8.3 The Pelvic Girdle and Pelvis

The Pelvic Girdle and Pelvis

The pelvic girdle is a bony ring that connects the lower limbs to the axial skeleton. It transfers the weight of the upper body to the lower limbs and provides protection for the organs housed within the pelvic cavity. Understanding its anatomy matters because so many structures pass through or attach to it: muscles of the trunk and thigh, major nerves and vessels to the lower limb, and the organs of the reproductive, urinary, and digestive systems.

The pelvis has two main regions: the greater (false) pelvis above and the lesser (true) pelvis below. These areas house different organs and serve as attachment points for muscles. The pelvis also shows clear differences between males and females, with the female pelvis adapted for childbirth.

Bones and Ligaments of the Pelvic Girdle

The pelvic girdle consists of the right and left hip bones (os coxae), the sacrum, and the coccyx. Each hip bone forms from the fusion of three separate bones during adolescence: the ilium, ischium, and pubis. These three bones meet at the acetabulum, the deep socket for the head of the femur. In children, the three bones are connected by hyaline cartilage at the acetabulum; by about age 23, they've fully fused into a single hip bone.

Strong ligaments bind the pelvic girdle together:

• Sacroiliac joint ligaments (anterior, posterior, and interosseous sacroiliac ligaments) stabilize the sacroiliac joint, which bears the full weight of the upper body. The interosseous sacroiliac ligament is one of the strongest ligaments in the body.
• Sacrospinous ligament extends from the lateral sacrum and coccyx to the ischial spine. It converts the greater sciatic notch into the greater sciatic foramen and helps support pelvic organs.
• Sacrotuberous ligament runs from the sacrum to the ischial tuberosity. It resists upward tilting (nutation) of the sacrum and converts the lesser sciatic notch into the lesser sciatic foramen.
• Pubic symphysis ligaments (anterior, posterior, superior, and inferior/arcuate pubic ligaments) reinforce the pubic symphysis, the fibrocartilaginous joint between the two pubic bones.

Regions of the Hip Bone

Ilium

The ilium is the largest of the three hip bone regions and forms the broad, superior portion you can feel at your waist.

• Body of the ilium contributes to the superior portion of the acetabulum.
• Ala (wing) is the broad, flat, fan-shaped portion that provides attachment for abdominal and gluteal muscles.
• Iliac crest is the thick superior border of the ala, running from the ASIS to the PSIS. It serves as an attachment site for abdominal muscles, the latissimus dorsi, and the thoracolumbar fascia. This is the ridge you feel when you put your hands on your hips.
• Anterior superior iliac spine (ASIS) is a prominent projection at the anterior end of the iliac crest. It's a key clinical landmark and an attachment point for the sartorius muscle and the inguinal ligament.
• Anterior inferior iliac spine (AIIS) sits just below the ASIS and serves as the origin of the rectus femoris (part of the quadriceps).
• Posterior superior iliac spine (PSIS) marks the posterior end of the iliac crest. It's an attachment point for sacroiliac ligaments and can be identified by the small skin dimples on the lower back.
• Posterior inferior iliac spine (PIIS) sits below the PSIS and forms the superior margin of the greater sciatic notch.
• Greater sciatic notch is a large indentation converted into the greater sciatic foramen by the sacrospinous ligament. The sciatic nerve, piriformis muscle, and several blood vessels pass through here.
• Iliac fossa is the smooth, concave internal surface of the ala. The iliacus muscle originates from this surface.
• Auricular surface is the ear-shaped area on the posterior ilium that articulates with the sacrum to form the sacroiliac joint.

Ischium

The ischium forms the posterior-inferior portion of the hip bone. It's the part you sit on.

• Body of the ischium contributes to the posterior-inferior portion of the acetabulum.
• Ischial spine is a pointed projection between the greater and lesser sciatic notches. The sacrospinous ligament and the superior gemellus muscle attach here. In obstetrics, the ischial spines are used as reference points to measure the descent of the fetal head during labor.
• Lesser sciatic notch is the smaller indentation below the ischial spine, converted into the lesser sciatic foramen by the sacrospinous and sacrotuberous ligaments. The tendon of the obturator internus and the pudendal nerve pass through here.
• Ischial tuberosity is the large, roughened projection at the inferior end of the ischium. This is the weight-bearing surface when you're sitting. The hamstring muscles (biceps femoris, semitendinosus, semimembranosus) and the sacrotuberous ligament attach here.
• Ischial ramus is a thin bar of bone that projects anteriorly and superiorly from the ischial tuberosity to join the inferior pubic ramus.

Pubis

The pubis forms the anterior portion of the hip bone.

• Body of the pubis contributes to the anterior portion of the acetabulum and forms the medial part that articulates at the pubic symphysis.
• Superior pubic ramus extends laterally from the body to connect with the ilium. The pectineal line runs along its superior surface.
• Inferior pubic ramus extends inferiorly and laterally to join the ischial ramus, together forming the ischiopubic ramus and the inferior border of the obturator foramen.
• Pubic tubercle is a small projection on the superior ramus near the body. The inguinal ligament attaches here, making it an important landmark for locating the inguinal canal.
• Pubic symphysis is the fibrocartilaginous joint where the right and left pubic bones meet at the midline. It allows very limited movement but can soften slightly during pregnancy due to the hormone relaxin.
• Obturator foramen is the large opening bounded by the pubis and ischium. Despite its size, it's almost entirely covered by the obturator membrane. The obturator nerve and blood vessels pass through a small gap (the obturator canal) at the superior margin.

Structure and Function of the Pelvis

The pelvic brim (linea terminalis) is the continuous bony ridge that divides the pelvis into its two regions. It runs from the sacral promontory, along the arcuate line of the ilium, across the pectineal line of the pubis, and to the upper margin of the pubic symphysis.

Greater (false) pelvis:

• Located above the pelvic brim
• Bounded by the alae of the ilia laterally and the lumbar vertebrae posteriorly; it's open anteriorly
• Contains lower abdominal organs such as the sigmoid colon and loops of small intestine
• Provides attachment for abdominal wall muscles (transversus abdominis, internal oblique)
• Functionally, this region is more a part of the abdominal cavity than the pelvic cavity

Lesser (true) pelvis:

• Located below the pelvic brim
• Bounded by the ischium and pubis anteriorly, the sacrum and coccyx posteriorly, and the hip bones laterally
• Contains the bladder, rectum, and reproductive organs (uterus and ovaries in females; prostate and seminal vesicles in males)
• Pelvic inlet (superior pelvic aperture): the opening into the lesser pelvis, bounded by the sacral promontory, linea terminalis, and upper margin of the pubic symphysis. The fetal head must pass through this opening to enter the birth canal.
• Pelvic outlet (inferior pelvic aperture): the opening at the bottom of the lesser pelvis, bounded by the pubic arch anteriorly, ischial tuberosities laterally, sacrotuberous ligaments posterolaterally, and the coccyx posteriorly. This is the final bony passage the fetus must clear during delivery.

Pelvic Characteristics and Function

Sexual dimorphism of the pelvis is among the most reliable ways to determine biological sex from skeletal remains. The key differences reflect the female pelvis's adaptation for childbirth:

Pelvic diameters are measurements used in obstetrics to assess whether the pelvis can accommodate vaginal delivery:

• The obstetrical conjugate (true conjugate) is the shortest anteroposterior diameter of the pelvic inlet, measured from the sacral promontory to the closest point on the posterior surface of the pubic symphysis. It's approximately 10 cm in females and represents the narrowest fixed space the fetal head must pass through. This measurement cannot be taken directly but is estimated from the diagonal conjugate during a pelvic exam.
• The transverse diameter of the pelvic inlet is the widest side-to-side measurement, typically about 13 cm in females.

Pelvic floor muscles form a muscular sling (the pelvic diaphragm) that supports the pelvic organs from below:

• Levator ani is the larger muscle group (composed of the pubococcygeus, puborectalis, and iliococcygeus). It supports pelvic viscera, maintains urinary and fecal continence, and plays a role in childbirth.
• Coccygeus (ischiococcygeus) lies posterior to the levator ani and supports the coccyx and pelvic organs.

The acetabular labrum is a ring of fibrocartilage attached to the rim of the acetabulum. It deepens the hip socket by roughly 20%, increasing the surface area for contact with the femoral head. This enhances joint stability, helps distribute pressure more evenly across the joint, and creates a partial seal that maintains negative intra-articular pressure to resist dislocation.