ISCHIUM: GENERALITIES, ANATOMY, PARTS, CLINICAL CONSIDERATIONS - ANATOMY AND PHYSIOLOGY - 2025

The ischium is an even bony structure, which is part of the skeleton of the pelvis or bony pelvis. It is found fused with two other bones, the ilium and the pubis. The union of the three pelvic bones is known as the innominate bone and is articulated in its posterior part, with the sacrum. This joint is strongly secured by means of strong and resistant ligaments.

In its lower internal part, it articulates with the pubis; In its upper part with the ilium and in its lower external part, it joins with the head of the femur to form the hip joint.

Human pelvis. From derivative work: LP (talk) Pelvis_diagram.png: Original uploader was Je at uwo at en.wikipedia - Pelvis_diagram.png, Public Domain, The pelvis is the part of the skeletal system that connects the trunk with the lower limbs. Through its joints with the spine and with the legs, it provides mobility to the body.

The ischium, like the rest of the bones that make up the pelvis, serves as an insertion point for the muscular bodies that make up the pelvic floor. For this reason, it has a fundamental function in the support of the internal organs.

Generalities

The ischium is a bone that forms the bony part of the pelvis. It is fused with the other two bones that make it up, the ilium above and the pubis below.

It is an even bone, it is found on both sides of the body. It is a fundamental part of the hip joint, since a large percentage of its body is articulated with the head of the femur.

Its structure is very similar to that of the pubis, as it consists of a body, a branch and a tuber. The branch of the pubis and the ischium are united, forming a pelvic foramen called the obturator foramen, through which important vascular and neurological structures pass.

The ischium also serves as a supporting structure for many muscles and ligaments that make up the so-called pelvic floor, which is a muscular base whose function is to contain the internal organs within the pelvis such as the bladder, rectum and uterus in women.

Embryology and development

The first cartilage outlines that will form the skeleton begin to be observed from the fourth week of gestation.

The ischium and ilium are the first bones of the pelvis to differentiate and find their position in the body of the fetus.

By the ninth week, the slow and progressive formation of these structures can already be seen.

The pelvic bones begin to fuse by the 12th week. The entire process of joining these bones happens slowly from birth through adolescence.

Between 15 and 17 years, the pelvis is completely fused and the muscles are almost fully developed.

Anatomy

Despite being a medium-sized bone, the ischium has a complicated structure due to its multiple projections, concavities, and muscle relationships. It consists of a body, an upper and a lower branch.

In addition to this, it has two prominences in its postero-inferior part that are of the utmost importance for movement.

- Parts of the ischium

Body

The body is the medial portion of the bone. From its posterior border there is a protrusion called the ischial spine. It is at this site that the superior twin pelvic muscle originates.

It represents an important structure because it forms more than half of the socket where the head of the femur will be installed to form the hip joint. This area is called the acetabulum.

The acetabular fossa is made up of the three bones of the pelvis, but the largest area is provided by the ischium.

Symphysis pubis exposed by coronal section. By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See «Book» section below) Bartleby.com: Gray's Anatomy, Plate 321, Public Domain, https://commons.wikimedia.org/w/index. php? curid = 85413

Upper branch

The superior or descending branch is a cubic surface on which some of the important muscles of the pelvic floor originate, such as the quadratus femoris muscle, the transverse perineal muscle, and the ischiocavernosus.

Lower branch

The lower or ascending branch, for its part, is the thinnest and flattest part of the bone. It is usually called the ischiopubic branch, because in its anterior part it meets the lower branch of the pubis and together they form the obturator foramen.

Lower branch. By BodyParts3D is made by DBCLS - Polygondata is from BodyParts3D, CC BY-SA 2.1 jp, The obturator foramen serves as a passageway for important vascular and neurological elements that nourish the pelvis and upper thigh.

Its surface is also the origin of several muscles of the pelvic floor, such as the internal obturator, adductor magnus, and transverse perineum.

Both branches connect with the upper part of the femur through ligaments that go from this bone to insert into the projections of the other. In this way, the pelvis is connected to the lower limbs through the hip joint.

- Ischial tuberosity

Ischial tuberosity or ischial tuberosity is called a robust and irregular convexity found in the posterior and upper part of the lower branch of each ischium. A smoother upper part and a rustic lower part are recognized.

These bony protrusions can be easily palpated with the patient in the fetal position, over the mid-buttock, at the same level as the hip.

Nélaton's line and Bryant's triangle By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See «Book» section below) Bartleby.com: Gray's Anatomy, Plate 1243, Public Domain, https: // commons.wikimedia.org/w/index.php?curid=564859

They have a mechanical and an anatomical function. From them originate the biceps femoris, semitendinosus and semimembranosus muscles, which are the ones that form the back of the thigh.

The origin of these muscles in this area make the ischial tuberosities a fundamental element for sitting.

In topographic anatomy, the union, through an imaginary line, of both ischial tuberosities to separate the pelvic floor anteriorly and posteriorly is used as a limit.

This allows the precise description of injuries and is also a guide to recognize, during surgery, the anatomical elements that are related to them.

Clinical considerations

The ischium is one of the bones that are fused to form the bony pelvis or pelvic girdle.

Since it is rich in blood vessels, and due to its important relationships with nearby muscles and neurological structures, the surgeon who operates this area must have a good understanding of the anatomy of the region.

The obturator artery, which is a branch of the iliac that comes directly from the aorta, makes its way through the obturator foramen. This is accompanied by the nerve and the vein of the same name.

These elements nourish the lower limbs, providing branches that mainly benefit the gluteal, pelvic and upper femur muscles.

References

1. Wobser, AM; Wobser, RW (2018). Anatomy, Abdomen and Pelvis, Bones (Ilium, Ischium, and Pubis). StatPearls, Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
2. Figueroa, C; Le, PH (2019). Anatomy, Bony Pelvis and Lower Limb, Pelvis Bones. StatPearls, Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
3. Buxton, JD (1959). Surgery of the ischium. British Medical Journal. Taken from: ncbi.nlm.nih.gov
4. Gold, M; Varacallo, M. (2019). Anatomy, Bony Pelvis and Lower Limb, Hip Joint. StatPearls, Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
5. Glenister, R; Sharma, S. (2018). Anatomy, Bony Pelvis and Lower Limb, Hip. StatPearls, Treasure Island (FL). Taken from: ncbi.nlm.nih.gov