The Pelvic Girdle
The pelvic girdle is a ring-like bony structure, located in the lower part of the trunk. It connects the axial skeleton to the lower limbs.
The bony pelvis consists of the two hip bones (also known as innominate or pelvic bones), the sacrum and the coccyx.
There are four articulations within the pelvis:
- Sacroiliac joints (x2) – between the ilium of the hip bones, and the sacrum
- Sacrococcygeal symphysis – between the sacrum and the coccyx.
- Pubic symphysis – between the pubis bodies of the two hip bones.
- Ligaments attach the lateral border of the sacrum to various bony landmarks on the bony pelvis to aid stability.
Functions of the Pelvis
The strong and rigid pelvis is adapted to serve a number of roles in the human body. The main functions being:
- Transfer of weight from the upper axial skeleton to the lower appendicular components of the skeleton, especially during movement.
- Provides attachment for a number of muscles and ligaments used in locomotion.
- Contains and protects the abdominopelvic and pelvic viscera.
The Greater and Lesser Pelvis
The osteology of the pelvic girdle allows the pelvic region to be divided into two:
- Greater pelvis (false pelvis) – located superiorly, it provides support of the lower abdominal viscera (such as a ileum and sigmoid colon). It has little obstetric relevance.
- Lesser pelvis (true pelvis) – located inferiorly. Within the lesser pelvis reside the pelvic cavity and pelvic viscera. The junction between the greater and lesser pelvis is known as the pelvic inlet. The outer bony edges of the pelvic inlet are called the pelvic brim.
The pelvic inlet marks the boundary between the greater pelvis and lesser pelvis. Its size is defined by its edge, the pelvic brim.
The borders of the pelvic inlet:
- Posterior – sacral promontory (the superior portion of the sacrum) and sacral wings (ala).
- Lateral – arcuate line on the inner surface of the ilium, and the pectineal line on the superior pubic ramus.
- Anterior – pubic symphysis. The pelvic inlet determines the size and shape of the birth canal, with the prominent ridges key areas of muscle and ligament attachment.
Some alternative descriptive terminology can be used in describing the pelvic inlet:
- Linea terminalis – the combined pectineal line, arcuate line and sacral promontory.
- Iliopectineal line – the combined arcuate and pectineal lines. This represents the lateral border of the pelvic inlet.
The pelvic outlet is located at the end of the lesser pelvis, and the beginning of the pelvic wall.
Its borders are:
- Posterior: The tip of the coccyx
- Lateral: The ischial tuberosities and the inferior margin of the sacrotuberous ligament
- Anterior: The pubic arch (the inferior border of the ischiopubic rami). The angle beneath the pubic arch is known as the sub-pubic angle and is of a greater size in women.
Adaptation for Childbirth
The majority of women have a gynaecoid pelvis, as opposed to the male android pelvis. The slight differences in their structures creates a greater pelvic outlet, adapted to aid the process of childbirth. When comparing the two, the gynaecoid pelvis has:
- A wider and broader structure yet it is lighter in weight
- An oval-shaped inlet compared with the heart-shaped android pelvis.
- Less prominent ischial spines, allowing for a greater bispinous diameter
- A greater angled sub-pubic arch, more than 80-90 degrees.
- A sacrum which is shorter, more curved and with a less pronounced sacral promontory. In addition to the bony adaptations, the sacrotuberous and sacrospinous ligaments can stretch under the influence of progesterone and increase the size of the outlet further.