The pelvic girdle is a ring-like bony structure, located in the lower part of the trunk. It is responsible for connecting the axial skeleton to the lower limbs. In this article, we will take a close look at the anatomy of the pelvic girdle, including its bony landmarks, functions, and its clinical relevance.
The bony pelvis consists of two hip bones, also referred to as innominate or pelvic bones, the sacrum and the coccyx. There are four articulations within the pelvis, including the Sacroiliac joints (x2) which are between the ilium of the hip bones and the sacrum, the Sacrococcygeal symphysis which is between the sacrum and the coccyx., the Pubic symphysis which is between the pubis bodies of the two hip bones, and ligaments which attach the lateral border of the sacrum to various bony landmarks on the bony pelvis for stability.
The pelvis is strong and rigid, and is adapted to a number of roles in the human body. Mainly, it is responsible for transferring weight from the upper axial skeleton to the lower appendicular components of the skeleton, especially during movements, as well as providing attachment for a number of muscles and ligaments used in locomotion. Further, the pelvic girdle also contains and protects the abdominopelvic and pelvic viscera.
The osteology of the pelvic girdle allows the pelvic region to be divided into two main parts. The first is the Greater pelvis, or false pelvis, which is located superiorly and provides support of the lower abdominal viscera. It has little obstetric relevance. The second is the Lesser pelvis, or true pelvis, which is located inferiorly. This is the area which contains the pelvic cavity and pelvic viscera.
The junction between the greater and lesser pelvis is known as the pelvic inlet. The exterior edges of the pelvic inlet are referred to as the pelvic brim. The pelvic inlet marks the boundary between the greater and lesser pelvis, and its size is determined by the pelvic brim. The posterior border of the pelvic inlet consists of the sacral promontory (the superior portion of the sacrum) and the sacral wings (ala). Its lateral border is the arcuate line on the inner surface of the ilium, and the pectineal line on the superior pubic ramus. The anterior border of the pelvic inlet is the pubic symphysis.
In addition to the anatomical terms, there is some alternative descriptive terminology used to describe the pelvic inlet. This includes the Linea terminalis which is the combined pectineal line, arcuate line and sacral promontory, and the Iliopectineal line which is the combined arcuate and pectineal lines, representing the lateral border of the pelvic inlet.
The pelvic outlet is located at the end of the lesser pelvis, and the start of the pelvic wall. The posterior border of the outlet consists of the tip of the coccyx. its lateral border is the ischial tuberosities and the inferior margin of the sacrotuberous ligament, while its anterior border is 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 generally larger in women.
When comparing the male and female pelvic structures, the most obvious differences lie in the gynaecoid pelvis, which is often the structure found in women. It has a wider and broader structure yet is lighter in weight. It is also oval-shaped at the inlet rather than the heart-shaped android pelvis. This slight difference in structure allows for a greater pelvic outlet, which is better adapted to aid the process of childbirth.
The pelvis is a complex region of the body, consisting of two distinct portions: the greater pelvis (also known as the false pelvis) and the lesser pelvis (also known as the true pelvis).
The greater pelvis is located superiorly and provides support for the lower abdominal viscera, such as the ileum and sigmoid colon; however, it is of little obstetric relevance. Meanwhile, the lesser pelvis is located inferiorly and houses the pelvic cavity and pelvic viscera. The point of division between the greater and lesser pelvis is referred to as the pelvic inlet, and the outer edges of the pelvic inlet are called the pelvic brim.
Due to the importance of the lesser pelvis during childbirth, it is of great clinical relevance to determine the diameter of this canal. This way, the childbearing capacity of the mother can be accurately assessed.
The diameter can be determined either through a pelvic examination or radiographically. When performing such an assessment, there are two measurements of special importance.
The obstetric conjugate (also known as the true conjugate) measures the narrowest fixed distance that the fetus must pass through in order to complete its journey. This distance is between the sacral promontory and the midpoint of the pubic symphysis (where the pubic bone is thickest).
Unfortunately, this measurement cannot be assessed clinically due to the presence of the bladder.
In order to compensate for this limitation, the diagonal conjugate measures the distance from the inferior border of the pubic symphysis to the sacral promontory and can be measured manually via the vagina.
To do this, the clinician places the tip of their middle finger on the sacral promontory and uses the other hand to mark the level of the inferior margin of the pubic symphysis. The distance between the index finger and the pubic symphysis then gives an estimation of the diagonal conjugate. Ideally, the measurement should be 11cm or greater.
In addition to measuring the diagonal conjugate, a mid-pelvis check is also performed. This analysis allows the clinican to inspect the straightness of the side walls and to measure the bispinous diameter, which is the narrowest part of the pelvic canal. Additionally, they can determine the width of the subpubic angle at the pelvic outlet by calculating the distance between the ischial tuberosities.
The pelvic inlet marks the boundary between the greater pelvis and lesser pelvis. Its size is defined by its edge—the pelvic brim. The posterior border is the sacral promontory (the superior portion of the sacrum) and the sacral wings (ala), the lateral border is made up of the arcuate line on the inner surface of the ilium and the pectineal line on the superior pubic ramus, and the anterior border is the pubic symphysis.
The pelvic inlet is of importance in childbirth, as its prominent ridges act as sites for attachment of muscle and ligaments. This region is also known by other names such as the linea terminalis (the combined pectineal line, arcuate line and sacral promontory) and the iliopectineal line (the combined arcuate and pectineal lines).
The pelvic outlet is located at the end of the lesser pelvis, and the opening of the pelvic wall. Its posterior border is the tip of the coccyx, the lateral border is the ischial tuberosities and the inferior margin of the sacrotuberous ligament, and the anterior border is 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 larger in women than men.
The majority of women have a gynaecoid pelvis, as opposed to the male android pelvis. This slight difference in their structures creates an adapted pelvic outlet which is better suited for childbirth. When comparing the two, the gynaecoid pelvis has a wider yet lighter structure, an oval-shaped inlet, less prominent ischial spines, and a greater angled sub-pubic arch (greater than 80-90 degrees). Furthermore, the sacrum is shorter, more curved and has a less pronounced sacral promontory.
In addition to the bony adaptations of the gynaecoid pelvis, the sacrotuberous and sacrospinous ligaments can be stretched under the influence of progesterone, widening the pelvic outlet even further.
The pelvic girdle is the bony structure that supports the lower limbs and contains the pelvic cavity. This is composed of the sacrum, the coccyx, and the two innominate bones, which include the ischium, ilium, and pubis. The bony landmarks of the pelvic girdle are important for a number of functions, including support of the axial skeleton and for providing attachment sites for muscles, ligaments, and tendons. The pelvic girdle also has important clinical relevance due to its adaptation for childbirth.
The lesser pelvis is the bony canal through which the fetus has to pass during childbirth. Therefore, it is essential to determine the diameter of this canal and assess the mother’s childbearing ability. This can be determined by a pelvic examination or radiographically. Two measurements are of particular importance- the obstetric conjugate and the diagonal conjugate.
The obstetric conjugate is the shortest possible distance between the sacral promontory and the midpoint of the pubic symphysis (the thickest part of the pubic bone). This measurement is also referred to as the true conjugate, and it cannot be assessed clinically due to the presence of the bladder.
The alternative is the diagonal conjugate, which is the line from the inferior border of the pubic symphysis to the sacral promontory. This measurement can be taken manually via the vagina, by using the tip of the middle finger to measure the sacral promontory and then using the other hand to mark the level of the inferior margin of the pubic symphysis on the examining hand. The distance between the index finger and the pubic symphysis is then used to measure the diagonal conjugate, which should ideally be 11 cm or greater.
In addition to measuring the diagonal conjugate, a mid-pelvis check is carried out. This is used to test for straight side walls and measure the bispinous diameter, which is the narrowest part of the pelvic canal. The width of the subpubic angle at the pelvic outlet can be determined by measuring the distance between the ischial tuberosities.
The assessment of the female pelvic girdle is of vital importance for childbirth, as it can determine the minimum antero-posterior diameter of the pelvic inlet and ensure that the fetus can pass through it with the least difficulty. Knowing the dimensions of the bony pelvic girdle can help clinicians in the diagnosis of medical issues related to the pelvic girdle and, in the case of childbirth, ensure that the delivery goes smoothly.
Therefore, measuring the obstetric conjugate and the diagonal conjugate is an important skill for any clinician. In addition, a mid-pelvis check and a measurement of the subpubic angle at the pelvic outlet are essential for assessing the female bony pelvis.
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