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The ischium forms the back lower part of the hip bone, known as the os coxae. The bone sits directly below the acetabulum. The hip bones help provide support for the lower spine and aid upper leg movement.
Each side of the pelvis contains one of these, which are divisible into three portions. The body of the bone reaches up toward the acetabulum. The superior ramus goes down and toward the back of the body. The inferior ramus is the thin, flat piece of bone that joins the ischium to the pubis, the lower center portion of the hip bones.
The ischium absorbs much of the weight of the human body when a person sits down. While this does not normally cause problems, some people experience bone pain when sitting for extended periods, especially if the hips or pelvis have suffered injury. Pain from this area is normally felt in the bottom of the buttocks, or in the hamstring muscles along the back of the thighs.
Ischial tuberosity pain, also called weaver’s bottom, is one of the most common types of bone pain felt in the ischium. The condition is most common in cyclists, runners, and soccer players. It usually develops over time, as the hamstring muscles and other ligaments pull at the ischial tuberosity, or the broad part of the front of the ischium. Sudden trauma or injury can also be responsible for the onset of such pain.
Most of the time, resting and icing the injured hip and taking over-the-counter anti-inflammatory drugs, such as ibuprofen, is sufficient to treat ischial tuberosity pain. Patients with pain that persists for several weeks or results from a serious injury may receive cortisone injections to manage swelling and pain. Physical therapy may also help strengthen the area after an injury.
A fracture is rare, though it can lead to severe pain and a long treatment and recovery phase. Avulsion fractures are the most common types that affect the ischium, and they usually occur in young athletes. These breaks happen when a portion of bone is pulled away from the main bone, usually due to a tendon or ligament contracting.
Fractures of the ischial tuberosity are usually treated with physical therapy and medications to reduce swelling and pain. Athletic activity is generally limited for several months while the fracture heals. If the bone is pulled more than a few centimeters away from its point of origin, an orthopedic surgeon may repair the fracture surgically, or move the fractured bone away from any growth plates that could be damaged by it.
@KoiwiGal - Well, I believe the shape of the ischium is very different depending on how you walk, so, for example, we probably have more in common with a tyrannosaurus rex in the shape of our ischiums than with a gorilla, simply because one usually walks upright and the other doesn't.
The shape of the pelvis was one of the things we studied in my course on evolution because you can tell a whole bunch of things about an animal by the shape of the pelvis, including the ischium.
So it doesn't surprise me that they classify dinosaurs according to their pelvic bones, particularly when they often don't find complete skeletons and the pelvis is large enough to be one of the more commonly found bones. They can define from the ischium, whether the animal was upright or on all fours, whether it was egg bearing and so forth.
One of the cool things about bones is that they are similar in almost all animals. So, when I looked up ischium on Wikipedia, it had a bit about the ischium in dinosaurs and how some different kinds of dinosaurs are classified according to hip structure.