ischion

Ischion is an anatomical term referring to the ischial bone, one of the three components of the hip bone (the others being the ilium and pubis). The word ischion appears in older or alternative texts, and modern usage more commonly uses ischium, though the two terms describe the same structure.

Anatomy and structure: The ischium forms the posterior and inferior part of the hip bone. It consists of a body, a ramus, and a prominent ischial tuberosity. The body contributes to the acetabulum, the socket of the hip joint, while the ramus extends toward the pubis. The ischial tuberosity, often called the sitting bone, is the bony prominence that bears weight when seated. Several muscles originate from or attach to the ischial tuberosity, including the hamstring group (semitendinosus, semimembranosus, and the long head of the biceps femoris). The ischial spine, located just above the tuberosity, serves as an attachment point for ligaments such as the sacrospinous ligament and is in proximity to the pudendal nerve.

Function and clinical relevance: The ischium contributes to the stability of the pelvis and the hip joint. Its tuberosity provides an important muscular attachment point and bears load during sitting. Clinically, fractures of the ischium can occur with pelvic trauma, and avulsion injuries at the ischial tuberosity are common in athletes. Knowledge of the ischial region is also relevant in obstetrics and pelvic surgery due to nearby neurovascular structures.

Etymology: The term derives from Latin ischion, itself originating from Greek iskhion, referring to the same anatomical region. In modern anatomy, ischium is the standard name, with ischion appearing mainly in historical or alternate usages.