Functional Anatomy
Certification

The Pelvis

/ THE PELVIS STRUCTURE & FUNCTION

PELVIS STRUCTURE

BONES	Hip bone (ilium, ischium, pubis), sacrum
JOINTS	Sacroiliac, pubic symphysis, lumbosacral, sacrococcygeal, hip joint
FUNCTIONS	Weight-bearing, ambulation, landmarks, organs support, labor and delivery

The pelvis is a basin-shaped complex of bones located at the base of the spine between the abdomen and the thighs. It consists of four bones: the right and left hip bones, the sacrum, and the coccyx.

The pelvis has several important functions. Its primary role is to support the weight of the upper body when sitting and to transfer this weight to the lower limbs when standing. It serves as an attachment point for trunk and lower limb muscles, and also protects the internal pelvic organs, such as the intestines, the urinary bladder, and the internal sex organs.

When standing, the pelvis is tilted slightly anteriorly in a ‘neutral position’. In this position, the anterior superior iliac spines and the pubic tubercles lie in the same vertical plane, and the anterior surface of the sacrum faces forward and downward. (For more details, see Pelvic Placement below).

FRONT

TOP

LATERAL

BACK

The pelvic region includes several structures: the pelvic cavity, the pelvic floor note , the perineum, and the bony pelvis.

The pelvic cavity is a body cavity that is bounded by the bones of the pelvis and primarily contains the reproductive organs and the rectum. The pelvic floor at the base of the cavity assists in supporting the organs of the abdomen.

The perineum is located below the pelvic floor. It is the space between the anus and scrotum in the male and between the anus and vulva in the female.

The bony pelvis (anterior part of the pelvic skeleton) belongs to the part of the skeleton embedded in the pelvic region of the trunk. It is usually divided into two separate anatomic regions: the pelvic spine and pelvic girdle.

PELVIC SPINE

The pelvic spine is the posterior portion of the pelvis skeleton below the lumbar spine, which is composed of the sacrum and coccyx.

The sacrum, part of the spine, consists of four fused vertebrae which do not move independently of one another. The joints formed by either side of the sacrum and the two pelvic bones are the sacroiliac joints (SIJs). note

The sacroiliac joints (SIJs) are synovial joints between the ala or wings of the sacrum and the auricular surface of the ilium. This solid joint allows very little mobility through slight gliding and rotational movements. In women, due to the conformation of their pelvis during pregnancy, the ligaments of the SIJs can sometimes soften (which enables an increase of the pelvic diameter during childbirth).

The coccyx is connected to the sacrum by an amphiarthrodial joint. This is a type of joint that functionally only allows for a slight amount of movement.

The coccyx functions as an insertion point of some of the muscles of the pelvic floor and serves as an attachment for several tendons and ligaments. The coccyx also supports and stabilizes the human body while it is in a sitting position note

PELVIC GIRDLE

The pelvic girdle consists of a pair of bones named the os coxae or hip bones, each of which contains three fused bones: the ilium, ischium and pubis.

HIP BONES - OS COXAE

The paired hip bones are the large, curved bones that form the lateral and anterior aspects of the pelvic girdle. Each adult hip bone is formed by three separate bones that fuse together during the late teenage years. These bony components are the ilium, ischium, and pubis.

ILLIUM

The ilium is the fan-like, superior region that forms the largest part of the hip bone. Its internal concave surface is termed the iliac fossa. It consists of the two main parts: the body and ala (wing).

The body of ilium is a smaller, inferior part of the ilium that contributes to the formation of the acetabulum.

The acetabulum is the point of fusion of the three bones: the Ilium, ischium and pubis; the cup-shaped socket that forms the hip joint with the head of the femur (thighbone) is also called the ‘hip socket’.

The superior part, called the ala, is a large, flat portion of the bone that has four borders and three surfaces.

The superior convex border of the ilium is named the iliac crest, and can be easily palpated along the waistline on the side of the body. The iliac crest is also used as a landmark to find the L4-L5 vertebrae.

The ilium has four major protruding areas:

Anterior superior iliac spine (ASIS)
Anterior inferior iliac spine (AIIS)
Posterior superior iliac spine (PSIS)
Posterior inferior iliac spine (PIIS)

These areas are commonly used as reference landmarks when locating other structures. The ASIS and PSIS are often especially mentioned as landmarks for training purposes.

Anterior superior iliac spine (ASIS)

The rounded, anterior termination of the iliac crest is the anterior superior iliac spine. This important bony landmark can be felt as bony prominences at the anterolateral hip. It serves as a point of attachment for the inguinal ligament.
This is the most prominent front part of the hip bone and it is key for evaluating pelvic symmetry and alignment. (See Pelvis Placement for more details.)
Anterior inferior iliac spine (AIIS)

Inferior to the anterior superior iliac spine is a rounded protuberance called the anterior inferior iliac spine. It is located anterior to the supra-acetabular groove and acetabular margin. It is separated from the ASIS by a short, vertical slope. The AIIS provides points of attachment for the rectus femoris muscle and iliofemoral ligament.

Posterior superior iliac spine (PSIS)

Located at the posterior end of the iliac crest, the PSIS is laterally related to the iliac tuberosity and sacropelvic surface. The PSIS is not accessible to be palpated but is commonly represented by a dimple at the medial gluteal region as the muscles and ligaments surrounding it do not cover this bony landmark.

Posterior inferior iliac spine (PIIS)

The PIIS is located inferior to the PSIS at the inferior end of a large area called the auricular surface of the ilium. The auricular surface articulates with the auricular surface of the sacrum to form the sacroiliac joint. Both the PSIS and PIIS serve as attachment points for the muscles and very strong ligaments that support the sacroiliac joint.

The shallow depression located on the anteromedial (internal) surface of the upper ilium is called the iliac fossa. The inferior margin of this space is formed by the arcuate line of the ilium, the ridge formed by the pronounced change in curvature between the upper and lower portions of the ilium.
The large, inverted U-shaped indentation located on the posterior margin of the lower ilium is called the greater sciatic notch.

ISCHIUM

The ischium is found posteriorly and inferiorly to the hip joint and it is often called the ‘ischial tuberosity’ or ‘seat bone’. It is formed by an ‘L’ shaped bone continuous superiorly with the ilium and anteriorly with the pubic bone.

The ischium consists of two parts: the body and ramus.
The ischium provides several points of attachment for the pelvic and lower limb muscles, like the piriformis, coccygeus and elevator ani muscles (medial surface).

It carries the weight of the body when sitting. You can feel the ischial tuberosity if you wiggle your pelvis against the seat of a chair.

PUBIS / Pubic Symphysis (PS)

The pelvis is connected at the front by the pubis symphysis (PS).

The PS is a cartilage disc that joins the pubis of each hip bone at the lower and front portion and at the back by the sacrum. It is important for shock absorbency during movements like walking.

The left and right pubic bones are each made up of three sections, a superior ramus (segment of bone that passes laterally from the pubic body to join the ilium), inferior ramus (extends downward and laterally from the pubic body) and a pubic body.

The narrow ridge running along the superior margin of the superior pubic ramus is the pectineal line of the pubis.

In contrast, the pubic tubercles are felt as prominences at the lateral edges of the pubis, precisely on the upper border of the medial portion of the superior ramus of the pubis.

The pubic tubercles function as a point of attachment for the inguinal ligament, and it provides points of attachment for several anterior abdominal, pelvic and lower limb muscles. The superior border of pubic bone provides attachment points for rectus abdominis and pectineus muscles. The external surface of the pubic bone supports the gracilis, adductor longus, adductor brevis and obturator externus muscles.

NOTE

Another important bony landmark of the hip/pelvis area is the greater trochanter.
It forms a large projection toward the top of the femur and faces outward. When you internally and externally rotate the leg, you can feel with one finger the greater trochanter moves beneath. note

THE DIFFERENCE BETWEEN THE MALE & FEMALE PELVIS

The shape of the adult pelvic girdle varies between the sexes among individuals and races. The fundamental differences in the pelvis of the two sexes are based on the shape of the pelvic inlet, the angle of the ischiopubic rami and the projection of the ischial spines.

The woman's pelvic bone is wider and shallower, reflecting the necessity in the female of providing an adequate birth canal for a large-headed fetus.

The male pelvic bone is heavier and narrower than the female pelvic bone, adapted for support of the male’s heavier physical build and stronger muscles.

The female sacrum is wider, shorter, and less curved, and the female pelvic inlet (pelvic brim) a more rounded or oval shaped as compared to the male pelvic bone, which is more heart-shaped.

Figure – left: male pelvis, right: female pelvis

PELVIS FUNCTIONS

Weight-bearing

The pelvic girdle is a robust structure that is designed to support the weight of the upper body. Its primary function is to transfer the weight of the upper body to the legs. It transfers the weight from the axial skeleton to the lower appendicular skeleton when standing and walking.

Additionally, its thick bones provide points of attachment for some of the largest muscles needed for adequate posture and locomotion.

Compared to the shoulder girdle, the pelvic girdle is stronger and more rigid.

Support

The bony pelvis holds in place and protects the organs located in the pelvic cavity including the urinary bladder, pelvic colon, reproductive organs, and rectum.

The bony pelvis also provides anchoring points for the muscles and ligaments of the pelvic floor and the perineum.

Labor and delivery

As previously mentioned, the anatomical features and biomechanical properties of the female pelvis are important for carrying out and facilitating a natural labor. In fact, the pelvic girdle provides a comfortable environment for the fetus during pregnancy.

CLINICAL ASSOCIATIONS

Two of the most common diseases of the pelvic bone are osteoporosis and osteomalacia, or bone cancer.

Common problems at the pelvic girdle can be:

Sprain strain at the sacroiliac joint. Because the pelvic girdle is a movable joint, it is covered by a large number of ligaments. Causes can be trauma such as a sports injury, car accident or slip and fall accident.
Pelvic fractures. People who play sports or elderly people who have osteoporosis have the highest risk of pelvic fracture. It is also common to see pelvic fractures in car accidents involving a high impact.
Ankylosing spondylitis (AS). As is a chronic inflammatory condition that causes stiffening of the spinal joints and other joints of the body. This inflammation can cause fusing of the joints such as the sacroiliac joints. People with AS often have pain in the sacroiliac region or lower back.