Pelvic Bones

The pelvic bones are the foundational structures of the bony pelvis, forming a sturdy basin that supports the weight of the upper body, anchors powerful muscles, and houses vital pelvic organs. This bony ring comprises the paired hip bones (ilium, ischium, and pubis), the sacrum, and the coccyx. Together, these bones not only create the pelvic cavity but also establish critical articulations for bipedal locomotion and childbirth.

Structure

The pelvic bones include the following:

• Two hip bones (ossa coxae) — each made up of the ilium, ischium, and pubis
• Sacrum — a wedge-shaped bone formed by fused sacral vertebrae
• Coccyx — a small, terminal bone formed by fused coccygeal vertebrae

Ilium

The ilium is the broad, fan-shaped superior part of the hip bone. It articulates with the sacrum at the sacroiliac joint and provides attachment for abdominal and gluteal muscles. Key landmarks include the iliac crest, anterior superior iliac spine (ASIS), and greater sciatic notch.

Ischium

Situated posteroinferiorly, the ischium forms the lower and back part of the hip bone. Its thick ischial tuberosity bears the body’s weight when sitting. The ischial spine and ramus are notable structural components contributing to the obturator foramen and pelvic outlet.

Pubis

The pubis is the anterior portion of the hip bone, consisting of a body and two rami. The left and right pubic bones meet at the pubic symphysis, a fibrocartilaginous joint. It forms the anterior boundary of the pelvic inlet and part of the acetabulum.

Sacrum

The sacrum is a triangular bone formed by the fusion of five sacral vertebrae. It articulates laterally with the ilium via the sacroiliac joints and superiorly with the fifth lumbar vertebra. Its anterior (pelvic) surface is concave, while the posterior surface is convex and marked by ridges and foramina.

Coccyx

The coccyx, or tailbone, consists of 3–5 fused coccygeal vertebrae. Though vestigial in function, it serves as an anchor point for ligaments and muscles of the pelvic floor, including the levator ani and coccygeus muscles.

Location

The pelvic bones are located at the base of the spine and form the lower boundary of the trunk. They sit between the abdomen and lower limbs, enclosing the pelvic cavity. The two hip bones meet anteriorly at the pubic symphysis and articulate posteriorly with the sacrum and coccyx.

Function

• Support: Transmits weight from the axial skeleton to the lower limbs when standing or walking
• Protection: Encases and shields pelvic organs such as the urinary bladder, rectum, uterus, and ovaries
• Locomotion: Serves as an attachment point for muscles involved in movement of the hip and thigh
• Posture: Maintains upright posture by acting as a base for the vertebral column
• Childbirth: In females, the shape and dimensions of the pelvic bones are critical for parturition

Clinical Significance

• Pelvic Fractures: Often result from high-impact trauma (e.g., motor vehicle accidents). Disruption of the pelvic ring can lead to severe bleeding and organ injury.
• Pelvic Inlet/Outlet Measurements: Important in obstetrics to assess whether vaginal delivery is feasible
• Developmental Dysplasia of the Hip (DDH): Abnormal formation of the acetabulum can lead to joint instability in infants
• Osteitis Pubis: Inflammation at the pubic symphysis, commonly seen in athletes due to repetitive stress
• Coccygodynia: Pain in the coccyx region, often due to trauma or prolonged sitting