrum (which forms the posterior surface of the bony pelvis). Through these foramina pass the anterior rami of the sacral spinal nerves.
- Greater and lesser sciatic foramina: These are the major foramina of the pelvis. In the bony pelvis, they are present as greater and lesser sciatic notches but by the attachment of sacrotuberous and sacrospinous ligaments, these notches are converted to respective foramina. From these foramina various structures enter and leave the pelvis.
- Obturator foramen: Each lateral wall of the pelvis has a large hole, called the obturator foramen. In living subjects, this hole is closed by the obturator membrane except for a small opening, which represents the foramen. Obturator nerve passes through this small opening.
Orientation of pelvis in body:
When seen in the form of a sample outside the body, it is very difficult to understand the correct orientation of bony pelvis in the body. For a person standing in anatomic position, the front of the symphysis pubis and the anterior superior iliac spine should be in the same vertical plane. To elaborate, place the sample of bony pelvis against a vertical wall. When the symphysis pubis and the two anterior superior iliac spines are all in contact with the surface of the wall, this would indicate the anatomic position. In this position, the anterior surface of sacrum is directed forward and downward, while the pelvic surface of symphysis pubis faces upward and backward.
Terms related to pelvis:
While studying pelvis, the following terms are of great importance:
- Pelvic inlet:
It is also called pelvic brim, brim of the true pelvis, and is the circumference of the true pelvis that serves as entrance into it. It is formed anteriorly by the symphysis pubis, laterally by the iliopectineal line, and posteriorly by the sacral promontory.
- Pelvic outlet:
It is the lower circumference of the true pelvis that serves as exit route. It is formed in front by the pubic arch, laterally by the ischial tuberosities, and posteriorly by the coccyx. Sacrotuberous ligament also form part of the margin.
- Pelvic cavity:
It is the space between the pelvic inlet and pelvic outlet.
- Axis of pelvis:
It is the imaginary line joining the central points of the anteroposterior diameters of pelvic inlet and outlet. This is the line followed by the head of baby during parturition.
Difference between male and female pelvis:
Male and female pelvis has some considerable differences, which can be summarized as follows:
- Pubic arch:
Male pubic arch is more acute and approximates the angle between index and middle finger. On the other hand the female pubic arch has a greater angle and approximates the angle between thumb and index finger.
- Pelvic Inlet:
The anteroposterior diameter of the pelvic inlet of male pelvis is greater than the same of female pelvis. The transverse diameter of the pelvis inlet of male pelvis is smaller than the same of female pelvis.
- Pelvic outlet:
The anteroposterior diameter of the pelvic outlet of male pelvis is smaller than the same of female pelvis. The transverse diameter of the pelvic outlet of the male pelvis is greater than the same of female pelvis.
- Pelvic cavity:
The male pelvic cavity is cone shaped while the female pelvic cavity is cylindrical.