Ischium
Ischium forms the posteroinferior portion of the hip bone and bears weight when sitting.
The ischium is the posteroinferior component of the hip bone and one of the three bones (alongside the ilium and pubis) that fuse to form the os coxae. It contributes significantly to the acetabulum and forms a major part of the pelvic floor and the boundaries of the obturator foramen. Most notably, the ischium bears the body's weight during sitting, thanks to its thick ischial tuberosity. Its structure supports locomotion, provides muscle attachments, and helps define the lower pelvic wall.
Structure
The ischium is composed of a body and a ramus. It is a strong, curved bone that forms the lower and back portion of the pelvis and blends with the other two hip bones at the acetabulum.
1. Body of Ischium
- Thick portion that contributes to the posterior wall of the acetabulum
- Includes the ischial spine and the greater sciatic notch (shared with ilium)
2. Ramus of Ischium
- Extends anteriorly to join the inferior ramus of the pubis
- Helps form the inferior boundary of the obturator foramen
Key Bony Landmarks
| Landmark | Description |
|---|---|
| Ischial Spine | Pointed projection separating the greater and lesser sciatic notches; attachment site for sacrospinous ligament |
| Ischial Tuberosity | Thickened, rough surface that bears weight during sitting; origin for hamstring muscles |
| Lesser Sciatic Notch | Indented area below the ischial spine; forms part of the lesser sciatic foramen |
Location
The ischium is located in the lower posterior part of the hip bone. It is inferior to the ilium and posterior to the pubis. The ischial tuberosity lies at the most inferior part and is easily palpable in seated positions.
| Position | Relation |
|---|---|
| Superior | Fused to the ilium at the acetabulum |
| Anterior | Meets the pubis at the ischiopubic ramus |
| Posterior | Forms part of the gluteal region |
| Inferior | Ischial tuberosity touches the seating surface |
Function
- Weight Bearing: The ischial tuberosities support the weight of the upper body during sitting
- Muscle Attachment: Provides origin points for the hamstring muscles and adductors of the thigh
- Pelvic Wall Formation: Helps define the boundaries of the lesser pelvis and obturator foramen
- Ligament Anchor: The ischial spine and tuberosity serve as attachments for ligaments like sacrospinous and sacrotuberous
Muscle and Ligament Attachments
| Structure | Attachment Area |
|---|---|
| Semitendinosus | Ischial tuberosity |
| Semimembranosus | Ischial tuberosity |
| Biceps Femoris (long head) | Ischial tuberosity |
| Adductor Magnus (hamstring part) | Ischial ramus and tuberosity |
| Quadratus Femoris | Lateral border of ischial tuberosity |
| Sacrospinous Ligament | Ischial spine |
| Sacrotuberous Ligament | Ischial tuberosity |
Clinical Significance
- Ischial Bursitis: Inflammation of the bursa overlying the ischial tuberosity, often seen in cyclists or rowers (“weaver’s bottom”)
- Hamstring Avulsion: Sudden contraction may pull tendons from the ischial tuberosity, especially in sprinters
- Pressure Sores: The ischial tuberosities are common pressure points in paraplegic patients who sit for prolonged periods
- Pelvic Outlet: The ischium helps define the boundaries of the pelvic outlet — clinically relevant in obstetrics
- Ischial Fractures: May occur during direct trauma to the pelvis and are often associated with high-impact injuries
Last updated on May 9, 2025
