Anatomy of the Pelvis

The pelvis forms a bony ring that sits between the vertebral column and the hip bones, the acetabulum. It is stronger and more massive than the wall of the cranial or thoracic cavities. The pelvis gains its strength through the ligaments and muscles.

The pelvis is symmetrical and each side is actually made up of three separate bones:

The top half is the ilium. Also known as the iliac crest.
The middle is the pubis. The front of the pelvis where the two sides join together and is called the symphysis pubis.
The bottom is the ischium. The ischium forms the lower and back part of the hip bone. It is situated below the ilium.

ANATOMY OF THE PELVIS back

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Symphysis Pubis

Anterior Sacroiliac Joint

The four pelvic shapes

Pelvic Shapes back

The female pelvis comes in four shapes:

Gynecoid: The gynecoid pelvis (sometimes called a "true female pelvis") is found in about 50% of the women.
Android: The android pelvis (sometimes called a "true make pelvis") is found in about 20% of women.
Anthropoid: The anthropoid pelvis is very long and almost "ovoid" in shape. It is more common in non-white females (it makes up about 25% of pelvic type in white women and close to 50% in non-white women).
Platypelloid: The platypelloid pelvis is very short (almost like a "flattened gynecoid shape"). Only about 3% of women have a true and pure pelvis of this type.

Pelvimetry back

Pelvimetry assesses the size of a woman's pelvis aiming to predict whether she will be able to deliver vaginally or not. This assessment can be done by either a clinical examination, conventional x-rays, computerized tomography scanning, or magnetic resonance imaging.

Symphysis Pubis back

"Symphysis" is a Greek word that means "growing together". The symphysis pubis is a non synovial amphiarthrodial joint, (a joint in which the opposing bony surfaces are covered with a layer of hyaline cartilage or fibrocartilage and in which some degree of free movement is possible).

At the time of birth the symphysis pubis is 9-10mm in width, with thick cartilaginous end-plates. By mid-adolescence the adult size is achieved. During adulthood the end-plates decrease in width to a thinner layer. Degeneration of the symphysis pubis accompanies aging and postpartum. Women have a greater thickness of this pubic disc which allows more mobility of the pelvic bones, hence providing a greater diameter of pelvic cavity during parturition. ;

Pelvic Ligaments back

The ligaments connecting the bones of the pelvis with each other may be divided into four groups:

Those connecting the sacrum and ilium.
Those passing between the sacrum and ischium.
Those uniting the sacrum and coccyx.
Those between the two pubic bones.

PUBIC LIGAMENTS

The Anterior Pubic Ligament

The anterior pubic ligament consists of several superimposed layers, which pass across the front of the articulation. The superficial fibers pass obliquely from one bone to the other, decussating and forming an interlacement with the fibers of the ses of the Obliqui externi and the medial tendons of origin of the Recti abdominis. The deep fibers pass transversely across the symphysis, and are blended with the fibrocartilaginous lamina.

The Posterior Pubic Ligament

The posterior pubic ligament consists of a few thin, scattered fibers, which unite the two pubic bones posteriorly.

The Superior Pubic Ligament (ligamentum pubicum superius)

The superior pubic ligament connects together the two pubic bones superiorly, extending laterally as far as the pubic tubercles.

The Arcuate Pubic Ligament (ligamentum arcuatum pubis; inferior pubic or subpubic ligament)

The arcuate pubic ligament is a thick, triangular arch of ligamentous fibers, connecting together the two pubic bones below, and forming the upper boundary of the pubic arch. Above, it is blended with the interpubic fibrocartilaginous lamina; laterally, it is attached to the inferior rami of the pubic bones; below, it is free, and is separated from the fascia of the urogenital diaphragm by an opening through which the deep dorsal vein of the penis passes into the pelvis.

Fibrocartilage back

Fibrocartilage is composed of small chained bundles of thick clearly defined type I collagen fibers. This fibrous connective tissue bundles have cartilage cells between them and to a certain extent resemble tendon cells. The collagenous fibers are usually place in an orderly arrangement parallel to tension on the tissue. It has a low content of glycosaminoglycans (2% of dry weight).

Glycosaminoglycans are long unbranched polysaccharides (relatively complex carbohydrates) consisting of a repeating disaccharide unit. Disaccharide is a sugar (a carbohydrate) composed of two monosaccharides. The two monosaccharides are bonded via a dehydration reaction that leads to the loss of a molecule of water.

Fibrocartilage does not have a surrounding perichondrium. Perichondrium surrounds the cartilage of developing bone, it has a layer of dense irregular connective tissue and functions in the growth and repair of cartilage.

Hyaline Cartilage back

Hyaline cartilage is the white shiny gristle at the end of long bones. This cartilage has very poor healing potential, and efforts to get it to repair itself frequently end up with a similar, but poorer fibrocartilage.

Sacroiliac Joint back

The sacroiliac joints are a C-shaped amphiarthrodial joints (joint permitting only slight motion) formed between the articular surfaces of the sacrum and ilium. The sacroiliac joint is a "viscoelastic joint", meaning that its major movement comes from giving or stretching. The joints are covered by two different kinds cartilage; the sacral anterior edge has a hyaline cartilage and the ilium anterior edge a fibrocartilage. The stability of the SIJ's are maintained by various muscles and ligaments. As we age the characteristics of the sacroiliac joint change. The joint's surface remains flat until sometime after puberty. In our thirties and forties there is an increase in the size and number of elevations and depressions on the sacral and iliac surfaces.

The SI joint's main function appears to be providing shock absorption for the spine through stretching in various directions. The SI joint may also provide a "self-locking" mechanism that helps you to walk. The joint locks on one side as weight is transferred from one leg to the other and through the pelvis the body weight is transmitted from the sacrum to the hip bone. These joints bear the weight of the twists and turns of the trunk of the body. It is common for the SI joint to become stiff and actually "lock" as we age. During pregnancy micro tears and small gas pocket can appear within the joint. Traumatic incidents, biomechanical mal-alignments and hormonal changes can all lead to SIJ dysfunction. The self-braced position of the SIJ can be altered by these factors and the joint can lose its stability. SIJ dysfunction puts abnormal pressures on the joint surfaces, ligaments and surrounding muscles. In some situations, pain can be felt at the front of the pelvis, down near the pubic bone.

Motions of the Sacroiliac Joint

The motions of the sacroiliac joint's are:

Sacral flexion
Sacral extension
Sacral forward torsion

Pubic Bone back

The pubic bone consists of the body and two ramus bones, the superior and inferior pubic rami. This surface generally is oval. The inferior pubic rami pass posterior and inferior to join the ramus of ischium to form half of the pubic arch. The and length of the pubic arch depends on the shape of the pelvis.

The Hip back

Walking takes place from the pelvis to the foot, not the other way around and occurs in a closed kinetic chain. When people walk, their pelvises don't drop dramatically on a non-stance side because gravity's adductor is balanced by an equal and opposite abductor movement. A major factor of the hip abductor is in a closed-chain motion to maintain a level pelvis in unilateral stance. Hip joint reaction force depends on the ratio of lever arm of abductor muscle force and gravity. Because the centre of gravity lies posterior to the joint axis, body weight also creates a bending moment, increased with hip flexion. Because hip abductor activity is necessary to stabilize the hip in the frontal plane during unilateral stance, people with weakness have a problem. One might see the pelvis drop on the unsupported side if we ask a person to stand briefly on the weakened limb.

The inability to maintain a level pelvis in unilateral stance is called a "Positive Trendelenburg Sign". The most direct way to orientate the line of application of the gravity vector is to shorten the arm movement with respect to the hip joint, leaning the trunk towards the side of the hip whose abductor muscles are weak. This movement and type of stance is called "Trendelenburg Gait Pattern". (Go to treatments to view Trendelenburg Test).

Hip adductor strength can be recommended to measure, disease and severity in pelvic instability. Decreased hip adduction strength appears to be caused by the inability to use the hip muscles rather than by weakness of the muscles. By using an aid to assist in stance and movement is to create an additional force that keeps the pelvis level. Additionally, the person needs adequate strength in the muscles of the wrist, elbow, shoulder girdle and trunk to transfer sufficient weight to the aid. (Go to treatments to view Hip Add / Abduction test).

Hip Anatomy The hip joint is a ball and socket joint between the head of femur and the acetabulum of the pelvis. The articular capsule encloses the head and the greater part of the neck of the femur. Anteriorly the capsule is thickened as the iliofemoral ligament. This prevents over-extension. Medially the capsule is thickened to form the pubofemoral ligament. The head of femur is connected with the hip bone by the ligament of head of femur which connects the pit on the head and the margins of the acetabular notch.